This article is part of the CNC Machining Materials Guide: Metals, Plastics & Superalloys on CNX Precision.
Vacuum casting materials define the look, feel, and function of your cast parts. Most vacuum casting uses two-part polyurethane resins. These resins can imitate production plastics and even flexible rubber. Therefore, material selection is just as important as mold design. CNX Precision uses a wide range of polyurethane systems for low-volume casting projects. This guide explains the key options and how to choose.
This guide reviews vacuum casting materials in practical terms. You will learn about Shore hardness, transparency, heat resistance, and color. In addition, you will find a comparison table for common resin families. As a result, you can specify the right resin for your product without guesswork.
Understanding Vacuum Casting Materials
Vacuum casting materials fall into two broad families: rigid and flexible polyurethanes. Rigid resins produce hard, impact-resistant parts similar to ABS or polycarbonate. Flexible resins produce soft, elastic parts similar to rubber or silicone. Between these extremes, many intermediate grades exist.
Every resin is a two-part system. Part A is the polyol, and Part B is the isocyanate hardener. The ratio, mixing, and curing temperature control the final properties. Therefore, a skilled casting technician follows the resin datasheet closely.
Polyurethane parts cure at low temperature. Consequently, you can cast them directly into soft silicone molds. The silicone captures fine detail, while the vacuum removes bubbles. This combination makes vacuum casting materials perfect for detailed enclosures, grips, and consumer products.
Durability depends on resin chemistry. Flexible grades resist repeated flexing, while rigid grades handle impact. In addition, some resins include glass or mineral fillers for stiffness. Therefore, ask your supplier for mechanical data before final selection.
Shore Hardness: The First Decision
Hardness is the fastest way to describe a cast part. In vacuum casting materials, hardness is measured on the Shore scale. Shore A covers soft, flexible materials from 0 to 100. Shore D covers rigid materials from 0 to 100.
Soft parts, such as overmolded grips, typically range from Shore A 30 to Shore A 70. These parts bend, compress, and absorb vibration. Medium parts, such as flexible gaskets, sit around Shore A 80 to Shore D 40. Hard parts, such as housings and structural covers, range from Shore D 60 to Shore D 80.
Higher hardness usually means higher stiffness and better wear resistance. However, very hard resins become brittle under impact. Therefore, match hardness to the part’s function, not just its feel.
Polyurethane Resin Options
The table below lists common vacuum casting materials and their typical uses.
| Resin Type | Hardness | Looks Like | Best Uses |
|---|---|---|---|
| Rigid PU | Shore D 60-80 | ABS, PC | Housings, covers, brackets |
| High-Impact PU | Shore D 55-70 | HDPE, PP | Ducts, handles, containers |
| Nylon-Like PU | Shore D 75-85 | Nylon | Gears, wear parts, clips |
| Flexible PU | Shore A 50-90 | TPE, rubber | Gaskets, seals, grips |
| Soft Foam PU | Shore A 20-40 | Foam rubber | Cushions, padding |
| Clear PU | Shore D 70-80 | PC, glass | Lenses, covers, display parts |
Resin cost varies with hardness and additives. Clear and flame-retardant grades cost more than standard rigid resin. However, the difference is small compared with tooling costs. Therefore, choose the grade that fits your function first.
When you compare vacuum casting materials, start with Shore hardness. Then, review other properties that affect your application.
Wall thickness also guides resin choice. Thin walls need low-viscosity resins that fill the mold easily. Thick walls need resins with low exotherm to avoid shrinkage marks. Therefore, share your wall thickness plan with the casting engineer.
How to Choose Vacuum Casting Materials
Begin with the part’s mechanical demands. Ask how much load, impact, and flex the part will see. Next, consider the environment. Temperature, chemicals, and UV exposure all influence resin choice.
Transparency is a special option. Clear polyurethane systems cast window-like parts for prototypes of lenses and light covers. Minor bubbles or flow lines can appear in thick sections, so design for even wall thickness.
Color is easy to control. Resin can be tinted to any custom color before casting. Alternatively, parts can be painted after demolding. In addition, textures and logos molded into the silicone surface transfer directly to the part.
Flame-retardant and food-contact grades exist for specific industries. However, they usually cost more and cure more slowly. Therefore, request them only when your specification requires it.
Properties That Matter in Production
Vacuum casting materials also differ in production behavior. Each resin has a pot life, a demold time, and a full-cure time. Short pot life means the technician must pour quickly. Long cure time slows the batch cycle.
Heat deflection matters for parts near motors or sunlight. Standard rigid PU handles about 60 to 90 degrees Celsius. Specialty high-temperature resins can exceed 120 degrees Celsius. For most consumer and industrial prototypes, standard grades are enough.
Shrinkage is another factor. Polyurethane shrinks slightly as it cures. The silicone mold compensates for this during design. Consequently, small adjustments at the master stage improve final accuracy.
Ask for sample chips before the full run. A small test part lets you check hardness, color, and surface texture. In addition, it validates the demold behavior of undercuts. Consequently, you avoid surprises in the production batch.
FAQ
What are vacuum casting materials?
Vacuum casting materials are two-part polyurethane resins that cure at low temperature. They come in rigid, flexible, clear, and foam-like grades. As a result, one process can produce parts that feel like ABS, nylon, rubber, or glass.
How do I choose between Shore A and Shore D?
Use Shore A for flexible parts that bend and compress. Use Shore D for rigid, load-bearing parts. If you are unsure, order sample chips in two candidate hardnesses. Then, test them with your actual assembly before full batch production.
Can vacuum casting materials match injection molding plastics?
Very closely. PU resins replicate the appearance and many mechanical properties of ABS, PP, PC, nylon, and rubber. Exact chemical and thermal properties still differ. Therefore, use vacuum casting for functional testing and pilot production, then validate with molding at scale.
Conclusion
Vacuum casting materials give you a fast, flexible path to realistic parts. Choose the right Shore hardness, transparency, and heat rating, and your prototypes will behave like production components. In short, material knowledge turns a good mold into a great part.
For reference, machining standards such as the ASTM material standards define the quality and tolerance requirements we follow.
CNX Precision helps you select the right resin for every project. We are ISO 9001 certified, and our casting team supports silicone molds, tinting, painting, and finishing in-house. Send us your 3D file today, and we will recommend the best vacuum casting materials for your product.