Part and Mold Design.pdf

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A Design Guide

Part and Mold Design

Engineering Polymers

THERMOPLASTICS


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INTRODUCTION

The manual focuses primarily onplastic part and mold design, but alsoincludes chapters on the design process;designing for assembly; machining andfinishing; and painting, plating, anddecorating. For the most part, it excludesinformation covered in the followingBayer companion publications:

Material Selection: Thermoplastics and

Polyurethanes : A comprehensive look atmaterial testing and the issues to consider

when selecting a plastic material.

Joining Techniques : Includes infor-mation and guidelines on the methodsfor joining plastics including mechanicalfasteners, welding techniques, inserts,snap fits, and solvent and adhesivebonding.

Snap-Fit Joints for Plastics : Containsthe engineering formulas and workedexamples showing how to design snap-fit joints for Bayer thermoplastic resins.

A product of the Bayer Design

Engineering Services Group, this manual

is primarily intended as a reference

source for part designers and molding

engineers working with Bayer thermo-

plastic resins. The table of contents and

index were carefully constructed to

guide you quickly to the information

you need either by topic or by keyword.

The content was also organized to allow

the manual to function as an educational

text for anyone just entering the field of

plastic-part manufacturing. Conceptsand terminology are introduced pro-

gressively for logical cover-to-cover

reading.

Contact your Bayer sales representativefor copies of these publications.

This publication was written specificallyto assist our customers in the design andmanufacture of products made from theBayer line of thermoplastic engineeringresins. These resins include:

Makrolon Polycarbonate

Apec High-Heat Polycarbonate

Bayblend Polycarbonate/ ABS Blend

Makroblend Polycarbonate Blend

Triax Polyamide/ABS Blend

Lustran and Novodur ABS

Lustran SAN

Cadon SMA

Centrex ASA, AES and ASA/AESWeatherable Polymers

Durethan Polyamide 6 and 66,and Amorphous Polyamide

Texin and Desmopan

Thermoplastic Polyurethane

Pocan PBT Polyester

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This publication was written to assistBayer's customers in the design andmanufacture of products made fromthe Bayer line of thermoplasticengineering resins. These resinsinclude:

- Makrolon polycarbonate- Apec high-heat polycarbonate- Bayblend polycarbonate/ABSblend- Makroblend polycarbonate/ polyester blend- Texin and Desmopan

thermoplastic polyurethaneFor information on these materials,please call 1-800-662-2927 or visithttp://www.BayerMaterialScienceNAFTA.com.

The following additional productshighlighted in this publication are nowpart of LANXESS Corporation:

- Cadon SMA- Centrex ASA, AES and ASA/AESweatherable polymers- Durethan polyamide 6 and 66, andamorphous polyamide- Lustran and Novodur ABS- Lustran SAN- Pocan PBT polyester- Triax polyamide/ABS blend

For information on these products,please call LANXESS in NorthAmerica at 1-800-LANXESS or visit:

http://techcenter.lanxess.com/sty/ americas/en/home/index.jsp forstyrenic resinshttp://techcenter.lanxess.com/scp/ americas/en/home/index.jsp forpolyamide resins


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Bayer CAMPUS : Software containingsingle and multi-point data that wasgenerated according to uniform standards.Allows you to search grades of Bayerresins that meet a particular set ofperformance requirements.

www.bayer.com/polymers-usa: BayerWeb site containing product informationon-line.

This manual provides general information

and guidelines. Because each productapplication is different, always conducta thorough engineering analysis of yourdesign, and prototype test new designsunder actual in-use conditions. Applyappropriate safety factors, especiallyin applications in which failure couldcause harm or injury.

Most of the design principles covered inthis manual apply to all of these resins.When discussing guidelines or issuesfor a specific resin family, we referencethese materials either by their Bayertrade names or by their genericpolymer type.

The material data scattered throughoutthe chapters is included by way of example only and may not reflect themost current testing. In addition, much

of the data is generic and may differfrom the properties of specific resingrades. For up-to-date performance datafor specific Bayer resins, contact yourBayer sales representative or refer to thefollowing information sources:

Bayer Engineering Polymers Properties

Guide : Contains common single-pointproperties by resin family and grade.

Bayer Plastics Product Information

Bulletin : Lists information and propertiesfor a specific material grade.

In addition to design manuals, BayerCorporation provides design assistancein other forms such as seminars andtechnical publications. Bayer also offersa range of design engineering servicesto its qualified customers. Contact yourBayer sales representative for moreinformation on these other services.

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Chapter 1PART DESIGN PROCESS: CONCEPT TO FINISHED PART

7 Design Process

8 Defining Plastic Part Requirements

8 Mechanical Loading

8 Temperature

8 Chemical Exposure

8 Electrical Performance

8 Weather Resistance

8 Radiation

8 Appearance

9 Agency Approvals

9 Life Expectancy9 Dimensional Tolerances

9 Processing

9 Production Quantities

9 Cost Constraints

10 Assembly

10 Thermoplastic Processing Methods

10 Injection Molding

11 Extrusion

12 Thermoforming

12 Blow Molding

13 Rotomolding

13 Optimizing Product Function

14 Consolidation

14 Hardware

14 Finish

15 Markings and Logos

15 Miscellaneous

15 Reducing Manufacturing Costs

15 Materials

16 Overhead

17 Labor17 Scrap and Rework

17 Prototype Testing

Chapter 2GENERAL DESIGN

19 Wall Thickness

22 Flow Leaders and R estrictors

24 Ribs

24 Rib Design

24 Rib Thickness

26 Rib Size

27 Rib Location and Numbers

27 Bosses

30 G ussets

30 Sharp Corners

32 Draft33 Holes and Cores

34 Undercuts

34 Slides and Cores

36 Louvers and Vents

37 Molded-In Threads

40 Lettering

40 Tolerances

42 Bearings and Gears

TABLE OF CONTENTS

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Chapter 4DESIGN FOR ASSEMBLY

83 Part Consolidation

84 Mechanical Fasteners

85 Snap-Fit Joints

88 Welding and Bonding

89 Ultrasonic Welding

90 Vibration and Hot-Plate Welding

91 Spin Welding

91 Solvent and Adhesive Bonding

92 Retention Features

92 Alignment F eatures

94 Orientation94 Expansion Differences

94 Tolerances

Chapter 5MACHINING AND FINISHING

97 Drilling and Reaming

99 Tapping

99 S awing

100 Punching, Blanking, and Die Cutting

101 Milling

101 Turning and Boring

102 Laser Machining

103 Filing

103 Sanding

103 Polishing and Buffing

104 Trimming, Finishing, and Flash Removal

Chapter 3STRUCTURAL DESIGN

45 Structural Considerations In Plastics

46 Stiffness

46 Viscoelasticity

48 Stress-Strain Behavior

50 Molding Factors

51 Short-Term Mechanical Properties

51 Tensile Properties

52 Tensile Modulus

52 Tensile Stress at Yield

52 Tensile Stress at Break

53 Ultimate Strength53 Poisson's Ratio

53 Compressive Properties

53 Flexural Modulus

53 Coefficient of Friction

54 Long-Term Mechanical Properties

54 Creep Properties

56 Stress Relaxation

56 Fatigue Properties

58 Structural Design Formulas

58 Use of Moduli

59 Stress and Strain Limits

60 Uniaxial Tensile and Compressive Stress

61 Bending and Flexural Stress

65 Shear Stress

66 Torsion

67 Designing for Stiffness

67 Part Shape

70 Wall Thickness

71 Ribs

73 Long-Term Loading

76 Designing for Impact78 Fatigue Applications

80 Thermal Loading


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Chapter 7MOLD DESIGN

121 Mold Basics

121 Types of Molds

124 Mold Bases and Cavities

125 Molding Undercuts

128 Part Ejection

130 Mold Venting

130 Parting-Line Vents

131 Vent Placement

133 Sprues, Runners, and Gates

133 Sprues

134 Runners137 Runners for Multicavity Molds

140 Gates

144 Other Gate Designs

145 Gate Optimization

147 Gate Position

149 Hot-Runner Systems

149 Hot-Runner Designs

149 Hot-Runner Gates

151 Valve Gates

151 Thermal Expansion and Isolation

152 Flow Channel Size

153 Mold Cooling

154 Mold-Cooling Considerations

155 Cooling-Channel Placement

158 Cooling-Line Configuration

159 Coolant Flow Rate

160 Mold Shrinkage

162 Mold Metals

163 Surface Treatments

164 Mold Cost and Quality

APPENDICES

165 Index

169 Part Design Checklist

Chapter 6PAINTING, PLATING, AND DECORATING

105 Painting

105 Types of Paints

106 Paint Curing

106 Paint-Selection Considerations

107 Spray Painting

108 Other Painting Methods

108 Masking

109 Other Design Considerations for Painting

109 In-Mold Decorating

110 Film-Insert Molding

111 Metallic Coatings111 Electroplating

112 Design Considerations for Electroplating

113 Molding Considerations for Electroplating

114 Vacuum Metallization

115 Design Considerations for Vacuum Metallization

115 EMI/RFI Shielding

115 Design Considerations for EMI/RFI Shielding

116 Printing

118 Labels and Decals

119 Texture

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Chapter 1

PART DESIGN PROCESS:CONCEPT TO FINISHED PART

DESIGN PROCESS

Like a successful play in football,successful plastic product design andproduction requires team effort and awell-developed strategy. When designingplastic parts, your team should consistof diverse players, including conceptualdesigners, stylists, design engineers,materials suppliers, mold makers,manufacturing personnel, processors,finishers, and decorators. Your chance

of producing a product that successfullycompetes in the marketplace increaseswhen your strategy takes full advantageof team strengths, accounts for memberslimitations, and avoids overburdeningany one person. As the designer, youmust consider these factors early instrategy development and makeadjustments based upon input from thevarious people on the design team.

