David O. Kazmer Injection Mold Design Engineeringfiles.hanser.de/hanser/docs/20071031_2713115331-97_9783446412668... · David O. Kazmer Injection Mold Design Engineering ISBN-10:

David O. Kazmer

Injection Mold DesignEngineering

ISBN-10: 3-446-41266-2ISBN-13: 978-3-446-41266-8

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4 Mold Layout Design

During the mold layout stage, the mold designer commits to the type of mold and selects thedimensions and materials for the cavity inserts, core inserts, and mold base. Mold bases areonly available in discrete sizes, so iteration between the inserts’ sizing and mold base selectionis normal. The goal of the mold layout design stage is to develop the physical dimensions ofthe inserts and mold so as to enable procurement of these materials. Mold material selectionis also an important decision, since the material properties largely determine the mold makingtime and cost as well as the mold’s structural and thermal performance.

The mold layout design assumes that the number of mold cavities and type of mold has beendetermined. To develop the mold layout, the mold opening direction and the location of theparting plane are first determined. Then, the length, width, and height of the core and cavityinserts are chosen. Afterwards, a mold base is selected and the inserts are placed in as simpleand compact a layout as possible. It is important to develop a good mold layout design sincelater analysis assumes this layout design and these dimensions are quite expensive to changeonce the mold making process has begun.

4.1 Parting Plane Design

The parting plane is the contact surface between the stationary and moving sides of the mold.The primary purpose of the parting plane is to tightly seal the cavity of the mold and preventmelt leakage. This seal is maintained through the application of literally tons of force (hencethe term “clamp tonnage”) that are applied normal to the parting plane. While the term“parting plane” implies a flat or planar surface, the parting plane may contain out-of-planefeatures. The mold designer must first determine the mold opening direction to design theparting plane.

4.1.1 Determine Mold Opening Direction

Examination of any of the previous mold designs (e.g., Figure 1.4 to Figure 1.8) indicates thatthe mold opening direction is normal to the parting plane. In fact, the mold usually opensin a direction normal to the parting plane since the moving platen of the molding machineis guided by tie bars or rails to open in a direction normal to the platen. Accordingly, guidebushings and/or mold interlocks are almost always located on the parting plane to guide themold opening in a direction normal to the parting plane.

It may appear that there is nothing about the mold opening direction to determine sincethe mold opens normal to the parting plane. However, it is necessary to determine the mold

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opening direction relative to the mold cavity. There are two factors that govern the moldopening direction:

1. First, the mold cavity should be positioned such that it does not exert undue stress onthe injection mold. The mold cavity is typically placed with its largest area parallel to theparting plane. This arrangement allows the mold plates, already being held in compressionunder the clamp tonnage, to resist the force exerted by the plastic on the surfaces of themold cavity.

2. Second, the mold cavity should be positioned such that the molded part can be ejectedfrom the mold. A typical molded part is shaped like a five-sided open box with the sidewalls, ribs, bosses, and other features normal to its largest area. If so, then the part ejectionrequirement again supports the mold opening direction to be normal to the part’s largestprojected area.

Consider the cup and lid shown in Figure 4.1. A section of the core and cavity inserts usedto mold these parts was previously shown in Figure 1.6. There are only two potential moldopening directions relative to the part. One mold opening direction is in the axial directionof the cup, while the second direction is in the radial direction of the cup.

Figure 4.1: Sectioned isometric view of cup assembly

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A section of a cavity block with an axial mold opening direction is shown in Figure 4.2. Thetwo bold horizontal lines indicates the location of the parting plane where the two halves ofthe insert are split to form the cavity insert (top) and the core insert (bottom).

Consider next the same cavity block but with a radial mold opening direction for a portionof the cavity insert as shown in Figure 4.3. For this design, four bold lines separate the sidesfrom the top and bottom. Since the metal core is located inside the molded part, there is noway to remove the core other than in the part’s axial direction. The cavity insert, however,can be separated into three pieces that move along two different axes in order to remove themolded part.

