Moulding for Bodgers

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AndyA
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Moulding for Bodgers

Postby AndyA » Mon Oct 10, 2011 11:30 am

There's been little progress on "Humby's Lane" over the weekend, but I have been working on a casting for a sleeper for the project (but first for a couple of others). It's been a while since I showed this, so I thought I'd stick a 'how I did it' up in case anyone else is interested.

Please note that what you'll see here isn't the posh resin casting of the likes of Steve B, Andrew M and others, it's the 'just good enough' approach, but it may encourage others, or at least convince people that the prices for proper castings out there in the real world are completely justified.

I'd had a go earlier in the year at a corrugated sleeper, but rejected the result. I was trying to figure out how to do a better one when we went to Ireland and I had a chance to photograph a Bord Na Mona one.

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Back home I could convert the measurements and make "drawings", from which I constructed a "master". This is about as simple as it gets, a piece of basswood sanded to size using sealer to eliminate the grain, with 'bolts' added from .5mm card. Just because it's simple, though, I don't skimp at this stage because the fewer imperfections now, the less effort later. My wife had difficulty understanding how I could spend three hours making a piece an inch long by a quarter wide. (Note that after all that work, matching the rail height for the APA challenge dimensions and the Bord Na Mona sleeper ends, I ended up with something suspiciously close in size to the Peco O16.5 crazy track. Ah me).

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Steve and others use RTV rubber and resin. This requires a degree of precision I've never mastered, although Steve has repeatedly told me it's quite simple once you've had a bit of practice. I use remeltable rubber (MIne from Alec Tiranti, there are other types available) and plaster. This limits what you can do but I have ways around a fair number of such problems. These sleepers won't slide onto the rail, but the rail will be glued on. I've done this before so it should work. Note the 'should'.:)

The next stage is to make an intermediate mould. Tiranti market a specialist sealing agent for porous surfaces, but I use dilute PVA adhesive, and that works well. I stick the master onto a convenient tile with a dab of PVA (otherwise it'll float) and make a 'fence' round it to hold the rubber in. There is a use for junk mail, after all.

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Now Tiranti also sell a thermostatically controlled pot for melting this stuff. I guess that's all well and good if you can write the cost off against tax, but... One gas ring, cast iron skillet to spread the heat (nine pounds from Dyas's, but I use it a lot for cooking as well), one semi-non-stick jelly mould (one pound for ten from Poundland, used only for this purpose).

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It takes a bit of trial and error to get the temperature right. Chop the rubber into small pieces and turn the temperature up until water on the skillet splitters and dances - this will be marginally too low but put the jelly mould full of rubber pieces in and turn it up until the rubber at the bottom starts to melt. Now leave it until most has melted and then just stir it once or twice, trying to avoid air bubbles.

If you overheat the stuff, it smokes. There is some anecdotal evidence on the web of people having adverse side effects: I've never had a problem other than getting grief from my wife, which I have cured by cooking curry that evening, but treat with caution if you know you're allergic to stuff like ACC. If it smokes, turn the gas down.

Now that the stuff is nice and runny (but not smoking:)), pick up the jelly mould with oven gloves and pour into the fence, a single smooth pour if you can. Now immediately start to vibrate the sides of the fence to try to eliminate any air bubbles.

Fault - many more bubbles than expected because I poured it carefully.
Possible problem - I forgot to seal the master (or in your case, not enough, since your memory is probably better than mine:)).
Remedy - follow through to the end, it'll probably be okay anyway

Fault - rubber shoots out from under the fence
Problem - insufficient tape holding fence together
Remedy - unless the fence has actually fallen apart, just keep pouring, the stuff will set eventually and no more stuff will pour out

Fault - lumps in rubber
Problem - not runny enough
Remedy - if there's enough rubber and it hasn't started to set, pick them out and add more rubber. Otherwise, let everything cool down, clean up the mess and start again, which is the advantage over RTV.

Anyway, now go and do something else (remembering to turn the gas off) for at least an hour. You can peel the mould off before then but trust me, you don't want to.

Return from pub, lift mould and fence from master (if the master comes with it, gently easy it out). Peel fence from rubber. Turn over and inspect your handiwork. (or, if you had an irremediable fault three, cut the rubber up again, make a new fence and start from phase three again, having learned that that was not quite hot enough.

Now for the moment of truth. This will produce the item from which the working moulds are made. Mix up the plaster according to the instructions. Yes, it's a stupidly small amount of plaster. That's why people who do this for a living always have other moulds into which the excess can go. I'm working on some playing pieces for a board game, so...

