CA2497457A1

CA2497457A1 - Method and apparatus for hauling off extrusion products, in particular for aligning the caterpillar take-off device in relation to the extrusionaxis

Info

Publication number: CA2497457A1
Application number: CA002497457A
Authority: CA
Inventors: Henning Stieglitz; Roman Laberer; Stefan Jovers; Andreas Weinmann; Karl-Heinz Forsch; Karl-Heinz Baston
Original assignee: Individual
Current assignee: Krauss Maffei Kunststofftechnik GmbH
Priority date: 2002-09-04
Filing date: 2003-08-14
Publication date: 2004-03-18
Also published as: RU2005109404A; AU2003260416A1; EP1695811A3; DE50304500D1; DE10240718A1; EP1695811A2; WO2004022310A1; ATE334795T1; CN1678446A; CN100366417C; EP1536937A1; DE10240718B4; EP1536937B1; AR041141A1; RU2314199C2; JP2005537161A

Abstract

The invention relates to a caterpillar take-off unit, in addition to a method for taking off extrusion products, in particular plastic pipes, using a caterpillar take-off unit comprising at least two caterpillar tracks. Said tracks can be arranged symmetrically in relation to a longitudinal axis of the caterpillar take-off unit, at least during the take-off process, and the distance from said unit can be varied. One disadvantage of known caterpillar take-off units is that they have previously had to be re-adjusted manually for each cross-sectional modification of the extruded product. According to the invention, a modification of the cross-section of the extrusion product is measured and the distance of the caterpillar tracks from the longitudinal axis is automatically adapted in accordance with said modification in such a way that the axis of symmetry of the extrusion product coincides with the longitudinal axis of the caterpillar take-off unit.

Description

METHOD AND APPARATUS FOR HAULING OFF EXTRUSION
PRODUCTS, IN PARTICULAR FOR ALIGNING THE CATERPILLAR
TAKE-OFF DEVICE IN RELATION TO THE EXTRUSION AXIS
The invention relates to a method and apparatus for taking off extrusion products according to the preamble of claim 1 and claim 6, respectively.
In the production of extrusion products, which also includes plastic tubes, the thermoplastic melt strand emerging from tan extruder is shaped in a molding tool, subsequently cooled under the constraints of the mold, and calibrated. Take-off devices disposed in the extrusion line downstream of the calibrating and cooling devices are intended to convey the melt strand, emerging from the extruder, through the various intermediately positioned devices. The extrusion product which is produced from the melt strand after being shaped and calibrated and has sufficient strength following the cool-down period, is grasped by the take-off device and transported jerk-free and at even speed or precisely controlled speed pattern through the various devices. The greater the friction forces encountered in these devices, the greater the take-off force of the system. At the same time, the take-off force has to be suited to the size, cross section, type, and wall thickness of the extrusion product being conveyed.
The most common take-off devices are the si-called caterpillar take-off devices, as described, e.g., in DE 33 05 175. The required take-off force is applied into the tube by means of circulating conveyor chains, so-called caterpillar tracks, which are provided with rubber shoes and have a conveyor surface in parallel relationship to the extrusion product being transported. The rubber shoes should be as wear-resistant as possible while still exhibiting a good adhesion of the caterpillar tracks upon the surface of the extrusion products. In order to avoid uneven wear of the caterpillar tracks and an unwanted twisting of the extrusion products during take-off, the caterpillar tracks engage the extrusion product as symmetrically as possible. In the event of a tube extrusion, the caterpillars are thus attached cylindrically symmetrical to a length axis of the take-off device which must coincide with the symmetry axis of the transported tube in order to prevent an unwanted twisting or uneven stress of the tube.
Depending on the tube diameter, it is also known to use in caterpillar take-off devices for the tube extrusion, for example, four caterpillar tracks which are arranged in the form of an X about a length axis of the take-off device. To keep the stress on the tube as small as possible, the tube has to enter the take-off device in midsection, i.e. the symmetry axis of the tube must coincide with the length axis of the take-off device. The lower caterpillars are hereby respectively movable via mechanical adjustment units along the leg of the X whereas the upper caterpillars are urged into contact with the tube by pneumatic cylinders which act also along the legs of the X. The pressure applied by the pneumatic cylinders is freely adjustable and must be matched to the dimension and wall thickness of the tubes. During production of tubes with very slight wall thickness, it may become necessary to impose on the pneumatic cylinders a counterpressure which opposes the gravity acting on the upper caterpillars, to ensure a sufficient force transmission of caterpillar tracks and to prevent damage to the extrusion product.
Known take-off devices suffer shortcomings because the caterpillar take-off device must be readjusted in the event of a change in production accompanied by a change in diameter of the extrusion product because otherwise the symmetry axis of the extruded material does no longer coincide with the length axis of the take-off device. As a consequence, the whole extrusion line must be halted, resulting in production downtimes.
It is an object of the present invention to provide a method and apparatus for hauling-off extrusion products which recognizes a cross sectional change of the extrusion product and so adjust the position of the caterpillar tracks that the symmetry axis of the extrusion product coincides with the length axis of the take-off device.
This object is attained by a method according to the features of claim 1, and an apparatus according to the features of claim 6.
In accordance with the present invention, the take-off device recognizes spontaneously changes in cross section of the extrusion product as a result of a production change and initiates immediately steps to correct the position of the caterpillar tracks so that the extrusion product continues to centrally enter the take-off device. This adjustment to a change in diameter or cross section occurs during operation of the apparatus so that the need for a shutdown and renewed start-up is eliminated and resultant uncontrolled changes of process parameters are avoided. A take-off device according to the present invention enables the operator to retrofit the apparatus in a very short time.
The system control unit of the extrusion line is required to merely communicate to the take-off device the line speed and the contact pressure and counterpressure to be applied by the caterpillar tracks upon the extrusion product. In addition to the fully automated adjustment to any changes in cross section, it is also possible to carry out the adjustment by hand at the take-off device, as already known.
According to a preferred method, the change in cross section of the extrusion product can be determined in response to a deflection of the caterpillar tracks from their original position, and reacts in dependence on the determined absolute variable by automatically adjusting the distances of the individual caterpillar tracks from the length axis of the take-off device so that the extrusion product runs centrally through the take-off device and all force impacts occur substantially symmetrical.

