In order to be able to manufacture parts for a R2-D2, the use of silicon moulds is recommended. The advantage of this method is that even if a part is needed several times, only one master model needs to be built. Stability, “paintability”, thermal characteristics etc. are of secondary importance because you only use this master model for building the silicon mould and you don't use it on the R2. I used only high-quality addition-interlaced silicone.
Very good elasticity
Self separating – this makes it possible to manufacture GRP parts without using releasing agents
Accurate weighing of the individual components is necessary
It is not compatible with all materials - this means that if you use the wrong modelling compound the silicon will no longer harden correctly
It is extreme expensive - I used silicon to the tune of more than 1,300 Euro (1,900 USD) during my construction period!!!
Basically preliminary tests should be carried out before using new materials. I worked mainly with polystyrene sheets, MDF sheets and Epoxyd filler.
Durability of silicone moulds
One important piece of information first: Silicon moulds only have a limited durability. How often a silicon mould can be used, depends on several factors. Moulds with only few undercuts and without "open" bubbles last longer than moulds with many holes, corners, slots and edges. Also the type of casting or laminating material used is important. The silicon that I use makes it possible to make 20 to 30 castings using PU resin and with epoxy resin the mould is damaged after approximately 10 castings. Using polyester gives even less castings, but I didn't use it as it stinks, it is more poisonous, and shrinks when hardening.
Supporting moulds support the silicon moulds, as the name implies. With very small silicon moulds no supporting moulds are necessary, since they can hold themselves "in shape". If the silicon moulds are somewhat larger, it is best to build supporting moulds using plaster, sand and water.
If the silicon moulds are still somewhat larger you have the choice between supporting moulds made of plaster bandage, eventually even using additional reinforcing fabrics such as those used on building sites or GRP supporting moulds. Both variants have their pro and cons: plaster supporting moulds do not bend but GRP supporting moulds don't break. If you consider the fact that an expensive silicon mould isn't useable without a supporting mould it is recommended that you make one supporting mould of plaster and one from GRP as a back up. Supporting moulds should consist of at least 20-25 layers of plaster bandage and eventually additional reinforcing fabrics. For this step at least 2 persons are necessary, because the plaster hardens extremely quickly. The finished plaster supporting moulds should dry for 3-5 days, before they are removed from the silicon mould. In the case of large silicon moulds such as the dome and body, you can work only with GRP, because by the size and weight of a plaster mould guarantees that it will break.
Building smaller silicon moulds
Depending upon the part there are several possibilities of the mould production. With simple, small parts the mould production is relatively simple.
First of all you need the appropriate master model. This must fixed on an even, smooth surface, usually a polystyrene sheet. Build an edge which then becomes the mould around it. This should be approximately 3-5 mm away from the master model and it shouldn't have any undercuts, otherwise the silicon mould can only be removed with great difficulty, if at all. I shouldn´t have to mention that the edge must be totally sealed, because if the silicon finds a way out of the mould, the whole process will have been in vain.
Next, you fill the mould with silicon and pay particular attention to make sure if possible that there are no bubbles in the silicon or on the master model. By blowing across the mixture with a straw or a compressed air gun fitted with a pressure-reducing valve, ascending bubbles can be removed at the surface. After hardening, remove the edge carefully. Here you should pay particular attention to the fact that the silicon mould is not removed from the master model, because you would not get the required accuracy any more.
Now pour over the mould with plaster, which you have mixed before with some sand, which reduces cracks in the plaster, and water. Note: the plaster hardens within few seconds - depending upon the temperature this can take 30-60 seconds.
To separate the silicon mould with the poured part more easily from the supporting and/or plaster mould, a small hole drilled in the supporting mould is recommended. If you blow from the outside with compressed air into the hole, the silicon mould jumps out automatically.
Building larger silicon moulds
With larger parts the whole
process becomes more difficult. The edge, namely, the mould is more difficult to
manufacture, and it must be arranged that the silicon mould doesn't "sag" later
in the supporting mould because of its dead weight. A consequence of this would
be distorted parts. After some miss-castings I built larger silicon moulds with
a "retaining” edge.
If undercuts between the silicon mould and supporting mould aren't able to be avoided, you must work with multipart supporting moulds.
Even with larger master models it is important that they are fixed firmly at the bottom, because they can float in the silicon and this would again entail the total loss of the silicon allowance.
Schematic cross section designs (not true to scale) of two silicon moulds with “retaining” edges.
For smaller and middle silicon moulds a simpler “retaining” edge is sufficient, so that the silicon mould doesn't sag in the supporting mould.
For larger moulds, such as the dome or body, a “retaining” edge must be made as shown in the lower illustration.
During the silicon mould production the moulds should not be lain upside down, as shown, because the rising bubbles in the silicon come into direct contact with the master model. Whenever possible, Master models are poured over with silicon from above, so that the rising bubbles can move away from the master model.
The mould for the middle foot was made of polystyrene sheets, Perspex and filler compound.
The art of pouring is to cast the part, namely the master model, as exactly as possible without internal bubbles. Pouring takes some time, because the silicon must use its dead weight to run downwards into the mould. It is advisable to mix several small quantities, otherwise the silicon begins to harden before it can be poured into the mould.
The use of Perspex makes monitoring possible and allows an inside view of the capping process.
The completely filled mould should be checked continuously after the casting procedure for at least 1-2 hours, because it is now that the most pressure is exerted on the mould. Leakages should be filled with hot adhesive immediately. Note that adhesive can no longer be used on surfaces, which have already come into contact with silicon, due to the separating effect of the silicon.
Despite everything the mould shouldn't be built too solidly, because it must be usually divided and/or destroyed after hardening the silicon, in order to get it from the silicon mould. Also here the silicon mould may not be removed from the part (master model), if no supporting mould has been manufactured or it will be completely useless.
The finished silicon mould in its plaster supporting mould.
Supporting moulds can be manufactured from plaster or GRP. GRP supporting moulds should be used with talcum powder between the finished silicon and GRP mould, because this enables them to be separated more easily.
This shows a GRP part which was manufactured using this silicon mould.