DE2536462A1 - PROCESS FOR PRODUCING RECTANGULAR HOLLOW PROFILES WITH NON-CIRCULAR CROSS-SECTION - Google Patents

Description

Pat.Dr.Hh / Emm / 9

Process for the production of stretched hollow profile strands with a non-circular cross-section

The mechanical properties and the weather and corrosion resistance of thermoplastics can be decisively improved if they are in the thermoelastic state be biaxially stretched. While the plastic is flowable in the thermoplastic state and its shape under the The effect of relatively small forces is permanently changed, but in the thermoelastic state it sets an imposed one Change in shape against an elastic restoring force. The forced change in shape, for example biaxial stretching a flat plastic plate, can be frozen by cooling to below the thermoelastic temperature range. The frozen molecular orientation caused by the stretching is responsible for the property improvements. The improved Properties disappear again when the stretched plastic is subsequently restored to its thermoelastic state is heated and shrinks back to its original shape. All thermoplastics that are between the glass transition temperature and the beginning of the thermoplastic state range a sufficiently pronounced thermoelastic Have state area. These are mainly thermoplastics with a high molecular weight, such as polymethyl methacrylate, Polystyrene, ABS plastics, polyvinyl chloride and cellulose ester plastics .

The production of sheet-shaped stretched plastics has been known for a long time. K. Richard, G. Diedrich and E. Gaube have ^{also described the biaxial reocing of pipes in "Kunststoffe 1} * Vol. 50, pp. 371 to 375 (1960). The pipes are by means of

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a dome or by means of compressed air on an enlarged one Brought cross-section and at the same time stretched in the longitudinal direction. The pipe wall is stretched biaxially and decreases - just like a flat plastic plate - in thickness. The stretching of pipes with compressed air is carried out by the Identified by authors as the preferred method, but has so far found little use in practice. This is believed to be due to technical difficulties associated with the process.

The stretching system works continuously and is connected to an extruder. The extruded tubular strand is first cooled below the softening temperature and then by means of a pair of rollers at a defined speed conveyed into a heated calibration sleeve, where it is uniformly adjusted to a temperature suitable for stretching in the thermoelastic State area is heated. The strand is in the calibration sleeve through the ruling inside Air pressure pressed against the wall of the calibration sleeve. In order to prevent the sliding of the To enable strand through the calibration sleeve, one leaves the heating medium, which is under approximately the same pressure like the DrudtLuft in the pipe string, directly on the string approach. The pipe string leaves the heating zone through a round outlet opening, which is caused by the onset of inflation of the pipe string is sealed against the outlet of the pressurized heating fluid. Immediately behind the In the heating zone, the strand is inflated by the internal air pressure and with an expanded scope it enters another calibration sleeve, where it is cooled below the softening temperature. It is then picked up by a pulling device that runs faster than the conveyor in front of the entrance into the heating zone. The speed difference between the two conveyors causes the stretching in the longitudinal direction. The interior of the strand is through a slidably suspended, connected to the extruder core

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Sealing plug sealed pressure-tight.

The internal pressure in the plastic strand must be very precise Be dimensioned so that the pipe string after exiting the heating zone and before entering the cooling zone inflates exactly to the required extent. If the pressure is too low, the required widening of the circumference will not be achieved achieved. If the pressure is too high, the strand expands to such an extent that it does so when it enters the cooling calibration sleeve must be constricted again. In this case, there is a high level of friction in the cooling calibration sleeve, as this occurs in the Heating zone acting as a lubricant liquid heating medium is almost completely stripped off when it exits the heating zone. The "method therefore requires constant, very careful monitoring in order to ensure uniform stretching.

The difficulties associated with the known method increase considerably when hollow profile strands of not circular cross-section, e.g. square profiles or hollow slabs with flat outer surfaces. When leaving from the heating zone, a hollow cord delimited by flat surfaces expands in the shape of a pillow, so that it is only compressed into a cooling calibration sleeve can enter. There are deviations of the desired geometry can no longer be avoided, especially if the hollow strand has inner walls, as is the case with, for example a hollow profile plate consisting of two cover layers and webs arranged vertically between them is the case.

It has been found that stretched hollow profile strands with a non-circular cross-section made of thermoplastic material pass through Expansion of a likewise non-circular hollow profile strand in the thermoelastic state with the aid of an in the cavity produce compressed gas enclosed in the profile strand when the hollow profile strand runs through a calibration device, whose free cross-section increases in the running direction of the strand to the extent of the desired stretching and in which the expanded hollow

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profile strand after reaching its final cross-section is cooled below the softening temperature. The cross-sectional expansion of the strand occurs through the conical expansion the calibration device independently of the internal pressure to the desired extent, provided that the internal pressure is at least that for the expansion is of the required size.

