How do I use Expanding Foams?


As with most polyurethanes, there is usually a mix ratio of 50% polyol and 50% isocyanate, but this ratio does vary and special care should be taken over checking your mix ratio and measuring out the correct proportions.


1. Using digital scales, measure out the polyol first. Remove from the scales and add pigment at this stage if required. Mix any pigment in thoroughly. Adding pigment to one part of the foam ensures the colour is distributed evenly before it is catalysed as it cures so quickly.


2. Replace the mixing cup with your polyol and pigment on the scales and add the isocyanate very carefully until the mix ratio is complete.


3. Speed is of the up most importance at this stage.

Stir the mixture quickly and very carefully. Pour into your mould without delay.


4. The reaction time for most foams is very short, you can expect to start seeing a reaction within the first 3-5 minutes after mixing. It is important you complete pouring your foam casting before this reaction begins or you will risk compromising the integrity of your foam casting.


5. De-moulding time varies according to the particular foam in use. Expect to be able to remove the casting around 30 minutes to an hour after pouring.


Tip: To minimise post cure distortion I recommend leaving the foam in the mould for at least 24 hours after pouring.

Once demoulded, any soft foam castings should be manipulated to stimulate air movement between the foam cells.

Expanding Foam

Health and safety


All Polyurethane liquids are very harmful to living organism’s. So, in order to protect ourselves from the these dangers, it is important we observe a few simple rules.


•Personal protection equipment- An approved urethane filter respirator should be used.

    As well as protective gloves, apron, goggles and long sleeves.

•Use the material in a careful and controlled manner.

•Use only in a controlled environment with approved extraction units and filtration.

Ensure your protection from any possible chance of over exposure, and minimise the risk to your health.

Always read the health and safety data sheet for each new material you use.

Polyurethane liquid foams are available from Bentley Advanced Materials


Prices vary:

Liquid form

£10.00 p/kg for rigid foam

£25.00 p/kg for most rubbers and soft foams


Pre-fabricated Sheet

(Available locally from John A Stephens, Nottingham.)

£20 per sheet

   Polyurethane rigid foam slab sheet 
 fabrication, clad in black plastic sheets
Polyurethane flexible foam castings taken from silicone skin moulds
   Polyurethane (‘Flexfoam-it V’) poured into a jesmonite shell
Polyurethane (‘Flexfoam-it X’)
casting taken from a silicone split mould
 Polyurethane rigid foam casting from a multi-part fibre-glass mould