Casting materials

I’ve updated prices, suppliers and added proper entries for principal materials in the casting section under Materials above. The ‘quick view’ comparisons page provides an overview for anyone not sure which of the various casting materials to use, while the other pages give more detailed information on the properties of each and how to work with them. Not all are there yet in detail, I am slowly working through them .. but so far the pages include polyurethane resin, polyester resin, polyurethane foam and polymer-modified plaster.

These more detailed pages begin with a summary ‘definition’; outline what the material is best at and not so good for; describe their ‘working life’ i.e. how to use them and how long for, and lastly what they cost and where to get them. At the end is a ‘worklog’ where I can add other bits of information as I have it.

Here is an excerpt from my page dealing with polyurethane foam. It’s the first one in this section to include photos, but I hope to do the same with the others.

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

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