polyurethane foam

These page entries are meant to be added to and usually start with general outline information, price guidance, suppliers and useful links followed by my worklog where I can put further info and photos as they come. Details of suppliers are listed in the Suppliers section.


Polyurethane foam exists in a number of different forms, but for many it may be more familiar either as the ready-made and rigid sheets (similar to styrofoam) and useful for sculpture or surfboard-making .. or as the instant expanding foam sold in canisters for repairs and insulation. The interior foam in Kapa-line foamboard is also a very fine-grade rigid polyurethane foam.

But since I’m writing this within the ‘materials for casting’ section, I’m only concerned here with the expanding foam. I’ve yet to try any of the versions sold in DIY stores though.

2-part expanding polyurethane foam

To be able to control it properly enough to use as a casting material it needs to be available as a liquid, in two parts, which can be carefully portioned and properly mixed, and which allows just a little interval of time after that before it starts expanding . Luckily it is available and there are two versions .. one which sets rigid like the sheet foam, and another which remains flexible like ‘cushion’ foam.

This is supplied as two liquids which are quickly mixed together in a certain proportion and the mix immediately poured into a mould (which is then usually closed). The liquid expands rapidly to fill the mould and the foam sets to create a foam casting of the object. There are rigid-setting and flexible-setting versions available, also ‘self-skinning’ meaning that the foam forms its own skin on the surface inside the mould. It is usually white but can be coloured by adding pigment (mixing into one of the parts first)

Advantages of using it

Either flexible or rigid foam can be used to fill out hollow castings done in other materials to give them much more strength while keeping them very light.

With the flexible foam castings can be made in rigid moulds i.e. plaster as long as the plaster surface has a suitable release coating such as Vaseline.


The ‘foaming’ may not be consistent throughout and imperfections to the surface should be expected.

Working life

Polyurethane liquids generally have a recommended shelf life of under a year, but I recently made a test with this flexible polyurethane foam bought at the end of 2012  .. so, more than two years old .. and it worked perfectly! For more advice on disregarding ‘shelf life’ have a look at the start of the ‘quick view’ comparisons page in this section.

self-skinning flexible polyurethane foam, old batch still usable

I always write the date or period when I buy materials, plus reminders if anything needs special handling .. as with part ‘B’ of the mixture here which needs to be shaken because the ingredients separate after it has been standing for some time. This is common with resins especially those that are pre-filled. Another thing .. not only common, inevitable .. is that the cap or lid for the ‘hardener’ component (usually part ‘B’) gets stuck because traces of the liquid crystallize. Something which has never failed me so far even with the most stubborn screw-caps is a strip of tough rubber to grip around the cap while turning.

strip of rubber to help unscrewing caps or lids

As with resins, polyurethane foams consist of two component liquids which are mixed together in a set proportion by weight, so having a good digital kitchen scales is essential. With this one from Tiranti the ratio is 2:1 part ‘A’ to part ‘B’. But in addition to being able to weigh accurately one also needs to judge the volume in this case, to be able to dose the right volume of liquid needed to completely fill the mould when it expands.

For example if the expanding foam has to fill a volume of 1,000 cubic centimetres (equivalent to a 10cm cube) and the foam is expected to expand up to 5-6 times its original liquid volume, then dividing 1,000 by for example 4.5 should ensure that the mould is filled, with a little surplus. This gives us close to 222 ml of liquid needed, of which two-thirds of the weight is part ‘A’ and one-third part ‘B’. So let’s say we need 148ml of part ‘A’ and 74ml of part ‘B’. We must now find out what these would weigh. Luckily the SG (specific gravity, written as the weight in grams of 1 cubic centimetre of ml of the substance) is often given on the containers. If not it will be on the MSDS (Material Safety Data Sheet) available online from the manufacturer or supplier. The SG of part ‘A’ of this expanding foam is 1.05 so 148ml would weigh 155.4 grams, and the SG of part ‘B’ is 1.13 so 74ml would weigh 83.62 grams. We need to round these figures off a bit but also adjust them back to a 2:1 ratio .. 156 grams of part ‘A’ to 78 grams of part ‘B’.

