Screw speed, increase

Shear rate When, a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner and cavity, or die wall, it is subject to a shearing force. As the screw speed increases, so does the shear rate, with potential advantages and disadvantages. The advantages of an increased shear rate are a less viscous melt and easier flow. This shear-thinning action is required to move the melt. 

Figures 4.20 to 4.22 show the relationship between screw speed, throughput, specific mechanical energy input, and melt temperature. It is apparent that melt temperature and specific mechanical input increase as screw speed increases and decrease as the throughput increases (see Fig. 4.20 and Fig. 4.21), while the temperature increase in relation to the screw speed is greater than the temperature decrease in relation to the throughput. Therefore, according to the above equation, the melt temperature ultimately rises as the specific energy input rises (see Fig. 4.22).

Die-block temperature too Increase screw speed. Increase temperature. 

Incomplete blow Extrusion rate too high Blow-up air pressure Blow-up time too short Parison is cut at pinch-off Reduce screw speed Increase blow air pressure Reduce mold closing speed... 

What happens to the melt temperature as the screw speed increases above 70 RPM Why ... 

Initial die gaps are set to about 20 percent greater than the final film thickness, and then adjusted to accommodate changes in polymer flow which are resin and rate sensitive. Higher screw speeds increase extruder output, overall film thickness, the tendency toward melt fracture, and may alter the flow pattern. Thus, extruder speed is not a recommended control. In contrast, increased chill-roll speeds decrease film thickness, reduce film width due to increased neck in, increase uniaxial orientation, and alter the optimum air gap or drawdown distance. The optimum air gap, which produces the best orientation, crystallization, and surface properties, depends on the material and chill-roll speed. At 23 to 30 m/min (75 to 100 ft/min), the air gap for low-density polyethylene is about 100 mm (4 in), but when the line speed increases, the air gap is found by trial and error, Since the chill-roll speed controls film stretching, the take-off speed has little effect on the film dimensions. 

In conventional extruders the melt temperature typically increases with screw speed. This often creates problems when we deal with high-viscosity polymers, particularly when they have limited thermal stability. In the example shown in Fig. 2.10 the melt temperature is essentially constant as screw speed increases from 200 to 1000 rpm. This is probably the result of two factors the residence time of the polymer melt is reduced with increasing screw speed and the volume of the molten polymer likely reduces with increasing screw speed as the melting length becomes longer with increased screw speed. 

The effect of increasing screw speed diminishes at higher screw speeds. Increasing the screw speed from 25 to 50 rpm increases the final temperature by about 5°C, while an increase from 100 to 200 rpm increases the final temperature about 4°C. 

Thus, the critical screw speed depends on the heat flux, channel depth, temperature coefficient, initial melt temperature, reference temperature, consistency index, screw diameter, and power law index. The critical screw speed increases with the heat flux and channel depth, and reduces with the power law index, consistency index, and screw diameter. The effect of the power law index is shown in Fig. 7.96. 

When a melt moves in a direction parallel to a fixed surface, such as with a screw barrel, mold runner, or die wall, it is subject to a shearing force. As the screw speed increases. 

In Elemans and Van Wunnick s trials with PBT/2% ultramarine blue, extrudates and injection moulded plaques showed significantly fewer agglomerates when screw speed was increased from a flood fed 40 rpm condition to a starved 60 rpm at the same feed rate of 40 kg/h (Figure 13.8). Further screw speed increases gave no further improvement. 

Figure 7 depicts a chart of extruder output vs. screw speed. Considerable increase in the extruder output was seen upon incorporation of SOLPLUS 3000/AX5 (modified version of S3000) as compared to the control formulation. As expected, the torque at the respective screw speed was much lower as compared to the control in the presence of a hyperdispersant. The results are shown in Figure 8. The torque for the control formulations at higher screw speeds increased exponentially. This is expected under normal circumstances. Use of a hyperdispersant allowed the formulation to be processed at relatively lower torque.

Deviation from LCL Deviations on extruder screw speed Increase in feeding rate Increase in feeding rate, operator change Increase in vacuum... 

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