Calibration (extrusion)
The calibration is a unit downstream of the extruder in order to fix the extruded material to the later product dimensions. The material , which is still molten, is cooled to a temperature at which subsequent deformation is no longer possible. The complete cooling takes place in the subsequent cooling section.
Differentiation between dry and wet calibration:
The wet calibration is primarily because of the reduction of the frictional forces are used, however, can only rotationally symmetrical parts (z. B. tubes ) are possible, because the water film can form uniform only in those parts. The dry calibration is mainly used for profiles or parts with a rectangular cross-section.
General
The caliber is usually liquid-tempered . If the profile enters the caliber, it is cooled from the outer profile surface . To ensure contact with the inner wall of the caliber, a vacuum is usually used, which is intended to press the material against the wall. For this purpose there are vacuum slots in the caliber so that the vacuum can take effect. Another possibility is dealt with under the point full profiles . Since the caliber has a major influence on the surface quality (if several calibres are connected in series, the first caliber has the greatest influence on the surface ), special attention must be paid to the surface quality of the caliber during manufacture. In addition, the entry area of the caliber is exposed to increased wear . Special materials or hardened or otherwise treated surfaces are recommended here . The friction forces in the caliber must be taken into account, especially with dry calibration, since the material comes into direct contact with the hardened surface due to the applied vacuum. On the one hand, the extraction device and the calibration table must be adapted to this . A distinction is made between different calibration methods:
Pipe calibration
In the case of pipe calibration, a basic distinction can be made between internal and external calibration. The external calibration is further subdivided into the vacuum water bath calibration and the vacuum calibration with the calibration housing closed.
Vacuum water bath calibration
The viscoelastic melt is fed from the extruder into a series of water-filled tanks, which can be hermetically sealed. When the extruder starts up, the pipe is guided through a series of calibrating lamellas. Then a vacuum of 0.3 to 0.8 bar is applied around the pipe and the basins are flooded. The tube is pressed into the calibration units by the vacuum. On the one hand, the water cools the molding compound and, on the other hand, lubricates the contact surfaces between plastic and metal through capillary action, which results in a reduction in the friction forces. Analogous to the plate packs, so-called slotted sleeves can also be used, which are either radially drilled or slotted. The vacuum water bath calibration is used for pipe diameters from 2 to 160 mm.
Vacuum calibration with closed calibration housing
Here the vacuum and cooling chambers are separated in caliber. This means that the water does not hit the material directly. The first cooling chamber prevents the plastic from sticking due to the cooling of the outermost edge layer of the pipe. Due to the poor thermal conductivity, however, the entire wall does not solidify and therefore remains flexible. The applied vacuum prevents sink marks and ensures compliance with the shape contour.
Compressed air calibration
In contrast to the previous methods, with compressed air calibration the calibration unit is flanged directly to the nozzle of the extruder . The caliber consists of a cooled case with a double jacket and forced circulation so that the temperature can be controlled with water, and a drag stopper. The overpressure of 0.3 to 0.8 bar compared to the ambient pressure is generated via a bore in the pipe head. The overpressure is maintained in a defined area of the pipe by the drag stopper, which is attached to the nozzle by means of a tow rope or chain. The overpressure in the area after the nozzle presses the material against the inner wall of the caliber, where the material cools down. Compressed air calibrations are mainly used for pipes with a diameter greater than 160 mm.
Profile calibration
Full profile calibration
With full profile calibration, particular care must be taken that calibration is only carried out on one side. This means that the opposite side of the profile must not touch the wall of the caliber, otherwise the still deformable material could become wedged and destroyed. The caliber slots should therefore be dimensioned with an oversize of approx. 15 to 20% . Another reason for the excess is that the pressure gradient to the vacuum side only exists if atmospheric air pressure can act on the non-calibrated side . Another possibility for full profile calibration is to let the upper part of the device act on the band to be calibrated by its own weight or by spring force .
Hollow profile calibration
The differential pressure required to apply the profile surfaces to be cooled is achieved with hollow chamber profiles by means of a vacuum. The calibration is fixed at a short distance from the nozzle. Due to the relaxation behavior of the plastic, the profile strand entering the calibration expands. This is used to seal the negative pressure zones. Thick-walled profiles, as well as those with supporting webs, require long passages. To ensure this, individual caliber tools are connected in series . Their dimensions must be reduced proportionally from step to step in order to take account of the shrinkage of the profile. Particular attention must also be paid to the exact design of the caliber tools.
Calibration table
Since pull-off forces of up to 30 kN can be effective with large profiles , it is necessary to ensure a very robust construction of the clamping table and the positional fixation of the profile tools. The caliber table must allow the following adjustment options:
- Axial adjustment to optimize the distance between the profile nozzle and the calibration inlet according to the working speed .
- Height adjustment so that calibrations with different cross-sections can be set up.
- Side adjustment to be able to align the calibration in the tool axis.
In order to be able to guarantee an optimal profile surface, all spindles , guide rods or slide guides used, which cause "chatter marks" due to vibrations , must be designed with almost no play .
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- Schaab, Herbert / Stoeckhert, Klaus: Kunststoffmaschinenführer, Verlag: Hanser, 1979
- Greif, Helmut / Seibel, Stefan: Technologie der Extrusion: Learning and workbook for training and further education, Publisher: Hanser, 2004