Cast Urethane Design Considerations

Generally speaking, cast urethane parts fall under the same design constraints as injection molded parts. Designs will need to factor ribs, drafts, undercuts, intersections, wall thicknesses and the joints between parts just as any conventionally constructed tool design would consider for molding. However, unlike injection molding, urethane casting can execute complicated features – such as varying wall thicknesses, smaller details and involved geometries – more seamlessly and provide end use parts faster than conventional means can achieve.

We covered the key differences between injection molds and cast urethanes in parts one and two, but for a quick recap:

Cast urethanes use a soft tool (Platinum Cured Silicone) instead of a hard tool (Aluminum or Steel) which saves on cost and time
The soft tool, or mold, is created using a 3D printed (or CNC machined) master pattern
3D printed patterns are an fast economical alternative to the creation of hard tooling because 3D printed patterns are capable of incorporating the gamut of desired features in a single build
The lower cost of the technology allows for lower volumes, as it negates expensive overhead tooling costs
Cast urethane materials simulate injection mold materials

It is because of the speed, low overhead cost and the less labor intensive master pattern creation that cast urethanes have been praised for completing certain, more complicated designs than injection molds. If you already have a part designed for injection molding, it can be produced with cast urethane manufacturing as low volume prototype or to launch a product (if you are planning on high volume production then injection molding will remain your final manufacturing method).

With cast urethane designs, varying wall thicknesses, small features and surface textures can be easily accommodated. When designing for these features, keep the below design considerations in mind.

DESIGN CONSIDERATIONS

WALL THICKNESS

Urethane casting has an advantage over injection molding in that wall thicknesses can vary, but a minimum wall thickness between 0.020” and 0.050” is required for most designs to ensure a complete fill and accuracy.

RIBS: Avoiding Voids & Shrinkage

RIBS are often used to increase bending stiffness without adding to wall thickness to cast urethane parts. To account for any shrink (or sink), we recommend rib thickness not exceed 60% of nominal wall thickness.

RADII AND FILLETS

To increase strength at the transition between surfaces add fillets. We recommend a 0.125” fillet at inside corners and .060” fillet at the inner corners.

LETTERS AND LOGOS

When incorporating letters and logos – raised or recessed – there are three factors to keep in mind: Height or feature depth, feature width and the radii on feature/ between features. The space between features should land around 0.050” at the minimum, though this can vary with design. The radii should be equal to half the height, or larger, and the width should be twice the height.

MICRO-WELDED INSERTS (Co-Molded)

Micro-welded inserts are incorporated with cast urethane manufacturing via a method developed by Stratasys Direct Manufacturing. The bond of Micro-welded inserts in urethane cast parts are on average 50-100% stronger than conventionally bonded inserts and over 100% stronger than heat staked inserts. We have developed the chart below as a reference guide for incorporating inserts with urethane cast parts.

urethane casting

ACCURACY

Cast urethane standard tolerances range around ± 0.015” or ± 0.003 “/”, whichever is greater. Tighter tolerances may be offered on case-by-case bases. Factors such as paint and EMI/RFI coatings add thickness and should be factored in your design.

EMI/RFI SHIELDING

Stratasys Direct Manufacturing offers Electro Magnetic Interference (EMI) and Radio Frequency Interface (RFI) shielding for the production of sensitive electrical part housings. RFI/EMI shielding uses copper, nickel or silver based coatings processed internally via a Stratasys Direct Manufacturing production site.

Beginning urethane casting can be an exciting time during a product’s lifecycle. It marks the beginning of the product launch, or the earliest functional part for sharing with customers and your project team members. Cast urethane parts undergo painting, sanding and quality inspection to result in beautiful units ready for market.