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Extruders for the feeding of calenders.

Extruders for the feeding of calenders

Up to now, 40 to 50 extruders are used under production conditions by the rubber industry to feed calenders. This means that mills are being predominantly applied for the pre-warning and plastifying of rubber compounds before calendering. Special lines are the exceptions to this rule, as for example the roller head lines, which have been developed for non-standard products.

The general discussion about the advantages and disadvantages of cold feed extruders took place in Europe 15 years ago and the same is happening now in the U.S. and Far East. Today's critical analysis supports the increased application of pin-barrel extruders because conventional cold-feed extruders have lower output capacities.

In the early days, rubber processing extruders were built with a screw diameter range of 60 to 250 mm and their capacity range would reach from 100 to 2,500 kg/h. If a comparison is made between these outputs and the pre-warming capacity needed to run a textile cord or steel cord calender, it becomes quite clear that conventional cold-feed extruders can only be limitedly used.

A completely new situation arose with the introduction of pin-barrel extruders. These extruders have screw diameters ranging between 90 and 300 mm and outputs of 800 to 9,000 kg/h, thus fulfilling all capacity requirements of the rubber industry. The three main arguments are depicted which result from the clear advantages offered by these machines:

* Higher output.

* Lower energy conversion and/or low specific drive power.

* Low melt pressure in the (machine's) processing section.

These three arguments were decisive in boosting the sale of pin-barrel extruders, since their introduction to the market between 1977-1978. More than 1,000 are now applied under production conditions worldwide and increasingly for the feeding of calenders.

The design of the extruders is so universal that they are suitable, on the one hand, for the processing of rubber compounds with a high content of natural rubber and with Mooney viscosities between 100-140 ML1+4 (100 [degrees] C) and, on the other hand, for compounds consisting of only 10% polymer content and a considerable amount of fillers. From today's point of view, there is not an application case which excludes the use of pre-warming extruders for the feeding of calenders. One of the few remaining fields for the mills is the laboratory where research is carried out for the development of compounds.

Comparative criteria: Pre-warming mill - cold feed extruder

The following differential criteria resulted from the comparison between rubber pre-warming lines equipped with pin-barrel extruders and mills:

* Process influence on the product's quality.

* Energy costs, personnel requirement.

* Investment costs, space requirement.

Processes (comparison)

Rationalization motives and improvement of the product's quality call for replacement of discontinuous processes with continuous ones. The first problem is already encountered during the production of the compound, as the raw materials used have their own quality tolerances. The compounding process with internal mixers or mills is effected batchwise. The result is that the compound batches - depending on the process's degree of mixing perfection - could entail an extensive tolerance range with regard to their physical properties.

This tolerance could increase with every next production step, depending on the process, degree of automation and the process control. Continuous production processes allow mostly an easier process control and are superior, in the majority of cases, to the discontinuous ones. Without any reservations, the extrusion process can be considered as a continuous process.

When using an extruder, there is an absolute certainty that every compound particle has the same residence time in the machine at a minimal tolerance only. A constant residence time in conjunction with a constant energy transfer is the basic prerequisite for a uniform decrease of viscosity and scorch time of the rubber compound during its pre-warming process.

Another advantage is the possibility of blending two or more compound batches. Usually, extruders can hardly improve the degree of carbon black dispersion of a rubber compound, but they can - fed simultaneously with several batches - considerably homogenize the physical properties of the compound itself. As a result, the possibility is offered to homogenize partially the batch tolerances after the mixing process. Conclusively, all these facts offer a decisive influence to the product's quality.

When pre-warming rubber compounds with mills, a control of the residence time of each particle and of the number of passes through the gap is almost impossible. The decision of the operating personnel whether the degree of pre-warming or plastifying of a rubber compound has been attained or not is usually subjective.

Energy costs, personnel requirement

Table 1 shows a comparison study of energy costs for a pre-warming extruder and a pre-warming mill line, on the basis of a pre-warming capacity of 4,500 kg/h. Such a line is normally equipped with a 100" breaker mill, an 84" homogenizing mill and an 84" cutting mill. For the same capacity, the pin-barrel extruder is equipped with a screw diameter of 250 mm and an L/D ratio of 12.

