IntroductionYou may find that the amount of distortion on casts taken from some rubber moulds, especially Latex, is not acceptable. If this is the case then you will need to manufacture a plaster support jacket for your mould. For some users this is probably a routine procedure, in which case you need read no further, for less experienced users, however, we outline our suggested method.
EquipmentHeavy card, hardboard or similar to make a support with a central hole in which to place the mould, plus a suitable container or support on which to rest the mould.
A suitable box large enough to enclose the mould and its support case, say at least 10mm bigger all round than the maximum mould dimensions.
Any grade of casting plaster If a multi piece case proves necessary then a further supply of card, hardboard or similar for flanges and some white petroleum jelly for use as a parting agent will also be required.
MethodMake up a thin (watery) plaster mix and pour inside the supported mould, swirl this around ensuring that the entire surface is covered and then drain off any excess, allow to dry.
Taking great care not to damage this brittle first skin, add a further two layers using the same method but with a slightly thicker mix on each occasion. When all layers are fully dried you should have a fully supporting hollow shell which will retain the exact fine detail of the original, next completely fill the mould with plaster and allow to fully dry.
You are now ready to add the outer support case. If this shape appears straightforward then a one piece case will be needed, as a general rule if you can imagine the case lifting off with no restriction, e.g. similar to a pudding basin, then one piece will work, if not, then a split case will be required.
For a one piece case set the mould, with its internal plaster support still inside, top down into the box and pour plaster over it, allow to dry, remove the internal support and you are ready to start production.
If you feel that the support case needs to be multi part, e.g. because of undercuts or the sheer size and weight of a one piece case then proceed as follows. Using the box as before make up one or more dividing flanges in thick card or similar, fit over the mould, pour plaster over the area behind the flange and allow to dry, remove the flange, smear the exposed face with white petroleum jelly before pouring the next section.
As you gain experience consider applying the outer case freehand using a thickish plaster mix and your hands, use plaster bandage or scrim as a reinforcement, this will give a lightweight jacket. For an ultra lightweight case use fibreglass following the same basic methods.
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