The Plasticating Extruder

The plasticating extruder can be divided into three main zones  

25 for blow molding, film blowing, and injection molding 

30 or higher for vented extruders or high output extruders 

The solids conveying zone. The task of the solids conveying zone is to move the polymer pellets or powders from the hopper to the screw channel. Once the material is in the screw channel, it is compacted and transported down the channel. The process to compact the pellets and to move them can only be accomplished if the friction at the barrel surface exceeds the friction at the screw surface. This can be visualized if one assumes the material inside the screw channel to be a nut sitting on a screw. As we rotate the screw without applying outside friction, the nut (polymer pellets) rotates with the screw without moving in the axial direction. As we apply outside forces (barrel friction), the rotational speed of the nut is less than the speed of the screw, causing it to slide in the axial direction. Virtually, the solid polymer is then unscrewed from the screw. To maintain a 

The melting zone. The melting or transition zone is the portion of the extruder were the material melts. The length of this zone is a function of material properties, screw geometry, and processing conditions. During melting, the size of the solid bed shrinks as a melt pool forms at its side, as depicted in Fig. 3.10 which shows the polymer unwrapped from the screw channel. 

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M. Kaufman, The Birth of the Plastics Extruder, Polym. Plast., 243-251 (June, 1969). 

The plastically extruded layer around indentations is observed more often in Ge than in Si [38, 121]. Such a layer (Fig. 26) is believed to represent a squeezed-out ductile metallic phase that forms under the indenter during the loading stage. In general, although much less effort was put into the indentation studies of Ge as compared to Si, it is acknowledged in the literature that the behavior of both group IV semiconductors is very similar. 

Therefore, the plasticating extruder has three zones (1) the feed zone, wherein the conveying of the solid particle is carried out (it is usually cooled in order to eliminate sticking) (2) the compression zone, in which the principal melting takes place and (3) the metering zone, in which the liquid melt... 

Extruder screws fit into the barrel and are supported by the thrust bearing. The screw s shank length fits into the thrust bearing, while the flighted length contacts the plastic. Extruder screws are specified by their outside diameter (D) and the LID, which is given by... 

Surface defects in the plastic extrudate can occur due to flow instabilities or degradation of the plastic during the extrusion process. Some common defects and possible causes are hsted in Table B.2. 

Extrusion is one of the most important operations in the polymer processing industry. A substantial part of all polymers actually passes through an extruder at least onee in its production path from the polymerization reactor to the finished product. Most screw extruders in current operation are plasticating extruders. The plasticating step is usually the primary function of the plasticating extruders and has a profound effect on extruder performance. A fundamental understanding of the plasticating phenomena has been briefly provided in several books [1-2]. 

Single-Screw Mixers Because of the growth of the plastics indiisti v, use of extruders such as that depicted in Fig, 18-47 are now... 

Pellet Mills Pellet mills operate on the principle shown in Fig. 20-92. Moist, plastic feed is pushed through holes in dies of various shapes. The friction of the material in the die holes supplies the resistance necessary for compaction. Adjustable knives shear the rodlike extrudates into pellets of the desired length. Although several designs are in use, the most commonly used pellet mills operate by applying power to the die and rotating it around a freely turning roller with Fixed horizontal or vertical axis. 

The process requires some specialized (expensive) equipment to heat and mix the waste and plastic matrices, but equipment for mixing and extruding waste plastic are commercially available. The plastic in the dry waste must be mixed at temperatures ranging from 130 to 230 C, depending on the melting characteristics of the material and type of equipment used. 

Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

In the intermittent processes, single or multiple parisons are extruded using a reciprocating screw or ram accumulator. In the former system the screw moves forward to extrude the parisons and then screws back to prepare the charge of molten plastic for the next shot. In the other system the screw extruder supplies a constant output to an accumulator. A ram then pushes melt from the accumulator to produce a parison as required. 

Barrels and Heaters These are also similar to those in extruder machines. In recent years, vented barrels have become available to facilitate the moulding of water sensitive plastics without the need for pre-drying. Water sensitivity in plastics can take several forms. If the plastic absorbs water then dimensional changes will occur, just as with wood or paper. The plastic will also be plasticised by the water so that there will be property changes such as a reduction in modulus and an increase in toughness. All these effects produced by water absorption are reversible. 

Find the operating point for the above extruder when it is combined with a die of length 40 mm and diameter 3 mm. What would be the effect on pressure and output if a plastic with viscosity 400 Ns/m was used. 

An extruder is coupled to a die, the output of which is given by (KP/ij) where P is the pressure drop across the die, i] is the visco.sity of the plastic and is a constant. What are the optimum values of screw helix angle and channel depth to give maximum output from the extruder. 

The exit region of a die used to blow plastic film is shown below. If the extruder output is 100 X 10 m /s of polythene at 170°C estimate the total pressure drop in the die between points A and C. Also calculate the dimensions of the plastic bubble produced. It may be assumed that there is no inflation or draw-down of the bubble. Flow data for polythene is given in Fig. 5.3. 

A lace of polyethylene is extruded with a diameter of 3 mm and a temperature of 190°C. If its centre-line must be cooled to 70°C before it can be granulated effectively, calculate the required length of the water bath if the water temperature is 20°C. The haul-off speed is 0.4 tn/s and it may be assumed that the heat transfer from the plastic to the water is by conduction only. 

Time, pressure, and temperature controls indicate whether the performance requirements of a molded product are being met. The time factors include the rate of injection, duration of ram pressure, time of cooling, time of piastication, and screw RPM. Pressure requirement factors relate to injection high and low pressure cycles, back pressure on the extruder screw, and pressure loss before the plastic enters the cavity which can be caused by a variety of restrictions in the mold. The temperature control factors are in the mold (cavity and core), barrel, and nozzle, as well as the melt temperature from back pressure, screw speed, frictional heat, and so on in the plasticator. 

Competition, at each stage in the plastics industry, is in their raw materials. Many monomers can be made from alternate raw materials such as polyvinyl chloride that may begin with either ethylene or acetylene. Most plastic products may be made from a variety of plastics such as pipe that may be extruded from PVC, polyethylene, ABS, and so on. 

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