New considerations for old extruders.

Worldwide economic conditions have forced rubber manufacturers to re-evaluate their investment programs and reconsider how best to make use of existing equipment, while at the same time addressing their need to reduce costs and improve quality. With old, but still useful equipment, limited cash resources, poor quality, high reject rates and the need to enter new world markets and at the same time overcome high production costs, the manufacturer is faced with seemingly insurmountable problems. Now, however, at least as far as extrusion processes are concerned, with the availability of the MCTD extrusion system, many of these problems pale into insignificance. In order to understand how hot feed and cold feed extruders may be combined, it is first necessary to understand the principles involved in the MCTD extrusion system.

MCTD extrusion system

The MCTD extrusion system is a new generation of MCT extruder which has had more than 10 years of proven practical use. In comparison with well known cold feed extrusion systems using pin-type screws or normal plasticizing screws, the plasticization and controlled temperature increase with the MCT system is not made by friction, but rather through flow division and rearrangement of the compound in the short mixing section.

The so-called MCT part (multi-cut-transfer) comprises opposing flights in screw and barrel with changing flight depths. In this way, with each revolution the compound is cut and moved many times, and in so-doing becomes plasticized and thoroughly mixed. The number of flights determines the effectiveness of this work. Because the whole length of the MCTD-zone is only about twice the corresponding screw diameter, the result is a very short compound dwell time in this region.

For extruders which are required to handle especially difficult compounds, e.g., highly-filled natural compounds, the, new MCTD generation has a changed screw geometry which, compared to the MCT, has double the plasticizing effect, with adiabatic performance. This is achieved through another partitioning of flights in the screw and barrel as well as a changing of the lengths of the first and second MCT zones. The main advantage with all MCTD extruders is to be found in the higher production output when compared with other extruder systems with the same screw diameter (figure 1).

MCTD screw and barrel design

Feed section

The patented 360 [degrees] double spiral undercut ensures optimum feeding, with no pins being needed for compound breakdown and therefore, no danger of pins breaking. In order to compensate for the usual dimensional changes in the feedstrip a constant torque feedroll is fitted, which guarantees constant output.

MCTD-zone

This unique and fully-developed plasticizing and mixing section is only 2-2,5 L/D and can handle all types of compound, even the most difficult, to produce low temperature profiles combined with superior homogeneity.

Throttle section

An adjustable throttle system enables the optimum extrusion characteristics to be obtained and guarantees constant flow without dead spots. This means that compounders are not limited by the extrusion process when compounding to meet product specifications. By simultaneously adjusting a series of vanes placed between two flights in the MCTD barrel where all the rubber is passing through the barrel, improved plasticization is achieved.

Depending on the position of this throttle, a back pressure is created in the first part of the MCTD zone. This means that the plasticizing work can be regulated during operation and therefore that the extruder can be adjusted specifically to any type of compound.

The effect is clearly visible in die photograph in figure 2, which shows three extrusions of a natural rubber compound. The irregular extrusion shows what happens with insufficient throttle use while the extrusion on the right represents the result of sufficient throttle application by reducing output to normal. The greatest advantage of the throttle element lies in the way it acts immediately.

The MCTD extruder contrasts with all other known extruders in that the action of the throttle produces an immediate reaction in the extrudate, without changing die temperature settings. In other extruders, when only the temperature controls of the screw and barrel can be regulated, the effect on the compound is much slower, since it is necessary to wait for conditions to stabilize. This means much time and material can be saved during start-up. It is even possible to establish a closed loop circuit to maintain the required compound temperature in the extruded profile.

Pressure build-up section

This is a metering section in the screw, where friction is limited to a minimum, ensuring pressure build-up at the die - without heat build-up. This special extruder design offers manufacturers of extruded rubber profiles several advantages, which include high production output for low energy requirements (maximum 0.08 kWh per kg of rubber on non vented or maximum 0.11 kWh/kg on vented extruders) and control over extrusion temperatures and plasticities relative to compound characteristics. These superior qualities may be given to old extruders by changing screws and barrels to the MCTD design. This can be done regardless of whether the existing machine is vented or non-vented.

The advantages of the MCTD extruder system are particularly proven on extruders with degassing capability. Because of its positive characteristics the MCTD vacuum extruder has, when compared with conventional vacuum extruders, the advantage that degassing is one hundred percent effective, even when extruding small profiles.

Function

The rubber compound is plasticized in the first part of the screw and subsequently forced through the slits of the degassing zone. In order to achieve perfect degassing the compound temperature has to be at a minimum of 90 [degrees] C. This temperature can be realized with the MCTD system, even on low extrusion speeds.

After passage through the slits the emerging gasses are evacuated in the degassing zone by means of the vacuum pump. To protect the vacuum pump from small pieces of compound a pump filter is provided.

In the second part, the so called conveying or compression part, the compound is re-compressed after the pressureless degassing zone and subsequently forced through the head, i.e., the die.

The conveying part has to be regularly filled, as is necessary for every extruder screw. The degree of filling nevertheless depends on the dimension of the profile and the kind of compound. This means, if the conveying part is not totally filled the output becomes irregular. If the conveying part is over-filled, the compound will emerge in the pressureless degassing zone and therefore degassing becomes ineffective.

Therefore, very often users have to change screws in order to avoid this. However, on MCTD vacuum extruders this is not necessary. With this type the quantity of compound to pass from the plasticization part to the conveying part can be adjusted by the overflow regulating system of MCTD vacuum extruders.

In practice, this can be automated by a pressure-sensitive control using a probe in the conveying section of the screw.

