Resins and Rubbers

Polyurethane rubbers, or elastomers, are an alternative to silicone rubber with many similar properties and offer us a wide range of hardness’ in which items can be produced. They range from 10 Shore A, which is softer than an eraser, to over 90 Shore D which is  about as hard as a golf ball. Polyurethane elastomers are a great way to produce rubber parts from a silicone rubber mould because, although polyurethane will stick to lots of surfaces, it does not stick to silicone rubber, even without a release agent.

This allows us to use a silicone rubber mould to cast polyurethane rubber parts. Which is an advantage, because silicone sticks to itself and is often incompatible with other types of silicone like addition cure silicone (making a silicone part from a silicone mould is almost impossible.)

So, most polyurethane rubbers do not need any release agents when used with condensation cured silicone. This is a big advantage when surface texture is of up-most importance. Any release agents you apply to your mould surface will reduce the amount of detail you can expect to capture in your casting.


Polyurethane rubbers usually cure fully after 24 hours although it can take 48 hours or longer to stabilize completely. ‘Post-curing’ polyurethane rubber is recommended to achieve full mechanical properties of the material. This takes place in the oven at 60 -70 degrees centigrade, for around 4 hours.

Do not try this in your oven at home, as harmful vapours are released during curing of any type of polyurethane.


Mixing ratio varies greatly because there are so many different polyurethanes, all with such different properties. Always check the mixing ratio and cure times and procedures before you start work.

Some people use measuring cups to mix materials like polyurethane especially when speed is of the essence. My personal preference is to always use digital scales when using any material which requires a specific quantity of catalyst or hardener. Obviously, this method is much more precise when working out ratios.


1.Remembering to zero the scales after you have put your mixing cup on, pour in the first part of your chosen polyurethane, pouring the less volatile of the two compounds (the polyol) first will minimise your exposure to dangerous isocyanates.

2.Now, work out how much of the isocyanate needs adding. Once added, the curing process begins; mix thoroughly, remember to scrape the sides and the bottom of the mixing pot as thicker materials will cling and make mixing difficult. Add a pigment if necessary. You will usually have plenty of time to mix in the pigment and work with the rubber before it cures, unless the working environment is particularly warm. Always check the ‘pot-life’ of the material before you start.

3.De-gas in the vacuum chamber for around 8 minutes or until the progressive popping of bubbles has slowed.

4.Remove from the vacuum chamber and pour into your mould.


To make moulds with polyurethane rubber, use steps 1-4 and see-

‘Mould-making Techniques’.
















How do I use Polyurethane Rubber?


Generally speaking polyurethane rubbers are more difficult to use than silicone rubbers, in a sense that more preparation of master model and mould is essential. Problems with the polyurethane rubber bonding to a variety of other materials including itself, untreated wood, plastics, metals and all porous objects means extensive knowledge of release agents and material compatibility can be a very intimidating notion for most novices.

Precautions to Take to Ensure Safe Usage


•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.

How do I use Fast-Cast Polyurethane Resin?


Fast setting polyurethanes resins are usually mixed at a 50-50 ratio and come in a variety of viscosities (thickness’). Primarily used for quick casting’s of prototypes with-in the product development industry, it has a very smooth texture and is easily pigmented and spray paint is easy to apply to its surface after cleaning and keying. It is machinable and can be tapped and threaded, it is also highly durable and can flex, but is prone to shatter if dropped onto a hard surface. Polyurethane ‘fast-cast’ resin cures so quickly, (know as a snap cure) that if timed right, the resin can solidify as it is being poured, creating an artificial state of suspended animation!


‘Fast-cast’ is also used to make reproductions of sculptures and figurines. With this particular resin, pot life is about 3-4 minutes and de-mould time is about 10-15 minutes. Although with slip casts with thin walls and casting of smaller objects it is recommended that extra time should be allowed for full curing as the resin relies on heat generated through the curing process to set.

Because of this short pot-life it is essential that you do not delay between mixing and pouring.


  1. 1.Pour the polyol into the mixing vessel and add pigment as required (pea sized amount 

    of pigment for a grapefruit sized amount of resin is usually plenty). Mix thoroughly.


  1. 2.Add the isocyanate to the polyol at the specified mix ratio and mix well for 30 seconds.

    Using the same mixing technique as used for mixing rubber, we scrape the sides and 

    bottom several times as we mix.

  1. 3.Now pour the well mixed resin into the mould at the lowest point, this allows the mixture to rise from the lowest point in the mould and should displace any possible air trapped against the surface and help minimize bubbles in the cured casting.

  2. 4.After 5 - 7 minutes, the resin will change colour from translucent caramel colour to a creamy white colour, as it solidifies.


Remember it is an exothermic plastic and must produce heat to cure, the volume of the cast dictates the speed of that build up of heat. More volume leads to more heat and the quicker it will set.


  1. 5.In about 20 minutes, the casting can be removed from the mould.
    Notice that the casting reflects all of the detail and texture from the mould.

    A perfect reproduction of the original master.


  1. 6.If you have chosen not to use pigments you may paint the resin surface to suit your needs. To paint your casting it is important that you remove any release agent, if used and any residue caused through the curing process. You can use acetone, or lighter fluid, and wash with dishwashing liquid soap. Afterwards, dry thoroughly. Apply two coats of an auto body primer, and it is ready for a colour of your choice.




Polyurethanes are available from Bentley Advanced Materials


Prices vary: £20-30.00 p/kg for most resins and rubbers

   

Layering technique with polyurethane rubber

   

Polyurethane rubber casting, taken from a silicone split mould

   

Polyurethane rubber jacket casting, taken from a multi-part mould

   

Polyurethane rubber cast, taken from a silicone split mould

   

Translucent polyurethane rubber

   

Polyurethane resin ‘dripping’ from ceiling

   

Polyurethane resin lights, slip cast from a silicone multi piece mould