What is Urethane Casting? Benefits, Working Process, Types?

Ah, urethane casting. The unsung hero of the manufacturing world—always there when you need it but seldom the star of the show. If you’re in the industry, you’ve probably encountered it. But do you know what it is?

Urethane casting, called Vacuum casting, is a versatile and cost-effective manufacturing process that uses a silicone mold to create parts from urethane materials. It’s ideal for producing high-quality, low- to medium-volume parts with excellent material properties. 

Hang tight because we’re about to dive deep. This is not just another tech talk; it’s a candid discussion from a CEO who’s been around the CNC block.

What is Urethane Casting?

Urethane casting is a manufacturing technique that produces intricate, high-quality parts in low to medium volumes. In this process, liquid polyurethane is poured into a silicone mold, which then undergoes a curing process to solidify the material into the desired shape. 

It’s like baking a cake, but instead of flour and sugar, we’re working with advanced polymers. This method is ideal for creating prototypes, one-offs, or limited production runs.

Why Choose Urethane Casting?

Speed

Firstly, the turnaround time for urethane casting can be rapid. From mold creation to the final product, you’re looking at a much shorter timeline than traditional methods like injection molding.

Cost-Efficiency

Creating injection molds is expensive and time-consuming. If you’re not planning to produce parts by the thousands, the cost per unit with injection molding can be prohibitively high. Urethane casting, on the other hand, uses silicone molds, which are much cheaper and faster to produce. Your CFO will thank you—trust me.

Material Versatility

Urethane comes in various formulations, allowing you to tailor the material properties to your needs. Need something tough and durable? No problem. Need a part with high heat resistance? Urethane Casting has got you covered.

Quality

Did I mention the quality? Oh, yes, I did. But it’s worth repeating. The parts from urethane casting can be incredibly detailed, with excellent surface finishes. Sometimes, the untrained eye might not even distinguish them from injection-molded parts.

Flexibility in Production Volumes

Whether you need a handful of parts for prototyping or several hundred for a limited run, urethane casting can adapt to your needs. That’s something you will only sometimes find with other manufacturing methods.

What is the Working Process of Urethane Casting?

Step 1: Create the CAD Design (Detailed Design Tips)

Ah, the nitty-gritty of CAD design. When designing a part for urethane casting, there are several critical factors you need to consider to ensure the end product meets your expectations. 

Wall Thickness

Ensure a consistent wall thickness across your design. Too thin, and you risk structural weaknesses; too thick, you’ll deal with issues like sink marks. A general rule of thumb is to keep wall thickness between 1.5mm to 3mm.

Draft Angles

Add draft angles to your vertical walls to facilitate an easier removal of the part from the mold. A 1-2 degree draft angle is usually sufficient for most designs.

Ribs and Radii

To add structural integrity without increasing wall thickness:

1. Consider adding ribs.
2. Ensure the rib base is at most 60% of the wall thickness.
3. Use generous radii at corners to improve flow and stress distribution.

Groove and Bosses

Grooves and bosses are often used for assembly purposes. Keep the diameter of bosses to a minimum and add fillets where they meet the walls to ensure structural integrity.

Threads and Holes

Threads can be directly cast into the part, although for a cleaner, more precise thread, consider adding them post-production. Holes should be teardrop or diamond-shaped rather than circular for easier moldability.

Undercuts

Try to avoid undercuts, as they complicate the molding process. If unavoidable, consult with your casting expert about possible solutions.

Texture and Finish

Urethane casting allows for a variety of textures and finishes. However, remember that adding a complex texture could make the mold more challenging to produce and may impact the ease of part removal.

Step 2: Produce the Master Pattern 

This master is typically created using high-precision processes like CNC machining or 3D Printing. Remember, this master pattern sets the standard for your silicone mold, so ensure it’s done with the highest precision possible.

Step 3: Create the Silicone Mold

The master pattern is placed in a box, and liquid silicone is poured around it. Once the silicone cures, you’ll have a mold that captures all the intricacies of your master pattern. This is your key to mass-producing parts consistent in size and shape.

Step 4: Pouring the Urethane

Liquid urethane is carefully measured and mixed before being poured into the silicone mold. It’s crucial to get this mix right; otherwise, the mechanical properties of the final product could be compromised.