Below, I didn’t need to make a specific volume calculation in this case because I was just testing whether the material still functioned normally. I poured an arbitrary amount of part ‘A’ .. 17 grams .. into the cup first, then 10 grams of part ‘B’, a little more than half the amount. I did this because I’ve come to expect that with ‘old’ materials it’s the catalyst that’s most often affected, becoming weaker.

part 'A' polyurethane foam being weighed

polyurethane foam part 'B' added 2:1 by weight

Above, 10 grams of part ‘B’ has been added. It is always important to think ahead when preparing for this work! Make sure that you have all your necessary tools etc. to hand .. i.e. as here, a mixing stick .. so that you don’t have to hunt around for them at short notice. As soon as the part ‘B’ is added it should be quickly stirred in because the reaction will start within a few seconds. If the mixture needs to be decanted into a form mixing should not be more than c. 10 seconds before transferring it. I’ve tried mixing the material directly in the mould form a few times in the past but this has often resulted in an uneven result with parts not properly curing.

2-part polyurethane foaming within 10 minutes

The foam will have fully risen within about 5-10 minutes. Out of interest I calculated exactly how much it had done so in this case. There was 17g of part ‘A’ .. so 17.85ml in volume using the above calculation, and 10g of part ‘B’ being 11.3ml in volume .. altogether 29.15ml. The foam rose to fill the cup with a little more on top so by measuring water in the cup and adding a little I estimated 210ml. So the polyurethane had actually expanded to 7.2 times its original volume! I don’t know whether the increased expansion was due to the age of the material or the fact that I added a touch more part ‘B’ .. but it’s worth experimenting with!

When I bought the foam in 2012 it was for making these forms shown below (I’ve placed an old casting in its mould) .. and I recorded at the time that the polyurethane only expanded 4.5 times its volume. The mould is plaster-jacketed silicone rubber (made a long time ago when I was living in Hamburg and I can’t remember why I used a translucent silicone). The silicone doesn’t need any release agent against the foam but plaster certainly will if you want to keep it clean .. and Vaseline will be fine. The surface of the cast didn’t achieve the smoothness of the mould .. but one shouldn’t expect it to.

form cast in flexible polyurethane foam, showing silicone rubber mould

What this photo shows well is how much polyurethane discolours over time. In this case it was just discolouration though .. after three years exposure the feel of the surface was just the same.

freshly foamed and 3-year old polurethane cast compared

Test 19/12/2012  72g (48g part A to 24g part B) expanded to fill the ‘Koerper’ mould with just a little pushing out of the top, which cut then be cut off. The volume of the ‘Koerper’ form was measured as 325 ml so expansion was 4.5 times (weight to volume). The foam took c. 25mins to reach tack-free curing

Additional info


What it costs and where to get it

Tiranti’s Self Skinning Flexible PU Foam  £26.30 per 1.5kg, £40.88 per 3kg (Tiranti 2017, inc. VAT). Mix Parts A and B in ratio 2:1 by weight. Expands 5-6 times volume. 5mins to rise, 15-20mins demould. White cure. H&S care taken against breathing in Part B ‘harmful by inhalation and contact with eyes’

Tiranti’s Rigid Foam 002 £37.88 per 2kg (Tiranti 2017, inc. VAT). 2 parts mixed 1:1 by weight. Expands up to 8 times, tack free in 10mins and demould in 15-45mins. This can be used as a fill material for hollow castings, giving extra strength without adding to weight.

Further info sources



23 thoughts on “polyurethane foam

    • Sorry, I don’t know because I’ve never cast PU into latex .. always silicone. With this I usually use Vaseline but I’m told that this will affect latex. It may be worth doing a test using baby oil.

  1. Hi David I want to cast anatomical shaped parts in rigid Polyurethane foam, as these parts have undercuts I need to use a softer silicone mold. I need to achieve a high density close to 500 ~ 600 kg/CBM so I need to close the mould after pouring to get that hardness. But I have had limited success as the expanding foam distorts silicone mold or leaks from the 2 part molds. I am aware many users are successfully doing this, can you guide how to get around this, please.

