The Maillefer Screw

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 width of the solids channel of the Maillefer screw can he written as  

The relationship becomes clear from examination of a picture of the unwrapped geometry of the Maillefer barrier section this is shown in Fig. 8.57. 

By comparing the channel width profile, Fig. 8.57, to the solid bed width profile in a standard screw with constant solid bed velocity. Fig. 8.56, it is clear that the two profiles are considerably different. If the solid bed is to occupy the full width of the channel W,s in the Maillefer screw, the velocity of the solid bed will have to change along the barrier section. This may require substantial deformations in the solid bed, but it will be assumed that the solid bed is capable of undergoing the required 

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The advantage of the Maillefer screw is primarily the physical separation of the melt pool and the solid bed. As a result, there is less chance of formation of a melt film between the solid bed and the screw and, therefore, there is less chance of solid bed breakup. Thus, the melting process can occur in a more stable fashion but not necessarily at a higher rate. 

The initial part of the barrier section is the same as the Maillefer screw. However, when the melt channel is sufficiently wide, the barrier flight starts to run parallel to the main flight. 

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 ... 

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 ... 

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. 

Fig. 9.40 Schematic view of the Maillefer barrier type screw.

The cold-feed extruder had shallow screw channels and a much longer length/diameter ratio. The original cold-feed extruder produced extrudates which had large temperature distributions and became distorted as they emerged from the die. The issue in cold-feed extrusion of rubber was to produce a thermally homogenized product in the extruder screw which would yield thermally uniform extrudate that does not distort. From the 1960s, there have been several improvements in cold-feed extruders. First, Maillefer [M6]... 

In the Maillefer type screw design, there is a spiral barrier or secondary screw flight extending over the entire length of the modified section of the screw. The mass and force balances across the Maillefer type screw are available [61] and can be used to estimate the increase in power requirements due to this modification. Additional barriers or flights may be introduced to split the flow into two or more streams, which in the limit arrives at a series of longitudinal grooves or flutes spread around the circumference of a cylinder. 

The barrier melting screw can be an answer to this problem. They are understandably often referred to as barrier mixing screws because their replacement for conventional screws will often give an overall improvement in mixing as a consequence of their improved polymer melting. The barrier screw was patented in Europe in 1959 by Maillefer, a Swiss manufacturer of wire and cable machinery [16, 17] and curiously [8] patented by Uniroyal in 1961 [18]. 

The Maillefer barrier screw s performance was first described in 1963 [21]. 

With the original situation that the barrier screw was patented in Europe by Maillefer, a Swiss cable machinery manufacturer, there was little interest in Europe in this technology other than by insulated wire and cable producers. When the patents lapsed, interest focused on designs emanating from developments which had taken place in the USA. 

A comparison of a Maillefer type barrier screw with a conventional screw in Section 7.5.1, showed for the barrier screw, a reduced output rate when used with PP and an increased output rate when used with LDPE. Following the discovery that the metering zone of a Maillefer type screw was running only partially filled when extruding medium density polyethylene (MDPE), Qui and co-workers [25] evaluated a grooved feed zone to ensure the metering channel was filled. 

With the apparent approaching maturity of the Maillefer and Uniroyal derived barrier screw developments, further advances will most probably be in more radical innovations. Their potential diversity is illustrated by the following examples. 

The inventor of the barrier flight extruder screw is Maillefer, a pioneer in the field of extrusion. He first appiied for a patent in Switzerland on December 31, 1959 [23], and later applied for patents in various other countries. Patents were granted, among other countries, in Germany [24] and in Engiand [25]. Maillefer filed a patent application in the U.S. on December 20,1960. However, Maillefer did not obtain a patent in the U. S. because of a particular provision in U. S. patent law. Geyer from Uniroyal filed a patent application for a barrier screw on Aprii 5, 1961, several months after Maillefer s filing date [26]. 

This represents the shortest possible melting length with a Maillefer-type barrier screw. A reasonable maximum value of E = 0.4 this results in the following melting... 

The early analysis of solids conveying in an extruder evolved through several simple models based on simple concepts such as the frictional forces between the polymer and the metal surfaces is credited to Decker [36] and Maillefer [37] and the concept of the differential torque at the screw and barrel surface was introduces by Simonds [38]. The rigor of the derivation was significantly improved by Darnell and Mol of DuPont and their analysis is most often cited as it gives a good description of the behavior observed. The model is based on the force and torque balance developed on the solid plug, which is assumed to completely fill the screw channel. The basic assumptions used for the derivation of the model are ... 

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