Hot air vulcanization of rubber profiles.
Complex proofing systems must also be achieved with extremely complicated profile forms. All too often such profiles have an extremely large surface together with a low cross-section density. They frequently consist of two or three rubber compounds and are steel reinforced. Sometimes they are flocked and coated with a low friction finish. Such high-tech seals require an adjustment of the vulcanization method.
The consistent trend in the nineties towards lower quantities of elastomer per sealing unit and the dielectric factor, especially with EPDM, has brought an old fashioned vulcanization method once more to the fore, a method developed over the past years to an extremely high standard, namely the hot-air method.
Curing methods and their importance
Curing in an autoclave (steam-bath) is a very old method and has no relevance to todays manufacture of sealing profiles.
It does still have a place in the manufacturing of profiles with a woven constituent such as cables, pressure hoses and manifold pipes for vehicles.
The inner construction of such products should not be required to be absolutely free of moisture and air inclusion, as one is working with a high steam pressure. In this process a peroxide vulcanization system is possible.
The curing is very labor-intensive and thus very expensive. It is barely possible with modern profile-geometries to hold the profile shape while packed in the autoclave as the curing time is very long. In most cases twenty minutes or longer.
This method of vulcanization is totally outmoded. Hot air is blown into a vortex - the speed of this air must be stringently controlled so that the glass beads which make up the bed of the vortex are not expelled.
The profile glides on moving glass beads and is heated simply on the facets. A surface impression develops. The glass beads imbed themselves easily in the surface depressions of the fine-lipped profiles and cleaning is very difficult.
The curing uses large amounts of energy as enormous quantities of air must be heated. Due to this necessary environmental protection measures are difficult to achieve. Also, among other things, the recirculation of surplus energy is made extremely difficult due to the condensation of the softening steam. This is certainly the dirtiest of all curing methods. Only limited running speeds can be attained.
These plants are seldom used nowadays to heat up rubber profiles. They are subject to the great disadvantage that the profile can almost never be heated up evenly, because of the known problems of energy dissipation over distance. Thus this method is now also rarely used for adhesive-drying of flocked profiles. Energy consumption is relatively low, but unfortunately this is also true of the throughput speed.
Vulcanization in molten salt is very common even today. The excellent heat transfer properties between the salt and the vulcanizate make peroxide curing possible even when a saltdrizzling oxygen is achieved on the profile surface.
Heating up of the profile is extremely even, as the molten salt contacts all surfaces equally. As this contact occurs within seconds one can reckon on a very smooth surface for sponge-rubber.
The energy required to maintain the molt at 240 to 248[degrees]C is not very great, although one should bear in mind that salt baths are usually electrically heated and so can carry quite heavy operating costs in spite of this. These costs are noticeably increased by the tremendous consumption of salt.
Together with the obvious advantages of the system the following disadvantages must be taken into consideration.
Large cross-sections of sponge rubber are difficult to manufacture as the cell structure deteriorates and loses its uniformity as it nears the center. If a metal conveyor belt is used one must be prepared for an alteration in the lip-placement of the profile because of the buoyancy.
The admittedly excellent heat transfer can scarcely be varied over the length of the plant, due to the very narrow running speed tolerances.
If sulfur is used for curing it is almost impossible to prevent the dangerous buid-up of nitrosamines.
Purification of the used salt in accordance with the present environmental requirements can only be achieved with an enormous capital outlay.
LCM vulcanization will be of less and less importance as time goes by. It will become practically impossible to fulfill the always stricter environmental requirements.
Shear head vulcanization
This curing method was publicized in 1979 and has in the meantime earned its place in the industry.
Electrical energy is converted from the motor to the rotation of the shear head mandrel directly in front of the extrusion die. The elastomer is processed through the extruder barrel into the shear head space, which has been created from the space between the rotating mandrel and the static and centered cylinder.
The amount of electrical energy transformed into molecular frictional heat is regulated by altering the rate of revolution of the mandrel.
The elastomer temperature can therefore be much higher than typical extrudate temperatures and be maintained while exiting the die.
