Extrusion screw speed

Figure 31.1 The processing of PET leads to loss of viscosity, while the addition of clays increases the viscosity at low contents. Extrusion screw speed is 50 rpm. Mold injection temperature is 7°C. The tests were made in a rheometer with parallel plates of 25 mm diameter. Source Reproduced with permission from Sanchez-Solis A, Romero I, Estrada MR, Calderas F, Manero O. Polym Eng Sci 2004 44 1094 [61]. Copyright 2004 John Wiley and Sons, Inc.

Primarily used in profile extrusion Screw speeds from 10-40 rpm... 

As plastics can have quite different viscosities, they will tend to behave differently during extrusion. Fig. 4.3 shows some typical outputs possible with different plastics in extruders with a variety of barrel diameters. This diagram is to provide a general idea of the ranking of materials - actual outputs may vary 25% from those shown, depending on temperatures, screw speeds, etc. 

These complex systems demand strict screw speed control for all the individual machines in the setup, to balance the extrusion characteristics of the different rubber compounds forming the multiunit component. 

Katta, S.K. et al., Elfect of temperature and screw speed on stability of fumonisin Bl in extrusion-cooked corn grits. Cereal Chem., 76, 16, 1999. 

The target discharge temperature for the extrusion is 240 °C, and the maximum screw speed is 130 rpm (N = 2.167 rev/s). What will be the expected production rates for both resins at 130 rpm At 240 °C the melt density of the resins is 735 kg/mT As presented in Sections 1.4 and 7.4, the net flow in the extruder is the difference between the rotational flow and the flow induced by the pressure gradient Qp. The data in Table 7.2 was calculated from the example in Section 1.5.1 and Table 7.1 ... 

The baseline extrusion process was numerically simulated using the processing conditions in Table 9.4 and the method described in Section 9.2.1, that is, with a rate of 77 kg/h, a screw speed of 27 rpm, and a discharge pressure of 10.6 MPa. The iterative calculation process was used to estimate a bulk temperature of 160 °C and a pressure of 13.1 MPa at the entrance to the meter section. The axial pressure and temperature profile for the simulation is shown in Fig. 9.5. 

The availability of power and torque to the shank of the screw is extremely important to the success of an extrusion process, especially if modifications to the line are planned to increase the rate. That is, the proper level of power must be available at the proper screw speed. Thus, the motor size and speed, belt sheaving if used, and gearbox reduction must be specified to provide enough power to the screw to plasticate and pump the resin at a high rate. 

To illustrate the barrel optimization process [22], an extrusion process was optimized first for rate and then for discharge temperature. Each of the experiments was conducted at a screw speed of 50 rpm. The initial extruder barrel tempera-... 

Figure 10.13 Extrusion rate and motor current for an HDPE resin at a screw speed of 50 rpm. The highest rate was achieved at a Zone 1 temperature of about 230 °C...

Figure 10.14 Extrusion rate and discharge temperature for an HOPE resin at a screw speed of 50 rpm. Discharge temperature increased when the Zone 1 temperature was increased...

Table 10.7 Extrusion Measurements for a Screw Speed of 60 rpm and Different Barrel Temperatures...

After the screw modification, the 148 kg/h rate was obtained at a screw speed of about 69 rpm with an extrudate temperature of 223 °C. Thus, the specific rate increased from 1.63 kg/(h rpm) before the modification to 2.14 kg/(h-rpm) after the modification, a specific rate increase of about 30%. At a screw speed of 69 rpm, the rotationai flow rate was calculated at 173 kg/h now the extruder was operating at about 86% of the rotational flow rate. The calculated axial pressure gradient required to maintain the flow of the extruder at the reported flow rate showed that pressures in the screw never decreased to zero, indicating that the channels were full as shown in Fig. 11.21. No adverse effects were experienced with the reduced discharge temperature (8 °C lower), no unmelted material was observed in the extrudate, and no gel showers occurred after the modification. A summary of the extrusion performance before and after the modification is shown in Table 11.5. 

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...

The scale-up factors depend on the specific event being scaled-up in extrusion. The cube rule for mixing (23) states that at constant screw speed, output and power consumption increase with ct when H/D ratio is constant. The square-root rule for conveying (24) of material states that when channel depth is increased and screw speed decreased with x/d, the output rate increases with S, while power consumption increases by One could obtain... 

The pressure exerted by the rotating extrusion heads on the screen as product is being pushed out in an A-R screw extruder (25) is shown in Figure 10. Each spike in the profile represents one revolution of the extrusion head while the frequency represents screw speed. Based on the shear stress developed during extrusion, wet mixtures of MCC-lactose had a lower yield stress in the radial screw extruder (130kN/m ) than in the ram... 

Figure 11 Torque of extrusion as a function of moisture content of wet mass of MCC at varying screw speeds a-19 rpm, -28 rpm, c-38 rpm, d-4S rpm. Source From Ref 26.

Screw speed is another variable that impacts extrudate quality via residence time. The effect of screw speed on extrusion shear stress is given (22) by the expression... 

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