Injection silicon rubber material

The first device integrated a microstructured multichannel plate fabricated by micro-injection molding from a two-component liquid silicon rubber material (Silopren LSR 4070) with an appropriately interfaced and temperature-controlled housing, as shown in Figure 3.1a. 

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. 

The injection port is the place where the sample is entered into the chromatograph. The injection port is fitted with a small septum made of materials that permit passage of a needle and then reseal after the needle is withdrawn. This is usually silicone rubber, 1 mm llutiv. .. d 6-8 mm in diameter. Figure 20-9 shows several septa. Typically, 20 to 30 injections can be made with common 26 gauge needles before they need to be replaced. 

Septum disc of silicone rubber or similar material that provides a barrier between the atmosphere and the carrier gas, allows syringe injections and is self-sealing. The under surface is purged by carrier gas to prevent a build-up of material and contamination of injections. 

Another way to classify polymers results from the consideration of their typical applications. Typical classes are Compression molding compounds, injection molding compounds, semi-finished products, films, fibers, foams (urethane foam, styrofoam), adhesives (synthetic adhesives are based on elastomers, thermoplastics, emulsions, and thermosets. Examples of thermosetting adhesives are Epoxy, polyurethane, cyanoacrylate, acrylic polymers), coatings, membranes, ion exchangers, resins (polyester resin, epoxy resin, vinylether resin), thermosets (polymer material that irreversibly cures), elastomers (BR, silicon rubber). 

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. 

Like thermosets the elastomers will only cross-link in the mold by introducing additional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the... 

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. 

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