These facts produce an important advantage for the user; the vulcanization begins to take place immediately before the forming process. Thus with the reduced viscosity and the compound partially cross-linked, a high degree of form definition and stability in sealing profiles can be achieved.
There is little or no wastage in the electrical energy used for heating and, because of the low consumption, costs are relatively low.
As with every curing method there are some disadvantages to be considered when deploying this method.
Adjustment of the elastomer compound can be problematic as the scorch point of the compound can only be established after the exiting of the rubber from the die. For reasons dictated by compound technology it is often only possible to work with low exit temperatures which once again places the onus of the greater part of vulcanization on the following, compulsory, hot-air channel.
Production of reinforced multi-component seals and sponge rubber is possible but the same problems still persist.
A particular drawback is the swelling of the elastomer when subjected to a variable temperature. Because of this, this curing method requires special extrusion dies which cannot be used with ordinary extruders. On the other hand, of course, profile-tooling from previous extruders cannot be used with shear head systems.
And yet for the majority of rubber sealing profiles where variations in form and compound are unimportant, shear head vulcanization appears to offer an advantageous technology.
Vulcanization using UHF energy at 2.45 GHz is widespread. If it is possible to set the dielectric loss or polarity of the compound high enough, then the microwave energy can be directly transformed to heat within the vulcanizate.
Unfortunately, however, the physical properties of EPDM formulas often do not lend themselves well to UHF. Polarities can be low due to the fillers or other agents in the compound recipe. This can, however, be overcome by the addition of special agents that will cause the formula to be more receptive to UHF. The drawback to this is that it makes the rubber compound more expensive.
The high technical requirements of, above all, the automobile industry leads to the ever declining deployment of UHF plants. Also, however, because the modern elastomer seals demand only limited quantities of raw rubber per unit and at the same time maximize the profile-seal surface. The applied heat is reflected back extremely quickly because the radiation emission factor is 0,95 W/m*d 253* K*d 253* and the total radiation grows to the power of four of the profile temperature.
The already installed, following, hot-air-channel tends increasingly to take over the task of heating the profile, a task which naturally demands of it specific qualities.
It then must be noted here that UHF machines can only be sensibly deployed if the compounds have a high polarity and the profiles have a small surface in relationship to the cross-sectional mass.
The operating costs of microwave installations are, however one may attempt to prove the contrary, relatively high. For example: With a rated power of output of 24 kw UHF, the total electrical requirement (including the far too weak support hot air) is 80 kw. This does not include the energy for the hot-air-channel in line after the microwave.
Taking into account only the UHF section - in United States alone for example - over a year's operation, running three shifts a day, one can expect the energy for the UHF alone to cost $35,000 ($.05/Kw). Additives to increase polarization to the rubber compound will increase your compound cost by an average of $0.05 per kilogram. With an extrusion rate of 300 kilograms of compound per working hour we have an additional cost of $131,400 per annum. The electrically heated post curing channel uses another $63,000 in energy costs, so that with the most favorable calculations and excluding the cost of maintenance, the yearly costs are in the neighborhood of $229,400.
Were we to compare this with the 180 kW natural gas requirements of a hot-air-channel of equal performance we would find an immediate saving of $205,700, the latter having an energy cost of $23,700 per annum ($0.015/Kw). This calculation has very similar results when applied to our European neighbors.
An extra, economically calculated, saving - nay, "profit" of $205,000 per installation per annum. This should be reason enough to think about vulcanization methods.
Thirty years ago rubber was already being vulcanized by the continuous hot-air-method. Success, however, was restricted as the machine development was not such as to overcome the well known inferior heat exchange between a gas and the vulcanizate.
The firm Gerlach Ltd., involved in the hot-air process particularly for the past 18 years, has, in the last seven years, improved this procedure to such a high technical standard that in most cases it outperforms the methods described above both technically and economically.
Not only high production standards recommend hot-air vulcanization. As already discussed, nowadays the user of elastomer profile seals demands compounds and forms which can simply be better produced by the hot-air method.
