Introductory Guide to Polyurethane Foams

Polyurethane foams are used across industries from automotive and marine, to the film and theatre industries for making high quality consistent parts. Below we have answered some common questions about polyurethane foams to help you choose the right material for your application.

What types of polyurethane foams are there? 

There are two primary types of polyurethane foam: rigid and flexible.

Rigid foams are closed cell and come in a variety of densities. At lower densities they are easy to sand and carve while at higher densities they can hold nails and staples or even be structural enough for lightweight furniture. Typically, rigid foams are used for flotation applications, prototypes, and lightweight back up tools.

Flexible foams are also available in different densities, either compressible cushion style foams or firmer self-skinning foams. These foams are used for movie props, prototypes, moulded cushions and bumpers.

What mould material can you use for polyurethane foam? 

Rigid moulds tend to work best for foam due to their expansion which places pressure on the mould, especially in closed moulds. With softer materials this pressure can cause distortion of the mould.

• Silicone – silicone moulds tend to be easy to make, capture high levels of details and are naturally self-releasing. However, they need rigid backup structures to handle the internal pressures and don’t have the longevity of other materials. Silicone moulds work well for low volume applications.
• Plaster – a material traditionally used in the special effects industry as it’s low cost and easy for one off moulds.
• Laminated fibre glass – ideal for larger tools and when used with a surface coat it forms a rigid lightweight tool that can be highly polished.
• Castable urethanes – a popular mould material for foams as they are rigid, with a fast turn around and low cost.
• Castable epoxies - great for foam, due to their increased thermoconductivity, ability to hold high pressures and maintained tool life.
• 3D Printed – newer technology allows for computer designing high detail and next generation models.

Castable foams are fast reacting so it can be tricky to get the material mixed, into the mould, sealed, and orientated in time. To help with this BJB recommends using mould which are hinged with quick clamp systems.

What can impact foam expansion? 

The expansion of polyurethane foams can be affected by environmental conditions, the mixing process, and the mould itself:

• Liquid temperature prior to mixing – the higher the temperature the more of a reaction.
• Different levels of reaction will occur if the material is hand mixed or machine mixed and how fast or thorough it is mixed.
• Relative humidity – moisture in the air can affect expansion, with more humidity resulting in more reaction.
• Geometry of the mould – how the mould is shaped and positioned, for example a tall skinny/narrow mould can cause less expansion.

How do you stop casted foam from shinking? 

Shrinking of casted foam parts occurs due to trapped gas in the cell structure slowly exiting the foam which causes cells to shrink. By squeezing the foam roughly 30-60 minutes after demoulding you can release the trapped gas and replace it with atmospheric air. Not only does this help reduce risk of shrinkage but also aid in reducing natural odours from the part.

Where can polyurethane foam be used? 

With the range of densities available and the ability to pigment, polyurethane foams are ideal for the film and special effects industries like allowing prop master’s to create weapons from spears to futuristic ones while staying well within safety regulations

Other applications include:

• Set pieces, sculptures, fantasy sets, city sets
• Padding and bumpers,
• Buoyancy and flotation applications (rigid foam)
• Furniture cushions, arm rests, and head rests
• Safe weapon props (rigid foam)
• Costumes and armour (flexible foam)
• Stunt props, and breakaway props (flexible foam)
• Medical training devices
• Prototypes
• Theme parks, haunted houses, and theatrical performance pieces
• Lightweight back up tools
• And many more…

How should liquid foam materials be stored? 

Once liquid materials are opened their shelf life becomes dependant on the environmental conditions hey are stored in. for example moisture from humidity can have a negative effect. To preserve always reseal as soon as possible and ensure threads are clean.

Always consult the material TDS for more information.

Other FAQs

What is Foam Density? Foam density is based on how much the foam weighs in a given volume.

Can polyurethane foams be shaped with a hot wire process? No, they should be mechanically shaped or sanded once cured.

What is self-skinning foam? Self-skinning foams are polyurethane foams which form a distinct rubbery skin.

What does free-rise and closed mould mean? Free rise is an open mould where the foam can expand without restriction. While closed moulds cause back pressure and require vents to help even expansion. Using less vents can help to allow for more back pressure to build and form a nicer skin, this is used for armrests for example.

Foam Materials to Consider:

Rigid – BJB TC-300, a rigid closed cell foam, with a quick work and demould time (available in densities 3, 6, 10 lbs)

Flexible Foam - BJB TC-274, a water blown flexible polyurethane foam with a medium to firm furniture cushion feel. (20–30-minute work time)

Fire retardant – BJB TC-296 FR, a fire-retardant flexible foam with medium density. (20-25 second worktime)

Self-Skinning – BJB TC-284, a self-skinning flexible foam which feels like an auto arm rest pad once cured. (20-25 second worktime)

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Want to learn more? 

Watch BJB's guide to expanding foam 

Watch BJB’s Video Guide to Polyurethane foams with demonstrations and technical information.

Contact our team for more information about polyurethane foams and help in finding the right one for your needs!