Author Topic: Taking Casts of Tunnel Portal - How?  (Read 571 times)

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Offline Peaky 556

Taking Casts of Tunnel Portal - How?
« on: November 14, 2017, 11:58:08 AM »
In the Newsletter I did ask for some advice on how could I replicate this Tunnel Portal in concrete, for use at various other locations around my track-to-be.  I’m sure someone must have tried something similar?  My approach in the absence of anyone’s experience would be to panel off the opening some way in (say about 2”), make up some suitable shuttering around the exterior and seal it to the brickwork with a mastic, then pour in what will become a huge quantity of silicon rubber moulding agent.  I am just a bit apprehensive though about the quantity and cost required, and just how well a large bulk of rubber like that would hold its shape for subsequent concrete casting work.
Any suggestions please?
Thanks, Tim



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Offline Traininvain

Re: Taking Casts of Tunnel Portal - How?
« Reply #1 on: November 14, 2017, 02:02:55 PM »
Perhaps make a wooden mould at the outset then cover in sump oil before pouring in the concrete so it releases easily?

Or perhaps not ...

Ian


Offline John Candy

Re: Taking Casts of Tunnel Portal - How?
« Reply #2 on: November 14, 2017, 02:11:14 PM »
Tim,

I have never cast in concrete because of risk of frost damage and fact that castings may easily break when coming out of the mould. If you are going to cast in that material, then the mould will have to be deep (for a thick casting) and the quantity of rubber required will be quite expensive.

My tunnel portals have been cast in resin and needed to be about 10mm thick max. and even that needed a huge mould and used a large amount of rubber for just a single track portal (I'll weigh it later ... it is stored in the garage at present). I think for cement/concrete you would need to be at least 1 inch thick.

There is a material called "ciment fondu" which is recommended for ornamental architectural casting and figurines/sculpture, etc.). I have a booklet on its use and a small bag which I bought a few years ago but have never got around to using. 

https://tiranti.co.uk/products/secar-71-white-ciment-fondu-25kg/

John.
My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline Peaky 556

Re: Taking Casts of Tunnel Portal - How?
« Reply #3 on: November 14, 2017, 05:28:20 PM »
When I cast the concrete I would be well advised to use some steel weldmesh as reinforcement, so that ought to solve the breakage problem.  I can make the mould sides deeper later on to increase the thickness of the concrete.
My main uncertainty though is how to create the rubber mould that transfers the shape to the new casting concrete.  It’s rubber that will be poured against the existing masonry shape, so do I need a release against here, and what grades of rubber should I consider, and will a new mortgage be required...? ???

Offline John Candy

Re: Taking Casts of Tunnel Portal - How?
« Reply #4 on: November 14, 2017, 06:16:47 PM »
I have weighed the portal mould used for Monkton Priors...it comes out at 7 lbs.

It is relatively thin to produce a 10mm casting and the cost of the RTV rubber was around 50GBP.





I reckon you will need around 10lbs for your portal at a cost of about 75GBP.

The (hardness) grade I use for resin is Shore A25 which is quite soft and flexible but (because of density/weight of concrete compared with resin) I think you will need a harder grade ..... the weight may distort and compress a soft mould.

Glass fibre matting is the suggested reinforcement in the booklet I have........ it suggests wire mesh has been displaced.

You will need a release agent ....... suggest you try a PTFE dry lubricant (spray can) or brush on a hard wax.

Regards,
John.
My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.

Offline cabbage

Re: Taking Casts of Tunnel Portal - How?
« Reply #5 on: November 14, 2017, 06:28:59 PM »
Speaking as an ex speed freak... What is wrong with glass fibre? This will produce  a strong negative. The original item can be protected from the GF by covering it with wax polish and cling film. The negative can be then filled with concrete to your hearts content!

regards

ralph

Offline Peaky 556

Re: Taking Casts of Tunnel Portal - How?
« Reply #6 on: November 14, 2017, 11:49:19 PM »
Ralph,
Thanks, but I’d already asked advice of my brother re fibreglass, as he’s been restoring a bubble car body shell.  His reply was:
“In order to get the mould off the original profile there has to be a taper angle on all parts just like in a casting process. Thus you can’t do it on brickwork like you show as also too many reverse cavities.”
So unless you think it is possible, a more flexible negative like silicone rubber is probably best.

John, thanks also for the info.  Re the release agent, I presume you mean against the brickwork, not the concrete secondary?

Regards, Tim

Offline John Candy

Re: Taking Casts of Tunnel Portal - How?
« Reply #7 on: November 15, 2017, 07:41:05 AM »
Tim,

The brickwork you use as the "pattern" will need a good coating (to seal the porous surface as much as anything else) otherwise you will find it difficult to remove without tearing the rubber mould.

On the topic of the Shore hardness scale following is general guide.
a. Shore 20A = Rubber Band
b. Shore 40A = Pencil Eraser
c. Shore 60A = Car Tire Tread
d. Shore 70A* = Running Shoe Sole
e. Shore 80A = Leather Belt
f. Shore 100A = Shopping Cart Wheel


I buy my materials from TOMPS.com .... very much cheaper than Tiranti and model hobby stores.

Regards,
John.

P.S. Having written above, it may be that you will have to use Tiranti : Shore A30 is the hardest grade listed by TOMPS but Tiranti have A40   see  https://tiranti.co.uk/product-category/mouldmaking/polyurethane-rubber/polyurethane-mould-rubber-shorea40/  and it specifically mentions suitability for concrete.
My fellow Members, ask not what your Society can do for you, ask what you can do for your Society.