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Reduce air bubbles in crystal clear casting resin

Without the right equipment, it can be difficult to cast crystal clear polyurethane casting resins such as the SG Clear Cast S, M, L and Poly Optic 14 series without the inclusion of air bubbles. These clear casting resins are a ‘two-component’ polyurethane casting resin that has been specially developed for applications where optical clarity is required.

Below is an example of a Poly-Optic casting (left) and the original model (right).

In this blog we discuss four different methods for casting Poly-Optic 14 series. The amount of air entrapment in the final model is different for each method.

Pouring methods:

  • Open casting
  • Pulling a vacuum
  • Vacuum Casting
  • Vacuum mixing and pouring

NOTE: Some of these methods require special equipment; always seek advice from a sales professional and the use of this type of equipment.

With each method, the first step is to measure and mix the casting resin. Poly-Optic 14series has a mixing ratio of 1A: 1B by volume (100A: 90B by weight).

All three of the sample methods below.

Open casting method

The open casting method does not require any special equipment. After mixing parts A and B, you can simply pour the resin into a mold and let it cure.

Vacuum “pulling” degassing method

Pulling a vacuum requires a vacuum compressor. This can be expensive, although a small vacuum compressor is available from €250. To create a vacuum, the mixed casting resin must be placed in the vacuum compressor. The mixing bowl containing the mixed casting resin must be minimal, must not be more than 1/3 full, because the mixture will rise when the vacuum is drawn.

The air is pulled out of the casting resin, causing air bubbles to rise and pop. This process usually takes a few minutes. CAUTION: Casting resins with a short pot life should not be vacuumed.

When the casting resin comes out of the compressor, the material is free of air entrapment.

The casting resin is then poured into the mold. Although the resin has been evacuated, air bubbles may still be trapped during this step.

Vacuum castingmethod

To vacuum cast, the casting resin must be mixed and poured into a mold prior to being placed in the vacuum compressor. Vacuum casting pulls almost all air out of the model.

NOTE: Molds must be made under the same pressure, otherwise deformation may occur.

We recommend setting the compressor pressure to 60 psi; The closer to 100 psi the better.

The mold must remain in the compressor for as long as the demoulding time of the relevant casting resin.

Vacuum mixing and pouring method

A vacuum mixing and casting machine is required for vacuum casting. These machines are very expensive, but prevent air entrapment in the model. In this process, only a good mixing ratio of A and B needs to be poured into the stirring beaker. The machine will mix under vacuum and then fill the mold using a funnel.

See Below the result…

The open casting method results in hundreds of tiny air bubbles that can be seen very clearly in the final result.

The vacuum drawing method gives a better result, but because there is no vacuum casting, small air bubbles have formed in the model.

With the vacuum cast method, there are virtually no air bubbles in the final model.

With the vacuum mixing and pouring method, the model will be completely free of air entrapment.

If you have any questions about the Poly-Optic casting resin, you can always reach us by phone or email.

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