Extruder output variation

Considering a power law dependency of melt viscosity for the range of shear rates of relevance to the extruder, an estimate of extruder output variation over time is given as a function of pressure variation by... 

Because the solid material is restricted to the solids channel, the barrier screw is inherently more susceptible to plugging. This occurs when melting cannot keep up with the reduction in the size of the channel in the compression section of the screw, resulting in the solid material getting stuck in the screw channel. This creates a momentary obstruction to flow and leads to surging. Surging is a variation in extruder output. 

A variety of AC and DC drives are available for extruders. DC motors with solid-state eontrollers and power supplies provide the most flexible seenario for control. A speed control feedback loop on the drive will minimize variation in the extruder output. The unwind roll should be equipped with a tension control device such as an electronic regulator. 

According to equation (12), it follows that when the L/Dq ratio is not significantly larger than e, small variations of the shear rate (and hence of the output) will directly affect the corrected residence time and therefore the extrudate swell. The shorter the time, the larger the effect. 

The melting process can be vmstable, with breaks occurring in the solid bed continuity. These cause pressure fluctuations at the die, and hence fluctuations in the volume output rate, which cause the pipe wall thickness to vary. Such fluctuations are more likely when the screw speed is increased the polymer residence time in the extruder can become insufficient for complete melting. Some screw designs place a barrier in the compression section that allows melt, but not granules, to pass the final metering section smoothes out any pressiu e variations. 

These on-off controllers are unsatisfactory for a loading having a long time constant, such as an extruder barrel, a die adapter, a die, and so forth. The temperature will oscillate violently at an amplitude that is set not by the characteristic of the controller, but by the delay in the load, as reviewed in Fig. 1-7. To reduce this variation, a proportional control was developed. It is similar to the on-off, but operates in between full on and off, with its output proportional to the deviation of temperature from the set point value (Fig. 1-8). Variations still exist with this system, but they are less than those of the on-off control. 

Today s extruder temperature control systems are capable of maintaining temperature within a range of 1 °E Precision of this level is acceptable for practically all extrusion applications however, when variation exceeds just a few degrees, variations in product output can result. Therefore, it is important that accurate temperature control circuits are employed. 

The complex interplay between the rate of viscous energy dissipation, temperature, and material viscosity in an extruder sometimes results in a variation in output (surging) even at constant screw speed. Where it is necessary to minimize surging, positive-displacement gear pumps are often added after the extruder. 

Many variations on the basic film-blowing process exist. Some of the more common variations include the use of rotating dies and coextrusion. Rotating dies serve to even out thickness variations around the circumference of the film, ensuring that rolls of film have no cumulative thick spots that would cause distortion. Coextrusion is used when it is desired to produce a film made up of layers of different polyethylene resins or of polyethylene in combination with other thermoplastics. In coextrusion, two or more separate extruders feed a single die designed in such a way that their outputs are combined concentrically. 

---