Of these two designs, the axial mold opening direction shown in Figure 4.2 is the simplestdesign and is usually preferred. However, the second design is sometimes used in practicesince it allows for a more complex part design as well as more options in locating the partingline. For instance, the second design might be required if a handle were added to the cup, orif it was necessary to move the parting line to a location away from the top lip. This seconddesign is known as a “split cavity mold” and is discussed in more detail in Section 13.9.1.

As another example, consider the laptop bezel shown in Figure 3.5. There are again twopotential mold opening directions. The first opening direction is in the screen’s viewingdirection, as indicated by the section view shown in Figure 4.4. In this case, the mold sectionis split by two horizontal lines into a cavity insert forming the outside surface of the bezeland a core insert that forms the inner surface and ribs of the bezel. When the core and cavityinserts are separated as indicated by the arrows, the molded bezel can be readily removed.

Figure 4.2: Axial mold opening directionfor cup

Figure 4.3: Radial mold opening directionfor cup

Figure 4.4: Normal mold opening direction for bezel

4.1 Parting Plane Design

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Alternatively, the cavity block for the PC bezel can be split as indicated with the three verticallines shown in Figure 4.5. In this case, the former cavity insert is split into two pieces, resultingagain in a split cavity mold design. The two halves of the former cavity insert must nowbe removed in oblique directions in order to remove the molded part; the mold openingdirection is inclined in order to allow the mold surfaces to separate from the molded partwithout excessive surface friction or shearing of features on the molded part. This movementrequires several additional mold components to control the moving cavity inserts, which addsignificantly to the cost of mold design, manufacture, and operation.

4.1.2 Determine Parting Line

The term “parting line” refers to the location at which the cavity insert, the core insert, andthe plastic molding meet. Since the core and cavity insert meet at this location, any significantdeflection of the cavity insert away from the core insert will result in a gap into which theplastic will flow and form a thin film of plastic known as“flash”. Imperfections in the core andcavity inserts at this location, for instance due to wear or improper handling, will also creategaps into which the plastic will flow. Even with new and well-crafted molds, the location ofthe parting line usually results in a very slight “witness line” along its length.

For this reason, the parting line should be located along a bottom edge of the part, or someother non-visual, non-functional edge. Consider the previous cup shown in Figure 4.1.Placing the parting line very close to the lip as indicated by the dashed line in the left drawingof Figure 4.6 would result in a witness line and possible flash that might make the moldedcup unusable. Alternatively, a better location for the parting line is at the bottom of the rimas indicated in Figure 4.2, corresponding to the parting line shown in the right drawing ofFigure 4.6.

Figure 4.5: Complex mold opening directions for bezel

Figure 4.6: Two parting line locations for cup

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For the laptop bezel, the parting line will be located around the bottom edge of the part asshown in Figure 4.7. It is observed that, unlike the cup, the parting line for the bezel is not ina single plane. Rather, the parting line follows the profile of the features on the side walls. Thisnon-planar parting line is required to fit the core insert which hollows out the mold cavityto form the holes required for the various connectors. As will be seen in the next section, thiscomplex parting line shape will cause a more complex parting plane.

4.1.3 Parting Plane

Once the parting line is identified, the parting plane is projected outwards from the part, soas to separate the core insert from the cavity insert. The preferred parting plane for the cup isshown in Figure 4.8. The cavity insert will form the outer and top surfaces of the part, whilethe core insert will form the rim and inner surfaces.

Figure 4.7: Parting line location for bezel

Figure 4.8: Parting plane for cup

4.1 Parting Plane Design

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For the laptop bezel, the parting line in Figure 4.7 can be radiated outward to form the partingsurface shown in Figure 4.9. It can be observed that all of the out of plane features along theparting line now become complex surfaces on the parting plane. These surfaces pose twosignificant issues during mold operation. First, any misalignment between the sharp featureson core and cavity inserts will cause wear between the sliding surfaces if not an outright impactbetween the leading edge of the core and the mating cavity surface. Second, the clamp tonnageexerted on the core and cavity inserts can cause the surfaces to lock together with extremeforce, causing excessive stress and potential mold deformation during mold operation.

