What Is Urethane Casting?
Urethane is a type of resin that’s used to make a range of plastic items. It’s end-use is highly versatile; you can create both flexible items that are almost rubbery in texture, and very rigid plastics.
Urethane castings in the manufacture of short-run, low-volume products such as movie props or bespoke models. It involves pouring the urethane into a mold and letting it cure – but we’ll talk more about the process later.
Urethane casting is a brilliant low-cost, high-quality way to create parts and whole models. It works well for detailed designs too, creating a high standard finish quickly and without the high price tag of other methods.
How Does Urethane Casting Work?
Urethane casting involves four key components: polyol compounds, diisocyanate compounds, chain extenders or curatives, and additives.
The mechanical properties of the cast depend on the formulation of the prepolymer resin (a mix of polyol and diisocyanate) and the curatives.
Additives enhance properties such as resin curing time, machinability, color, and UV protection. However, additives must be carefully measured as excessive amounts can weaken the urethane cast.
One concern in urethane molding is the coefficient of friction (COF), which indicates how much plastic materials stick to each other.
To reduce COF, “slip additives” are used. These organic compounds, such as acid amides, erucamide, and oleamide, coat the surfaces of plastic forms to minimize friction. Erucamide is a slow-moving additive, whereas oleamide is fast-moving.
The polyurethane reaction involves forming a polymer chain by reacting a polyol component with a diisocyanate component.
This reaction results in a molecule with a reactive alcohol at one end and a reactive isocyanate at the other.
The alcohol end links to another isocyanate, while the isocyanate end reacts with chain extenders (curatives like hydroxyl and amines), continuing the formation of long-chained polyurethane.
Formulating urethane can be achieved through different methods: single shot, prepolymer, and quasi-prepolymer processes.
In the single shot process, all components are kept in separate chambers and then blended by a mixing head before being poured or injected into the mold.
The prepolymer process involves mixing polyols and diisocyanates before pouring them into the mold, which helps to manage the heat generated from the exothermic reaction.
The quasi-prepolymer process uses polyols that are partially reacted with diisocyanates, making the formulation easier due to the lower viscosity and reduced processing temperature required.
What Is Urethane Casting Process?
Casting urethane can be tricky, so we recommend ordering a tester sample before a big order. You’ll be able to get to grips with the process, work out whether you like the resin, and avoid losing too much in failed attempts.
Most of us mess up a few times when trying a new resin, so don’t worry if it doesn’t go to plan!
The process starts with a silicone mold. In order to create this mold a master model of the parts will be manufactured by 3D printing or CNC Machining, then by pouring the silicone into a tool imprints of the parts will be shaped.
You then have to mix your urethane, which comes as two separate liquids. You should mix the parts in a 1:1 ratio or as directed on the brand instruction. As soon as you mix the two, a chemical reaction starts that turns them from liquid to solid – this is curing.
You may also hear curing being referred to as room temperature vulcanization. It may sound more advanced, but it really is the same thing.
If you’re adding a tint to your resin, remember to do it before combining the two liquids. It can take a while to mix the tint so this will give you more time. If you add the tint to the combined resins, the urethane may start to set before you’ve thoroughly mixed it.
Once you’ve mixed the tint, you can combine parts A and B of your urethane. Stir this for between 10-20 seconds. You can then carefully pour it into your mold.
Leave your urethane for around 1 hour to cure – although you can follow the brand’s guidance for timing. Remember to always read their safety information, too, and they can tell you more about pot life and pouring techniques.
Once the resin is properly cured, remove it from the silicone mold – easy! They can repeat the operation up to 20 times.
What To Remember When Using Urethane
Urethane can be affected by many factors. Remember these so that they don’t affect your final parts:
- Urethane hates moisture. As soon as urethane comes into contact with moisture, it reacts. Even the water in the air can ruin your urethane, which would be a pain after you’ve just bought two large jugs. So, always remember to put the caps back on your urethane as soon as you’ve poured it. This will help it to last longer and work better.