Solicit simultaneous input from the var-ious players early in product develop-ment, before many aspects of the designhave been determined and cannot bechanged. Accommodate suggestions forenhancing product performance, or forsimplifying and improving the variousmanufacturing steps such as moldconstruction, processing, assembly,and finishing. Too often designs pass

sequentially from concept developmentto manufacturing steps with featuresthat needlessly complicate productionand add cost.

Many factors affect plastic-part design.

Among these factors are: functional

requirements, such as mechanical

loading and ultraviolet stability;

aesthetic needs, such as color, level of

transparency, and tactile response; and

economic concerns, such as cost of

materials, labor, and capital equipment.

These factors, coupled with other

design concerns such as agency

approval, processing parameters,

and part consolidation are discussed

in this chapter.

Early input from various design andmanufacturing groups also helps tofocus attention on total product costrather than just the costs of individualitems or processes. Often adding aprocessing step and related cost in onearea produces a greater reduction intotal product cost. For example, addingsnap latches and nesting features mayincrease part and mold costs, and at thesame time, produce greater savings inassembly operations and related costs.

Likewise, specifying a more-expensiveresin with molded-in color and UVresistance may increase your raw-material cost, while eliminatingpainting costs.

When designing and developing parts,focus on defining and maximizing partfunction and appearance, specifyingactual part requirements, evaluatingprocess options, selecting an appropri-ate material, reducing manufacturingcosts, and conducting prototype testing.For the reasons stated above, theseefforts should proceed simultaneously.

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Chemical Exposure

Plastic parts encounter a wide variety of chemicals both during manufacturingand in the end-use environment,including mold releases, cutting oils,degreasers, lubricants, cleaning sol-vents, printing dyes, paints, adhesives,cooking greases, and automotive fluids.Make sure that these chemicals arecompatible with your selected materialand final part.

Electrical Performance

Note required electrical property valuesand nature of electrical loading. Forreference, list materials that are knownto have sufficient electrical performancein your application. Determine ifyour part requires EMI shielding orUL testing.

Weather Resistance

Temperature, moisture, and UV sunexposure affect plastic parts propertiesand appearance. The end-use of a productdetermines the type of weather resistancerequired. For instance, external automo-tive parts such as mirror housings must

withstand continuous outdoor exposureand perform in the full range of weatherconditions. Additionally, heat gain fromsun on dark surfaces may raise the uppertemperature requirement considerablyhigher than maximum expected temper-atures. Conversely, your requirements

DEFINING PLASTIC PARTREQUIREMENTS

Thoroughly ascertain and evaluate yourpart and material requirements, whichwill influence both part design andmaterial selection. When evaluatingthese requirements, consider more than

just the intended, end-use conditionsand loads: Plastic parts are often sub-

jected to harsher conditions duringmanufacturing and shipping than in

actual use. Look at all aspects of partand material performance includingthe following.

Mechanical Loading

Carefully evaluate all types of mechanicalloading including short-term staticloads, impacts, and vibrational orcyclic loads that could lead to fatigue.Ascertain long-term loads that couldcause creep or stress relaxation. Clearlyidentify impact requirements.

Temperature

Many material properties in plastics impact strength, modulus, tensilestrength, and creep resistance to name a

few vary with temperature. Considerthe full range of end-use temperatures,as well as temperatures to which the partwill be exposed during manufacturing,finishing, and shipping. Remember thatimpact resistance generally diminishesat lower temperatures.

may be less severe if your part isexposed to weather elements onlyoccasionally. For example, outdoorChristmas decorations and other season-al products may only have to satisfy therequirements for their specific, limitedexposure.

Radiation

A variety of artificial sources such

as fluorescent lights, high-intensity dis-charge lamps, and gamma sterilizationunits emit radiation that can yellowand/or degrade many plastics. If yourpart will be exposed to a radiationsource, consider painting it, or specifyinga UV-stabilized resin.

Appearance

Aesthetic requirements can entail manymaterial and part-design issues. Forexample, a need for transparency greatlyreduces the number of potential plastics,especially if the part needs high clarity.Color may also play an important role.Plastics must often match the color of other materials used in parts of anassembly. Some applications require theplastic part to weather at the same rate

as other materials in an assembly.

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Chapter 1

PART DESIGN PROCESS:CONCEPT TO FINISHED PART continued

contact, United States Department of Agriculture (USDA) for plastics inmeat and poultry equipment, andNational Sanitation Foundation TestingLaboratory, Inc. (NSF) for plastics infood-processing and potable-waterapplications. Always check forcompliance and approval fromappropriate agencies. Determine ifyour part requires flame resistance inaccordance with UL 94. If so, noterating and thickness.

Life Expectancy

Many functional parts need to meetcertain life-cycle expectations. Lifeexpectancy may involve a time duration as in years of outdoor exposure time at a specific set of conditions such as hours in boiling water orrepetitions of an applied load orcondition as in number of gammasterilization cycles or snap-armdeflections. Determine a reasonablelife expectancy for your part.

Dimensional Tolerances

Many applications have featuresrequiring tight tolerances for proper fit

and function. Some mating parts requireonly that mating features have the samedimensions. Others must have absolutesize and tolerance. Consider the effectof load, temperature, and creep ondimensions. Over-specification oftolerance can increase productcost significantly.

In resins, custom colors generally costmore than standard colors, particularlyfor small-order quantities. For certaincolors and effects, some parts may needto be painted or decorated in the mold.Depending upon the application, partswith metallic finishes may requirepainting, in-mold decorating or vacuummetallization. Surface finishes rangefrom high-gloss to heavy-matte.Photoetching the mold steel can impartspecial surface textures for parts.

Styling concerns may dictate the prod-uct shape, look, and feel, especially if the product is part of a component sys-tem or existing product family. Note allcosmetic and non-cosmetic surfaces.Among other things, these areas mayinfluence gate, runner, and ejector-pinpositioning.

Many part designs must include mark-ings or designs such as logos, warnings,instructions, and control labels.Determine if these features can bemolded directly onto the part surfaceor if they must be added using one of the decorating methods discussed inChapter 6.

Agency Approvals

Government and private agencies havespecifications and approval cycles formany plastic parts. These agenciesinclude Underwriters Laboratories(UL) for electrical devices, Military(MIL) for military applications, Foodand Drug Administration (FDA) forapplications with food and bodily-fluid

Processing

Determine if your part design placesspecial demands on processing. Forexample, will the part need a moldgeometry that is particularly difficultto fill, or would be prone to warpageand bow. Address all part-ejection andregrind issues.

Production Quantities

The number of parts needed mayinfluence decisions, including processingmethods, mold design, material choice,assembly techniques, and finishingmethods. Generally for greater productionquantities, you should spend money tostreamline the process and optimizeproductivity early in the design process.

Cost Constraints

Plastic-part cost can be particularlyimportant, if your molded part comprisesall or most of the cost of the final product.Be careful to consider total system cost,not just part and material cost.

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THERMOPLASTICPROCESSING METHODS

A variety of commercial methods areused to produce thermoplastic products.Each has its specific design require-ments, as well as limitations. Usuallypart design, size, and shape clearlydetermine the best process.Occasionally, the part concept lendsitself to more than one process. Becauseproduct development differs depending

upon the process, your design teammust decide which process to pursueearly in product development. Thissection briefly explains the commonprocesses used for thermoplastics fromBayer Corporation.

Assembly

Address assembly requirements, such asthe number of times the product will bedisassembled or if assembly will beautomated. List likely or proposedassembly methods: screws, welds,adhesives, snap-latches, etc. Note matingmaterials and potential problem areassuch as attachments to materialswith different values of coefficient of linear thermal expansion. State any

recycling requirements.

The Part Requirements and DesignChecklist in the back of this manualserves as a guide when developing newproducts. Be sure not to overlook anyrequirements relevant to your specificapplication. Also do not over-specifyyour requirements. Because partsperform as intended, the costs of over-specification normally go uncorrected,needlessly increasing part cost andreducing part competitiveness.

