Extrusion, continuous vulcanization of profiles.

For the most part, rubber profiles are employed as sealing components in the automobile industry between fixed and moveable body parts such as windows, doors and boot (trunk), for related functions in industrial applications and in the building industry as window and door seals.

The conventional production process took place in three stages. The rubber compound was plasticized in the extruder, before being formed to the required profile and calibrated in the extruder die heads. Subsequently, the semi-finished profiles were discontinuously vulcanized in autoclaves. A third stage was then necessary to wind the profile onto a roll or to cut the parts to a finished length.

On modern lines, all three process stages are combined into one.

Situated after the extruders are continuously working vulcanization lines, working predominantly on the ultra-high frequency/hot air principle or salt bath heating systems, cooling units, cut-to-length machines and wind-up stands.

Using the same basic principles, hollow, multi-component profiles with complicated sealing lips, sometimes consisting of solid and sponge rubber profile sections and equipped with steel or wire inserts for strengthening, are manufactured on today' s equipment.

The range of products extends from mini-profiles with weights of 10-20 g/m up to large sections weighing up to 3035 kg/m, for example to be used as fenders in dockside applications.

Line speeds depend on the profile cross section and the vulcanization characteristics of the rubber compound and vary between one and 40 m/min.

The limit to production speed is to be found not so much in the processing equipment as in the handling of the finished product. Typical profiles with normal production line speeds are shown in table 1.

Table 1 - typical profiles with normal production line speeds

Profile   Rubber       Compound   MW size       Production   Weight
          type                                  m/min.       kg/m

??        Compact      EPDM       2 x 6 kW      25-30        0.1
          sponge

??        Compact      EPDM       2 x 6 kW      10-25        0.25
          sponge
          steel
          reinforced

??        Compact      EPDM       2 x 6 kW      15-20        0.35
          sponge
          steel
          reinforced

??        Compact      EPDM       4 x 6 kW      12           0.45
          sponge

??        Compact      SBR        2 x 6 kW      15           0.2
          steel
          reinforced

??        Compact      SBR        2 x 6 kW      20           0.2

??        Compact      SBR        4 x 6 kW      2-2.5        4.65

??        Compact      SBR        72 x 1.2 kW   1.5-1.8      9.5

??        Compact      SBR        20 x 1.2 kW   0.3-0.5      30

Recent developments in degassing extruders

Over the last 20 years, the technology of the degassing extruder did not display any particular high points.

The standard requirements for such machines, such as the highest possible degree of plasticization of the extrudate, pulsation-free extrusion and evacuation of moisture and plasticizer vapors from the compound, are fulfilled by most extruders available today, albeit at comparatively low throughput rates.

These extruders suffer, however, from the common disadvantage that varying degassing quality is achieved at different die backpressures (resulting from different product cross sections). This is because of the differing degree of fill in the screw in the area of me barrel degassing anti the resultant varying free extrudate surface, which is directly relevant to the degassing process.

Hitherto, attempts were made to compensate for this disadvantage by the alternate use of low-performance screws, normal rated screws and high-intensity screws, and the additional costs of changeover coupled with the duplication of investment had to be accommodated to achieve a constant product quality.

Against the background of the increasing production of larger-section profiles for the construction industry as well as for general industrial applications, the achievable output from the extruder, which had not always been the first priority, is now gaining in importance.

This has made necessary the development of a high-performance vacuum extruder, which is free of the compromises involved in screw changes for different production requirements, and also the development of vulcanization systems with sufficiently large treatment chamber cross sections. Whereas previously the capacity of a conventional profile line might have been between 100 and maximum 400 kg/h, lines with throughput capacities of up to 1,500 kg/h are now being asked for.

This means that, for the degassing extruder, ever larger screw diameters have had to be used. Now, 150 mm and 200 mm diameter screws are employed where once 60 mm, 90 mm and 120 mm were the largest. In connection with these larger screws, an adjustable section has been developed, guaranteeing the largest free extrudate surface in the area where the vacuum is applied.

For this special profile of applications, the PinConvert extruder, introduced in 1993 and further developed since that date, is particularly well suited for extrusion applications with barrel degassing.

As can be seen in figure 1, the hydraulically or mechanically operated throttle pins in the processing section of the machine can be continuously adjusted during production. The throughput of the extruder per screw revolution in the feed section and before the degassing dome can thus be controlled. The output of the extruder per screw revolution in the metering zone is, on the other hand, dependent only on the degree of plasticity (viscosity) of the compound and on the die back-pressure. In this way, a balance can readily be achieved with this type of extruder between the performance of the extruder in the feed-zone and processing section on the one hand and the metering section on the other. The degree of fill in the degassing zone can be preset and, in this way, the well-known problem with conventional machines of extrudate exiting the degassing dome is avoided and the effectiveness of the degassing is predetermined.

