Blown film installation, method for producing a blown film strip and film produced therewith

Abstract

In blown film installations, it is known to provide longitudinal stretching of the produced double layer film strip downstream of the draw-off, more precisely downstream of the reversing unit and upstream of the winder. It is also known to stretch the down-drawn film, wherein the film must then be preheated owing to the long cooling path from the draw-off. According to a first feature, the present invention specifies warming the film above the draw-off and then treating it mechanically. The film can thereby be brought with only little energy from a first level of warmth to a temperature level at which it can easily be worked. According to a second feature, the invention specifies providing a tractive force breakdown brake.

Claims

1. A blown film installation comprising: an annular nozzle for extruding a film tube; a tube formation area for longitudinal and transverse stretching of the film tube; a cooling means for the film tube moving in the machine direction; a collapsing section of the film tube into a double layer film strip; and a pair of haul-off rollers beyond the cooling means for hauling off the film tube, wherein a processing roller line is provided above the pair of haul-off rollers for longitudinally stretching the double layer film strip, wherein the processing roller line has a first stretching roller and a second stretching roller following the first stretching roller, wherein the pair of haul-off rollers is adapted to be driven at a haul-off roller speed, wherein the second stretching roller is adapted to run at a stretching speed that is higher than the haul-off roller speed and the speed of the first stretching roller, so that, during operation of the blown film installation, the double layer film strip is stretched in the machine direction between the first and the second stretching roller, and wherein a tensile force reach through brake impeding a penetration of a tensile force from the second roller to the pair of haul-off rollers is provided in the processing roller line before and/or on the first stretching roller.

2. The blown film installation according to claim 1, wherein the tensile force reach through brake has a speed-regulated holding roller.

3. The blown film installation according to claim 2, wherein the holding roller has a speed regulator, which is adapted to set a speed that is closer to the speed of the haul-off rollers than to the stretching speed.

4. The blown film installation according to claim 1, wherein the tensile force reach through brake has a pressing roller.

5. The blown film installation according to claim 1, wherein the tensile force reach through brake has a wrap angle guide for wrapping around a first roller in the processing roller line with an angle of at least 160.

6. A method for producing a blown film strip in a blown film installation, the method comprising: a. extruding a film tube; b. inflating the film tube in a tube formation area for longitudinal and transverse stretching of the film tube; c. cooling the ascending film tube with a cooling means; d. collapsing the film tube into a double layer film strip by means of a collapsing section; and e. hauling off the double layer film strip with a pair of haul-off rollers comprising: guiding the double layer film strip in the processing roller line around a first stretching roller and around a second stretching roller following the first stretching roller, wherein the pair of haul-off rollers is driven at a haul-off roller speed, wherein the second stretching roller is driven at a stretching speed, which is higher than the haul-off roller speed and the speed of the first stretching roller, so that, during operation of the blown film installation, the double layer film strip is irreversibly stretched in the machine direction between the first and the second stretching roller, and wherein a tensile force reach through brake impeding a penetration of a tensile force from the second roller to the pair of haul-off rollers is provided in the processing roller line before and/or on the first stretching roller, so that the tensile force reach through speed forms a hearing for absorbing the tensile force.

7. The blown film installation according to claim 3, wherein the holding roller has a speed regulator, which is adapted to set a speed that is the speed of the haul-off rollers.

8. The blown film installation according to claim 5, wherein the tensile three reach through brake has a wrap angle guide for wrapping around a first roller in the processing roller line with an angle of more than 180.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the following, the invention is described in more detail based on six exemplary embodiments with reference to the drawings:

(2) FIG. 1 schematically shows a vertical section perpendicular to a pair of haul-off rollers through a first variant of a processing roller line with five rollers and a reversing unit disposed above it,

(3) FIG. 2 shows the same view as in FIG. 1 onto a second variant of a processing roller line with five rollers and a reversing unit disposed above it,

(4) FIG. 3 shows the same view as in FIG. 1 onto a third variant of a processing roller line with five rollers and a reversing unit disposed above it,

(5) FIG. 4 schematically shows a vertical section through a pair of haul-off rollers of a fourth variant of a processing roller line with five rollers and a reversing unit disposed above it, having a lesser construction height, more specifically as an embodiment for a MDO unit,

(6) FIG. 5 schematically shows a vertical section through a pair of haul-off rollers of a fifth variant of a processing roller line with four rollers and a reversing unit disposed above it, more specifically as an embodiment for a flattening package, and

(7) FIG. 6 schematically shows a possible sixth variant of a processing roller line with a construction height that is as small as possible.