- Urethane is exothermic. An exothermic reaction is when heat is created, and this happens to urethane whilst it cures. Because of this, areas with a higher volume of concentrated urethane will cure faster than thinner areas. Just because one area is ready doesn’t mean the whole part is, so be sure to leave it for long enough for everything to dry. The exothermic reaction will also evaporate heat from your mold which can cause damage.
- It will shrink. As your resin turns from liquid to solid, it’ll shrink. With urethane casting you expect shrinkage of around 0.15%, which you’ll have to bear in mind when creating your mold.
- Your tint will be affected by the base color. The base color of urethane is usually white. It can also be black, translucent, or colored. Remember that whatever base color you pick will affect your final color when you use a tint. So if you choose a white base and red tint, for example, it’ll more likely come out pink.
- You can paint urethane. Don’t worry too much about tint if you’re finding it too hard to color match. You can also paint your urethane parts. We still advising tinting your parts though to a similar color; this will help if your paint chips and reveals the urethane beneath.
Each urethane brand will be slightly different, so always read the advice and guidance from your brand before using it. This will help you get the best finish and make sure your urethane lasts for its full pot life.
Designing For Urethane Casting
Designing for urethane casting is slightly more forgiving than designing for injection molding, as there is less risk of part shrinkage and less risk of sink marks developing due to the lack of a heating-cooling cycle in cast material.
However, wall thickness, draft angle, and construction of ribs and bosses can all affect the result. It can also be helpful to incorporate design changes for injection molding if large-volume production will be the eventual end goal.
Wall thickness
The typical minimum wall thickness for cast urethane parts is 0.040″. However, wall thicknesses as thin as 0.020″ can be possible on smaller parts. Larger parts should use thicker walls to ensure the part is strong enough.
While urethane casting can accommodate variations in wall thickness better than injection molding, a uniform wall thickness can help reduce the risk of any deformation.
Draft
While incorporating a draft angle is not strictly necessary in urethane casting, a 3-5 degree draft can help the part release from the mold much easier, extending the life of the mold.
As a draft angle is necessary for an injection molded part, it often makes sense to include one if you intend to replace it with an injection molded equivalent eventually.
Ribs
Ribs can help reinforce parts without significantly adding to their thickness. Rib height should be no more than three times the thickness of the rib, with rib spacing at least twice the thickness of the rib away from each other.
A fillet radius of at least one-quarter of the rib thickness can help reinforce the ribs. Make sure to position the ribs so that they increase the part’s bending stiffness.
Bosses
Bosses are often used to secure mating parts such as pins or screws. The base diameter of the boss should be at least half the thickness of the part.
The wall thickness of the boss should be less than 60% of the thickness of the part to minimize shrinkage. A fillet on the inner radius of bosses of 0.060″ can help reduce the risk of sinking.
Fillets
A fillet is a rounded corner that is used to smooth out sharp edges. It not only improves appearance but also can reduce stress. Interior fillets of 0.125″ should be added to all inside corners to increase the part’s strength.
Inserts and through holes
During the mold-making process, metal dowels are placed into the holes of the master part to transfer through holes into the fabricated parts.
Threaded inserts are also added to a urethane casting mold if the part will be fastened using screws.
Blind holes and overhangs
While it may be possible to design and print a 3D printed part that contains overhangs and blind holes, often, these features do not translate well to the urethane casting process. Make sure that the part you design will be manufacturable.
Material selection
Most common urethane resins resemble ABS; however, various resins have different material properties.
Resins are available that are flexible, rigid, heat resistant, or UV resistant. Whatever material requirements you need, there is likely a cast urethane resin that meets your needs.
Urethane Casting Material Selection Factors
Urethane casting is compatible with a wide range of both rigid and flexible materials so cast urethane parts can be produced in a variety of colors, textures, and finishes.
However, with so many options available, it’s important that you adhere to design for manufacturability (DFM) best practices when selecting a material. You’ll be weighing a complex set of factors, including:
- Application requirements: Does the part need to store or dissipate energy? Does it require tear resistance, load-bearing, or sound-dampening? Each of these factors necessitates different physical and chemical properties, which should help refine the pool of viable material options.
