MOld Making and Resin Casting

Odds are you're already familiar if you're reading this, but if you're not: A mold is a manufacturing device created in any number of ways using any number of materials for the purpose of casting something in a liquid or semi solid state, which then cures or hardens into a desired part. This, of course, means that the mold itself relies on the negative space being shaped in such a way to create the desired positive form. 
While even steel and other rigid materials are commonly used for professional and industrial casting, this guide will cover rubber molds.






Common Mold Rubbers


Tin Cure Silicone: Anecdotally considered "standard" grade silicone, tin based silicone is cheaper than platinum cure, cures well at a wide range of temperatures, can be used to cast a wide variety of materials, and is not very sensitive to cure inhibition. It's easy to work with and produces great looking casts. The biggest disadvantage of tin silicone is that due to the catalyzing process shrink will occur and the mold will eventually deteriorate and breakdown over time. This makes it well suited for smaller runs of casts and over shorter periods of time.      
"Tin" is a condensation cure silicone, essentially meaning that the compounds within the rubber catalyze in the presence of small amounts of water. This means that higher ambient humidity levels can significantly increase the rate of cure. Shrink occurs because of byproducts released during the curing process. 

Platinum Cure Silicone: Ideal for capturing fine details better than any other material, and with the added benefits on longer shelf life and minimal shrinkage, platinum based silicone is the ideal choice for any part with lots going on, or for casting at quantity and over time. The major disadvantage is that the price point of this material sits at the top of the list. Platinum silicone also has the reputation of being very sensitive to cure inhibition. This is true. Plastic is best used for all mixing containers and utensils, and mixing accurately is critical. Any surface contacting the rubber should be kept clean, as should the environment the curing rubber is within.  
"Platinum" is an addition cure rubber, essentially meaning the mixed material catalyzes more rapidly at higher temperatures, and slowly at cooler ones. 

Urethane: Offering a lot of all-around benefits, urethane molds tend to work well with a variety of masters and casting materials, are usually mixed in simple 2-part ratios, and do tend to last over time as long as they are stored properly. Urethane is noticeably cheaper than silicone and often "tougher" Urethane's largest disadvantage comes from the fact it tends to adhere very well to whatever it contacts, meaning release agents are critical. Urethane is another rubber sensitive to contaminates, in particular the oils on your skin. Rather than causing cure inhibition, a more common problem is layer delamination on molds in which layers of rubber were built one upon the other. 

Latex: Readily available and among the cheapest options, latex is usually simple to work with, as it does not require mixing and cures when exposed to air. Its disadvantages come from the facts that creating a mold can be a very time consuming endeavor as layer upon layer must be built up to the necessary thickness, it is fragile, does not hold up over time, and can easily deteriorate in temperatures too hot or too cold.  

-Most rubbers do not gain properties of full cure for 24 hours, even if they look and feel cured before that time.
-Over-accelerating the cure of any rubber can significantly decrease its usable life and result in deterioration



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