For design and processing of Liquid Silicone Rubber (LSR) parts, in the silicone injection molding vulcanization process, the curing and respective flow behavior must be known. The two components begin to cure immediately after mixing. During curing the chemical bonds formed release heat which causes a temperature rise. Increasing the temperature of the resin decreases the viscosity, allowing better mold flow and exhaustion of air bubbles.
An informative presentation of the silicone liquid rubber curing incorporating the degree of cure is the Time-Temperature-Transformation (TTT) diagram, developed by Enns and Gillham. It can be used to relate the material properties of thermosets, such as Silicone Liquid Rubber (LSR), as a function of time and processing temperature. The diagram visually presents various lines representing constant degrees of cure. The curve labeled c=,c-g. represents the gel point and c=1 represents the fully cured resin.
Liquid Silicone Rubber (LSR) is often utilized for seals, valves and diaphragms, due to its high elongation (between 400 and 700% at room temperature) and tensile strength over a wide temperature range. The tensile properties of thermoset rubbers and thermoplastic elastomers need to be measured/tested in order to verify that the quality control standards of Silicone Rubber Liquid are met, as well as to determine whether or not the material is fit for its purpose. The test should be performed according to ASTM D412, which describes two test methods: A and B.
Injection molding of Liquid Silicone Rubber (LSR) has existed and evolved over the past 20 years. Due to the thermosetting nature of the material, injection molding of liquid silicone rubber requires special treatment, such as uniform distributive mixing; in addition, the material must be maintained at a constant temperature up until it is pushed into the heated cavity and vulcanized.