Continuous production of rubber profiles - state of extrusion line technology.

This article will discuss the line technology development for the continuous production of rubber profiles from a practice-orientated point of view. We would especially like to show the options and advantages with the new technology according to the following judgment criteria:

* Application-specific choice of systems, i.e. definition of the application range of the presented technology.

* Output uniformity and its impact on product quality, scrap reduction, etc.

* Energy consumption as a way to reduce manufacturing costs.

* Improvement of productivity by optimizing material consumption and labor.

* Environmental protection as a current topic on the example of the optimized choice of primary energy and systems.

I will cover the two machine groups in regard to the following for continuous extrusion: the extruder, the gear pump and the extrusion head. For vulcanization: shearhead, microwave system, hot air system and salt bath.

I will give a short outline of new developments with regard to their technology and adjacent production methods for profile extrusion presented. Two typical examples of product optimization are given.

Systems for continuous extrusion of profiles

For continuous profile production with unpressurized vulcanization, vacuum extruders represent the state-of-the-art technology, although with advancing compound development, the normal extruder without degassing becomes of increasing importance. The main functions of a vacuum extruder are plasticizing, degassing and conveying the material with optimal temperature profile and consistency of compound flow. The final quality is mainly determined by the type and geometry of the extrusion screw. There are various characteristics of rubber compounds that influence production - beginning with the very soft microcellular rubber compound that is fed in strips up to the very hard compact compounds in granulate form - the influence of the screw geometry of the above mentioned process parameters becomes immediately apparent. The requirements - which are to a certain extent opposite such as highest capacity with best degassing - further restrict the screw geometry. For best possible utilization of the system, a certain classification of screws is generally adopted today:

* Low-output screws for low compound flows, high operating pressures and/or highest degassing requirements.

* Standard screws for (as the name says) standard applications.

* High-output screws for high capacity with best possible degassing.

In most cases the homogenizing capacity given by the vacuum barrier and the minimum length of approximately 16 D makes additional plasticizing elements in the conveying area of the screw - such as mixing zones, pins or shearing parts - unnecessary or obsolete.

To free the layout of the extruder from the output uniformity and pressure build-up or degassing safety parameters, the optional use of a gear pump is essential.

The conveyance of the gear pump is largely independent of the tool pressure. The gear pump has the ability to provide a surge free flow of material to the die regardless of pressure fluctuations from the extruder screw. In the gear pump the material is conveyed by intermeshing gears through the gear pump housing.

This results in the following characteristics:

* Safe degassing by avoiding flooding of the vacuum zone of the extruder, independent of the conveying behavior of the compound (figure 1).

* Highest possible compound flow consistency, resulting in dimensional stability even in inconstant production runs (starting-up and shutting-down of the line) (figure 2).

* Low temperature load due to friction-independent pressure build-up, especially with high operating pressures (figure 3).

* Conservation of energy by reduced build up of frictional heat (figure 4).

Disregarding the need for degassing, the three last mentioned points should also receive special attention in connection with a pin-type extruder as used, e.g., for the production of hoses. Here too, optimal compound flow properties (capacity, temperature, consistency, etc.) can be achieved with a gear pump. With this extruder/gear pump combination, the material is treated gently at high operating pressures.

Independently of this, the extruder requirements with regard to compound plasticizing and degassing must still be fulfilled. Here the following principle can be applied: A chain is only as strong as its weakest link.

After plasticizing the compound in the extruder, the final shape is given by the extrusion die. If the compound needs purification, the installation of a screen pack is possible, but not recommended in production lines because of problems that can result from this. The known disadvantages of the straining procedure are not only increased temperature and increased constructional length of the adapter piece. Compound flow variations due to the pressure changes as the screen pack becomes clogged cannot be completely using a screen changer.

This device offers the option of changing screens without production stops, although there will be a short-time variation of the profile cross section. For this reason, with regard to the compound purity, attention should also be directed to the compound mixing process and to all activities, processes and handling up to the extruder (transport, storage, etc.).

If the desired standards cannot be kept, one has to attempt to carry out screen and die changes simultaneously and install a new set of screen packs. Here of course, the use of a gear pump also offers the above advantages, especially the constant compound flow independent of counter pressure.

