Extrusion process control

The melt index of extrusion or thermoforming grade of plastics is typically less than 1 gram per 10 minutes. The melt index for injection grade plastics is typically greater than 1 gram per 10 minutes. 

The transportation of the plastic in the extruder is retarded by the pressure that builds up at the die. The plastic flows along the extruder until it reaches the die where the plastic flows through the restrictive opening and conforms to the shape of the die. The pressure in the extruder is highest at the die exit and lowest at the feed zone at the hopper. Thus, the faster the screw is turning, the higher the pressure at the die. The pressure at the die will restrict the flow of the plastic. The pressure drop across the die can contribute to the quality of the plastic in the extrudate. 

The residence time in the extruder can also affect the quality of the plastic in the extrudate. Polymer degradation can occur in the extruder if the plastic is heated too long in the extruder. Degradation can result in 

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Concentrating on the control of the extrusion process, it is clear from the literature [54-68] that true total extrusion process control is quite complicated and has not been fully achieved in practice. Under true total process control, the process is considered a multivariable system and the interaction between the variables is known and fully taken into account in the control scheme. One can therefore assume that many commercial microprocessor-based controllers that control melt temperature, pressure, and extrudate dimensions are most likely built with more or less independent control loops, each controlling only one variable. These controllers more or less consolidate multiple discrete controllers into one convenient package without major changes to the individual control characteristics. This may reduce cost but does not necessarily improve overall control performance. 

The first requirement in the development of a true extrusion process control is a dynamic process model. The goodness of the process control will depend very strongly on the accuracy of the process model. However, obtaining a good dynamic process model is quite complicated in practice. The dynamic process model can, in principle, be derived from extrusion theory, and various attempts in that direction have indeed been made [50-54]. As will be discussed in Chapter 7, extrusion theory to date has not been developed to a point that the entire process can be predicted... 

Table 8.6. Fluoropolymer Extrusion Process Control Variables...

Nield, S.A., Budman, HM., Tzoganakis, C. Control of a LDPE Reactive Extrusion Process, Control Engineering Practice, 8,911-920 (2000). 

For several basic reasons, the extrusion process does not have the large number of possible process product interactions that the preceding molding methods presented. Due to this situation it can not fabricate the complex shapes and tighter tolerances obtained from molding. The process is a steady-state continuous production operation that can be brought to a condition of control. However it has its share of potential problems (Chapter 8, EXTRUSION). 

The operating pressures and shear rates in the extrusion process are considerably lower than they are in molding. As it exits the die, but not necessarily when it leaves the process, the material is in an essentially stress-free condition. Depending on the wall thickness of the material and the particular material, there is orientation of the plastic to a greater or lesser controllable degree. Thin walls produce higher orientation in materials such as PP, that is a highly crystalline polyolefin, and which orients much more than materials such as PVC. 

Insistence on all dimensions as critical will result in the process being restricted to one narrow set of operating conditions, usually at a low production rate, with a high scrap rate and high product costs. In cases where the dimensions are critical, it may be that the extrusion process should be discarded in favor of molding or machining of the product. However controlling all dimensions can be accomplished at a cost. 

Subject to the limitations indicated, control over extrusion process products is consistent enough to make a uniform repeatable product once the limitations are accommodated. Here, as in other processing, good communication between the processor and the designer will help make for a successful economical product. 

By extrusion parison control it is possible to minimize the wall thickness variation and the extent of stretching and stretch orientation. These are the province of the processor when the designer is not familiar with BM. Knowledge is required to provide information on what is possible and to select the specific BM process that has the capability to mold the product. The designer should be aware of the possible failure modes and compensate for them in the design. There is little else the designer can do but select the best material and process to make the product. 

The extrusion process has fewer process control problems with TPs than does injection molding but has greater problems in dimensional control and shape. During... 

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 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... 

Thermal quenching of the crosslinking reaction In an actual reactive extrusion process, the degree of crosslinking can be controlled by adjusting the residence time at elevated temperature. 

In many cases drying operations are critical to the production of successful commercial catalysts. Close control of the drying process is necessary to achieve the proper distribution of the catalyst precursor within the pore structure of the support. Drying also influences the physical characteristics of the finished catalyst and the ease with which subsequent pelleting or extrusion processes may be carried out. 

The first task was to produce carriers from different recipes and in different shapes as shown schematically in Fig. 8. The raw materials diatomaceous earth, water and various binders are mixed to a paste, which is subsequently extruded through a shaped nozzle and cut off to wet pellets. The wet pellets are finally dried and heated in a furnace in an oxidising atmosphere (calcination). The nozzle geometry determines the cross section of the pellet (cf. Fig. 3) and the pellet length is controlled by adjusting the cut-off device. Important parameters in the extrusion process are the dry matter content and the viscosity of the paste. The pore volume distribution of the carriers is measured by Hg porosimetry, in which the penetration of Hg into the pores of the carrier is measured as a function of applied pressure, and the surface area is measured by the BET method, which is based on adsorption of nitrogen on the carrier surface [1]. 

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...

Figure 13.1 Schematic of a tandem extrusion process for foams. The secondary extruder or cooling extruder will be the rate-controlling step for a properly designed process...

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. 

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... 

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