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As discussed in the May 2025 issue of this magazine, two very important screw design parameters are the metering channel depth and the compression ratio. In that article, we showed data for melting capacities, specific rates, pressure profiles and discharge temperatures for the extrusion of linear low-density polyethylene (LLDPE) resin pellets as a function of the compression ratio. 

The optimal compression ratio was shown to be 2.8, consistent with commercial operations. For screws with identical lead lengths in the feed and metering sections, the compression ratio is simply the feed channel depth divided by the metering channel depth. The compression ratio specifies the feed channel depth needed to keep the metering channel full of resin and pressurized. The optimal compression ratio depends on the resin and feedstock bulk density.

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To illustrate the importance of the compression ratio, let’s look at two commercial processes. First, a polystyrene (PS) resin running in an injection molding process, and second an acrylonitrile-butadiene-styrene terpolymer (ABS) resin running in an extrusion process. PS resins are brittle but can be toughened with small amounts of rubber. These toughened resins are referred to as high-impact polystyrene (HIPS) and are often used as replacements for ABS products which don’t need high-impact performance. A process comparison for the extrusion of ABS and HIPS is provided as a third case.

PS Injection Molding and Splay

Injection molded PS parts were afflicted with splay, reducing the capacity of the plant and increasing the cost to make the product. Splay is defined as a cosmetic surface defect that causes an off-color streak which is often in the flow direction. The splay defect in the part is shown in Figure 1. Here, the defect looked like it was caused by entrained air or moisture. About 5% of the parts produced had to be rejected due to the splay defect.

The plasticating screw was examined for design defects. The screw was designed with a compression ratio of 2.4, a ratio which is too low for PS resins. Here, the plasticator was 63 mm in diameter, and the feed and metering channel depths were 0.325 and 0.135 inch, respectively.

PS pellets plasticate the best with a screw designed with a compression ratio of 3.0. The low compression ratio was suspected to be the root cause of the splay defects. The low compression ratio would not be able to fully compact the resin, enabling small amounts of entrained air to discharge to the parts. The air was likely the root cause of the splay.

To eliminate the splay problem, the compression ratio of the screw was increased from the original ratio of 2.4 to 3.0. The higher compression ratio should enable the resin to compact and force the entrained air between the pellets to escape out through the hopper and not be entrained with the injectate. The higher compression ratio would also increase the melting capacity of the screw by increasing the pressure in the transition section. 

This modification was made by increasing the feed channel depth from 0.325 to 0.405 inch by removing small amounts of metal from the feed channel of a production screw. To maintain a constant compression rate on the transition section, deepening the feed section to 0.405 inch also decreased the feed section length by about 3.3 diameters and increased the length of the transition section by the same length. A summary of the channel dimensions for the original and modified screws are shown in Figure 2. This figure was constructed such that the zero point is the top horizontal line, corresponding to the barrel position.

PS pellets plasticate the best with a screw designed with a compression ratio of 3.0.

The modification to the screw eliminated the defects and decreased the cycle time by 8%, and increased the plant capacity by 14%. Here, the higher compression ratio caused higher channel pressures, complete compaction of the solid bed and the expulsion of the entrained air.

ABS Extrusion

The compression ratio can also be too high and lead to process instabilities. For example, a 6-inch diameter, two-stage, vented extruder was running an ABS resin. The screw had a first-stage metering channel depth of 0.33 inch and a compression ratio of 3.1. A compression ratio of 2.5 is used commercially for ABS pellet feedstocks. The compression ratio was too high for this application. A spiral dam was positioned in the first-stage metering section to mitigate low levels of solid polymer particles from incomplete melting in the transition section. The process was extremely unstable and was operating with a high specific energy from the motor. This screw would have worked well for a PS resin.

The high compression ratio caused the feed section to overfeed the extruder to a point that it could not completely melt the resin. Moreover, the high level of solid polymer fragments that were not melted in the transition section would flow downstream and plug the spiral dam. With the spiral dam plugged, the motor was inputting a higher level of energy into the resin, and the plug caused a decrease in the discharge pressure and the rate. 

This effect is shown in Figure 3 where at the 45-minute mark, the motor current increased substantially, indicating a high level of energy input, and the discharge pressure decreased, indicating a decrease in the rate. After the plug was melted, the line would become relatively stable until the spiral dam became plugged again. The technical solution for this problem was to build a new screw with a compression ratio of 2.5.

Solids Conveying Forces

It is clear from the case studies above that the solids-conveying ability is considerably higher for ABS resin as compared to PS resin. That is, a PS screw must have a deeper feed section to offset weaker solids conveying forces as compared to an ABS resin. To maintain acceptable extruder performance, the compression ratio is adjusted such that air entrapment does not occur, melting is acceptable and the metering channel is maintained full and pressurized.

As presented earlier, the optimal compression ratio for ABS resin pellets is 2.5. To reduce resin cost, some parts made from ABS are commonly switched to a lower cost HIPS resin. Screws designed for ABS, however, have feed channel depths which are too shallow to run HIPS effectively. Although the equipment required to extrude these two resins are nearly identical, a screw designed for ABS will run HIPS at reduced rates, especially at high screw speeds. 

Every resin is unique with different solids conveying abilities.

The difference in the rates is caused by the solids conveying behavior of the resins and the low compression ratio for the ABS screw. HIPS resins extrude optimally with a compression ratio of 3.0. Moreover, if ABS resin is extruded using a screw made for HIPS resin, the higher solids conveying rate for the ABS resin and the deeper feed channel can overfeed the metering channel, and solid polymer fragments are likely to occur in the discharge or flow downstream and plug a mixer such as in the previous case.

The solids conveying abilities of these resins depend on the channel geometry, process conditions, and frictional and viscous properties of the resin sliding on the barrel and screw surfaces in the feed section. The frictional and viscous forces are directly proportional to the stress at the interface. The stress at the interface is shown for HIPS in Figure 4 and for ABS in Figure 5. The stress at the interface is the same for PS and HIPS resins. Optimal solids conveying will occur when the forwarding forces and the stress are the highest at the barrel wall, and the retarding forces and stress at the screw surfaces are the lowest. 

As shown by Figure 4 for HIPS resin, the stress at the optimal conditions of the screw will occur at temperatures between 70 and 100°C where the retarding forces (and stress) at the screw are small. The stress at the interface is about 0.20 MPa. The stress is a maximum at the optimal conditions at a barrel wall temperature near 150°C. Here the stress is about 0.47 MPa. The difference in stress between the optimal screw and barrel conditions is about 0.27 MPa, as shown in Figure 4. This difference in stress is an indicator of the solids conveying ability of the resin. 

As a comparison, the difference in optimal stress levels between the barrel and screw for ABS resin is considerably higher at about 0.37 MPa, as shown in Figure 5. Thus, the solids conveying forces are higher for ABS than those for HIPS resin. To compensate, the solids channel depth and compression ratio are lower for ABS resins.

Every resin is unique with different solids conveying abilities. The feed section of the screw must be able to convey the resin at a rate high enough to maintain the metering channel full and pressurized while not allowing solid fragments to plug a mixer. Screw designers know how to set the compression ratio for commercial resins.

About the Author: Mark A. Spalding is a fellow in Packaging & Specialty Plastics and Hydrocarbons R&D at Dow Inc. in Midland, Michigan. During his 39 years at Dow, he has focused on development, design and troubleshooting of polymer processes, especially in single-screw extrusion. He co-authored  with Gregory Campbell. Contact: 989-636-9849; maspalding@dow.com; .