Quimica, Chemical: liquid silicone rubber (LSR)

Products from liquid silicone rubber (LSR) are to be found in numerous applications. And its

range of uses will no doubt extend as further and new developments come on the market.
Especially where thermoplastic elastomers no longer meet requirements, LSR provides the capability profi le to fi ll the need. An added advantage is in the fact that LSR will combine with a host of other plastics and with metals. Low viscosity creates high flexibility Liquid silicone rubbers belong to the group of heat-curing rubbers. A characteristic feature is their low viscosity during processing compared to solid silicones or elastomers.
Two-component mixtures increasingly crosslink by the addition process. This means that the reaction takes place without any decomposition product forming. This is a great benefit i

injection moulding because there is no concern of deposits forming on the moulds. In order to define processing time until crosslinking starts, one of the two components hasan inhibitor added, which is also referred to as a ‘pot-life regulator’. The other component has a catalyst added. Crosslinking – better known as curing – starts due to the infl uence of the mould temperature which, depending on the Vehicle and Medical Food machine building devices industry mix, is between 160 °C and 220 °C. But then the reaction proceeds fairly rapidly at approximately 5 s/mm of wall thickness. Consequently, much shorter cycle times can be achieved with LSRs than, for instance, with classic elastomers.• Food industry: Dosing devices, covers, baby bottle teats
• Kitchen and plumbing: Baking moulds, keypads,
seals and gaskets
• Electricals and electronics: Mobile phone keypads,

Major fields of application
Despite their comparatively high price, liquid silicone rubbers (LSRs)
cut across almost all areas of every day life and industrial production.
Primary applications are in
• Vehicle and machine building: O-rings, gaskets,
spark plug connectors
• Medical devices: Mouthpieces for anaesthetic devices,
clinical nipples

keyboard mats
• Sports and leisure: Diving masks, toys

LSR moulding solutions in practice
APPLICATIONS
The processing of liquid silicone rubber involves vulcanisation or curing under heat. This implies three particularities of LSR injection moulding: metering of two different liquids by means of a special metering unit; homogenisation and delivery of the paste-like rubber in the injection moulding machine; and curing in the mould at around 160 °C to 220 °C. As a single-component material, LSR is used in many technical applications, such as keyboard mats, plug connectors, O-rings and gaskets. Recently, it has also found application for such common consumer products as dummies and dispensers in food packaging articles. Moreover,
LSR is used in medical devices, such as tubing, respirator masks, and body electrodes. The most dynamic growth areas today are fl exible baking moulds, self-ceramifying products, e. g. sheathing
of cables in fi re-hazard areas and lift wells – and technical parts made by twocomponent injection moulding for use in the automotive and electronic industries. Tips and tricks from LSR pros 1. In the case of thermoplastic/LSR combinations, continue processing the thermoplastic immediately – whether direct by transfer into a second injection moulding machine or by overmoulding in a two-component mould. This way you will be able to avoid changes in surface tension on the thermoplastic part by oxidation or water absorption. 2. “Try out and adjust“ new moulds for some time with standard LSR in order to “siliconise” the surface so that zero-loss demoulding of the parts is ensured in commercial production. 3. You can signifi cantly shorten the time for adhesion build-up between the thermoplastic and LSR by allowing a longer dwell time after injection moulding or by post-curing. This is the way to quickl achieve optimum fi nal bonding.