Making plaster castings lighter

[47137]

I am wanting some rock faces on my new layout and trying use plaster and keep down the weight at the same time. This technique actually works - fill the back of the casting with air:

 

1/ Cut a piece of expanded aluminium mesh an inch or so bigger than the mould. Press the mesh down into the mould, bending and stretching and folding it as needed so it is about 6 to 9 mm inside the mould all the way round.

 

2/ The plaster is Lightweight Hydrocal mixed 3 parts plaster into 1 part water. Pour plaster into the mould to make the mould about two-thirds full.

 

Wait until the plaster just begins to set (around 15 to 20 minutes), and then roll and turn the mould so the plaster runs up the sides. Use a lolly stick or similar to persuade the plaster to go into nooks and crannies. Keep doing this for a few minutes until the plaster has enough viscosity to stay put and not run back down again. Sorry no photo - you don't get long to do this.

 

Then, wait another five minutes or so. The plaster will now be firm enough to hold itself in place, but has no strength. Use the lolly stick to pare away the plaster from the back of the mesh. Don't disturb the mesh itself.

 

Wait another full hour. Peel the mould off the casting, and then peel the mesh away too.

 

In this example I ended up with about 90 ml empty space in the back of the casting, saving around a third of the weight compared to a solid cast. I have tried embedding pieces of foam board instead and also cutting out surplus plaster with a disc in a mini drill. The mesh is the easiest and makes the neatest job.

 

Obviously the plaster which is pared away is unusable and goes to waste, but the quantity involved is quite small if you get the initial pour right.

 

Hope this is useful to someone.

 

- Richard.