Continuous extrusion curing

A continuous extrusion process, as well as molding techniques, can be used as the thermoforming method. A more rapid rate of cure is then necessary to ensure the cure of the rubber before the cellular structure collapses. The stock is ordinarily extruded at a temperature high enough to produce some curing and expansion and then oven-heated to complete the expansion and cure. 

Glass fibres dominate this field either as long continuous fibres (several centimetres long), which are hand-laid with the thermoset precursors, e.g., phenolics, epoxy, polyester, styrenics, and finally cured (often called fibre glass reinforcement plastic or polymer (FRP)). With thermoplastic polymers, e.g., PP, short fibres (less than 1 mm) are used. During processing with an extruder, these short fibres orient in the extrusion/draw direction giving anisotropic behaviour (properties perpendicular to the fibre direction are weaker). 

Continuous screw extrusion is another way to form PVC plastisol propellant into the desired size and cross-sectional shape. Simultaneous curing of the propellant is also accomplished. This process has been described fully by Rossen and Rumbel (9) and is only summarized here. Mixed propellant is fed from a casting pot into the feed end of a continuous worm-screw extruder which is fitted at the discharge end of the screw with a device for constricting propellant flow. The action of the... 

The bulk of the cost of raw materials is the base polymer itself at approximately 13%, depending upon the blend. Extrusion (as well as compression molding) and curing will add the bulk of cost to a polymer at around 40% (see Figure 5.2). When a material is extruded, it is forced through a die of fixed cross-sectional area, often heated while a continued cure or vulcanization takes place. 

If the filaments are not wound onto a mandrel but simply gathered into relatively large bundles and led slowly into a heated consolidation zone, emerging from a die partly or completely cured, the primitive essentials of pultrusion are present (Fig. 2.3(b)). This process began as the thermosetting resin equivalent of extrusion, that is, it was a continuous operation for manufacturing profiles, such as rod and channels. The method has been extended to fibre reinforced thermoplastics. 

A unique process for chemical stabilization of a cellular elastomer upon extrusion has been shown for ethylene-propylene rubber the expanded rubber obtained by extrusion is exposed to high energy radiation to cross-fink or vulcanize the rubber and give dimensional stability (113). EPDM is also made continuously through extrusion and a combination of hot air and microwaves or radio frequency waves, which both activate the blow and accelerate the cure. 

Processing temperature °C 200-316 (vulcanization) 0-50 (moisture cure) 250-500 (extrusion vulcanization) 180-200 (continuous vulcanization in steam) 60-90 followed by 130-200 (peroxide cure) ... 

Continuous Curing of Extrusions - The direet, continuous curing of extrusions as the rubber exits the extruder is growing in popularity. In addition to minimizing handling and labor eosts, very fast line speeds can be obtained. These faetors make for economical extruded articles when a particular item can be run for long periods without changing compounds or profiles (dies). 

Continuously cured profiles are based on very fast curing compounds, a cure of Imin at 200 °C being typical so that thiourea acceleration is necessary, with DETU (Iphr) most commonly used. Frequently the magnesia is reduced from the standard 4 phr to 2 phr to obtain a faster onset of cure. The processing safety of these compounds is obviously limited and requires careful control of all processing steps to limit the heat history. The compounds should be two stage mixed with the accelerators added shortly before extrusion. 

Silicones generally have less green strength (see Section 2.5) than organic rubbers and should be cured immediately after extrusion on a continuous basis. 

The general necessity for silicones to be vulcanised continuously at the point of extrusion presents no problem. With the correct choice of peroxide, cure can be obtained in less than 1 min for thin section profiles by passing through an infra-red heated unit or hot-air vulcaniser (HAV). Pressure is not necessary to suppress porosity and many simple heating techniques work satisfactorily. Both hot liquid baths and fluidised bed... 

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