Extruder gear-pump

Polymer processes, whether batch or continuous, rarely run under stable equilibrium conditions. However, in order to operate such processes safely and in order to set the characteristics of the products optimally, a set of process manipulated variables must be kept constant or systematically modified over the duration of the reaction or in the course of the various reaction steps. The main process variables of this type are temperature, pressure, concentration, amount, flow, and level. Speeds (agitator, gear pumps, extruders), power input and viscosity can be of substantial importance also. 

Mnciple materials and the same between material and screw case of single screw extruder on gear pump principle... 

Fitted to an extruder a gear pump is particularly useful for difficult compounds and process conditions. The compound is softened and plasticised in the extruder and fed into the gear pump under low intake pressure. The gear pump intake pressure regulates the speed of the extruder so that the gear pump is always filled. The gear pump builds up the pressure required at the die. A screen can be fitted if desired. 

The gear pump can be fitted to a normal cold-feed, pin or vacuum extruder and enhances the accuracy and quality of the product. 

In this process, a vacuum extrusion step (melt treatment) is followed by a continuous SSP (solid treatment). The process starts with PET flakes which are introduced without pre-drying into a ring extruder. The flakes are dried, melted and degassed inside the extruder. A gear pump builds the necessary pressure for melt filtration, where solid particles are removed. After granulation the pellets are fed to a continuous three-step SSP unit [10]. 

Two new LDPE resins are proposed to be processed on a similar extruder as described in the example in Section 1.5.1. The extruder here is configured with a downstream gear pump such that the discharge pressure for the extruder is targeted at 7 MPa. The resin manufacturer has provided viscosity data in the form of a viscosity relationship ... 

The new commercial process was based on the lab process and has the target performance shown in Table 9.2. To achieve this performance, an 80 mm diameter extruder with an L/D ratio of 28 was specified. The extruder was built with a 100 kW motor and a gear box such that the maximum speed of the screw was 117 rpm. A gear pump was positioned downstream of the extruder such that the extruder only needed to generate 6 MPa of pressure at all rates. 

Figure 12.4 Schematic of a two-stage extruder with a downstream gear pump...

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 12.6 Pressure at the inlet to the gear pump for the data presented in Fig. 12.5. The pressure increased during manual control because the flow rate of the extruder was slightly higher than the rate of the pump...

A portable electronic data acquisition system was transported to the plant site and connected to the extruder panel. All available instrument outputs from the panel were connected in parallel with the acquisition system. Data collected included barrel zone temperatures, screw speed, motor current, pressure at the entry to the pump, transfer line temperature, and gear pump temperature. Process data were collected at a frequency of once every five seconds. 

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