This comparison study shows that the mill requires 540 kW approx. for an output of 4,500 kg/h, whereas the extruder only 300 kW; thus a clear energy cut of 45%.

Furthermore, extruder lines require minimal operating staff. For example, two pre-heating extruders could be operated by one man; thereby, each extruder would feed one of the two gaps of the four-roll calender in case of a high-performance textile-cord or steel-cord calender. A comparable line with mills could be hardly operated by one man only, even if it had a high degree of automation.

Investment costs, space requirement, noise level

Other comparative calculations have shown that the investment costs for a rubber compound pre-heating line equipped with mills are usually 30 to 40% higher than the ones for a comparable extrusion line, covering as well the complete new investment for all units pertaining to the line. However, practical experience has shown that this seldom arises due to the following disadvantages regarding the procurement of an extrusion line:

* The mill line can be easily equipped with second-hand equipment.

* The extrusion line requires a complete new machine investment. This is often the difficulty encountered by users when their decision is to be made.

The question of space requirement offers a totally new aspect. The extruder line requires only 25% of the space a comparable mill line takes.

An additional criteria supporting the increased use of pre-warming extruders, in conjunction with the rules for work protection becoming more and more strict, is the extruder's high accident prevention degree and low noise level. Due to their compact design, extruders can be easily insulated, thus increasing their noise-absorbing properties. It is also to be emphasized that the pre-warming process takes place in the closed processing section avoiding the loud bursting of air bubbles, as in the case of mills.

Capacity data of the pre-warming extruders

The diagrams of figures 1, 2 and 3 show the capacity data of pre-warming extruders (150, 200 and 250 mm screw diameters) based on a pure natural rubber compound and a blend of natural rubber and SBR. The illustrated sizes are the ones most commonly used by the rubber industry for pre-warming tasks.

The diagrams show individually the output, specific energy and stock temperature after the die as the function of the screw rpm. There is a general tendency that the outputs of natural rubber compounds are nearly 10-15% lower than the ones of synthetic rubber blends; here a stock temperature of a maximum of 100 [degrees] C was only allowed, contrary to conventional extrusion processes. It remains to be mentioned that, compared with the extruder of a 250 mm screw diameter, today's largest machine (300 mm screw diameter) has an output increase of approximately 45%.

Design criteria of pre-warming extruders

The specific requirements for pre-warming extruders (high output, low thermal stress of the rubber compound in the machine's processing section) are design criteria not to be ignored. Normally, extrusion processes allow a maximum stock temperature of 120 [degrees] C. This reflects a technological necessity for the sufficient plastification of the rubber compounds needed to extrude semi-products having often even profiled cross sections.

During the feeding process, the calender transfers energy to the compound when forming the compound to sheets in the gap. This has to be taken into consideration during the pre-warming process to avoid a thermical overcharge of the compound. Therefore, investigations were carried out, during the design of pre-warming extruders, about the stock temperature increase in the extruder's processing section.

Figure 4 shows the measuring data record of a standard machine with a 150 mm screw diameter and an L/D ratio of 16.

The record shows in detail the stock pressures and temperatures in the extruder's processing section on the basis of L/D = 4, L/D = 9, L/D = 16 (in the area of the screw tip) to be considered as a function of the screw rpm. In this respect, it is clear that, at 80% approx. of the extruder's nominal speed of [n.sub.s] = 30 1/min, the stock temperature of 100 [degrees] C relevant to the calender feed, is attained already at 2/3 of the processing length - namely at L/D = 9-10, while the temperature at the screw tip (L/D = 16) is 120 [degrees] C.

These co-relations led to the decision to design extruders or pre-heating tasks with shortened L/D ratio. The obtained results for comparative tests of extruders with graduated shortened L/D ratios are indicated in figure 5.

It was ascertained, additionally, that shortening the machine not only caused a considerable reduction of the stock temperatures and specific energy consumption, but also an increase of the output.

