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SECTION 4PLASTICS DESIGN
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COMPRESSION MOLDS FOR THERMOSETS
INJECTION MOLDS FOR THERMO-PLASTICS
EJECTOR PIN MARKS
WARPAGE ALLOWANCE
LONG FLAT STRIPS
SINK MARKS
TAPPING IN PLASTICS
SHARP EDGES
INSERTS
TAPERED OPENINGS AND PROJECTIONS
LIVING HINGE
MISMATCH
HINGE PIN DESIGN
DRIVE PINS
HOT STAKE OR PRESS
CEMENTING PLASTICS
SONIC WELDING
ENGRAVING AND STAMPING
DRAFT FOR MOLDS
WALL THICKNESS
RIBS
(continued on page 2)
TRANSFER MOLDS FOR THERMOSETS
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SECTION 4PLASTICS DESIGN
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MOLDED BOSSES
HOLES IN PLASTIC PARTS
In General
The Structural Foam Process
Applications
Material Flow
Ribs
Bosses
Wall Thickness
Wall Thickness Transition
Inserts
Fillets and Radii
Draft Angles
Fabrication
Molds
Self-Tapping Fasteners
Ultra-Sonic Inserts
Expansion Inserts
(continued on page 3)
IMBEDDING INSERTS
STRUCTURAL FOAM
Staking
Spot Welding
Vibration Welding
Solvent Bonding
Adhesive Bonding
14
14
14
14
14
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SECTION 4PLASTICS DESIGN
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Wall Thickness
Radii
Cross-Ribbing
REACTION INJECTION MOLDING (RIM)
164 Page of
SECTION 4PLASTICS DESIGN
COMPRESSION MOLDS FOR THERMOSETS
The 3 basic types of a closed compressionmold are;
These molds are used to make plastic
parts with very heavy sections.
force
cavity
P/L
Semi-Positive or Landed Type
force
cavity
P/L
Positive Type
P/L
force
cavity
Flash Type
P/L
Center Gate
CYLINDER
INJECTION MOLDS FOR THERMOPLASTICS
The 3 basic types of injection molds are;
1. Three-plate mold (center gate)2. Edge Gate3. Center Gate
These molds are excellent for making parts
of thermoplastic materials.
force
force
cavity
CYLINDER
P/L
Edge Gate
THIRD PLATE
cavity
force
Three-Plate Mold (Center Gate)
P/L
1. Semi-positive or landed type
2. Positive Type
3. Flash Type
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SECTION 4PLASTICS DESIGN
P/L
TRANSFER MOLDS FOR THERMOSETS
The 3 basic types of transfer molds are;
1. Injection of thermoset
2. Plunger transfer3. Pot transfer
P/L
P/L
cavity
force
Injection Of Thermoset
These molds are generally used to produce
plastic parts requiring inserts and small
diameters and those parts having very
complicated shapes.
CYLINDER
cavity
force
Injection Of Thermoset
PLUNGER
SUPPORT
PAD
PLUNGER
cavity
pot
force
Pot Transfer
EJECTOR PIN MARKS
In order to avoid surface imperfections
caused by gates and ejector pins, the
drawing should specify those areas whichare not to be marred.
NO SURFACE
IMPERFECTIONS
WARPAGE ALLOWANCE
One method to assure the best results
for a finished product is to indicate
warpage direction and allowance on the
drawing.
INDICATE WARPAGE
DIRECTION
WARPAGE ALLOWANCE
LONG FLAT STRIPS
If warpage is not permissible on a long,flat surface, ribs should be added, as
shown.
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SECTION 4PLASTICS DESIGN
SINK MARKS
One method to reduce the possibility of
developing sink marks in thermoplastic
parts is to make the thickness of adjacent
walls and ribs thinner. Adjacent walls should be about 60of the main wall.
.100 .060
.100
SINK MARK
TAPPING IN PLASTICS
When molding holes to be tapped or for
having molded in threads, the hole should
be countersunk to avoid chipping.
INITIALLY COUNTERSINK
TAPPED
SHARP EDGES
There should be no sharp edges on plastic
parts to prevent the possibility of chipping.
AVOID SHARP EDGES
INSERTS
An excellent method for securing an insert
is by hot rolling the plastic over theinsert.
Before After
INSERT
MOLD
INSERTSPIN
±.001O.D.
±.001I.D.
Mold
INSERTS (Tight Tolerances)
Tight tolerances: i.e. ±.001 with
maximum T.I.R. .003 can be held by
using either the inside or outsidediameter of the insert.
In most cases, assembling inserts into a
pre-molded hole is preferred. This
technique reduces the possibility of plastic
flowing over metal surfaces and prevents
the scratching of plated inserts during flashremoval.
