Design Guidelines for Urethane Casted Parts
Urethane castings are excellent choices for production-grade plastic parts without the cons of expensive hard tooling, design complications, and delayed turnaround periods. When designing cast urethane, it is imperative to stick to some basic guidelines. These rules are important to ensure that your part functionality and production lead times remain intact. To help you create master patterns that match your desired specifications and function as intended, Premium Part Manufacturing Solutions have put together these simple but actionable tips to help design higher-quality urethane casted parts.
1. Keep the walls uniform
As is the case in the most manufacturing process, maintaining uniformity in wall thickness is equally important for a dimensionally stable urethane casted part. The wall thickness of your cast urethane part should be consistent all-round the part to enable the molten urethane to flow into the mold and cure at a uniform rate.
Designing parts with varying wall thickness can cause parts to fail, inhibit functionality and even result in a total product defect on production. The design guideline recommended for wall thickness is to have them at least 0.020’’ – 0.050’’. This range can be defined as at least 0.20’’ for smaller parts and at least 0.40’’ for larger parts. Remember that the wall thickness should be consistent with the size of the product to provide the required support. Most parts have their average wall thickness ranging from 0.80’’ to 0.150’’.
2. Draft angles and Undercuts
Unlike injection molding, urethane castings designs need not place much emphasis on drafts. This is because liquid urethane is free-flowing and will take the shape of any mold to be used in production. To mitigate the risks of breaking and warpage however, consider using at least one degree of a draft for your cast urethane part. This will also aid the extraction process when removing the final part from the mold.
3. Factor in shrinkages
Shrinkages are very natural in urethane casting and metal die-casting operations. They naturally occur as a result of a change in the density between the liquid urethane and the solidified part. Because of this, the final dimensions of any cast urethane part will largely depend on the precision and allowance put into the design of the master model. For best results, you must factor in some percentage as shrinkage rate. This number often ranges between 0.10 to 0.20%.
4. Consider accessibility and manufacturing
Having a firm understanding of your design, the important, functional and purely aesthetic features can go a long way in helping you attain the best master model. Before incorporating certain features into your design, evaluate the importance of the feature and how such feature can have an impact on the functionality and moldability of your final part. As for corners, try to use radii of at least 0.125’’ to ensure that your part remains strong and mold flow remains seamless. For the boss corners, radii considerations should be around 0.060’’ to retain part strength even at slightly lower wall thickness. For part tolerances, keep your urethane casting design within +/- 0.010” for the first inch and half of that for every subsequent inch.
As much as possible, try to do away with design features that may simply complicate the mold flow or increase lead times and overall production cost.
5. Finishing considerations
Finally, considerations around finishing are very important with cast urethane parts. This is because urethane castings can be made in a variety of finishes. The standard finishes include matte and sanded, however, polished surfaces, lettering, embossing, or engraving may come at some extra cost.
If your urethane-casted parts are to be lettered, ensure that the logo or lettering is at least 0.040 inches thick. It should also be elevated to this same dimension for the raised visual effect or lowered likewise for a sunk-in, recessed effect.
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