At first, this result may appear illogical, but it can be explained as follows:

The geometry of the feeding zone of a modern high performance extruder usually dictates the output. Especially in the case of low die pressures, as it is apparent when feeding calenders with relatively large-sized strips or strands, the conveying zone annexed to the feeding zone - which normally ensures the pressure build-up capacity of a screw - is overrun by the feeding zone. A shortening, therefore, of the conveying zone leads to a drop of the stock temperature at the end of the feeding zone and to an increase of the output. Individual investigations of the feeding zone by using a lab extruder with an L/D ratio of 3 led to double or triple the outputs, at comparable parameters but with low product quality.

Peripherals

The efficiency of a modern high-performance extruder is considerably influenced by the equipment it is being fed with. In order to have optimum extrusion conditions, special extrusion feeding systems have been developed.

These systems are d.c. driven feed conveyors and their speed is synchronized with the main drive of the extruder. A control loop at the outlet of the conveyor and in front of the hopper maintains the feeding conditions constant, i.e. the conveyor's speed is either reduced in case of overfeeding or increased in the opposite case.

Aiming at the protection of the extruder, the feed conveyors are often equipped with metal detectors and cross cutters which go automatically into operation as soon as metals are traced in the feed slab. Machines of higher screw diameters - from 150 mm onwards - are fed with slabs 600-1,000 mm wide, whereas smaller sized extruders (90 or 120 mm screw diameter) require narrower slabs in order to avoid any overfeeding. If narrower slabs cannot be manufactured due to the machine equipment in the mixing shop, feed conveyors equipped with zig-zag cutters can be used, thus allowing an adaptation of the feeding slab's width to the extruder's own intake capacity.

Examples of application

Figure 6 shows the allocation of a calender line, fed with strands, for the coating of fabrics to produce semi-finished products for conveyors. The pre-heating equipment for this line is a pin-barrel extruder with a screw diameter of 120 mm. An extruder with a post-arranged conveyor can also be equipped with a metal detector.

The machine allocation of a high-performance cord calender line is indicated in figure 7. Both roll gaps are fed by two pin-barrel extruders GE 300 KS x 12 D. Each extruder has an output of 5,500-6,000 kg/h.

The most advantageous manner to feed this line proved to be two feeding strips, by each extruder produced parallel to one other; each is led to the roll gap on both sides and very close to the stock guides. Having taken this measure, the result is a more uniform roller back compared with the one of a single strip fed with a cross traversing conveyor. Moreover, the gap load is lower, as well as the roll bending and calender's drive power. This factor causes, of course, consequences for the thermal product load, but their extent cannot be as yet determined.

Another field of application, in which pre-warming extruders are used more and more, is the feeding of calenders for cushion rubber as they are used in tread lines. The capacity required hereto is relatively low so that extruders of a 90 mm screw diameter are installed. Figure 8 shows a sectional view of a tread line equipped with a calender for cushion rubber and a pre-warming extruder. Since the working width is rather small, the rubber extrudate is distributed into the roll gap by a cross traversing conveyor.

Conclusions

A market analysis clearly emphasizes the increased application of pre-warming extruders. This trend is justified by the following criteria, opposed to that of pre-warming mills: Higher from stability of the rubber compound with regard to its viscosity and temperature constancy; Lower energy costs; Less personnel; Under certain conditions, minimized investment costs. Nowadays, there is a limited number of pre-warming extruders being applied in production in the rubber industry. This is due to the fact that a certain amount of time is needed in order to evaluate the advantages that a pin-barrel extruder offers for various tasks. However, the prognosis is that this situation will fundamentally change in the very near future. [Tabular Data Omitted]

[Figure 1 to 8 Omitted]
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Title Annotation:Machinery & Equipment 1990
Author:Capelle, Gerd
Publication:Rubber World
Date:Jul 1, 1990
Words:2354
Previous Article:Chemical exposure at work.
Next Article:Practical applications of the short, adjustable MCT cold-feed mixer-extruder.
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