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"Questions about vulcanised moulds" Topic

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one hundred eyes24 Oct 2017 3:06 a.m. PST

Hi, thanks in advance for any help. This forum has been a great source of information for me.

I am experiencing a few production issues and wondered if anyone can help out.

Firstly, my moulds appear to be slipping during vulcanisation. The top half is pushed backwards and causes the mould to be unbalanced and much thicker at the back edge. This then causes problems with the pewter flow as well as with venting etc. The vulcaniser's plates are level (I have checked) and it doesn't happen every time I create a mould, but it does happen most of the time now. How much rubber should be forced out of the can during the vulcanisation? I think maybe too much is coming out. Can I use a paper / cardboard gasket to help seal the can or is it time to purchase a new can?

Also, it takes quite a few spins to get my moulds up to temperature and this seems like quite a lot of wasted time and effort. I am using a Centricast and so each spin takes a little while to tighten and loosen the four screws. Does anyone use an oven or similar to heat the moulds up prior to spinning?

Thanks again for any help.

IUsedToBeSomeone24 Oct 2017 4:24 a.m. PST

There shouldn't be any rubber forced out of the can during vulcanisation…

I use a small oven to preheat moulds to 120 degrees C before casting


shaun from s and s models24 Oct 2017 4:28 a.m. PST

hi, we have been using a centricast machine for years and have little problems, yes casts can fail and i would not expect 100% good casts everytime, infact i always expect a small failure rate.
we also vulcanise our own moulds and some do and up thicker on one side, but mainly due to having several larger objescts on one side, nver been a real problem however.
moulds will always take time to get up to temp and that is part of the process and i have never preheated any moulds.
if you want to give me a ring i may be able to help
(01278) 780193
but i am out most of today but will be in weds

Master Caster24 Oct 2017 5:57 a.m. PST

Sounds to me that there is failure to seal the mold frame during the vulcanizing process.
If some of it is caused by large pieces then part of the raw rubber should be cut away on one of the interior surfaces of the mold and the ragged surface being sealed with lacquer thinner. This action will help these pieces to assist in the alignment and sealing of the mold frame (your term is 'can') during the vulcanizing process.
If the breaching always occurs toward the back of the mold press and mold frame you probably have unequal pressure being applied in that area during the pressing even if your vulcanizer platen plates are level.
The total pressing process takes at least ten minutes – nine if all the pieces being pressed are small. The first six minutes of vulcanizing are critical – and should be done in increments – whereby the majority of the cavities are formed. During this first six minutes you can relieve the pressure (click the jack to lower the platen) and spin the mold around. Then re-apply pressure slowly to check the seating all around the mold frame. The last four minutes should be where full pressure is gradually applied.
Good luck!

olicana24 Oct 2017 6:49 a.m. PST

If some of it is caused by large pieces then part of the raw rubber should be cut away on one of the interior surfaces of the mold.

All wise words indeed from Master Caster, though I can't remember using lacquer.

That's how I was taught to make them (years ago). Cutting into one half of the rubber to make an impression matching the general shape of that half of the master, into which the master fits I was making elephants, amongst other things but impressions were made for most of the masters so that the both halves of the mold were nearly closed and level before placing into the 'can'.

I was also taught to use the outcut bits of rubber to raise other surfaces slightly where necessary, so the moulding lines could be chosen rather than left somewhat to chance and providing the mold was kept clean during the process, and the edges of the raised bits were scraped down into place before both halves were sprayed with the release agent, the rubber all became 'one' in the 'can'.

Taking time for the vulcaniser to come to temperature and the importance of the incremental addition of pressure also rings a very loud bell.

Molds take a few spins to come up to temp when casting, all molds do (even drop casts ones). I'm not sure if there is a way around that one. I've never had to tighten screws though (sounds irritating); the tightening mechanism on the machine I used were clamps with centrifugally driven arms that tightened the clamps as the mold was spun the faster it span the more the arms were forced out and the tighter the clamps got, when it stopped the arms went slack and there was no clamping.

shaun from s and s models24 Oct 2017 11:23 a.m. PST

the centricast machine has thumbscrews for securing the plates, 3 on a 6in mould.
they were origianly m5 thread but i have replaced mine with m6.

MechanicalHorizon24 Oct 2017 2:10 p.m. PST

Basically what's happening is that your mold boxes aren't milled properly.

So the lid won't fit tightly when closed and under pressure, so the gap allows the rubber to squeeze out of the gap.

You can prevent this by using newsprint or aluminum foil covering the mold before you close the lid. That extra material will help seal the gap.

Or you can have new mold boxes made. Personally I hate using filler materials, since they tend to stick to the mold after vulcanization.

Personal logo Dye4minis Supporting Member of TMP24 Oct 2017 7:21 p.m. PST

Have you checked to see if your vulcanizer is sitting level? Often, the cause of this dilemma is that the lower half (when jacked up to meet the top plate) does not engage the "can" evenly due to the vulcanizer legs being askewed.

I always preheated my molds while they were inserted on pieces of cardboard. 10 minutes per side, flipped twice. The side to start and end with would be the sides you will embed your production masters in. Be sure to talc both sides to keep from sticking for the pre-heat.

Good luck!

Tom Dye

IUsedToBeSomeone25 Oct 2017 2:08 a.m. PST

I always top and tail my moulds with a circle of newspaper.

It gives you a surface to write on and something to read while casting…


one hundred eyes26 Oct 2017 4:05 a.m. PST

Fantastic information as always. Thank you all very much.

I have a mould in the Vulcaniser now and will see how it comes out. Trying a paper gasket.

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