Cast Urethane

Cast Urethane is a process used to bridge the gap between prototyping and highvolume production tooling.  It’s ideal for quantities from 1 to in the 100s, and both flexible and rigid parts can be used for prototyping or lowvolume production. 

If threaded inserts or other components are needed for your project, they can be insertmolded into parts during the casting process. 

Cast Urethane Mold line drawing

Table of Contents

How Does Cast Urethane work?

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 is made by encasing 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 put in a chamber that is pressurized to help removes air bubbles. When the material has cured, the part is removed, and the process is repeated.

What are the advantages to using 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 solutions@midwestproto.com, or give us a call at 608-437-1400.

Best Practices For Designing Parts For Cast Urethane At Midwest Prototyping.

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.

Ribs

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.

Bosses

Radii and Fillets

Radii and fillets are used to increase the strength of a fillet radius of .125″ are recommended for all corners.  One can use a .060″ radius on internal corners to reduce wall thickness.

Joints Between Parts

Consider adding reliefs between parts.  Clearance between the tongue and groves should be allowed for tolerances and paint thickness.  The larger the part, the more significant the reveal gap and interlock.

Reveal Gap

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.  

recesses and embossing

Snap-Fits

Snap-fits should follow guidelines for thermoplastic resins.  They should be considered to speed up assembly time, save hardware costs, and be used to attach components.

Considerations For Finishing Cast Urethane Parts At Midwest Prototyping.

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 solutions@midwestproto.com.

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 making the part.  One can use the two methods 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.

Midwest-Prototyping-Logo-combo-white.

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