Injection Molding

The most common processing methodfor Bayer thermoplastics, injectionmolding, involves forcing moltenplastic into molds at high pressure. Theplastic then forms to the shape of themold as it cools and solidifies (seefigure 1-1). Usually a quick-cycleprocess, injection molding can producelarge quantities of parts, accommodatea wide variety of part sizes, offer

excellent part-to-part repeatability,and make parts with relatively tighttolerances. Molds can produce intricatefeatures and textures, as well as structuraland assembly elements such as ribs andbosses. Undercuts and threads usually

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The injection molding process can quickly produce large quantities of parts inmulti-cavity molds.

Figure 1-1Injection Molding


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Chapter 1


of each part. The same mold producing500,000 parts would contribute only$0.10 to part cost. Additionally, mold

modifications for product designchanges can be very expensive. Verylarge parts, such as automotive bumpersand fenders, require large and expensivemolds and presses.

require mold mechanisms that addto mold cost.

The injection molding process generallyrequires large order quantities to offsethigh mold costs. For example, a$50,000 mold producing only 1,000parts would contribute $50 to the cost

Extrusion

In extrusion forming, molten materialcontinuously passes through a die thatforms a profile which is sized, cooled,and solidified. It produces continuous,straight profiles, which are cut tolength. Most commonly used for sheet,film, and pipe production, extrusion alsoproduces profiles used in applicationssuch as road markers, automotive trim,store-shelf price holders, and window

frames (see figure 1-2). Productionrates, measured in linear units, such asfeet/minute, ordinarily are reasonablyhigh. Typically inexpensive for simpleprofiles, extrusion dies usuallycontribute little to product cost. Partfeatures such as holes or notchesrequire secondary operations thatadd to final cost.

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Extrusion

The extrusion process produces profile shapes used in the manufacture of window frames.

Figure 1-2


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Thermoforming

Thermoforming creates shapes from athermoplastic sheet that has been heatedto its softening point. Applied vacuumor pressure draws or pushes the softenedsheet over an open mold or form whereit is then cooled to the conformingshape. The process of stretching thesheet over the form or mold causesthinning of the wall, especially alongthe sides of deep-drawn features. Mold

or form costs for this low-pressureprocess are much lower than for injectionmolds of comparable size.

Thermoforming can produce large parts(see figure 1-3) on relatively inexpensivemolds and equipment. Because theplastic is purchased as sheet stock,materials tend to be costly. Material

selection is limited to extrusion grades.Secondary operations can play a largerole in part cost. Thermoformed partsusually need to be trimmed to removeexcess sheet at the part periphery. Thisprocess cannot produce features thatproject from the part surface such asribs and bosses. Cutouts and holesrequire secondary machining operations.

Blow Molding

Blow molding efficiently produceshollow items such as bottles (seefigure 1-4), containers, and light globes.

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The automobile industry has taken advantage of the production efficiency, appearance, lightweight, and performance of thermoformed engineering thermoplastics for many OEM andafter-market products like this tonneau cover.

Figure 1-3Thermoforming

Blow Molding

This large water bottle was blow molded inMakrolon polycarbonate resin.

Figure 1-4


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Rotomolding

In rotomolding , a measured quantity of thermoplastic resin, usually powdered,is placed inside a mold, which is thenexternally heated. As the mold rotateson two perpendicular axes, the resincoats the heated mold surface. Thiscontinues until all the plastic melts toform the walls of the hollow, molded

shape. While still rotating, the mold iscooled to solidify the shape.

Design permitting, the process mayalso produce hollow shapes such asautomotive air ducts and gas tanks.Wall thickness can vary throughout thepart and may change with processing.Blow molding cannot produce featuresthat project from the surface such asribs and bosses. Part geometrydetermines mold and equipment costs,which can range as high as those forinjection molding.

The two most-common types of blowmolding are extrusion and injection. Inextrusion blow molding , mold halvespinch the end of a hanging extrudedtube called a parison until itseals. Air pressure applied into the tubeexpands the tube and forces it againstthe walls of the hollow mold. Theblown shape then cools as a thin-walledhollow shape. A secondary step removesthe vestige at the pinch-off area.

Injection blow molding substitutes amolded shape in place of the extrudedparison. Air pressure applied frominside the still-soft molded shapeexpands the shape into the form of thehollow mold. This process eliminatespinch-off vestige and facilitates moldedfeatures on the open end such as screwthreads for lids.

This process is used for hollow shapeswith large open volumes that promoteuniform material distribution, includingdecorative streetlight globes (see figure1-5) or hollow yard toys. Mold andequipment costs are typically low, andthe process is suited to low-productionquantities and large parts. Cycle timesrun very long. Large production runsmay require multiple sets of molds.

OPTIMIZINGPRODUCT FUNCTION

The molding process affords manyopportunities to enhance part function-ality and reduce product cost. For exam-ple, the per-part mold costs associatedwith adding functional details to thepart design are usually insignificant.Molds reproduce many features practi-cally for free. Carefully review allaspects of your design with an eyetoward optimization, including partand hardware consolidation, finishingconsiderations, and needed markingsand logos, which are discussed inthis section.

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Rotomolding

Rotomolding can produce large hollowparts such as this polycarbonate streetlight globe.

Figure 1-5

Chapter 1



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Consolidation

Within the constraints of good moldingpractice and practical mold construction,look for opportunities to reduce thenumber of parts in an assembly throughpart consolidation. A single molded partcan often combine the functionality of two or more parts.

Hardware

Clever part design can often eliminateor reduce the need for hardware fastenerssuch as screws, nuts, washers, andspacers. Molded-in hinges can replacemetal ones in many applications (seefigure 1-6). Molded-in cable guidesperform the same function as metal onesat virtually no added cost. Reducinghardware lessens material and assemblycosts, and simplifies dismantling forrecycling.

Finish

Consider specifying a molded-in colorinstead of paint. The cost savings couldmore than justify any increase in mater-ial cost for a colored material with therequired exposure performance. If you

must paint, select a plastic that paintseasily, preferably one that does notrequire surface etching and/or primer.

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Molded-in hinge features can eliminate the need for hinge hardware.

Figure 1-6 Hinges

SlightUndercut

PL


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Chapter 1


REDUCINGMANUFACTURING COSTS

Although many factors contribute tocosts of producing plastic parts, mostcosts fall into one of four basic categories:materials, overhead, labor, and scrap/ rework. This section highlights potentialmethods for reducing these manufacturingcosts. Carefully evaluate the effect eachcost-reduction step may have on yourproducts performance and overall cost.

Materials

To reduce material costs, you mustreduce material usage and obtain thebest material value. Within the limits

Markings and Logos

Secondary methods of adding direc-tions, markings, and logos includinglabels, decals, printing, stamping, etc. add cost and labor. Molded-in tech-niques, when applied properly, producepermanent lettering and designs at avery low cost (see figure 1-7). Mixturesof gloss and texture can increase contrastfor improved visibility.

Miscellaneous

Look for opportunities to add easily-molded features to simplify assemblyand enhance product function such asaligning posts, nesting ribs, finger grips,guides, stops, stand-offs, hooks, clips,and access holes.

of good design and molding practice,consider some of the following:

Core out unneeded thickness andwall stock;

Use ribs, stiffening features, andsupports to provide equivalentstiffness with less wall thickness;

Optimize runner systems tominimize waste;

Use standard colors, which are lessexpensive than custom colors;

Compare the price of materials thatmeet your product requirements, butavoid making your selection basedupon price alone; and

Consider other issues such as materialquality, lot-to-lot consistency, on-timedelivery, and services offered bythe supplier.

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Molded-In Illustrations

This molded in schematic is a cost effective alternative to labels or printing.

Figure 1-7


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This last option requires carefulevaluation to determine if machine-cost-per-part savings compensate forthe added mold cost.

Mold costs, usually amortized over aspecified number of parts or years, canalso make up a significant portion of part cost. This is particularly true ifthe production quantities are low. Thecomplex relationship between moldcost, mold quality, and molding

efficiency is covered in Chapter 7.

Overhead

Hourly press rates comprise a significantportion of part cost. The rate varies byregion and increases with press size.Some options to consider whenevaluating overhead costs include:

Maximizing the number of partsproduced per hour to reduce themachine overhead cost per part;

Avoiding thick sections in your partand runner system that can increasecooling time;

Designing your mold with goodcooling and plenty of draft for easyejection; and

Increasing the number of cavities ina mold to increase hourly production.

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Chapter 1


Scrap and Rework

Part and mold design can contribute toquality problems and scrap. To avoidrework and minimize scrap generation,consider the following:

Follow the part design recommenda-tions and guidelines outlined inChapter 2;

Avoid specifying tighter tolerances

than actually needed; and

Adjust the mold steel to produceparts in the middle of the tolerancerange, when molding parts withtight tolerances.

In the long run, this last suggestionis usually less expensive than tryingto produce parts at the edge of thetolerance range by molding in a narrowprocessing window.

Do not select your mold maker basedon price alone. Cheap molds oftenrequire costly rework and frequentmold maintenance, and are prone topart quality problems.

Labor

When looking to maintain or lower yourlabor costs, consider the following:

Simplify or eliminate manual tasksas much as possible;

Design parts and molds for automaticdegating or place gates in areas thatdont require careful trimming;

Keep parting lines and mold kiss-off areas in good condition to avoidflash removal;

Design parting lines and kiss-off points to orient flash in a less criticaldirection; and

Streamline and/or automatetime-consuming assembly steps.

