Extrusion screw channel depth

Shear in the channel of the screw is equal to KDN/6Qh (where D = average barrel inside diameter, N = screw RPM, and h = average screw channel depth). This formula does not include the melt slippage between the barrel wall and screw surfaces, but the shear rate obtained is still useful for purposes of comparison. A l in. screw with a 0.140 in. channel rotating at 100 RPM results in a shear rate of 93.5 reciprocal seconds (rsec). This value is approximately the desired value in most extrusion processes, with 100 rsec generally the target. 

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... 

Extrusion moulding is mainly used in making wire insulation and elastic profiles. For slow extrusion, pastes with viscositoes of about 15-18 Pa s (150-180 P) are normally used for fast extrusion (hear rates of 103—104 s 1), plastisols with viscosities of about 20-25 Pa s (200-250 P) are suitable. Extruding machines for pastes should have long screws with narrow thread (small depth of screw channel). The cylinder temperature is kept at about 150 °C, and the head (die) temperature at about 180 °C. 

Determining the effect of viscous dissipation in the metering section of a single screw extruder. Consider a 60 mm diameter extruder with a4 mm channel depth and a screw speed of 60 rpm. The melt used in this extrusion system is a polycarbonate with a viscosity of 100 Pa-s, a thermal conductivity of 0.2 W/m/K and a heater temperature of 300°C. To assess the effect of viscous heating we can choose a temperature difference, AT of 30K. This simply means that the heater temperature is 30K above the melting temperature of the polymer. For this system, the Brinkman number becomes... 

Fig. 9.33 Calculated SBP versus reduced length of melting in a single-section screw A/ j/ — 0 denotes a constant channel-depth section. The SBP becomes increasingly concave as A/ j/ increases, either as a result of an increasing taper (i.e., increasing A) or a decreasing rate of melting (i.e., decreasing ij/). [Reprinted by permission from Z. Tadmor and 1. Klein, Engineering Principles of Plasticating Extrusion, Van Nostrand Reinhold, New York, 1970.]...

The channel depth decreases in the extruder compression section. Screws are designed with different numbers of turns of flight, and different compression ratios (the ratio of the channel depth in the feed section to that in the metering section) to suit the rheology of the polymer being extruded. The pressure generated here either squeezes out any gas bubbles, or causes gas to dissolve in the melt. Unless a foamed extrusion is required, bubbles must not be allowed to reform in the melt when it returns to atmospheric pressure after the die. Consequently, there may be a vent to the atmosphere or to a vacuum line just before the compression section to aid degassing. 

Extrusion instabilities] screw speed too high/screw temperature too high/barrel temperature at delivery end too high/channel depth too high in the metering sec-... 

Finally, increasing the channel depth will reduce the shear rate and viscous heating as discussed earlier this will result in lower melt temperatures. In fact, the channel depth is one of the most critical screw design parameters to control melt temperature. Deep-flighted screws are used when the viscous dissipation and melt temperatures have to be minimized. That is why screws used to extrude rubbers generally have deep channels. The same is true for cooling extruders in tandem extrusion lines for foamed polymers. 

High stock temperatures are likely to be a problem in extrusion operations where the extruder is run at high screw speed and where the polymer melt viscosity is high. The main screw design variable that affects viscous heating is the channel depth. Increasing the channel depth will reduce the shear rate and thus the viscous heating. There are limits to how deep the screw can be cut. One limit is the physical... 

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