Plastification screw

A reciprocating-screw machine is an exunder system that has the advantages of screw plastification and ram injection in the same unit. In this design, as the screw rotates, it is pushed backwards by the molten polymer that collects in front of the screw. At a predetermined point, rotation stops and the nozzle moves in the forward position, acting as a ram to force the melt into the mold. It remains forward until the mold gates freeze, then pumps back to repeat the cycle while the mold opens. 

GriinschloB, E., A New Style Single Screw Extruder with Improved Plastification and Output Power, 7/it. Polym. Process., 17, 291 (2002)... 

Each processing step is linked to the next. Therefore the different steps cannot be considered independently from each other. For example, coloring processes already take place in the plastification zone, the incorporation of fibers added to the melt takes place not only in the designated dispersing zone but also in the discharge zone and in partially filled screw channels. 

Up to 80 % of the overall mechanical energy input in the twin screw extruder takes place in the plastification zone. Unfortunately, currently only rudimentary modeling capacities for the melting process are available. 

The energy input via the screw elements and kneading discs results in splaying forces, particularly in the plastification zone, which can cause wear to the cylinders (see Fig. 4.10). This can be reduced by an appropriate design of the plastification zone and by heating the cylinder wall, which results in the formation of a melt film [8]. 

The feed section takes the product from the feeding system and conveys it to the plastification section. At the same time, there must be sufficient free volume to allow gas reflux, e. g., air or nitrogen, to escape from the process section upstream. If the throughput rate of the extruder is less than the input product flow, the product will back up in the feed hopper, indicating that the intake limit has been reached. Along with the known extruder data, such as the screw speed, the screw pitch, and the available volume in the screw channel, other influencing factors, such as the fill rate of the screw channel, the conveying efficiency, the bulk density, and other bulk characteristics of the product sometimes affect the... 

The intake behavior is also substantially influenced by the resistance of the downstream plastification section. If discharged at atmospheric pressure, e.g., via screw feeders, the throughput rate is considerably higher than if the same feed section is succeeded by a plastification section (Fig. 11.4). For instance, if metering a product with a bulk density of 0.35 g/cm3 and a melt phase density of 0.7 g/cm3, it is clear that considerable quantities of gas will have to be extracted upstream through the intake hopper. 

Figure 12.15 Screw elements with large pitches as plastification elements...

Different types of wear occur in the plastification zone, in the additive intake zone, or in the discharge zone of the twin screw extruder. These include ... 

Figure 16.2 shows abrasive wear on screw elements from the plastification zone when... 

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

Liquid additives are mostly added downstream of the plastification section because they tend to lubricate the pellets or cause powders to agglomerate in the feed throat. If a significant amount of liquid is to be incorporated, it can be added at several locations. The most effective method for low viscosity liquid incorporation is to inject into a fully filled distributive mixing section. This requires a pressure injection valve and positive displacement pump. For small amounts of compatible liquid, non-pressurized injection into a low degree of fill area of the screw configuration may also be acceptable. 

Plastification of polymeric material requires energy to be transferred from an outside source into the material. In the twin-screw extruder, this energy transfer occurs through both mechanical and thermal mechanisms. However, as the extruder gets larger, the surface to volume ratio decreases significantly. Therefore, mechanical energy transfer is the dominant mechanism for plastification. 

The processing zones in the extruder are the solid zone - feed section, transformation or plastification zone, the pumping or metering section (screw) and the mold. Feeding in the solid zone is the main determinant of machine output. 

In the screw preheat zone in particular, and at the onset of plastification, microwave technology offers promising solutions to the process engineering problems in extruder design mentioned above. Energy transfer in this technology does not involve heat conduction and makes for an efficient transfer of energy into the compacted bulk powder materials with poor heat conductance properties present in the extrude in this phase of the plastification process (For details, see Diemert [20]). 

Initial tests of microwave-supported plastification run in a test kneader, the kneading chamber of which has the same geometry as the counter-rotating twin-screw extruder, showed very good microwave transmission to the PVC to be plasticized. These tests also demonstrated that plastification is possible even at a clearly reduced kneading chamber temperature and that this clearly reduces the plastification time. 

With all the varieties of materials available, and their individual grades with many different formulations, it has not yet been possible to design a screw in advance, based on the melt or the rheological/flow physical relationships involved in plastification and conveying. Trial and error, with an observant processor using reliable and reproducible controls, makes the screw perform to its maximum efficiency. 

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