PROTOTYPE TESTING

Prototype testing allows you to testand optimize part design and materialselection before investing in expensiveproduction tooling. Good prototypetesting duplicates molding, processing,and assembly conditions as closely aspossible. Molded prototype parts canalso be tested under the same range of mechanical, chemical, and environmen-tal conditions that the production parts

must endure.

Simplifying or eliminating prototypetesting increases the chance of problemsthat could lead to delays and expensivemodifications in production tooling.You should thoroughly prototype testall new designs.

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Chapter 2

GENERAL DESIGN

WALL THICKNESS

Wall thickness strongly influencesmany key part characteristics, includingmechanical performance and feel,cosmetic appearance, moldability, andeconomy. The optimum thicknessis often a balance between opposingtendencies, such as strength versusweight reduction or durability versuscost. Give wall thickness carefulconsideration in the design stage toavoid expensive mold modificationsand molding problems in production.

In simple, flat-wall sections, each10% increase in wall thickness providesapproximately a 33% increase in

While engineering resins are used

in many diverse and demanding

applications, there are design elements

that are common to most plastic parts,

such as ribs, wall thickness, bosses,

gussets, and draft. This chapter

covers these general design issues,

as well as others you should consider

when designing parts made of

thermoplastic resins.

stiffness. Increasing wall thickness alsoadds to part weight, cycle times, andmaterial cost. Consider using geometricfeatures such as ribs, curves, andcorrugations to stiffen parts. Thesefeatures can add sufficient strength,with very little increase in weight, cycletime, or cost. For more informationon designing for part stiffness, seeChapter 3.

Both geometric and material factorsdetermine the effect of wall thicknesson impact performance. Generally,increasing wall thickness reducesdeflection during impact and increasesthe energy required to produce failure.In some cases, increasing wall thickness

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Figure 2-1

I Z O D I M P A C T S T R E N G T H

( f t l b / i n )

THICKNESS (in)

Izod impactstrength of Makrolonpolycarbonate vs.thickness at varioustemperatures.

140 F (60 C)

20

18

16

14

12

10

8

6

4

2

0

0.100 0.140 0.180 0.220 0.260 0.300 0.340

73 F (23 C)

-4F (-20 C)

Critical Thickness


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Avoid designs with thin areassurrounded by thick perimetersections as they are prone to gasentrapment problems (see figure 2-2);

Maintain uniform nominal wallthickness; and

Avoid wall thickness variationsthat result in filling from thin tothick sections.

Thin-walled parts those with mainwalls that are less than 1.5 mm thick may require special high-performancemolding equipment to achieve therequired filling speeds and injection

can stiffen the part to the point that thegeometry cannot flex and absorb theimpact energy. The result can be adecrease in impact performance. Somematerials, polycarbonate for example,lose impact strength if the thicknessexceeds a limit known as the criticalthickness . Above the critical thicknessparts made of polycarbonate can show amarked decrease in impact performance.Walls with thickness greater than thecritical thickness may undergo brittle,

rather than ductile, failure duringimpact. The critical thickness reduceswith lowering temperature and molecularweight. The critical thickness formedium-viscosity polycarbonate atroom temperature is approximately3/16 inch (see figure 2-1).

Consider moldability when selectingthe wall thicknesses for your part. Flowlength the distance from the gate tothe last area fill must be withinacceptable limits for the plastic resinchosen. Excessively thin walls maydevelop high molding stresses, cosmeticproblems, and filling problems thatcould restrict the processing window.Conversely, overly thick walls canextend cycle times and create packingproblems. Other points to considerwhen addressing wall thickness include:

pressures. This can drive up themolding costs and offset any materialsavings. Thin-wall molding is generallymore suited for size or weight reductionthan for cost savings. Parts with wallthicknesses greater than 2 mm can alsobe considered as thin-walled parts if their flow-length-to-thickness ratios aretoo high for conventional molding.

Usually, low-shrinkage materials,such as most amorphous or filled resins,

can tolerate nominal wall thicknessvariations up to about 25% without sig-nificant filling, warpage, or appearanceproblems. Unfilled crystalline resins,because of their high molding shrinkage,

20

Non-uniform wall thickness can lead to air traps.

Figure 2-2 Racetracking

ConsistentWall

Thickness

Correct

ThickThin

Air Trap

Incorrect


22/174

Chapter 2

GENERAL DESIGN continued

Many designs, especially those convertedfrom cast metal to plastic, have thick sections that could cause sinks or voids.When adapting these designs to plastic

parts, consider the following:

Core or redesign thick areas tocreate a more uniform wall thickness(see figure 2-3);

can only tolerate about half as muchthickness variation. These guidelinespertain to the parts main walls. Ribsand other protrusions from the wall

must be thinner to avoid sink. For moreinformation about designing ribs andother protrusions, see the section onribs in this chapter.

Make the outside radius one wall-thickness larger than the inside radiusto maintain constant wall thicknessthrough corners (see figure 2-4); and

Round or taper thickness transitionsto minimize read-through andpossible blush or gloss differences(see figure 2-5). Blending alsoreduces the molded-in stresses andstress concentration associated withabrupt changes in thickness.

In some cases, thickness-dependentproperties such as flame retardency,electrical resistance, and sound deaden-ing determine the minimum requiredthickness. If your part requiresthese properties, be sure the materialprovides the needed performance at thethicknesses chosen. UL flammabilityratings, for example, are listed with theminimum wall thickness for whichthe rating applies.

21

Core out thick sections as shown on right to maintain a more uniform wall thickness.

Figure 2-3Coring


23/174

should extend from the gate withoutrestrictions.

To avoid possible warpage and shrink-age problems, limit the added thicknessto no more than 25% of the nominalwall for low-shrinkage, amorphous orfilled materials and to 15% for unfilledcrystalline resins. Carefully transitionthe flow leader into the wall to minimizeread-through and gloss differences onthe other side of the wall.

FLOW LEADERS ANDRESTRICTORS

Occasionally designers incorporatethicker channels, called flow leaders orinternal runners , into the part design.These flow leaders help mold fillingor packing in areas far from the gate.Additionally, flow leaders can balancefilling in non-symmetrical parts, alterthe filling pattern, and reduce sink inthick sections (see figure 2-6). For

best results, the flow-leader thickness

Flow restrictors , areas of reducedthickness intended to modify the fillingpattern, can alleviate air-entrapmentproblems (see figure 2-7) or moveknitlines. When restricting thick flowchannels as in figure 2-7, use thefollowing rules of thumb in your design:

Extend the restrictor across theentire channel profile to effectivelyredirect flow;

22

Internal and external corner radii should originate from the same point.

Figure 2-4 Corner Design

Too Thin

Too Thick

t

R2 = R1 + t

R1R2

Blend transitions to minimize read-through.

Figure 2-5 Thickness Transitions

Incorrect

Correct

Correct

Correct


24/174

Chapter 2


Flow leader and restrictor placementwere traditionally determined by trialand error after the mold was sampled.

Reduce the thickness by no more than33% in high-shrinkage resins or 50%for low-shrinkage materials; and

Lengthen the restrictor todecrease flow.

Today, computerized flow simulationenables designers to calculate thecorrect size and placement beforemold construction.

23

Corners typically fill late in box-shaped parts. Adding flow leadersbalances flow to the part perimeter.

Figure 2-6Flow Leaders

Flow Leader

Flow restrictors can change the filling pattern to correct problemssuch as gas traps.

Figure 2-7Flow Restrictors

Gate


25/174

This section deals with general guide-lines for ribs and part design; structuralconsiderations are covered in Chapter 3.

Rib Design

Proper rib design involves five mainissues: thickness, height, location,quantity, and moldability. Considerthese issues carefully when designingribs.

RIBS

Ribs provide a means to economicallyaugment stiffness and strength in moldedparts without increasing overall wallthickness. Other uses for ribs include:

Locating and captivating componentsof an assembly;

Providing alignment in matingparts; and

Acting as stops or guides formechanisms.

Rib Thickness

Many factors go into determining theappropriate rib thickness . Thick ribsoften cause sink and cosmetic problemson the opposite surface of the wall towhich they are attached (see figure 2-8).The material, rib thickness, surfacetexture, color, proximity to a gate,and a variety of processing conditionsdetermine the severity of sink. Table 2-1gives common guidelines for rib thick-

ness for a variety of materials. Theseguidelines are based upon subjectiveobservations under common conditions

24

Sink opposite thick rib.

Figure 2-8 Sink

Offset rib to reduce read-through and sink.

Figure 2-9 Offset Rib


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Chapter 2


often tolerate ribs that are thicker thanthe percentages in these guidelines. Onparts with wall thicknesses that are 1.0mm or less, the rib thickness should beequal to the wall thickness. Rib thicknessalso directly affects moldability. Verythin ribs can be difficult to fill. Because

and pertain to the thickness at the baseof the rib. Highly glossy, critical sur-faces may require thinner ribs. Placingribs opposite character marks or stepscan hide rib read-through (see figure2-9). Thin-walled parts those withwalls that are less than 1.5 mm can

of flow hesitation , thin ribs near the gatecan sometimes be more difficult to fillthan those further away. Flow enteringthe thin ribs hesitates and freezes whilethe thicker wall sections fill.

