Feed-controlled extrusion

Controlling extrusion processes takes two types of systems, one on the extruder and another on the finished extruded product. In turn, these controls have to be interfaced. Extruder controls have been reviewed (heats, pressures, hopper feed rate, screw RPM, back pressures, etc.), as have product controls (wall thicknesses, rate of travel, width indicators, weight, etc.). Extensive efforts are continually being made to achieve the maximum in speed and precision. 

Operating to give dimensionally accmate extrudate, the MVX had limitations. At relatively low throughput rates the dimensions could be held accurately, but as soon as output was increased dimensional accuracy became inadequate for section extrusion. This was probably caused by the system of feed control which uses an oscillating ram in the feed hopper. 

Understand the critical properties of the mixed compound that affect processability. It should be understood that the properties and acceptable operating values of those properties, which control the compound processability, are dependent on the processes under consideration, e.g., hot or cold feed extrusion, compression or injection molding. 

The success of any continuous extrusion process depends not only upon uniform quality and conditioning of the raw materials but also upon the speed and continuity of the feed of additives or regrind along with virgin plastic upstream of the extruders hopper. Variations in the bulk density of materials can exist in the hopper, requiring controllers such as weight feeders, etc. 

For example, if a two-stage vented extruder has a screw that was designed for resins with an Mi range of 2 to 20 dg/min, and plant personnel are asked to manufacture a resin with an MI of 100 dg/min, the extruder may not process the resin acceptably. For this case and flood feeding, the extruder is likely to have resin flowing into the vent because the first stage can deliver more resin than the second stage can pump. One practical solution to this problem is to operate the slide valve in a partially open manner such that it controls the rate of the extrusion process. This will allow the extruder to operate at a lower specific rate and allow operation without the flow of resin into the vent. 

Fig. 7 Extruder and extrusion. 1 Drive unit, 2 barrel inlet, 3 temperature control, 4 feed hopper and dosing unit, 5 vacuum vent, 6 barrel, 7 screw. (Reproduced with permission from Reifenhauser)...

The attributes of the extrudate, such as density, thickness, and moisture content, are controlled by variables due to the formulation, extrusion process, and equipment design. The properties of the wet feed material and its ability... 

Among the formulation variables that control extrudability of a product, several studies (25,26,33-35) point to the fluid or moisture content of the wet feed material being more critical than others. The force or torque of extrusion and power consumption are often inversely proportional to moisture content of the extrudate as seen from Figures 11 and 12 (26,36-38), while the particle size of pellets increases linearly with water content when extruded with gravity feed basket and roll extruders (33). Using instrumented gravity feed and radial screw extruders, a three- to fourfold decrease in the force has been noted, with a 10% increase in water content of feed material (37). 

Pressure pellets sink when placed in water, whereas under the proper conditions, floating pellets can be produced through the extrusion process. That is accomplished when the feed mixture contains high levels of starch that expands and traps air as the cooked pellets leave the barrel of the extruder. This gives the pellets a density of less than 1.0. Floating pellets are desirable for species that come to the surface to feed since the aquaculturist can visually determine that the fish are actively feeding and can control daily feeding rates based on observed consumption. 

Control feeding devices to the hopper of primary equipment (injection molding, extrusion, etc.) is important to provide products that meet performance requirements at the lowest cost. Equipment manufacturers have increased the feeding accuracy using different devices such as microprocessor blender/mixer controllers. Also materials are being reduced in size with more uniformity to significantly improve uniformity in melt. Processors can use blenders and other devices mounted on hoppers that target for precise and even distribution of materials. 

A variable speed feeding screw must be used to continuously and uniformly discharge material from the bin and feed it to the extruder, as uniform feeding of the raw material is essential for the consistent and uniform operation of an extruder. The extrusion rate of the food extruder is typically controlled by a feeder screw or other metering devices. Therefore, the production of the single screw is dependent on the screw speed. This feeding screw must meet a variable speed to achieve the desired feed rate for extrusion. This feeder screw can feed directly into the extruder inlet throat or into a preconditioning cylinder (31). 

---