Practical applications of the short, adjustable MCT cold-feed mixer-extruder.

Practical applications of the short, adjustable MCT cold-feed mixer-extruder

MCT stands for Multi-Cut Transfermix, covered by worldwide patents. It represents the second generation of the already well-known Transfermix. It was developed to provide a much more concentrated mixing and plasticizing action in only one transfer zone in order to:

* get away from the need for lengthening of extruders in response to increasing demands of processing, and

* to overcome the problems of scale-up in plasticizing from cold feed.

In a paper read in 1983[1], the author compared the development potential of the cold-feed plasticizing systems maillefer (Plastiscrew in the U.S.), E.V.K., pin-barrel and MCT by arranging them in a hierarchy according to basic features and by quoting comparative results.

The MCT features of making the helix-angle in the barrel complementary to that in the screw and making the number of helical grooves in the screw and the barrel change inversely in one or more steps within the same transfer-zone, were adopted to obtain the necessary high number of grooves without making any groove too deep to maintain positive forward transport in both the screw and the barrel.

This is illustrated in the scale-up of an MCT 250 shown in figure 1. The object is to do the work of plasticization in the first transfer zone from screw to barrel. In the developed view of the interface between the screw and the barrel, the first transfer-zone shows as dense a concentration of well distributed cut-points between crossing lands of screw and barrel as exists in the MCT 95. Such a concentration of cut-points cannot be approached with any other plasticizing systems.

Scale-up

This solution of the scale-up problem for the MCT of L/D=7 has been proven in practice up to 150mm (6") diameter units. The drawing shows that this can be carried to any size of cold feed extruder, for example, the next bigger sizes being covered by four groups of complementary subdivisions of the annular space between the screw and the barrel in the first transfer-zone.

The sections in figure 1 provide a better visualization of the three such groups in this example. It will be seen that the even distribution of cut-points extends also radially through the annular flow of the compound and that the overall flow cross-section remains unrestricted.

Self-cleaning

The indication of the grooves being of opposite hand in the screw and the barrel will make it clear that although only the screw rotates, the relative circumferential motion must induce positive forward transport in the barrel as well as in the screw. For this reason the MCT is at least as self-cleaning as the best single-screw extruder and perhaps more so as its individual grooves with their changing depths are mostly shallower than a plain screw. This has been amply proven in practice where compounds having sufficient green shear- and tensile-strength clean out completely to the end of the screw. Compounds tending to smear and which at the end of a run hang up on any screw will be removed easily by means of a cleaning batch. Referencing back to figure 1 shows that there are no locations for material to hang up.

Mixing and plastification

The developed and exploded view of figure 1a shows flow division and rearrangement, i.e. mixing in principle, in a Transfermix. In this, the geometry of screw and barrel grooves, i.e. their width and depth, determines the size of the element being worked upon in the transfer.

One may consider, in a simplified view of the plasticization process for different compounds, that these differ in the size of the element necessary to be worked, i.e. the more difficult the compound, the smaller the element which has to be worked while an easier compound can, without harm, also be worked on a smaller than its `characteristic' element size.

Practice has shown that the Multi-Cut features are essential to provide the necessary concentration of interacting helical grooves in a transfer-zone of about 1.2 x D to cover the plasticization of a wide range of compounds, with mixing being included anyway.

This shows up in a production-application of blending in one pass through an MCT two compounds of dissimilar rheogram-characteristics, besides extruding a tire-tread, from the two different feed-strips fed simultaneously.

Similarly, final mixing from a preblend of masterbatch pellets and vulcanizing chemicals in powder form has been demonstrated to yield as good results in every test criterion as the same operation on a mill.

The MCT has demonstrated superior plasticization on compounds with high NR content, high filler contents and on thermoplastic rubbers, i.e. on the compounds which are difficult or impossible to run on conventional extruders.

Adjustability

The adjustability-feature (throttle) is utilized for these more difficult compounds by starting from an underplasticized condition with excessive output to arrive at a minimum plasticized temperature for the compounding concerned as the degree of homogenization required by the process. This economizes on mechanical work input and substantially reduces the need for cooling insofar as this is possible to any great extent with rubber at all.

The graph in figure 2, reporting results on an MCT 150, emphasizes the effects of throttling to the purpose of keeping down the extrusion-temperature and thereby achieving a very respectable output. Figure 3 summarizes graphically some results in these respects for a range of compounds, both for the MCT 95 and the MCT 150. Outstanding characteristics are the flat temperature curves.

For some compounds these indicate that the maximum screw speeds of the drives provided actually still limit the possible outputs. It will be noted that MCTs generically run at at least 50% higher screw-speeds than conventional, long extruders. This is in itself an aid to smoothness or uniformity of extrusion.

Uniformity

The results in table 1 show that it is necessary to use the control-parameter of throttling, as well as other parameters concerned with the production of a really stable and controlled rolling bank in the extruder-inlet to provide uniformity to the ever more exacting standards of statistical process control for tread, hose and other profile extrusion. It proved possible to achieve this without loss of the higher throughputs and/or lower extrusion temperatures already reported for the MCT in 1983, and on a widening range of compounds.

It is worth noting that the process capability index Cpk based on 1% above and below the mean represents, for the example of tire-treads, a considerably higher standard than an index based on 2% above and below the mean, as appears to be accepted in production today.

