The technique for making a large mould has already been described in the post on making large moulds. A variation on this technique involves the addition of a sculpted polystyrene section to make the armature for the large mould. The object that was made using this technique was a large cactus.
Additional steps for the large mould
Again the basic technique is a plaster and vermiculite mix over a hessian covered timber armature. The only extra steps to make the large mould for the cactus involves using polystyrene to sculpt a part of the cactus armature that would be too difficult to make using templates. In this case the rounded top of the cactus was sculpted in polystyrene.
Instead of using ply template ribs to define the shape we use thin strips of 4mm plywood. These are stapled between the top and bottom cross section shapes.
You could add additional cross sections if needed but this shape is simple and only needs a top and bottom.
The rounded top of the cactus was a little difficult to make in wood so it was roughly shaped from polystyrene. This was also covered in a vermiculite mix after being glued to the top of the base section after the main sculpting was finished.
After the timber frame has been completed it is covered with a tight fitting layer of hessian. Next a thick layer of plaster is applied to the hessian. After this sets it makes a solid base onto which the sculpting mix can be applied.
It is a good idea to do as much rough shaping as you can when applying the plaster base coat at this stage. This will minimise the amount of sculpting mix that will need to be applied afterwards and also reduce the amount of sculpting work that will need to be done later.
Make sure you have thought through the logistics of how you will handle the armature as it will now start getting quite heavy.
After the base section has been covered with a thick layer of vermiculite and plaster the finer sculpting can start. You can start before the mix has dried because it is a little easier to work. When all the detail has been sculpted the whole thing is given a light sanding.
Trace out the shape of the top and cut a block of polystyrene about 25mm smaller all around. This is roughly carved to the approximate shape which will then be fitted to the top of the timber frame.
After the base section has been finished the polystyrene block that has been prepared is glued to the top of the base.
A layer of vermiculite and plaster is applied over the polystyrene and shaped to blend in with the bottom section.
The whole thing can be given a light sanding with fine sandpaper and put aside for several days so the plaster can thoroughly dry out.
Once it is totally dry the surface can be sealed with shellac in preparation for fibreglassing.
When totally dry the armature is sealed with several coats of shellac. A wax mould release is now applied and polished to get a smooth surface on the large mould.
Since the cactus has undercut issues with the curved rib shapes in its surface a two part fibreglass mould cannot be used. To solve this problem a three part mould is used which will allow the mould sections to be removed easily from the armature.
The picture shows the completed fibreglassing of the first section of the large mould.
The other two sections of the large mould are fibreglassed and, after bolt holes are drilled in the flanges so that it can be reassembled accurately, are removed from the armature.
The completed mould sections are given a sanding to remove any fibreglass splinters from the outside surface. This makes handling the large mould sections much easier and safer. This is an important step because, while not life threatening, fibreglass needles in the hands are very painful and you will be itching for days until they are out of your body.
Next the three large mould sections are bolted together ready for casting. Below is a completed cast of the cactus. The little horns were added after the main body was cast. Because the cactus was going to have lights inside clear surfboard resin was used so the fibreglass would be translucent.