It is hereby possible, to measure the deflection of caterpillar tracks only which are located above a plane, defined by the horizontal and the length axis of the take-off device, and to move the caterpillar tracks, positioned below or in this plane, in dependence on the measured deflection until a symmetric disposition of the caterpillar tracks about the extrusion product and the length axis is reestablished.
According to especially preferred procedure, the upper caterpillar tracks are urged against the extrusion product by controllable, elastic means, so that an even pressure is generated on all caterpillar tracks. This contact pressure is set by the system control unit of the extrusion line in dependence on material and dimension.
According to a further preferred procedure, the measured deflection is directly transmitted to a control unit for computing a needed change in the distance and for initiating this change in distance. With the assistance of this control unit, it is also possible to simultaneously control the contact pressure.
This method can be preferably carried out in a caterpillar take-off device in which the at least two caterpillar tracks can be disposed in symmetric relationship to a length axis of the take-off device and pressed against the extrusion product.
According to known caterpillar take-off devices, the caterpillar tracks should be arranged, at least during operation, i.e. during conveyance of the pulled-in extrusion product, in symmetric disposition to a length axis of the caterpillar take-off device and apply a contact pressure upon the extrusion product so as to enhance the force transmission between caterpillar tracks and extrusion product.
When the extrusion product has very small cross section or thin wall thickness, the contact pressure applied by the gravity of the caterpillar tracks situated above the extrusion product can be reduced also by applying a counterpressure so as to prevent damage to the extrusion product. Still, sufficient take-off force must be transmitted in order to be able to transport the extrusion product through the devices between extruder and take-off device.
According to the invention, means are provided to enable measurement of the deflections of caterpillar tracks, caused by changes in cross section of the extrusion product, substantially orthogonal to the length axis of the take-off device.
The contact pressure and counterpressure upon the caterpillar tracks can be generated by, providing piston and cylinder units which can operate at a predetermined pressure and have piston rods moveable orthogonal to the length axis of the take-off device, whereby their effective lines intersect on the length axis. It is especially preferred, when only those caterpillar tracks, which are arranged above a plane defined by the horizontal plane and the length axis of the take-off device, are elastically and controllably supported by the piston and cylinder units, while the caterpillar tracks situated below or in this plane are mechanically movable and controllable.
The use of automatically controllable piston and cylinder units has been shown effective which require only the desired pressure and the pressure to be maintained. Especially preferred is the use of pneumatic cylinders.
A mechanical change of the distance between those caterpillar tracks that are not linked to the piston and cylinder units and the length axis of the unit spontaneously can be realized by using at least one motor. As all caterpillar tracks should have a same distance to the firmly defined length axis, and thus involves a symmetric distance change, the use of a single motor is sufficient which is operatively connected to the caterpillar tracks, situated in or below the horizontal plane, via a gear mechanism.

