Distractions and demands on my time having eased a little for the time being, I can report on some progress with the making of moulds to reproduce as nearly as possible in cast resin the exquisite master parts that Mike Trice so kindly printed earlier in the year.
I started with the side frames for the 11'9" wheel base Doncaster 6 wheeled carriage bogies. Mike had tried out several versions of these, and the evolution of design had resulted in both a flat-backed variety designed to stick onto a thin etched fold-up inner frame such as Dave Sutton's or possibly Bill Bedford's (the latter I've not tried) and a version with locating pips and tabs on the back to permit an all-resin assembly for which Mike had also printed an inner frame master. I was trying to make most efficient use of time especially and of materials, also trying to guess what was most likely to suit any likely users. Being keen to initially make only one set of the minimum necessary number of moulds, but to keep options open in case my expectations proved to be wrong, I decided to base my side frame moulds on the version with the pips and tabs on the back, reasoning that these are easy enough to trim off to suit fixing to a metal inner frame and are essential for use on a printed / resin inner frame. At first I cursed the thought that I would therefore have to make a two part mould in order to create the pips and tabs on the back, but then I realised that a two part mould would have been necessary even for flat-backed frames if I wanted to at least produce dimples in the correct places to accommodate axle bearings, whether those are soldered into a metal inner frame or glued into the resin side frame! I decided, after some agonising, to postpone production of a resin inner bogie structure pending convincing evidence of demand.
After examining the printed master frames I decided to add some little rectangles of plastic to the backs of the spring hangers where those project below the lower edge of each frame. This was to ensure that when casting there would be good access for resin to find its way in from the back, equally for air to find its way out. I also filled the fully formed bearing holes almost completely with plasticine so as to leave just a shallow conical dimple (sorry Mike, I wasn't confident that they would be reproduced reliably if left full depth). I then made a plastic base for a moulding box, with some slots and holes to accept the tabs and pips on the backs of the frames, then stuck the frames down as firmly as was possible on a couple of layers of double-sided tape. I also added some blobs of plasticine around the edges to later create alignment aids for the two parts of the mould.
The printed master frames had all cured with slight matching curvature which I did not want to reproduce in the cast resin versions. Once stuck to the base of the moulding box alone, that also tended to curve. I therefore made the sides and the joints in the completed box strong enough and stiff enough to hold the base flat. That was good as far as it went, but I hadn't thought about what would happen when I had poured the first part of the mould, stripped down the moulding box and kept (or reinstated) the frame masters in the flexible rubber. Of course, the frame masters assumed curved form again and so did the rubber mould! I had to think of a way around that problem, and it occurred to me that if I allowed the first part of the mould and the masters to remain in a curve while I produced a suitably thin and fully cured second part to the mould, everything would be okay again when the completed mould was straightened out by simply placing it on a flat surface. In order to allow the first part of the mould to sit temporarily in the same curvature as the master sides I supported it as shown below on two round paint brush handles and very carefully built sides around it once again.
Here's a look at the first part of the mould which has to capture all of the surface detail:
Here's the thinner second part with the recesses that create the tabs and pips, plus the shallow mounds that produce the bearing hole dimples. I've also bored some little breather holes to ensure that the recesses fill with resin rather than trapping air:
The mould certainly produces straight castings, although the first attempt to cast in it revealed some problems. The most obvious one was that it was a struggle to thoroughly coax the resin in and the air out of all of the recesses within the viable working time of the resin mixture. Four frames in a single mould were too many. At least one of the spring hangers failed to form properly as a result. Air bubbles had also been retained in the mould rubber in the narrow spaces around the leaf springs, resulting in annoying little beads of resin appearing in those areas in the castings. As a one-off defect in the first casting attempt I also had a badly defective axlebox cover, the result of a stray flake of rubber that had fallen into the mould....
The resin beads can, with some care and patience, be largely picked and scraped away with a small sharp tool, and the hidden backs of the castings are adequately formed.
After slicing the mould into two smaller moulds for separate production of two frames apiece it is easier to get the resin in fully. I may in the end decide to re-make this mould completely, taking care to dislodge the air bubbles before the rubber sets, but that is for later consideration.
More about the separate bolster end pieces for the bogies, the buffers, and those nice bucket seats when I can find more time to type and upload!