[Figure 1 ILLUSTRATION OMITTED]

The first machines of this type have been working in production conditions since 1996.

The gear pump as auxiliary equipment for high-performance degassing extruders in profile manufacturing

Gear pumps are used as accepted practice in the field of thermoplastic extrusion. All the known makes of pump adopt plain bearings for the rotors, using the thermoplastic itself as the lubrication medium and accepting a certain leakage of the melt.

The use of gear pumps of this kind for vulcanizable rubber compounds is questionable, firstly because rubber compounds in their plasticized state are more viscous than thermoplastics by a factor of about 10 and, as a consequence, flow less readily. Secondly because, due to the high shear velocities experienced by the rubber in the bearings, the compound tends to overheat and starts to vulcanize.

The benefits of a gear pump, situated after an extruder, are well known and are:

* Build-up of static stock pressure with low specific energy input and a high conveying efficiency with low thermal loading of the extrudate; and

* constant delivery performance with optimal filling of the inter-tooth cavities, giving pulsation-free extruder output and improved profile dimensional tolerance.

To put a quantity on this, a gear pump with optimally designed teeth geometry requires only approximately 25-30% of the specific energy input of an extruder for the equivalent output, including stock pressure build-up, and results in under a third of the temperature increase for this process stage.

Output pulsations from the extruder, and thereby variations in profile tolerances, are damped by a factor of 4-10, depending on the absolute value of the variations. In other words, the higher the variations in output, the higher the damping effect, because the extrudate which is pressed into the teeth cavities of the pump under pressure from the extruder is, to all extents and purposes, incompressible.

In order to eliminate the problems inherent in the pumps available on the market, mentioned earlier, a pump has now been developed which has roller bearings in place of the usual plain metal bearings.

The following details were also incorporated into the new design:

* Both rotors have a double-helical tooth arrangement, so that the bearings have to take no axial loadings, only radial.

* The protection of the bearings is by means of back pumping thread sections with free exit for any leakage between back-pumping threads and roller bearings.

* The rotor shaft diameter is very nearly the same as the rotor diameter itself, so that there is no radial gap between the rotor sides and the housing.

* The rotors as well as the housing are fitted with rotary joints for temperature control.

* The main pump housing, including all the bearing housings, is split in the center, making the whole unit very service-friendly.

Figure 2 shows a VacGE 90 SC X 16D Extruder (PinConvert version) fitted with a gear pump of the design described above. This machine configuration is designed for a profile extrusion output of 400-600 kg/h. Furthermore, the pump is designed to give a static stock pressure increase of 250 bar between input and output flange.

[Figure 2 ILLUSTRATION OMITTED]

Normally, the extruder delivers extrudate to the pump to fill the inter-tooth cavities completely at a pressure of 15-30 bar, set by the extruder screw speed. Thus, the extruder works completely independent of the die backpressure (profile cross section) and always under constant process conditions. This, in turn, leads to the technological advantages mentioned earlier of lower specific energy consumption, reduced thermal loading of the compound, high dimensional stability and optimal extrudate degassing.

Figures 3-5 emphasize the relationship between rotor speed of the pump and its output, rotor speed and extrudate temperature increase, extrudate temperature increase in connection with the pressure difference between inlet and outlet (die pressure) at constant output, specific energy consumption of the pump as a function of the pressure difference at constant output, and stock pressure variations from the extruder at the pump inlet compared to the residual variations at the outlet while extruding high-viscosity EPDM compound, designed for profile dimensional stability.

[Figures 3-5 ILLUSTRATION OMITTED]

Microwave lines

As is generally well known, vulcanization of rubber compounds entails an irreversible chemical process, during which semi-finished products in a plastically deformable state are converted to elastomers, generally by the introduction of thermal energy and often with the application of external pressure. In conventional vulcanization processes, whether carried out on a batch-basis or continuously, the energy transfer into the product takes place by conduction. Microwave systems, on the other hand, work on the principle of high frequency magnetic energy and the production of heat through internal friction between the molecules of the product.

The microwave vulcanization process is thus well suited to the heating of large-volume profiles, as the magnetic energy is simultaneously introduced across the whole of the product cross section. The danger of over-vulcanization at the product edge or surface and under-vulcanization at its core, which is present with all other processes depending on heat transfer by conduction, are thereby eliminated.

There are two basic types of microwave vulcanization equipment:

* Hollow conductor systems, in which the magnetic energy takes the form of a so-called standing wave, with maximum energy concentration at the points of energy introduction; and

* resonator chamber systems, in which a distributed magnetic field is formed, without locally defined maximum and minimum energy points.

Hollow conductor systems have declined in importance over recent years, largely because, for physical reasons, the treatment chambers are limited to a maximum cross section of 50 x 50 mm, which severely limits the products which can be handled and because the maximum energy concentrations cause problems with overheating of the product.