For measuring the deflection measurement of the caterpillar tracks in response to an encountered change in diameter or cross section of the extrusion product , the use of displacement pick-ups is preferred. It is, however, also possible to use at least one angle position transducer. This angle position transducer is attached on the gear mechanism or shaft of the motor and allows implementation of a precise movement of the caterpillar tracks, positioned below or in the horizontal plane, along the connecting lines to the length axis.
Suitably provided on the piston and cylinder units are displacement pick-ups by which the deflection of the caterpillar tracks, which are elastically supported on the piston and cylinder units, is measured in the event of an extrusion product with different cross section, and transmitted to a control unit of the take-off device. On the basis of the measured deflection, the control unit computes the required modification in distance between the caterpillar tracks in or below the horizontal plane and the length axis of the take-off device and causes the caterpillar tracks to shift accordingly. The caterpillar tracks above the plane are also moved along the connecting line to the length axis and orthogonal thereto, as a consequence of the applied contact pressure when the caterpillar tracks in or below the plane change their position, so that all caterpillar tracks are again arranged symmetrically about the length axis.
In addition to the fully automated recognition of a cross sectional change and the subsequent adjustment and recalibration of the relative position of the caterpillar tracks, it is, of course, still possible, to adjust the take-off device according to the invention by hand, as required, e.g., when the production parameters dictate a complete change-over of the extrusion line.
The mode of operation of an apparatus according to the invention and of a method according to the invention for taking-off tube should now be illustrated by way of example with reference to the following drawing. It is shown in:

FIG. 1 a tube extrusion line with take-off device according to the invention;
FIG. 2 a schematic sectional illustration of a take-off device according to the present invention with a tube of a first diameter;
FIG. 3 the caterpillar take-off device of FIG. 2 immediately following a dimensional change; and FIG. 4 the caterpillar take-off device of FIG. 3 in the readjusted state.
FIG. 1 shows an extrusion line 1 for producing plastic tubes 8. Following an extruder 2 is a tubular die 3, which is followed by a vacuum tank 4 for calibrating the extruded molten tube. This is followed by a cooling device 5 by which the tubes 8 must cool down far enough to have sufficient stable shape to withstand the strains applied by a downstream take-off device 6 according to the present invention. The take-off device 6 applies upon the tube 8 a take-off force which must be greater than the friction forces encountered beforehand in the line, in particular in the calibrator, on device transitions and seals. The take-off device 6 is followed by a separating device 7 by which the tubes 8 are cut to size, and a stacking device 9.
FIG. 2 shows greatly simplified a schematic cross section through a take-off device 6 according to the invention in which the extruded tube 8 is transported by caterpillar tracks 10, 10' arranged in symmetric relationship in the form of an X.
The caterpillar tracks 10, 10' are arranged symmetrically to a length axis 12 of the take-off device, whereby the length axis coincides with the symmetry axis of the tube 8. The contact pressure is applied by pneumatic cylinders 14 and effects that all caterpillar tracks 10, 10' are pressed against the wall of the tube 8.
In the event of a change in diameter of the extruded tube 8, both upper caterpillar tracks 10' are deflected in opposition to the pressure applied by the pneumatic cylinders 14 so that the disposition between the caterpillar tracks 10', positioned above the plane defined by the horizontal and the length axis 12, and the caterpillar tracks 10, positioned below the plane, is momentarily no longer symmetric in relation to the machine length axis 12. The symmetry axis of the tube 8 does no longer coincide with the length axis 12 of the take-off device.
This situation is shown in FIG. 3. This deflection by the caterpillar tracks 10' is detected by - unillustrated - displacement pickups on the cylinders 14, measured and transmitted to an - unillustrated - control unit which instantly modifies the distance of the lower caterpillar tracks 10 from the machine length axis 12 with the assistance of an angle position transducer on the motor 18 so as to reestablish a symmetric disposition of the caterpillar tracks 10, 10' and the machine length axis 12, as shown in FIG. 4, for ensuring an optimum advance of the tube 8 at minimum wear of the caterpillar tracks 10, 10'.
The advantage of the present invention is the capability to automatically recognize any change in cross section during production of extrusion products in extrusion lines and to spontaneously make the appropriate optimum readjustment. As a result, the operation runs smoothly, without experiencing cost-intensive production downtimes, as caused by manual adjustments.