Step 5: Curing Process

After pouring, the mold is placed in a pressure or vacuum chamber to remove any air bubbles and ensure the material fills all the intricate features of the mold. The part then undergoes a curing process to solidify the urethane. Depending on the type of urethane used, this could take several hours to a few days.

Step 6: Demolding and Post-Processing

The mold is carefully opened, and the part is removed. Any excess material or flash is trimmed off, and the part may undergo additional post-processing steps like painting, drilling, or even additional CNC machining for ultimate precision.

What Types of Urethanes Are Used?

Silicones

Silicones are widely known for their excellent temperature resistance and flexibility. If you’re working on something that needs to withstand high temperatures or require a softer, more rubber-like feel, silicones are the way to go.

Common Materials:

• RTV Silicone: Room-temperature vulcanizing silicone, ideal for molding applications.
• LSR (Liquid Silicone Rubber): Offers excellent thermal, chemical, and electrical resistance.

Elastomers

Elastomers provide that sweet spot between flexibility and durability. They’re best for applications that need materials capable of returning to their original form after being twisted, stretched, or deformed.

Common Materials:

• TPU (Thermoplastic Polyurethane): Offers excellent abrasion and chemical resistance.
• TPE (Thermoplastic Elastomers): Known for flexibility and mechanical strength.

Rigid Urethanes

As the name suggests, these urethanes are rigid. If you need a part that can withstand high impacts or heavy loads, rigid urethanes are your go-to.

Common Materials:

• MDI (Methylene Diphenyl Diisocyanate): Provides excellent tensile strength and is often used in structural components.
• TDI (Toluene Diisocyanate): Known for its rigidity and is typically used in applications like insulation and furniture.

What Are the Mechanical Properties of Polyurethane?

Tensile Strength

Polyurethane offers a broad range of tensile strength, usually 500 to 5000 psi. This allows it to be tailored to applications that require resistance to stretching and breaking under tension.

Flexural Strength

If you’re looking for material that can withstand bending without cracking, polyurethane is your go-to. Its flexural strength typically ranges from 1000 to 8000 psi, making it suitable for various applications.

Impact Resistance

Have you ever heard the saying, “takes a licking and keeps on ticking?” That’s polyurethane in a nutshell. Its high impact resistance makes it perfect for parts that must withstand repetitive or sudden force.

Hardness

Polyurethane is available in a range of hardness, from soft and rubbery to stiff and rigid. The Shore hardness can range from 20A to 85D, depending on your requirements.

Elasticity

Polyurethane exhibits excellent elastic properties, making it ideal for components that need to return to their original shape after applying stress.

Thermal Stability

Withstand the heat with polyurethane. It usually remains stable in temperatures ranging from -30 to 150°C, although specific grades can tolerate even higher temperatures.

Abrasion Resistance

In applications where wear and tear are a concern, polyurethane excels due to its superior abrasion resistance.

Chemical Resistance

Polyurethane offers excellent resistance to oils, solvents, fats, and greases. This makes it ideal for industrial settings where chemical exposure is a concern.

What Are the Limitations of Urethane Casting?

Urethane casting, while versatile, doesn’t offer the full spectrum of material options that you’d find in processes like injection molding or CNC machining. Your choices are limited to the types of urethanes available.

Also, silicone molds have size restrictions. This means there might be better choices than urethane casting for overly large or complex parts. When it comes to “going big,” you might have to look elsewhere.

Silicone molds, while fabulous for capturing intricate details, are less durable than metal molds. They wear out more quickly, mainly when producing parts with complex geometries or harsh undercuts.

While urethane casting is relatively quick, it’s not the speed demon that some other processes are. The curing time can take up to a few days, making it less ideal for extremely high-volume production.

What Applications Are Used for Urethane Casting?

Medical Equipment Enclosures

Medical devices often require exact, yet small-scale, production. Urethane casting excels here, especially when creating equipment enclosures that must meet stringent safety standards.

Automotive Prototypes

In cars, form often needs to meet function before production can ramp up. Things like dashboard assemblies, interior components, and even aerodynamic parts are prototyped using urethane casting.

Architectural Models

The architecture world often demands incredibly detailed scale models to visualize a project before it’s built. Urethane casting can replicate complex designs down to the tiniest detail.

