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Aerodyne 15th Jan 2010 11:26 am

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.


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

Another comprehensive supplier.

For Gelflex, try a Google search.

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


Retired 15th Jan 2010 11:40 am

Re: Copying a control knob by moulding
Hi Tony,

Many thanks for adding this very useful and well written thread. In desperation I've had new knobs made in the USA but the next time I require a knob I'll be trying your method.

Regards, Col.

oldticktock 15th Jan 2010 12:18 pm

Re: Copying a control knob by moulding
Thanks Tony


FRANK.C 15th Jan 2010 1:02 pm

Re: Copying a control knob by moulding
Thanks Tony for a very informative thread. It's something I would like to try sometime.


Radio_Dave 15th Jan 2010 1:09 pm

Re: Copying a control knob by moulding
Thanks Tony,

In the past I thought about making replacement knobs, but I decided against it due to the high cost of the products:(.


richrussell 15th Jan 2010 2:24 pm

Re: Copying a control knob by moulding

Originally Posted by Radio_Dave (Post 299376)
Thanks Tony,

In the past I thought about making replacement knobs, but I decided against it due to the high cost of the products:(.


Ditto - but then I've just remembered that I've got several tubes of RTV Silicone in my garage, from when I last rebuilt a car engine (specifically Rhodorseal 5661). It's just sitting there doing nothing, so I might give it a go at some point.

Paul LS 15th Jan 2010 3:34 pm

Re: Copying a control knob by moulding
I use a two part Siligum silicone moulding compound from HobbyCraft or similar stores. You mix the two rubbery parts and wrap it around the knob and after 5 mins it sets. The resultant mould is similar to Latex rubber.. pliable and fairly tough. It is expensive, however they say the mould can be used up to 50 times. But then again who wants 50 knobs? :)


Aerodyne 15th Jan 2010 4:02 pm

Re: Copying a control knob by moulding
Good point, Paul. Unfortunately most of the available moulding materials are expensive to by on the retail market, and the quantities are often very small. The cheapest way is to use Alginate, I believe, for the odd couple of knobs, at least.
It is only when you have a set of value with a damaged or missing knob that cannot be replaced and has to be specially manufactured that moulding one for yourself really becomes viable in financial terms. In other terms, however, such as pleasure in creating, moulding and casting your own has more going for it.
Siligum sounds interesting, though...

Aerodyne 15th Jan 2010 4:06 pm

Re: Copying a control knob by moulding
Rich, though not specifically designed for mouldmaking, Rhodorseal sounds as though it might well make a useful mould. Well worth a little experimentation.

richrussell 15th Jan 2010 4:47 pm

Re: Copying a control knob by moulding
I'll give it a go and report back. It's meant for sealing the sump and various other seals in Renault C and F series engines. I'm not sure how long it'll take to set at room temperature though, as it generally cures quickly when you start running the engine.

I've also got some Threebond 1215 which is similar and actually a lot nicer to use than the Rhodorseal, which is sticky beyond belief until it sets and gets everywhere. That's a Subaru recommended sealant for the same purposes.

Both are very thick and viscous, as they're designed to be squashed by the two things you're sealing. So I would imagine the best way to take a mould of a knob would be to over fill a cylindrical mould and squash it into the gaps with a plunger of some sort to ensure it gets into all the crevices.

joe 15th Jan 2010 7:14 pm

Re: Copying a control knob by moulding
Thanks Tony (Aerodyne) for an excellent and informative write-up on the subject.

I approached a company called "Tiranti" on the recommendation of my dentist's mechanic. They deal in all kinds of moulding materials and its probably worth while getting hold of their "Sculptors Catalogue" (free), "The Polyester Resin Booklet" and "the silicone rubber booklet"(About 2 to 3 each).

The shop & mail order dept. are at:70, High Street, Theale Reading Berks RG7 5AR Tel:0118 930 2775 or the London Shop is at 27 Warren Street W1 ( just off Tottenham Court Road) 020 7636 8565

I have no connection with them !


Aerodyne 15th Jan 2010 8:37 pm

Re: Copying a control knob by moulding
Yes, Joe... I know Tiranti. A long established company. Years ago I dealt with them when buying tools and materials for school casting and moulding (sand moulds, aluminium casting). Those days - late 1970s - they had a very full range of goods and a thick catalogue. I imagine their ranges will have reduced due to the trimming of educational budgets plus the far-reaching effects of punitive safety laws.
I think that buying from them should be pretty safe!


benjamin77 16th Jan 2010 8:55 pm

Re: Copying a control knob by moulding
I have an Ekco U29 that is missing the volume knob. When you cast a repro knob is it smaller than the original ?