- Environmental considerations: If the application requirements will expose the part to extreme temperatures, humidity, UV radiation, or chemicals, these environmental considerations should also inform your material selection.
- Expectations: Other factors like cosmetic considerations and managing the customer’s expectations when it comes to part life, durability, and cost may play a significant role in determining the final choice.
Ultimately, determining the best-fit material for a given application is vital to ensuring the entire production cycle is positioned for success.
Urethane casting materials including silicones, foams, and urethane casting resins can be categorized by their hardness according to durometer scale.
Here’s a quick look at durometer, the Shore scale, and some of the most common urethane casting materials.
What Are The Materials Used In Urethane Casting?
As with most manufacturing projects, the desired qualities and characteristics of the final part application should inform the material selection process.
For example, gaskets, overmolds, and skateboard wheels while all commonly manufactured via cast urethane have widely differing performance requirements.
In order to provide better sealant and fit, gaskets and overmolds should be made from softer, rubber-like materials, while skateboard wheels need to offer a combination of durability and noise- and shock-absorption.
The urethane resin family is highly versatile and can provide material and performance characteristics comparable to many engineering-grade plastics.
Resins can often be augmented with additives, such milled glass fibers and other particulates, to improve the part’s strength, anti-static qualities, flame resistance, or other desirable properties.
The material and additive offerings of individual suppliers will vary, but urethane resins are often broadly categorized according to the Shore durometer hardness scale
Rigid Polyurethanes
ABS-like polyurethanes are low-cost, general purpose resins that provide comparable if not better strength and impact resistance to acrylonitrile butadiene styrene (ABS), a thermoplastic polymer commonly used in vehicle exteriors.
With typical durometers of 80-85 Shore D, these resins make excellent product enclosures and components that will stand up to frequent handling (such as game console controllers).
Urethanes are susceptible to UV radiation and will therefore require stabilizers or coatings if used in exterior applications.
Acrylic-like polyurethanes are another common casting material. These resins are stiff and transparent, offering similar properties to polymethylmethacrylate (PMMA), a lightweight acrylic commonly used as a glass substitute.
Offering durometers around 87 Shore D, these materials are excellent for creating durable, transparent parts such as light tubing.
Softer rigid resins (such as those that fall between 60 and 75 Shore D on the durometer scale) can also be used to create firm parts with a degree of flexibility, such as car tires or hard hats. These resins have low viscosity and are therefore excellent for filling complex mold designs.
Elastomeric Polyurethanes
Elastomeric polyurethanes offer similar properties to flexible materials like TPE, TPU, and silicone rubber, making them ideal for manufacturing products such as wearable components and bumper pads.
Much like rigid resins, elastomeric polyurethanes come with a number of mechanical advantages.
Those with durometers of 50 Shore A and below provide high tensile strength, toughness, and flexibility, while those between 60 and 80 Shore A are well-suited for producing abrasion-resistant parts and fixtures with a degree of flex.
Shore A resins can also be used to create urethane casting molds that are more durable than their silicone counterparts.
Hard elastomeric urethane materials include durometers ranging from 90 Shore A to 60 Shore D, and are ideal for demanding industrial applications like forming dies and gears. In some cases, these resins are able to outlast metal patterns.
What Are The Applications Of Urethane Casting?
Due to their diverse range of properties, cast urethanes are utilized across various industries. Their versatility and low initial manufacturing costs make them a popular choice for many applications. Here are some common uses of urethane casting:
Product Design
Urethane casting is frequently employed in product development to create prototypes for design validation and testing.
This method provides a cost-effective and rapid way to iterate and refine designs before investing in expensive tooling for mass production.
Wheels and Rollers
Urethane wheels and rollers are known for their high load-bearing capacity, excellent wear resistance, and superior traction.
They are also capable of absorbing shock and reducing noise levels, making them ideal for various industrial applications.
