Making hollow casts in open or ‘closed’ moulds – Part 1

This post follows directly from the last one in which I featured one of the simplest ways of making a complete mould for a puppet head .. making a 2-piece block mould in silicone rubber. At the end of the post I included a couple of photos of a hollow casting using filled polyurethane resin and now I want to explain how to do this in more detail. I will also deal in later posts with making hollow casts using other materials such as Jesmonite and the advantages of being able to make a hollow cast in a ‘closed’ mould .. i.e. without having to set up a pouring hole at the mouldmaking stage.

mould with casting

Jumping forward for the moment above and below, the hollow resin cast is almost finished and just needs a little cleaning up on the seam line. Polyurethane resin normally cures a white-to-beige colour dependent on the type and this cast is light grey because I added a filler called Fillite when mixing it. Fillers are added to resins for many different reasons (see Common fillers for resin casting in the Materials section) but in this case it is specifically to thicken the resin to help it stay put on sloping surfaces. Fillite also makes the resin easier to carve or sand without reducing its strength too much. Unlike polyester resins, there is no thixotropic or ‘gelling’ additive available for polyurethane resins.

casting nearly finished

For this test piece I used Fast Cast Polyurethane Resin from Tomps (see Quick view comparisons of casting materials for current prices) which is particularly thin to begin with, but the slower version has the advantage of a slightly longer working time and the ‘turning’, i.e. when the resin changes from liquid to solid, is not so abrupt. Below is the equipment needed for correct mixing. Polyurethane resins come in two equal parts, almost always mixed 1:1 by weight but in the past I’ve often got by without any problems by judging equal volumes in two disposable plastic cups, even though the weights of the two parts are slightly different. Now that I’m even more grown-up I prefer to measure properly by weight, using a fairly inexpensive kitchen weighing scales. Because the cans or bottles resin comes in are never designed to assist the pouring of small amounts .. the manufacturers would prefer that we use it all up in one go! .. I always decant some of each part into plastic cups and then pour from those when measuring. To avoid knocking these over while working, I made the cup-holder shown out of foamed Pvc.

materials for hollow resin casting

I usually mix Fillite with resin in the proportion 1:2 .. that is, equal amounts by weight of all three parts. This is easiest to remember and it also usually results in a thick sludge which is still easily spreadable and which will still manage to fill fine detail. It’s best to mix the Fillite thoroughly into part ‘A’ of the resin first (which is the ‘resin’ part of most polyurethane brands). The resin will combine with the Fillite surprisingly smoothly, to form a thick paste, which of course becomes thinner and more manageable when part ‘B’, the hardener, is added. Mixing must then be both thorough .. and fast! .. but shouldn’t need more than 30 seconds or so for small amounts like this. I usually mix up 10g part ‘A’, 10g Fillite and then 10g part ‘B’. The best mixing sticks I’ve found are disposable chopsticks because they’re very resilient, clean easily and can be re-used.

mixing Fillite with resin

The best practice is to pour most or all of it into the open mould-half straight away and sway the mould to let the slushy liquid cover the surface naturally first. In my experience it’s rare that air gets trapped with this method and using this mix, but if you’re concerned about deep detail there’s always a bit of time to poke around with a cocktail stick or small brush to make sure air is freed. An alternative is to take the extra time to brush on a thin detail coat all over first and let this firm up before pouring in more. For a few minutes the resin will pool back into the centre, but I work round the mould with a soft but rigid brush (synthetic is good) pulling it back up the sides for as long as I can until it starts to change. I try not to take it over the mould edge i.e. the outline of the form, but it doesn’t matter if this happens because this line can be cleaned up before the resin is fully hardened. Obviously, with the mould I’m featuring the neck part is completely open .. I had to edge the mixture very carefully into this part at first, but as it congeals it’s easier to build up a thickness.

close-up of mould being filled

After about 5mins or so (though this will vary with different resins) it can no longer be distributed so easily with the brush .. and it’s very important to stop trying at this point! .. because in doing so one risks separating the now gum like resin from the mould surface. In this state it’s possible though to press it, almost model it, with the fingers. Also at this stage if you want to use the brush again you need to clean it quickly in acetone.

filled halves of mould

I never try to do the two halves of the mould at once, however simple the form is .. most often the quick curing of the resin doesn’t give enough time for this. It is inevitable that the cast is much thicker in the deeper parts, but I’ve always found that if I follow the procedure described even the thin sections end up strong enough. They’ll get an extra covering during the next stage anyway. Resins have a so-called green stage (polyurethane having a longer one than polyester) when the thinner sections of the resin remain quite flexible. This can be taken advantage of .. let’s say it’s been 20-30mins since pouring .. for going round the mould edge with a scalpel or fine wooden modelling tool and peeling away anything that’s crept over the line. If not, the mould halves won’t fit together tightly! Now that they’re ready, the two halves of the hollow cast are going to be joined together from the inside .. by closing the two mould halves together and pouring in just enough resin to fill the seams.

Vaseline between mould halves

Above, I am brushing Vaseline on the remaining silicone surface, but being careful not to get it on the resin .. not so clear in the photo above. Vaselining the ‘seam faces’ (that is, the parts of the mould which come together to form the seam) is not an essential move when using silicone, but I’ve found that it often helps a lot! It provides an extra seal which halts the seepage of resin (this time mixed with a little less Fillite, to make it more liquid) out of the mould. I’ve also found that the Vaseline helps the silicone halves to align better.

For this internal coating I mixed up a small amount of resin and Fillite in the same way as before, but this time 10g part ‘A’, just 7g Fillite and 10g part ‘B’. I poured most of the mixture immediately into one half of the mould, placed the other half of the mould on top, made sure that the mould was secure and then rotated the mould carefully along the axis of the seam line to concentrate resin in this area. Obviously I had to be careful not to tip too far when running the resin close to the open neck part. Basically one has to continue in this fashion, ‘see-sawing’ around the whole seam and back, until one’s fairly certain that the resin has become too thick to move much more. Here it helps to have some of the resin remaining in the cup, as shown below, to indicate how thick it’s become. Another option, for those who have a little less patience, is to accelerate the curing with heat. Below I’ve set up a small heat gun to blow into the mould. I’m holding the mould because it needs to be moved around .. if left static it would get too hot. Once there’s no more obvious movement of the resin I usually leave the mould alone for a while, only demoulding the form once the extra resin in the cup is completely hard to the touch. If you don’t have this as a control, it should generally be safe to demould 1 hour after pouring, whichever brand of polyurethane resin you’re using.

heating resin to quicken curing

Obviously the advantage of being able to make a hollow cast like this is that it is lightweight, while still being strong. It also saves on material. If strength is of particular importance, more than one coating of resin can be applied or strengtheners such as glassfibre matting or scrim can be integrated into the two halves before the mould is put together. This is not a method of speedy mass-production .. it takes considerably more time than pouring a cast .. but manually ‘applying’ the cast in sections, as it were, does ensure that you can make perfect, blemish-free casts every time. If for any reason a solid cast is preferred, it’s easy to fill the hollow casting with more resin .. although it may be better to do this in stages for forms larger than this one because the heat produced by larger amounts of resin could cause tensions during curing which have been known to crack the casting.

removing flashing with scalpel

As I’ve said in the previous post, there’s always a seam to be cleaned up .. but in this event the work was minimal. With polyurethane resin the flashing (as the excess is called) is particularly easy to remove .. but trimming and sanding is made even easier by the addition of Fillite. My preferred method is to scrape with a scalpel, in the direction away from the blade edge, because I find this easier to control.



3 thoughts on “Making hollow casts in open or ‘closed’ moulds – Part 1

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s