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as they will add cost to the mold
MINIMIZING STRESS IN CORNERS
• Mold makers may not be advanced enough and will put
limits on undercuts
Corners - sharp corners can cause molded-in stress from
resin flow. It is important to minimize this stress by using
rounded corners which also helps to maintain consistent wall
thickness. Make the outside radius one wall thickness larger
than the inside radius to maintain constant wall thickness
Transitions - sometimes it’s necessary to transition from
thicker walls to thinner ones. Again, sharp corners can
cause molded-in stress from resin flow. Round or taper the
thickness of your transitions (Figure 4) to minimize molded
in stresses and stress concentration associated with abrupt
changes in thickness.
Adhere to best practices in designing your part for
the plastic injection molding process. Consider the
following “checklist” as a baseline to meeting your
• Maintain uniform wall thickness throughout
• Utilize ribs to reinforce walls without adding
• A 10% increase in thickness = 33 percent increase in stiffness
• Core out unneeded thickness and wall stock
• Maintain a minimum of 0.5° draft angle on all features
perpendicular to the parting line. 1° - 2° is ideal.
• Utilize low-shrinkage materials for parts with tight tolerances
Ribs & Bosses
• Design ribs and bosses to approximately 60 percent of the
joining wall thickness for minimum risk for sink marks.
• Undercuts will add cost to the mold. Minimize them when
you can. Otherwise, there are no limits.
Corners and Transitions
• Use gradual transitions if wall thickness must change.
• Corners: R1 + T = R2