Ribs usually project from the main wallin the mold-opening direction and areformed in blind holes in the mold steel.To facilitate part ejection from themold, ribs generally require at leastone-half degree of draft per side (see

figure 2-10). More than one degree ofdraft per side can lead to excessive ribthickness reduction and filling problemsin tall ribs.

Thick ribs form thickened flow channelswhere they intersect the base wall.These channels can enhance flow in therib direction and alter the filling pattern.The base of thick ribs is often a goodlocation for gas channels in gas-assistmolding applications. The gas-assistprocess takes advantage of these channelsfor filling, and hollows the channelswith injected gas to avoid problemswith sink, voids, or excessive shrinkage.

Rib thickness also determines thecooling rate and degree of shrinkage inribs, which in turn affects overall partwarpage. In materials with nearly

uniform shrinkage in the flow andcross-flow directions, thinner ribs tendto solidify earlier and shrink less thanthe base wall. In this situation, the endsof ribbed surfaces may warp toward the

25

Rib Thickness as a Percentage of Wall Thickness

Resin Minimal Sink Slight Sink

PC 50% (40% if high gloss) 66%

ABS 40% 60%

PC/ABS 50% 66%

Polyamide (Unfilled) 30% 40%

Polyamide (Glass-Filled) 33% 50%

PBT Polyester (Unfilled) 30% 40%

PBT Polyester (Filled) 33% 50%

Table 2-1

Rib design guidelines.

Figure 2-10Rib Design Guidelines

T

Draft*

Radius = 0.125T

*Minimum 0.5 Per Side

0.5T


27/174

direction becomes more aligned alongthe length of the ribs, this effectdiminishes. Warpage can reverse asthe ribs become thicker than the wall.

opposing wall (see figure 2-11). As ribthickness approaches the wall thickness,this type of warpage generally decreases.However, ribs that are the same thick-ness as the wall may develop ends thatwarp toward the ribbed side. To preventthis warpage, design extra mold coolingon the ribbed side to compensate for theadded heat load from the ribs.

For glass-filled materials with highershrinkage in the cross-flow versus flowdirection, the effect of rib thickness onwarpage can be quite different (seefigure 2-12). Because thin ribs tend to

fill from the base up, rather than alongtheir length, high cross-flow shrinkageover the length of the rib can cause theends to warp toward the ribs. As ribthickness increases and the flow

Rib Size

Generally, taller ribs provide greatersupport. To avoid mold filling, venting,and ejection problems, standard rules of thumb limit rib height to approximatelythree times the rib-base thickness.Because of the required draft for ejection,the tops of tall ribs may become too thinto fill easily. Additionally, very tall ribsare prone to buckling under load. Ifyou encounter one of these conditions,

consider designing two or more shorter,thinner ribs to provide the same supportwith improved moldability (see figure2-13). Maintain enough space betweenribs for adequate mold cooling: forshort ribs allow at least two times thewall thickness.

26

Warpage vs. rib thickness in unfilled resins.

Figure 2-11Warpage vs. Rib Thickness

Thin Rib Thick Rib

Warpage vs. rib thickness in glass-filled resins.

Figure 2-12 Warpage vs. Rib Thickness

Thin Rib Thick Rib


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Chapter 2


to mold-cooling difficulties and warpage.Typically much easier to add thanremove, ribs should be applied sparing-ly in the original design and added asneeded to fine tune performance.

BOSSES

Bosses find use in many part designsas points for attachment and assembly.The most common variety consists of

Rib Location and Numbers

Carefully consider the location andquantity of ribs to avoid worseningproblems the ribs were intended tocorrect. For example, ribs added toincrease part strength and preventbreakage might actually reduce theability of the part to absorb impactswithout failure. Likewise, a grid of ribsadded to ensure part flatness may lead

cylindrical projections with holesdesigned to receive screws, threadedinserts, or other types of fasteninghardware. As a rule of thumb, the outsidediameter of bosses should remain within2.0 to 2.4 times the outside diameter of the screw or insert (see figure 2-14).

27

Replace large problematic ribs with multiple shorter ribs.

Figure 2-13Multiple Ribs

t

2t

0.5t

t

2.0 to2.4D

D

0.060 in(1.5 mm)

0.3t max.

t

d

Typical boss design.

Figure 2-14Boss Design


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To limit sink on the surface opposite theboss, keep the ratio of boss-wall thick-ness to nominal-wall thickness the sameas the guidelines for rib thickness (seetable 2-1). To reduce stress concentra-tion and potential breakage, bossesshould have a blended radius, ratherthan a sharp edge, at their base. Largerradii minimize stress concentration butincrease the chance of sink or voids.

For most applications, a 0.015-inch blend (fillet) radius provides agood compromise between strengthand appearance.

Specifying smaller screws or insertsoften prevents overly thick bosses.Small screws attain surprisinglyhigh retention forces (see the BayerJoining Techniques manual). If theboss-wall thickness must exceed therecommended ratio, consider adding arecess around the base of the boss(as shown in figure 2-15) to reduce theseverity of sink.

Avoid bosses that merge into sidewallsbecause they can form thick sectionsthat lead to sink. Instead, position thebosses away from the sidewall, and if needed, use connecting ribs for support(see figure 2-16). Consider using open-boss designs for bosses near a standingwall (see figure 2-17).

28

A recess around the base of a thick boss reduces sink.

Figure 2-15Boss Sink Recess

30

0.3tt

Connecting bosses to walls.

Figure 2-16Bosses

Incorrect

Correct

Normally, the boss hole should extendto the base-wall level, even if the fulldepth is not needed for assembly.Shallower holes can leave thick sections,resulting in sink or voids. Deeper holesreduce the base wall thickness, leadingto filling problems, knitlines, or surfaceblemishes. The goal is to maintain auniform thickness in the attachmentwall (see figure 2-18).

Because of the required draft, tallbosses those greater than five timestheir outside diameter can create afilling problem at their top or a thick section at their base. Additionally, the


30/174

Chapter 2


cores in tall bosses can be difficult tocool and support. Consider coring a tallboss from two sides or extending tall

gussets to the standoff height ratherthan the whole boss (see figure 2-19).

29

Open bosses maintain uniform thickness in the attachment wall.

Figure 2-17Boss in Attachment Wall

A

A

Section A-A

Figure 2-18Boss Core Depth

Core Too Short

Correct

Core Too Long

Radius Too Large

Boss holes should extend to the base-wall level.

Incorrect Correct

Options to reduce the length of excessively long core pins.

Figure 2-19Long-Core Alternatives

CoreTooLong

Correct Correct


31/174

is a concern. Because of their shape andthe EDM process for burning gussetsinto the mold, gussets are prone toejection problems. Specify properdraft and draw polishing to help withmold release.

The location of gussets in the moldsteel generally prevents practical directventing. Avoid designing gussets thatcould trap gasses and cause filling andpacking problems. Adjust the shape

or thickness to push gasses out of thegussets and to areas that are more easilyvented (see figure 2-21).

Other alternatives include splitting along boss into two shorter mating bosses(see figure 2-20) or repositioning theboss to a location where it can be shorter.

GUSSETS

Gussets are rib-like features that addsupport to structures such as bosses,ribs, and walls (see figure 2-21). Aswith ribs, limit gusset thickness to one-

half to two-thirds the thickness of thewalls to which they are attached if sink

SHARP CORNERS

Avoid sharp corners in your design.Sharp inside corners concentrate stressesfrom mechanical loading, substantiallyreducing mechanical performance.Figure 2-22 shows the effect of rootradius on stress concentration in asimple, cantilevered snap arm. Thestress concentration factor climbssharply as the radius-to-thicknessratio drops below approximately 0.2.

Conversely, large ratios cause thicksections, leading to sinks or voids.

30

Excessively long bosses can often be replaced by two shorter bosses.

Figure 2-20 Mating Bosses

Contour lines show flow front position at incremental time intervals.Squared gussets can trap air in the corners.

Figure 2-21 Gussets

Incorrect Correct

Incorrect Correct

Air Trap Positionof flowfront atregular timeintervals


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Chapter 2


A radius-to-thickness ratio of approximately 0.15 provides a goodcompromise between performanceand appearance for most applicationssubjected to light to moderateimpact loads.

Initially use a minimal corner radiuswhen designing parts made of high-shrinkage materials with low-notchsensitivity, such as Durethan polyamide,to prevent sink and read-through. Inside

corner radii can then be increased asneeded based upon prototype testing.

In critical areas, corner radii shouldappear as a range, rather than a maximumallowable value, on the product drawings.A maximum value allows the mold makerto leave corners sharp as machinedwith less than a 0.005-inch radius.Avoid universal radius specificationsthat round edges needlessly andincrease mold cost (see figure 2-23).