Pour it into the mould and once it's partly set, clean off the excess with SWMBO's store card. Let the gash plaster and stuff set as well. Don't wash it down the drain, wait until it's dry, sweep it up and put it into the bin. Now leave it. Leave it longer than the instructions say - in fact going down to the pub for a swift one helps.

Hopefully, when you return, you'll be able to pull a perfect casting from the mould. Strangely enough I managed this at only the second attempt, the fist having numerous air bubbles due to my impatience. Examine it carefully. If there are a couple of bulges where there are air bubbles in the master mould, you can sand them off, if there are hundreds, don't bother, start again from stage three. slight imperfections can be fettled at this stage. Why use an intermediate mould? Several reasons:

Firstly, the mould will deteriorate with time, and if you go back to the master top make new ones, the master will quickly fall apart (I think Steve is better at this than me). Secondly, with it all the same colour it's easier to see any faults. Thirdly repeat casting is tedious in the extreme (this may be why Steve is approaching the 10,000 posts mark), so I'm going to make two moulds of four each. Still not a lot of plaster, but easrrier to handle. So, I made three more from the first mould and put it aside.

Image

Now repeat the process with the mould (don't cut up the first one, although if you have several scrap ones, re-use those after checking very carefully which is the good one.

For good measure, since I forgot to photograph this stage first time around, here's what the mould looks like in situ.

Image

Make some trial castings from this mould. If they have significant air bubbles or suchlike, start again, don't press on with a bad mould, it'll only lead to tears before bedtime. Yours and mine.

I have enough castings now to experiment with the 'real rust' technique someone posted ages ago. When I find the rust I made then. I'll report back when I do.

You can, in extremis, cast simple things in resin this way. I don't recommend it except if you're stuck for a dozen axle-box covers or something. Steve tells me that the lifetime of an TV mould with resin is by my standards frighteningly low, I suspect that this stuff will die by about, oh, say, Tuesday. But you can of course cut it up and start again. I have no idea how many times you can re-use the stuff. Enough times, anyway.

Storage. to be on the safe side, wrap the moulds in paper. The plasticisers attack polystyrene foam and are in turn attacked by certain other types of plastic including the stuff cheap cuttng boards are made of. Don't ask me, I'm not a chemist. They also appear to degrade in bright sunlight. This isn't a problem for me since sunlight never enters the stack of boxes where 'stuff' is stored.

regards
Andy A
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gnine: less is the gnew more
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Gerry Bullock
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Postby Gerry Bullock » Mon Oct 10, 2011 12:16 pm

Great story, as always, Andy. I think I'll pass. :wink:
So little time, so many ideas!!!!! GerryB.
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andrew milner
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Postby andrew milner » Wed Oct 12, 2011 2:47 pm

Very entertaining Andy!! Must admit though, that looks no easier than RTV and resin and the bit about having to guess the temperature etc makes it more difficult - with RTV and resin you just need some decent scales - no smell, no smoke etc

If it works for you then that's fine but I can't help thinking perseverance with resin would pay off in the long run :wink:

By the way, you can re-use RTV a bit - if you chop up old moulds and mix them with fresh rubber you can use it to bulk out moulds.
Andrew Milner, still trying to figure it out....

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AndyA
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Postby AndyA » Wed Oct 12, 2011 3:10 pm

Andrew Milner wrote:Very entertaining Andy!! Must admit though, that looks no easier than RTV and resin and the bit about having to guess the temperature etc makes it more difficult - with RTV and resin you just need some decent scales - no smell, no smoke etc

If it works for you then that's fine but I can't help thinking perseverance with resin would pay off in the long run :wink:

By the way, you can re-use RTV a bit - if you chop up old moulds and mix them with fresh rubber you can use it to bulk out moulds.


Maybe I overdid the smoke/smell stuff - I no longer have any problem because the trial and error is over. I suspect that the biggest problem for most people here would be that resin casting really does destroy the things quite quickly. I have tried to persevere as you suggest, but it's cheaper just to buy stuff off you and Steve, by the time I've got an acceptable result.

regards
Andy A
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gnine: less is the gnew more

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michael
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Postby michael » Sat Oct 22, 2011 5:07 am

Andy I had to wipe the tears of laughter away a few times reading your post, you never fail to capture those moments we all go through at one time or another with real and genuine illumination.

Thanks for the Step by Step of the funny rubber moulding that I used when I worked for Allmodels in Acton making moulds of a Cathedral for a model village somewhere, I can't remember where now. It works for fiberglass too.
Regards Michael
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