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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 17th Feb 2019, 10:30 am   #1
Ti Pwun
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Default Casting a set of Vidor knobs

I thought some may be interested in seeing how I'm reproducing some replica knobs for knobs for Vince (FIXITNOW), so I started this thread.

The sample knob Vince sent to me was slightly damaged so I first needed to 'repair' that damage using clay to fill in the missing plastic. I used foam core board and a hot-melt glue gun to build a rigid casing. I then softened the clay in the microwave oven and built up the clay to the centre line of the knob.

I then used a wood screw and a steel rod to press some registration marks into the clay. As you can see, I added clay to build up the area where the plastic is broken.

I sprayed this half with a mould release spray, degassed some Silicone and poured the first half of the mould, before putting it in the pressure pot overnight.

Once dry, I removed the knob to reveal the first part of the mould. This worked very well. A very smooth, bubble free mould.

In the final image here I have adjusted the contrast so that the actual damage to the knob can be seen against the first half of the mould. I again used clay to 'repair' this damage. I also filled in the indicator mark indent because not all of the set of knobs will need this indent. I can drill that later for those knobs where an indicator is required.

With that done, I gave it a blast with mould release spray and poured the second half of the mould, before putting this in the pressure pot at 40 PSI overnight.

Later today I will continue the process when the Silicone has cured. With the two halves of the mould fitted together I will first need to determine where best to cut my resin pour spout into the Silicone. This will be somewhere in the underside of the knob where it cannot be seen.

One thing worthy of note is that this knob is extremely thin, the plastic is old and brittle, so if anyone is thinking about making their own reproduction knobs in this way, you MUST soften your clay so that you can easily shape it without applying much pressure against the plastic. The plastic WILL break if you do not take great care at this point.

To be continued...
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Old 17th Feb 2019, 6:26 pm   #2
Ti Pwun
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Default Re: Casting a set of Vidor knobs

FAIL ONE:

Well, this initial mould was a definite fail so I'm not going to pretend otherwise. I think my problem was that I didn't use enough mould release spray, or perhaps I didn't allow it to dry for long enough.

Try as I might, I could not find my separation line at all. When a small line eventually started to open up I assumed I had found it so I separate it there.

As you can see, this was way off. The two halves were well and truly bonded together.

So, as far as the Silicone I used and the method of degassing and pressurising in the pressure pot, it worked perfectly - I don't see a single bubble where it matters.

Clearly, I need to start again with this, but I'm confident that when I sort out the release spray thing, it will work out well. Maybe I need to get a better quality release spray. I used one at 6 per can, rather than the higher priced recommendation at 14 per can, so I will look into that.

Watch this space, as they say. No point skipping this failure to show a perfect mould because that isn't what I got, but even though this was a failure, it was a partial success and I now know that the mould is good other than the fact that both halves became bonded together. I'll go away and work on that and I will return when I'm ready to colour some resin and cast a knob.
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Old 17th Feb 2019, 6:40 pm   #3
merlinmaxwell
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Default Re: Casting a set of Vidor knobs

I admire your dedication.
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Old 17th Feb 2019, 6:56 pm   #4
Ti Pwun
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Default Re: Casting a set of Vidor knobs

Failure is a part of the process, as we in the real world know, where a quick edit to something perfect isn't going to help anyone, so I don't want to skip that.

I will get it done and if that prevents someone else making similar mistakes, that's exactly the point of uploading this. At the end of it I'll be able to create a simple tutorial to put on the website for anyone who wants to give casting a go themselves.

Thanks for the comment.
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Old 18th Feb 2019, 2:21 am   #5
batterymaker1
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Default Re: Casting a set of Vidor knobs

That's sharp. Two part molds are hard to make.
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Old 18th Feb 2019, 8:48 am   #6
FIXITNOW
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Default Re: Casting a set of Vidor knobs

A big learning curve
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