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Grooves in rubber mold

Can anyone tell me why I’m getting grooves in my molds? And how to fix them?
There are lines and grooves where I can see the different layers of packing/rubber. I can see in both the mold and the wax model.

In particular, I have a size 11, square, wide band ring I am trying to make into a successful mold. Twice I have used Castaldo Gold, 7 layers. I took time to cut pieces to make sure the pack is tight. Both times I put in vulcanizer at 307 degrees (as close as possible with readout markers of 300, 325, 350… The first on for 50 minutes, the 2nd 55 minutes, thinking it didn’t get hot enough.
Any suggestions? Info?
Thank you!

HTemp dial not accurate. Not compressed enough. Not pre-heated plates.
I drill holes in my mold frame and insert a temperture probe to monitor. Not into the rubber area but along the side of the frame. I have a multimeter that will measure temp with an inexpensive thermocouple
Regards RLW

Thank you rwade,
and sigh.
Just bought this vulcanizer. What a headache I face with returning (if possible after a few months) and finding another I can afford…and one that will work right! And I don’t have skill set, knowledge or inclination to drill holes and check temp…
The search and determination to get this right continues…


Give this a try before you start drilling holes etc. Purchase a cheap ($10) digital kitchen thermometer the type with a metal stem. ‘Close’ the platens so they are holding the stem.

Let the platens heat till you see they have peaked and the temperature starts dropping. See what temperature you have from both platens on the high end of the temperature swing. Adjust the temp to around 300-310F for ‘both’ platens.

We don’t make a lot of molds anymore so we purchased the Economy Vulcanizer from Rio which has only “one” temp control for both platens. The high temperatures on the platens are not even close but it works OK for Castaldo ‘Gold’.

A vulcanizer with only one control dial for both platens will not work for a more sensitive mold material like Castaldos No Shrink Pink which needs the top and bottom platens to be calibrated ‘very close’ to the same temperature.

Also when you are spinning the wheel to clamp the mold… lower the top platen just till it stops and is touching the mold frame and then give it a ‘half turn’ more. You will have to experiment with how much to turn it but you want to be able to repeat the process close to the same pressure and temp each time once the molds are working for you. Also with a mold frame 1/2 inch deep I use 6 layers of Castaldo Gold. You can pm me if you need more details. Hope this helps…

Greg Miller

Dear Eblitz,

You’ve gotten a lot of good advice so far, but I think I might have something to add.

I’m not sure what you mean exactly by " grooves" in the rubber, but I suspect you are seeing the lines where each piece of rubber is laid down in the mold frame.

If that is the case there are two possible reasons.

One is that the rubber is old and has started to cure in the box. If that’s what happened it will not melt and flow into one solid mass as it should normally.

The other common reason is that you may have under-packed the mold in an effort to save a bit of rubber. The rubber needs to be under pressure in the mold frame as it liquefies and then cures. I’m not talking about the pressure applied by the press, but the pressure that develops inside the mold frame.

If you don’t have enough rubber - even 1/2 piece too few will do this --the rubber will melt and then cure, but you’ll see traces of the separate layers.And perhaps areas of no-fill against the model.

Better to waste a 1/2 piece of rubber than an entire mold . .

Michael Knight

Maybe a foolish question… but did you remove ALL the paper/plastic from the sheets of rubber prior to packing. And were they clean?

Please excuse any typos-- curse my clumsy digits…

Thank you, Greg.
I’ve got the economy ARBE with the one dial. It looks very similar to the Rio Grande economy, Dura Bull model. I will try this with my kitchen thermometer.
I spin the top platen down to resistance for about 3 minutes, release and then tighten.
I think I am putting enough layers in. Believe me, I’m going through the sheets like crazy. If looks flush I add a 1/8" layer. I will contact you via PM if I have more questions, Thanks again

Oh yes! No paper or plastic between. And very clean :slight_smile: Thanks

Thank you Michael @CASTALDO.Yes, I am speaking of the lines between the layers. Unless Rio keeps old mold material around to sell at this time, this is new material. I’m not skimping on the mold material layer wise. I’m also packing in material all around the piece and tamping it in until it sticks in place. I start with cuts outside the mold and then add the whole “sandwich” into the mold. This takes time and effort to get it to fit in, again tamping it down securely. I actually take a setting graver with a rounded tip to tamp it in, puncture it.


All of our dealers, even Rio, sometimes sell old rubber without knowing it. Boxes get pushed to the back of the shelves, new boxes are added and five years later someone grabs the old box. It happens.

Another thought – if your vulcanizer is running much too hot the rubber will cure before it has a chance to flow properly.

You might want to check out my " How to Test Your Vulcanizer" article on our website at


Thank you Michael. I did perform this test. The top plate averages about 5 degrees hotter than the bottom. Would that be enough to cause an issue?