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Old 15th Jan 2010, 11:26 am   #1
Aerodyne
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Join Date: Jun 2004
Location: Hampton Vale, Peterborough, UK.
Posts: 1,698
Default Copying a control knob by moulding

In response to a request by a forum member, here is a brief outline of methods used to mould and cast copies of control knobs.
There are numerous moulding techniques, each with plus and minus points. Forget plaster moulds - they are too rigid to allow easy removal of any but the most flared knob. Also they are porous and when used with casting resin they must be sealed thoroughly or the resin will penetrate and you will lose shape and definition. Any plaster mould is fragile and logically will not withstand more than one use.

What's left? Alginate - the dental moulding compound - is both easy and quick to use. It does not require heating and is easily pourable, setting rapidly. It is also inexpensive in small quantities. It is delicate and will not last more than a few days before drying out and distorting. It will take poured resin and I've used it several times with success.

Gelflex and Vinamold require very careful heating until liquid. This is safest when done in a double-walled pot like the old scotch glue pots, but for a one-off a normal pan can be used. Overheated, the stuff self-destructs. This should never be heated in the home as the smell is unpleasant and long-lasting. With care it can be remelted and re-used many times over. It does suffer from trapped air bubbles so it should not be vigorously stirred during melting and once fully liquid it should be allowed to stand for a while, cooling very slowly perhaps, to help any air escape before pouring. Professionally, a vacuum pump is used to extract the air but at home the best you can do is to pour slowly over the control knoballowing the liquid to rise smoothly up the sides of the knob until finally covering to at least a half-inch, preferably more, for casting stability. Expect to find the odd air bubble which must be cut or filed from the surface of your subsequent casting.

RTV silicone is a cold curing material which cannot be re-used and is expensive but has the merit of simplicity in use.

Latex rubber is the favoured mould making material of those who copy chess pieces and complex figurines as the stuff is capable of recording very fine detail. In use, the item is suspended and repeatedly dipped in the liquid latex, in between times being allowed to air cure. Alternatively it can be applied by brush, again allowing curing between coats. It is a very slow process in my experience and the finished mould must be supported before castings are taken. I use a large mass of child's Plasticene but putty would be as good. The moulds are delicate and there is a limit to the number of copies that can be taken before deterioration.

Not every knob is suitable for single-stage moulding. Those with a complex shape may need a two-part mould. Polish the knob with brasso then wax polish. Leave the grub screw in place, slightly below the surface, or you will have problems

Assuming the knob you are copying is a relatively simple one with the widest part at the very back, secure this with double-sided tape into a small plastic container (for moulding with Alginate) or a metal or wood container (for hot-melt moulding). The alternative is to construct a simple timber box with a strut across the open top to acrry a control. Secure the knob to be moulded to the control shaft which can then be lowered into the box and the moulding material poured around it. Gentle tapping of the sides of the box will encourage release of air bubbles.

Casting method

The same box can be used for casting. Ensure it is on a level surface. Pour sufficient to create a slight meniscus. Resin can be tinted with pigments and filled with a variety of fillers. Filling reduces the inevitable slight shrinkage of the setting resin and can change the feel and quality of the finished cast. Alternatively the casting can be sprayed to requirements afterwards.

Finishing tips

With coarse abrasive paper taped to your bench surface or with a belt sander, abrade the meniscus until level with the edges of the cast knob. Find the centre of the back and drill a pilot hole. This is best done using a pillar drill and with the casting supported by the mould. Slowish speed, don't force the drill and lubricate with paraffin or WD40. Most holes reach well into the knobs to within 2mm of the front. This cannot be achieved with a standard drill's cutting angle, so an end-mill cutter can be used finally to clear the material remaining due to the drill angle or you might consider regrinding an old quarter-inch or 6mm drill to a very shallow angle.

The grub screw hole can be drilled and tapped using the point on the casting where the original was.
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www.resin-supplies. co.uk

The above site has information sheets and a comprehensive stock of materials.

www.fredaldous.co.uk

Another comprehensive supplier.

For Gelflex, try a Google search.

Further info, see my article in Radio Bygones issue 97, Oct/Nov 2005.

-Tony
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