What are casting liquid silicone rubber?
Simple Analogy
Think of it like a two-part epoxy resin, but instead of hardening into a rigid plastic, it sets into a flexible, rubbery solid. You mix two liquid components (Part A and Part B), pour the mixture into a mold, and after a while, it turns into a durable silicone rubber part.
Technical Definition
Casting Liquid Silicone Rubber (LSR) is a two-part, platinum-catalyzed silicone system that is liquid at room temperature and cures (vulcanizes) into a solid elastomer (rubber) when heated. It is specifically designed for making molds, prototypes, and production parts through a process called liquid casting or potting.
Key Characteristics
Two-Part System:
Part A: Contains the base silicone polymers and often the platinum catalyst.
Part B: Contains the base silicone polymers and the crosslinker (usually silicone hydride).
Mixing Part A and Part B in a precise ratio (commonly 1:1) initiates the curing chemical reaction.
Low Viscosity: As a liquid, it has a viscosity like honey or motor oil. This allows it to easily flow into complex molds, capturing extremely fine details without trapping air bubbles.
Platinum Catalyst: This is a high-purity, additive-cure system. Platinum-cure LSR is known for being:
Low Shrinkage: It shrinks very little during curing (<0.1%), ensuring highly accurate parts.
Chemurally Inert: Once cured, it's non-toxic, odorless, and biologically compatible, making it safe for medical and food applications.
Inhibition Sensitive: The platinum catalyst can be "poisoned" or inhibited by contact with certain chemicals (e.g., sulfur, tin, some plastics like Bondo or certain 3D prints), preventing it from curing properly.
How is it Used? (The Casting Process)
Mold Making: A master model (the object you want to copy) is placed in a container. A different type of silicone (often tin-cure) is poured over it to create a negative mold. For LSR casting, molds are typically made from metal or high-temperature plastics.
Mixing: Parts A and B of the LSR are precisely measured and thoroughly mixed together.
Degassing: The mixed liquid is placed in a vacuum chamber to remove trapped air bubbles. This is a critical step for creating bubble-free parts.
Pouring/Casting: The degassed liquid is slowly poured into the mold cavity, often in a thin stream to minimize introducing new air bubbles.
Curing: The filled mold is placed in an oven or heated press. Heat (typically between 100°C - 150°C / 212°F - 302°F) accelerates the chemical reaction, causing the liquid to solidify into rubber. This can take from several minutes to a few hours.
Demolding: Once cured, the flexible rubber part is removed from the mold. It is often fully cured and ready for use immediately.