The Science Behind Silicone: Understanding Cure Chemistry for Better Molding

LSR cures via an addition reaction, also known as hydrosilylation. This is fundamentally different from the radical-based peroxide cure used for High Consistency Rubber (HCR). The reaction involves a platinum-based catalyst facilitating the addition of a silicon-hydride (Si-H) group to a vinyl (C=C) group on another silicone polymer chain. This forms a direct silicon-carbon bond, cross-linking the polymer chains into a three-dimensional network.

The Two-Part System: LSR is supplied as a two-component system (A and B). Part A contains the vinyl-terminated polymers and the platinum catalyst. Part B contains the polymers with the Si-H crosslinker and an inhibitor. They are stored separately and only mixed in the precise 1:1 ratio at the machine's metering unit. The inhibitor temporarily prevents reaction at room temperature, giving the mixed material a usable "pot life."

The Cure Triggers: Heat and Kinetics. When heat is applied in the mold, the inhibitor is deactivated, and the platinum catalyst becomes active. The rate of this crosslinking reaction is not linear; it follows cure kinetics. Initially, the reaction is slow (induction period), then accelerates rapidly to a point of vitrification where the material solidifies, before slowing down as it reaches full cure. The mold temperature is the primary driver: too low, and the cure is incomplete, leading to poor physical properties; too high, and it can cause porosity (from by-product gases) or even "burn" the catalyst.

1. Drives the cure to completion, maximizing tensile strength, tear strength, and compression set.

2. Drives off any low molecular weight volatiles, which is critical for achieving the required levels of biocompatibility (e.g., USP Class VI) and preventing odor or taste transfer.

3. Stabilizes the material's properties, ensuring they remain consistent over the product's lifetime.

• Why vacuum degassing is needed: The mechanical mixing of A and B can introduce air, and the reaction itself can produce hydrogen gas. Trapped air causes bubbles. A vacuum applied to the mixing chamber or the mold itself removes these gases.

• The danger of contamination: Substances like sulfur, tin, amines, and some plastics (like PVC) are "catalyst poisons." Even trace amounts from a dirty mixer, contaminated tools, or nearby processes can inhibit the platinum catalyst, leading to spots of uncured, sticky material.

• The importance of temperature uniformity: A variation of just 5-10°C across the mold surface can lead to inconsistent cure states, causing differential shrinkage, warpage, or areas of weakness.