The latter is particularly important for products with steel inserts.

Resonator chamber systems are, by comparison, more universally applicable. This is because, on the one hand, with modern systems the width of the chambers can be from 100 mm to 700 mm wide and up to 200 mm in height and, on the other, because the possibility of coupling a number of resonating chambers behind one another, enables a step-by-step introduction of the energy into the profile to be made. This, in mm, allows a defined control to be exercised over the vulcanization process.

Normally, microwave systems work with 6 kW magnetrons, which can be regulated to vary the energy output or, more recently, and in particular in connection with wider treatment chambers, with the so-called 1.2 kW technology. The regulation of the energy output is achieved in these lower-cost magnetrons by switching them on or off for varying amounts of time, for example: 0.2 seconds on, 1.8 seconds off in a cycle time of two seconds, gives a nominal power rating of 20%.

In addition, in a 1.2 kW system, the magnetrons are arranged diagonally displaced in the working direction, in order to optimize the distribution of the energy. The chambers themselves are placed at distance equivalent to half a wavelength, inclined to the direction of working in order to achieve further improvement of the energy distribution.

Because the pressure-free vulcanization is dependent not only on the reaction temperature of the crosslinking process, but also on the treatment time, the microwave section of a plant is followed by sections known as heating chambers, using either infrared energy or gas as their source of heat. Vulcanizing temperatures lie, as a rule, in the range 160-200 [degrees] C. For rubber compounds, vulcanizing times are generally between 1.5 and 2.5 minutes. The installed ultrahigh frequency power and the total length of the heating chambers are thus determined by the required production speed and the weight per meter of the profile.

The installed power of microwave systems lies typically between 6 and 24 kW, but for special applications handling large-volume building sections, total powers of 85 kW have been installed.

The heating chambers are equipped with 2.5 to 5 kW per meter heating power, depending on the width of the chambers. The most usual heating chamber lengths are 9, 12, 18, and 24 m, and often several chambers are arranged in line to achieve the requisite vulcanization times.

In order to limit the total length of a plant, especially when the profile being treated is not rigid, multi-pass chambers are employed.

Salt bath lines with effluent salt recovery

Although, in the past, salt baths have, for environmental reasons, been regarded as an outdated method of continuous vulcanization, they have enjoyed something of a renaissance over the last decade.

This is accounted for by the fact that certain physical characteristics of profile products, such as optimal compression set, have only been possible up to now with rubber compounds which have to be peroxide vulcanized, and because this can only be done in an oxygen-free atmosphere if unwanted surface reactions are to be avoided. It follow that UHF and hot-air systems cannot, by their very nature, be employed for vulcanization in such cases.

Furthermore, salts, which are more environmentally friendly, and methods of recovering the salt after vulcanization, have been developed in the meantime, so that the disadvantages mentioned earlier now only apply to limited extent.

A further argument against such systems, referring to the effect of salt pressure on the geometric integrity of the profile, has also been largely dispelled by machinery improvements. Modern salt bath lines are equipped at the inlet end with so-called salt sprinkling systems, which have the effect of prevulcanizing the surface and thereby stiffening the profile section.

Only after this prevulcanization is the profile immersed in the salt and then not, as before, on a driven steel belt, which tended to work against the desired action of the salt and was in contact with the product during the whole vulcanizing process, but instead by means of driven rollers a defined distance apart, which make only point contact with the profile while pressing it under the salt and thus allow the memory of the product to ensure that the original geometry is retained, while running freely between two guide rollers.

The salt, which remains on the surface of the profile, is removed in combined washing and cooling units and the salt-bearing water is passed to an evaporator, where the water is driven off to regain the majority of the salt.

Downstream equipment

The machinery which is situated downstream of the vulcanization system, such as cooling units, cut-to-length machines, wind-up stations, etc., are in principle very similar from one line to another and only differ in their layout depending on the particular product specification in question, profile cross section, line speed, etc.

The most costly downstream equipment is integrated into lines where flocking of the profile is called for, as, for example, in automotive window channels and door profiles.

As profiles with smaller dimensions are sometimes produced in a parallel multi-strand arrangement, multiple double tandem wind-up stations are to be found at the end of such lines, with individual guide system for each profile.

Summary

The increasing demand for large-section profiles makes necessary, for economic reasons, the further development of continuously working vulcanization systems.

This aim sets new trends in regard to the development of high-performance degassing extruders, with the integration of gear pumps for the extrusion of rubber compounds, as well as a new and wider spectrum of demands on the vulcanization plant.

These demands are being largely met by the application of 1.2 kW microwave technology, whereas on the other hand, certain product quality aspects have resulted in an until quite recently unexpected re-emergence of salt bath lines.