LIST OF REFERENCE CHARACTERS
1 extrusion line 2 extruder 3 tubular die 4 vacuum tank 5 cooling device 6 caterpillar take-off device 7 separating device 8 tube 9 stacking device 10 lower caterpillar tracks 10' upper caterpillar tracks 12 machine length axis 14 pneumatic cylinder 18 motor

Claims

1. A method of taking-off extrusion products, in particular plastic tubes, by means of a caterpillar take-off device which has at least two caterpillar tracks, wherein the caterpillar tracks can be arranged at least during take-ff in symmetric disposition to a length axis of the caterpillar take-off device, and their distance from the length axis can be modified, characterized in that a change of the cross section of the extrusion product is measured and the distance of the caterpillar tracks from the length axis of the caterpillar take-off device is automatically adjusted in dependence on this change so that the symmetry axis of the extrusion product coincides with the length axis of the caterpillar take-off device.

2. Method according to claim 1, characterized in that the caterpillar tracks arranged above a horizontal plane through the length axis of the caterpillar take-off device are deflected in the event of a change in cross section, and this deflection is measured, and the tracks situated in and/or below this plane are moved in correspondence to the measured deflection that the symmetry axis of the extrusion product coincides with the length axis of the caterpillar take-off device.

3. Method according to claim 2, characterized in that the caterpillar tracks arranged above the plane are pressed against the extrusion product by controllable, elastic means.

4. Method according to one of the claims 1 to 3, characterized in that the measured deflection is transmitted to a control unit which computes the necessary change in distance and initiates the change in distance.

5. Method of producing extrusion products, in particular plastic tubes, by means of extruding a melt strand emerging from an extruder, shaping the melt strand in a die, in particular a tubular die into a molten tube, cooling down and calibrating the extrusion product and taking off the latter with a caterpillar take-off device having at least two caterpillar tracks, as well as cutting the extrusion product into single pieces, characterized in that the distance of the caterpillar tracks from a length axis of the caterpillar take-off device is set automatically based on a measured change in diameter of the extrusion product in dependence thereof so that a symmetry axis of the extrusion product coincides with the length axis of the caterpillar take-off device.

6. Caterpillar take-off device for taking-off extrusion products, in particular plastic tubes (8), comprising at least two caterpillar tracks (10, 10'), wherein the caterpillar tracks (10, 10') can be arranged at least during the take-off operation in symmetric relationship to a length axis (12) of the caterpillar take-off device (6) and can be pressed against the extrusion product, as well as means for changing the distance between a number of the caterpillar tracks (10, 10') and the length axis (12) of the caterpillar take-off device (6), characterized in that means are provided for measuring deflections of caterpillar tracks (10, 10') orthogonal to the length axis (12).

7. Caterpillar take-off device according to claim 6, characterized in that piston and cylinder units are provided for pressing the caterpillar tracks (10, 10') and have piston rods moveable orthogonal to the length axis (12) of the caterpillar take-off device.

8. Caterpillar take-off device according to claim 7, characterized in that the piston and cylinder units are provided exclusively for the caterpillar tracks (10') arranged above a horizontal plane through the length axis (12).

9. Caterpillar take-off device according to claim 7 or 8, characterized in that the pressure of the piston and cylinder units is automatically controllable.

10. Caterpillar take-off device according to one of the claims 7 to 9, characterized in that pneumatic cylinders (14) are provided as piston and cylinder units.

11. Caterpillar take-off device according to one of the claims 7 or 10, characterized in that at least one motor (18) is provided for changing the distance between the caterpillar tracks (10), which are not moved by the piston and cylinder units, and the length axis (12).

12. Caterpillar take-off device according to claim 11, characterized in that the at least one motor (18) is connected to the caterpillar tracks (10) which lie in or below the horizontal plane.

13. Caterpillar take-off device according to claim 11 or 12, characterized in that only one motor (18) is provided which moves the caterpillar tracks (10), which lie in or below the horizontal plane, via a gear mechanism.

14. Caterpillar take-off device according to one of the claims 6 to 13, characterized in that the means for measuring the deflection includes displacement pick-ups.

15. Caterpillar take-off device according to one of the claims 6 to 14, characterized in that the means for measuring the deflection includes at least one angle position transducer.

16. Caterpillar take-off device according to claim 15, characterized in that an angle position transducer is provided on the motor (18) for measuring the deflection of the caterpillar tracks (10) connected to the motor (18).

17. Caterpillar take-off device according to one of the claims 14 to 16, characterized in that displacement pick-ups are provided on the piston and cylinder units.

18. Caterpillar take-off device according to one of the claims 6 to 17, characterized in that a control unit is provided for controlling the distance of the caterpillar tracks (10, 10') from the length axis (12) of the caterpillar take-off (6) on the basis of signals received by the means for measuring the deflection.

19. Caterpillar take-off device according to one of the claims 6 to 18, characterized in that means for manual control of the distance between the caterpillar tracks (10, 10') and the length axis (12) of the caterpillar take-off (6) are provided