The final shape of the rubber profile is given in the extrusion head or die. Here, the aim of new developments is to reduce the head volume (to reduce compound waste when changing) and set-up times. Ease in mounting and dismounting of the dies, for example swing out dies and quick connectors from die to adapters, allow very fast product changes and short runs. The profile geometry also provides limitations (single-component or multi-component profiles) as do the customers' existing tool systems. In this respect, new developments can only be made in close cooperation between the customer and the manufacturer. In the field of hose and cable extrusion we have succeeded in developing a new crosshead with an especially compact design.

Here, the set-up and adjustment times, as well as the head volume, could be reduced considerably by corresponding measures. This concept, when adopted accordingly, can also be used for large pintle openings and double cross heads. The main features of this construction are: Easy centering by hand levers only; very quick tool change connections; easy flow divider removal using swing bolts and hydraulic cylinder; and very compact design.

Systems for vulcanization

When processing rubber, the stabilization of the final shape always means heat vulcanization. Different methods can be applied for this according to the product requirements and the characteristics of the material. There are two different methods - peroxide cross-linking and sulfur cross-linking. The majority of profiles, for example, for motor vehicles today are tight-cured by sulfur cross-linking so that the choice of the vulcanization method is almost unlimited. The peroxide cross-linking required for construction profiles necessitates heating without oxygen to avoid stickiness of the surface.

One option for increasing the temperature before the shaping process is the shear head. This technology, whereby the compound temperature is increased through high shear forces in the gap between shear mandrel and housing, is easily controlled today. This apples also to problems with eventual leakages and to problems with temperature - speed control. In production, there are imitations with larger profiles due to the disadvantages caused by temperature differences normally occurring in the shear head, and in case of long flow paths like in cross heads or heads for multi-component profiles. Apart from that, die partly necessary adaptation of the compound to the shear head technology means some expenditure and effort to set up the compound formula and requires corresponding experience.

In this example, very short flow paths could be realized by the compact design of the two-component cross head. The measurement of the material outlet temperature allows a precise process control through regulation of die speed of the shearing mandrel. The leakage of material towards the gearbox side is prevented by efficient feed-back spirals or reverse thread and, if necessary, by die additional infeed of PE granulate. Quick cleaning is possible by easy disconnection of gearbox block with shear - mandrel and housing. For compound heating directly in the material, but after extrusion through the die, a microwave system UHF) is employed.

This is a system whereby the molecules are stimulated to oscillate by the energy of the microwaves. A sufficient compound polarity is required for this. Systems are available today with magnetrons of 3 to 6 kW or mini-magnetrons of 1.0 to 1.2 kW. An advantage of the bigger magnetrons is higher efficiency despite higher technical investment reflection tuning, water cooling).

The often stated argument that production is safer with many small magnetrons which allow continued operation after failure of one or two magnetrons. Units which consist of few large size magnetrons do not have to be shut down if one magnetron fails, since in most circumstances the applied power ratio is in the 60% to 70% range. If one magnetron should fail, the power ratio will be increased and will make up for the loss of power until repairs can be made. The construction of a modem microwave system has the following characteristics:

* Smooth channel cross sections and easy access for easy cleaning.

* Uniform field distribution through an optimized channel volume, thus no demand for adjustment of the belt height which is not practicable.

* High security against radiation leakage.

* Variable adjustable supply of microwave power (ratio).

* Automatic and continuous minimization of reflection during operation by means of arc-resistant and low-wear templates, ensuring lowest energy loss and even profile heating.

* Efficient air heating of the channel to avoid condensation build up on the channel walls.

* Simple belt change and easily accessible drive unit mounted outside the channel.

* Modular construction - very low maintenance.

In contrast to the larger version, the technical expenditure on a mini-microwave unit should be kept as low as possible to make a simple construction possible of such a system which is intended as a preheating section, e.g., for an already existing hot air unit. For this purpose, several mini-magnetrons with a total power of approximately 6 kW are used, the proportional energy supply can be considerably simplified, be more effective and the use of water cooling and tuning unit becomes unnecessary. Adding on a mini-microwave unit the capacity of an existing profile line could be increased by up to 30%.

As a heat retainer and to a large extent also as a sole heating unit, the hot air system is employed.

