Breaking the mould

Phil Bulmer shows Airgun Shooter some tips on casting your own gun-grip

Many airgunners, like myself, love lavishing a few hours of TLC on their airguns – and that often extends to a spot of DIY, too. Whether for customisation or restoration, one special technique to have in your armoury is that of making ‘pattern’ parts. As long as you have access to an original part, you can make a duplicate copy surprisingly easily with the moulding technique I’m about to show you – and it’s a workshop trick that’s good for a variety of components as long as they’re not under any great strain or require significant threading. It’s not a cheap option – but when it comes to restoring golden oldies, it could be a lot less expensive than buying a genuine part… assuming you can find one in the first place.

Here, I’m showing how I used the technique to make grip for a Webley Hurricane pistol. Of course, you need a good original from which to copy – so if you’re trying to replace a broken part, you may need to repair it sufficiently (with Milliput or similar epoxy putty) to at least make it usable as a ‘master’ template.

MAKING THE MOULD

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First, the master should be attached to a firm, flat base, like a block of wood [1]. As the grips on my Hurricane had seating lugs on the inside, I had to drill holes into the wood to accommodate them – and to hold them firmly in place, I also screwed the grips to the wood. (Obviously, I’d have to ultimately drill out the holding screw that would be cast into the final moulding.) Next, build a wall around the part to make a casting tray [2] into which you’re going to press in the silicone putty moulding material – I used wood, affixing it with PVA glue. As silicone is expensive, don’t make the tray too generous – just enough to give it a little thickness where it’s most needed. The walls of the tray should be perfectly vertical and of even height as you’ll need to compact and smooth the top of the silicone with a roller of some description after it’s pressed in.

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I’ve found that silicone putty can stick to porous parts of certain components (and is then difficult to remove), so it’s worth ‘testing’ an unobtrusive area first. There was no issue with my Hurricane’s grips; the silicone peeled off easily once cured. The instructions recommend using Vaseline as a release agent, but I used a wax furniture polish from an aerosol can and had no sticking problems.

The silicone putty I used is called Siligum [3] (from Hobbycraft) – a two-part mix you make into a soft putty, which you work with your hands until all the ‘marbling’ between the two compounds has blended into a single colour [4]. You get roughly fi ve minutes before it starts to cure and go hard, so mix only what you need and push it into the mould,taking care to work it well into any detailed areas first. Once the moulding tray is full, flatten the top surface with a roller [5]; I used an aerosol can. This is important – because when the silicone mould is removed and filled with the modelling polymer clay, you will need to push it into the silicone moulding with a little pressure. If the back of the silicone mould isn’t fl at it will flex… and whatever you’re casting will then go wrong

Once the silicone in the mould has cured, it can be peeled out from the casting tray [6], and it’s extremely satisfying if the master part peels out cleanly – which it will do if you pushed it properly into all the detail areas in the first place [7]. In my case, the mould had only a few, very slight imperfections and was good enough to go to the casting stage.

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THE CASTING

108_DSC02089 copyTo begin casting the pattern part, I took the casting tray apart and reassembled it around the silicone mould in a mirror image [8]. This is because the silicone goes back in a reversed configuration, such that the image of the moulded product now faces you. The tray helps support the mould as the casting material is pushed into it and also allows for that rolling pin to be used again.

You have a number of choices when it comes to the material used to cast the final product. There are two-part resins that can be mixed and poured in as a liquid, and these produce a fine moulding that’s sure to pick up every detail – but there can be air bubble issues in the final product if the mixing is too vigorous. Liquid resins will also need a colouring agent, as they tend to be transparent – more expense.

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For the Hurricane’s pattern grips, I chose a modelling clay called Premo [9] – a cross between clay and plastic, which I bought in Hobbycraft. Premo Clay is available in many different colours, which can be mixed if desired. It’s also flexible until it’s baked in an oven, so you have unlimited time to work with it. Simply take it from its packet, work it in your hand until fl exible and push it into the silicone mould, ensuring it’s pushed in well enough to pick up all the mould detail. A rolling pin helps compress it down, and keeps the back fl at [10].

112_DSC02099 copyOnce the clay has been fully pressed in to the mould, you could try and coax it out for baking… but that runs the risk of it being flexed out of shape – and as silicone is remarkably heat resistant, I put themoulding complete with its silicone tray straight into the oven [11]. The Premo’s instructions suggest 30 minutes at 130C for 6mm thickness, and as my grip was about 9mm its widest point, I gave it 45 minutes – and also allowed it to cool slowly after baking to prevent warping. Thankfully there was no appreciable shrinkage during the bake and, once cool, the pattern part was easy to remove from the mould.

DIY: PATTERN MOULDING

The last stage involved a bit of a tidy-up to the fi nished part, using a fine emery board, drilling out the hole for the screw, and then applying a layer of varnish to match the glossy look of the Webley’s original, factory-finished grip [12]. To stop the new grip from slipping out of position (as it’s held by only one screw), I also glued some lugs on the reverse.

Overall, the effect is quite remarkable. I showed the gun to a fellow shooter who didn’t notice the grip as being a facsimile part – and although it’s not as tough as the original, visually the results with Premo Clay are pretty reasonable. I’ve got more than half that silicone left, too, so I should have enough to cast a few more grips – ormaybe even have a go at replacing the broken rearsight on one of my other airguns…

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