All inserts, regardless of method of
assembly, should be surrounded with
reasonably thick plastic since thin wallscan crack and also show sink marks.
167Page of
SECTION 4PLASTICS DESIGN
CRITICAL TAPERED OPENINGS
AND PROJECTIONS
Ordinarily, on a plastic part drawing, it is
adequate to specify a particular degree of
taper. Hower, if a tapered projection or
opening is critical, the following method
of dimensioning is acceptable.
.405
.395
.500
.422
.412
Tapered Projection
Polypropylene Hinged Box
Hinge Closed
LIVING HINGE
Very often, plastic is molded, coined
or cold formed to create a living hinge.
As illustrated, the cover, hinge and
box were molded as a single unit.
COVER
BOX
BOX
COVER
HINGE PIN DESIGN
The following design requires the drilling
of holes or the use of cams in the design.
MISMATCH
When the misalignment of two molded
parts appear, it is called a mismatch.
This is usually due to shrinkage problems
or to warpage of a part.
Often a mismatch is designed into the
parts so as to make their appearance
less objectionable.
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SECTION 4PLASTICS DESIGN
BOX
COVER
HINGE PIN DESIGN (no cams)
In a simpler hinge design, there is no
need for drilling or cams in the mold.
DRIVE PINS AND SELF-TAPPING
SCREWS
Metals as well as plastics can be
assembled to plastic parts using drive
pins and self-tapping screws.
METAL
or Drive pin
Self-Tapping Screw
HOT STAKE OR PRESS
This method can be employed to attach
metal, glass, leather etc. to plastics.
METAL
Plastic Plastic
CEMENTING
The use of solvent cements, epoxy etc.,
must be carefully considered. Consultwith molder.
SONIC WELDING OF PLASTICS
This method of assembling plastics to
plastics is widely used. It is especially
suited to joining hard plastics. With
proper fixturing, the process can be
fast and inexpensive. However, particular
attention must be paid to the design
of joints.
Before After
ENGRAVING AND STAMPING
Raised letters can be molded on plastic
parts by the engraving and stamping ofmolds.
In order to have depressed lettering on
a molded part, there must be removable,
engraved metal pads inserted into the
mold and the depressed lettering canbe filled with ink or paint.
Another technique for decorating flat
surfaces is by means of silk screening
and hot stamping processes.
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SECTION 4PLASTICS DESIGN
DRAFT FOR MOLDS
Many factors determine the draft for a
mold such as type of material, surface
area, length of draw, and method of
ejection. Ordinarily, the draft may vary
from .25° to 4°. In special cases a no
draft condition may even be possible.
.25°-4°NO DRAFT
Preferred Avoid
AvoidPreferred
WALL THICKNESS
In order to prevent uneven shrinkage,
causing internal stresses, the walls of a
plastic molded part should have a uniform wall thickness.
The minimum wall thickness of an injection
molded part should be .050 inch. The
minimum wall of a compression or transfer
molded plastic part should be .0625 inch.
RIBS
A preferred method for designing ribs isshown below.
2 TO 5 TAPER
R
T
1 1/2 T
T
2
If 4 boses are being used to mount a
component, it will probably be necessaryto machine one of them in order to
achieve a flat surface. On the otherhand,
if 3 bosses are used, machining may not
be necessary.
1/32 R MIN
3/32 MIN
5 TAPER
2D MAX
D
MOLDED BOSSES
The height of bosses should be kept to a
minimum to prevent the trapping of gases
and weakening of the boss.
TAPER
1610Page of
SECTION 4PLASTICS DESIGN
All inserts should have diamond knurling
when subjected to both tension and torsion.
1/4 minimum
1/8 minimum
3/32 minimum
TD
Less than 1/4
Over 1/4 to 1/2
Over 1/2
HOLES IN PLASTIC PARTS
In order to facilitate removal of a part
from a mold, holes should be designed
perpendicular to the parting line.
PARTING LINE
AvoidPreferred
PARTING LINE
DRILL
MOLD
Holes which are difficult to mold should
be drilled; i.e. holes under 0.062 dia.
Sometimes it is not feasible to mold very
long, slender holes. In such cases it is
advisable to mold only a portion of thehole and drill the remainder.
It is always preferable to mold through
holes rather than blind holes. The molding
of through holes is easier because core
pins can usually be supported at bothends.
Holes larger than 0.062 Dia should not be
deeper than twice their diameter.
D
T
2D MIN
1/16MIN
BOTTOM OF INSERT
INSERTS IN PLASTIC MOLDED PARTS
Inserts should always be placed on the
same side of the parting line and at
right angles to it.
Both Inserts Placed
In Same Half Of Mold
Also, there should be adequate clearance
between the insert and a neighboring hole
and between the insert and the edge of
the part as tabulated below.