To avoid excessive stress, interlocking features on the parting plane should be inclined at leastfive degrees relative to the mold opening direction. The parting surface is now typically createdvia three dimensional computer aided design (“3D CAD”) using lofted surfaces. Each lofted

Figure 4.9: Parting plane for bezel

Figure 4.10: Modified parting surface for bezel

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surface blends a curved feature along the parting line to a line of corresponding width onthe parting plane. The result is a surface with the needed profile at the parting line and thenecessary draft down to the parting plane. The lofted surfaces are then knit together with theparting plane to provide a parting surface, as shown for the bezel in Figure 4.10.

4.1.4 Shut-Offs

Shut-offs are contact areas between the core insert and the cavity insert that separate portionsof the cavity formed between the core and cavity inserts. A shut-off will need to be definedfor each window or opening in the molded part. Conversely, if a part has no windows, like thecup, then no shut-offs are defined. Each shut-off is defined by a parting line, which shouldbe located in a non-visual area where a witness line or slight flashing would not reduce thevalue of the molded part.

For example, the laptop bezel has one large opening above the parting plane for the display.A shut-off is necessary across the entire area of the opening. As indicated in Figure 4.11, thereare essentially two possible locations for the shut-off ’s parting line, corresponding to the topand bottom of the shelf that supports the display.

Either location (or even any location in between) would likely be acceptable since the entireshelf is hidden from view. If the parting line is placed at the top of the shelf as indicated atthe right of Figure 4.11, then a shut-off surface as shown in Figure 4.12 will result.

Figure 4.12: Shut-off surface for bezel

Figure 4.11: Shut-off surface for bezel

4.1 Parting Plane Design

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4.2 Cavity and Core Insert Creation

With the definition of the parting plane and all necessary shut-offs, the core insert and thecavity insert have been completely separated. To create the cavity and core inserts, the length,width, and height of the inserts must be defined.

The length and width of the cavity and core inserts must be large enough to:

enclose the cavity where the part is formed,

withstand the forces resulting from the melt pressure exerted upon the area of thecavity,

contain the cooling lines for removing heat from the hot polymer melt, and

contain other components such as retaining screws, ejector pins, and others.

All of these requirements suggest making the core and cavity inserts as large as possible. Forsmaller molded parts, increasing the sizing the core and cavity inserts may have little addedcost. However, the cost of larger core and cavity inserts can become excessive with increasesin the number of cavities or molded part size.

4.2.1 Height Dimension

The height dimension is often determined by two requirements. First, the core and cavity insertshould have enough height above and below the molded part to safely pass a cooling line.Cooling line diameters typically range from 4.76 mm (3/16″) for smaller molds to 15.88 mm(5/8″) for large molds. Generally, large inserts with larger cooling lines will provide fasterand more uniform cooling as will be analyzed in Chapter 9. While cooling line design willbe later discussed, the minimum height dimension between the molded part and the top or

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Figure 4.13: Insert height allowance

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bottom surface of the insert is typically three times the diameter of the cooling line to avoidexcessive stress as analyzed in Chapter 12. The initial height dimensions for the core andcavity inserts are shown in Figure 4.13.

Second, the core and cavity insert should have a height that is matched with the heightof available cavity and core insert retainer plates (the “A” and “B” plates). These plates arecommonly available in ½″ increments in English units, and in 10 mm increments in metricunits. As such, the insert heights should be adjusted up such that the faces of the cavity andcore inserts are flush or slightly proud with respect to the “A” and “B” plates on the partingplane. It should be noted that the height of the core insert as indicated in Figure 4.13 is notits total height but rather the height dimension from the rear surface to the parting plane.For materials procurement and cost estimation, the total height of the core insert should alsoinclude the height of the core above the parting plane.

