Designing with plastics

Diseño de piezas plásticas, para Ingenieros de Diseño

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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
Radius
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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 can
create 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 lines
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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.
Draft
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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 undercuts
A) 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 line
that 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 ejection
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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
Bosses
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• Wall thickness < 60 % of nominal wall
• 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 followed
Comment 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 developed
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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 inserts
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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 design
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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 hub S
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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 plastics
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Building
a New Electric World