(8) FIG. 7 shows a diagram regarding the development of the longitudinal tension as a function of a longitudinal extension of a plastic film,

(9) FIG. 8 schematically shows a vertical section through a pair of haul-off rollers of a sixth variant of a processing roller line with five rollers and a reversing unit disposed above it, and

(10) FIG. 9 schematically shows a vertical section plane through a pair of haul-off rollers of a seventh variant of a processing roller line with six rollers for four processing stations and with a reversing unit disposed above it.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(11) The blown film installation 1 (only its upper area is shown) in FIG. 1 consists substantially of an extruder, a blown film die with an annular nozzle, an ascending section disposed above it for an extruded film tube, a calibration cage, a collapsing unit and a pair of haul-off rollers 2 above the collapsing unit, wherein a first haul-off roller 3 is held by a first holder 4 and its position is finely adjustable, whereas a second haul-off roller 5 is mounted on a sliding bearing support 7 in such a manner that it is horizontally displaceable by an adjusting cylinder 6. The adjusting cylinder 6 can thus horizontally move the second haul-off roller 5 toward and away from the first haul-off roller 3.

(12) A processing roller line 8 is provided above the pair of haul-off roller 2. A total of five rollers, namely a first roller 9, a second roller 10, a third roller 11, a fourth roller 12 and a fifth roller 13 are located there.

(13) The five rollers of the processing roller line 8 are alternately disposed on different sides of a virtual plane 14, which lies parallel to central axes 15 (exemplarily labeled) of the two haul-off rollers and runs vertically through the nip of the two haul-off rollers. The virtual plane contains a surface, which the flattened film would have to cross if the flattened film coming out of the nip of the haul-off rollers would simply rise vertically upwards. In a vertical section perpendicular to the central axes 15, the virtual plane 14 is therefore represented as a vertically running line, namely beginning at the nip of the pair of haul-off rollers and following the vertical direction of ascension of the film.

(14) The first roller 9, the third roller 11 and the fifth roller 13 are located on a first side 16 of the virtual plane 14; the second roller 10 and the fourth roller 12 on the other hand are located on an opposite second side 17 of the virtual plane 14.

(15) At the same time, all five rollers of the processing roller line 8 are vertically placed so close to each other, that a horizontal projection of the rollers onto the virtual plane 14 results in a respective overlap of the first roller 9 with the second roller 10, the second roller 10 with the third roller 11, the third roller 11 with the fourth roller 12 and the fourth roller 12 with the fifth roller 13, namely respectively by a third of the total diameter of the five rollers.

(16) The first four rollers 9, 10, 11, 12 of the processing roller line 8 have the same size, whereas the two haul-off rollers 3, 5 of the pair of haul-off rollers 2 and the fifth roller 13 are larger.

(17) The distances 18 (exemplarily labeled) between the five rollers of the processing roller line 8 respectively amount to at least 50 mm.

(18) The first roller 9 is equipped with a speed regulator, so that its surface speed can be adjusted quite precisely to a given amount during rotation.

(19) The second roller 10 is equipped with a drive and a closed loop control, which are able to adjust the second roller 10 to a significantly higher circumferential speed than the first roller 9.

(20) The third and fourth roller 11, 12 can be drivable, in which case they are driven for example at the same speed as the second roller 10 or preferably slower than the second roller 10.

(21) The fifth roller 13 can also be drivable, for example at least substantially at the same speed as the fourth roller 12 or preferably slower than the fourth roller 12.

(22) It must be pointed out that it can be advantageous to slow down after a stretching in order to let the tensions in the film dissipate.