Here, direct heating with gas, in this case also with bottles or a gas tank, is prevalent for reasons of costs. Gas heating additionally makes for very short heat-up times and good uniformity in temperature. If further energy savings are desired, the hot air system can be equipped with a heat exchanger which absorbs heat from the outgoing air and transfers it to the inlet - fresh air. Such hot air systems of one-belt or multi-belt design can also be used with a cooling unit being installed below.

A further development for the improvement of the product quality is die hot air shock unit of 3 m lenght only, with very even air contribution and efficient temperature to allow excellent profile heating.

The special construction of the transport mechanism provides for careful support of the profile whereby through the active heating, a very quick tight curing of the surface is realized. By this, especially in case of microcellular rubber profiles or those being sensitive to impressions of the belt a very good surface quality can be achieved. Heating of the hot air shock system is also by means of gas whereby for better utilization of the energy, the hot air is heated again and again in a direct circuit (closed loop) and only a small quantity of fresh air required for combustion is fed in. For the vulcanization of peroxide cross-linked compounds which must take without oxygen, heating is done in a salt bath.

In spite of stricter environmental regulations, no other procedure turned out to be an acceptable alternative. On the other hand, new developments have also been made in salt bath technology and led to a very efficient improvement of this procedure. Here die following points should especially be mentioned:

* Energy-saving heating, in the present example by gas with air as heat transfer medium which makes possible a cost and space saving construction and operation. Compared to salt bath heating with thermal oil, no separate oil collection pan and no change of the heat transfer medium are required, nor any official acceptance. At the same time the lower energy cost offers a considerable advantage compared to electrical heating. A reduction of up to 65% can be attained.

* Efficient insulation to avoid the formation of thermal bridges and undue dissipation of heat through the housing.

* Thermic and constructional separation of bath, frame and moving elements of die construction in order to avoid distortion damage and to simply the installation and maintenance.

* Integrated salt cascade for complete curing of thin crosssections and pre-curing of the surface to avoid deformations and damage to the profile.

* Easy exchange of parts which come in contact with the material - e.g. of the immersion rollers - for quick cleaning, if required.

* Profile deflection, knock-off device and hot-air blow-off unit largely prevent the salt melt from being dragged out of the salt bath area.

The downstream equipment for such a salt bath system has two functions, namely to clean the profile intensively and to cool it effectively. This is realize in a cascade washing station which is designed with three basins. For better removal of the salt that is dragged along with the profile, the water in the first basin is heated. Changing the water depends on the salt content in the counter flow whereby the water/salt solution is separated in a salt recovery station and reused. The following cooling of the profile is effected in a spray cooling station.

Applications

The following is a short introduction of two lines. The first example is a line for die production of windshield wiper profiles.

Highest accuracy of the compound flow is essential to achieve the desired quality. A gear pump is installed to meet this demand. Before the the, a strainer with a screen pack is provided and necessary. In this configuration, the gradual pressure build-up in die gear pump to overcome the additional resistance does not only increase the consistency of compound flow, but also has a favorable effect on the temperature profile and distribution. In the following vulcanization procedure, the required quick vulcanization of the fine cross sections takes place in a hot air shock channel of 3 m length and complete curing is done in a 3-belt channel of 3 x 9 m length, which has a cooling unit mounted below for space reasons. Exact length cutting of the profiles is done by a cutting device at the end of the line with the help of encoders. The required system for surface treatment of the windshield wipers is provided by the customer. The second line is provided for the manufacture of construction profiles.

Due to the peroxide cross-linking a salt bath is necessary for this production. The desired capacity of the line demands an extruder of the size GS-Vak 120/k-20 D. Moreover, a screen changer must be provided as the desired purity of the compounds cannot be guaranteed and the production lengths are very big. The salt bath is heated by means of bottled gas or gas tanks as there is no local gas supply. The cascade washing station with salt recovery system and the spray cooling unit are provided downstream from the vulcanization unit. Haul-off unit and siliconizing facility for preparatory treatment for the assembly and the coil winders - wrapping complete the line.

Conclusion

New and upgraded vulcanization systems provide the producer with several choices and more opportunities to achieve the best production results. More complex the designs as well as multi-component extrusions are possible today with the implementation of upgraded extrusion systems to satisfy even complicated profile - shape production at high output rates to compete against thermoplastic materials.