The expansion of the profile diameter and the stretching of the hollow profile walls are not always strictly proportional to one another. In particular, the inner walls of multi-chamber profiles are used not only by increasing the profile diameter, but also by reducing the thickness of the outer walls. There is hence cases in which a flat multi-chamber hollow profile is only stretched in length and width while its thickness remains unchanged, but the inner walls are stretched to the same extent solely by the decrease in thickness of the outer walls become like the outside walls.

The calibration device used for the method of the invention consists of three zones; in the first zone the hollow strand is adjusted to the temperature suitable for stretching. In the second, In the conically enlarged zone, the stretching takes place at a substantially constant temperature. In the third zone that like that first zone has a constant clear cross-section, the strand is cooled below the softening temperature.

In the method of the invention - as in the known method - a lubricant is used, which is a allows even flow of the thermoelastic, pressurized strand in the calibration device. As in the known method, the lubrication can be effected by the heating or cooling medium, which is introduced through openings in allows the wall of the first zone of the calibration device to approach the plastic strand directly, but it is preferred introduced separately from the heating or cooling medium.

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The strand drags a film of lubricant into the following Zones with. In the stretching zone, this film is expanded to the same extent as the plastic strand and is accordingly thinner. At the same time, the strand is accelerated. All of these factors increase frictional resistance between the strand and the wall of the calibration sleeve. The friction has a considerable influence on the process "On" the stretching process. As soon as irregularities in frictional resistance occur at different points on the strand, the result is unevenness in the stretching that occurs at the finished strand as visually unsteady zones, mechanical weak points or in the form of distorted partition walls in appearance step. While these problems are easy to control with a round pipe, they take them away with non-circular ones Extruded profiles with their complexity too. With such cross-sections and especially at the attachment points of Different restoring forces arise on the inner walls of the hollow profile and consequently also different pressures between the strand and the calibration sleeve wall. The lubricant collects preferentially in the zones of reduced pressure and changes the frictional resistance there.

It has been found that the local differences in stretching caused by different friction are largely avoided when using a lubricant with a viscosity of at least 5000 cP under operating conditions. That The lubricant system is then of course separated from the heating or cooling system and the lubricant at the inlet of the Tempering zone of the calibration device via a lubrication groove brought to the strand surface. In contrast to the low-viscosity ones used as lubricants in the known process oils and glycols form the highly viscous lubricants used according to the invention even at the low feed rate of the strand from a hydrodynamic lubricating film, which is caused by fluctuations in the pressure exerted by the hollow profile strand Pressure is not noticeably affected. One therefore obtains

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at every point of the calibration device the same flow and friction conditions so that no distortions occur in the cross-sectional geometry and in the position of the partition walls.

The lubricant must be liquid at the operating temperature, can be solid at room temperature, thereby reducing the mechanical Detachment from the strand surface is facilitated. Suitable as lubricants are substances that have a have a sufficiently high viscosity and have a corrosive effect neither on the extruded plastic nor on the calibration device. For example, you cannot use waxes, paraffins, polyethylene glycols or solutions of high molecular weight substances in one Use corrosive solvents. Aqueous solutions of water-soluble polymers are also suitable, for example 0.1 bis 10 $ solutions with a viscosity of at least 5,000, preferably between 5,000 and 150,000 cps. For the production of the aqueous lubricant solutions are water-soluble polymers with a molecular weight of about 50,000 to a few millions. They are preferably wholly or predominantly composed of acrylic acid, methacrylic acid, itaconic acid, maleic acid or their salts or from acrylamide, methacrylamide, vinylpyrrolidone, vinyl alcohol, hydroxyalkyl esters or Blalkylaminoalkylestera acrylic or methacrylic acid, salts or quaternization products of these esters or of mixtures of such monomers. Also cellulose ethers and other natural or synthetic water-soluble polymers are suitable. Since the solutions are applied under pressure, it evaporates water does not serve as a solvent under the operating conditions. When leaving the calibration device The film of the permeating agent dries up quickly and can in most cases be removed from the plastic surface by brushing will. If you intend to use paper or foil to protect the hollow profile strand produced, this can be applied to the lubricant film before it dries can be applied using the lubricant as an adhesive serves for the protective layer.

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The compressed gas used to expand the strand is introduced through the core of the extruder nozzle into the cavity of the profile strand. The required gas pressure depends on the degree of stretching and the restoring forces that occur.If the profile circumference increases linearly by 50 to 200 % , a pressure between 1 and bar is used.Compressed air can be used as the compressed gas To cool the extrusion nozzle quickly and intensively, liquid nitrogen can also be introduced.