The aim to transfer heat to a rubber profile as quickly as possible, depends on three criteria:
* The temperature difference between the heat-transferring air and the rubber profile should be as great as possible. Technically it is easy to achieve this "delta T."
* The hot air must be as heavy as possible in order that it can store and give out large quantities of heat. It should also be as damp as possible.
* The heat absorbing elastomer profile should be exposed to the air in such a way that the high energy air molecules come into contact with its surface as much as possible.
Lastly, the heat exchange should be linearly fashioned over the whole period spent in the hot-air-channel, difficult to attain because the dela T sinks, owing to the warming up of the profile.
Possible working principles and their effects
Figure 1 shows a very old method in which cold air is extracted from the working channel and is then reintroduced through the electric air heater. The air-stream direction is at a ninety degree angle to the profile axis.
The effect with regard to the profile warming and the temperature constancy is very limited. The profile can, as is shown in the illustration, only be heated weakly on one side and simultaneously gives off a great part of the acquired energy (see UHF installations). The air is permanently contaminated with softening agents, and these condense or coalesce in the working channel and in the supply routes, leading to an obvious deterioration in the quality of the process.
This so-called cross-current recycling principle comes from the textile industry and was originally developed for fabric drying.
It is totally unsuitable for the vulcanization of rubber profiles but is, unfortunately, used again and again for that purpose.
Figure 2 shows a similar principle. Here cold air is drawn from the working channel and finally fed through a weak electric air heater.
Then the heated process air is returned to the working channel passing infra-red rods, which complete the heating up process, immediately before its re-entry.
The infra-red heating rods are situated behind long drawn out and narrow meshed performated plates which, understandably, barely allow the infra-red energy access to the working channel. The air is led over a relatively long course, parallel to the axis of the profile, and is recirculated. With every in and ou-ttake of air, however, a quantity escapes onto the factory floor and this must be dealt with.
The improvement in efficiency of this method over the previous one described is noticeable. In spite of this the heating up efficiency is still too limited. Of the criteria previously listed, only the first point is covered. At its best, therefore, this machine can only be deployed as an after-heater for UHF plant, should the desired profile temperature be attainable with UHF.
This is, however, exactly where the problem lies (see UHF installation) and there is no reason to suppose it will become any easier with time.
The convenience is restricted by the soiling of the installation. In order to combat this problem the installation of heat-exchange plates in which the emollient can partially condense has been tried. This has failed to achieve total success, however, since maintenance now concentrates on daily cleaning of the exchange plates. This also brings problems as the cleaning is done with compressed water which creates environmental problems.
Hot-air installations which work on this principle are sadly still deployed, although seldom. The method has been extant for more than twenty years, and in this time it has never met the expectations of the profile producer.
The Gerlach hot-air concept
This concept meets the criteria as listed previously. An optimum delta T is reached and the air is damp. The heat carrying air molecules provide an optimum heat exchange directly on the profile surface through their extremely high crossmotion.
A linear heat transfer over the total length for the complete duration is achieved by a modification of the molecular movement.
As in figure 3, process air is forced into the air/air heat exchanger through the ventilator. Here it absorbs a large proportion of the exhaust warmth and is fed via pipes 3 and 4 to the burner unit, 5. This gas heating can be effected with natural or propane gas.
The air is fed further into the working channel 6. An intake booster jet every three meters is foreseen.
The air is removed in varying amounts at stages 7, 8, 9 and 10 and led to heat exchanger 2. Here it is cooled to a temperature of 65[degrees]C with emollient containing exhaust air.
Through the cooling of the air on one side, and the air diversion-separation effect, so-called, on the other, a large quantity of the condensation separates out and can be led away.
The still contaminated air is fed through pipe 11 into a cooler and intensive separator. Here it is further cooled and another portion of emollients can be condensed and led away. Then the air is chanelled further into a centrifugal filter where 99.7% of the remaining impurities are removed. A special feature of this filter is that it is self-cleaning.
Lastly, the air is fed into the exhaust ventilator, 14, from whence 85% returns to the individual burner.
