Vacuum Degassing vs Pressure Potting: Are They Necessary?

Vacuum degassing and pressure potting are techniques professionals use to get clear, bubble-free molds and castings. Either process removes or reduces these weak spots and imperfections from liquid materials that would otherwise affect the look and durability of the final product.

Many new casters use an open pour method that involves mixing A & B components together, pouring them into a mold, and then allowing them to cure at room temperature. The issue is that hundreds of tiny air bubbles are introduced into the material and will be visible in the final casting.

While using a vacuum chamber or pressure pot may seem unnecessary or costly, for those who create molds and parts regularly, it is almost essential to achieve professional results. If you are in a situation where it is critical to make bubble-free castings, using both will ensure a clean mold and part. Various industries use these processes to ensure purity, structural integrity, and performance.

What Causes Bubbles in Liquid Materials?

Air or gas becomes trapped in liquid resins and creates nodules, cavities, and hollow parts in the finished cast for several reasons:

1. Mixing two components (resins and hardeners) together quickly can introduce air into the material. While mixing faster may save time during the rest of your molding process, it can produce voids in your final product. Use a gentle folding method to prevent air from getting trapped during this process.

2. Depending on what they’re made of, mixing sticks and mixing containers can play a part in introducing moisture and air into your material. It’s best to use plastic and metal mixing tools, especially with polyurethane rubber or plastic.

3. Air can also become trapped due to improper casting techniques. If you pour your material too quickly or in thick streams, it can lead to bubble formation. To combat this, pour the material into the mold or form in a high, thin stream. Trapped air escapes more easily when the material is poured this way.

4. If the material or environment is too cold, it can increase the viscosity of the resin. Thicker viscosity materials with high surface tension have a harder time releasing trapped air since they have more resistance to flow. Using thinner viscosity and warmer materials is generally recommended. Warming the material can reduce the viscosity, making it easier to mix without introducing extra air.

5. Over-applying a release agent in your mold box or mold can also cause champagne bubbles or pinholes in your cured material. Be sure to let your release dry before casting, or use one that you can gently brush onto the surface.

What Is Vacuum Degassing?

Vacuum degassing is a process that uses a vacuum pump to pull air out of a closed chamber to reduce and remove trapped air in materials for a higher-quality product. In the controlled environment, the air pressure is reduced, which pulls the bubbles in the material to the surface, where they foam over and pop or release. For the best results, the vacuum pump should be capable of pulling up to 29 inches of mercury (Hg).

The vacuum chamber is usually a steel container with a clear lid for visibility while vacuuming. These chambers can come in various sizes and designs to accommodate a range of projects.

How It Works

Degassing is done after mixing and before pouring the material into a mold. Some casters choose to degas individual components after dispensing the amount needed and then also degas the combined mixture.

The container placed in the chamber should be large enough to allow for as much as five times the expansion of the material. If this space is not supplied, the material may spill over, leaving the mold or container only partly filled. While stopping and starting the chamber can prevent this, you would have to pay close attention during the degassing process.

The speed of this technique and how easily air escapes will depend on the viscosity of the material used. High-viscosity materials take longer for all bubbles to release. Vacuum degassing can also be more time-consuming, which is why certain fast-cure resins cannot be degassed traditionally before pouring.
Since degassing happens before you pour the material into a mold, you must be careful not to introduce air back in. If you are worried about bubbles forming in the pouring process, a pressure pot may be used instead or after degassing.

When to Use and Not Use

This method is best for materials with longer pot lives because quicker setting materials may cure with a foamy texture if they start curing while degassing. It also works well for materials with high viscosities and high surface tension. Even if the material is thick, if it has a long pot life, there should be enough time for the bubbles to rise and escape.

It’s an effective way to remove air bubbles from hard and flexible rubber materials. It’s also good for materials that must form intricate designs and complex shapes on the surface.

An important note is that materials that have a flash point under 200°C are prone to flashing off during the degassing process. This causes the product to be inconsistent and can release harmful vapors. So, any materials that contain solvents are not recommended for degassing.

Materials that can be degassed: silicone rubber, urethane rubber, urethane resins, epoxy resins, etc.

What Is Pressure Potting?

Pressure potting is a process that uses an air compressor to push air into a concealed chamber to create bubble-free molds and castings with liquid rubber and plastics. Depending on the material, the amount of pressure added is between 40-60 psi and should not exceed the pressure limit of the pot to prevent safety hazards from occurring.

Adding pressure will alter the viscosity and flow characteristics of the material. The pressure forces the material into tight spaces, ensuring cavities are filled and minimizing surface imperfections. Unlike degassing, the pressure pot will not completely remove the trapped air but shrinks it, so bubbles are invisible to the naked eye.

How It Works

Pressure potting must be done after mixing, degassing, and pouring the material into a mold. Most people will fill the mold outside the pressure pot before they transfer it. The lid is then tightened on the chamber, and air is slowly introduced through the attached air compressor line. The material and mold will sit in the pressure pot to cure before the pressure is released. If the pressure is removed before full cure, it will not work and may increase the number of imperfections.

Note: Ensure your air compressor line is dry. Bubbling or foaming may occur if there is moisture in your line, especially if you are using urethane resins or rubbers. Also, molds must be made under the same conditions, including pressure, as the casting material or deformation may occur.

A way to combat spillage is to fill your mold outside the chamber about ¾ of the way. Fill it the rest of the way once it is transferred into the pot. You’ll also want to make sure the mold fits inside the container before you start pouring. A pressure pot can also be used on its side, depending on manufacturer’s specifications.

When to Use and Not Use

A pressure pot should be used if you need a perfectly clear, bubble-free casting. Materials that can be pressure potted are those that cure to a solid or hard state.

Pressure pots can remove bubbles from materials that have either a long or short pot life. Using materials with short pot lives is better since they must cure while still in the pressure chamber, so you can process your parts faster than if you used a vacuum chamber.

When using a silicone mold, a pressure pot is almost mandatory for urethane parts. This is due to the surface tension on the silicone causing bubbles in the urethane. Additionally, urethane will cure better when pressure potted.

It typically does not work as well for materials with thicker viscosities. When a material has a low viscosity, bubbles are able to rise faster. Some liquid resins have relatively low viscosities, which makes them ideal for pressure potting.

Benefits of the Equipment

• Bubble reduction: Either method will eliminate bubbles from your castings. This is especially helpful for clear, transparent materials that must be perfect or nearly there throughout.
• Improved surface finish: By eliminating any bubbles in opaque materials, you gain parts and castings with smooth finishes and zero to minimal imperfections on the surface.
• Enhanced material quality: Both methods are crucial to achieve high-quality castings. They will reduce defects like porosity, voids, and other surface imperfections for better reliability of your finished products.
• Enhanced material properties: The material properties may be slightly modified as pressure potting occurs. The castings exhibit better performance characteristics as the material is better fused together with minimal voids, if any.
• Extended lifespan: When materials don’t have trapped air, gases, or impurities, they are less susceptible to degradation over time.

VFI Molding Rubbers and Plastics

VFI has various molding and casting materials that can be degassed, pressure potted, or both. We recommend using either or both of these methods with our materials if you want a bubble-free cast. Our VFI-4580, 4581, and 4582 clear plastics require you to degas and pressure pot to achieve perfect castings. If you have any questions about these processes, reach out to VFI today.