Making rubber

[exvalvesetdabbler]

Hi Folks,

Just thought I would share my experiences molding bushes and meter cushions in polyurethane.

The meter cushions that I have are sagging and disintegrating, they were barely useable 20 years ago when I tidied up the instrument console.   Not to be put off by what seems impossible I have tried to come up with a solution and I'm too tight to buy the right cushions the for job under the suspicion that NOS stuff might not have much life left in it any way  and on one day there will be none around.

I can see why Mr. Honda did it the way he did - single piece to assemble on the production line and time is money.  My time is free and I can spend it drinking beer,  staring at the telly or casting rubber bits.

The ring that the meters sit on is to provide a cushion and stop the meter bowls filling up with water.  How many clocks has anyone pulled from an old bike and found the lowest part of the meter case crusty despite there being a drain hole. The rest of the OEM cushion has some volume lower down to center up the clock and provide additional cushioning.

Job #1 mold a bug rubber ring


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[exvalvesetdabbler]

Job #2 mold some new mounting grommets and turn out a few collars in stainless.

[exvalvesetdabbler]

The sides and bottom can be padded out with 4mm sticky backed neoprene strip.

I'll post more on this in the next day or so if there's any interest

[JamesH]

This is brilliant - I’m definitely interested in more details of how you approached this, what materials you used etc…thanks for sharing!

[exvalvesetdabbler]

Still experimenting, but here's what I have so far.

I have started off using Shore A50 hardness rubber for the meter cushions and we could probably go a bit softer for these. I  prototyped using some left over bath sealer silicone but that was tricky to de-mould without tearing and a bit soft. 

The mounting grommets are not yet fully cured, but my gut feeling would be a Shore A70-75  for these as well as the tail light mounting grommets. Shore A70 is the hardness of a standard nitrile O ring.  Once fully cured I'll assemble and post some pics.

The resin I used was  Black Poly PT Flex 50 Fast Cure - Shore A50 Polyurethane Casting Rubber, mainly because it was available in small quantities.  The molds were machined from scrap bits of plastic lying around.  People who do this on a regular basis would first make a silicone mold of the original part and then use that to cast numerous copies in whatever resin they choose.  There is shed loads of stuff on
u tube how to do this.   The black pigment is in a separate tiny pot and you need to mix that into the resin before adding the activator.  The gel time with this stuff is about 5 minutes so you have to work fast once the ingredients are mixed.

I have spent a lot of time in my career  molding in polyurethane for jointing under water cables and gone to great lengths to ensure that the resin bonds to the cable jacket and connector using specialist primers as needed.  Trying to make sure the resin doesn't bond to the mold is a new challenge for me.  So far I have tried silicone grease or Vaseline dissolved in white spirit, a few failures using Vaseline where I hadn't let the white spirit evaporate fully and that just got into the resin as as a grease bubble. Silicone grease is quicker a you only need to wipe on the thinnest of film.

Oh - and its messy!

Pics of molds tomorrow so long as my hands are clean enough to pick up my phone.

Regards
Dave. 

[K2-K6]

Interesting work Dave, and very useful for some of these component.

The silicone sealant one you tried .... we did mould some of this successfully back in the 70s for bespoke support on such things as brake lines tbat were non std fit. Broadly speaking "cotton reel" shape, plus some rear light mount etc.

We added acetic acid as catalyst/hardener and getting  it to set. Either nylon or ptfe turned on lathe to form mould, release successful.

Some are on a friend's Honda still. Think they're OK but can check.

Maybe of use in this project.

[exvalvesetdabbler]

Mold for the rubber ring.  Machine a groove in a bit of scrap polypropylene sheet.

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[exvalvesetdabbler]

Mold components for the rubber bushes. In the top of the white piece there is a hole to allow excess resin to escape when it is tightened down.

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[exvalvesetdabbler]

Clock assembled to the bowl.

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I am having a look to see what can be done in silicone but hardness options look limited. UV stability is supposed to be excellent though.

The long term stability is still a bit of an unknown with PU, compression set and UV stability,  probably no better than nitrile rubber so I will assemble using an old clock and leave it in the glasshouse over summer and see what happens.

There are some suppliers offering a UV additive but not in the small quantity I would need.


Regards Dave.