4.2.2 Length and Width Dimensions

The length and width dimensions are similarly determined by two requirements. First, if acooling line is needed around the exterior of the mold cavity, then the inserts should be sizedlarge enough to accommodate such a cooling line. As for the height allowance, length andwidth allowances of three cooling line diameters per side are typical. Second, the width andlength dimensions of the inserts should provide side walls, also known as “cheek”, that arethick enough to withstand the lateral loading of the melt pressure exerted on the side wallsof the mold cavity. This requirement will become dominating for deep parts with large sidewalls. While the structural design will be discussed in detail in Section 12.2.4, a safe guidelineis that the thickness of the side wall in the length and width dimension should equal thedepth of the mold cavity.

Figure 4.14 demonstrates an allowance that should be added to the length and width of themold cavity to derive the length and width of the core and cavity inserts. It can be observedthat for the laptop bezel, the requirement of fitting a cooling line will exceed the structuralrequirement. For the molded cup, however, the insert length and width dimension are drivenby the structural requirement.

Figure 4.14: Insert length and width allowance

4.2 Cavity and Core Insert Creation

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4.2.3 Adjustments

The core and cavity inserts can now be created with the prescribed dimensions. However, it issometimes desirable to adjust the cavity insert dimensions to provide a more efficient molddesign. In general, the length and width dimensions of the inserts are more critical than theheight dimension, since these dimensions will

drive the size of the mold base in multi-cavity applications, and

contribute more to the material and machining costs.

As such, these dimensions may be decreased somewhat by effective cooling and structuraldesigns, which will be supported by later engineering analysis.

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Figure 4.15: Core and cavity inserts for cup

Figure 4.16: Core and cavity inserts for bezel

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Figure 4.15 provides the core and cavity inserts for the cup. Since the molded part is round,the design of the core and cavity insert may also be round. This shape provides a benefit withrespect to ease of manufacturing, since both the core and cavity inserts can be turned ona lathe. While the allowances in the axial and radial dimensions are sufficient to fit coolinglines, the allowance in the radial dimension may not be sufficient to withstand the pressuresexerted on the side wall by the melt.

There is no fundamental requirement on the external shape of the core and cavity inserts.While the insert design in Figure 4.15 showed round inserts, the mold design for the cupshown previously in Figure 1.4 used square inserts. Rectangular inserts with or without filletedcorners are also quite common. The design of the insert should be dictated by the shape ofthe molded part, the efficiency of the mold design, and the ease of manufacture.

The core and cavity inserts for the laptop bezel are shown in Figure 4.16. In this case,rectangular inserts are designed. The length and width dimensions of the inserts havebeen designed quite aggressively. While the bezel is quite shallow and the inserts arestructurally adequate, the thickness of the surrounding cheek may not allow for sufficientcooling around the periphery of the mold cavity while also providing space for other moldcomponents.

4.3 Mold Base Selection

After the core and cavity inserts have been initially sized, the mold layout can be furtherdeveloped and the mold base selected. It is critical to order a mold base with appropriately sizedplates and materials, since any mistakes in the mold base selection can consume significanttime and expense. To determine the appropriate size, the mold designer must first arrangethe mold cavities and provide allowances for the cooling and feed systems. Afterwards, themold designer should select a standard size from available suppliers and verify suitabilitywith the molder’s molding machine.

4.3.1 Cavity Layouts

The goal of cavity layout design is to produce a mold design that is compact, easy tomanufacture, and provides molding productivity. If a single cavity mold is being designed,then the cavity is typically located in the center of the mold, though gating requirements maynecessitate placing the mold cavity off center. For multi-cavity molds, there are essentiallythree fundamental cavity layouts:

cavities are placed along one line

cavities are placed in a grid, or

cavities are placed around a circle.

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4.3 Mold Base Selection