(23) The first roller 9 is furthermore equipped with an active heating means and a temperature sensor (both not shown), namely a piping for a heating fluid with a heat conducting connection to the surface of the first roller 9, while the temperature sensor can measure either the surface temperature of the first roller 9 and/or of the moving double layer film strip by way of a contactless measurement.

(24) The temperature sensor is preferably disposed in a return pipe of the heating fluid, so that, while accepting a certain imprecision, it can be assumed that the film will run off at a slightly cooler temperature than the return temperature of the heating fluid.

(25) The exiting temperature of the double layer film strip corresponds ideally exactly to the temperature of the roller surface. In practice however, as the film moves away, it will be slightly warmer or cooler, depending on whether it has been cooled down or heated up by the roller.

(26) In order to implement a particularly precise embodiment, the person skilled in the art can measure the exiting temperatures of the film at each individual roller, for example contact-free by way of an infrared sensor, and adjust the roller temperatures based on the actual film temperatures.

(27) Independently from each other, the second roller 10, the third roller 11 and the fourth roller 12 can respectively also be equipped with such an active heating means.

(28) One roller at any rate, here the fifth roller 13, is equipped with a temperature sensor and an active cooling means.

(29) Two non-driven deflecting rollers 19 (exemplarily labeled) are disposed above the processing roller line 8 on the way toward a reversing unit 20 disposed above the pair of haul-off rollers 2 and the processing roller line 8, wherein the deflecting rollers 19 and the reversing unit 20 are sufficiently known from the prior art and thus do not need to be defined any further.

(30) The two haul-off rollers 3, 5 of the pair of haul-off rollers 2 as well as the five processing rollers of the processing roller line 8 and also the deflecting rollers 19 are borne at their front sides on a machine frame 21.

(31) Two temper devices 22 (exemplarily labeled) are disposed laterally from the machine frame 21. The tempered rollers of the processing roller line 8 are connected with the temper devices 22 by way of coolant conduits, respectively heat medium conduits, preferably also by way of temperature sensor data lines (not shown). Electronic micro controllers (not shown) are also provided in the temper devices 22 or at least have access to the temper devices 22, said micro controllers being able to execute the adjusted control of the temperature of the tempered rollers by means of the fluid return flow.

(32) During operation of the blown film installation 1, a film tube (not shown) is extruded by the extruder (not shown) through the annular nozzle (not shown). The film tube is pulled upward along the blown film installation 1, through the calibration cage (not shown) and the collapsing unit (not shown). At the end of the collapsing unit the film tube is mostly flattened and runs in that form into the pair of haul-off rollers 2. From there on, it must be referred to as a double layer film strip 23.

(33) The double layer film strip 23 can optionally be passed in a straight ascending direction above the pair of haul-off rollers 2, which is congruent with the virtual plane 14, through the rollers 9, 10, 11, 12, 13 of the processing roller line 8 and immediately onto the deflecting rollers 19 and from there led towards the reversing unit 20.

(34) In that case, the blown film installation 1 is a customary blown film installation. During operation of the blown film installation 1, the reversing unit rotates and creates a film wrap that is as uniform as possible on the roll (not shown) on the floor of the installation site (not shown).

(35) In an alternativeand preferredfilm conducting line on the other hand, the double layer film strip 23 runs respectively around the five rollers 9, 10, 11, 12, 13 of the processing roller line 8, wherein the geometry of the rollers relative to each other results in a wrap angle of more than 180 at least on the second roller 10, the third roller 11 and the fourth roller 12. The wrap angle of the first roller 9 depends more specifically on the positioning height of the first roller 9 relative to the pair of haul-off rollers 2, as well as on the diameters of the three rollers and moreover on the distance between the first roller 9 and the virtual plane 14. In the embodiment chosen here the wrap angle at the first roller 9 amounts to approximately 170.

(36) The same applies to the fifth roller 13, the relevant criteria being here more specifically the positioning relative to the virtual plane 14, to the first deflecting roller 19 and the diameter between the fifth roller 13 and the first deflecting roller 19.

