Plastic article or earthenware shaping or treating: apparatus – Shaping orifice and downstream work contacting gaseous...
Reexamination Certificate
2000-09-07
2004-08-31
Eashoo, Mark (Department: 1732)
Plastic article or earthenware shaping or treating: apparatus
Shaping orifice and downstream work contacting gaseous...
C425S141000, C425S144000, C425S387100, C425S392000
Reexamination Certificate
active
06783344
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims priority to German Patent Application 199 42 999.5 filed Sep. 9, 1999, and German Patent Application 100 29 175.9 filed Jun. 19, 2000, which applications are herein expressly incorporated by reference.
BACKGROUND OF THE INVENTION
The invention relates to a method of controlling and automatically controlling the thickness profile in the production of blown film. The invention also relates to a device for carrying out the method.
The thickness profile and especially the circumferential thickness profile of a film tube emerging from a forming tool or annular nozzle of a blown film extruder, in the circumferential position, has thicker and thinner regions. These regions are in largely fixed positions, which adversely affects the winding and further processing of the film tube.
During the production process, to reduce any deviations of the thickness profile as early as possible, hotter regions are stretched to a greater extent and cooler regions to a lesser extent. In prior art devices, the differences in the thickness of the blown film across the circumference are influenced by varying the cooling power across the circumference. The film tube is divided into individual associated circumferential sectors as a function of the number of control elements.
If the cooling power is higher in one circumferential sector, the blown film in that sector cools more quickly. Thus, that sector is stretched to a lesser extent and remains relatively thicker. If the cooling power is lower in one circumferential sector, the blown film retains a higher temperature and can be stretched to a greater extent. As a result, the film thickness is reduced to a greater extent. The differences in film thickness across the circumference are determined by a measuring device. This data is fed into a control device. As a rule, the measuring device is arranged in the direction of production behind a freezing region of the film material.
DE 40 01 287 C2 and EP 0 508 167 A2 disclose cooling air rings. The rings are divided into segments. The volume or the temperature of the cooling air is changed by devices built into the segments.
In DE 40 01 287 A1, to change the individual volume streams, partial streams of the cooling air are branched off and blown off. The partial streams do not participate in cooling the blown film. This is disadvantageous from an energy efficiency viewpoint. The large number of control units fitted directly to the cooling air ring leads to a very tight arrangement of parts.
In EP 0 508 167 A2, a cooling air ring has nozzles in two exit planes. A smaller partial stream is supplied, uncontrolled, to a first plane, relatively close to the annular nozzle of the blow head. A larger partial stream of cooling air is supplied in a second plane in the direction of production. The temperature of the larger partial stream is controlled segment by segment. The heating elements fitted directly in the cooling ring again lead to a tight arrangement inhibiting the flow conditions. The reaction of the heating elements suffers from inertia.
The main disadvantage of the above-mentioned systems is that the devices have to be fitted in the cooling ring. This constitutes interference with the sensitive aerodynamic system of the cooling air ring. Furthermore, because of the influence on the air quantity or temperature in the cooling ring, losses may occur in the overall cooling power.
DE 26 58 518 C2 discloses influencing differences in the film thickness by corrective air nozzles. The nozzles are arranged on the outside above the cooling ring (external air blowing ring). The disadvantage of this device is that the corrective nozzles are arranged above the cooling ring. In this case, thickness corrections can only take place to a limited extent because the film material has already been subjected to cooling air by the main cooling ring. Thus, the temperature of the film material has already been reduced.
It is the stretching process which allows the corrective air to exert its influence. The stretching process has partly already taken place above the main cooling ring. This reduces the influencing potential with respect to thickness corrections in this region. A further disadvantage of this device is that the corrective air nozzles are arranged above the main cooling ring. Thus, due to the space taken up by the corrective air nozzles, there is only very restricted access to the film bubble (when starting the plant), to the nozzle gap (deposits have to be removed regularly from melt exit), and to the main cooling ring (cooling air ring lips to be set as a function of the respective production process). This device has a further disadvantage. Depending on the required end dimension of the film tube, the position of the corrective air nozzles has to be adapted to the film tube diameter behind the main cooling ring.
DE 39 20 194 C2 discloses a method and device to control the thickness profile of a blown film in the course of production. The device uses an additional cooling ring divided into segments. The ring is arranged in the direction of production behind/above a main cooling ring. This means that in this device, the cooling air streams influence the film thickness first flowing downstream into the main cooling air stream. As a result, they only exert a slight influence on the thickness correction of the film tube which has already been cooled.
DE 196 29 076 A1 discloses a cooling ring to cool a film tube emerging from the annular gap of a film blow head. The cooling ring is supplied with cooling air at two different temperatures. Inside the head, the cooling air is variably mixed independently for each segment for thickness controlling purposes. This device requires more sophisticated equipment to generate cooling air with a certain temperature and to mix the cooling air. It also has to have a very compact and complicated design due to the large number of mixing valves closely arranged inside the cooling ring.
DE 44 28 212 A1 discloses a blow head to produce tubular film. Here, an additional cooling ring is arranged between the exit nozzle of the blow head and an uncontrolled main cooling ring. The additional cooling ring includes small pipes. The pipes start from an outer annular line, extend radially inwardly and each pipe contains a heating cartridge to heat the additional cooling air. The cooling air is uncontrolled in the volume stream. The reaction behavior of the heating cartridges is affected by inertia. Thus, the overall control behavior is expected to be bad. The design of the additional cooling ring with the large number of heating cartridges is extremely complicated. Additionally, the cartridges can be adversely affected by dirt. Further, to carry out maintenance work on the heating cartridges, the cooling rings have to be completely dismantled.
A further disadvantage of the above device is that, due to the required differences in temperature, the heating cartridges require a very effective thermal insulation. The cartridges must be insulated relative to each other and relative to the heated annular nozzle of the blow head.
When producing blown film, not only do differences occur in thickness in the circumferential direction, but they also occur in the direction of production. The fluctuations in the direction of production are uniformly superimposed on the differences in circumferential thickness. Thus, the circumferentially determined thickness is subject to periodic fluctuations in the direction of production. The fluctuations have different causes, such as output fluctuations of the extrusion unit or driving fluctuations of the extraction unit which pulls and stretches the tubular film in the longitudinal direction. To minimize the fluctuation causes, gravimetric throughput control systems are provided in the prior art. These systems connect the throughput of raw materials, the conveyor worm speed of the extrusion unit and the driving speed of the extraction unit in a control circuit in order to keep the mean film thi
Eashoo Mark
Kdesign GmbH
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