There are four main causes of bubbles forming on the surface of a part

1. Inaccurate weighing of the components

Take care to measure the components out accurately

Ensure the scales used are calibrated regularly.

2. Incomplete mixing of the components

If the components have not been thoroughly mixed, soft spots or bubbles can appear on the surface or within the part.

3. Air may have been entrapped during the mixing process, and not had time to disperse fully before the resin cured.

Given enough time, air entrapped through overly enthusiastic mixing would slowly come out of the mix; in practice the resin may start to thicken and trap the bubbles in first.

The best way to remove bubbles is to put the mixed resin into a vacuum chamber, which will pull the air out much more quickly.

4. Excessive moisture can lead to bubbles forming, both on the surface and within the resin. In extreme circumstances it can make the resin ‘foam‘.

Ensure the mould is dry and free of condensation. Keep the polyurethane in its original containers, sealed, until you are ready to use it.

When resealing containers, if possible, flush the air space at the top of the container with nitrogen.

Ensure lids are tightly secured.