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In addition to selecting feeding equipment to best suit the mate- rials, take time to consider feeder control systems. One example is the automatic hopper- refill control. If the hopper is refilled too frequently, the controller spends less time measuring the loss in weight over time, which can reduce the accuracy of the feeder. On the other hand, if the level in the hopper is allowed to drop too low, the sudden pressure of material dropping into the hopper during a refill may cause a surge of material to slip through the screw, especially for low-bulk-density and easily fluidizing materials. When this surge of material falls into the extruder, it will cause a momentary increase in load on the motor, manifesting itself as an increase in torque on the extruder control screen. These torque spikes cannot exceed 100% of the extruder's available torque or else safety interlocks will shut down the extruder to avoid an overload condition. As a result, the normal operating torque of the process must be kept lower to create room for these potential spikes. The increase in profitability with feed-system optimization can be calculated by the increased productivity. An example of this calculation is shown in Table 1 for a 70-mm twin-screw extruder processing a polyolefin master- batch with a profit margin of 10ยข/ kg. In this example, a feeder that was experiencing surging during refill was replaced with one better suited for the material, allowing the throughput to be increased from 2000 kg/hr to 2300 kg/hr (4409 to 5070 lb/hr). This results in a possible $180,000 in additional annual profit, more than four times the cost of the new feeder. Additionally, since the degree of fill in the twin-screw extruder increases, specific mechanical energy input to the material decreases. This results in an energy savings of 5% or 138 MWh/yr FEED INTAKE After the feed system delivers material to the extruder, it must be conveyed downstream in the feed intake zone. A limit to the throughput rate occurs when the intake zone has insufficient capacity to convey the materials. The conveying capacity of the feed intake zone is determined by the free area of the twin screw and the pitch of the screw elements in this zone, along with screw speed and a few product-related parameters. The free cross-sectional area of the extruder is fixed in a standard twin-screw extruder. Extruders with a larger outer-diam- eter to inner-diameter ratio will have more free area. It is impor- tant to consider this when procuring a new extruder. The product parameters are tied to the selection of raw mate- rials. Changes are often complicated for purchasing or product- quality reasons. Therefore, the parameters that can be optimized on an existing machine are the pitch of the screw elements and the screw speed. Increasing screw speed will typically result in a higher energy input to the material. This increase in energy can be partially offset by an increase in feed rate. However, this increase tends to trail off at increasing screw speeds. This results in a higher melt temperature, which among other downsides could cause issues in pelletizing or in product quality. Increasing the pitch of the screw elements in the feed intake zone is the most innocuous change. As a rule of thumb, it is recommended to always start with the widest-pitch screw elements in the feed intake zone. Feed intake limitations can result when feeding powders, espe- cially low-bulk-density or easily fluidizing ones, due to air becoming entrained as the powders drop from the feeder 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 down- stream past the melt seal with the molten polymer. Instead, it is 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. Optimized Layout of Feeder and Vent on the Extruder Feed Hopper FIG 3 FIG 4 How Feed Enhancement Technology Works The FET insert is a porous, gas-permeable wall section, on which a vacuum is pulled externally. Drawing out the air surrounding the polymer increases the bulk density to improve feed rate. Air Filter Cake: Densified Powder Vacuum FET Insert 58 APRIL 2017 Plastics Technology PTonline.com T ips & Technique s