Plastics extruder capacity

It should be noted that melting takes place along most of the extruder. Indeed, the production capacity of plasticating extruders is frequently determined by their plasticating capacity. Further visual analysis of the experimental results reveals a tendency of the melt pool to penetrate under the solid bed and, occasionally, to completely surround it the continuity of the solid bed is frequently broken and a melt filled gap appears (e.g., turn 15.5, Fig. 9.23). This tendency for solid bed breakup seems to originate in the tapered sections of the extruder, and it appears to be a source of surging (i.e., fluctuation in time of temperature, pressure, and flow rate) of the extmdate at the die, as well as a source of entrapping some air bubbles into the melt stream. 

In the flight depths illustrated in Figure 12.12, a distinction is made between standard screws and shallow-flight screws. Shallow-flight screws pick up less material and, as a result, the residence time is the plasticizing unit is shortened. This can be advantageous when utilizing thermally sensitive additives, such as colorants, or when the part production rate is a small compared with extruder capacity. 

Extruders make pellets by forcing powders, pastes, and melts through a die followed by cutting. An 8 in. screw has a capacity of 2000 Ib/hr of molten plastic and is able to extrude tubing at 150-300 ft/min and to cut it into sizes as small as washers at 8000/min. Ring pellet extrusion mills have hole diameters of 1.6-32 mm. Production rates cover a range of 30-200 lb/(hr)(HP). 

Given that extruders of various designs, types and geometries are available in the marketplace, let us examine some aspects that drive selection of the appropriate extruder for a product. The key factors relevant in selecting the extruders are formulation factors (selection of excipients, thermal stability, plasticity, aqueous solubility, and moisture holding capacity) and equipment factors (batch versus continuous operation and scalability of the extruder). 

Screw extruders usually are built with a single screw as shown, but may have as many as four screws and the die may have multiple holes of various cross sections. An 8 in. dia screw can have a capacity of 2000 Ib/hr of molten plastics. Tubing can be extruded at 150-300 ft/min. To make pellets, the extrudate goes to cutting machines in which parts as small as washers can be made at rates as high as 8000/min. The extrusion of plastics is described at length by Schwartz and Goodman (Plastics Materials and Processes, Van... 

Extruders are available with capacity of more than 1 tonne/h, to mix and melt plastics and provide profiles, tubes, sheet, etc. They are mostly heated electrically, or else using circulating thermal oil, and can be used in pyrolytic systems for feeding plastics, straining the molten feed, eliminating HCl from PVC at temperatures up to some 400°C. Professor... 

A 20 t/day plant (6000 t/yr) is equipped with two pyrolysis vessels (with dimensions of 2800 mm ID and 2000 m height). The vessels are fed with molten plastics by four extruders each with a capacity of 250 kg/h. The plant runs continuously and can feed waste plastics and discharge the solid residue while the plant is running. The liquid fuels are fractionated in a fractionation tower. The plant produces a liquid fuel yield of up to 80% (by weight), depending on the nature of the feedstock. 

In the injection molding process, setting the temperature involves optimization of the temperature profile of the plasticating unit (extruder barrel), temperatures of the mnners and gates, (aU these determine the molten polymer temperature) as well as the mold temperature. The temperature setpoints depend on the material type (viscosity profile, thermal and shear stability, thermal properties) as well as machine or process considerations (machine capacity to shot size ratio, screw design, mold and part design, cycle time, etc.). Temperatures of the two basic units, the injection system and the mold, should be discussed separately since their selection stems from very different considerations. 

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