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Working with Vac-Formed Parts & Kits


Please note: I wrote this article some time ago for another site that I used to own. It's not really up to TG standards as it has no images to illustrate it. This is an issue that I hope to rectify as and when I have time to take suitable photographs but in the meantime I offer it here in it's original text only format:

Introduction

Vacuum-forming is a technique where a piece of sheet plastic is heated so that it is pliable and then pulled down over a mould using a vacuum device such that the plastic takes on the shape of the mould. The tooling costs are relatively low enabling the production of niche market kits that would not be available otherwise. It is not uncommon for such kits to use vacuum formed parts for the larger components and halves of large bodies while resin, brass and other materials are used for smaller parts.

Cutting Out

Vacuum-formed parts will usually arrive as a rectangular sheet of plastic with bulges (the actual parts) in it and the first job is to remove the parts from the sheet.

In cases where the whole part is a single thickness of plastic, for example with the sails on sailing ships, it is possible to cut the parts out flush. However, if the parts represent the front and back of what will eventually be a solid body, the chances of them matching up are not good if this method is used. Instead, the technique is to remove the part by cutting the base sheet close to the part and then sanding off the resulting lip.

In either case, begin by scoring the plastic with a sharp knife and then gently flexing it until the pieces separate. When appropriate, the lip is removed by taping a sheet of sandpaper to a flat surface and rubbing the part on it until the lip is removed. Great care is needed while doing this as the part is likely to flex resulting in uneven sanding and a bad join.

Assembly

Vacuum-formed parts can be glued with liquid plastic cement or cyanoacrylate glue. The most difficult situation occurs when halves of a shell are to be joined because only a very thin plastic edge is available for glueing. The trick is to make small tabs from the leftover plastic and glue them to one half of the shell. When these are set, the halves should be test fitted again to make sure that everything is in order and then glued.

When the glue is dry, small gaps can be filled with epoxy or cyanoacrylate. It may also be necessary to fill or sand small dimples or pimples left on the surface from the moulding process by the small holes in the mould that were used to suck the plastic into contact with the mould.

Bodies made by joining two vacuum-formed parts will usually benefit from some form of internal filling to give them extra strength. Small parts can be filled with epoxy before they are joined together while larger bodies can be filled with aerosol insulating foam which is sprayed inside after assembly and expands to fill the cavity.


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