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H> Organization - 06/25/2004
Product & Technology H> GESC Bangalore Quality • Excellence • Support
1
Global Engineering Support Center
Designing with plastics
10.09.2004
Product & Technology
H> Organization - 06/25/2004
Product & Technology H> GESC Bangalore Quality • Excellence • Support
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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
H> Organization - 06/25/2004
Product & Technology H> GESC Bangalore Quality • Excellence • Support
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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
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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
shutoff_cam_anim.gif
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