Critical screw speed

When Bo is taken as zero, Eq. 7.397 represents the screw speed at which the viscous dissipation equals the heat transfer. At this critical screw speed, the melt temperature does not change at all along the length of the extruder screw. This critical value of the screw speed is not dependent on the mass flow rate and specific heat it can be written as follows ... 

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. 

Even relatively small increases in the power law index, e.g., from 0.3 to 0.5, can reduce the critical screw speed significantly. This indicates that small increases in the power law index can cause significant increases in viscous heating and melt temperature. This is known in practice when we consider the extrusion characteristics of LLDPE relative to LDPE [325]. The power law index of LLDPE is considerably higher than that of LDPE. As a result, LLDPE tends to have more power consumption, higher melt temperatures, higher diehead pressures, and is more susceptible to melt fracture. 

Figure 7.96 Critical screw speed vs. power law index for heat flux of 10,000 W/m ...

Figure 4.12 Critical screw speed and/or minimum throughput for the extrusion of polyamide...

The material of construction for the screw is critical in order to provide the proper strength in the event of an improper startup procedure such as a cold start and to minimize cost. The choice of the material of construction depends on the motor size, the maximum screw speed at the base motor speed, and the depth of the feed section. The maximum torque that the motor and gearbox combination can supply to the screw, (Nm) is computed as follows ... 

The drive unit usually is based on an electric motor and turns the screw at a suitable speed set in advance the electric elements in the cylinder are connected to temperature controllers that maintain the levels required. While screw speed and temperature are of critical importance the ability to extrude a given material continually to a satisfactory standard depends also on more fundamental considerations—like the suitability in design and construction of an entire line, including the operation of the die and of any haul-off or other technique applied after material passes the die. 

Older SSE s have been equipped with minimum instrumentation a pressure gauge at one point along the barrel (usually at the head) and a thermocouple in the hot melt region. For non-critical operations the operator would record pressure, temperature, screw speed, and mass flow rates (typically determined by the sample weight-time method). 

For impact-modified blends (Xenoy 1200 and 5720, Stapron E), screw speed should be set at minimum, as low as possible (less than 50 rpm), just sufficient to assure a reasonable cycle time. Melt temperature should be kept as low as possible, without binding the screw. When temperature control is critical, as is the case here, the acmal melt temperature should be checked with a pyrometer or digital thermometer, rather than relying on barrel set points. The material runs best in thick parts with oversized gates. Injection speed of 25 mm/sec is suggested. When running a large shot (>70% of machine capacity), a flat or reverse barrel heat profile may help to convey the melt. 

Melt fracture where the critical shear stress of polymer (about 0.1 to 0.4 MPa) exceeded in the die excessive shear stress at the wall >0.1 MPa] exit speed at the die is too fast/melt too cold/throughput excessive/die land too short/die opening too small/entrance to die not sufficiently streamlined/screw speed too high/molar mass and melt viscosity too high/cross section area in exit flow channel too small/ external lubricant additive missing. 

If the diameter D = 120 mm, the screw speed N = 1.5 rev/s, the elongational viscosity T e = 1,500 Pa-s, a = 30°, and the critical elongational stress a nt = 100,000 Pa, then the maximum flight clearance is = 2.45 mm. The expressions above can be used for a first order approximation of the critical geometrical parameters of the mixer. For accurate determination, numerical techniques are necessary to capture the complexity of actual flow. 

The polarity of the insulating matrix plays a critical role in the miscibility of the insulating matrix/conducting polymers blends. The adhesion between non-polar PE and polar ICPs suffer due to this disparity. In order to improve the compatibility between the polymer components, low density PE was blended with p-toluenesulfonic acid, phenolsulfonic acid or camphorsulfonic acid doped polyaniline in the presence of functionalized metallocene PE, a compatibilizer [56]. The components were fed into a co-rotating twin-screw extruder at 200 C. Blending was xmdertaken at a screw speed of 60 rpm under nitrogen atmosphere. Castillo-Castro et al. (2011) have prepared n-dodecylbenzene sulfonate doped polyaniline/ low... 

Because extruders were initially developed to serve industries other than the pharmaceutical industry, they were designed without GMP considerations. Therefore, changes are made to ensure pharmaceutical regulatory compliance, including qualification requirements. At the minimum, all the parts of the extruder that come in contact with the product must be of high-quality stainless steel, and the construction must be such that cleaning requirements are easily met. Whenever possible, the machine should permit easy monitoring and documentation of all critical process parameters such as pressure at the die plate, product temperature, power consumption or torque of the drive unit, and rotational speed of the screw. Some of... 

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