It is not advisable to mold 2 holes at
right angles to each other. The hole
perpendicular to the parting line should be molded and the side hole should be
drilled.
PARTING LINE
1611Page of
SECTION 4PLASTICS DESIGN
STRUCTURAL FOAM, IN GENERAL
Structural foams are preferred when
designing large, complex, rigid shapes
such as housings for mechanical and
electrical components. These moldings
are extremely light and rigid and the
cellular core provides improved thermal
insulation and noise attenuating qualities.
THE STRUCTURAL FOAM PROCESS
The process is similar to injection molding
and the foam is obtained by using either
of the following techniques:
a) By introducing inert gas directly into the melt.
b) By pre-blending the resin with a chemical blowing agent which, when
heated, causes inert gas to be
released which disperses through the
polymer melt.
In either case, the gas/resin mixture is forced into the mold cavity, under
pressure, and the mold is filled as the
gas expands within the material.
At the mold face, a tough external skin
is produced which covers a rigid internalcellular core.
CELLULAR CORE
EXTERNAL SKIN
APPLICATIONS
Typical applications for structural foam
include computer panels, drive frames,
complex doors, mounts for instruments
and latching mechanisms, molded one
piece card cages, card guides with 0° draft etc.
Structural Foam offers greater design
freedom than is possible with metal andat a much lower cost.
MATERIAL FLOW
In order to avoid difficulties in molding,
grille work and slotted areas should be
located perpendicular to the flow of material in the mold.
Proper Grille Orientation
If perpendicular orientation is not possible,a flow runner across the back is advisable.
FLOWRUNNER
FLOW
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SECTION 4PLASTICS DESIGN
RIBS
Whenever it is necessary to increase the
rigidity of a part, ribs are preferred tothick sections.
Intricate ribbing can be designed on the
back side of large appearance surfaces
because structural foam allows a part tobe free of sink marks.
For proper rib design, the base must not
be too thick nor the tip too thin.
BOSSES
On structural foam parts, bosses are
easily designed to accept fasteners and
support components.
When there are heavy loads on bosses,
they should have adjacent walls and ribs
for support.
Also, these bosses should be provided
with generous fillets.
T
1 PER SIDE MIN
1 1/2T max.
1/2T R.
DRAFT ANGLE ON RIBS
1/2TMIN.
D
2D
2/3TMIN.
WALL THICKNESS
Structural foam provides greater rigidity in
large parts than is possible with ordinary
injection molding because the normalwall thickness is .250 in.
For special applications, the .250 in. wallcan be increased.
Wall thickness of less than .250 in. is not
recommended because part stiffness wouldbe reduced. Also, a thinner wall tends to
increase molding problems such as
warpage, unfilled parts and sink marks.
WALL THICKNESS TRANSITION
A uniform wall thickness is preferred as
in the case of injection molded parts.
Varying wall thickness without sink marks
and stresses is possible, however, if thetransition from thick to thin walls is
smooth and generous radii are provided.
Good Rib Design
Good Boss Design
1613Page of
SECTION 4PLASTICS DESIGN
INSERTS, IN GENERAL
A variety of metal inserts are available
for use in structural foam products.
Threaded fasteners are used successfully
for repeated assembly and disassembly
of parts and can provide higher pull out
strength than can be obtained with self
tapping screws.
The method of installing inserts can be by
ultrasonic welding, press fits, or moldingin inserts.
FILLETS AND RADII
As an aid in filling the mold and to reduce stress concentration on inside and
outside corners, the largest possibleradii should be used.
The optimum fillet and radii for a .250 in.wall is .150 in. radius but a .125 R is
acceptable. However, the radii shouldnot be less than .06 R.
DRAFT ANGLES
Generally a draft angle of .5° to 3° is
adequate to release a part from a mold.
In certain situations, a draft angle of 0°
is possible, i.e. molded 1 piece card cagesor as outside ribs.
ZERO DEGREE DRAFT
NORMAL DRAFT ANGLE
FABRICATION
For simple testing and appearance, partscan be fabricated from structural foam
panels which can be readily purchased.
Such panels can be cut, machined and
bonded to produce samples at the lowestcost.
MOLDS
Small numbers of actual parts can be
molded in structural foam by making low
cost molds made of epoxy, kirksite orcast aluminum.
The epoxy mold is suitable for producing
from 1 to 25 parts whereas the kirksite
or cast aluminum molds are adequate for
limited production.
SELF-TAPPING FASTENERS
The compressible cellular core of
structural foam allows for the satisfactory
use of self-tapping fasteners. In this
application, thread rolling screws are
preferable to the thread cutting variety.
In order to maximize performance, cored holes should be molded.
PILOT HOLE
NO LESS THAN HALF
DIA. APPROX.
EQUAL TO
PITCH DIA.