Consumer Electronics Housings

Your latest smartphone or smartwatch? The protective case was a urethane casting prototype before going into mass production.

Handheld Devices

From barcode scanners to specialized medical devices, the ergonomic and aesthetic aspects of handheld devices are often prototyped using urethane casting.

Custom Footwear

Specialized athletic or medical footwear often begins life as a urethane cast prototype, especially when customization is required for individual fit.

Marine Components

Boat hulls and specialized marine equipment that need to withstand the rigors of saltwater and intense weather conditions often use urethane casting for initial tests.

Seals and Gaskets

In machines and engines where sealing is crucial, customized urethane seals and gaskets are often utilized due to their durability and chemical resistance.

Mannequins

Fashion retailers require dummies that are not only durable but also highly detailed, and urethane casting fits the bill perfectly.

How Does Urethane Casting Compare to Injection Molding?

Cost-Effectiveness

In the battle of the budget, urethane casting comes out on top for short runs. The initial setup costs for injection molding can be high, mainly due to the metal molds. Urethane casting uses silicone molds, which are significantly cheaper to produce.

Speed

If you want to pump out a few thousand parts, injection molding might be your speed demon. But for smaller runs and prototypes, urethane casting can be quicker because it skips the lengthy mold-making process.

Material Options

Injection molding offers a broader range of materials, including various plastics. Urethane casting, on the other hand, is limited to, well, urethanes and similar polymers.

Part Complexity

Both processes can produce complex parts, but urethane casting has a slight edge when it comes to intricate details and undercuts. Silicone molds are more forgiving than their metal counterparts used in injection molding.

Tolerance Levels

In terms of precision, injection molding takes the cake. It can achieve higher tolerances due to the rigidity and temperature control of the metal molds. Urethane casting is slightly behind but may require post-production finishing for extremely tight tolerances.

Scalability

Injection molding reigns supreme for large-scale production, hands down. But if you’re looking for smaller batches or limited series, urethane casting can be more cost-effective and quicker to set up.

Surface Finish

Both processes offer good surface finishes, but injection molding can sometimes lead to weld lines, a result of material flowing around an obstacle within the mold. Urethane casting doesn’t typically have this issue.

How Does Urethane Casting Compare to 3D Printing?

Material Versatility

3D Printing has a broader range of available materials, including plastics, metals, and ceramics. However, if you’re specifically interested in polyurethane or its derivatives, urethane casting offers a more focused material range optimized for mechanical properties and part longevity.

Dimensional Accuracy

3D Printing can achieve excellent dimensional accuracy, particularly with high-end systems like stereolithography (SLA) or selective laser sintering (SLS). Urethane casting also offers high accuracy but may require post-processing to meet stringent tolerances.

Production Speed

For single or very low-volume parts, 3D Printing can be faster because it doesn’t require a mold. Urethane casting, on the other hand, makes more sense for medium-volume runs. Once the mold is created, producing multiple parts is quicker and more cost-effective than 3D Printing them individually.

Scalability

Urethane casting has a distinct advantage in scalability for medium-volume runs (around hundreds to thousands of parts), especially considering the time and cost of 3D Printing each part individually. However, for highly high-volume production, neither method is generally preferable to injection molding or other traditional manufacturing methods.

Surface Finish

3D printing methods like FDM (Fused Deposition Modeling) often leave visible layer lines that require post-processing for a smooth finish. Urethane casting naturally yields a high-quality surface finish that usually meets project requirements without additional processing.

Tooling Costs

One of the most prominent advantages of 3D Printing is eliminating tooling costs, as each part is built layer by layer from a digital file. Urethane casting requires the production of a silicone mold, which adds to the upfront cost but can be amortized over a medium-volume run.

Conclusion

Navigating the maze of manufacturing methods can be challenging, but having a solid understanding of urethane casting and how it measures against injection molding and 3D Printing equips you with the knowledge to make informed decisions. Each method has its unique strengths and limitations, tailored for specific project requirements—be it materials, volume, complexity, or budget constraints. 

Armed with this insight, you can select the most efficient and cost-effective process for your needs. If you still need to figure out which method suits your project, feel free to contact us at Worthy Hardware. Our team of experts is always ready to offer tailored solutions that align with your objectives. Contact us today for a consultation.