Urethane’s flexibility in durometer selection allows for tailored hardness, providing a perfect balance between grip and durability.
Additionally, these cast components exhibit excellent resistance to chemicals and abrasion, ensuring longevity even in harsh environments.
Automotive Vehicles
Urethane castings can be engineered to be shock and vibration resistant, making them ideal for automotive applications.
Additionally, their ability to withstand high temperatures allows them to serve as a replacement for steel in certain applications.
Shock Absorbers
High vibration from rotating equipment causes rigid materials to crack. Urethane castings can be made to absorb vibrations, as seen from shock absorbers and dampers.
Medical Device Components
Urethane formulations that are FDA compliant are available, making urethane casting a suitable method for producing medical devices. This method is particularly advantageous for low-volume production of specialized or uniquely designed medical products.
Consumer Products
Due to their versatile properties, urethane castings are used in a variety of consumer products. Examples include shoe soles, sports equipment, electronics casings, and more.
Advantages Of Urethane Casting
Urethane casting offers several advantages, making it a popular choice for various applications, including rapid prototyping, custom part production, and more. Some of the advantages of urethane casting include:
#1. Cost-Effectiveness. Urethane casting is an economical alternative to traditional manufacturing methods such as injection molding, particularly for low to medium production runs. It involves less expensive tooling and setup costs, making it ideal for small-batch production and prototyping while producing parts that possess similar qualities to injection-molded parts.
#2. Rapid Prototyping. Urethane casting enables designers and engineers to test and refine their designs before committing to more expensive production methods, saving time and resources.
#3. Material Versatility. There are many available cast urethane materials, each with unique properties. The casting process can replicate various material properties, from rigid to flexible, transparent to opaque, making it ideal for various applications. Unlike injection molding, which requires high temperatures and pressures, you can cast urethane resins at room temperature. This feature provides flexibility in creating parts with specific material qualities.
#4. Complex Geometries. Urethane casting excels at reproducing complex and intricate geometries. It can replicate fine details and undercuts in molds that are challenging for other manufacturing processes.
#5. Quick Turnaround. The process is relatively fast, with short mold creation and part production lead times. This speed is advantageous for meeting tight project timelines and getting products to market faster. Cast parts, especially larger ones, can be made faster than 3D-printed ones.
#6. Low-Volume Production. Urethane casting is well-suited for low to medium-volume production runs. It allows for the economical production of a limited number of parts without needing expensive tooling adjustments.
#7. High Precision. Urethane casting can achieve high levels of precision and accuracy, ensuring that the cast parts meet tight tolerances and quality standards.
#8. Mold Reusability. Silicone molds used in urethane casting are durable and usable for multiple casting cycles. This reusability further reduces the per-part cost, especially for larger production quantities.
#9. Customization. Urethane casting offers the flexibility to tailor material properties and finishes to specific project requirements, meaning you can produce parts with varying hardness, color, texture, and other attributes.
#10. Reduced Waste. Urethane casting generates minimal waste compared to other manufacturing processes, as it only uses the material necessary for the part, minimizing material waste and environmental impact.
#11. Risk Mitigation. Urethane casting can help mitigate the financial risk of expensive injection molding tooling. By first prototyping and testing designs with urethane casting, you can refine your product and reduce the risk of costly production errors.
#12. Broad Industry Applicability. The adaptability and versatility of urethane casting make it a valuable manufacturing method in various industries, including aerospace, automotive, medical, and consumer goods.
#13. Replicate lost Designs. One unique application of urethane casting is that it can replicate parts when the design is lost. In this situation, the original object becomes the master part. Since the same master can make multiple molds, you can recreate the original part without designing it from scratch.
Disadvantages Of Urethane Casting
While urethane casting offers several advantages, it also has its limitations and disadvantages, such as:
#1. Limited Production Volume. Urethane casting best suits low to medium production volumes. It may not be cost-effective for large-scale manufacturing, where other processes, like injection molding, become more efficient.