The burner, 5, is constructed in such a way that it can burn off the remaining impurities.
The efficiency of the hot-air concept can be summed up as follows: The system is virtually closed and environmental protection is totally integral. As the individual burners are fed relatively cold air there is a strong pressure difference between the burner in-take and outlet, which makes it possible to modify the air in the working channel in such a way that here also differing pressures can be attained.
Figure 3 shows extremely clearly that the pressure values, delta p, are greater at the end of the installation.
The Gerlach hot-air system establishes that parallel to the increase in pressure there is an increase in the cross-motion of the molecules immediately in contact with the profile surface. Due to the decreasing delta Ts between the air temperature and the profile temperature a huge increase in the heat exchange values is achieved.
The heat transfer runs from beginning to end of the channel, parallel to the pressure curve.
The burner units are constructed in such a way that they can release large quantities of water into the working channel. Thus, as explained above, the air becomes particularly heavy and the heat transfer properties are once more improved.
Purely theoretically, the working temperature can be increased to 500[degrees]C. For safety reasons it is regulated to a maximum of 340[degrees]C.
The Gerlach hot-air concept offers a modular system in which software can be modified so that hardware can be processed as desired. For the user this offers the possibility of growth, both in the area of special elastomers and also of specific profiles, for years to come.
The hot-air installation concept
When we are absolutely clear about the product range to be cured by means of a hot-air vulcanization plant, then it is simple to create a working concept for the installation.
Figure 4 shows a selection of hot-air installations with the average running speeds and the projected energy consumption.
Each installation presupposes a vacuum extruder preceding and a regulating and measuring unit following, the latter capable of measuring the profile and regulating the raw material feed to the extruder. It must be pointed out, however, that where a coextrusion is present, regulation of the material in this way is not possible.
After the measuring and regulating unit, a high-speed-hot-air channel is basically always deployed which serves to heat up the rubber temperature from the extrusion temperature to the desired vulcanization temperature.
The running speeds "V" shown in the table are compiled from the operating records of customers who have installed Gerlach hot-air channels. To establish the values, EPDM profiles - as used overall in the automobile trade - were used.
In cases where the rubber profile is rigid because of its steel reinforcement, two hot-air channels in sequence are often deployed. In such cases the second machine invariably consumes less energy than the first.
It is often possible to divert steel reinforced profiles over corresponding minimum radii. In this case the second machine should be a triple-banked hot-air channel with integral cooling system. This machine is shown in the illustration; marked 114.
The most compact installation, worldwide, in deployment, is a combination of types 109 and 114.
Elastomer seals for the automobile industry are frequently cured on the systems indexed in figures 5, 6 and 7.
The installation 9 is very commonly used for the production of high volume profile seals for the building industry.
Due to the compact construction of the machine, extremely good running speeds can be attained, as in a total length of 25 meters, a heating channel of 50 meters and a cooling plant of 25 meters are combined.
The throughput figures given are an average and are easily adjusted heavily upwards or downwards if the relationship of the surface area of the profile to the cross section is altered.
Special requirements are not included in the installations illustrated. There are, however, because of the modular construction, virtually no special orders which cannot be fulfilled.
Which compounds and profiles can be cured with a hot air vulcanization
Here we can give a very simple answer: All profiles which consist of a sulfur-cured compound and are heat-curable can be vulcanized with a hot-air channel.
Sealing profiles with a silicon base can be cured extremely fast with a hot-air channel type 109.
Textile or metal reinforced low- middle- and high-pressure hoses can be vulcanized extremely efficiently in a hot-air channel. Special aggregates are available which make it possible to control these supported forms and to adjust the air content, for example, to eliminate damp.
If, in the broadest sense, we can consider a fire-hose, with or without polyester sheating, as a profile, then it must be a perfect example for warming by hot air.
Peroxide curing with hot air
Unfortunately, peroxide curing in hot air cannot be achieved without assistance, because of the affinity between the peroxide particles on the surface of the profile and the oxygen in the process air vulcanization is virtually impossible.
