Designing a part for cast urethane

Cast urethane is forgiving compared with injection molding, but a few choices on the drawing make the difference between a part that pours clean every time and one that fights the operator. These are the guidelines we apply when reviewing a print.

Draft

Urethane is flexible, so parts can demold with far less draft than rigid plastics. One to two degrees is comfortable. Zero draft is possible on shallow features and softer durometers. Tall, hard parts benefit from more.

Wall thickness and section

Cast urethane fills thick sections happily, so there is no need to core out a part for the sake of the process. Very thin walls, under about 0.060 inch, are possible but fragile in softer durometers. Keep wall thickness reasonably uniform to avoid differential cure and distortion.

Radii

Sharp internal corners concentrate stress and start tears. Add a radius, 0.030 inch or more, at every internal corner. External corners can be sharp if the design needs it.

Undercuts

Because the material flexes, modest undercuts demold without side actions. Deep undercuts in hard durometers may need a split mold, which adds a little tooling cost but is routine.

Molded-in inserts

Steel, stainless, aluminum, and brass inserts bond well when prepared and primed. Design considerations:

• Give the urethane something to grip: knurls, grooves, flanges, or through holes in the insert.
• Keep at least 0.060 inch of urethane over the insert where possible.
• Locate the insert on features the mold can hold, such as a bore or a through hole.
• Specify the insert material and finish. Plated or coated inserts may need different priming.

Venting and air traps

Liquid urethane traps air at high points in the mold. Blind pockets that face down in the pour orientation may need a vent. We handle this in tooling design, but a part that avoids deep blind pockets pours easier.

Tolerances

Cast urethane shrinks slightly on cure, and tolerances loosen with part size and softer durometers. As a starting point:

Tighter tolerances are achieved by post-machining critical features. Call out which dimensions actually matter for fit and function so we can focus tooling and inspection there.

What to put on the drawing

• Hardness with a tolerance, such as 85 ±5 Shore A
• Color, if not black
• Insert material, finish, and any orientation requirement
• Critical dimensions marked
• Environment: temperature range, fluids, load
• Annual quantity and any packaging requirements

Send it early

The cheapest time to change a design for casting is before the tool is cut. Send the model or print for review and we will mark up anything that would pour better another way. Request a quote.