  2. Hi David I have a project to cast parts in Rigid Polyurethane Foam PUF, wherein the parts are anatomical in shape (odd shape like human bones)

  3. Hello David… I want to create a mould using polyurethane foam of a beach scene where the sand ridges are pretty firm yet still damp. I want to be able to capture the details of the ridges and plan on building the frame and having a slats of wood to put on top to cause compression so the foam will be pressed into all the areas but will also have holes dripped in these slats so there is not too much compression. The mould I want to do is 60″ x 36″ x 2″ since the ridges have about 1″ depth to them. These sand ridges are a mile away from where I can park so I need the foam to cure completely in 30-90 minutes so I can cart it out. Will the foam provide sufficient detail of the ridges? Will the dampness of the sand be a problem and, if so, can I spray something on the sand that will provide a waterproof barrier and also help act as a release agent? If this works as a mould, then my plan is to cast with perhaps plaster of paris (?)… still deciding on this but creating the mould is the big hurdle. Thanks for your advice. Dusty

    • Sorry Dusty, but I have no idea what you could use. Anything polyurethane is no good re the moisture and I don’t think ‘waterproofing’ would help in this environment even if were possible. If I had the time at the moment I would think about this .. but I’m afraid I don’t.

      • David…

        Thanks for the prompt response!

        I am just starting to read your article ‘Beginner’s Basics’ – Mouldmaking and casting’ and, after your reply about the polyurethane foam, wonder if there is another product that will work? I have called companies that have silicone products but given the size of this mould, the cost to use these products seem cost prohibitive.
        Any other ideas you may have would be appreciated…


      • David…
        I understand you did not have time in January to give my inquiry any time but checking back to see if you can now? Or help me find someone that has knowledge of a casting material that will work with damp form sand.
        This is a very important personal project for me so any assistance or guidance would be appreciated.
        Dusty Butler
        Durham, NC

      • Hi Dusty,

        I looked into this and .. as you might know from crime dramas, when forensics want to preserve footprints, even in sand, they’ll use plaster over them to make a reverse impression. Of course, they don’t need to ‘cast back’ into that .. but it suggests that maybe making a plaster mould is the only way. The trick is to do the first plaster layer as thinly and carefully as possible .. I don’t know whether it’s possible to avoid disturbing the sand at least a bit! Once done the mould can then be made thicker/heavier using more plaster. Then you’ve got a rigid mould .. then the choice of making a cast in polyurethane foam, foamed-latex, or something similar. What you could do beforehand though is find out what forensics specialists use on fragile surfaces .. there must be information from specialist forensics material suppliers?

  4. Hi David,
    This is probably a very stupid question, but I am very new to all this so please excuse my ignorance.
    My question is, can I use expanding foam (flexible, something like flexit 17, which is self-skinning as well) to create foam sheets that I can use for surface making, instead of kapa line foamboards because they are not available here in Australia and I am having a real hard time finding alternative pu based foam sheets. Help will be highly appreciated. Thanks.

    • Hi Waqas, This is not a stupid question because .. you never know! No-one ever knows totally because there’ll always be an infinite number of things that haven’t been tried yet! But in this case .. unfortunately not .. mainly because that form of 2-part expanding polyurethane creates a very different kind of foam to what’s in the Kapa-line foamboard (even the ‘rigid’ 2-part foam, it’s too hard). Just two thoughts .. there may be something in the ‘comments’, from Australia, at the end of other articles I’ve written about using Kapa-line foam? Maybe, can’t remember .. and second, do a search for ‘Depron’, it’s not the same, not as good but similar.

  5. Hi, Thanks for this blog posting. I want to mold an item in Ultra-cal then cast it using rigid foam. I don’t want to use Vaseline as a release agent. I was thinking that if I treated the plaster with a clear coat that would help, but is there a paint I can use that will adhere to the foam but easily pull away from the plaster?