WALL THICKNESS
1614Page of
SECTION 4PLASTICS DESIGN
ULTRASONIC INSERTS
Repeated assembly and disassembly of
parts is best achieved by use of the
Ultrasonic Insert. Wherever possible, theholes used to accomodate these inserts
should be molded into the part. Drilledholes can be used but are not as reliable
as molded holes. The manufacturer of
such inserts should be consulted for
the proper hole geometry.
EXPANSION INSERTS
The principal advantage of this insert is
that it is cheaper than the Ultrasonic
Insert. They are placed in pre-formedholes and when the screw is installed,
the insert expands against the side of the
hole to retain it in place.
The proper use of expansion inserts in
structural foam requires consideration of
the following factors:
1. A boss diameter should be at least
two times the diameter of the insert.
2. The insert and not the plastic should
carry the load when designing component
parts for clearances.
3. If a final assembly procedure required
any sort of heating cycle, the heat can
possibly loosen the insert from the foam.
ULTRASONIC BONDING
STAKING
Staking is the process of melting and
forming a portion of a plastic stud in
order to retain another part as shown.
SPOT WELDING
Spot welding is the process of creating
small localized bonds between two partsof structural foam.
VIBRATION WELDING
Vibration welding of structural foam parts
is capable of producing strong pressure
tight joints. This process is especially
suitable for parts that require permanent
loads over large areas.
SOLVENT BONDING
Solvent Bonding is also possible for
bonding structural foam parts to each
other. In some cases, other plastic parts
may even be solvent bonded to foam.
ADHESIVE BONDING
Adhesive Bonding has many applications
such as cementing structural foam resins
to themselves, to other plastics and tobrass, aluminum, steel etc.
ULTRASONIC HORN
Final Configuration
Initial Configuration
SLIP FIT
METAL
PLASTIC
ULTRASONIC
STAKING
1615Page of
SECTION 4PLASTICS DESIGN
CORE
RIM MOLDING
RIM (Reaction Injection Molding) is a production method which results in a
product made of polyurethane structuralfoam. When molded, this material results
in a component with an integral, solid skin
and a low-density microcellular core.
OUTER SKIN
AIR
FOAMING
ENTRAPMENT
DIRECTION
Avoid Sharp Corners
Polyurethane allows designs of greater
versatility than is possible with the usual
materials and methods of production.
WALL THICKNESS
The wall thickness can vary from a minium
of 1/4 up to 1 inch. However, when thegeometry of a part requires a variation inwall thickness, the transition should be
gradual and generously radiused, in order
to provide a better flow of material.
As a rule, wall thickness between 3/8 and1/2 inches are most desirable. A thickerwall increases cycle time and a thinner
wall may result in improper fill and
poorly formed parts.
RADII
Every effort should be made to eliminate
sharp edges on a part. A minimum radius
of 1/8 in. is advisable in all cases. Otherwise, there is the possibility of air
entrapment and damage to the outer skin.
If the outside wall of a part is less than
8" high, a no draft condition is possible.If more than 8" high, the outside wallsof the mold should be removable.
Nevertheless, it is always advisable to
design parts with a taper of at least 1.5°;
otherwise, it is likely to be difficult to
demold the part.
UNDERCUTS
Undercuts should be avoided so that in
production a simple two piece mold can
be used. If undercuts are a necessity,
automatic side pulls must be built into
the mold, thereby increasing over-allcost.
Undercut
REMOVABLE INSERT
SIDE PULL
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SECTION 4PLASTICS DESIGN
CUT-OUTS OR HOLES
The same rules which apply to undercuts
apply, as well, to cut-outs. Inserts and
pulls may be necessary. Occasionally, if
the parting plane is chosen wisely, there
can be cut-outs even though a two part
mold is contemplated as shown below.
CAVITY
BOSSES
Molded-in-bosses are used to provide for
assembly of components. A boss should
not have an excessively thick cross section.The I.D. should be half the O.D. with a
minimum .25 in. wall thickness.
Bosses should be attached to the outer
wall of a part to prevent air pockets from
developing during the molding process.If this is not feasible and a boss must
stand alone, the taper of the boss
should be as large as possible.
AVOID
NO VENTING
DESIRABLE
CROSS-RIBBING
One method of stiffening the flat side
walls of a housing is by means of cross-
ribbing as shown. However, it must be
remembered that the simpler the design,
the less expensive it will to be to produce.
It is not advisable to just throw in ribs
because it looks like they ought to be there. However, if there is doubt that
the part will be stiff enough without ribs,
a properly designed rib can be of benefit.
Generally, excessively thick cross sectionsof a rib should be avoided.
Preferred
Avoid
In addition, properly designed ribs can
maintain minimum part weight and cycletime, whereas an increased wall thickness
adds weight and may increase the
cooling time of the part.