Urethane Casting Design Considerations
Bosses
Bosses are used to facilitate the registration of mating parts, for attaching fasteners such as screws, or for accepting threaded inserts.
Wall thickness of bosses should be less than 60 percent of the nominal wall to minimize sinking. The base radius should be a minimum of 0.25 x thickness. Bosses can be strengthened by incorporating gussets at the base or by using connecting ribs attaching to nearby walls.
Draft & Undercuts
Draft and undercuts are not a concern when designing for the urethane casting process itself. However, if you are building prototypes that are intended to move on to another production methods such as injection molding where hard tooling is required, you should create your design as intended for production tooling. The urethane casting process is still ideal for prototypes and short run of parts that will move on to other production technologies.
Letters and Logos
Both raised and recessed letters and logos can be molded into cast urethane parts. The key factors are: height or depth of the feature, width of the feature, radii on the feature and space between features.
The recommended minimum space between features is 0.050″. Width to height ratios should be at least 2. The radii should be at least equal to half the height. Larger radii are better.
Radii and Fillets
A fillet radius of 0.125″ is recommended on inside corners to increase strength. The inner corners of bosses can use 0.060″ radii to help reduce wall thickness. Use these radii whenever you make transitions between surfaces. Radii are important because rigid urethanes are notch sensitive materials.
Ribs
Ribs are used in a design to increase the bending stiffness of a part without adding thickness. Ribs increase the moment of inertia, which increases bending stiffness. Rib thickness should be less than wall thickness to minimize shrink and sink marks.
The recommended rib thickness should not exceed 60 percent of the nominal thickness. Plus, the rib should be attached with corner radii as generous as possible. Because the thickness of the material will be greater at rib intersections, coring or another means of reducing material should be employed to avoid excessive sinking from occurring on the opposite side.
The height of a rib should be limited to less than three times its thickness. It is better to use multiple ribs to increase bending stiffness than use one very tall rib. A rib is oriented to provide maximum bending stiffness to the part. By paying attention to part geometry, designers must be conscious of the orientation of the rib to the bending load or there will be no increase in stiffness.
Snap Fits
Snap fits can be used in all urethane casting processes following design practices used for thermoplastic resins.
Voids and Shrinkage
In most molding processes, troublesome shrinkage problems can be caused by intersection of walls that are not uniform in wall thickness. Examples might include ribs, bosses, or any other projection of the nominal wall. Since thicker walls solidify slower, the area they are attached to at the nominal wall will shrink as the projection shrinks. This can result in a sunken area in the nominal wall. Such shrinking can be minimized if a rib thickness is maintained between 50 – 60% of the walls they are attached to.
Wall Thickness
Minimum wall thickness for cast urethane parts is 0.020 in. While a good design practice is to maintain a uniform wall thickness, you can vary your wall thickness in the urethane casting process without impacting the resulting parts. This is the perfect process when you require non-typical wall thicknesses.