MIMF

I need to copy some old plastic knobs for early clock radios and electric alarm clocks. Functionality is not as important as replicating the original appearance. Has anyone here done this with guitar knobs? What material would you use for the casting and how do you color it? Any information would be helpful and much appreciated.

Al,

You need to go to one of the places that sells silicone casting materials and stock up on all of it. It's not that cheap but well worth it if you need to make multiples in the tens to hundreds.
http://www.smooth-on.com is a good place to start
Also http://jgreer.com has the basics.
http://tapplastics.com also has some really basic videos.
Youtube can probably answer all the rest of your questions.
For best results you'll want a small vacuum pump that can pull 29" Hg to get all the bubbles out of the silicone and urethane.

Getting the color just right may be the trickiest part of the whole deal.

There is a really good thread in the old library on casting techniques.

http://www.mimf.com/old-lib/robinson_plastic_parts.htm

That's as good a description as you're going to find I bet. If I were doing knobs I'd be tempted to cast them solid and machine the backs of them after the fact but it may be totally unnecessary if everything registers correctly

As a suggestion, you might take a look at the ALLPARTS catalog, as they have a LOT of knobs to choose from. Their website is easily found by searching for "allparts music".

Thanks, I remember the old discussion now that I see it. These are fairly small, about 3/8" diameter by 1/2" tall so maybe I could do it without the vacuum. I'll just have to try and see. I will check Alllparts but I would be pretty surprised if they had knobs for Telechron alarm clocks. This plus somehow coming up with a nest of 8 tuned glass bells should keep me off the streets for awhile. The glass bells will be a challenge but I have found a couple of local glass blowers that are willing to give it a go. I'll post a new discussion on this as we get into it. Happy Holidays to all.

Al, I'm sure you know already but there are web businesses that are exclusively concerned to replicating old radio knobs. They already have hundreds of molds and they sell very nice looking knobs for what seems to me to be a very reasonable price. Moreover they seem quite willing to make new molds of whatever part you need.
http://www.renovatedradios.com/parts.html
http://www.antiqueradioknobs.com/index.html
http://bensradioattic.com (Scroll down to see the hundreds of old knobs for sale)
There also seem to be several forums devoted to old radios when you might get more specific info.

One interesting aside to this knob business that I just discovered and might be of interest to luthiery is that the old Zenith radio knobs were made of wood but using a mysterious proprietary process, Zenith managed to mold a letter "Z" in to the top of the wood. I'd love to know more about this as I love cases of "lost technology"
You can see examples of these knobs here here:
http://antiqueradio.org/art/Zenith12A5831.jpg
The process supposedly involved steam and a hot press to permanently emboss the design into the end-grain. It looks almost laser carved to me.

That does not look pressed. Did you happen to see Chris Paulick's discussion on carving a peghead here: http://www.mimf.com/phpbb/viewtopic.php ... ing#p20997
The effect looks very similar.

Chris does beautiful work there but I'm not sure RCA would have had the time, the resources or the skill to accomplish that on the many thousands of knobs they must have produced. I agree the RCA knobs don't looked pressed but they must have add a way to etch the wood mechanically, perhaps with sandblasting over a mask or some vibratory mold cutter that could erode the surface. A sort of EDM for wood?

Hey all – I will have to update with some new molding trick and tech. I have been molding my own tuning key heads, to match my pickup covers, knobs, switches, etc. You know how I am about that. I will upload a new series of molding with material specs and tips.
dr

David, that would be great if yoiu could post some new material. The problem with some of the repros I see is the color. They look like new knobs and just don't blend in on older objects.

Well replicating color is nearly impossible because every batch of the old plastic was probably different and will age differently depending on climate, smoke, ozone, UV exposure etc.
I'm guessing that there is also color-shift from the liquid mix to the solid cast part. It's going to take a lot of trial and error to get it perfect and perhaps some final fine-tuning with salt dye or coffee after the parts are scuffed to age them.
Just look at how terrible the "ivory" colors on reissued LP parts are when thousands must have been spent trying to get it right.

Alumilite "tan" looks more ivory than most ivory colors i have seen.
http://www.alumilite.com/ProdDetail.cfm ... %20Regular

Here are some knobs and a modified P90 cover in the Ivory.

hmm... having problems with the attachment.

here it is resized.

That's lovely David,
I agree Alumilite natural looks pretty good. Where would to go for the silicone mold material? Is the tin cure stuffgood enough for these types of molds?

I use silicone from polytek.com - mostly the Platinum based 73 series. Plat based silicones work best with urethane resins and foams over the Tin based variety.
It holds up better in the long run, you don't need mold release when pouring urethane resins, and if you store the molds in sealed plastic boxes, the molds last for years.

I have been using this one. 60 durometer, pretty stiff so smaller parts won't need a mother (support structure)
http://www.polytek.com/index.php?dispat ... ory_id=422

As well as the Alumilte tan for "Ivory" and opaque color parts, I also use Polytek's Easyflo Clear for transparent and tinted translucent parts.
http://www.polytek.com/index.php?dispat ... ory_id=247

Here is the Polytek PlatSil 60 (in blue) about to pour the second half of a 2 part mold from SLS masters.

Thanks for the links! Not having to use release agent seems like a huge plus.

No Mold release. I made some molds off of those chrome plated plastic P90 soap bars you see on ebay. The silicone mold came out so glossy, that if I had to use any type of release, the release would have shown up on the part.
You must use mold release when you are molding silicone onto silicone. I use Synlube 531 non silicone release agent. It is basically vaseline in a spray can.