The 11 Victorian period brick arches at the back of the layout, 'supporting' the high level lines, are based on these images taken in 2001. This is near the Thames, where the Victoria station lines cross the Waterloo lines.
Design
From what I had seen in general, the top of an arch is always to the same dimensions, just the height of the buttress varies with any given viaduct height. On the drawing board I first drew the top of the arch, from the photos, by counting bricks from the parapet top down. The 5-ring brick arch meets the buttress at an angle and by projecting these lines, the radius point for the ring can be found and the rings drawn in. I estimated the track bed line as being 8 courses above the top of the rings, it may be wrong, but it works for me! Our layout’s high-level track bed height had already been selected and so the ground level line was then drawn this distance from the track bed line.
At this point, I decided that the master would be of a ½ arch plus a buttress plus another ½ arch and would not include the parapet or the 'stringer' courses. This would make the butt joints between arches a mere 21 mm and easy to blend in and by making the 5 courses above the rings 'bond' interlocked, the blending in of the joints would be even easier. The parapet and 'stringer' courses can be added from simple brick card later.
This design would give a casting that is 190 mm wide and 140 mm high.But what thickness should I make it? I planned for 5 mm, as this is the thickness of the parapet wall above. Since I needed materials I popped over to the Tiranti shop to stock up and as well as advising on the moulding rubber variant to use (of the 3 types) they agreed that 5 mm was OK for thickness in the chosen casting resin. One thing to look out for in the design is 'Undercut' on the master, that is when you look down on it anything that is out of view by more than a couple of millimetre is excessive undercut, this can cause tear in the rubber.
Making the Master
I copied the drawing in pencil onto a sheet of SE Finecast 'brick' plastic sheet, making sure I was aligned to the brick bond. Using a sharp divider I could then 'score' the plastic at the top radius of the 5-ring brick arch and then with a scalpel and a steel rule I finished the cutting out. On a sheet of plain 0.5 mm plastic, to match the brick sheet, I 'scored' the radiuses of the 5-ring brick arches and cut out the 2 halves. These 3 parts of the master are too flexible and need to be mounted on a 1 mm sheet of plastic cut to the same shape (using solvent). The master is now 1.5 mm thick, moderately strong but needing to be built up to 5 mm thick and I did this with 3.5 mm strips of 1 mm plastic. These strips I fixed edge-on to the underside of the master, all the way around the edge, ensuring there are no gaps and flush with the edge of the master and then crossing the underside with strips, so that no unsupported area of more than 25 mm existed. I could have made the master up to strength and thickness by more layers of plastic card, but I think the plastic could have warped badly this way. Now the master needs careful inspection looking for any holes or cracks between the component parts and filling any found and smoothing down the sides. Also check that when the master is placed on a flat surface, you see no gaps underneath!
Making the Mould
The master needs a floor and sides to enable the mould to be made. I use 1mm plastic sheet for the floor and mounted the master on this, again carefully inspecting for and filling any holes. The sidewalls for this item are 15 mm high, again from 1 mm sheet. As the rubber is expensive, usage needs to be minimised, so the sidewalls are mounted between 10 mm and 20 mm from the sides of the master. Now this is important, the surface where you place the master for the rubber pouring must be flat and LEVEL and you must be able to leave the mould there, undisturbed, for up to 24 hours whilst curing. Also be prepared for the plastic master to warp, it almost certainly will at this size, so place weights around the outside floor of the master to keep it flat. Using the kitchen scales I estimated the weight required by weighing water in the mould and then, into a used yoghurt pot sitting on the scales, poured in the moulding rubber and then topped up with (1:20 as per these instructions) of catalyst. This is then fully stirred and mixed and then poured onto the master. The problem is air bubbles trapped on the surface of the master and to clear these I carefully run an old blunt screwdriver blade all around the master. Of course it's best efforts since you can't see the bubbles or the master through the rubber! For the next hour I intermittently rattle the tabletop to vibrate the bubbles and indeed, you can see the odd bubble break the surface.
24 hours later you can break/snap off the sidewalls and ease the rubber mould carefully from the master. The use of force will tear the rubber, so tease it off. The backside (topside in the master dish) of the mould will need any meniscus edges trimming off, this is important since the mould must be able to lie flat during casting operations.
Casting
The mould needs to be placed on a flat and level surface and since the resin is only workable for a matter of a couple of minutes you need to get everything ready before you start the resin. The casting resin comes as 2 packs which are mixed 50:50, so again using a yoghurt pot on the scales, pour in the resin and then top up with the same amount of hardener. Working quickly mix the resin well and pour into the mould and this time the bubble problem is easy to deal with, using your blunt object and under a light, since it is a brown clear liquid. Note that once this resin is mixed it will get warm, within 5 minutes it starts to cloud and then set.
