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Page 1: Sect 4 Plastics

161Page of

TABLE OF CONTENTS

PAGE NO.CONTENTS

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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

Page 2: Sect 4 Plastics

162Page of

TABLE OF CONTENTS (continued)

PAGE NO.CONTENTS

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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

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14

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Page 3: Sect 4 Plastics

163Page of

TABLE OF CONTENTS (continued)

PAGE NO.CONTENTS

SECTION 4PLASTICS DESIGN

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Wall Thickness

Radii

Cross-Ribbing

REACTION INJECTION MOLDING (RIM)

Page 4: Sect 4 Plastics

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

Page 5: Sect 4 Plastics

165Page of

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.

Page 6: Sect 4 Plastics

166Page of

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.

Page 7: Sect 4 Plastics

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.

Page 8: Sect 4 Plastics

168Page of

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.

Page 9: Sect 4 Plastics

169Page of

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

Page 10: Sect 4 Plastics

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

Page 11: Sect 4 Plastics

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

Page 12: Sect 4 Plastics

1612Page of

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

Page 13: Sect 4 Plastics

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

Page 14: Sect 4 Plastics

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

Page 15: Sect 4 Plastics

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

Page 16: Sect 4 Plastics

1616Page of

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.