In addition to reducing mechanicalperformance, sharp corners can causehigh, localized shear rates, resulting inmaterial damage, high molding stresses,and possible cosmetic defects.

31

Effects of a fillet radius on stress concentration.

Figure 2-22Fillet Radius and Stress Concentration

3.0

2.5

2.0

1.5

1.00.2 0.4 0.6 0.8 1.0 1.2 1.4

R/h

S t r e s s

C o n c e n

t r a

t i o n

F a c

t o r

R

P

h

Round Edges

Easy

Easy Difficult

Difficult

Avoid universal radius specifications that round edges needlessly and increase mold cost.

Figure 2-23


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DRAFT

Draft providing angles or tapers onproduct features such as walls, ribs,posts, and bosses that lie parallel to thedirection of release from the mold eases part ejection. Figure 2-24 showscommon draft guidelines.

How a specific feature is formed in themold determines the type of draft needed.Features formed by blind holes or

pockets such as most bosses, ribs,and posts should taper thinner as theyextend into the mold. Surfaces formedby slides may not need draft if the steelseparates from the surface before ejection.Other rules of thumb for designingdraft include:

Draft all surfaces parallel to thedirection of steel separation;

Angle walls and other features thatare formed in both mold halves tofacilitate ejection and maintainuniform wall thickness;

Use the standard one degree of draftplus one additional degree of draft forevery 0.001 inch of texture depth asa rule of thumb; and

Use a draft angle of at least one-half degree for most materials. Designpermitting, use one degree of draftfor easy part ejection. SAN resinstypically require one to two degreesof draft.

32

Common draft guidelines.

Figure 2-24 Draft

Correct

Parts with Draft

Correct

Correct

0.5 Min.

0.5 Min.

0.5 Min.

Draw Polish

Incorrect

Parts with No Draft

Incorrect

Incorrect

Less draft increases the chance ofdamaging the part during ejection.

Additionally, molders may have toapply mold release or special mold sur-face coatings or treatments, ultimatelyleading to longer cycle times and higherpart costs.

The mold finish, resin, part geometry,and mold ejection system determine

the amount of draft needed. Generally,polished mold surfaces require less draftthan surfaces with machined finishes.An exception is thermoplastic poly-urethane resin, which tends to eject easierfrom frosted mold surfaces. Parts withmany cores may need a higher amountof draft.


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Chapter 2


add slides or hydraulic moving coresthat can increase the cost of moldconstruction and maintenance (seesection on undercuts).

During mold filling, the advancingplastic flow can exert very high sideforces on tall cores forming deep orlong holes. These forces can push orbend the cores out of position, alteringthe molded part. Under severe conditions,this bending can fatigue the mold steeland break the core.

Generally, the depth-to-diameter ratiofor blind holes should not exceed 3:1.Ratios up to 5:1 are feasible if fillingprogresses symmetrically around theunsupported hole core or if the core isin an area of slow-moving flow.Consider alternative part designs that

Some part designs leave little room forejector pins. Parts with little ejector-pincontact area often need extra draft toprevent distortion during ejection. Inaddition to a generous draft, some deep

closed-bottomed shapes may need airvalves at the top of the core to relievethe vacuum that forms during ejection(See figure 7-13 in Chapter 7).

HOLES AND CORES

Cores are the protruding parts of themold that form the inside surfaces of features such as holes, pockets, andrecesses. Cores also remove plasticfrom thick areas to maintain a uniformwall thickness. Whenever possible,design parts so that the cores can separatefrom the part in the mold-openingdirection. Otherwise, you may have to

avoid the need for long delicate cores,such as the alternative boss designs infigures 2-19 and 2-20.

If the core is supported on both ends,

the guidelines for length-to-diameterratio double: typically 6:1 but up to10:1 if the filling around the core issymmetrical. The level of support onthe core ends determines the maximumsuggested ratio (see figure 2-25).Properly interlocked cores typicallyresist deflection better than cores thatsimply kiss off. Single cores for through-holes can interlock into the oppositemold half for support.

Mismatch can reduce the size of theopening in holes formed by mating cores.Design permitting, make one coreslightly larger (see figure 2-26). Even

33

The ends of the long cores should interlock into mating surfacesfor support.

Interlocking CoresFigure 2-25

Reduced Hole Correct Through-Hole

When feasible, make one core larger to accommodate mismatch inthe mold.

Core MismatchFigure 2-26


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part can flex enough to strip from themold during ejection, depending uponthe undercuts depth and shape and theresins flexibility. Undercuts can onlybe stripped if they are located awayfrom stiffening features such as cornersand ribs. In addition, the part must haveroom to flex and deform. Generally,guidelines for stripping undercuts fromround features limit the maximumamount of the undercut to a percentagedefined as follows and illustrated in

figure 2-28 as:

Generally, avoid stripping undercutsin parts made of stiff resins such aspolycarbonate, polycarbonate blends,

% Undercut =D d

x 100D

with some mismatch, the required holediameter can be maintained. Tight-tolerance holes that cannot be steppedmay require interlocking features on thecores to correct for minor misalignment.These features add to mold constructionand maintenance costs. On shortthrough-holes that can be molded withone core, round the edge on just oneside of hole to eliminate a mating coreand avoid mismatch (see figure 2-27).

UNDERCUTS

Some design features, because of theirorientation, place portions of the moldin the way of the ejecting plastic part.Called undercuts, these elements canbe difficult to redesign. Sometimes, the

and reinforced grades of polyamide 6.Undercuts up to 2% are possible in partsmade of these resins, if the walls areflexible and the leading edges arerounded or angled for easy ejection.Typically, parts made of flexible resins,such as unfilled polyamide 6 or thermo-plastic polyurethane elastomer, cantolerate 5% undercuts. Under idealconditions, they may tolerate up to10% undercuts.

Slides and Cores

Most undercuts cannot strip from themold, needing an additional mechanismin the mold to move certain componentsprior to ejection (see Chapter 7). Thetypes of mechanisms include slides,

34

Rounding both edges of the hole creates a potential for mismatch.

Mismatch No Mismatch

Mismatch Figure 2-27

Undercut features can often successfully strip from the mold duringejection if the undercut percentage is within the guidelines for thematerial type.

Figure 2-28Stripping Undercut Guidelines

d

D

30 45 Lead Angle


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Chapter 2


split cores, collapsible cores, split cavi-ties, and core pulls. Cams, cam pins,lifters, or springs activate most of theseas the mold opens. Others use externaldevices such as hydraulic or pneumaticcylinders to generate movement. All of these mechanisms add to mold cost andcomplexity, as well as maintenance.They also add hidden costs in the formof increased production scrap, qualityproblems, flash removal, and increasedmold downtime.

Clever part design or minor designconcessions often can eliminate complexmechanisms for undercuts. Variousdesign solutions for this problem areillustrated in figures 2-29 through 2-31.Get input from your mold designerearly in product design to help identifyoptions and reduce mold complexity.

35

Figure 2-29Sidewall Windows

Bypass steel can form windows in sidewalls without moving slides. Snap-fit hook molded through hole to form undercut.

Figure 2-30Snap Fit

Snap Fit Draw

Simple wire guides can be molded with bypass steel in the mold.

Figure 2-31Wire Guides

Draw


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Carefully consider the molding processduring part design to simplify the moldand lower molding costs. Extendingvents over the top of a corner edge canfacilitate straight draw of the vent coringand eliminate a side action in the mold(see figure 2-32). Angling the louversurface can also allow vent slots to bemolded without side actions in the mold(see figure 2-33).

LOUVERS AND VENTS

Minor variations in cooling-vent designcan have a major impact on the moldingcosts. For instance, molds designed withnumerous, angled kiss-offs of bypasscores are expensive to construct andmaintain. Additionally, these moldsare susceptible to damage and flashproblems. Using moving slides or coresto form vents adds to mold cost andcomplexity.

Consult all pertinent agency specifica-tions for cooling vents in electricaldevices. Vent designs respond different-ly to the flame and safety tests requiredby many electrical devices. Fully test allcooling-vent designs for compliance.

36

Extending vent slots over the corner edge eliminates the need for a

side action in the mold.

Figure 2-32 Vent Slots

Louvers on sloping walls can be molded in the direction of draw.

Figure 2-33 Louvers on Sloping Wall

Direction of Draw

MoldCore

MoldCavity

Mold

Mold

Part

Direction of Draw


38/174

Chapter 2


cost. Typically, threads that do not lieon the parting line require slides or sideactions that could add to molding costs.All threads molded in two halves areprone to parting line flash or mismatch.

Thread designs requiring unscrewingdevices add the most cost to the mold.Most of the mechanisms for moldinginternal threads such as collapsibleand unscrewing cores significantlyincrease the molds cost and complexity.

MOLDED-IN THREADS

The molding process accommodatesthread forming directly in a part, avoidingthe expense of secondary, thread-cutting

steps. The cost and complexity of thetooling usually determines the feasibilityof molding threads. Always comparethis cost to the cost of alternative attach-ment options, such as self-tapping screws.

