Moulded rain cover   Back to Wind Power

After some mixed success trying to heat form a rain cover, I decided to try to make a mould.


The first step was to prepare a positive, and this was done using 3mm plywood. The next step was to make a box to fit around it, and secure the plywood to the base of this box.

I decided to try Plaster of Paris for the mould, since it was relatively cheap and easily available. I found that chemists and hardware stores sell low quality Plaster of Paris - you get a much better product, and much cheper, from an art supplies store (like Artsource in Obs).
After wiping some vaseline on the plywood, the Plaster of Paris was mixed and poured into the box. I completely underestimated the amount of matieral that would be need. In the end I used about 25kg! This despite using extra pieces of wood as shown to reduce the amount needed.

Once it was all set the mould box was flipped over, and the base removed, exposing the edges of the plywood. With some difficulty, the plywood was then removed, and some imperfections repaired with a little more Plaster of Paris.

The plywood was then reinserted, it it was time to pour the inside section. This was done in two halves, to make it easier to remove once set.

Wire was used as reinforcing, expecially where a flange would be formed. Wire hooks were also suspended so that the inside plugs could be lifted out once set.

There were quite a few imperfections due to bad mixing of the Plaster of Paris. An old credit card was the perfect tool for filling these holes.

Note the registration bumps, which ensure the pieces fit back together perfectly aligned.

Then it was time try it all out! Two part polyurethane resin was used, and once set, the Plaster of Paris plug needed to be removed. This was not easy, and after trying various methods, a small jack was used to force it apart. In the process, both the Plaster of Paris and the polyurehtane cracked, although it was all a success as the polyurethane positive could be glued together, touched up and used for the next step.


So the Plaster of Paris mould was obviously not reusable. The crux of the issue is that using a rigid mould to make a rigid object is asking for troublle.

The solution is to use a rubber mould, but the catch is rubber will not keep its shape very well. So a rigid shell needs to be made around the rubber. So, after cleaning and sanding, the holes were covered as shown, and the positive then attached firmly to a base plate. A dam was built using modelling clay (sulphur free), and a thin layer of rubber painted on.
Its important to eliminate bubbles, as these wwill show up on the casting, so the first layer is brushed on very thinly. For the second layer, to make things go a bit quicker, some thixotropix additive was used to make the rubber a bit thicker.

It can be very difficult to see how thick the layer of rubber has become, and so some depth register blocks were stuck onto it while it was still sticky.

The third layer was thickended more, so that it could be spread on like peanut butter.
On the last layer, some keys were added (with a slightly negative draught) to allow the rubber to lock into the rigid shell. When it was starting to set, a soultion of water and washing liquid was used to smooth it down.

The rigid shell needed to be in two halves to take apart easily. So a dam was build with plywood, sealed with a bit of clay, and then foil and vaseline used to create a slean edge. The foil is great to work with - the vaseline makes it stick well, and it can be smoothed and worked into a neat corner.
Now for the rigid shell. First, many small squares of fibreglass mat were cut, and placed all around the keys. It took a surprising amount of time and material to build up to a smooth layer. The idea is that when a complete layer is added next, there wont be big air gaps around the keys.
Next the dam was removed, a bit more vaseline added, and the second half of the shell completed. Holes were drilled for 6mm bolts to position and hold it together.
The same process is needed for the inside. First a thin layer of rubber, then depth registers, then more rubber, and the keys for the inside rigid shell. Then fibreglass mat and polyester resin, and finally prising it all apart.


The first time I tried to do a casting, I mixed the polyurethane too thoroughly, and is started to set while being poured. What a waste! The next time I was more careful, and mixed smaller batches at a time. Once it had set, the shell comes off, and the outer rubber is peeled back.

The cast was then quite firmly attached to the inner mould shell, so I cut a thread into a bicycle type valve insert, drilled a hole in the top of the cast, and screwed the valve in. I then used a bicycle pump to force air into the gap between the cast and the inner rubber, which made the cast easier to remove.
Finally, some trimming of the edges, a coat of aliminium paint for protection from UV, and to match the stainless steel bracket, and its ready to go!
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