Wax casting: rubber mold vulcanization

Neither is it. Vulcanization is caused by heat. (Vulcan, god of volcanos, fire, etc.)

The pressure is used to ensure, as the rubber expands with heat, that it is forced into and picks up the finest detail from the master. The finished mould will usually not be precisely the same gross size as the master when it cools (although very close), but all the detail will be faithfully reproduced. Without vacuum degassing prior to curing, it's difficult to exactly reproduce tiny details, even with clamping and heating.

RTV molding compounds work just as well as heat-vulcanizing types. It's a matter of selecting the compound for its properties relative to what material you wish to cast. Some of the RTVs are good to 500F for short periods of time.

LLoyd

LLoyd:

Ok that makes sense. So the vulcanization machines I saw are basically a press with a heating element and vacuum? Trying to understand what each piece of equipment does and why.

For my immediate need RTV should work just fine. It's purpose will only be to be the master mold for wax castings.

Thank you very much for your reply.

- Ben

Pourable mold-making silicone, or a tube of gasket gunk? I use two different silicones for hot glue release beds. One is an (almost) RTV, curing at about 140F in fifteen minutes, the other requires about 250F for eight hours. The slower cure material is stronger, more elastic, and less prone to cuts growing from abrasion, but rated at about the same working temperature as the other, once cured.

Both are liquids about the consistency of honey, and designed to purpose. Neither can withstand sustained heat above 520F (Which, when you think about it - over the temperature required to ignite wood/paper - is pretty damned hot.

BTW... when I say "sustained", I don't mean days/weeks/months, but minutes. Both materials begin to lose mechanical properties at above 450F, and degrade in a few hours at 500+.

LLoyd

Yes, it is possible using C-clamps and an oven. Procedures vary, but in general, the item to be molded must be stable at vulcanizing temperature, and compatible with the mold material. The vulcanizing material is cut up and packed around the item over-filling the flask. As the vulcanizing temperature is reached, the clamps or press is tightened a little at a time forcing the material into and around the item. Air and excess material is expelled through vents or seams in the flask. When fully vulcanized and cooled, the finished mold can be cut to remove the item. The RTV type is often used where the item cannot withstand the vulcanizing heat. Most RTV silicones require degassing in a vacuum chamber, but there are some which don't, meaning very little equipment is needed.

All you need to cure the rubber is heat. At work we cure about 3 million pounds of rubber per day. The mills, mixers and extruders are all temperature controlled, if the rubber gets too hot for too long, it will cure in the machines. I saw a small piece of rubber that was layed on a curing press and cured without pressure, it was spongey inside. Heat and pressure will help the unvulcanized rubber to flow around your part, picking up the detail. The time to vulcanize is experimental and will vary alot with the thickness of rubber and other variables. The ideal method would be to find out what temperature the rubber needs to be and use a temperature probe to monitor when it gets there. For temperature controlled preheated molds and platens at 360 deg. F, cure time varies from ~12 minutes to ~50 minutes depending on the thickness of the rubber.

Lloyd is right on the money. Just to clarify, using rubber you need a steel tough container capable of resisting the huge pressures created by the expanding rubber while cooking. C clamps will do and you can do it in your kitchen oven. The rubber I use is Castaldo and it does not require vacuum.

The pourable RTV is my favorite. But I do have a vacuum setup which is a 'must have' for this kind of material. I'll put the master halfway into the RTV and let it set overnight, then sprinkle talcum and pour the cope and let it cure. I use Silpac Silicone RTV. You will also need a very accurate scale because the 10% activator ratio is crucial. I think I got my last batch from McMaster but I'm not sure. The nice thing about this material is that it is almost like Teflon in that very few chemicals or waxes will stick to it so mold releases are rarely used. I use just enough to keep Murphy's law at bay.

Wayne

IIRC, the SilPac brand is poisoned by zinc salts. Some use zinc compounds to cure them. Beware of brass, copper, or zinc-plated (galvanized, too) masters in contact with any two-part silicone resin.

LLoyd

Just get some 2 part silicone. Works everytime and lasts for many castings. Buy it from Diversified Materials here in San Diego ( la Mesa)

I should have added that some are cured by tin salts, some poisoned by them. Read the data sheets carefully. The same dimethyl tin dilaureate used to cure one may cause the other to never, ever cure.

LLoyd