Choosing Tin or Platinum Cure RTV Silicone When Making Molds for Resin Casting by Specialist Steve Levine
Let’s talk about why you need to choose tin or platinum when buying RTV silicone for mold making. But first you should know tin is “tin cure silicone” and platinum is “platinum cure silicone”. When you choose the wrong one it could mess up your costly master beyond repair by not curing against the substrate material and remaining sticky no matter what you do. It could also not cure for days if you’re in an extreme dry climate giving you the impression all is lost when it probably is not. What does this mean for you? Read on and you’ll know which to choose up front to avoid all the issues or carry on without reading and probably get lucky, the odds are with you; well do you feel lucky? It’s a short read come on…
Tin Silicone is pretty foolproof. Most are 10:1 mix ratio by weight and once mixed usually cure against most mold making media. It takes the condensation from the air and gives off a byproduct of alcohol or ethanol. Since there is evaporation in the form of the alcohol then there is shrinkage of the same amount, usually in the .3-.5% range which is acceptable for most applications. Tin cure is robust, high in tear strength and a great temporary tool. Some companies offer two catalyst slow, usually a 60-90 minute work time and overnight cure or fast (20 minute work time, 6-8 hour cure). Raw Material Suppliers offers three catalysts slow 60-90 minute, medium 35 minute and fast 15 minute work time. Other additives available are a thixotropic additive i.e. thickener usually a liquid added in small percentages. This allows for easy brush up. With all these advantages and ease of use you would wonder why some might choose platinum over tin. Tin is best for most hobbyists.
Platinum Silicone is usually either 10:1 mix or 1:1 mix ratio by weight and is a chemical reaction cure with no byproduct or leaving group. This means that there is nil shrinkage or .1% or less making it a great choice for precision parts. This silicone likes heat when curing. For every 10 degrees above 70F you can reduce the cure tie in half. So in an oven at 140F 24 hour cure silicone should be ready in less than 20 minutes. Rapid Prototypers use it for parts which require a high level of accuracy and parts that need to snap together or for fit and clearance studies in other words platinum cure silicone is the best and most widely used for the rapid prototyping industry. The silicone is also best for higher temperature applications such as low melt metal alloys. They are highly durable when used with polyester, urethane and epoxy resins but are more finicky when mold making subject to surface inhibition caused by “poisonous” substrates like, tin, sulfur, oil clay, double side adhesives, synthetic rubber and more.
Both Tin cure silicone and Platinum cure silicone rubbers are an appropriate choice for casting resins of most types. The master model that often dictates which rubber to use. The application and specific casting resin can also determine which to choose but I’ll save that for another article or your phone calls.
For a list to see if your substrate is on the inhibition list and for what to do in the desert when making molds as well as for technical assistance go to www.RawMaterialSuppliers.com or call 877-RTV-MOLD (877-788-6653)