Mould silicone elastomer

There are three principal methods of moulding silicone elastomers ... 

The silicone elastomer is hand mixed initially and then vacuum degassed. It is then poured into the mould cavity in a manner to avoid, as far as possible, the inclusion of air. 

It must be remembered that although silicone elastomer is selfreleasing from epoxy resins and from clean metal surfaces, it will bond to silicone release agents. It is therefore essential to mould the rubber prior to treating any parts of the mould with release agent. 

The expanding range of use of two-component hot-crosslinked silicone elastomers is leading to the appearance of injection machines and moulds in which a thermoplastic melt is injected using HR and a hardening plastic from a separate cylinder. 

Fig. 3.13. A mould manufactured from low consistency (RTV) silicone elastomer is used in the epoxy encapsulation of capacitors. (Replica Materials Ltd...

Silicone elastomers are encountered in a variety of applications, such as sealants used for baths and showers as well as other applications where gaps are to be filled with a water impervious flexible material, mouldings used in medical applications, fuser rolls in printers, etc. SiUcone elastomers are usually based on polydimethylsiloxane (PDMS). Linear PDMS is normally a hquid at ambient temperatures. Even the very high molar mass materials show viscoelastic rather than solid behaviour.The polymer is a simple flnear chain with terminal hydroxyl groups at each end. The of the backbone is about —90°C. 

RTV silicones face strong competition from moulded silicone gaskets used by US manufacturers, and European manufacturers have preferred to use moulded rubber gaskets in most applications. Furthermore, many applications are developing for liquid injection-moulded gaskets that that can be injected and cured into preformed grooves in an assembled component. Up until now, this has only been possible with silicones, but Freudenberg NOK G.P. has announced a family of non-silicone liquid elastomers ... 

The characteristic feature of these elastomers is that they are based on liquid materials which can be fabricated by injection moulding. The basic components are low viscosity materials, supplied as a two-pack system, which are blended before injection. The mixture is then cured by heating in the mould at 150-200°C. Liquid silicone elastomers are based on polydimethylsiloxanes containing vinyl groups which are cross-linked by compounds containing silanic hydrogen. In the presence of a platinum catalyst, the following addition reaction occurs ... 

Moulds should include provision for the release of entrapped air and closed-end flow paths should be avoided unless a small bleed hole is incorporated. Most silicone elastomers will effectively push the air from a mould cavity during filling, but air pockets entrapped in the preform may give rise to soft uncured spots in the moulded part. This can normally be... 

As silicone elastomers flow easily under pressure, the forces involved in moulding can be lower than other elastomer types. Many prototype or short production-run moulds can be made of relatively soft materials, though for normal production situations, hardened steel is preferred. Highly polished cavity surfaces may well prove adequate, but chrome plating is often specified. 

The use of heat curable liquid silicone rubbers to manufacture a range of supported extrusions and moulded parts is a recent innovation in the development of silicone elastomer processing. For some years, addition cured RTV silicones have been available as heat curable liquid materials, but have never been widely used in this way for a number of reasons ... 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

A method of manufacturing rubber articles by pouring a compounded latex into an absorbent hollow mould the skin of rubber thus formed is removed, dried and vulcanised. The term is also apphed to the pouring into moulds of liquid polymer systems based on silicone or polyurethane elastomers. 

Silicone polymeric materials are used in a variety of forms such as pure fluids, emulsions, solvent-based formulations, resins, elastomers and so on. They are formulated in numerous products for various applications involving different processes across many different industries. Some of these products where adhesion plays a major role are adhesives, coatings, encapsulants, hair conditioners. Pressure-sensitive adhesives, mould Release agents. Sealants, skin creams and so on. 

Further processing in which appropriate fillers, catalysts, pigments and other additives are incorporated leads to silicone greases, elastomers and resin-based transfer moulding compounds. A broad list of final silicone products is given in Table 3.1. 

Most silicone fluids and silicone greases are used in the form in which they are supplied and require no further polymerisation or curing by the customer. The other products listed, elastomer- and resin-based materials, usually require further curing in the course of extrusion, moulding, encapsulating or similar processes. They are therefore formulated with appropriate reactive chemical groups, as already described, which can be activated by heat, atmospheric moisture or other means to produce the desired end product. 

Liquid silicone rubber (LSR) for the production of elastic parts in injection moulding technology is gaining an ever increasing significance in the processing of elastomers. The characteristic for liquid silicone rubber is the low viscosity in comparison to solid silicone rubber and other elastomers. 

This section will introduce material selection issues for multi-shot moulding as well as providing some introductory information on two classes of materials which may not be familiar to the non-specialist moulder namely thermoplastic elastomers (TPE) and liquid silicon rubbers (LSR). This section will begin, however, with consideration of adhesion. 

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