Screw melt performance

Compression rates typically vary between 0.0025 and 0.0055 for pellet feedstocks. If the compression rate is too low, then the compaction rate of the solid feedstock may not be high enough to force the entrained gas out through the hopper. If the compression rate is too high, poor melting performance and solid bed breakup can occur. For example, LDPE resins can be melted very easily using a screw with a compression rate of 0.0055, while LLDPE resins perform best with a compression rate near 0.0030 [3, 4]. If an LLDPE is extruded using a screw with a compression rate of 0.0055, then solid bed breakup and solid polymer particles in the extrudate are likely to occur. 

Elbirli, B., Lindt, J.T., Gottgetreu, S. R., and Baba, S.M., Mathematical Modeling of Melting of Polymers in a Single-Screw Extruder, Polym. Eng. ScL, 24, 988 (1984) Lindt, J.T. and Elbirli, B., Effect of the Cross-Channel Flow on the Melting Performance of a Single-Screw Extruder, Polym. Eng. ScL, 25, 412 (1985)... 

As the feeder is adjusted, the screw s performance will at some point deteriorate so that the output consistency or the melt efficiency is threatened. Adjusting the screw design to preclude the need to starve is a better alternative, which usually is economically feasible. 

These results are for a 63-mm extruder running a 0.2 melt index HOPE at a screw speed of 60 rpm. ft is clear that increased flight clearance significantly reduces melting performance, ft is important, therefore, to keep the flight clearance small in the melting zone of the extruder. 

The following analysis of the melting performance of various barrier screws is based on the analysis developed by Meijer and Ingen Housz [27]. This analysis provides a clear and logical approach to the determination of the melting capacity as a function of the barrier section geometry. 

The melting performance of the Maillefer screw geometry can be analyzed by a down-channel mass balance ... 

The right-hand term, the melting rate per unit down-channel distance, is given by Eq. 7.109(c). In the analysis of the melting performance, a complication arises in that the solid bed velocity cannot be assumed constant. This can be appreciated by comparing the solid bed width profile of a standard screw with constant channel depth to the channel width profile of the solids channel in a Maillefer screw. The width of the solid bed in a standard zero-compression screw as a function of down-channel distance z can be written as ... 

The melting performance of the Barr screw can be analyzed by the same procedure followed for the Maillefer screw. The initial portion of the barrier section can be analyzed just as a Maillefer screw. If Zji is the length of the initial Maillefer portion of the barrier section, the solid bed velocity at z = Zj, is approximately ... 

In the parallel portion of the barrier section, the width of the solids channel is constant. The highest melting performance will be reached if the width of the solid bed fills the entire width of the solids channel. If the solid bed velocity is assumed constant, the total melting length can be found by using the equations derived from the standard extruder screw see Eq. 7.116. The total melting length for the parallel barrier portion is simply ... 

If typical values are used for cpb and cpf, the melting length of the Dray and Lawrence screw will about 10 to 20% longer than the ideal compression screw. The melting performance of the Dray and Lawrence screw is thus about the same as the Barr screw and slightly better than the Maillefer screw. A patent on this barrier screw... 

Again, the melting performance can be analyzed by the procedure used for the Maillefer screw. Because of the continuously varying helix angle, the analysis is rather involved. Ingen Housz and Meijer [27] found for the total melting length of the Kim screw ... 

This means that the melting performance of the Kim screw is slightly lower than the Dray and Lawrence screw and the Barr screw. It has an advantage over the Dray and Lawrence screw in that the transition from feed to barrier section occurs more smoothly. However, at the end of the barrier section the same difficulty arises as with the Dray and Lawrence screw. 

The characteristics of the various barrier screws are summarized in Table 8.1. The Maillefer screw has many desirable characteristics despite the fact that its melting performance is not quite as good as the other barrier screws. The Ingen Housz screw clearly has the best melting performance however, this is at the expense of geometrical simplicity. 

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