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Hot tin roofs - more uses for home vac forming

Posted by whart57 , 09 February 2018 · 447 views

layout building layout scenery modelling technique 3mm scale asia thailand

I won't say I have mastered the technique of home vacuum forming, but once I was getting consistently acceptable results for my market stall canopies my thoughts turned to other uses for this technique. And I didn't have to look far. Across the tracks from where the market was starting to take shape is the large corrugated iron shed that in reality belonged to the SRT Permanent Way division. A photo of this appears in an earlier blog entry.


Now, when I built this I was quite pleased with it. I thought it captured the spirit of the original quite nicely. In isolation it still does, but now that other buildings are starting to appear in that corner of the layout and providing context, the overscale corrugations of the Slaters sheet is becoming more noticeable. The Slaters sheet is after all intended for 4mm scale and I model in 3mm scale. I was also finding that the Slaters sheet looked completely wrong if I used it to put a roof on a smaller building, and given that corrugated iron roofs are very common in Thailand this was an issue I couldn't duck for much longer.


Over the years various techniques have been tried to produce representations of corrugated iron. I have heard for example of a roller built by Stewart Hine, a giant of creative solutions to modelling problems back in the 60s and 70s, that gave thin copper sheet 3mm scale corrugations but I have never seen one, nor any of its output. A decade or so ago too I came across some 3mm scale corrugated panels made from steel that were being sold at a 3mm Society event. (Edit: I have since been informed it was the late Dave Martin who sourced these items and sold them at Society events) These were generally being used as wagon loads or for standing in yards, and being made of steel could be given a natural tint by leaving them out in the garden for a few weeks. I bought some as I was building a model of Bodiam station at the time. However not only were these sheets expensive, they proved very hard to work with. (Bodiam station was eventually built as an etched brass kit, I believe some are still available to 3mm Society members through the Society Shop).


Etched brass was of course another possibility but the success I had had of vac forming suggested this was worth a try so I dug out these corrugated sheetlets


My first step was to check that these steel pieces were actually correct to scale. No point expending effort on making a mould from them if they were badly out. I thought a quick Google would turn up a BSI or other standard, but amazingly, no. Builders' merchants were happy to state overall dimensions of the sheets they sold but they presumably assumed that all their customers knew what the pitch between ridges would be. Eventually I found a tender from the Nepalese government for the supply of corrugated iron for the construction of rural housing that set out a detailed specification. My experience of government and corporate tenders is that they are usually written to come up with the answer you first thought of, so I felt I could assume that this specification matched what the world's steel manufacturers could supply and wasn't the Nepalese government devising something from first principles. However, my little corrugated steel sheetlets turned out to be spot on for corrugation pitch and sheet width and were long enough to represent most of the standard lengths required in Nepal.


The next step then was to create the mould around which I would form the plastic. The chamber of my home made device is some 10cm x 8cm so the mould has to fit inside of that. I don't have to worry about air leaking round the outside of the mould because the main seal is where the plastikard sheet is clamped down on top, so the base of the mould can be a loose fit with a working area of around 8cm x 7cm. How much clearance you need at the sides depends on how deep your mould is, obviously in this case it is shallow so not much space has to be allowed for the plastic to get into the deeper crevices.


My basic mould is a 4mm ply base with a 4mm or 6mm high strip underneath to hold it above the exhaust holes and give a clear exit for the air. Pieces cut from a cheap mouse mat (£1 from Wilco) provide a better seal so the greatest suck is applied where I want it. The formers for the shape I want to mould are then stuck on top. In this case I was able to stick 16 of the steel sheetlets onto the play base. The final step is the vital one of drilling holes for the air to be sucked through. These do not have to be big, 1mm or 1.5mm diameter is enough, but they do have to be fairly closely spaced around the formers. I drilled twelve holes around each sheetlet.


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Then came the moment of truth. A 11cm x 11cm piece of ten thou sheet was clamped over the vacuum chamber, heated up with the hot air gun until it started to flop onto the formers and then the vacuum cleaner was switched on for a few seconds. The result was everything I hoped for, nice corrugations and in pieces wide enough to use. The suction also makes the plastic thinner so that the 10 thou is closer to five thou over the formers. I checked this with a micrometer to make sure I wasn't imagining things. The result therefore approaches prototypical thinness, though not thin enough to overlap pieces in prototypical fashion. Five thou is the equivalent of half an inch in 3mm scale, still quite a slab.


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Clearly then the product of this technique can only be used for cosmetic detail. That's not really an issue, as model buildings need to have a good solid core anyway. So as a test I build a model of a small shack, a structure typical of the small squatter's dwellings that line the tracks in much of Bangkok. This shack would have corrugated iron walls and roof with an open living area under cover in front. I constructed a basic core out of 40 thou Plastikard and then laid the corrugated iron representations on top of that.


Each "sheet" needs cutting out individually. A sharp scalpel does this nicely, with a steel rule as guide and following the path of the corrugations. Prototype sheets are three foot wide, 9mm in 3mm scale, but in order to allow for the fact that sheets are mounted with one corrugation overlap, these model sheets are cut a little over 8mm wide. I follow the corrugation that is just over 8mm over from the first edge, and this makes it 8.3 mm wide according to the Vernier. The piece is then cut to length, normally a scale eight foot for walls or nine foot for roofing (that Nepalese document specifies 2.75m - aka nine foot - for roofing) and then I sand a little bit off the back at one edge and sand across the edge at the other to create a tiny lip. This means that when pieces are butted up against each other there is a small but perceptible height difference to simulate the overlap. I did try a prototypical overlap, as can be seen on the back wall of the model shack in the photograph, but it looks a bit crude to me.


Then finally painting. Photographic evidence suggests that roofs suffer far more from the elements than walls. I have painted the walls to suggest the galvanising is holding up. This is a mix of matt grey and metallic steel. A lighter shade of grey and steel was later dry brushed over in order to highlight the corrugations. On the roof though the combination of rain and hot sun has stripped off the protective coating and the steel is now open to the air. And rusts as steel does. I still need to dry brush a redder oxide colour over the roof as well as add more details to the front - next problem is how do I simulate the washing hanging up to dry - but this test looks good to me. That PW shed is going to have to be rebuilt unfortunately.


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