Replicating vulcanised rubber bands?

[davebullock]

How did you find PF?: Via Google search.

Hi everyone,

I am an enthusiastic vintage wireless restorer/hobbyist attempting to preserve rare and unloved wireless sets from extinction.
There seems no limit to the effort I will go to in attempting to faithfully reproduce missing parts in my restorations.

For instance I will carefully remove the workings from 'leaky' capacitors and hide modern components inside, rendering
them undetectable from original, but now working.
I am really struggling with a 'materials' issue and hope someone can help me solve my problem?

I have a 1930's portable valve (tube) radio that has black moulded rubber valve retaining bands.
These are quite firm (but stretchy) rubber that have holes at each end to hook over lugs either side if the valve holder.
Stretched over the top of the valve they firmly hold the valves in place, with a slit in the centre for the 'top cap' of the valve to protrude through.
These rubber retainers are all perished and useless and I have been attempting to make replacements.

I have tried 3D printing with alleged 'elastic' filament, I have printed a 3D mould and tried various silicone rubber compounds.
The results have been either too hard (and not elastic at all) or too soft and split when under tension.

What I need is a way to engineer original vulcanised rubber to produce a material like bicycle / car tire inner tube.
The difficulty is I need to mould it as the holes at each end and the slit in the center has a thicker reinforcing annulus.
(See attached)

I have scoured the web and failed miserably to discover how to reproduce DIY moulded vulcanised rubber. I can make the required aluminium mould, I can make the heater but where do I get/ how do I make the moulding material?
Thanks in anticipation.

Dave

 

[berkeman]

Welcome to PhysicsForums Dave! It's great to have you here.

Paging @dlgoff who is one of our best electronics restoration specialists...

https://www.physicsforums.com/threads/beauty-of-old-electrical-and-measuring-things-etc.755959/

 

[Baluncore]

https://en.wikipedia.org/wiki/Nitrile_rubber#Production
For higher temperatures; https://en.wikipedia.org/wiki/Viton
google
'nitrile rubber casting supplies'
'viton rubber molding'

 

[dlgoff]

Paging @dlgoff who is one of our best electronics restoration specialists...

Thank you Sir. That's a very nice compliment.

 

[Vanadium 50]

I think I know the material you want - it's kind of like leather, kind of like rubber. It has a lot of flexability but not so much stretch. You can't stretch this to twice or three times (or more) of its length like a rubber band - just a few mm. Home Depot has sheets of material like this that may work.

To be honest, as a few off, I'd be thinking of taking a sheet and cutting or milling it to the shape I need. Milling will be horrible, so I'd be looking to pay someone to do it. If I were cutting it and doing it myself, I'd do the holes first, and then the notches, and there would be a lot of fixturing.

 

[dlgoff]

How did you find PF?: Via Google search.

Hi everyone,

I am an enthusiastic vintage wireless restorer/hobbyist attempting to preserve rare and unloved wireless sets from extinction.
There seems no limit to the effort I will go to in attempting to faithfully reproduce missing parts in my restorations.

For instance I will carefully remove the workings from 'leaky' capacitors and hide modern components inside, rendering
them undetectable from original, but now working.
I am really struggling with a 'materials' issue and hope someone can help me solve my problem?

I have a 1930's portable valve (tube) radio that has black moulded rubber valve retaining bands.
These are quite firm (but stretchy) rubber that have holes at each end to hook over lugs either side if the valve holder.
Stretched over the top of the valve they firmly hold the valves in place, with a slit in the centre for the 'top cap' of the valve to protrude through.
These rubber retainers are all perished and useless and I have been attempting to make replacements.

I have tried 3D printing with alleged 'elastic' filament, I have printed a 3D mould and tried various silicone rubber compounds.
The results have been either too hard (and not elastic at all) or too soft and split when under tension.

What I need is a way to engineer original vulcanised rubber to produce a material like bicycle / car tire inner tube.
The difficulty is I need to mould it as the holes at each end and the slit in the center has a thicker reinforcing annulus.
(See attached)

I have scoured the web and failed miserably to discover how to reproduce DIY moulded vulcanised rubber. I can make the required aluminium mould, I can make the heater but where do I get/ how do I make the moulding material?
Thanks in anticipation.

Dave

I would probably use something like a rubber strap for a hard hat.

Image compliments of https://www.jharlen.com/p-10638-rubber-strap-for-hard-hat-headlamp.aspx

 

[davebullock]

Hi,
Thanks for your swift replies and thoughts.
The flexibility of the finished straps have to be similar to a strip of bicycle innertube or thick elastic band. Strong, stretchy and black.

As you see from my pictures they are most likely Vulcanised India Rubber and my reproductions have to look concourse. My restorations have to end up looking as original as possible as some of these wireless sets may be the last surviving example. My reproductions must have the correct shape, elasticity and include all the inscribed lettering that is on the originals (patent number/manufacturers logo etc)

I have seen that there is a process called 'compression rubber moulding' in which an 'ingot' of uncured rubber is compressed under heat into a mould, forming the required shape and 'curing/vulcanising' the rubber in one operation.

This is most likely how these were originally manufactured I just need an expert who can tell me the secret 'spell' to DIY this operation :-)
Thanks
Dave

 

[Baluncore]

You can buy the material needed for Hot Vulcanizing.
https://www.nilos.com/en/products/industrial-rubber.html
Click on L1000. See the instructions.

 

[davebullock]

Hi Baluncore
I tried following your link but the instructions are for many 'cold' curing products.
The only mention of L1000 seems to be for splicing conveyor belts which would only use a thin layer of the 'vulcanising' solution.
There doesn't seem to be any data on Shore hardness or elasticity and if it is for repairing conveyor belts then I suspect it will tend to be 'flexibility' rather than elasticity that is required?
I will drop them a line and see what they suggest.
I still believe I need some insight into hot compression rubber moulding though?
Thanks
Dave

 

[Baluncore]

Google “pre-vulcanised natural rubber latex”.
I have seen it used. Put thick liquid in a mould. Close the mould. Bake it in the oven for a time at a specified temperature. Let it cool, then open mould and remove article.
Also see;
https://www.timcorubber.com/rubber-resources/rubber-molding-process/

 

[davebullock]

Dear Baluncore,
Wow! pourable would be ideal, though I don't quite know how to get my wife out the house whilst I bake it ...LOL! I have emailed a few UK links brought up with the Google search you suggested. Pity the Timcorubber people are in the US?
Thanks once again for entertaining my almost impossible quest.
Dave

 

[Matcon]

If pourable and longevity are important, may I suggest that you take another look at silicone elastomers?

I am guessing from your application description that you are looking for about a Shore 30A hardness, that has a typical elongation to breakage of 300%. You can get silicone in anything from a soft gel insole to a hard hat

There are some really excellent online resources for moulding silicones and, in my experience, suppliers are generally technically clued up and willing to advise. I'm not in the UK, so I can't suggest a specific supplier. I use an imported American product.

You may need to experiment with a carbon filler if you want a matt black finish.

Also, you won't have to chase your wife out of the house; they cure at room temperature