Table of Contents
Advantages of Cast Urethane
Whether you need multiple prototypes or are ready to jump into low-volume production, there are many advantages to using Cast Urethane. Cast urethane produces high-quality, precise, and consistent parts. The molds are low-cost and suitable for 20 – 25 reproductions. For larger production orders, we create more molds and maintain low costs for parts produced in the 100s. There is no need for expensive tooling. Cast Urethane is a relatively quick method. Once the CAD files for the master pattern are received, the products will be made and shipped in as little as three to ten days. The design flexibility is similar to injection molding, with a few exceptions. We’ll go into more detail below in the design guidelines section. We also offer a wide range of materials in our cast urethane department. For more information on materials, please contact one of our solutions managers at [email protected], or call us at 608-437-1400.
Process
Once Midwest Prototyping receives the CAD file, we use our in-house Laser Sintering or Stereolithography machines to create the master patterns. We offer various materials for the master pattern that can be found in our Material Pro database. A mold encases the master pattern in silicone or other soft materials. Once the mold material has cured, it is cut in half, and the master pattern is removed. The two halves are put back together, and a urethane material is poured into the mold. The mold is then placed in a pressurized chamber to help remove air bubbles. When the material has cured, the part is removed, and the process is repeated.
Design Guidelines
Wall Thickness
A minimal wall thickness of 0.040″ is recommended; however, a wall thickness of 0.020″ can be achieved on small parts. The larger the part, the larger the wall thickness should be for proper support. Good design practices use consistent wall thicknesses whenever possible.
Drafts and Undercuts
Drafts are unnecessary unless cast urethane is being used for prototyping for alternative production methods such as injection molding. If the plan is to use hard tooling down the road, follow design guides for those production methods. Using draft angles of 3 -5% may extend the life of the mold.
Ribs
Ribs are used to increase bending stiffness without adding thickness to the wall and to increase the moment of inertia. Keeping rib thickness at no more than 60% of the wall thickness will prevent shrinkage and sinking. Fillets should be used where the ribs meet the wall. The height of the rib should be less than three times the thickness of the wall. Multiple ribs can be more effective than one large rib. When adding ribs, pay attention to the geometry and where stiffness needs to be increased.
Voids and Shrinkage
Voids and shrinkage can occur at intersections where the walls aren’t uniform. Ribs at 50 – 60% of the wall width can minimize voids and shrinkage. Voids are less likely in cast urethane than injection molding. If injection molding is the next step in the production process, incorporate bosses into your design.
Bosses
Bosses are features used when two parts intersect, for example, fasteners, screws, or threaded inserts. Wall thickness like ribs should be 60% of the walls. If they are not visible, the thickness can accept self-tapping screws or micro-welded inserts. Bosses should be 0.25 times the thickness. To strengthen the boss, gussets can be used.
Radii and Fillets
Radii and fillets are used to increase the strength of a fillet radius of .125″ are recommended for all corners. To reduce wall thickness, a .060″ radius can be used on internal corners.
Joints Between Parts
Consider adding reliefs between parts. Clearance should be allowed between tongue and groves for tolerances and paint thickness. The larger the part, the larger the reveal gap and interlock should be.
Letters and Logos
Letters and logos can be molded by either raising or recessing letters and logos. The recommended minimum distance between features is 0.050″, width to height ratio is 2, and the radii are at least half the height, if not more significant.
Snap-Fits
Snap-fits should follow guidelines for thermoplastic resins. They should be considered to speed up assembly time, saves cost on hardware, and can be used to attach components.
Finishing
Texture
From rigid to flexible, many materials can be used in cast urethane. For more information about adding textures to these materials, contact one of our solutions managers at [email protected].
Color
Getting the part color you desire can be achieved by using cast-in colors, paint, or both. Cast-in colors are added to the urethane material; paint is applied after the part has been made. The two methods can be used subsequently if the desired color needs to last.
Cast-in Colors
Paint
Pros:
- No chipping
- No peeling
Cons:
- Surface texture finish is reliant on master pattern finish
- Color may differ in shade depending on the thickness of the wall
- It may not be UV stable
Pros:
- Gloss level
- Uniform surface appearance
- Eliminates visible lines left from the parting object from the mold
- UV stable
Cons:
- Can be scratched
- Can be chipped
- Can be peeled
- Additional labor resulting in a higher cost
EMI/RFI Shielding
Electro-Magnetic Interference (EMI) or Radio Frequency Interference (RFI) shielding are used in plastic enclosures designed to house sensitive electrical components. Copper, nickel, and silver can be applied to specific surfaces of cast urethane parts to shield against interference from other devices or power supplies in your device.
