Making a plug to draw hot acrylic down over for elements of cupola

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Timothy Huff
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Making a plug to draw hot acrylic down over for elements of cupola

Postby Timothy Huff » October 11th, 2018, 7:47 pm

I'm gradually approaching a tricky job. I need to make an accurate set of Perspex "windows" for my FN5 turrets. I know I need to procure and use 0.5mm thick acrylic, and then heat it in a frame before drawing it down over a plug. My intention is to then cut it into smaller panels, painting some to create the curved opaque surfaces and doors at the rear of the turrets. The various panels will then be screwed through brass straps onto the supporting internal structure.

The bit that I'm unsure as to how to proceed, is making the plug. I have a CNC router, so with luck should be able to shape a plug in (say) 50mm layers which can then build up the required shape. I figure some sort of "blue foam" will be a suitable material (I've not used this stuff before). Has anyone hear made canopies using this method, and if so, can you give me any guidance as to particular materials, and how best to smoothly surface a bare polystyrene plug so it doesn't fuse to the hot acrylic being drawn down over it. I figure urethane might do the job if I can stop it attacking the polystyrene.

If anyone has experience in this area, I'd be most grateful for any guidance, as experience had taught me experimentation is expensive!

supporting structure.jpg
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Stuart Solomon
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Re: Making a plug to draw hot acrylic down over for elements of cupola

Postby Stuart Solomon » October 11th, 2018, 8:46 pm

Never use any type of foam for a plug. A plug must be rigid enough to withstand any pressure on it as well as the heat. many years ago I worked on the models for the film 'Force ten from Navarone' We pulled around 100 perspex canopies/blisters etc in one afternoon. All the plugs were made from either very hard balsa finished in a two pack paint or any timber roughed out and then covered in polyester body filler which can be fine sanded and polished to the final shape and size, allowing for the thickness of the perspex. Hope this helps.

Timothy Huff
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Re: Making a plug to draw hot acrylic down over for elements of cupola

Postby Timothy Huff » October 11th, 2018, 10:34 pm

*thankyou*, that has likely saved me a costly and time-consuming cock-up, not having drawn acrylic before, I hadn't considered that the force required might collapse even a dense polystyrene plug. "Heavy" Balsa it is then - or something similar.

Phil Clark
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Re: Making a plug to draw hot acrylic down over for elements of cupola

Postby Phil Clark » October 17th, 2018, 8:56 pm

See here - http://www.fighteraces.co.uk/portfolio/14-scale-lancaster-mouldings-updated-on-111018/

Patterns from glassed balsa and/or Chemiwood………..2 part epoxy glass female moulds then take before the moulds are filled (poured) with an aluminium powder filled epoxy casting resin. Temperature resistant & dimensionally stable with no 'grain' that will transfer to the inside of the clear moulding.

Phil

Timothy Huff
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Joined: January 23rd, 2018, 4:12 pm
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Re: Making a plug to draw hot acrylic down over for elements of cupola

Postby Timothy Huff » October 22nd, 2018, 9:58 pm

Many thanks all, I'm deeply grateful for the advice and help. Now talking this over with Phil.


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