Liquid silicone rubber flows easily at room temperature and begins cross linking at very high temperatures. In the molding process, this could be anywhere from 350-400F. The high temperature not only transforms the material into a cross-linked elastomer which can stretch up to 600%, but one of the material's key properties is its high temperature resistance. Once molded, the liquid silicone rubber is able to withstand up to 600F prior to degradation.
There are several applications which are heat sensitive and require an elastic but resilient material. For example, if you needed to over mold or encapsulate a heat sensitive substrate such as a polypropylene insert, flexible electronics component, or a wearable device. Another example would be molding a silicone connector on Pebax medical tubing which has a Vicat softening temperature below 200F, meaning the traditional molding process would melt the tubing.
The current solution in the silicone industry is an RTV or room temperature vulcanizing material. The RTV material can take anywhere from 1-24 hours to fully cure. RTVs also have a short pot life or working time once the two parts of material are mixed. Hence RTVs are never recommended in the injection molding process.
Once cured, the RTV material does not have the same mechanical properties that a silicone elastomer would have. Apart from being the most expensive material per pound, the RTV material tends to have the longest cycle time with the worst properties. Below is a basic comparison of the properties:
|Material||Durometer||Cure Time (minutes)||Tear Strength (ppi)||Tensile Strength (psi)|
|RTV||50||60 at 23° C||50||450|
|LSR||50||0.5 at 175° C||270||1225|
|UV LSR||30||0.5 at 23° C||57||942|
|UV LSR||60||0.5 at 23° C||114||1812|
The next generation solution and alternative to the current RTV material is UV curing silicone which has LSR properties and can cure quickly at low room temperatures. One big setback to the adoption of UV cured silicone is that many traditional molding companies are not equipped to use this material.
Through extensive R&D, Konark has constructed a unique molding apparatus for the new LSR material. Conventional molds are made with metals with heater rods, with this UV curing material, there is no need the heat but instead the ability to transmit UV light. Much of the innovation was around developing the mold, material delivery mechanism, UV cure source, cooling systems and primers for adhesion. By implementing this new method, we successfully over molded silicone onto USB flash drives. This is something which is not feasible with conventional LSR and HCR materials and cost prohibitive with RTV materials. The most cost was under $2000 with a molded piece price under $0.50 (excluding the flash drive insert costs). The same part if made with RTV silicone would cost $5 just to cover the raw material costs.
The benefits of this UV LSR are fast cycle times even through thick cross sections of rubber. Ability to over mold and encapsulate at low temperatures and low pressures protecting the substrate or insert.