Easily molded in both mold halves,external threads centered on the moldparting line add little to the molding

Occasionally, threads in parts made of flexible plastics, such as unfilledpolyamide 6 or polyurethane elas-tomers, can be stripped from the moldwithout special mechanisms. Rarelysuited to filled resins or stiff plasticssuch as polycarbonate, this optionusually requires generously roundedthreads and a diameter-to-wall-thicknessratio greater than 20 to 1. Usually,molding threads on removable coresreduces mold cost and complexity

but adds substantially to the costs of molding and secondary operations.For this reason, limit this option tolow-production quantities or designsthat would be prohibitively complex tomold otherwise.

Thread profiles for metal screws oftenhave sharp edges and corners that canreduce the parts mechanical performanceand create molding problems in plasticdesigns. Rounding the threads crestsand roots lessens these effects. Figure2-34 shows common thread profilesused in plastics. Although less commonthan the American National (Unified)thread, Acme and Buttress threadsgenerally work better in plastic assemblies.Consider the following when specifyingmolded-in threads :

37

Common threadprofiles used inplastic parts.

Figure 2-34Thread Profiles

American National (Unified)

Acme

Buttress

60

P

29

P

0.371P

0.69P 50

50

5

P

0.125P

0.5P

R = 0.125P

0.5P

0.5P

0.125P


39/174

Avoid tapered threads unless youcan provide a positive stop thatlimits hoop stresses to safe limitsfor the material.

Tapered pipe threads , common inplumbing for fluid-tight connections,are slightly conical and tapered and canplace excessive hoop stresses on theinternal threads of a plastic part. Whenmating plastic and metal tapered

Use the maximum allowable radiusat the threads crest and root;

Stop threads short of the end to avoidmaking thin, feathered threads that caneasily cross-thread (see figure 2-35);

Limit thread pitch to no more than32 threads per inch for ease ofmolding and protection fromcross threading; and

threads, design the external threads onthe plastic component to avoid hoopstress in plastic or use straight threadsand an O ring to produce the seal(see figure 2-36). Also, assure thatany thread dopes or thread lockers arecompatible with your selected plasticresin. Polycarbonate resins, in particular,are susceptible to chemical attack frommany of these compounds.

38

Design guidelines to avoid cross threading.

Figure 2-35 Threads

Incorrect Correct

Incorrect Correct

Standard NPT tapered pipe threads can cause excessive hoopstresses in the plastic fitting.

Figure 2-36 Pipe Threads

Plastic PipePlasticFitting

StraightThread O-Ring

CompressionSeal

Plastic Pipe Metal FittingNPT

Recommended

Metal or Plastic Pipe Plastic Fitting

Tapered threads createlarge hoop stress.

NPT L

T

t

F

Not Recommended

Bulge

Bulge


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Chapter 2


For best performance, use threadsdesigned specifically for plastics. Partsthat do not have to mate with standardmetal threads can have unique threadsthat meet the specific application andmaterial requirements. The medicalindustry, for example, has developedspecial, plastic-thread designs forLuer-lock tubing connectors (see figure2-37). Thread designs can also besimplified for ease of molding asshown in figure 2-38.

39

Luer-lock thread used in medical applications.

Figure 2-37Medical Connectors

Examples of thread designs that were modified for ease of molding.

Figure 2-38 Molded Threads

Internal

External


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LETTERING

The molding process adapts easily formolding-in logos, labels, warnings,diagrams, and instructions, saving theexpense of stick-on or painted labels,and enhancing recyclability. Deep,sharp lettering is prone to cosmeticproblems, such as streaks and teardrops, particularly when near the gate(see figure 2-39). To address thesecosmetic issues, consider the following:

Limit the depth or height of letteringinto or out of the part surface toapproximately 0.010 inch; and

Angle or round the side walls of theletters as shown in figure 2-40.

TOLERANCES

Many variables contribute to thedimensional stability and achievabletolerances in molded parts, includingprocessing variability, mold construction,material characteristics, and partgeometry. To improve your abilityto maintain specified tolerancesin production:

40

Deep, sharp lettering can cause teardrop defects as shown ontop photo. The bottom shows the improvement with rounded,shallow lettering.

Figure 2-39 Lettering

W

W

dRR

30 W 2 d d = 0.010 in (Max.)

Design suggestions for the cross-sectional profile of lettering.

Figure 2-40 Lettering


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Chapter 2


Use low-shrinkage materials in partswith tight tolerances;

Avoid tight tolerances in dimensionsaffected by the alignment of the moldhalves or moving mold componentssuch as slides;

Design parts and assemblies to avoidtight tolerances in areas prone towarpage or distortion; and

Adjust the mold to producedimensions in the middle of tolerancerange at optimum processingconditions for the material.

41

Unlike standardtolerancing,geometrictolerancing canallow a feature

position to varywith the size of the feature.

Figure 2-41Tolerances

0.7500.003

1.0000.003

0.500 Dia. 0.003

StandardTolerances

0.750

1.000

0.500 Dia. 0.003

GeometricTolerances

0.006 M

To avoid unnecessary molding costs,specify tight tolerances only when need-ed. Generally, the size and variability of other part features determine the actualtolerance required for any one componentor feature within an assembly. Ratherthan dividing the allowable variabilityequally over the various features thatgovern fit and function, allot a greaterportion of the total tolerance rangeto features that are difficult to control.Reserve tight tolerances for features

that can accommodate them reasonably.

Geometric tolerancing methods canexpand the effective molding toleranceby better defining the size and positionrequirements for the assembly. Ratherthan define the position and size of fea-tures separately, geometric tolerancingdefines a tolerance envelope in whichsize and position are consideredsimultaneously.

Figure 2-41 shows the size and positionof a hole specified in both standard and

geometric tolerances. The standard tol-erances hold the position and size of thehole to 0.003. The geometric toler-ances specify a hole size tolerance of


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0.003 but allow the position toleranceto vary within a 0.006 tolerance zonewhen the hole is at its smallest diameter(maximum material condition). Whenthe hole is larger than the minimumsize, the difference between the actualhole size and the minimum hole sizecan be added to the tolerance zone forthe position tolerance. At the maximumhole size, 0.503, the position tolerancezone for the center of the hole is 0.012or 0.006 from the stated vertical and

horizontal positions. As the holebecomes larger, the position can varymore without restricting the requiredthrough-hole for the post or screwthat passes through the hole (seefigure 2-42).

BEARINGS AND GEARS

Material friction and wear propertiesplay a key role in the performance of bearings and gears made of plastic. Forinstance, Durethan polyamide resinsexhibit properties suitable for many gearand bearing applications. Used frequentlyas over-molded, gear-tooth liners, Texinthermoplastic urethane elastomersdemonstrate excellent abrasion resistanceand shock-dampening properties.

Because plastic parts exhibit complexwear behavior, predicting gear andbearing performance can be difficult.However, certain trends prevail:

42

Tolerance Zonefor Largest Hole

Tolerance Zonefor Smallest Hole

Required Through-Hole Size

As the hole size increases, the position tolerance can increase without restricting thethrough-hole clearance.

Figure 2-42Tolerances

When the mating components of abearing or gear are made of the samematerial, the wear level is muchhigher, unless the load and temperatureare very low;

When both contacting plastics areunfilled, usually wear is greater onthe moving surface;

When plastic components willwear against steel, use glass fillers

to increase the life of plasticcomponents; and

When designing bearing parts forlongevity, keep frictional heatinglow and ensure that heat dissipatesquickly from the bearing surface.


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Chapter 2


Avoid soft-metal shafts when theloads or rotational speeds are high;

Add holes or grooves to the insideof the bushing to capture debris andprevent premature wear;

Protect the bearings with seals orguards in dirty environments; and

Check the compatibility of lubricantswith your specific plastic.

The PV factor , a major factor in theformation of frictional heat, is the prod-uct of the pressure (P) exerted on theprojected area of the bushing and thesurface velocity (V) of the shaft. Testingshows that plastics exhibit a sharpincrease in wear at PV values above alimit characteristic of the specific resin(see table 2-2). The PV factor for thebushing must not exceed the PV limit(minus appropriate safety factor)established for the selected resin.

Many factors influence the effective PVlimit and actual bushing performance.For instance, bushings made of plasticlast longer when the shafts are hard andfinely polished. Other points to consider:

If chemically compatible, lubricantscan more than double the PV limitand greatly increase the life of gearsand bearings.

Differences in the coefficient of linearthermal expansion between the shaft andthe bushing can change the clearanceand affect part life. Calculate theclearance throughout the servicetemperature range, maintaining aminimum clearance of approximately

0.005 inch per inch of diameter. Alwaystest your specific shaft and bushingcombination under the full range of temperatures, speeds, loads, andenvironmental conditions beforespecifying a bushing material or design.

43

Approximate PV Limitsat 100 Feet/Minute

Polycarbonate 500

Thermoplastic PU 1,500

Polyamide 6 2,000

Polyamide 6/6 2,500

Polyamide 6 30% GF 8,500

Table 2-2


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44


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Chapter 3

STRUCTURAL DESIGN

STRUCTURALCONSIDERATIONSIN PLASTICS

When designing parts made of plastics,be sure to consider not only the magni-tude of mechanical loads but also theirtype and duration. More so than formost materials, plastics can exhibitdramatically different behavior dependingon whether the loading is instantaneous,long term, or vibratory in nature.

