I restore vintage wireless sets and need to replace some perished rubber thermionic valve (tube) retainers. They are like long (dogbone shaped) rubber bands (that have a hole in the centre for the top ‘pip’ of the glass to locate in) and a hole at each end to loop over hooks either side of the valve base.
The material is thicker round these holes than elsewhere and would be a perfect candidate for laser etching/engraving to achieve the different thickness profiles. They also have manufacturer’s logos moulded along the length that could be incorporated into the engraving process. Finally the overall shape could then be laser cut from the rubber sheet.
However they need to be stretchy and vendors of ‘laserable rubber’ (for ink stamps) tell me their rubber is NOT stretchy.
So my questions are:-
Has anyone used a safe stretchy laserable rubber that would be suitable for my restoration project?
Does silicone rubber laser etch/cut or will it break/split when slightly stretched?
I need something like rubber car tyre innertube 3mm thick but that I can safely laser cut/engrave.
Thanks in anticipation…
You could make a mold and then cast silicon in a shape like I imagine you are describing.
You could also make a logo insert.
Once the mold(s) is made you could make them in volume ;)!
The silicone molding material is easy to make for this.
There are various recipes but I have found: 1/3 corn starch 1/3 100% silicon caulk to work and the proportions are not critical some use 1/2-1/2.
However, it must be 100% silicon, if you smell vinegar then its the wrong stuff.
You can add dyes for color.
You can find the caulk in big box stores for 3-$4
I have made a number of parts like this and they are strong and stretch.
I make the mold out of glued layers of acrylic. I use acrylic because it needs no release agent and leaves a flat smooth surface.
The object’s shape for the mold can be cut by hand, cnc, or laser.
Mold (acrylic) Construction
The bottom layer is usually just a solid base
The middle layer(s) is a cutout of the shape being molded.
One or more middle layers can be added if the object needs to be thicker than the acrylic.
Multiple thickness acrylic can be used to build up the cavity to the right depth
The layers are glued together with CA.
The last (top) layer which is not glued to the other layers has air holes in it to allow the silicon to express out excess material and air. The entire mold assy is bolted or clamped until it sets which is very quick (15-30min).
Note: the air hole(s) can be placed in any layer(s) on any of the side(s) of the mold based on where you want the excess to squeeze out for the easiest and most invisible removal.
Add the mixed silicone into the mold area overfilling it. Press on the top expressing out the silicon.
Thanks for your reply and thoughts on my problem.
I attach a picture of a broken original and a couple of 3D printed ones in TPU which is sold as ‘high elasticity’ but at 2mm thick I can’t make it move lol!
As I hope you can see from the picture the bands are 2mm thick with thicker 3mm raised areas around the holes at each end and the slot in the centre.
[quote=“daveb, post:5, topic:81731”]
What do the results of your recipe look like?
A: It easily matches the mold surface. If the surface has texture you will need to use a release agent.
What does the corn starch do?
A. I believe it speeds up the silicone’s drying process. @NedMan will know the chemistry
Do you think your recipe would be strong enough to stretch about 1/3 longer than at rest?
A: I am sure the strength vs stretchability depends on the thickness but at 3mm I would expect it to stretch easily
I would make a test mold from acrylic the width and thickness you need then test if it has the strength/stretch properties you need. If so move on to making a 3D printed mold.
Hi there … me again,
So I printed a 3d mould in the shape of the vintage valve (tube) retainers and made some corn
starch silicone rubber compound.
I pressed it into the mould and after a while prised the moulded shape out of the mould and it looked great.
Now to the chemistry question?
What determines the Shore A hardness of the silicone rubber, and does the corn starch raise or lower the Shore A value?
I ask because whilst the shape of my moulded valve retainer is great, the composition of it is too soft and limp
Can I add something to the ‘mix’ to adjust the Shore A value?
Is there a supplier that stocks different Shore A grades of silicone rubber compound?
What durometer do you think you need?
Do you know what the durometer of the original piece is?
I think getting an estimate of the hardness of your application will be important.
These aren’t to $$$, mm I may get one.
Looks like a good place to research is under “mold making” as they use different hardness silicon for various molding applications.
Interesting video on hardness in mold making:
This stuff runs about 30A: " A 30A hardness is in the soft to the medium-soft range, between the flexibility of a rubber band and pencil eraser"… reads like the range you are looking for. A rubber band is about 25A.
I have used these folks stuff before, high quality and $$$.
A search on “increasing silicon hardness” pulled up a lot of references.
Adding silicated materials, like silica aerogel, probably interact with silicone rubber at the molecular level to alter the properties. Adding other materials like, carbon black, could alter the macro properties, but it’s unknown to me (polymer chemistry isn’t my strong suit).
I would also suggest looking at mold making suppliers to see if you can an appropriate rubber.
Wow thanks guys,
The RTV video was great and really gave me an idea of what Shore hardness I am looking for.
I will follow up the links you sent and see if I can actually get some samples.
I will have to try to try to source these or similar products here in the UK but at least I now have a much better understanding of Shore A hardness.
You have been most helpful