How To Choose RTV Silicone When Making Resin Casting Molds

Let’s discuss why you need to choose tin or platinum when buying silicone for mold making. First you should know tin cure is also known as “condensation cure silicone” and platinum cure is also known as “addition cure silicone”. When you choose the wrong one it could mess-up your costly master model beyond repair. In one instance the silicone might not cure against the substrate material and remain sticky no matter what you do and in another, it doesn’t cure for days because you’re in an extreme dry climate giving you the impression all is lost when it probably is not. What does this mean to you? Read on and you’ll know how to choose before buying avoiding potential costly issues.

Tin cure silicone is almost foolproof. Usually a 10:1 mix ratio by weight. Once mixed it will cure easily against most master model making media (oil clay, plastic, wood, stone, etc.). To cure it takes condensation from the air and gives off a byproduct of alcohol or ethanol. Since there is evaporation in the form of alcohol there is shrinkage of the same amount, usually in the 0.3-0.4% range; acceptable for most applications. Tin cure is robust, high in tear strength and a great tool making rubber. Bronze Foundries and Sculptors love it for its ability to make molds of fine detail and for the security of knowing it will cure against their precious masterpiece. Others love it for being easy to use and for its great strength.

Some companies offer two catalysts:

  1. Normal / Slow, usually a 60-90 minute work time and 12 hour+ cure / de-mold
  2. Fast 15-20 minute work time 6-8 hour cure / de-mold
  • Some fast catalysts are not mix-able or compatible with the normal cure catalyst

Other companies offer three catalysts:

  1. Normal / Slow, 60-90 minute work time and 12 hour+ cure / de-mold
  2. Quick 30-35 minute work time 4-6 hour cure / de-mold
  3. Fast 15-20 minute work time 6-8 hour cure / de-mold
  • All three catalysts can me mixed or blended together to create the cure speed desired

Additives available are for both types of silicone and include:

  1. Thixotropic additive a liquid thickener added in very small percentages. This allows for easy brush up of the silicone.
  2. DMF silicone fluid thinner. Softens and thins all silicone’s
  3. d’Limonene solvent thinner. Special effects paint base and to thin for spraying.
  4. Silicone pigments

With all the Tin cure advantages and ease of use you would wonder why some might choose platinum cure instead. Tin cure is used by a broad range of business and is the best choice for most hobbyists and beginners.

Platinum cure silicone can be either 10:1 or 1:1 mix ratio by weight. It is a chemical reaction cure with no byproduct or leaving group giving it the addition cure name. This means that there is nil shrinkage or less than 0.1% making it a great choice for precision parts. This silicone likes heat when curing.

Rule of thumb: For every 10 degrees above 70°F you can reduce cure time in half. For example in an oven at 140°F a 24 hour room temperature cure silicone would be ready in less than 20 minutes.

Rapid Prototyper’s use Addition Cure for parts that require a high level of accuracy and parts that need to snap together and for fit and clearance studies. Platinum Cure silicone is well suited for and most widely used for the rapid prototyping industry. The mold making silicone is also best for higher temperature applications such as low melt metal alloys (lead, pewter, jewelry applications). They are highly durable when using polyester, urethane and epoxy casting resins. Platinum cure is used for FDG Food & Drug applications such as food molds. They are a bit finicky when making a mold due to surface inhibition caused by “poisonous” substrates like, tin, sulfur, oil clay, double side adhesives, synthetic rubber and a few more.

Silicone is the best mold making material in the world capturing detail beyond belief. Both tin cure and platinum cure silicone rubbers are an appropriate choice for casting resins of most types. The master model and what its made from often dictate which rubber to use. The application and specific casting resin being used to make your part can also determine which silicone to choose but the end results are the same, a great silicone mold with fine detail. Some manufacturing industries use both tin cure and platinum cure. Both come in a variety of hardnesses. Delicate parts need a softer silicone while the most commonly used mold making silicone rubber is a “25-28 shore A”. For larger heavier concrete parts you may want something stiffer like a “40 or 50 shore A”.


Post time: 12-14-2016