Plastics Technology - Dedicated to improving Plastics Processing.
Issue link: http://pty.epubxp.com/i/802323
into the extruder. In typical polymer processes, there is a melt zone comprised of a section of kneading blocks fully filled by polymer. Since this section of the extruder is fully filled, the entrained air cannot move downstream past the melt seal with the molten polymer. Instead, it is forced to escape through the nearest upstream opening, which is usually the feed opening (Fig. 2). As a result, there is a competing flow of the raw materials entering and the entrained air exiting. One way to reduce the amount of entrained air is to place the powder feeder on the same vertical level as the extruder, as close to the feed hopper as possible. This minimizes the drop and reduces the amount of air that gets entrained. Another strategy is to design vents into the system to give the air alternate escape routes. A vent can be installed in the feed hopper itself. To increase the effect, the hopper should be designed so that the feeder discharges material down the side of the hopper on the same side as the down-turning screw in the twin-screw pair, with the vent stack on the other side of the hopper. This allows the air to flow away from the polymer on its way down to the extruder (Fig. 3). The other location where a vent can be installed is in the twin- screw, upstream of the feed barrel. The entrained air is easily able to flow backwards in the twin-screw to the upstream vent barrel, while the solid powder is conveyed by the screws downstream. These three strategies can be employed individually or in parallel with each other. Another highly effective technique for getting powders into a twin-screw extruder is with the use of Feed Enhancement Technology (FET). In this technology (Fig. 4), a vacuum is applied through a porous filter in the barrel wall. Air is removed, while the powders are kept in the barrel. It works primarily by forming a cake of material on the barrel, increasing the coefficient of friction on the barrel wall. This increases the conveying efficiency. The secondary effect is to remove some of the entrained air from the powder, reducing its volume. The magnitude of the effect of FET is not the same for all powders, and depends on attributes such as bulk density and particle size. For talc, it has been proven to increase throughput rate by over 250%, while for calcium carbonate, the increase in throughput is mostly negligible. The limitation is that FET can only be used No FET With FET Operating Hours/Yr 6000 6000 Rate, Kg/hr 600 1100 Total Kilotons/Yr 3.6 6.6 Energy Cost*, $/kWh 0.072 0.072 Specific Energy Input, kWh/kg 0.134 0.129 Specific Energy Savings/Kiloton, $ - 360 Energy Savings/Yr, $ - 2376 Profit/Kg, $ 0.2 0.2 Increased Profit/Yr, $ - 600,000 *U.S. Energy Information Administration/August 2016 With a profit margin of 20¢/kg, a 500 kg/hr increase in rate made possible by using FET could mean an increased profit of $600,000 annually, offsetting the cost of the unit several times over. At the same time, the specific energy requirements are reduced by 4% or 33 MWh annually. TABLE 2 Impact of FET on Profitability (ZKS45Mc 18 ) BOY Machines, Inc. | Exton, PA 19341 | Ph 610.363.9121 | Fx 610.363.0163 | E-Mail: sales@boymachines.com BOY Machines, Inc. | Exton, PA 19341 | Ph 610.363.9121 | Fx 610.36 Spritzgiessautomaten www.boymachines.com Industry 4.0 BOY VV Insert moulding Procan ALPHA ® ( Servo – Drive EconPlast EconPlast Well proven design – new technologies More than 45,000 successful machines BOY has become the leading supplier for injection molding machines up to 110 U.S. tons clamping forces. With efficient Servo-Drive, the EconPlast technology and the control Procan ALPHA ® BOY sets standards world-wide. 60 APRIL 2017 Plastics Technology PTonline.com T ips & Technique s