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Figure 1: Various factors which can limit the throughput rate on a twin screw extruder. (Source: Coperion)
Figure 1: Various factors which can limit the throughput rate on a twin screw extruder. (Source: Coperion)
Photo: 123

One of the overriding goals among all businesses is to increase profitability. Companies can deploy many strategies that can fall across various areas of the business. The capital equipment is often already in place and therefore a sunk cost. Increasing profitability in this case usually depends on maximizing productivity of these assets. For twin screw extrusion processes, this is most easily achieved by increasing throughput rate, thereby spreading operating costs over a greater amount of product produced in that same unit of time.

When it comes to increasing throughput, first find out what is causing the current limitations. For twin screw extrusion processes, limitations can exist anywhere from the feed system through the process section and to the downstream equipment. For example, difficult to handle materials may stick or flow poorly through feeding equipment, limiting the rate at which they can be discharged into the twin screw extruder. In the process section, an improperly designed melting section may not be able to fully melt the polymer above a certain throughput rate. Limitations in the downstream equipment include high discharge pressure in the case of high viscosity materials and restrictive dies and high pellet temperature in the case of inadequately sized cooling equipment.

Feed System

To achieve the highest throughput rates, it is important that the feeding equipment delivers an accurate and consistent feed to the extruder. In order to do this the feed system must be designed according to the materials and throughput rates being fed.

In addition to selecting feeding equipment to best suit the materials, consideration should also be given to the controls of the feeder. One example is in the automatic hopper refill control. If the hopper is refilled too frequently, the controller spends less time measuring the loss-in-weight over time, and 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 issues can be reduced by selecting feeding equipment best suited to the materials.

Extruders with a larger outer diameter to inner diameter ratio will have more free volume, which is important to consider when procuring a new extruder, however the parameters which can optimize 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 ability depreciates as screw speed increases further. This results in a higher melt temperature, which among other downsides could cause pelletizing or product quality issues. Increasing the pitch of the screw elements in the feed intake zone is the most innocuous change.

Feeding powders can manifest feed intake limitations due to air becoming entrained as the powders drop from the feeder into the extruder. In typical polymer processes, there is a melt zone, comprising 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 to the nearest upstream opening, which is usually the feed opening. 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 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, and 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. The other location which 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.

Another highly effective technique for getting powders into a twin screw extruder is with the use of Feed Enhancement Technology (FET). 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.

The examples given in this article are just a few common limitations in feeding twin screw extrusion processes. Factors such as energy cost, profit and ability to sell excess production should be evaluated for your specific business.

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