Specific rotational flow rate

At the startup of the line, the extruder was operated at 91 rpm to produce the required rate of 148 kg/h for a specific rate of 1.63 kg/(h-rpm). The temperature of the extrudate was measured through the transfer line wall at 232 °C. Due to process safety constraints the extrudate temperature could not be measured using a handheld temperature sensor. The extrusion rate was required in order to maintain the downstream take-away equipment at its maximum rate. At first the extruder appeared to be operating well except that the specific rate was lower than predicted. That is, the screw was rotated at an rpm that was higher than expected to produce the 148 kg/h. At 91 rpm, the rotational flow rate was calculated at 228 kg/h the specific rotational flow rate was calculated at 2.51 kg/(h-rpm). Thus, the line was operating at only 65% of the rotational flow rate. A barrier design... 

The lead length was 172 mm for the main flight of the barrier section and 114.3 mm for all other sections of the screw. The main flight width and clearance were 11 and 0.11 mm, respectively, in all sections of the screw. The first 2.7 diameters of the screw were inside a water-cooled feed casing. The compression ratio was 2.5 and the compression rate of the barrier section was 0.0030. The depth of the solids channel at the end of the barrier section was 3.2 mm. The specific rotational flow rate was calculated at 5.9 kg/(h-rpm). ... 

The injection-molding press was producing a part and runner system that had a mass of 2.15 kg. The mass was plasticated using a 120 mm diameter, 8L/D screw. The screw used for the process had a barrier melting section that extended to the end of the screw, as shown by the specifications in Table 11.9. That is, the screw did not have a metering channel. Instead, the last sections of the barrier section were required to produce the pressure that was needed to flow the resin through the nonreturn valve and into the front of the screw. The specific rotational flow rate for the screw for the IRPS resin was calculated at 9.3 kg/(h-rpm) based on the depth of the channel at the end of the transition section. The screw was built with an extremely low compression ratio and compression rate of 1.5 and 0.0013, respectively. For IRPS resins and other PS resins, screws with low compression ratios and compression rates tend to operate partially filled. The compression ratio and compression rate for the screw are preferred to be around 3.0 and 0.0035, respectively. The flight radii on the screw were extremely small at about 0.2 times the channel depth. For IRPS resin, the ratio of the radii to the channel depth should be about 1. 

Several mechanisms could cause the specific rate of the screw to be considerably less than the calculated specific rotational flow rate for the screw. These mechanisms include (1) normal operation for a screw with a very short metering section and a low-viscosity resin, (2) the screw is rate-limited by solids conveying, causing the downstream sections of the screw to operate partially filled, and (3) the entry to the barrier section is restricting flow (see Section 11.10.1) to the downstream sections of the screw and causing the downstream sections to operate partially filled. The goal was to determine which of the above mechanisms was responsible for the low specific rates for the plasticator. 

The entrance and exit regions in the spiral dam were also modified to eliminate the stagnant sections of the channel. The modification Is shown In Fig. 11.40. This modification allowed a relatively small amount of resin to flow Into the smaller channel at the entry such that stagnation of the resin cannot occur. A similar modification was made at the exit to allow a small amount of resin to flow out of the smaller channel into the main flow channel. To eliminate the unmelted particles or the particles that appeared to be more viscous because they were at a lower temperature, the clearance to the spiral dam was decreased from 0.76 to 0.25 mm. Since the meter channel depth was unchanged, the specific rotational flow rate for the modified screw was unchanged at 0.94 kg/(h-rpm). 

Flight width and flight clearance were 13.5 and 0.12 mm, respectively, in all sections of the screw. The extruder had a smooth-bore feed section. Three rings of pin mixers were evenly positioned in the middle portion of the metering section of the screw. All pins in a single ring had the same axial position as the type shown by Fig. 8.25. The specific rotational flow rate for the metering section was calculated at 5.0 kg/(h rpm). The compression ratio was 2.7. 

Because the processing specific rate was extremely lower than the calculated specific rotational flow rate, it was hypothesized that the metering section of the screw was too deep for this resin. Although the resin produced here was extremely vis-... 

Flight width and flight clearance were 38 and 0.38 mm, respectively, in all sections of the screw. The specific rotational flow rate for the metering section was calculated at 410 kg/(h rpm). ... 

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