7/8ths Building System

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-steves-
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Re: 7/8ths Building System

Post by -steves- » Tue Apr 02, 2019 6:57 pm

Not boring at all and very well engineered and well thought out :)
The buck stops here .......

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philipy
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Re: 7/8ths Building System

Post by philipy » Tue Apr 02, 2019 8:23 pm

The Oily Rag wrote: Tue Apr 02, 2019 4:28 pm I understand my detailed explanations are using up my ration of posts.
I think you are being more than a little over-sensitive.
Philip

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Re: 7/8ths Building System

Post by Jbs » Mon May 06, 2019 10:07 am

Ian, would something like copydex work? You can peel it off when you are finished and it might seal the base. You might have to run round the base of the pattern with a scalpel after fixing to remove excess and ensure a clean cast when you pour the rubber. Not tried it but thinking about my own future casting projects inspired by your posts.
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John
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John Smith

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Re: 7/8ths Building System

Post by GTB » Mon May 06, 2019 4:44 pm

Oily Rag wrote: Sun May 05, 2019 5:28 pm For the final moulds, new rubber in date essential and I have to think of a better way to seal the patterns down to mould box base. Will report progress when it happens.
It's many years since I last did any casting with silicone rubber moulds, I used to make two part moulds for metal casting of detail parts and open moulds for resin casting rolling stock bodies in HO scale. I used linotype metal for casting, but I would guess that is no longer obtainable. My resin experience was with polyesters (awful stuff) and epoxies (which attack the rubber). Polyurethanes were just coming into use when I stopped, but I remember them as being sensitive to moisture when casting. I used Silastic 3120 rubber for both metals and resins, as I didn't do enough casting to keep two types on the shelf. The stuff was too expensive and had too short a shelf life for that and I did more metal casting than resin casting.

Silicones are dilatant fluids, the harder you stir, the higher the viscosity becomes and from memory that got worse as they got older and it got very hard with the 3120 to get the red oxide filler back into suspension. Technically they aren't liquids, but are a colloid until the rubber is cured, hence the mixing behaviour you observed.

I made my patterns from polystyrene sheet and for open top moulds I built them up on a piece of 60 thous. sheet. I was generous with the solvent so there were no gaps for the rubber to creep into. When it came time to pour the mould, I built a fence out of strips of polystyrene to form the mould box, using plenty of solvent to stick them to the baseplate. Same reason, no gaps for the stuff to leak out.

I vaguely remember using double sided tape once to hold some rolling stock body patterns down to the base and was distinctly unimpressed with the results.

I think you've mentioned rubber inhibition......? Silicone rubber can be very sensitive to the materials it is in contact with while curing and one of the Silastic rubbers I used at work would never cure if it was in contact with plasticene. They can also be affected by mould release materials and stopping the rubber sticking to itself when pouring two part moulds can get interesting.....

When making two part moulds for metal casting, I used powdered graphite as the parting agent and unscented talc as a wetting/realease agent to get parts with a good surface finish.

Hope some of that is of interest/help.

Regards,
Graeme

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Re: 7/8ths Building System

Post by GTB » Tue May 07, 2019 1:20 pm

Oily Rag wrote: Tue May 07, 2019 9:17 am Just a quick note, it appears that linotype metal is still available over here somewhere.
One of the local metal refiners still lists it as well, but I don't think I'd live long enough to get through a 5kg ingot. There must be another use for it now, as it is still available. I've still got a few sticks of type metal left, if I take up casting again.

I do use Milliput, but only in a limited way, as I don't like handling epoxies. For filling polystyrene I use Squadron White putty from the model kit shop. It can be thinned with MEK and brushed into small gaps that a spatula can't reach. Tamiya putty works the same way but dries in the tube very quickly once opened.

Regards,
Graeme

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