(37) In the described configuration, the double layer film strip 23 then runs in the extrusion direction, i.e. in the machine direction, upwards through the pair of haul-off rollers 2 and is then first led around the first roller 9 in a clockwise rotation (indications relative to a clockwise or counter-clockwise rotation are based on the sectional view of the figures). The first roller 9 serves as a holding roller. At the same time, a first of a total of three heat medium or coolant circuits in the processing roller line 8 flows through the first roller 9, namely a heating circuit.

(38) In a configuration of the first variant of the blown film installation 1 according to FIG. 1, the double layer film strip 23 can arrive from the pair of haul-off rollers 2 with a film infeed temperature of about 60 C. to about 80 C.

(39) The first roller 9 is adjusted in such a manner that its circumferential speed is the same as that which the double layer film strip 23 is subjected to at the pair of haul-off rollers 2. Thus the double layer film strip 23 is not subjected to a mechanical manipulation in the gap between the pair of haul-off rollers 2 and the first roller 9.

(40) Due to the high wrap angle of the double layer film strip 23 around the first roller 9, the double layer film strip 23 runs on the first roller 9 with a static friction, i.e. exactly at the same speed as the roller surface, even if the static friction is not provided along the entire wrap angle.

(41) The first temperature circuit, i.e. the heating circuit, which passes through the first roller 9 serving as a holding roller, is for example set to implement a temperature difference in the film of plus 5 K and plus 10 K as compared to the temperature of the film as it exits the previous first haul-off roller 3. The double layer film strip 23 is thus heated up by 5 K to 10 K while running around the first roller 9. This small difference in temperature is already sufficient to significantly increase the processability of the double layer film strip 23 because the blown film installation 1 (below the area shown in FIG. 1) is setup in such a manner that the film is only cooled down during its ascending movement and thus still has a rather high temperature when running through the two haul-off rollers 3, 5, comprised between 60 C. and 80 C. in the tested example.

(42) Based on the initial heat of the film, the temper device 22 can thus very quickly heat the film back up to an easily processable temperature level with only very little energy, in order to facilitate a longitudinal stretching.

(43) In the shown embodiment, the second roller 10 is configured as a stretching roller. In positive tests, it has been driven at three times to four times the circumferential speed of the first roller 9. Stretching ratios around 1:3 resulted film quality that seemed to be adapted for silage pre-stretch films; higher stretching ratios on the other hand, i.e. ratios around 1:4 or more, resulted in a broader processability, more specifically with regard to the optical quality of the film.

(44) In those positive tests, the second roller 10, i.e. the stretching roller, served as the first of a total of three rollers of a second temperature circuit, namely the tempering circuit. The tempering circuit flows through the second-roller 10, the third roller 11 and the fourth roller 12. The temperature in the return conduit of the tempering circuit was set at plus 5 K to plus 20 K as compared to the return conduit of the previous first roller 9, i.e. the holding roller.

(45) Thus the second roller 10 has two functions: on the one hand it serves as a stretching roller, on the other hand as a tempering station in form of a tempering roller.

(46) The third roller 11 and the fourth roller 12 are configured as tempering rollers, which means that they at least substantially maintain the relatively high temperature level of the stretching roller and thus lead to a relief of the tensions of the stretched double layer film strip 23, which helped to reduce a memory effect of the otherwise occurring shrinkage.

(47) The fifth roller 13, configured as a cooling roller, is connected to the third of the three temperature circuits, namely a cooling circuit. The temperature level in the return conduit of the cooling circuit ideally amounted to between minus 10 K and minus 20 K as compared to the return conduit of the previous roller, i.e. the last tempering roller.

(48) The double layer film strip 23 runs around all five rollers of the processing roller line 8 mostly with static friction. Therefore, the five rollers are ideally surface-coated, more specifically a spiral-shaped groove or a silicone coating.

(49) It shall be understood that each roller can be provided with at least one contacting roller or pressing roller. In prototype tests however, an operation without contacting rollers has proven to be sufficient.

(50) Water has proven of value as a heating and cooling medium for the temper devices 22 and the three tempering circuits.

(51) At the run-out, the blown film installation 1 was operated with a film speed between 94 m/min and 340 m/min and with a stretching ratio between the first roller 9 and the fifth roller 13 between 1:2 and 1:3, wherein it must be mentioned again that a lower stretching ratio of around 1:2 seemed to be better suited for silage pre-stretch products.

