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Cast Urethane is a molding method of producing plastic thermoset parts of varying hardness (durometer) and color. Compared to injection molding, cast urethane has a lower up-front cost but a higher price per piece. The urethane casting process begins with creating a master pattern with machining or additive manufacturing. The master pattern is then placed in a mold box and encapsulated in liquid RTV silicone. Once then RTV silicone cures, the mold box is split in half and the master pattern is removed. The remaining silicone material is now a functional mold. To create an individual cast urethane piece, a polyurethane material, with an appropriate durometer and color for the application, is poured into the mold. The mold is then cured using a combination of heat, vacuum and pressure. The cast piece is then removed from the mold, and is ready for use or further finishing, such as metal coating, bonding, or painting. Cast urethane components can be produced with a variety of colors, textures, mechanical properties, and surface finishes, making the process an ideal alternative to injection molding for low to medium volume production.
Why use cast urethane
Companies generally choose cast urethane for its quick turnaround time and low up-front cost. Material selection is also a factor. Polyurethane materials are durable, robust, and available in a wide range of colors, durometers, and textures. Urethane casting is a good option for many applications because it can create components with specific levels of elasticity, density, and dimensional stability. Cast urethane parts are particularly suited to absorbing sounds and impact, making them excellent acoustic insulating materials.
- Fast turnaround: The RTV silicone molds used in urethane casting can be fabricated significantly faster than the alternative metal tools.
- Low up-front cost: Cast urethane molds are significantly less expensive and complex than molds for injection molding. Similarly, RTV silicone is an affordable alternative to steel or aluminum, and it requires less effort to modify than metal materials, cutting down significantly on up-front labor and mold material cost.
- High impact and sound absorption:
- Low durometer polyurethane products are capable of cushioning both noise and impact.
- Elasticity and shape recovery: Polyurethane provides a wide array of material properties from lightweight and elastic, to dense and rigid.
- Resistance to extreme temperatures: Because polyurethane is a thermoset material, components retain their properties and dimensional stability at high and low temperatures.
- Joints: Cast urethane is ideal for producing mechanical joints with overlapping features due to the flexible, resilient material.
- Logos: 3D logos with raised or recessed lettering can be embossed in a variety of textures using urethane casting.
- Prototyping: The low up-front cost and quick turnaround time make cast urethane a good choice for prototype parts.
- Low volume production runs: Urethane casting is a good choice for producing plastic components needed in small quantities.
Key advantages of cast urethane
- Low up-front investment: Soft tools made with cast urethane are thousands of dollars cheaper to produce than steel or aluminum tools, and they take less time to manufacture.
- Quick turnaround times: Cast urethane parts can be produced in less time compared to the typical eight- to ten-week turnaround time for injection molded parts.
- Quality finish: Cast urethane parts can be post-processed with custom colors and surface finishes. A range of post-processing options are also available, such as metal plating, bonding, and painting.
- Injection molding options: Components produced with urethane casting can be insert molded or overmolded, allowing them to be bonded to metal, plastics, and composites according to the needs of the design.
- Ease of making alterations: If design changes are needed during production of a part, a new tool can be created relatively inexpensively compared to an injection molding tool.
Key considerations and challenges with using cast urethane
- High cost per part: Cast urethane parts have a higher per-unit cost compared to injection molding. That cost may or may not be offset by the cost saved in tooling.
- Short life expectancy of tool: A single mold used in urethane casting will produce substantially fewer parts than a metal tool used in injection molding. The number of parts a soft tool can produce depends on its material and design, but it will never match the number of pieces that could by produced a single injection molding tool.
- Thickness requirements: Cast urethane parts require a minimum 0.5mm wall thickness in order to sustain against pressure.
Cast urethane materials can mimic the following common plastics
- High Heat ABS
- High Strength ABS
- 94V0 ABS
- Acrylic or polycarbonate
- Elastomers (25A – 90A)
Urethane casting design guidelines
A cast urethane design must include dimensions, layout, material (or required material properties), color, surface finish, and any other specific considerations for the design. Please note all parts must be at least 0.5mm thick and adhere to the specific minimum feature size requirements for the given material.
Surface finishes for cast urethane
Cast urethane parts do not require additional finishing after release from the mold. Depending on the material, the as-is surface finish can be soft or rigid, textured or smooth, transparent or colored. There are also numerous finishing options to give the piece specific qualities and aesthetics.
Cast urethane pieces can also be plated to achieve certain qualities. When cast urethane is used as an enclosure for sensitive electronics, a metal plated coating in specific areas prevents electromagnetic interference (EMI) and radio frequency interference (RFI) from impacting sensitive electronics within the enclosure.
Initially smooth, cast urethane parts can be sanded or bead blasted to create a rough or matte surface, enabling strong bonds with adhesives, resins, and paint. Tougher, low durometer polyurethanes are the best candidates for these surface finishes.
Cast urethane materials are available in a variety of colors, so painting isn’t often necessary to achieve a color match, but it is an option. If a part needs multiple colors, cast urethane parts can be prepped and painted in multiple processes.