Liquid silicone rubber
Products of Liquid Silicone Rubber (LSR) (dt .: liquid silicone ) are versatile due to the universal material properties. In the automotive industry, for example, it ranges from protective jackets for spark plug plugs, switch covers and rain sensors to membranes for central locking systems and headlight seals, and in electrical engineering from safety mats to anode caps and electrical plugs . LSR parts are also used in dental and medical technology , for example, as cardiac catheters, drinking and pacifiers, contact lenses or ventilation masks. Another focus is household and sanitary technology. Sealing rings, jet inserts for shower fittings or seals for mixer taps are made of LSR.
When producing injection molded parts from LSR, it must be ensured that the injection molding machine is specially designed for processing liquid silicones.
Typical components of an LSR silicone rubber:
- Linear siloxanes: approx. 70%
- Fillers: approx. 30%
- Additives: approx. 1%
Versatile material types
The constantly increasing number of material types and the possibilities for a connection with various metals and plastics make liquid silicones usable for a variety of areas of application. In general, many LSR material types are available in hardness from 20 to 70 Shore A available. A distinction is made between:
- Standard types
- Electrically conductive types
- Fluorosilicone types
- Types for medical technology
- Oil-exuding silicones
- Fast curing types
- Adhesion-modified types
- Highly transparent types for optical applications
Basic production process
The low-viscosity and hot-vulcanizing two-component silicone is provided by the material supplier in 20-liter or 200-liter container units A and B ready for use. The two addition-crosslinking components are fed to a mixing block under pressure using a multi-component mixing and metering system. The material that has been conveyed separately until then is mixed in this unit in a ratio of 1: 1. For color design or to change certain material or product properties, colors and additives can also be added in a proportion of 0.3% to approx. 4%. The reactive material mixture is fed to the LSR cylinder under pressure by a static mixer in the mixing block. To prevent premature vulcanization in the cylinder, the special LSR screw unit is tempered to approx. 20-25 ° C. Depending on the screw diameter or thread pitch, dynamic mixing takes place in the cylinder. The LSR enters the mold from the cylinder via a cold runner. The tool is heated to between 170 ° C and 220 ° C. Due to the high temperatures, the LSR networks in seconds. Due to the low viscosity and the high injection pressures, tools have to be manufactured with very low tolerances . Too high tolerances lead to the formation of burrs and webbing. As a result of the narrow tolerances, the tool seals very well under high pressure, which is why it must be evacuated before injection . Air inclusions would lead to burn points, surface and filling problems. Due to the high purchase price of over € 8 per kilogram, attempts are made to manufacture sprues-free using cold runner technology. This leads firstly to a money saving, and secondly to reduced cycle times , as the de-molding eliminates the sprue. Either handling devices or removal robots are used to demould the often very soft and unstable articles. The handling devices are either equipped with compressed air or brushing devices.