Vulcanization in inert atmospheres is also impractical as the unavoidable traces of oxygen find the undercured profile surface and thus lead to a volatile surface.
Past attempts to distance the oxygen from the profile surface by powdering the latter have failed.
Recently a chemical treatment of the surface has been developed which among other things accelerates the curing process.
We are involved in the research project and believe that the results will be ready for publishing for the technical market within a few months.
Can transport marks on the surface be eliminated?
The elastomer profile is lying on the conveyor belt when it reaches its highest plasticity and the contact surface acquires any irregularities inherent in the above belt.
These unwanted irregularities can be eliminated by the use of an absolutely smooth conveyor belt. Unfortunately the market does not yet offer such a belt, which at the same time is resistant to extremely high temperatures.
This surface marking can also be reduced by the use of a coating for the profile - with talcum for instance - this however, as is known, creates other problems.
Profiles with especially balanced cross-sections can indeed be suspended on a cushion of air, but handling is very difficult.
An excellent result can be obtained with the deployment of conveyor belt consisting of rollers. The development of this solution is complete and is being met with great interest. This new approach not only removes belt marking but it also helps aid in a more uniform heating around the part. Without the hinderance of the belt, more direct heating of the underside can be achieved.
The curing of sponge rubber by means of hot air
It is essential when curing sponge rubber to achieve a good cellular construction over the whole cross section and also to produce a surface area which is as smooth as possible. Naturally, to a large extent, both of the foregoing criteria are dependent on the compound used. However, it is also a question of the type and adjustment of the installation.
Index 9 in figure 4 shows a typical hot-air plant for sponge rubber. Here the rubber profile is fed through a high speed channel, type 109, with a length of 12,000 mm. The channel is set up in such a way that an extremely large heat transfer from the air to the rubber profile takes place in the first channel zone, so that the surface can be heated up and vulcanized as quickly as possible.
After approximately 3.5 meters the heat transfer rate is then, once more, raised in the third and fourth channel zones in order to initiate the foaming process.
In such an installation the blowing point is set at the end of the machine and the expansion of the profile can be balanced by raising the conveyor speed of the following machine. This subsequent machine is, incidentally, a type 114 twelve meter long, triple banked machine with an integral cooling plant built under.
This manufacturing process ensures that the profile has a uniform honey-combing over the whole cross section and that the surface is equivalently smooth. The relatively large cooling plant under the machine (requiring no extra floor space) provides a good cooling of the product, and this, as is well known, cannot be achieved with sponge rubber without a correspondingly long stay in the cooling medium. In a twelve-meter machine the cooling plant is twenty-five meters long.
With large cross sectioned profiles it is effective to station a very short UHF machine with a low output, possibly 3kw, between the high speed channel and the following triplebanked appliance or straight single pass ovens. It is now possible to integrate such a UHF back-up in a high-speed installation.
This type of low KW microwave is possible due to the extreme high energy that can now be put into the profile through the surface.
The addition of the new super jet air channel can now greatly improve the surface of the sponge profile due to the rapid skin cure that can be achieved.
Further possibilities for the deployment of hot-air plants
It is not only vulcanization which requires a heating of the elastomer profile, but also, often, drying out of primers of rubber coated steel reinforcements. For such applications very short installations are required, situated in line before the extruder. Optimally they are gas heated and the temperature of the process air can be set up to 400[degrees]F (248[degrees]C).
Coachwork seals are frequently flocked with a polyester fiber. For safety reasons it is best to use an adhesive which dries particularly fast providing it is subjected to a temperature of 170[degrees]F 248[degrees]C, and that the surrounding air is damp.
A high-speed channel, type 109, can fulfill all expectations, and at the same time it can polymerize the adhesive faster and more evenly than can be achieved with infra-red installations. More evenly because the hot air is fed equally to all surfaces, even in deep crevices, where infra-red radiation has particular problems, (also, the intensity of the infrared rays sinks in cubic proportion to the radiation distance, which results in assorted degrees of polymerization at different levels). Production is faster because of the better heat transfer and because air humidity is extremely high.
