Compression melting zone

There has been a general trend to move the Maddock element out of the compression/ melting zone and re-position it in the metering section or sometimes at the screw tip. 

Completion of melting of the solids bed moves towards and beyond the end of the compression/melting zone as screw speed is increased. It may even result in incomplete melting at the screw tip and lumpy extrusions. 

Tmnsition/Melting Zone It is the section, also called the compression zone, of a screw between the feed zone and metering zone in which the flight depth decreases in the direction of discharge. In this zone the plastics starts in both solid and molten state with target to have all molten upon leaving this zone. 

Equation 7.165 is comparable to Eq. 7.131 both equations are closed-form analytical solutions. However, Eq. 7.165 is more compact and easier to use. A word of caution is in order. Equation 7.165 has been experimentally verified with the screw simulator. The results of the screw simulator may not fully apply to actual melting in a single screw extruder. For instance, the screw simulator uses a molded solid polymer sample of one cubic inch. The solid bed in the extruder consists of compressed, partially sintered, polymeric particles. It is clear that an actual solid bed, as occurs in the melting zone of an extruder, may have different characteristics in terms of heat transfer properties and deformation behavior, as compared to a molded solid block of the same material. 

Compression (channel depth reducing) melting zone... 

In conical twin-screw extruders, a restrictive zone, follows the plastication or melting zone. The restrictive zone seals off the plastication zone from the devolatilization zone. Mixing will occur in the restrictive zone compressing the polymer in the narrower... 

The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. 

The pressure build-up which occurs along a screw is illustrated in Fig. 4.2. The lengths of the zones on a particular screw depend on the material to be extruded. With nylon, for example, melting takes place quickly so that the compression of the melt can be performed in one pitch of the screw. PVC on the other hand is very heat sensitive and so a compression zone which covers the whole length of the screw is preferred. 

The appearance of air bubbles in the polymer melt may occur under certain circumstances during processing. This phenomenon is rarely related to obvious faults in the polymer, but sometimes gas bubbles can be observed in cases of decreased thermal stability. Gas bubbles appear due to a certain amount of dispersed gas in the polymer matrix. Insufficient removal of gas from the extruder, particularly from the compression zone, can also cause the problem of air bubbles in the melt. An influence of the extruder screw could be established, because gas bubbles can be removed to some extent by using special screws or changing the extrusion conditions, along with the application of a vacuum. 

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