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Silicon Injection Moulding | Multi-Station Molds: Rotational Molds

In 2-Shot injection molding, a polymer is injected over a molded plastic insert to combine the best features of different materials while reducing or eliminating additional assembly operations. This reduction or elimination of post-processing steps is one of the biggest economical advantages of the 2-Shot silicone injection moulding process. For this kind of silicone injection moulding, a multi-shot mold is needed. There are several types of multi-shot silicone injection moulding available, such as over-molds, core-back, and multi-station molds. The right mold selection depends on part geometry, volume, quality, and molder capacity/capability.

Silicone Injection Molding: Process of LSR Vulcanization

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.

Glossary

Abrasion: the process of scraping or wearing away.

Silicone Liquid Rubber Time-Temperature-Transformation Diagram

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.

Why Only Hollow LSR Parts Float on Water

The density, or its reciprocal, the specific volume, is a commonly used property for polymeric materials. Density is primarily used as a material control tool, as well as to compare the buoyancy of different materials, but it can also be used to calculate the mass of rubber required to produce a given volume of material. The density of compounds with fillers or different materials can be computed at any temperature using the rule of mixture. Density measurements are performed following the standard tests ISO 1183 and ASTM D792. The density of Liquid Silicone Rubber (LSR) is typically in the range of 1.10 – 1.50 g/cm3 (density of natural rubber: 0.92, EPDM: 0.86 g/cm3), so it will sink in water (density of water: 1 g/cm3).