All polyurethanes shrink as they cure. They also give out heat as they cure, known as “exotherm”. The amount of shrinkage depends on several factors:

1. Keep to maximum thickness specification

Each material has a specified maximum thickness before excessive shrinkage occurs. Please refer to the product technical data sheet.

Check that your application falls within this specification.

2. Workshop temperature

Higher workshop temperatures increase shrinkage.

Keep the workshop between below 25°C.

3. Minimise section thickness

Thicker sections create more heat and so increase shrinkage.

Try to build up the part in layers, or fill the material with specified powders.

Use a core to minimise the volume of resin required.

4. Tool material

The thermal conductivity of the tool material also affects shrinkage;

Materials with high conductivity like metal take the heat out of the part fast and reduce shrinkage.

Insulating materials, such as wood, tend to increase shrinkage and will also invite moisture resulting in a poor surface finish.

5. Allow the material to gel

If heat-curing a tool in an oven, allow the material to gel before putting it in the oven. This will allow the chemical reaction to take place.

6. Use a filler

Fillers act in two ways

They absorb heat, reducing the temperature of the mix

They displace resin which would otherwise be adding more heat

Aluminium powder or beads make very successful fillers