(52) In an implementation of the various aspects of the invention the blown film installation 1 provides a blown film line 8 above the pair of haul-off rollers 2 with a heating means for the double layer film strip 23, namely with the heated fluid circuit in the first roller 9 and additionally with the tempering circuit in the second roller 10, in the third roller 11 and in the fourth roller 12.

(53) Thus several active heating means for the double layer film strip 23 are provided here, namely a total of four different rollers.

(54) Providing a heating means in several rollers, at least in two rollers, especially with two different fluid circuits is also advantageous as such.

(55) The second aspect of the invention is implemented by a transverse orientation of a processing roller line disposed, respectively as a partial section, above the pair of haul-off rollers 2, in that the angle between one roller and the next roller in the processing roller line 8 is about 35 to 40 relative to the horizontal, so that they are oriented more horizontally than vertically relative to each other. This leads to the fact that due to their sufficient lateral offset, the rollers can be disposed so low that they overlap in a projection on the virtual plane 14, and are therefore less high than if the diameters of the five rollers were added.

(56) In the second variant of a blown film installation 1 in FIG. 2, the installation can be configured identically or similarly up to the pair of haul-off rollers 2.

(57) Five rollers are again disposed above the pair of haul-off rollers 2 and below the reversing unit 20, namely a first roller 9, a second roller 10, a third roller 11, a fourth roller 12 and a fifth roller 13, upstream of a subsequent deflecting roller 19 disposed at the top.

(58) The five rollers fulfil the same function as described in the first variant according to FIG. 1 and also include three tempering circuits.

(59) In the second variant, the second roller 10 and the fourth roller 12 are however disposed on the same side of the virtual plane 14 as the first roller 9, the third roller 11 and the fifth roller 13. Thus all the rollers are disposed on the same side of the virtual plane 14.

(60) A direct connection between the nip of the pair of haul-off rollers 2 and the deflecting roller 19 is free, so that the double layer film strip 23 can be optionally guided directly vertically upwards without surrounding the rollers in the processing roller line 8.

(61) However this variant is preferably also arranged as an MDO installation, i.e. for longitudinally stretching the film beyond the yield point of the double layer film strip 23.

(62) The five rollers of the processing roller line 8 have a very small clearance between them, which lies at any rate under 5 cm. Therefore, when starting the blown film installation 1, it is very difficult to thread the film between the rollers of the processing roller line 8 in the shown operating position of the rollers, even if the rollers are driven in the respectively opposite direction of rotation than their predecessor.

(63) In order to thread an end (not shown) of the double layer film strip 23, two rollers, namely the second roller 10 and the fourth roller 12, are displaceable toward the left hand side, i.e. onto the opposite side of the virtual plane 14. The double layer film strip 23 can then be simply threaded vertically through the five rollers. The second roller 10 and the fourth roller 12 are then brought back onto the same side 17 of the virtual plane 14 as the other three rollers and the extrusion process can be operated as a steady state process.

(64) The second roller 10 can preferably be optionally displaced up to the imaginary plane formed by the axes of the roller 9, 11, 13 or even beyond it, so that a continuously adjustable stretching section is provided. Preliminary tests have shown that an adjustability of the stretching length is advantageous for technical reasons, for example because it resulted in a higher fault tolerance with regard to flaws while passing through the imaginary plane.

(65) In the threading position of the second roller 10 and the fourth roller 12, the blown film installation 1 can be operated like a customary blown film installation.

(66) The third variant of a blown film installation 1 according to FIG. 3 has substantially the same structure as the second variant of the blown film installation 1 according to FIG. 2. However, the five rollers of the processing roller line 8 are all located on the first side 16 of the virtual plane 14. Since an entry side into the reversing unit 20 is also located on the first side 16 of the virtual plane 14, the film can be led directly from the fifth roller 13 to the reversing unit 20. A deflecting roller 19 is not necessary.

