Silicone rubbers injection moulding

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. 

Injection moulding of silicone and polyurethane rubbers differs, because the material mixed and injected is usually liquid. 

Haberstroh et al. (2002) modelled the injection moulding of liquid silicone rubber via the use of curing kinetics, flow models and the pressure-volume-temperature behaviour. 

HTV and liquid silicone rubber LR, are used in moulding at elevated temperatures (press, transfer or injection moulding). 

Section 11.5. will explain the three alternatives for bonding, where mechanical anchorage (undercuts) or self-adhesive silicone rubber allow for two colour injection moulding. 

It is also possible to bond silicone rubbers to various substrates which are not injection moulded. Such materials range from steel and aluminium to ceramics, glass or any other solid. 

The application of primers does not allow for the injection moulding of the substrate on the same machine as the silicone rubber part. 

Self-adbesive silicone rubbers for injection moulding are restricted to LR mainly. The most peculiar property of these rubbers is the fact that the mould does not have to be treated with a special release agent (which is expensive and complicated to use) when working with these silicone rubbers. 

These test pieces were prodnced by injection moulding. They break at a maximum load of 4 kg or even above, if good adhesion has been built up. In many cases the silicone breaks. In other words, its mechanical properties are weaker than the force of adhesion between plastic and rubber. 

As mentioned earlier, the insert parts can be produced by any apparatus. This can be extmsion with subsequent cutting, injection moulding, two or multiple colour (also two or multiple component) injection moulding, press moulding, punched and bent metal sheets, cast iron, sintered metal, ceramics in any shape, cut glass, etc., or moulded or extruded silicone rubber (as in catheters and insulators). 

For the injection moulding of the plastic parts, hot runners allow a waste free production. The plastic is injection moulded as in a conventional process. When sufficient solidification is reached, the mould opens, and an index plate (this is a rotating platen which takes the plastic parts from the first cavity and puts them into the liquid rubber cavity by rotating them through 180 degrees), robot, etc., extracts the plastic moulding and transfers it into the silicone cavity. 

During injection moulding, i.e., on a very short time scale, adhesion is quite weak. Once the part is carefully extracted from the mould and stored for a certain period of time (weeks or months) the adhesive force increases significantly - sometimes to an extent where silicone and substrate cannot be separated and the silicone rubber will break before the hard and soft components can separate. This quite long storage time leading to the desired adhesion can be shortened substantially by using a heat treatment. 

Welding is widely used for bonding silicone rubbers, and is closely related to injection or press moulding. However, usually one does not apply a complicated mould technology... 

A silicone rubber can be bonded to a solid substrate if it is clamped into or onto it. A very good example is the baby soother. The silicone nipple is injection moulded by a common LR process and then post cured. After that the nipples are clamped into a plastic construction or even overmoulded by thermoplastic. 

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 ... 

A review is presented of developments by a number of companies in rubber processing machinery, including injection moulding machines (with particular reference to presses for the processing of liquid silicone rubbers), extruders, mixers and calenders. 

It can be seen from Figure 2.2, that carbon atoms form the backbone of the polymer. Many polymeric systems are made of long chains of carbon atoms such as this. But polymers are not eonfined to carbon forms and injection moulding materials such as liquid silicone rubbers (LSR) have different chemical structures as the backbone of the polymer chain. This will be illustrated later in Chapter 7. The repeat imit structures of a number of common polymers are shown in Table 2.1. 

This chapter has detailed common properties for a number of amorphous and serrri-crystalline materials. It has also highlighted any major processing issues attached to using these materials for injection moulding. In conjunction with the processing chapters (8 and 9), this should contain all the necessary information to successfully mould these materials. The next chapter will consider other types of injection mouldable materials, for example, thermosets and liquid silicone rubber. 

This section will discuss issues relating to the injection moulding of rubbers, specifically to the moulding of liquid silicone rubber (LSR) which is of large commercial interest. Firstly however, a quick introduction to rubber materials is given. 

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. 

The dosing system for liquid silicone rubber, which was described previously, is designed for only one machine. In order to save costs on space and investments, central material supply units are also available. Here, multiple injection moulding machines are supplied from one dosing system. Modem material conveyance systems operate on the concept of central material supply with a decentralised supply by volume at every injection moulding machine. Figure 7.8 depicts this type of system in a schematic overview. 

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