The main advantages of MCTD vacuum-extruders are:

* High output equalling extruders without vacuum;

* Adjustable to each compound and/or extrusion speed;

* No screw changing necessary;

* Due to better homogeneity and controlled temperature of the compound it is possible to shorten die vulcanization time, which results in an increase in capacity of the following continuous vulcanization line.

MCTD vacuum screws and barrels are available for converting existing coldfeed extruders.

One example of a conversion was made to an old 120 mm Bandera, non-vacuum extruder. This machine, with a screw speed of 58 rpm., originally gave an output of 400 kg/h. After conversion to a vacuum MCTD-120, at the same screw speed, 710 kg/h output was obtained. Suitable modification to increase the screw speed to 80 r.p.m. would show a corresponding output increase up to 1,000 kg/h.

Smaller space requirements, together with the ability to control plasticity, give MCTD extruders the unique ability to be combined with existing hot feed extruders, enabling multi-component extrusions to be produced. When MCTD extruders are combined with a main extruder (hot or coldfeed), the term Underpack (UP) is applied.

Conversions of existing extrusion lines

The RCM Underpack co-extrusion system provides the tire manufacturer with the possibility of modifying existing warm or coldfeed extrusion lines so that the benefits of modem co-extrusion can be realized for low capital investment. The proven compact construction of MCTD cold feed extruders gives the user the ability to extrude combined profiles. The hydraulically operated underpack head with its excellent material flow and accurate temperature control, enables extruded profiles to be produced to close tolerances and high output, matching the extrusion characteristics of dissimilar compounds.

Some advantages of the Underpack system are:

* Low capital investment;

* Modification of existing warm or cold feed extrusion lines;

* Minimum dimensional tolerances, resulting in material savings;

* Compact construction ensures minimum space requirements;

* Suitable for all types of compounds;

* Improved product quality;

* Simple installation;

* High output.

Conversions of existing tire extrusion lines have been made in several parts of the world, in which MCT extruders have been combined with existing warm feed extruders to permit co-extrusion of tread, sidewall and cushion. As an example a conversion was made to an 8 [inches] hot feed extruder. The equipment and its operating features met the following production requirements for all the existing shapes, as well as the newly-combined two-compound co-extruded shapes and the multiple extrusions of single shapes.

Production and dimensional/weight requirements were met and were based on: Maximum output - 30 kg/min. for MCT-120 and 40 kg/min. for die 8 [inches] hot feed; maximum line speed was 16 meters/min. for each.

There was an increase in tire production because of:

* Time savings in extrusion;

* Time savings in tire building;

* Time savings in plant processing;

Based on the following profiles, shapes and combinations: Two-compound shapes - abrasion/sidewall, tread/sidewall, cap/base and cap/base-sidewall; one-compound shapes - tread, sidewall and passenger filler, radial passenger tires had an overall capacity increase of approximately 16%. Approximate capacity increase for bias truck tires was: Building - 6% cap/base and 7.5% for cap/base/sidewall; extrusion - 25% for cap/base and 50% for cap/base/sidewall.

Additional savings realized were:

* Elimination of the pre-assembly operation of abrasion/ sidewall;

* Reduction in change time in extrusion;

* Increased extrusion line speed;

* Energy savings.

Besides a capacity increase, an additional increase in tire production of about 10% is estimated, due to less handling, storage and transportation, without taking into account improved quality and reduced rejects.

It is important to note that in such modifications, the existing warm feed extruder may be the only production machine in the tire plant on which complete production of tire profiles has to be made. Furthermore, when the modifications described above were made, at no time was it possible to shut down the extruder for an extended period, nor, what is more time-consuming, was it possible to run-in the new co-extrusion profiles.

The MCTD-120 cold feed extruder was mounted underneath the existing 8 [inches] warm feed extruder. A new co-extrusion head, in this case designed for extrusion widths of up to 500 mm, connected the two extruders to a co-extrusion line. The extruder head itself is hydraulically-operated to facilitate exchange of the die package and to open the head for compound changing and cleaning.

In order not to interrupt actual production more than necessary, the new extrusion head was designed in such a way that the old dies could be used in the new head for single extrusion. The co-extrusion dies were able to be developed during short breaks in production.

In the example cited, only one Underpack extruder was required, but in other cases it may be desirable to combine two Underpacks with the existing extruder. The compact, hydraulically operated triplex head is designed for the production of all these profiles, for which the dies are tailor-made. They are used in packages of several elements.

With world markets demanding ever higher quality in tire production comes the recognition that in order to survive and prosper the subject of quality improvement in all production elements has to be addressed. One way of looking at quality is to consider that end product quality is due to the sum of all variations in the production process. When this is done areas of need can be identified which will be different for different factories. Where one may consider a final tread cutting unit to be a priority, another will wish to place most importance on shrinkage and cooling to ensure constant weight per meter. Statistical software on final profile cutting for weight control may be desired. But wherever the priority lies, the control of all the parameters of the whole process is of paramount importance and must always be the first consideration. The addition of MCTD-UP extruders at the start of a tread or sidewall line introduces the initial elements of quality control that are so necessary to get started on the road to improvement. The design of these extruders incorporates systems which allow temperatures and plasticities to be controlled and starts the whole sequence of line improvement. Furthermore, all such conversions are designed so that at some later stage the old hot-feed extruder can be exchanged for a new cold feed machine.

As well as the economic advantages which can be realized this way for a very moderate capital investment ($500,000 - $1 million) additional economic and technical advantages result. These could normally only be achieved with an investment of some millions of U.S. dollars. There are even some occasions when conversion of hot feed extruders to the MCTD coldfeed system may be considered. If the gearbox is strong enough, with a new motor and controls, it may be possible.