Applications made possible by the short MCT

A very compact triplex installation comprising one MCT 95 and two MCT 50s with an RCM triplex extrusion head is possible. This is for tread with wings and cushion-gum or other tire components improved by co-extrusion. With each of the MCT 50s capable of an output up to 200 kg/hr and the MCT 95 from 600-800 kg/hr (see figure 3 and table 1) the combined output, depending on the relative cross-sectional areas in the extrudate, could be up to 1 t.p.h.

Examples of MCT Underpack conversions of existing tread extrusion lines are:

* One MCT 50 with an adaptation to the existing extrusion head converts a hot feed 8" extruder to a duplex for tread with cushion-gum or with mini-wings.

* Two MCT 50s with a new RCM triplex extrusion head convert a hot feed 8" extruder to produce tread with wings and cushion gum.

* An MCT 120 (4-1/2") with a new RCM duplex head for a 10" hot feed tread line.

All the RCM extrusion heads feature hydraulic opening for cleaning and change of die-packs in very short change-over times, partly on account of being so compact that they fitted into the space of the original single tread heads.

On the same scheme is a duplex co-extrusion line for bicycle and moped tires which has provided the user with a considerable improvement in quality in tread-to-sidewall adhesion and uniformity.

Special extrusion heads

From a tread die-head, a computer adjustable die head for treads - optionally with co-extrusion of cushion-gum - has been developed in which interchangeable side- and top-pieces (to provide for different shapes) are moved to provide different widths and depths of extrudate. This applies to any blanks. Die-sizes are called up by number in the computer.

As optional features, the length of the extrudate can be measured by the wide horizontal roller, to effect a change in die-size after a predetermined length.

Additionally, the upper roller measures the tread depth and, by means of a closed loop, causes computer adjustment of the depth to compensate for different swell-factors caused by changes in viscosity. These could be due to compound from different ends of a batch, or due to feeding in off-spec (previously extruded) compound with the fresh feed.

A number of these heads have been in use with satisfied customers for a considerable time.

MCT vacuum extruder

This has a very short plasticizing section followed by the degassing section and a metering section of whatever length may be needed for building up the extrusion pressure, in this case a high one.

The short plasticizing section, equivalent of a first MCT transfer-zone from screw into barrel followed by an adjustable over-flow regulation, leads to:

* Stability of degassing with any die-resistance and screw speed, also slow speed;

* Elimination of the need for screw-changes;

* High output, so that an MCT degassing extruder approaches the performance of the same size of cold-feed extruder without degassing;

* Retrofits with MCT Vac screws and barrels resulting in upgrading of existing installations.

Computer control

So far computer-control with set programs for different compounds and processes, including start-up and cleaning sub-programs, has been used on MCT-lines, as it has been on conventional ones.

However, while on conventional extruders computer process control can only influence the screw-speed or sectional tempering which are slow-response parameters, on account of its low hold-up volume and quick-response throttling the MCT is far better suited to process automation involving closed control loops.

Summary

MCT stands for Multi-Cut Transfermix, patented worldwide. New visualizations clearly explained its concentrated plasticizing action and why it is at least as self-cleaning as the best single-screw extruder.

The mixing function is illustrated by a production application of blending two dissimilar compounds fed as strip, as well as work done on final-mixing/extrusion. This mixing action improves the plasticization of compounds with nervy natural rubber, high filler contents or thermoplastic rubbers.

The adjustability feature (throttle) is utilized for the more difficult compounds by starting from an underplasticized condition with excessive output to arrive at a minimum plasticized temperature for the compound at the degree of homogenization required by the process. This economizes mechanical work input, widens the range of compounds and contributes to the achievement of extrusion-uniformity to equal or better standards of statistical process control than at present used.

The short MCT plasticizing section makes possible an L/D of 7 which is characteristic for tire-industry applications. Positive results are reported for the diameter-range from 40-150mm (1-3/4"-6"). This results not only in very compact duplex and triplex extruders, but also to upgrading of existing cold-or hot-feed tread extruders to low cost duplex and triplex lines. Bicycle and motorcycle tread co-extrusion lines are also quoted as examples.

The compact build and good plasticization of the MCT facilitates:

* The use of multiple extrusion-heads with much shortened internal passages providing quicker cleaning and much reduced change-over times;

* The use of a computer-programmable die-head for simple sections, such as treads or blanks;

* For technical rubber goods, the use of the Multex co-extrusion system for complex cross-sectional shapes from two or more compounds. This comprises a profile extrusion head in which the dimensions of parts of the shape can be corrected during operation, to avoid trial and error metalworking shape adjustment in the lead-in portion of the die itself.

In an MCT vacuum extruder, the short and effective plasticizing section with an adjustable overflow-regulation leads to:

* Stability of degassing with any die-resistance and screw speed;

* Elimination of the need for screw changes;

* High output, so that an MCT degassing extruder approaches the performance of the same size of cold-feed extruder without degassing;

* Retrofits with MCT Vac screws and barrels resulting in upgrading of existing installations.

While on conventional extruders computer process control can only influence the screw speed or sectional tempering which are slow-response parameters, on account of its low hold-up volume and quick response throttling the MCT is far better suited to process automation.

Conclusion

The practice of applying the MCT seems to show that it covers a range of compounds and applications overlapping but also going beyond those open conventional extruders. [Figure 1 to 3 Omitted] [Tabular Data 1 Omitted]

Reference

[1]Meyer P., "The development potential of different concepts of rubber extruder," Rubber World, July 1984.