Trying to make my first silicone rubber mould I mixed and poured the first 110 grams of rubber into the mould box and in/around the object to mould. The first pour spread over a layer of soft filleting and filling wax. I continued with another 3 or 4 110 mixes of rubber to fill the mould box. When I removed the mould box after a couple of days at room temperature all the rubber had cured except the first layer which had been in contact with the wax. This remains soft even after a couple of more days in the air.

Is the wax the cause of this not curing or something else? If I should not be using a layer (5mm thick) of wax to pour on to is clay ok for this purpose please?

Regards

Hi Ray,

Thanks for the post, this is a very interesting problem to be because I was not aware that our yellow wax could inhibit the cure of the Addition Cure Silicone Rubber although it sounds like it could be the case from what you're describing. I have used the wax in conjunction with the Addition Cure Silicone Rubber myself but it's only ever been very small quantities of the wax which I've used to help secure polypropylene sheet sidewalls down onto the baseboard so I guess the interaction between the wax and the silicone would be minimal and perhaps wasn't enough for cure inhibition to occur. I will get into the lab this morning and conduct a test in two sample trays, one with a yellow wax filling the base and the other without. This will re-create your conditions. It will take around 12hrs to get a definitive answer which if necessary we'll use to update our technical information.

If the wax is the problem then I would think clay would make a good alternative. There's nothing on the TDS for the silicone to suggest that clay would inhibit cure but we do know that just about any material with a high sulphur content will inhibit addition cure silicone so I would strongly recommend conducting a small test, much like the one I described above that I'll be doing on the wax.

I'll keep updating this thread as answers develop.

Best regards, Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Matt

Many thanks, I await the results with interest. Obviously I am holding off pouring any more silicone rubber until resolved due to the cost.

Regards

Ray

Hi Ray,

Sorry for not posting the response on Friday but the cure took a little longer than anticipated (cool room where we did the test). What we found was that the Addition Cure Silicone cured fine in contact with the yellow wax; there was no inhibition at all.

• Incorrect catalyst ratio
• Not thorough mixing / insufficient mixing of the catalyst
  
• Something else caused the cure inhibition (some other material present on the bottom layer only)

It really can't be anything else. The subsequent pours cured fine so we know the silicone and catalyst are OK and that the ambient cure temperature is OK. We've tested silicone curing against yellow wax and that's fine too so it can only be one of the above causes.

I hope this helps.

Matt

Matt Statham
Easy Composites / Carbon Mods - Technical Sales

Thanks Matt

I accept your findings and will try again. It was my first mix but I did the same for the next 3 or 4 mixes which were fine so I'm surprised.

Regards