January mouldmaking exercises – ‘Two legs good..’ Part 2

I left ‘Part 1’ (posted January 29) with most of the mould completed, but with the ‘clever’ bits still to do. I had a thick silicone ‘skin’ covering most of the prototype, encased by a plaster jacket (made in two detachable halves, otherwise the silicone part within would be impossible to take out later).

Next the whole setup (plaster jacket, silicone, prototype within and plasticine ‘plug‘) needs to be detached from the baseboard and turned over. All the plasticine can now be removed, meaning that the space occupied by the plug now becomes a void.

In this case the plasticine was very easy to remove. Silicone rubber doesn’t normally stick to it and there was only a little cleaning up to do on the underbelly and legs of the plastic prototype. Now this part of the mould is almost ready for filling, with pourable silicone this time, to ‘re-create’ the plug as a separate silicone piece. Almost ready, that is, apart from the crucial step of ‘greasing’ the void with Vaseline to prevent the new silicone addition from sticking to the previous. In the photo below I’ve bound the plaster halves together and set the mould in a bed of rice so that its top edge can be adjusted for a true ‘water’ level. If this is not done the liquid silicone would not fill the void completely.

making a silcone mould plug - 1I’ve chosen to use a slightly softer silicone for the plug than the outer skin, because during demoulding the plug will have to squash/flex a little more than the rest. The V1069 (green) has a Shore A of 30, whereas the Lukasil 429 (pink) has a Shore A of 20. In an overall Shore A scale of 0-70 this does make a difference. The photo below shows the void filled, waiting for the silicone to set (again at least overnight, or 24hrs as recommended by the manufacturer).

making a silicone plug - 2

After this has set it means that the whole prototype form has been moulded and it is safe to start dismantling the whole thing. Remember that the first (green) silicone skin was applied around the whole rhino form (except the underside masked by the plasticine) but it would not be possible to simply remove the prototype (or future casts) from this silicone skin as it is, because of the strong undercutting (jutting out) of the head part. The silicone skin must first be split with a sharp knife, just up to the point where the head can be eased out without much strain on either the form or the silicone. Although silicone can be very flexible there are definite limits.

splitting the silicone skinIf the mould parts are put together again with care and the plaster jacket is firmly in place the two parts either side of the cut will ‘find themselves’ again and may not even create any noticeable seam line on the cast. If in doubt and as an extra measure the cut can be cemented with a little Vaseline (or even .. just a passing thought, though I haven’t tried it yet.. denture paste!).

The separate mould parts are displayed below (with the softer plug shown re-fitted back onto the prototype for the moment, hopefully to make it clearer) and then the mould assembled as if ready for casting. I should mention that there’s something of common practice that I haven’t done here, and that is to include pouring cones (or funnels, or ‘reservoirs‘) in the design of the mould, which would have been built at the plasticine stage. Since I intend to cast in resin these are not so necessary here.

parts of the multi-piece mould

multi-piece mould with plug ready for casting

I had mentioned in ‘Part 1’ that there were two significant challenges with this form, the first being the space under the legs which has been solved by creating a separate plug in silicone. The second is the inevitable problem that, whichever casting material one chooses, the chances are that the head will not fill properly especially considering the relative slenderness of the horns and the ears. There are just too many places where air can get trapped. The traditional solution for this is to cut thin channels in the silicone leading from the outermost points of the horns and ears to the outside of the mould so that the air can escape that way. In the first place this may not be so easy with a split skin mould of this type and in the second place I wanted to try another idea.

separate mould part for headHere I have made a separate mould just for the part of the head that would cause the problems. I intend to make a resin cast in this and then insert this pre-cast part into the larger mould which can then be filled as normal.

Part 3 … when it comes .. will show this, along with some other aspects of casting in resin.


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