The fact that the process air is clean allows to be removed through evaporation the adhesive solvents. Although very high air turbulence is available in a channel, the flock-fiber remains very stable.
Hot-air installations as a back-up to a UHF machine
UHF machines, such as shearhead extruders, need hot-air installations as back-ups so that the achieved heat can be held constant for the necessary time. Overall, however, it is a fact that simply to maintain the heat is not enough and that extra heating is required. This has already been explained heretofore. At high throughput speeds hot-air installations like those in figure 4, index 9, are recommended. If high volume profiles are cured, then installations of the type 114 are sufficient - with an overall length of twelve meters, a heating length of thirty-eight meters and a cooling length of 25 meters, as the extrusion speed is already governed by the extruder.
The hot-air channel - often only one unit in a long line
Often the hot-air vulcanization plant is only one small unit in a very long production line. Even more then, it must be fashioned in such a way that it is compatible with other machines and its performance approaches 100%.
For instance, universally compatible computer controls are now offered which can identify and deal with a problem before this leads to a breakdown of the entire plant. Here we must not forget that a breakdown in the hot-air curing installation can bring the whole production line to a halt.
In our hot-air vulcanizing installation we measure not only the temperature but also what the rubber profile is actually experiencing - and nothing less is acceptable for successful vulcanization. The Siemens SPS Control can be spliced into the main computer via a RS-interface. The application of the stored data-values occurs automatically.
Filtering of the exhaust air
We believe one can no longer ignore one of the most basic human requirements - clean air to breathe. Therefore, Gerlach is developing at the moment a collecting filter which it is envisioned will mechanically remove all but .3% of the contaminants and return these to the burners to be incinerated.
The piece de resistance of this centrifugal filter unit is that an aerosol phase is generated and the fine spray is then mechanically collected. However, because the whole filter unit has a centrifugal motion, the collected vapor can be expelled immediately after its entry to the filter layer. The filter unit is therefore largely self-cleaning and thus works to maximum efficiency.
Energy costs - a savings concept
Naturally, we will pay less for energy if we use less; and if we can achieve a saving in energy then this is obviously very positive. If, however, at the same time as these energy savings the production rate falls, then the process has usually already degenerated into inefficiency.
We maintain that energy should never be squandered, and that the primary energy used in our process must be that which is the cheapest on the market and over and above that the cleanest. So, we come to natural gas. As we use direct and not indirect heat we find it particularly easy to meet the emission requirements laid down by law.
For most countries of the world, the cost of natural or propane gas is only ten to twenty-five percent of that of electricity. The small side effect, namely, the extremely fast heating of the process air from 20 to 340[degrees]C inside 30 to 60 seconds, is often almost ignored in comparison to the cost advantages. As shown previously, the investment costs of a well thought-out vulcanization installation can be recouped by the savings in energy costs over a comparatively short period of time.
Waste heat must be recycled
We do this in that we let if flow into our heat exchanger, and this in its turn provides for it to be carried over, in a most sophisticated fashion, to the clean process air.
Obviously, condensation is formed. This, however, can be allowed to run off freely, and cleaning, necessary perhaps only every 6-12 months, demands only about 30 minutes.
The trend to the introduction of EPDM sealing profiles with limited elastomer quantities per unit and the future demand for complex sealing units force the pace of integration of hot air processing units into industry.
In order to be more flexible with the future's complex seals, the Gerlach company is working on supporting the high speed hot-air channel with additional UHF. In this concept the polarity of the compound plays a less important role as the dielectric loss is usually increased considerably by the heating with hot air. The electromagnetic field can be shifted over the channel cross section by computer control so that an optimal adaptation to the profile is possible.
Only very low powered UHF units of about 2.4 kw are required for the most part as the primary heating is still achieved by the hot-air installation.
Our use of the superjet air oven eliminates belt marking and plays a great roll in better sponge surface conditions. It also reduces line length requirements which can be of big concerns in many older plants.
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|Date:||Jul 1, 1995|
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