DESIGN SMARTER: Practical Tips for Design for Molding

You can design the most beautiful CAD model, but if your part can't be molded efficiently it will never leave the screen. Here’s what I've learned to make sure our designs not only look good but also pop out of the Mold on the first try.

Understand the process

Injection molding isn't just "plastic goes in, part comes out." It's a set of cavities, cores, runners and gates that deliver molten material under pressure. Think about where your part will be fed, how the material will flow, and where the parting line should be. The earlier you think about these things, the fewer tooling headaches later.

Design tips for Molding

Uniform wall thickness

Keep your walls consistent wherever possible. Thick sections cool slowly and can cause sink marks or warping; thin sections may not fill completely. When you need transitions, taper gradually to avoid abrupt changes in thickness.

Using Draft angles is a must

Parts need to release from the mold. Add a little draft (usually 1–3 degrees) to any wall that pulls along the direction of the mold opening. The taller the wall or the more textured the surface, the more draft you should allow.

Fillets and chamfers

Sharp corners make for stressed parts and stressed mold-makers. Adding fillets to inside corners improves material flow and reduces stress concentrations. Radii also help the mold last longer by removing sharp edges on the tool.

Plan for undercuts and side actions

Undercuts add complexity and cost. Try to design your part without them, or if they are essential, plan the slides or lifters needed to release the part. Avoid surprises by discussing any potential undercuts with your mold maker early.

Mind the parting line

The parting line is the seam where the two halves of the tool meet. Place it in areas that are either hidden or easy to clean up. Avoid having cosmetic surfaces cross the parting line.

Material and finish

Not all plastics behave the same. ABS flows differently than nylon; glass-filled materials need larger radii and thicker walls. Choose the right material for your application and communicate it to your mold maker so they can adjust gating and cooling accordingly.

Material	Properties	Molding Considerations	Common Finishes	Applications
ABS	Tough, impact resistant, easy to machine	Flows easily, moderate shrinkage	Glossy or matte, can be painted or plated	Consumer products, housings, enclosures
Nylon	Strong, wear resistant, absorbs moisture	Needs drying before molding, can warp if unevenly cooled	Smooth, semi-gloss	Gears, mechanical parts, bearings
Polycarbonate	Transparent, high impact resistance	Requires high temperature, sensitive to moisture	Polished for clarity	Optical parts, covers, safety gear
Polypropylene	Flexible, chemical resistant, low cost	Shrinks more than ABS, requires draft for easy ejection	Textured, matte	Packaging, containers, automotive clips
Glass-Filled Nylon	High stiffness, temperature resistant	Needs thicker walls and larger radii, wears tools faster	Matte or lightly textured	Structural components, brackets
POM (Acetal)	Low friction, dimensionally stable	Good flow, minimal shrinkage	Glossy or smooth	Gears, precision parts, mechanical joints
TPE / TPU	Rubber-like flexibility, good grip	Sensitive to cooling rate, needs smooth mold surface	Satin or matte	Seals, grips, overmolding

Collaborate early and often

The best designs come from collaboration. Don’t wait until your CAD is "final" before talking to the mold maker. A five-minute chat can save weeks of revisions.

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