So that the compressed gas does not escape at the end of the hollow profile strand, the hollow space of the extruded profile is closed off from the atmosphere in a suitable manner · This can, for example be effected with a plug inserted into the hollow profile, which is attached to the core of the with a chain Extruder nozzle is attached and is held at about the level of the end of the cooling zone of the calibration device. There However, it is difficult to insert such a plug into the hollow pipe when starting up,

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drawn to weld the expanded hollow strand behind the exit from the calibration device each time a Sufficiently long piece of the expanded strand has been promoted, and that behind the welded parts separate pieces. The thermoplastics used to manufacture the hollow strands can usually be welded pressure-tight by moving heated press jaws together.

If the hollow strand is welded shut at its free end in the manner mentioned last, the in its The gas pressure prevailing inside not only widening the cross-section, but at the same time stretching in the longitudinal direction. One can choose the gas pressure so high that the stretching in the longitudinal direction is as great as the stretching in the transverse direction, so that the preferred biaxially uniform stretching is achieved. The gas pressure required for this, however, is so high that it is very considerable Transferring compressive forces to the wall of the calibration sleeve and this must be dimensioned accordingly. It is therefore preferred to have just such a gas pressure to be used, which causes the desired widening of the cross-section and at most triggers a slight longitudinal stretching. In order to still achieve a biaxially uniform stretching, the free end of the strand is cooled in its cold Area drawn off by a pair of pulling rollers at a higher speed than the strand in the calibrating device entry. The stretching in the longitudinal direction results from the speed difference. When the compressed gas in the hollow profile is sealed against the surrounding atmosphere by a drag stopper, can only be done by such a pulling device a longitudinal stretching can be achieved.

The extruded strand sets in the thermoplastic area immediately behind the extrusion nozzle of the longitudinal stretching the slightest resistance and would almost only be stretched there, if not by special precautions is prevented. The stretch in the thermoplastic area

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does not lead to the property improvement that the stretching is achieved in the thermoelastic range. In the known method described above, the stretching in the thermoplastic area is prevented by that the strand is initially below the softening temperature cools and can run through a braked pair of rollers in a heating device, where the strand again up to the thermoelastic State is heated. Then it can be stretched in the longitudinal and transverse directions, the longitudinal stretching from the speed difference between the pair of braking rollers and the pair of pulling rollers results in '.

The use of a highly viscous lubricant prevents the undesired longitudinal stretching of the strand in the thermoplastic area in a particularly simple manner. The highly viscous lubricant film exerts such a braking effect on the strand flowing through the calibration device that the tensile force causing the stretching in the longitudinal direction is opposed by an equally large frictional force, which is only effective in the thermo-elastic area of the strand between it and the wall of the calibration sleeve. The strand is thus gradually accelerated in the thermoelastic range, the degree of longitudinal stretching resulting from the speed difference of the take-off rollers and the strand speed in the area where the thermoelastic state is reached.This speed difference is determined by the frictional forces between the strand and the calibration sleeve wall. The friction is the viscosity fiss lubricant at the 3strisfcst3mp £ * rv'rar _r . dsp SfcranggssohMiaeiigkeit and Reifomgs ~ never surface dss thsrmoelastischen part of the strand with the Kalibrierbtiohsenwand dirskt and thickness nasty Sohmierfilms inversely proportional = 'Dz the Fli ^ Sgesoh ^ indigo ^ dit and di-.s friction surface during Reolc / organges au "ad Sofe When the film is thrown, the friction in the resistance becomes strong © 2? ht and srtj da £ dis to the longitudinal reok?

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in technical operation depending on the viscosity independently by the amount of lubricant, which the strand takes with it from the lubrication groove. If the lubricant viscosity is properly rated, it will be thermoplastic Plastic strand is drawn into the calibration sleeve in the longitudinal direction without stretching and only when the Stretched friction. Because of the strong influence of the lubricant viscosity on the stretching process, it is important to keep this viscosity constant.

The outer walls of the hollow profile, and in the case of several chambers also the partition walls, generally have a thickness between 0.5 and 2 mm in the stretched state. With a stretching of 70 % in each spatial direction, this wall thickness is achieved with an initial thickness of 1.5 to 6 mm.

According to the method of the invention, hollow profiles of any shape can be produced with high calibration accuracy. she can have one or more chambers. While the design of the contour shape of the cross section is practically no limitation is subject, the partition walls take on the position and shape that result from the elastic stress conditions during inflation results. The possibility of setting different pressures in the individual chambers of a hollow profile, for example to create arched partition walls should not go unmentioned.

