SFVs to the rescue of co-rotating twin-screw extruders

Evolution of extruders
The ratio of the Do to Di is the factor that decides the free volume. The evolution of extruders based on the Do/Di ratio is outlined in Table 1.
During the initial days, machines were designed with a ratio of 1.27 and had a three-lobed profile. The need to improve the capability of an extruder has resulted in the ratio being increased all the way to 1.71. The most important aspect to be considered while selecting the ratio is the torque carrying capacity or alternatively specific torque of the extruder. A higher ratio is advantageous only if it does not compromise on the torque carrying capacity of the extruder.

The profile of the intake zone elements plays an important role in the conveying efficiency and capacity. Traditionally, profiles in the feed zones may be normal bi-lobed with leads 1.0 to 1.5 times the diameter (Figure 1).
Modified elements called SK elements (Figure 2) have 20 per cent more free volume and provide corresponding improvement in capacity.

Need for SFV elements
There are now many applications where fillers are added at very high percentages. Most applications with talc or extremely fine carbon are difficult to process because of severe limitations .in' intake capacity of the main feeder or the side-feeder.
                       
While many applications are torque-limited, these specific applications become feed-limited. A new element with SFV profile can be used to substantially improve the intake capacity while starve-feeding. These elements find application especially in the deep-flighted, high-speed extruders, which can handle feed limited applications in the best possible way.

The SFV elements (Figure 3 - patent pending) are specifically designed to improve the conveying efficiency. At low screw speeds, these elements are capable of compacting the material during conveying. Increase in intake capacity of nearly 200 per cent is achievable at rpm range of 150-900.

In an experiment (Figure 4) conducted with a premix of 50 per cent talc and 50 per cent LLDPE granules, the speed versus output was found to be linear with constant torque up to 900 rpm. There was no increase  In output with further increase in speed. Beyond this point, the extruder becomes torque limited instead of feed limited.

With a 40 mm extruder, a 300 per cent capacity increase has been experienced with SFV elements, running at 1100 rpm. The material is sucked in from the hopper by the partial vacuum created by the elements and compacted while conveying. With a different formulation containing 40 per cent talc, a sudden raise in starve-feeding capacity was observed at 1100 rpm, till it was limited by the torque carrying capacity of the extruder.

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