Why do many mold makers choose flexible rubbers?

The majority of modern moldmakers use flexible rubber products for a variety of reasons. We've outlined the advantages and disadvantages of the four different rubber products:

I. Latex is natural rubber extracted from rubber trees found mainly in Southeast Asia. To make this rubber usable as a mold material, the raw rubber is usually processed with ammonia and water. Latex is almost always brushed onto an original model (not poured). 

Advantages -- Latex is a one-component system (no weighing necessary) that is ready to use right out of the container. Latex is relatively inexpensive. Latex is an elastic mold rubber and molds are generally thin-walled, strong and exhibit good abrasion resistance. Because of its high elasticity, a feature unique to latex is its ability to be removed from a model like a glove. A latex mold will retain its shape after being repeatedly rolled up and away (and turned inside out) from an original model or casting – like a glove. Because of this feature and its resistance to abrasion, latex is commonly used for making “glove molds” in the reproduction of ornamental concrete (lawn ornaments and statuary). Latex molds are also good for casting wax and gypsum. 

Disadvantages -- Low-cost latex products generally shrink – on the order of 10 to 20% depending on product. Making molds with latex rubber is slow and time-consuming. Brush-on molds made with latex require as many as 20 brush coats, with 4 hours of drying time between each coat. Time factor for making a brush-on latex mold is ten days or more. Many latex products have an ammonia odor (however, there are new latex products on the market with lower shrinkage and no odor). Latex molds are generally not suitable for casting resins.  

II. Polysulfide rubbers (Smooth-On’s FMC Series) are two-component systems (base plus curative; A+B) that have been the favorite mold rubber of bronze foundries around the world (for casting wax) for years. They are available for making molds that are poured or brushed on.  

Advantages – polysulfide molds are very soft, “stretchy” and long lasting (some molds still in production are over 40 years old), and are good for making molds with severe undercuts and/or very fine detail. Unlike other mold rubbers, polysulfide rubber is not inhibited by sulfur or water based modeling clays.Model preparation is minimal. Once cured, polysulfide molds are good for casting wax (lost wax process) and gypsum plasters.  

Disadvantages – the most common polysulfide rubbers with lead curatives have an offensive odor. Newly made polysulfide molds may stain plaster. Polysulfides have poor abrasion resistance (not good for casting concrete), and are not suitable for production casting of resins. Polysulfides (A+B) must be mixed accurately by weight (scale required) or they will not work. They are of moderate cost; higher than latex and urethanes but lower than silicones.

 III. Silicone rubbers (Smooth-Sil Series) are two-component systems (base plus curative; A+B) available in a hardness range of very soft to medium. Silicones

can be cured with either a platinum catalyst or a tin catalyst. They are available for making molds that are poured, brushed or sprayed on to a model and have performance characteristics that no other mold rubber has.  

Advantages – Silicone rubber has the best release properties of all the mold rubbers, which is especially an advantage when doing production casting of resins (polyurethanes, polyesters and epoxy). No release agent is required, so there is no post-production cleanup. Silicones also exhibit very good chemical resistance and high temperature resistance (400°F / 205°C and higher). High temperature resistance makes silicone the only mold rubber suitable for casting low melt metal alloys (i.e. tin, pewter, lead). The combination of good release properties, chemical resistance and heat resistance makes silicone the best choice for production casting of resins.  

Disadvantages - Silicones are generally high in cost - especially platinum-cure. They are also sensitive to substances (sulfur clay for example) that may prevent the silicone from curing (referred to as cure inhibition). Silicones are usually very thick (high viscosity), and must be vacuum degassed prior to pouring to minimize bubble entrapment. If making a brush-on rubber mold, the time factor between coats is long (longer than urethanes or polysulfides, shorter than latex). Silicone components (A+B) must be mixed accurately by weight (scale required) or they will not work. Tin catalyst silicones will shrink somewhat and do not have a long library life.  

IV. Polyurethane rubbers (PMC Series) are two-component systems (base plus curative; A+B) that cover a wide variety of applications at a relatively low cost. They are available for making molds that are poured, brushed or sprayed onto a model. 

Advantages - polyurethanes are easy to use, with many having a simple mix ratio by volume (i.e. 1A: 1B) – no scale required. Flexible urethanes are available in a wide hardness range from gel-like to harder than a car tire and everything in between. Urethanes have relatively low viscosity and “de-air” themselves – no vacuum degassing required. Urethanes have good abrasion resistance and are used to cast abrasive materials like concrete. They are less expensive than silicones and polysulfides.  

Disadvantages – As silicone rubber has the best release properties, urethane rubber has the worst release properties and will adhere to just about anything. Thorough model preparation (we’ll cover this topic later) is essential to successful mold making with urethane rubber. Urethanes are moisture sensitive and may bubble if exposed to too much moisture (making molds outside on a very humid day, for example). Limited shelf life after opening – remaining product may be affected by ambient moisture in the air. (Smooth-On makes a product called “Xtend-Ittm” that greatly extends the shelf life of unused urethanes).

(Courtesy of Smooth-On)