Everything You Need to Know About Urethane Casting

Did you know that urethane resin first appeared in the 1930s? Since then it’s used everywhere, from Hollywood movie studies to small craft rooms.

It’s definitely tried and tested – but so have many other resins. With a whole heap of options to pick from, how do you know urethane is the right one for you?

We’ve put together a run-through of everything you need to know about urethane casting. From curing times to how it compares to other resins, we’ve got the facts so you can make the right decision.

Take a look and start casting.

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 casting’s 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.

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.

Things 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.

What is Plastic Injection Moulding and How Does it Compare?

Choosing the right process for your project is vital to its success, even if they seem similar.

If you’ve been doing some research, you might have noticed that plastic injection molding and urethane casting look quite alike. But, this is only on the surface.

To start with the process itself, it uses very different techniques. Urethane casting involves the use of hand-operated vacuum machines to draw liquid material into a mold.

There’s also a difference in the molds used. Whilst urethane uses silicone, injection molding usually uses metal. The materials used for the cast are also very different. Urethane is a polymer that mimics the production-grade plastics (like PVC and ABS) used in plastic injection molding.

Finally, there’s quantity. Plastic injection molds usually have a ton of cavities in one mold so they can create more products in one cycle. Urethane molds typically only have a few – or one – cavity and make fewer pieces.

Urethane Casting Vs. Plastic Injection Moulding Differences

Now you know more about the processes, let’s take a look at some of the other differences you should consider:

• Upfront costs – Injection mold can be far more expensive than urethane casts as they’re made from metal. This makes this system more of an investment and better for long-term, mass-production. The molds for urethane casts can be made for me much less, helping you reduce upfront costs. This makes it better for one-off designs or low-volume batches.
• Cost of materials – Just like the molds, the production-grade plastic that you’ll need for injection molds is much more expensive than urethane. This includes PVC, ABS, acetal, and LPDE. Once again, this makes urethane the better choice for prototypes and low-volume work.
• Lead times – The molds for injection molding can take a very long time to make – sometimes even months! This increases lead times and can prevent quick delivery of products. Usually, the molds are 3D metal printed to try and speed up the process. Urethane silicone molds are much quicker to make. Usually, they can be created with a 3D printer or even by hand, reducing lead times and helping you get your product to market quicker.
• Per-part cost – Usually the bottom line, per-part cost will be higher for urethane than injection molding. This is because you can only make a few pieces with each mold. On the other hand, the higher volumes produced by injection molding make them much cheaper. This reduces those previous higher costs, like for materials and tools. In this way, injection molding is usually the most cost-effective molding for long-term production.

Plastic Injection Modelling or Urethane Casting?

So, which one’s better for you? That comes down to what you need to make – there isn’t one superior option.

The cheaper upfront costs and quicker lead times will always make urethane better for porotypes – unless injection molding is essential. Whereas for product runs of larger numbers, you’ll get a much better ROI with injectable molding and the steel molds used will last far longer than the silicone alternatives.

Types of Urethane

If you’re looking to buy urethane, Smooth-Cast is a great option. Tried, tested, and trusted, their resins are made to a high standard of quality. They have plenty of different options available, too, including:

• Smooth-Cast 300 – For an all-rounder, general-purpose urethane, this is your go-to.
• Smooth-Cast 65D – Ideal if you’re looking for a higher viscosity.
• Smooth-Cast 325 – This cures to an almost transparent finish. Add a tint for translucent color.
• Smooth-Cast Onyx – A unique urethane that cures to a jet black finish.

Of course, there are tons of other suppliers out there, but this gives you a better idea of what’s on the market. Take a look at MBFG for other options.

Benefits of Urethane

Still not sure if the urethane casting process is the option for you? Let’s run through some of the benefits:

• You can test out your design and ensure there is a market for it before going into mass-production. It’s a working prototype that isn’t too expensive. Once your product switches to higher volumes you can change to a different process.
• It gives you low-volumes of high-quality, production-grade parts.
• You can create very detailed models and have more freedom with your designs. This is ideal for bespoke props and models. You can even create internal, sharp corners, which can be trickier with other processes.
• You can bring your product to the market quicker with short lead times.
• It can cure to a variety of finishes, from very rigid to incredibly flexible.
• It’s available in tons of different texture, finishes, and tints can be added to change the color. This means you can tailor your casting to suit your end-use.
• The shrinkage rate of 0.15% is much less than many other processes.
• Urethane castings can be FDA approved, which is essential if you’re making items for the food, medical, and pharmaceutical industries.

Drawbacks of Urethane

There are some drawbacks to urethane casting. Make sure you know what they are before making your decision:

• It can be hard to get your exact requirements with the urethane casting process. This includes color-matching, which can be very difficult to get exactly right.
• You can paint your parts, but this doesn’t always get the finish you want. It can chip and scratch, revealing the surface underneath. It also means you’ll have to cover extra costs for materials and labor.
• Your mold may break after a few moldings. This is mainly because the heat created from the resin’s chemical reaction will draw water out of the mold. But you can make it stronger by spraying it with mold release spray after about 5 castings.

Is It For You?

After all of that, is urethane casting for you?

If you’re still not sure, we recommend taking a look at other options available and seeing how they compare – like we did here with plastic injection molding. Once you’ve read up on enough processes, you should be able to match your needs to the right one.

If you think you might need help, feel free to get in touch with us at Prototech Asia.

We’re experienced in creating prototypes for a range of industries. From injection molding to vacuum mold casting, we do it all. Get in touch with any questions and we’ll do our best to help.