EXTRUDER HEAD

Molding of flat film canvas is usually performed by pouring a melt coming out of a flat-slit extruder head (spinneret) on the receiving drum (a roll with sufficiently... 

Extrusion is a common way for solid products such as plastics to emerge from closed manufacturing systems. Normally a polymer is hot when extruded and may contain additives and oligomers that are volatile at elevated tempera-tures. The result is fuming at the extruder head. These fumes can result in employee annoyance, housekeeping problems, and, at worst, depending on composition, health hazards. 

Fig. 14.15 Schematic representation of the blow molding process, (a) The extruder head with the blowing pin and open mold (b) the extrusion of the parison (c) the mold closed with the parison pinched in the bottom and sealed at the top (d) the inflated parison forming a bottle.

Commercial instruments are available the Brabender Plastograph, and Plasticorder, this latter allowing use of an extruder head the RAPRA variable torque rheometer [5]. The advantages of these instruments are based on the similarity of their action to full-scale extrusion equipment and on the fact that they can be operated at shear rates appropriate for factory equipment. But because of the difficulty of matching exactly the range of shear rates which exist in full-scale plant, successful scaling-up is difficult. Elongation flow measurements have been reported by several workers, in which a sample is stretched in uniaxial tension at a constant strain rate [6]. 

The blown-film technique is widely used in the manufacture of polyethylene and other plastic films [14,15]. A typical setup is shown in Figure 2.23. In this case the molten polymer from the extruder head enters the die, where it flows round a mandrel and emerges through a ring-shaped opening in the form of a tube. The tube is expanded into a bubble of the required diameter by the pressure of internal air admitted through the center of the mandrel. The air contained in the bubble carmot escape because it is sealed by the die at one end and by the nip (or pinch) rolls at the other, so it acts like a permanent shaping mandrel once it has been injected. An even pressure of air is maintained to ensure urufoim thickness of the film bubble. 

Screw speed (rpm) Mass flow rate (kg/h) Extruder head pressure (har) Mw/M Conversion (%)... 

Melt pumps are most appropriate when the screw and die characteristics combine to give a relatively poor pumping performance by the total system. This can happen when die pressures are low but more often occurs when they are extremely high (5,000-8,000 psi), or when the melt viscosity is extremely low. When pumps are used to increase the production rate by reducing the extruder head pressure without a corresponding increase in the screw speed, the extrudate solids content often is increased. The result is an inferior product. This problem often necessitates additional filtration, which serves only to increase pressure and may counteract many of the benfits expected from the pump, as well as increasing the financial investment even further. 

While the measurement of extruder head pressure is very important for product quality purposes, it is also perhaps the most important measurement from a safety standpoint. Excessive pressure can cause rupture of the barrel, damage to head and die components, and injury to personnel from projected hardware and hot polymer. Therefore, accurate pressure readings, high-pressure motor shut-off capability, and properly rated rupture disks are essential for safe operation. 

The main causes of reduced output are increased flow restriction, commonly a result of clogged screens and screw wear. As screens perform their function properly and capture contaminants in the melt stream, they create an increased restriction to flow through the system. This increased restriction will result in higher head pressure. Additionally, there will be more recirculation of melt in the screw channel and less throughput. As discussed above in the section on high melt temperature, changing the screens should alleviate this problem. It is possible that other sources of flow restriction could exist, such as screens with an incorrect mesh size or a valve in the extruder head. 

Angle head (offset head, and crosshead). An extruder head so designed that the principal direction of the extrudate makes an angle with the (extended) axis of the screw. See also crosshead. 

Sheet die n. A heavy-walled, extremely rigid steel structure, bolted to an extruder head, whose inner passages form the molten plastic leaving an extruder screw into the shape of a flat sheet. Most modern sheet... 

Extruder head temperature Calender bowls water cooled... 

The metering zone provides the extruder head with plastic melt at constant flow rate and pressure. Ideally, all of the pressure buildup along the... 

Pressure valves are used to increase or decrease back pressure and thus control pressure at the extruder head. Pressure valves can provide good pressure control and remove this function from die design and screen-pack arrangement. Two types of pressure valves are available, internal and external. 

Rogers, M.G. (1970) Rheological interpretation of Brabender plasti-corder (extruder head) data, Ind. Eng. Chem. Process Des. Develop., 9,49-52. 

A post-mixer dump extruder has a large feed hopper which is often equipped with a pusher to force the material into the screw. The mixer discharges directly into this feed hopper. The mix is then taken into the screw and extruded down the screw flights to the discharge end of the extruder (head end). 

Fugitive emissions such as those around pipe flanges and pump seais Process operations, in modem chemical operations, automation and computer control have greatly reduced contact with feedstock, intermediates, and products Extrusion of solid products such as a solid polymer that normally is hot when released through an extruder head and may give off vapors of residual monomers, oligomers, and additives that may be inhaled... 

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