H&GT Organization - 06/25/2004 Product & Technology H&GT GESC BangaloreQuality Excellence Support 1 Global Engineering Support Center Designing with plastics

H&GT Organization - 06/25/2004

Product & Technology H&GT GESC Bangalore Quality • Excellence • Support

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Global Engineering Support Center

Designing with plastics

10.09.2004

Product & Technology



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Design guidelines• Uniform wall thickness • Coring out • Shrinkage and warpage• Radius• Parting lines• Draft• Undercuts• Flow pattern & weld lines• Gate size and location

Strengthening features• Bosses• Ribs and Gussets• Structural reinforcements

Special Features• Threads and inserts• Snap fits• Living hinges• Press fits

Contents



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• Generally range from 2 mm to 4 mm. Thin wall injection molding ~ 0.5 mm

• Thinner wall faster cooling of part, short cycle times =>lowest part costs

Uniform wall thickness

Uniform wall thickness eliminates air entrapment.

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• Avoids warpage and build-up of stress

• Easier to fill in the mold cavity

• Any transitions should be uniform



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Coring out aids in eliminating air entrapment

Examples of coring in injection molded parts

Coring out• Coring out helps maintain uniform wall thickness

• Leads to lesser usage of material

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• Often stronger design

• Lesser cost



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Uneven sections cause distortion, warpage, cracks, sinks, and strains because of differences in shrinkage from one section to another

Shrinkage & Warpage• Shrinkage affects the dimensional accuracy

• Shrinkage occurs in two forms - initial and post-molding

Shrinkage across the diagonal is higher than along the edges, tension force along the diagonal causes corners to curl up or down

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• Initial shrinkage occurs when the part cools

• Post-molding shrinkage is defined as shrinkage occurring after 24h of the molding

• Non-uniform shrinkage lead to warpage of parts

Comparison of a normal and a warped part



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• Generally 1xThickness

• Fillet radiuses provide streamlined flow paths for the molten plastic

• Sharp corners greatly increase the stress concentration

• From Graph R/T over 0.5 the stress concentration factor gets lower

RadiusD

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Parting lines• Parting line is the place at which the core and cavity meet

• More specifically the moving and stationery parts of the mold

• The part should be designed so as to present a good parting line to the mold maker

Parting line selection for different parts

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• A flash is usually produced at the P/L and has to be finished manually or mechanically

• If the part is aesthetically important the P/L should be concealed on a thin inconspicuous edge of the part

Parting line selection



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Excessive wear on mold parting lines cancreate a mismatch on a molded part that will appear greatly exaggerated

Ways to avoid flash at P/L

Tolerances have to relaxed for dimensions across P/L

Possibility of parting line mismatch

Parting linesD

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Where would you place the parting line?

Where would you place the parting line?

Exercise



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draft_eject_anim.gif

• Draft is the angle between the direction of ejection of a part from the mold and the surface of the part

• Facilitates part removal from the mold

Example of using multiple drafts to permit ease of part removal

Using parallel drafts with heavy walls.

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• General draft 0.5 - 2 degrees

• 1.5° min. per 0.025mm depth of texture



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Undercuts• Undercuts are any design features that inhibits the normal opening of the mold

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Undercuts

Mold opening direction



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Undercuts• Inside undercuts can be made with removable wedges or ejector pins and ejector wedges

• Undercuts can be avoided by simple changes in design of part

• Complicated undercuts need special mechanisms in mold

Ways to avoid undercutsA) mold knock-out pin through opening in part B) cavity cutoff approaches.

Split-mold design eliminates the undercut problem but the part develops a parting line.



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Undercuts

Creating undercuts with simple tooling.

Mold opening direction

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Creating undercuts with cams



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Identify the undercuts & parting line

Exercise



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fillpattern_anim.gif

Meld line with parallel flow fronts

Flow patterns• Incorrect design leads to flow patterns that lead to defects

• Weld lines caused by opposite flow fronts

• Meld lines are caused by parallel flow fronts

• Appropriate changes in design and gate location can lead to better flow patterns

Opposite flow fronts produce a weld linethat could also contain entrapped air.

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Part failed on a weld line

weldline_anim.gif



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Recessed gate in a hidden area of a handle

Gates and ejectionD

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Some types of gates

• Gate is the opening through which the melt enters the mold cavity

• Correct selection of type and position of gate ensures defect free part

• Aesthetic requirements need to be specified by the designer

Gate and ejection system designed for a part

• Ejection of the part is also to be considered during design

• An ejector pin "landing pad" can be integrated into the wall of a part design in order to provide material to seat the ejector pins

Ejector pin "landing pads



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• Bosses are projections on a plastic part designed to

– add strength

– facilitate alignment

– attach fasteners such as screws or accept threaded inserts

BossesS

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• Isolate bosses to eliminate sinks

Isolate bosses from nominal walls



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Bosses

Design guidelines for molding bosses.Guidelines for designing a gusset



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Ribs• Ribs increase the bending stiffness of a part

• Rib thickness should be ~60% the wall thickness

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Examples of rib design characteristics.



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Exercises

Identify Bosses

Identify Strengthening features used



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Examples of geometric structural reinforcement techniques.

Structural reinforcements

(A) Circular or diagonal ribbing eliminates twisting(B) Bi-directional ribbing eliminates sagging or bending

A B

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Summary



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Comment on guidelines followedComment on guidelines followed



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Examples of internal and external threads.

Threads and inserts• Threads can be molded or tapped into a plastic

• Internal threads usually need a rotary action in the mold

• External threads can be molded splitting the mold

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• Screw threads should be of the coarse type and have the outside of the thread rounded

• Metal inserts can be used for quality threads



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Molded-in countersinks avoids chipping and burrs

Examples of self-threading screws.

Counter bore are often better than countersinks and avoids breakage due to hoop stress developedS

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Threads and inserts

Knurls and grooves anchor inserts in molded products



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Hot-roll plastic technique over inserts

Other ways to assemble inserts

Threads and insertsS

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Snap fit design• Snap fits rationalizes the assembly of plastic parts

• Takes over other functions like bearing, spring cushioning, fixing

• Fixing of the joined parts is weaker than in welding, bonding, and screw joining

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Examples of different snap-fit designs



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Snap fit designS

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Core and cavity for creating the snap



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Example of basic polypropylene integral hinge design (All dimensions are in inches.)

Living hinges• Materials such as Polypropylene can be molded with a living hinge

• Direction of flow of material should be perpendicular to hinge

• Used primarily in packaging products but can have engineering applications too

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Some examples of living hinges.



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Press-fits• Press fit is when a plastic boss accepts a plastic or metal shaft or pin

• The features should be dimensioned correctly so as to get the right interference

Press-fit conditions for two typical situations

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Press-fits

Alternate press-fit designs for metal pins in a plastic hubSp

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Joining plastics• Ultrasonic welding

• Hot-plate welding

• Spin welding

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• Hot stamping

• Tampo printing

• Offset printing

• In-mold decorating

• Textures and colors

Decorating plasticsS

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Buildinga New Electric World

Documents

H&GT Organization - 06/25/2004 Product & Technology H&GT GESC BangaloreQuality Excellence Support 1 Global Engineering Support Center Designing with plastics