Many terrain modellers never think of making a mould because they do not want their tables populated by lots of identical models, however making a mould to cast parts of a model which can then be modified and assembled in different ways is a very useful technique. This still has to be balance against the cost of creating a mould (most mould making compounds are not cheap) however dlmos has come up with a method of using craft foam that is so cheap and easy that cost is not an issue.
The building to the right has been made using two such panels which have then been broken, distressed, and painted to create an individual building.
Over to dlmos:
If you're needing to produce a fairly simple piece like a wall with windows or maybe a blast door here's a quick and cheap way to do it. First go to you local crafts store and find the neoprene sheet foam. I found mine in the scrap-booking area. It comes in a couple thicknesses, usually 2mm a 3mm. I bought a bulk pack of the 2mm variety (LOTS of half sheets) for this project. It cost me 2 dollars.
A quick list of other supplies you'll need:
A cutting tool - I used an X-Acto knife
A steel ruler
A template (a drawing of the part)
Spray glue - cheap crappy stuff that barely holds. I got mine at Target for 3 bucks.
Something to catch overspray, I used wax paper
And the plaster of your choice, I use Merlins Magic
Ok, on to the mold making:
First make a template of the thing you want to cast. You can make this on graph paper or on the computer. I used MS Word to make mine and printed a copy for each layer. Color coding these also helps to Identify what you need to cut out.
This is the template I used for this piece:
After cutting all the templates to fit the foam sheets I applied glue to the foam, and stuck the templates down.
With the templates glued down I began cutting, starting with the top layer and working my way back. By doing it this way I knew I had to cut out anything that I has already cut out on the previous layer, plus whatever need to be cut on this layer. When you cut, use a metal straight edge, and score thought the paper so that it won't get dragged in, screwing up your line. It is important to apply pressure to the straight edge to compress the foam and hold it still.
Once you've made all your cuts for the layer, flip it over and check for spots that didn't get cut all the way though, normally the corners. Don't pull these apart! It will leave little knots that are a pain to cut out and ruin the clean line in the mold. Instead, slip your knife into the cut and with a slight sawing motion slice though the 'knots'.
Once you have cut all the layers they should look like the picture on the left below. The blank one is the base of the mold, I use two sheets for this, as well as two sheets for the first layer (bottom left) to increase the depth of the mold.
The image to the right (above) shows the layers glued together. Note that 'blocks' of foam have been inserted into the window openings.
Make sure that the glue is dry before pouring in your plaster, starting at the lowest point, until the mold is full. Like this:
The plaster should be allowed to thicken to the consistency of tooth paste before scaping the top to remove any excess. Then let the plaster set up until it is ready to de-mold. I waited 20 minutes start to finish.
When you're ready to de-mold flip the mold over and peel each layer off one at a time. This helps to reduce the chances of breaking the piece. Be easy though and try not to tear the foam, it should come off with gentle tugging.
When you get to the layer with the windows and door blocks simply pop them out the back. When you peel back the last layer you should have your finished piece:
When it's finished drying you can remove any flashing where the layers were and you're good to go. To use the mold again just rub off any plaster under running water, dry each layer, and glue them back together for the next cast. From the time I sat down to draw my template to the time I had my first piece out, with the mold cleaned and re-glued for the next cast, took a little over two hours. But now I can pump out a complete wall every thirty minutes, and most of that I'm watching TV while the plaster sets!