#2. Labor-Intensive. The urethane casting process can be labor-intensive, particularly during mold-making. Preparing and maintaining molds can be time-consuming, affecting overall project timelines.
#3. Mold Wear and Tear. Although silicone molds for cast urethane parts are easy to make, their quality deteriorates after several uses and may require maintenance or replacement; typically, molds last for only 25-50 casts.
#4. Limited Material Options. While urethane casting can simulate various material properties, it still has limitations regarding the range of materials available compared to other processes like injection molding. Some specialized materials may not be suitable for urethane casting.
#5. Part Size. There is also a limitation on the maximum size of the part so that the mold can fit into the pressurized curing oven. Most molders can accommodate parts up to 30″ in length.
#6. Lead Time for Mold Creation. The time required to create the silicone molds can impact project schedules. Mold design, fabrication, and curing can extend the lead time for part production.
#7. Post-Processing Requirements. Urethane cast parts often require post-processing, such as trimming, sanding, or surface treatments, to meet desired specifications. These additional steps can add time and cost to the production process.
#8. Inconsistent Wall Thickness. Achieving consistent wall thickness in urethane cast parts can be challenging, especially for more complex geometries. Variations in wall thickness can affect part strength and durability.
#9. Limited Heat Resistance. Urethane materials have limited heat resistance, which may restrict their use in high-temperature applications compared to other plastics.
#10. Not Suitable for Mass Production. Urethane casting is unsuitable for mass-producing identical parts at high volumes. Injection molding is typically a more efficient option for such scenarios.
#11. Complexity and Skill Requirements. The process of urethane casting requires skilled personnel for mold making, casting, and post-processing.
Urethane Casting Vs. Plastic Injection Moulding Differences
Injection molding can produce high volumes of parts, including components with tight tolerances.
That makes it a good choice for automotive and aerospace parts, electronic components, and medical devices. Urethane casting is limited to lower part volumes, typically with less complexity.
Before starting to injection mold parts, the appropriate tooling needs to be designed and manufactured via CNC machining.
Your part’s geometry will directly impact your tooling, so if you have a complex component, you’ll need complex tooling.
Complex parts may need to incorporate slide actions, lifters, or removable cores into the mold’s design. During the injection molding process, the core and cavity will come together.
Molten plastic will then be injected into the tooling, cooled, hardened, and ejected to create a usable part.
Urethane casting begins with a master pattern that is created via CNC machining or 3D printing. The operator places the master pattern in a mold box, fills the box with liquid silicone, and then cures it.
After the curing process is complete, the mold is cut in half to remove the master pattern. What’s left behind is a cavity in the shape of the part that will be molded.
Then, the cavity is filled with urethane casting resin and placed in a heated vacuum chamber for curing.
While both processes involve filling a mold’s cavity with a plastic material, there are some key differences between urethane casting and injection molding.
Molds for injection molding are often made of steel or aluminum, making them far more durable than the silicone molds used in urethane casting.
Creating a urethane casting mold is less expensive than creating one for injection molding. Complex injection molds can easily cost tens of thousands of dollars, but fabricating a urethane casting mold typically costs hundreds or thousands of dollars.
While it may take several months for an injection mold to enter the production stage, urethane casting molds can be tooled in under two weeks.
Urethane casting is more forgiving with regard to wall thickness and undercuts than injection molding.
If you’re planning a high-volume production run or have tight tolerances, consider injection molding. Metal molds are more durable and rigid than silicone molds, and metal will continually deliver consistent part quality.
Plus, you’ll have far more control when injection molding compared to urethane casting. Not only can the manufacturer control material flow and cooling rates, but also injection temperature and location when injection molding.
While the cost of creating a mold for injection molding may seem high initially, it becomes cost-effective when divided among thousands of parts.
On the other hand, urethane casting is ideal when developing prototypes or executing low-volume production runs, as soft tooling offers speed, flexibility, and affordability.
If you have a high demand for your product but your permanent tooling isn’t ready yet, you can start production with urethane casting.