Large Voids in Parts

Upon demolding, there may be large voids either within the part or on the surface of the part. These voids are visible with the naked eye, and they usually occur at thin-walled sections, corners and other transition areas.

Part with large void due to air entrapment.

Potential Causes

Air Entrapment During Pour
As the batch is poured into the mold, it displaces air from the mold. If the air becomes trapped within various cavities and has no mechanism to escape, it will form a void. This can happen in thin-walled areas if the prepolymer viscosity is too high.

Short Pot Life
As the batch is poured into the mold and it displaces the air in the mold, some air will become trapped there temporarily. This air will then begin to work its way to the upper surface of the part. This process takes a finite amount of time, and if the pot life of the system is too short, the air bubbles will not have a sufficient amount of time to escape.

Leaking Mold
If some of the material poured into the mold leaks out, a void will form at the leaking area. Leaking is more likely to occur in systems with a long pot life and low viscosity.

Solutions

Redesign the Mold/Vents
A proper mold design with adequate venting will generally prevent problems with large voids.

Use Lower Viscosity Prepolymers
Using a lower viscosity prepolymer will make it easier for the air to escape from the mold. Using a polyether prepolymer instead of a polyester prepolymer will also decrease voids, if the application allows for this substitution. In addition, using a system with a lower viscosity and longer pot life, such as a system containing an Airthane® prepolymer, will help reduce void formation.

Tilt Mold and Pour Down Its Side to Minimize Splashing
Often, tilting the mold upon filling, especially in the case of large rollers, will allow air to escape more easily. Also, pouring down the side of the mold will decrease splashing, another source for air bubbles in the system.

Add a Degassing Agent
The addition of a degassing aid will reduce the surface tension of the system. With higher viscosity materials, this is often enough additional help to reduce the formation of voids. Typical degassing aids are SAG-47 by Witco, and X-Air by Ciba-Geigy. Use levels are typically recommended at 0.2 parts per hundred parts prepolymer.