Extruder process control

Computer-aided flow-simulation programs are also available for dies. All the programs can successfully predict a certain amount of shrinkage under specific conditions that can be applied to experience. The actual shrinkage is finally determined after molding or extruding the products. When not in spec process control changes can meet the requirements unless some drastic error had been included in the analysis. 

A major difference between extrusion and IM is that the extruder processes plastics at a lower pressure and operates continuously. Its pressure usually ranges from 1.4 to 10.4 MPa (200 to 1,500 psi) and could go to 34.5 or 69 MPa (5,000 or possibly 10,000 psi). In IM, pressures go from 14 to 210 MPa (2,000 to 30,000 psi). However, the most important difference is that the IM melt is not continuous it experiences repeatable abrupt changes when the melt is forced into a mold cavity. With these significant differences, it is actually easier to theorize about the extrusion melt behavior as many more controls are required in IM. 

For uniform and stable extrusion it is important to check periodically the drive system, the take-up device, and other equipment, and compare it to its original performance. If variations are excessive, all kinds of problems will develop in the extruded product. An elaborate process-control system can help, but it is best to improve stability in all facets of the extrusion line. Some examples of instabilities and problem areas include... 

Figure 12.5 An extrusion process with a downstream gear pump with the screw operating in inlet pressure control and followed by the screw in manual operation (constant screw speed). The large level of variation in the motor current during constant screw speed control suggests that the extruder process is unstable, and the control algorithm is not the root cause for the variation in the motor current...

Information on dies and process control is in Chapter 3. Different control systems are used to process the different extruded products. Simplified examples of different controls are provided in Figures 5.3 and 5.4. 

Although there are very few twin-screw (TS) extruders in comparison to the many more single-screw extruders, they are used also to produce products such as window and custom profile systems. Their major use is in compounding applications. The popular common twin-screw extruders (in the family of multi-screw extruders) include tapered screws or parallel cylindrical screws with at least one feed port through a hopper, a discharge port to which a die is attached, and process controls such as temperature, pressure, screw rotation (rpm), melt output rate, etc.143... 

Initially, the focus has been on the bulk properties of the extruded mass, such as rheology and rheological change within the processed materials. This allows predictive performance of the raw material formulation and a primary means of specifying ingredient performance and process control, and will be examined further below. 

Most attention has been paid to applications of absorption and reflection spectroscopy in the NIR region for industrial process control (Dallin, 1997, Kammona et al, 1999, Fernando and Degamber, 2006). This arises firom the robust nature of the optical components and the availability of commercial systems that are based on gratings as well as interferometers. Transmission and reflectance probes that are designed to interface with extruders, and in particular reactive extruders, are commercially available, and their performance has been assessed quantitatively (Hansen and BQiettry, 1994). The interface for a rugged NIR probe to monitor the molten flow of an extruder is shown in Figure 3.44. 

Reactive Compatibilization has been discussed in earlier reviews [Brown and Orlando, 1988 Tzoganakis, 1989 Brown, 1992 Liu and Baker, 1992a]. As practiced commercially. Reactive Compatibilization is a continuous extruder process with material residence time usually 1 -5 min. Such a process permits large scale preparation of a polymer blend as needed ( Just-In-Time inventory control). 

The melt mixers are either batch or continuous type. The formers require lower investment cost, but are more labor-intensive, have low output and poor batch-to-batch reproducibility. Recent developments in process control and automation eliminated some of these disadvantages [Utracki, 1991]. The continuous melt mixers comprise extruders, continuous shaft mixers and specialty machines — these will be discussed in the following part of this chapter. A brief overview of the melt mixing devices is given in Table 9.8. 

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