    • Hi, Before Vaseline came along mouldmakers used to use ‘mouldmaker’s soap’ as a barrier/release on plaster moulds. It’s still obtainable from some places, but washing up liquid will also work. This can be washed off the plaster afterwards. I think that should be ok against self-skinning expanding foam. Another way of creating a non-stick barrier in a plaster mould is to dust it (a bit like a dough baker ‘flouring’ the work surface) either with talc, or perhaps just a dry plaster dusting. This should prevent the foam from sticking to the plaster.

  6. Hi David,

    I’m looking to mould the Tiranti self skinning foam from a CNC milled mould. The mould would be also be made from polyurethane. Do you think this would work or will the heat from the mixture melt the mould and would the foam release well?

    Many thanks,

    • If it’s a CNC milled mould it’s likely that it’s reasonably thick .. yes? If that’s the case there’s not much danger from the heat, and 2-part foam doesn’t get as hot as regular PU resin anyway in my experience. You’d need a good release agent inside .. usually a greasing of Vaseline should do it. The mould could also be put in a cold water ‘bath’ if this is possible, to minimise the danger even further.

  7. Hello David,

    I have a fibreglass mould of a ‘life like’ mannequin and would like a polyurethane cast. However, because of the size of the mould, I cannot mix sufficient volume of polyurethane liquid in time to fill the mould.

    Could you advise what I could do to get around the issue of the mix curing before pouring please?

    • Are you being serious? Have you over £1,000 or equivalent to spare for that amount of PU resin? In practise objects of this size are always hollow, and most often these days in fibreglass. It is possible to do this even in fibreglass moulds with the right barrier agent. Filling large volumes with PU resin, when its done at all, can only be by mixing smaller amounts and filling bit-by-bit.

  8. I just tried something similar with the DIY type aerosol product. It failed but I think I know why. The product both expands and cures due to exposure to moisture. I stuck the long tube you get on the aerosol can into the bottom of the mould and sprayed WAY too much product in there…as a result it had no room to expand and excluded all the moisture.

    I’ll do some more tests but I think if I withdraw the tuibe pretty sharpish as I squeeze the trigger, it should give results similar to yours.

  9. Hi David, I am due to start a degree course in Modelmaking in September. I understand you’re busy but I would appreciate some advice on a project I am working on.

    I am building a habitat for my turtle. When designing his basking area I designed a box to hold sand but quickly realised it wouldn’t work as it can get very messy as he goes in and out of the water. I am searching for a substitute.

    I want the material to be soft or cushioned underfoot when cured and dry. I am looking at the cushioned version of the Polyurethane Expanding Foam, Polyurethane Resin and Epoxy Resin. I am not sure which one to go for. I would like to add some sand as a filler to get a sand effect surface.

    The important thing is I need this surface to last for a long, long time, (he is going to live for at least 40 years). I will be protecting it with a seal, probably epoxy and will do this as often as needed. I don’t care about colour degradation, only that the surface remains intact even with a turtle walking on it over a long period of time. It is going to be exposed to direct heat and artificial sunlight.

    I was just wondering if you have any idea of which material I should use?

    Thank you,

    • Hello Aisling,

      Yes, it sounds like you want a soft ‘self-skinning’ polyurethane foam (I don’t think there is an ‘epoxy’ one .. certainly never heard of it). Something like this one .. http://www.mbfg.co.uk/foam-materials/foam-soft.html But I have my doubts about using something like this! Firstly polyurethanes can degrade noticeably under normal UV light; I can’t vouch for how long the foam might cope with the ‘abrasive’ parts of your average turtle (no personal experience, just imagining!) and thirdly, I’m not sure about ecotoxicity. Have a look at the Safety Data Sheets on that page .. especially for Part B. Maybe you can make sense of them. Better still (maybe) I would contact MBFG if in doubt. It may be fine, but personally if it were my turtle I’d much prefer to find some ‘organic’ solution like .. even if it were using fine aquarium ‘gravel’ instead of sand (surely less messy) or even having some kind of net covering over it to keep it in .. sort of like making a ‘bean-bag’ terrain?

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