High-consistency silicone rubber (HCR) is also known as solid silicone or gum stock silicone, which is a kind of HTV silicone rubber, is cured at high temperatures with either organic peroxides or a platinum catalyst. Which is structurally different from the rubber elastomer in the conventional sense, it’s a kind of solid block of rubber as pre-cure material and which contains polymers with a high molecular (400,000 to 800,000) weight and long polymer chains, which provides the rubber with a solid consistency.
Properties of HCR
- Odorless, tasteless
- Hypoallergenic, inorganic backbone
- Hygienic, resistant to bacteria and mold growth
- High biocompatibility
- High thermal stability
- Ozone/UV Safe
- Extreme temperature range performance
- Unique colors and visual effect possibilities
- Electrically conductive grades
- UL component recognition
High-consistency rubber silicone (HCR) is a versatile material. Commonly found in a wide variety of end-use applications, HCR silicone rubber is used in a wide range of static and dynamic applications, including:
- Electrical transmission and distribution
- Wire and cable
- Food processing
- Molded rubber goods
HCR Silicone Molding
One of the key appealing characteristics of HCR Silicone is its versatility. HCRs can be processed using different types of fabrication methods, including extrusion, calendering, and compression or transfer molding. With compression molding, material is placed between two plates. The heated plates are compressed, and the excess seeps out along the parting line. This is an older technology but can still be a cost effective method for low volume parts. Transfer molding differs from compression molding in that the press transfers the material through a runner, sprue and gate system into the cavity of the mold. Transfer molding can be a preferred processing method for silicone parts requiring unique material properties or low to mid-range annual volumes.
HCR Silicone Formulation
Polymers are the bulk of the formula for Silicone Elastomers. Specialty grades of HCR polymers also include a Fluoro Vinyl Methyl Silicone (FVMS). These polymers come in a range of product lines that will cover the wide range of material hardness. The main type to be used is a Vinyl Methyl Silicone, or VMQ, material. The higher the level of vinyl, the more crosslinking that will occur within the material. Controlling the level of vinyl helps polymer manufacturers to design in the degree of cure they want in an HCR material.
Fume- treated silica will typically be used because it has a very small particle size, treating the silica in this way helps it to better disperse in the elastomer. The purpose of these fillers is to add tensile and tear strength to the elastomer, as well as to raise the durometer or hardness of the material. Unlike an organic rubber, carbon black is generally avoided as a filler except in rare circumstances.
heat stabilizer is the ingredient that helps to protect the polymer backbone from being debilitated by heat. These are usually some form of a metal oxide, such as Magnesium Oxide.
When it comes to curing HCR material, a peroxide is typically the curing agent of choice. One benefit of peroxide use is that it is always used up during the curing process. The reason for this is that the peroxide bond gets cleaved apart as the energy created from the polymers exposure to high temperatures creates crosslinks.
The crosslinks are made as this process attacks the vinyl double bonds, and incorporates the peroxide so none is left behind. This again aids to more heat resistant properties, since no catalyst material is left behind to attack the crosslinks and weaken the material under extreme conditions.
The most common pigment is the Red Iron Oxide pigment, but just about any standard color option is available and can also be blended together to create custom colors.
Other specialty ingredients can be added to the materials as the need for supplementary features arises or depending on the specificity of the situation. An example of this would be internal mold releases, which help silicone elastomers become easier to mold and process. Another ingredient added could be metal particles, which allow silicone to be used in electronics, or simply to shield electronics from interference.