Urethane Casting: The Complete Guide

But here’s the thing – the world of resins is vast, with a plethora of options available today. With such a variety to choose from, how can you be certain that urethane is the ideal choice for your needs? That’s where we come in. We’ve compiled an informative guide covering everything you need to understand about urethane casting. From curing times to comparisons with other resins, we provide the facts to empower you to make the right decision.

What Is Urethane Casting?

Urethane casting, also called Vacuum casting, is a process that involves injecting a blend of polyurethane and additive resins into a pliable silicone elastomer mold, distinguishes itself from traditional injection molding by sidestepping the use of rigid, tooled metal molds. It specializes in short production runs and is ideal for low volume manufacturing and Rapid Prototyping. This suitability arises from the gradual wear and tear experienced by silicone molds over time. Urethane casting offers a more cost-effective and expedient alternative compared to its metal mold counterpart, without compromising the exceptional quality of the finished molded products.

What is the process of urethane casting? 

The urethane casting process involves creating detailed, high-quality parts by injecting liquid polyurethane or urethane resin into a mold. Here are the steps involved in the urethane casting process:

1. Master Pattern Creation: A master pattern of the desired part is crafted using methods like CNC machining or 3D printing.
2. Silicon Mold Making: A silicone mold is created by pouring liquid silicone over the master pattern and allowing it to cure. The mold is usually split into two halves for easy removal of the cast part.
3. Material Preparation: Urethane casting materials come in liquid form, consisting of two main components: a polyol and an isocyanate. These components are mixed together in specific ratios to initiate a chemical reaction that results in the solidification of the material.
4. Casting: The mixed urethane material is poured into the mold cavity. The mold is then closed and secured to prevent leakage.
5. Curing: The chemical reaction between the polyol and isocyanate components causes the urethane material to cure and harden within the mold. The curing time can vary based on factors such as material composition and part complexity.
6. Demolding: Once the material has cured, the mold is opened, and the cast urethane part is carefully removed. The silicone mold can typically be reused for multiple casting cycles.
7. Finishing: Depending on the specific requirements, the cast part may undergo post-processing steps such as trimming, sanding, painting, or other surface treatments.

What materials are used for urethane casting?

A multitude of urethane casting materials is available, each offering distinct properties related to flexibility, elasticity, stiffness, and hardness. These materials cater to a wide range of applications.

brands offering urethane casting materials include FastCast urethane casting resin, Smooth-On, and SRC Specialty Resin. These materials empower engineers and manufacturers with a wide array of options to meet the specific requirements of their projects.

• ABS-Like General Purpose Resin: Ideal for versatile applications where balanced properties of strength and flexibility are required.
• Glass-Filled Nylon-Like Material: Offers superior rigidity, making it suitable for parts that demand exceptional strength.
• Elastomeric TPE-Like Material: Perfect for wearables, hand grips, buttons, gaskets, and seals, thanks to its elastomeric properties that provide flexibility and resilience.
• PC-Like (Polycarbonate): Provides high-temperature resistance and durability, making it apt for demanding environments.
• Translucent or Water-Clear Casting Urethane (PMMA-Like): Offers optical clarity and is reminiscent of polymethyl methacrylate (PMMA) in terms of appearance.
• POM-Like or PU-Like Heat-Resistant Materials: Tailored for applications requiring resistance to high temperatures.

Moreover, there are several exotic urethane casting materials to explore, each with unique characteristics:

• Smash Glass: Specialty material with distinctive visual effects.
• Wax: Suitable for patterns and intricate detailing.
• Epoxy: Offers strong adhesion and versatility.
• Ceramic: Ideal for high-temperature applications.
• Composites: Combine multiple materials for specific performance attributes.

Urethane Vs. Polyurethane

Urethane and Polyurethane both are used for casting materials. Although the terms “urethane” and “polyurethane” are often used interchangeably, these are two distinctly different compounds.

Urethane

• Urethane is a crystalline compound represented by the chemical formula C3H7NO2. It’s essentially an ester of carbonic acid.
• This material is known for its flexibility and malleability, making it suitable for objects with diverse shapes. It’s typically used in its liquid form.
• Urethane finds applications in various areas, including insecticides, veterinary medicines, pharmaceuticals (although it can have side effects like nausea), and even as a solvent. It’s also a component in the formation of some plastics.

Polyurethane

• Polyurethane gets its name from being composed of multiple (“poly”) urethane units that are chemically joined through a process called polymerization.
• In contrast to urethane, polyurethane is stiff and rigid, making it ideal for creating firmer items.
• It’s commonly used in the production of solid materials like shoes and foams. Additionally, certain types of polyurethane are incorporated into paints to provide long-lasting coatings.
• Polyurethane biodegrades very slowly and is generally associated with a low level of toxicity risk.

What mold material for urethane casting?

In urethane casting, silicone rubber is the go-to material for making molds because it’s flexible, durable, and excellent at capturing fine details. There are two types of silicone rubber: tin-cure and platinum-cure. Tin-cure silicone can shrink a bit (1-4%), so for high-precision work, it’s better to use platinum-cure silicone or high-temperature vulcanization (HTV) rubbers.

A silicone with a hardness of 40-50 Shore A is ideal. It makes long-lasting molds that are easy to remove parts from, even for complex shapes. Translucent silicone is recommended because it lets you see inside the mold, which is helpful for spotting any issues during casting.

