Polyurethane casting, also known as vacuum casting, is a common process in rapid prototyping. It is a process that uses a cost-effective silicone mold to reproduce plastic and rubber parts, the same as the master part in weeks.
The materials used for polyurethane casting have properties similar to that of plastic injection molding. However, they are different in several factors.
This article discusses all the materials involved in polyurethane casting and discloses the differences between polyurethane casting materials and plastic injection molding materials. It aims to provide comprehensive and honest information for engineers and designers considering polyurethane casting for prototyping or small-series trial production.
The polyurethane casting involves making the master part, the silicone mold, and the vacuum-cast parts. Each process involves different materials.
What Are the Materials for Polyurethane Casting Masters?
A master part is used to make the silicone mold. The most common ways to make the master part are SLA (Stereolithography Apparatus) 3D printing and CNC (Computer Numerical Control) machining.
SLA uses photosensitive resin to make the master part. It is faster and cheaper than CNC machining, while the surface is rougher and has less strength.
CNC machining usually uses ABS plastic. Aside from being different from the form of ABS used in injection molding, the properties are identically consistent. The ABS used for CNC machining comes in plates, while the ABS for injection molding is in granules.
When deciding between SLA and CNC for a master part of vacuum casting, the key consideration is which method better suits the product's structure. Cost is also crucial, and SLA is generally the more affordable option.
What Is the Material for Polyurethane Casting Molds?
The mold material used for polyurethane casting is usually liquid silicone, also known as RTV (Room Temperature Vulcanizing) silicone. This material can cure at room temperature, forming a soft and flexible mold that can accurately replicate the details and textures of the master prototype.
What Is Polyurethane Casting Resin?
Polyurethane casting resin is widely used in various applications, including prototyping, model making, and manufacturing. It is a type of polymer created by mixing two liquid components: a polyol and an isocyanate. When these components are combined, they undergo a chemical reaction that results in a hard and durable plastic.
Polyurethane casting resins include a series of materials. Each has different properties. The common types of polyurethane casting resins are as follows:
ABS-like PU(polyurethane): PU 8150
Acrylonitrile-butadiene-styrene (ABS) copolymer is a commonly used thermoplastic engineering plastic with good overall properties, such as ease of processing, dimensional stability, and excellent impact resistance.
PC-like PU: PU 8751
Polycarbonate (PC), which has excellent impact resistance and transparency, is often used to manufacture clear or semi-transparent parts.
PA-like PU: PU 2180
Nylon (PA) has high wear resistance and good mechanical strength. It is suitable for making high-strength parts such as bearings and gears.
PMMA-like PU: PU PX521-HT
Polymethyl methacrylate (PMMA), commonly known as acrylic or plexiglass, has high transparency and can be used to make clear covers or windows.
PP-like PU: PU 8550
Polypropylene (PP) is a lightweight, heat-resistant, and chemically resistant material. The material is commonly used in household appliances and automotive components.
Rubber-like PU: PU 8400
Rubber is a versatile, elastic material derived from natural sources or synthesized from petrochemicals.
Listed above are the properties of the original materials. Every polyurethane casting resin has its properties. Some polyurethane casting resins can also offer special properties like fire and thermal resistance. Ask your vendor for the material datasheet of the PU resin so you can determine if it meets your requirements.
Many brands produce polyurethane casting resins. The famous ones are Hei-Cast (from Japan), AXSON (from France), Pelnox (from Japan), and DPI (from France).
Advantages of Polyurethane Casting Resin
High Performance: The performance of silicone-molded parts is close to that of injection-molded parts, making it very suitable for prototype testing and customized small-batch production.
Customizable Properties: The physical properties of polyurethane can be tailored to meet specific requirements, such as hardness, flexibility, and durability.
Smooth and Detailed Casts: Polyurethane casting resins can produce parts with fine details and smooth surface finishes, reducing the need for additional finishing work.
Color and Texture Options: The resins can be painted and textured to achieve desired aesthetic qualities.