The method of the invention is suitable for plastics, which can be in a thermoplastic state and extrude through a pronounced thermoelastic state on cooling, for example, ^J polymethyl methacrylate, polystyrene, polyvinyl chloride or ABS plastics. Polymethyl methacrylate is preferred and can also contain flame retardant additives.

The invention is based on a method and a device for producing a three-dimensionally stretched

Hollow chamber profile made of polyraethyl methacrylate explained in more detail. X) poly carbonate plastics,

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Figure 1 shows the cross section of the hollow profile after extrusion.

FIG. 2 shows, on the same scale, the cross section of the same hollow profile strand after stretching.

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Figure 5 is a - / section through the complete extrusion and stretching system including the welding and cutting device.

The thermoplastic is fed into the extrusion nozzle 1 through the opening 2 Molding compound 3 fed from an extruder (not shown). Between the nozzle lips 4,5 and the core pieces a hollow profile strand is formed, which consists of two cover layers 7 and 8 and webs 9 arranged at right angles between them consists.

When starting up the system, the cavities 10 of the profile strand, which are closed at the front end by welding in the thermoplastic state, with a low Positive air pressure, which is passed via the line 11 and the channels 12 into the chambers 10, is supported. Those from parts 15 to 20 constructed calibration device can be divided along the plane A-A and the upper parts 13, 15. » 17 and 19 are open during Approaching the lower parts 14, 16, 18, 20 released and raised so that the emerging strand initially without stretching can be carried out approximately to the end of the calibration device. Now the tops 15, 15, 17 and 19 are on the Lower parts put on and locked with them. The pressure in the chambers 10 is gradually increased to about 5 bar, so that the Strand inflates in the stretching zone I7 / I8. At the same time the air pressure in the gap 21/22 increased to the same extent as in the chambers 10. When flowing through the channel 23, the hollow profile strand is with a Lubricant film 24 surrounds and enters the zone with this film

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The calibration ζ one 13/14 is surrounded by a cooling jacket that is covered by Cooling water is flowed through. When leaving this zone, the outer sides of the cover layers 7 and 8 are at a temperature cooled below 100 °, while the surface of the outer layers and webs located inside has about 14o °. The following Calibration zone 15/16 is thermostatted to approx. 14o ° C using pressurized water.

At the end of this zone, the strand has a uniform temperature of 140 ° C., so that it is uniform in the subsequent zone 17/18 is stretched three-dimensionally.

After the stretching has been completed, the strand is placed in zone I9 / 20, around which a cooling medium of around 25 ° C flows, is cooled below the softening temperature. From the exiting strand the lubricant layer 24 is stripped off by means of rubber lips 25/26 and the lubricant is returned for reuse. The remnants of the lubricant film are removed with the hot air blower 27/28 dried and removed from the strand surface with the brush rollers 29/50 removed. The roller pair 31/32 causes the longitudinal stretching in zone I7 / 18 of the calibration device and conveys the strand to the separating device 33/34 * which contains two heated welding jaws 35/36, which are moved together hydraulically and weld the hollow strand. The part 37 of the hollow strand is at the Weld 38 peeled off in the thermoplastic state and separated from the strand. In the case of this, the chambers of part 37 burst in the area the still thermoplastic weld seam so that the pressure gas can relax. During the welding process, the Separating device 33/34 on the rails 39/40 with the strand.

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Claims (4)

Röhm ^2536A62 GmbH Darmstadt Claims Process for the production of stretched hollow profile strands of non-circular cross-section from thermoplastic Plastic by expanding a likewise non-circular hollow profile strand in the thermoelastic state Using a compressed gas enclosed in the cavity of the profile strand, characterized, that the hollow profile strand runs through a calibration device, the free cross section of which is in the running direction of the strand increases to the extent of the desired stretching and in which the expanded hollow profile strand after reaching its final Cross-section is cooled below the softening temperature. 2. The method according to claim 1, characterized in that the hollow profile strand simultaneously with the cross-sectional expansion is stretched in the running direction by having a higher Speed is deducted from the calibration device as it enters it. 5. The method according to claims 1 and 2, characterized in that a lubricant with a viscosity of at least 5,000 oP (under operating conditions), - which has a braking effect at the same time, is introduced between the hollow strand and the inner wall of the calibrating device. 4. The method according to claim 3, characterized in that the liquid lubricant is a 0.1 to 10 £ aqueous solution of a polymer with a viscosity between 5,000 and 50,000 cP. 709808/0980