You can find quality two-component silicone from brands like SmoothOn, Elastosil, V-Sure, and Silicones and More (S.A.M.). Another option is ordering directly from chemical factories abroad, which can be cost-effective and tailored to your needs, especially for large orders.

urethane Casting Machines

A vacuum resin casting machine automates mixing, casting, and curing. Operators control parameters like pressure and temperature. They come in different sizes and can work with various materials, including resin, nylon, ceramics, wax, and metals. They are ideal for pre-production runs but can be expensive. Some options include:

• Renishaw 5/01 PLC: Handles molds up to 530 x 450 x 420 mm
• SLM VCM 04: For molds up to 750 x 750 x 900 mm.
• MCP 5/01: Suitable for parts up to 400 x 400 x 400 mm. Offers larger models.
• Scott AM 1000: Handles molds up to 500 x 450 x 425 mm.
• Multistation CSV 340: Capable of molds up to 340 x 340 x 435 mm.

Advantages of Urethane casting

Uncertain whether the urethane casting process is the right choice for your needs? Let’s delve into some of its advantages:

• Rapid Production: Multiple components can be made within 24 hours, reducing product development time and costs.
• Versatile: Suitable for small quantities of high-quality parts, both for prototypes and low-volume end-use parts.
• Material Variety: Urethane casting resins come in various types, including transparent, rubbery, flame retardant, food-grade, and colored resins.
• Easy Material Evaluation: Resin types can be changed without retooling, allowing for material testing.
• Complex Forms: Multi-segment molds and cores can handle intricate shapes and features.
• Precision: Inserts made of aluminum or brass enhance accuracy for certain features.

Disadvantages of Urethane casting

Besides advantages, there are also some Disadvantages to urethane casting. Make sure you know what they are before making your decision:

• Mold Wear: Soft silicone molds wear out quickly, typically lasting for 30 to 50 components.
• Shrinkage Effects: Shrinkage can result in uneven, abnormally thin or thick features, similar to injection molding.
• Shrinkage Rate: A 15% to 25% shrinkage rate occurs due to thermal expansion, so it’s essential to discuss tolerances with the manufacturer.
• Surface Limitations: Surface finish is often confined to the exterior, making some interior aspects unreachable.
• Softened Edges: Sharp corners and lettering might have softened edges.
• Additional Components: Some features may require connectors and cores.
• Color Matching: Precise color matching can be challenging.

Urethane Casting vs. Alternatives

Urethane Casting vs. Injection Molding

Urethane casting suits low to medium runs with rapid turnaround and complex shapes. It’s cost-effective with flexible material options. Injection molding excels in high-volume, consistent production but demands longer setup and costlier tooling.

Urethane Casting vs. 3D Printing

Urethane casting delivers high-quality parts in small quantities, ideal for prototyping and end-use. It offers diverse resins. 3D printing is for rapid prototyping but has volume constraints. Both excel in complexity, with casting offering more material choices.

Urethane Casting vs. CNC Machining

Urethane casting is faster for small to medium runs, cost-effective, and versatile. CNC machining provides precision for low to medium runs but has longer lead times and higher setup costs. Choice depends on precision and volume needs.

Applications of cast urethanes

Rapid Prototyping

Urethane casting has emerged as a rapid prototyping champion. Its ability to swiftly transform CAD designs into physical prototypes is invaluable for engineers and product designers. It enables the creation of intricate and detailed models that closely mimic the final product, facilitating design validation and testing. Urethane prototypes, with their range of material properties, can accurately simulate end-use conditions. This agility in prototyping helps teams iterate designs, identify flaws, and refine concepts before committing to mass production. It significantly shortens the product development cycle, saving both time and resources.

Low-Volume Manufacturing

Beyond prototyping, urethane casting finds its sweet spot in low-volume manufacturing. When production runs are too small to justify the expense and time associated with traditional injection molding or CNC machining, urethane casting steps in as the cost-effective alternative. It excels in crafting limited quantities of high-quality components. Industries ranging from automotive to medical devices benefit from urethane casting’s ability to produce low-volume batches efficiently. Its flexibility in materials, colors, and hardness levels ensures that these end-use parts meet performance standards and aesthetic requirements.

Industries Using Urethane Casting

Urethane casting’s versatility has led to its adoption across various industries. Aerospace leverages it for creating lightweight and durable parts. In the automotive sector, it aids in crafting functional prototypes and low-volume production components. The medical field employs it for medical device development, from surgical instruments to wearable technologies. Urethane casting also serves the consumer goods industry, allowing for rapid prototyping of consumer electronics and household appliances. It’s an asset in the architectural realm, producing intricate models and fixtures. Whether for prototyping or manufacturing, urethane casting has firmly established its presence in these diverse sectors, offering a bridge between concept and reality.

Conclusion

After all of above, is urethane casting the right choice for your project? If you still have doubts, we recommend exploring alternative manufacturing processes, as we did when comparing it to plastic injection molding. By familiarizing yourself with various techniques, you can better align your project needs with the most suitable method.

Should you require assistance or have further inquiries, please don’t hesitate to reach out to us at KULSA prototype. With our extensive experience spanning multiple industries, including injection molding and vacuum urethane casting, we are well-equipped to address your questions and provide the support you need for your project. Feel free to get in touch, and we’ll be delighted to assist you.