Temperature and other environmentalconditions can also dramatically affectthe mechanical performanceof the plastic material. Many aspectsof plastic behavior, including visco-elasticity and sensitivity to a variety of processing-related factors, make pre-dicting a given parts performance in aspecific environment very difficult. Usestructural calculations conservativelyand apply adequate safety factors. Westrongly suggest prototype testing for allapplications.

Plastic part design must also take intoaccount not only the structural require-ments anticipated in the end-useapplication, but also the less obviousmechanical loads and stresses thatcan occur during operations such asmanufacturing, assembly, and shipping.

These other sources of mechanicalloads can often place the higheststructural demands on the plastic part.Carefully evaluate all of the structuralloads the part must endure throughoutits entire life cycle.

This chapter assumes the reader has

a working knowledge of mechanical

engineering and part design, and

therefore focuses primarily upon those

aspects of structural design that are

unique or particularly relevant to

plastics. Two main goals of this chapter

are to show how to use published data

to address the unusual behavior of

plastics in part design, and to show how

to take advantage of the design freedom

afforded by molding processes to meet

your structural requirements.

The mechanical properties of plasticsdiffer from metals in several importantways:

Plastics exhibit much less strengthand stiffness;

Mechanical properties are time andtemperature dependent;

Plastics typically exhibit nonlinearmechanical behavior; and

Processing and flow orientation cangreatly affect properties.

The following sections briefly discussthe relevance of these differences whendesigning plastic parts. For more onthese topics, consult the BayerCorporation companion to this manual:

Material Selection: Thermoplastics

and Polyurethanes .

45


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Voight-Maxwellmodel simulatingviscoelasticcharacteristics.

Figure 3-1Voight-Maxwell Model

Spring A

Dashpot A

Dashpot BSpring B

Maxwell

Voight

Stiffness

Designing parts with adequate stiffnesscan be difficult, particularly if your partwas made of metal originally. If yourdesign needs the strength and/or stiffnessof a metal part, you must account forthe large disparity between plastic andmetal mechanical properties (see table3-1). Increasing wall thickness maycompensate for the lower stiffness of plastic resins. In practice, however, the

molding process limits wall thickness toapproximately 0.25 inch in solid, injection-molded parts. More typically, wallthickness ranges from 0.060 to 0.160inches. Generally, good part designsincorporate stiffening features and usepart geometry to help achieve requiredstiffness and strength. These designconsiderations are covered in greaterdetail in the section Designing for Stiffness on page 67.

46

* Conditioned

Steel 28.5 70 40 0.29Copper (Annealed) 15.6 32 5 0.36

Aluminum 10.0 56 34 0.33

SAN 0.47 4 5 0.35

Polycarbonate 0.35 10 9 0.38

ABS 0.34 6 0.39

PA* Unfilled 0.16 8 6 0.40

PA* 30% Glass 0.72 15 0.34

PC/ABS 0.35 7 8 0.38

Property Comparison of Metals and Plastics

Modulus of Tensile YieldElasticity Strength Strength Poissons

Material (10 6 psi) (1,000 psi) (1,000 psi) Ratio

Table 3-1

Viscoelasticity

Plastics exhibit viscoelastic behaviorsunder load: they show both plastic andelastic deformation. This dual behavioraccounts for the peculiar mechanicalproperties found in plastics. Under mildloading conditions, plastics usuallyreturn to their original shape when theload is removed, exhibiting an elasticresponse. Under long-term, heavy loadsor at elevated temperatures, this same

plastic will deform, behaving more likea high-viscosity liquid. This time- and

temperature-dependent behavior occursbecause the polymer chains in the partdo not return to their original positionwhen the load is removed. The Voight-Maxwell model of springs and dashpotsillustrates these characteristics (seefigure 3-1). Spring A in the Maxwellmodel represents the instantaneousresponse to load and the linear recoverywhen the load is removed. Dashpot Aconnected to the spring simulates thepermanent deformation that occurs

over time.


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Chapter 3

STRUCTURAL DESIGN continued

stress relaxation , the reduction in stress

over time in a part under constant strainor deformation. To account for thisbehavior, designers should use datathat reflect the correct temperature,load, and duration to which the part willbe exposed. These topics are discussedmore fully in the section Long-Term

Loading on page 73.

Viscoelasticity causes most plastics

to lose stiffness and strength as thetemperature increases (see figure 3-2).As a plastic part is exposed to highertemperatures, it becomes more ductile:yield strength decreases and the strain-at-break value increases. Plastic partsalso exhibit creep , the increase indeformation over time in parts undercontinuous load or stress, as well as

47

Figure 3-2

S T R E S S

STRAIN (%)

100

80

60

40

20

0

14,000

12,000

10,000

8,000

6,000

4,000

2,000

6,370

4,900

Makrolon 2658 PC

0 2 4 6 8 101.35 2.301.75

-20 C

0C

23 C

40

60 C

90 C

120 C

Stress-Strain vs. TemperatureMPa

psi


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Stress-Strain Behavior

A simple tensile test determines thestress-strain behavior of plastic materi-als. The results, usually expressed as acurve, show the relationship betweenstress , the force per original cross-sectional area, and strain , the percentageof change in length as a result of theforce. Nearly linear at very low stressand strain levels, the stress-strainbehavior of plastics tends to become

increasingly nonlinear as these loadsincrease. In this context, the term non-linear means that the resulting strainat any particular point does not varyproportionally with the applied stress.

48

Viscoelastic

(Voight-Maxwell)

Elastic (Hookean)

Metals

UnreinforcedPlastics

Metals usually function within the elastic (Hookean) range of mechanical behavior.Unreinforced plastics tend to exhibit nonlinear behavior represented here by thecombination of springs and dashpots.

Figure 3-3

S T R E S S

STRAIN

Viscoelasticity


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Chapter 3

STRUCTURAL DESIGN continued

Figure 3-3 shows typical stress-straincurves for steel and unreinforcedthermoplastic materials. While metalscan exhibit plastic behavior, they typicallyfunction within the elastic (Hookean)range of mechanical performance.Because of viscoelasticity, unreinforcedplastic materials tend to exhibit non-linear behavior through much of theiroperating range. Even at low strainvalues, plastics tend to exhibit somenonlinear behavior. As a result, using

the tensile modulus or Youngs modulus ,derived from stress over strain in thelinear region of the stress-strain curve,in structural calculations could lead toan error. You may need to calculate thesecant modulus , which represents thestiffness of a material at a specific strainor stress level (see figure 3-4). The useof secant modulus is discussed in theexample problems later in this chapter.

49

The Youngs modulus derived from the stress-strain behavior at very low strain can overstatethe material stiffness. A calculated secant modulus can better represent material stiffness ata specific stress or strain.

Figure 3-4

S T R E S S ( )

STRAIN ( )

Secant Modulus

ActualCurve

secant secant secant =

secant

secant

Youngs

secant


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50

Figure 3-5

S T R E S S ( N / m m

2 )

STRAIN (%)

This graph shows the stress-strain performance parallel to fiber orientation at various temperatures for a 30% glass-filled PA 6material after conditioning.

Stress-StrainParallel to Orientation

120

100

80

60

40

20

0

Durethan BKV 130

0 1 2 3 4

23 C

40 C

60 C

120 C90 C

150 C30

1.3

Figure 3-6

S T R E S S ( N / m m

2 )

STRAIN (%)

This graph shows the stress-strain performance perpendicular to fiber orientation at various temperatures for a 30% glass-filledPA 6 material after conditioning.

Stress-StrainPerpendicular to Orientation

80

70

60

50

40

30

20

10

0

Durethan BKV 130

0 1 2 3 4

23 C

40 C

60 C

120 C90 C

150 C

a given part can endure. Always addreasonable safety factors and testprototype parts before actual production.

In glass-filled resins, fiber orientation

also affects mechanical performance:fatigue strength for a given fiber-filledresin is often many times greater whenthe fibers are aligned lengthwise, ratherthan perpendicular to the fatigue load.Stress-strain performance in the directionof fiber orientation can also differ greatlyfrom the performance in the directionperpendicular to the fibers. Figures 3-5

Molding Factors

The injection-molding process introducesstresses and orientations that affectthe mechanical performance of plastic

parts. The standard test bars used todetermine most mechanical propertieshave low levels of molding stress. Thehigh molding stresses in an actual partmay reduce certain mechanical properties,such as the amount of applied stress

and 3-6 show stress versus strain for a30% glass-filled PA 6 in the parallel-to-fiber and perpendicular-to-fiber directions.

Unless otherwise stated, most mechanical

properties derive from end-gated testbars that exhibit a high degree of orien-tation in the direction of the applied testload. Mechanical calculations basedon this kind of data may over-predictmaterial stiffness and performance in


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Chapter 3

STRU

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