March casting exercise – ‘Two legs good..’ Part 3

Part 2 of this exercise, posted at the beginning of February, finished with the completion of a multi-piece silicone rubber mould of a plastic rhino toy used as a practise form. The form had two special requirements; the first being the need for a separate ‘plug’ piece to fill the area under the belly (between the four legs) and the second, as an extra safeguard, a separate mould piece for the top half of the head because otherwise the details of horns and ears would be difficult to fill without trapping air. This head portion will be cast first and then inserted into the main mould before casting the rest.

This technique of casting in stages, let’s call it additive casting, works best when working with resin because this usually binds strongly to itself. Resin was also necessary for this form because it is a relatively small size, and the horns and ears could be especially fragile. I am using a standard, low-viscosity (thin and flowing) polyurethane resin from Sika called ‘Biresin G26’. Most polyurethane resins are reasonably thin and can flow into quite intricate moulds. Most also share the following characteristics: they are supplied in two parts which are mixed in equal measure; these two parts are usually translucent until mixed, turning opaque as they cure to anything from white to beige dependent on the brand; initial hardening is quick (5-20mins); they are strong when fully cured but rarely brittle, retaining a little flexibility; they can easily be coloured or filled with other materials. In fact it is common to add a filler to resin, partly to economise. For this exercise I am using a specially manufactured ash called Fillite, a certain amount of which can be added to the resin without making it difficult to pour. It also gives it an interesting surface quality. Many powdered materials (such as talc, marble or slate dust) can be used as fillers as long as they are inert i.e. not affecting the chemical reaction which has to take place for the resin to cure.

The photo above shows equal amounts of Parts A and B of the polyurethane resin measured by eye in disposable plastic cups. The same amount of Fillite (light grey) is also portioned. I have found that this proportion, in total 2:1 resin to Fillite is best if one still wants the mix to flow. One can add much more filler to the resin if one wishes but this results in a thicker mix which is more suitable for spreading than pouring. This for example might be ideal for laminating, that is, building up a shell inside the mould parts in order to make a hollow cast instead of pouring a solid one.

The Fillite must be mixed into one part of the resin ( I’ve found it doesn’t really matter which) before the two resin parts are mixed together. This is because once the resin parts are mixed the time available to pour is very limited, basically just a couple of minutes. In the photo below the small head mould is supported on a bed of rice (so that its position can be easily adjusted). Working quickly, a little of the resin mix was poured into the recesses of horns and ears first and a cocktail stick poked around to displace trapped air, then the rest was poured. It is difficult to mix very small amounts of resin properly, particularly if there are three parts to the mix so I usually mix up larger, more measureable amounts and, if there’s time, use the excess for filling something else.

Above, these details on the rhino head (especially the ears) would never have filled properly if pouring the whole form from the top in one go. Small castings in resin may take a little longer to cure (the standard term for setting by chemical reaction rather than hardening simply by evaporation of water) since the greater the volume of resin the more heat is generated, which in turn accelerates curing. Large resin casts (whether polyurethane or polyester) can become extremely hot during this process!

The head portion has been eased into its position in the main mould. It will of course fit snuggly since it’s the identical form. When the mould is reassembled and secured tightly more resin can be mixed to fill the rest of the form. As long as the same proportion of Fillite is used the colours will match and the join should be invisible. It doesn’t matter how fully cured the head portion has become in the meantime; the new resin will bind firmly to it. Below is the mould set-up ready for pouring, couched on a bed of rice so that the mould top can be checked with a spirit level and adjusted. Since I have not incorporated pouring funnels etc. in this particular case, an extra precaution was to grease the plaster part of the mould top with Vaseline so that in the event that resin overflowed it could be more easily peeled off once hardened.

The form was carefully demoulded (taken out of the mould) after about 20mins. The plug was kept in place untill last of all partly to protect the legs which would be in a slightly softer state than the body untill fully cured. Full curing of resin varies with type but can usually take a few days.

However well-fitting the mould pieces might be there will always be wafer thin  ‘flashing’ where the resin has seeped into the mould seams. Polyurethane resin cures very quickly and can normally be safely demoulded after about 15 minutes, though various brands may differ if they’ve been developed for a specific use i.e. hollow casting. With most there is a so-called ‘green stage’ fresh out of the mould when the resin should be firm but still a little rubbery. This is the best time to trim or sand away such things as seam lines.

Below, if you imagine the rhino form feet-up and as a hollow cavity which is gradually filled through one of the legs, the liquid resin rises comfortably until it gets to the belly which becomes a ‘celing’ to the cavity. Air can easily get trapped here as it can no longer be pushed so easily upwards. A certain amount of rocking and tapping while pouring can help a lot but may not get rid of all. Often a channel would be drilled in the silicone plug (the pink part here) from the problem area to the outside top of the mould, to allow air to escape. Normally a test cast is made (or a few) to locate these problem areas first. Otherwise, a little more of the resin/filler mix can be made and the holes patched up, then trimmed or sanded. Polyurethane resin can be tooled (worked with power tools, carved or sanded) very easily, especially if a lightweight filler such as Fillite has been added.

Finally below, the finished cast (with the original behind it). I’d mentioned above that the Fillite gives the resin mix a sympathetic colour (which is also slightly speckled). This is one common purpose of fillers (especially marble or slate dust), to impart an appearance of other materials. But resins discolour in time; the UV (ultra violet) component of daylight causes yellowing. This is noticeable even if a filler or pigment has been added. In the case of polyester resins a UV blocker is available which can be added to the mix to prevent this. So far I don’t know of any equivalent additive for polyurethane resins. What this means is that if any permanence is needed for the surface effect polyurethane resin needs to be painted and, common to all plastics, that can mean quite a bit of preparation especially if the form will be handled.

Painting is another subject in itself, which I will probably deal with at some point, but for the moment.. the polyurethane cast should ideally be given a few days to fully cure, after which it should be carefully scrubbed in warm soapy water. It is better still to use a scouring powder which will make the surface ever so slightly rougher and more paint- receptive. Lastly the form needs a good priming coat using a spray primer, the best of which is Simoniz Acrylic primer. The most reliable, hard-wearing and versatile paints I’ve found, all of which are suitable on plastic if primed, are; Humbrol enamels, Rosco ‘Supersaturated’ scenic acrylics or Osmo oil/wax wood paints.