Aerodyne 17th Jan 2010 11:38 am

Re: Copying a control knob by moulding
Ben, some shrinkage is unavoidable and of course, it shows more the smaller the original knob is. The shrinkage occurs most during the casting process but adding fillers to the resin does, apparently, reduce the effect.
Much depends upon the final spacing of the knobs as, well apart, slight differences are not as apparent as wehen the knobs are close together.

I should also point out that Alginate isn't recommended for use with resins but when only one or two castings are required, I find it works well. It does tend to 'sweat' water and GENTLE application of warmth to the mould immediately before pouring the resin is one way to minimise water ingress.

The best way is of course to use hot-melt materials when copying heat-resistant items such as Bakelite knobs, or alternatively Latex, but these are much more involved and time-consuming and also rather more expensive.

Mike Phelan 17th Jan 2010 12:48 pm

Re: Copying a control knob by moulding
This is an excellent and informative thread, Tony, so I've made it a Sticky (!)

PS: Moving it to Hints, Tips and Solutions.

Dave Moll 17th Jan 2010 5:01 pm

Re: Copying a control knob by moulding

Originally Posted by Aerodyne (Post 299867)
some shrinkage is unavoidable and of course, it shows more the smaller the original knob is.

Would it be possible to coat the original knob with anything to oversize the mould slightly to compensate for shrinkage?

Aerodyne 17th Jan 2010 7:02 pm

Re: Copying a control knob by moulding
In theory, I suppose it might be - but you'd run up against loss of detail and the problems of heat damage to the 'coating' if moulding with meltable compounds and the difficulty of removal of the extra layer from the original afterwards.
The amount of shrinkage isn't that great, usually. Many moulding and casting processes have a built-in degree of enlargement to compensate for shrinkage. In the case of radio knobs, provided the shrinkage is even over a batch of mouldings, it won't be important.

Plastics will sometimes continue to shrink slightly for years; the American Catalin castings can be found to have quite a large amount of shrinkage, which may be the result of poor storage, UV light etc. Bakelite doesn't seem to suffer from the same problem, though it can gradually - almost imperceptibly - distort over the years.

Billy T 8th Jun 2010 11:19 am

Re: Copying a control knob by moulding
2 Attachment(s)
I have an EKCO SW86 to undergo restoration and it has one knob missing. I can't see myself finding a replacement knob (or set, see below) so I am thinking of casting a replacement, but perhaps I am missing something here? After twice reading through all the very comprehensive advice, I see a number of alternative materials for making a mould, but I can't see any recommendation on the preferred material to actually cast the knob itself. There is mention of fillers for colouring but they are not detailed either, and as a novice in this area, trial and error does not appeal. The Resin Supplies link seems to be non-functional at present.

So, my question is, what is the most successful (recommended) casting medium to use, and what filler or colourant has been found to give the best match for dark brown bakelite? As you will see from the rear image below, this surviving knob has a bad crack across the middle as well. It will probably be ok if the surrounding channel is cleaned and filled with araldite, but for colour matching I'll probably have to make three.



Aerodyne 8th Jun 2010 11:51 am

Re: Copying a control knob by moulding
Amended link. More to follow on casting...

Aerodyne 8th Jun 2010 12:00 pm

Re: Copying a control knob by moulding
Hi Billy

The reason I didn't specify a particular type of resin is because the best approach is to read the maker's literature via the web sites. There are many sources for casting resins and related materials.
However, I can tell you that I've used Isopon liquid resin (from car accessory shops on the high street) with success. It is best to use a very small amount of a filler material to modify the 'feel' and the opacity of this type of resin. Fillers can be talcum powder or even a little plaster of paris (or Polyfilla). Experimentation is essential before using a delicate mould.
As for colour, pigments are available from the sources mentioned and many others too. I have found that the best 'brown bakelite' effect comes from a mix of red, black and yellow colour. I've used ordinary paint - Humbrol, in fact - for this, but I have to warn that there is a definite limit to the amount as more than a few percent of paint to resin creates problems with setting and strength of final casting. Powdered pigment designed for the job is to be preferred.
Hope this helps a little.

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