Flexible Range of Hardness: Polyurethane resins are available in several hardness levels, from soft and rubbery to hard and rigid.
Simple Processing: Polyurethane resins are relatively easy to mix and pour, making the casting process straightforward.
Low Cost: The resins used for polyurethane casting are relatively economical, especially for up to 200 pieces, which offers a significant cost advantage. It largely reduces costs, making vacuum casting more economical than injection molding, particularly in small-batch production.
Disadvantages of Polyurethane Casting Resin
Limited Choices: Compared with plastic injection molding, polyurethane casting resin choices are limited.
Moisture Absorption: Polyurethane resins are highly sensitive to moisture, which can lead to foaming and defects in the final cast.
Storage and Handling: Special care must be taken to store and handle polyurethane resins in dry conditions to prevent moisture contamination.
Degradation Over Time: Polyurethane resins can degrade over time, especially when exposed to air and moisture, which limits their shelf life.
Prone to Shrinkage and Deformation: Products made through vacuum molding may deform during the cooling process after being heated to high temperatures due to material shrinkage. This deformation typically results in a certain degree of error, usually around 0.2mm. Such deformation not only affects the dimensional accuracy of the products but may also impact their functionality and appearance.
Air Bubbles and Voids: Improper degassing and pouring techniques can form air bubbles and voids, leading to surface defects and reduced part quality.
Mold Release Issues: Improper use of mold release agents can result in surface imperfections or difficulties in demolding the parts.
Relatively Poor High-Temperature Resistance: Products made through vacuum molding generally can only withstand temperatures around 80 degrees Celsius, which may limit their use in some applications. Compared to CNC-machined prototypes, vacuum-cast products have lower strength and hardness, which somewhat restricts their range of applications.
Besides the limited choices and relatively poor high-temperature resistance of polyurethane resins, experienced polyurethane casting vendors can avoid all the above disadvantages. Selecting the right supplier matters a lot.
Polyurethane Casting Materials VS Injection Molding Materials
Polyurethane casting can be considered an economical alternative to injection molding, but there are differences in materials.
Simply speaking, polyurethane casting typically uses two different liquid components, A and B, which are blended and solidified through chemical and physical reactions, while injection molding uses engineering plastics that support a wider range of materials, including those crucial for strength, durability, and chemical resistance.
The detailed differences lie in the below factors:
#Polyurethane Casting Materials
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Versatility: Polyurethane casting materials can be formulated to achieve a wide range of properties, from flexible and rubbery to hard and rigid.
Durability: Excellent wear and abrasion resistance, making them suitable for high-stress applications.
Chemical Resistance: The materials have good resistance to oils, solvents, and other chemicals.
Thermal Stability: Generally good, but some formulations may degrade at high temperatures.
Elasticity: Many polyurethane materials offer excellent elasticity and resilience.
#Injection Molding Materials
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Diverse Options: There is a wide range of thermoplastics and thermosetting plastics available, including ABS, polycarbonate (PC), nylon (PA), polypropylene (PP), and more.
Mechanical Strength: High strength and rigidity, suitable for structural components.
Thermal Resistance: Some materials, like PC and PA, offer excellent thermal resistance.
Chemical Resistance: Varies by material, with some offering excellent resistance (e.g., PP) and others less so.
Surface Finish: This can achieve very smooth and precise surface finishes.
Color: Vacuum-cast products tend to have dull, non-glossy colors, and their color consistency is relatively poor, especially when the number of replicated products exceeds a limit. In contrast, injection molding ensures color consistency by mixing color powders with plastic pellets.
Performance: While the hardness and flexibility of vacuum-cast products are similar to those of injection-molded products, they still fall short in performance. Because vacuum-cast products are made from materials that solidify through liquid fusion, while injection molding uses engineering plastics better suited for applications with strict performance requirements.
Conclusion
After knowing the basics of polyurethane casting materials and their differences from injection molding ones, you can decide which process suits you better.
