Single Fixed Nozzle

The prime advantage of a single fixed-nozzle thermocompressor is its compactness. Single nozzle units are preferred when large compression ratios are required. Single nozzle units are used for small capacity application and when several parallel units are needed to provide some degree of control. 

The multiple nozzle unit offers a substantially higher efficiency than a single nozzle unit operating under the same conditons. In most cases steam use can be reduced 10 to 15% with greater savings possible in some applications. Multiple nozzle units are more compact than single nozzle units when large capacity is required. 

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Held in place by the head assembly is an adapter, which guides the melt stream from the barrel exit into the die entrance. There may be a fixed nozzle in the head, which can be fitted with a custom adapter for each die employed. For the best temperature control, the nozzle and adapter should each have their own heater band. Usually a single temperature control circuit is used for the nozzle/adapter zone, but for a long transfer pipe to the die, multiple circuits maybe necessary. 

Fig. 13. Injection mol fling machine A, hydraulic motor for turning the screw B, hydraulic cylinder and piston allowing the screw to reciprocate about three diameters C, hopper D, injection cylinder (a single-screw extruder) E, nozzle F, fixed platen G, tie rods H, mold I, movable platen J, hydraulic cylinder and piston used to move the movable platen and supply the force needed to keep the mold closed and K, machine base.

The construction of a multiple-nozzle fixed-orifice thermocompressor is the same as that of the single nozzle unit, except that it employs several steam nozzles as shown In Figure 18-2(c). The usuai configuration has one nozzle on center with the remaining nozzles equally spaced peripherally around it. This type of unit can be used for both critical and noncritical applications. Its characteristic performance curve is similar to that of a single nozzle fixed orifice unit. 

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