Why Does My Urethane Plastic Have Bubbles?

When making industrial or prototyping parts out of thermoset urethane plastic, the last thing you want to see in your casting is bubbles. For some applications, like clear, see-through parts, having no bubbles is critical. Not only are they an aesthetic problem, but they can also affect the integrity and performance of the part. The most prevalent reason a part develops bubbles is due to moisture sensitivity.

Other non-moisture-related issues include extreme temperatures, excessive mold release, application techniques, and tooling shape. It wastes time, money, and materials when you have to scrap the part and make a new one. However, if you follow the manufacturer’s instructions, you should be able to avoid these problems and produce bubble-free castings.

Reasons Bubbles Form in Urethane Plastic

1. Moisture

Urethane is a moisture-sensitive material. This means that when it comes into contact with moisture, it will react, resulting in bubbles or foaming. The following elements can all lead to moisture issues:

• Environment. The most obvious source of moisture comes from humidity. If the plastic has a long enough pot life to react with the moisture in the air, bubbles will form.
• Mixing equipment. Bubbles can form when mixing the material if you use mixing equipment that absorbs and holds moisture, like wood or paper. The moisture can then transfer into the urethane.
• Release Agents. Using a water-based release with a urethane plastic will cause a reaction that produces bubbles.
• Mechanical equipment. To remove bubbles with specialized equipment like a vacuum chamber or pressure pot, they must be hooked up to an air compressor line or a vacuum pump. If moisture enters these lines, it will react with the resin and affect the equipment’s ability to remove bubbles.
• Casting surface. If your tooling material is wet for any reason, maybe from cleaning it, it may cause bubbles to form on the face of your part. A tooling material like silicone that hasn’t fully cured before you cast into it can also produce bubbles.
• Open containers. The more you open the A and B side materials, the more likely it is that moisture will be introduced into the mix when they are not sealed. If you don’t use the entirety of both materials, especially on a humid day, you should expect bubbling.
• Wet foreign material. Adding an additional component to the system such as pigment, thickeners, or other foreign materials can cause foaming because of entrapped moisture within the product.

2. Extreme Temperatures

When casting urethane plastic, ambient, material, and surface temperature play a role in the speed of its cure. If bubbles are unable to reach the surface before cure, they will be trapped in the plastic.

Extremely cold temperatures can slow curing or prevent a full cure. Extremely warm temperatures can cause rapid cure, which doesn’t allow much time for the material to release bubbles.

Temperature also affects the material’s viscosity, which affects its ability to release bubbles. Cold temperatures increase the viscosity, and warm temperatures decrease the viscosity. The thicker the material, the slower it will eliminate bubbles.

Thick liquid plastic materials with short pot lives will have a harder time releasing air bubbles at high temperatures. Liquid plastics with long pot lives and high viscosities should be able to release most bubbles at room temperature (77°F). Low-viscosity materials with long pot lives should have no problem releasing air bubbles at room temperature or higher.

3. Excessive Release

Depending on the tooling material you use to cast your urethane plastic, you’ll probably need to use release. Be sure you’re using the right amount of release; otherwise, it could cause problems for your cast part. Excessive application on a surface that is not allowed to dry before casting will cause small pinholes or champagne bubbles on the face of your part.

4. Pouring & Mixing Technique

Poor application techniques can lead to various surface defects. For starters, the way you mix can cause issues. Because these are two-component materials, they must be mixed to generate a chemical reaction. If you rapidly mix the material, you can introduce air into your mix and cause bubbles.

Even the way you pour your material into your mold can create air bubbles. Pouring too quickly and moving around as you pour doesn’t allow trapped air to escape as easily.

5. Tooling Shape

Removing bubbles can be difficult if the tool you’re casting into has complex details or undercuts. Some undercuts can trap air when the material is poured. The air may not be able to rise to the surface and escape fast enough before it starts to cure. This will cause large surface bubbles that are unacceptable for certain parts.

Also, if you’re product thick parts, trapped air might take longer to rise in the mold, especially if it has a high viscosity.

Solutions

Follow the manufacturer’s recommendations for temperature and humidity. Room temperature (77°F) allows most materials to maintain a workable viscosity. This makes it easier to mix and less likely to trap air. Also, ensure your workspace and equipment are dry, especially during the application process.

Warming your material to at least 65°F can also help lower its viscosity and allow air to escape more easily. Warming the tooling material or making sure you are not casting onto a cold surface can also prevent bubbles. Be aware that warmer conditions will result in a reduced pot life.

To prevent the urethane part from adhering to your tool, you’ll need to use a compatible, non-water-based release. Don’t overdo it on the mold release, and allow the release to dry before casting to prevent bubbles.

Mix the two components slowly, consistently, and thoroughly. The best mixing equipment you can use, especially for large amounts of material, is a paddle mixer attached to a drill. This equipment can reduce the amount of air that is introduced while you are mixing. It mixes the entire container, including the corners and bottom. Also, when adding any foreign material to the plastic, make sure it is chemically dry with the lowest moisture content possible to avoid incorporating trapped air.

Even with good mixing techniques, it’s likely that some air will be introduced into the material. During the pouring process, you want to pour slowly, in one place, and allow the material to flow into the mold. Pouring in a high, thin stream can help to minimize air bubbles as well.

The two most effective ways to get rid of bubbles in your urethane parts are:

• Vacuum degassing is a method of removing bubbles before you pour the resin into the tooling material. The vacuum chamber sucks out all the trapped air, making the material rise and then collapse as it is removed.
• Pressure potting is a method to force material into the face of the tooling to remove any imperfections caused by the surface or bubbles. The part will cure fully in the pressure chamber giving a consistent finish. Note: your tooling material should be made under the same pressure to avoid distortion.

For our VFI-4500 UV Stable Clear Polyurethane series, these methods are required to get rid of bubbles for a crystal-clear part. Other plastics, like VFI-110 75 D Injectable Plastic or VFI-1678 80 D Injectable Plastic, are too hard to vacuum degas due to their short pot life. However, because they are injectable and do not incorporate air, they fill the mold quickly with little to no air entrapment.

If you don’t fully use your material, put the container lids back on as soon as possible. Make sure to store them in a temperature-controlled, dry location. You can nitrogen purge both sides to extend the shelf life as well. However, the pot life will be greatly reduced once they are open.

If none of these methods work, use a different material. You might not have chosen the best urethane for your project. If getting a bubble-free cast is crucial, consider finding a low-viscosity material with a long enough pot life that provides ample time for bubbles to rise and escape naturally or through mechanical means.

Contact VFI if you are having other issues when casting urethane plastics.