(67) In addition, the fifth roller 13 is disposed relative to the four previous rollers in such a manner that its edge oriented toward the virtual plane 14 protrudes beyond the previous four rollers, so that the double layer film strip 23 can be led around the four previous rollers without threading and without passing through the processing roller line 8. However, the wrap angle of the double layer film strip 23 around the fifth roller 13 amounts already to almost 90 even without threading, in the threaded state even almost 180 so that a sufficient guidance is provided.

(68) Like in the second variant of the blown film installation 1 according to FIG. 2, the third variant of the blown film installation 1 according to FIG. 3 also provides a meshing of the second and the fourth roller so that the end of the film can be threaded by simply guiding it straight through the rollers when starting the installation.

(69) The fourth embodiment of the blown film installation 1 according to FIG. 4 also has the same configuration as the previously described variants, with the exception of a pair of haul-off rollers 2.

(70) However, in the fourth variant of the blown film installation 1, the first haul-off roller 3 disposed on the same side 16 as the entry into the reversing unit 20 is configured to be displaceable for threading and closing the pair of haul-off rollers 2. On the other hand, a second haul-off roller 5 located on an opposite side 17 of the virtual plane 14 is, in principle, configured to be fixed.

(71) Above the pair of haul-off rollers 2, four rollers of a processing roller line 8 are disposed in a horizontal arrangement next to each other, and a fifth roller 13 is offset laterally as well as in height relative to said rollers.

(72) With regard to the virtual plane 14 above the pair of haul-off rollers 2, three rollers of the processing roller line 8 are thus located on the second, here right hand side 17, whereas two rollers of the processing roller line 8 are disposed on the first, here left hand side of the virtual plane 14, i.e. on the same side as the entry into the reversing unit 20.

(73) To this end, a machine frame 21 for the rollers of the processing roller line 8 has a collar 24, which protrudes laterally beyond a main body of the machine frame 21. The first roller 9 is mounted on the collar 24.

(74) Due to the horizontal arrangement of several rollers of the processing roller line 8 next to each other, i.e. here a total of four rollers of the processing roller line 8, the blown film installation 1 as a whole is very low, in spite of the fact that the reversing unit 20 is disposed above the pair of haul-off rollers 2 and the processing roller line 8.

(75) Two of the rollers in the processing roller line 8, here the second roller 10 and the fourth roller 12 are again configured to be displaceable in a meshing manner, so that the threading into the installation during start-up is facilitated. A pivoting motion can however also be provided, especially when space is limited, so that the second roller 10 for example can be configured to be pivotable around the first roller 9, while the fourth roller 12 for example is configured to be pivotable around the fifth roller 13 or around the third roller 11.

(76) A straight route 25 for the double layer film strip 23 is left open in a straight prolongation above the pair of haul-off rollers 2, i.e. in the virtual plane 14, so that the double layer film strip 23 can also be produced without running through the MDO processing roller line 8. In that case it runs straightly upwards toward a straight course deflecting roller 26 and from there into an entry 27 to the reversing unit 20.

(77) The double layer film strip 23 can be alternately led along an MDO route 28, which pivots directly into the opposite side 17 of the virtual plane 14 relative to the entry 27 and runs externally around the first roller 9. There it runs through the next four rollers as described above, which also have the same functions as described above.

(78) From the fifth roller 13, the double layer film strip 23 finally runs either over a further deflecting roller 29 or, if the wrap of the fifth roller 13 is sufficient, directly to the entry 27 into the reversing unit 20.

(79) The fifth roller 13 and/or a possibly provided other deflecting roller 29 serving as a cooling roller, or for example other cooling units can be displaceable together or relative to each other, individually or together, so that the cooling path is easily adjustable. For example, the fifth roller 13 and the other deflecting roller 29 can be mounted together on the machine frame and rotate around an axis, which lies parallel to the axis of the shown rollers; or the other deflecting roller 29 for example can be displaceable or pivotable vertically downwards, so that the wrap angle of the double layer film strip 23 along the MDO route 28 around the fifth roller 13 can be adjusted with quite simple movements and practically continuously. Even with a predetermined cooling temperature it is then possible to adjust the cooling effect before entry of the double layer film strip 23 into the reversing unit 20.

(80) A similar idea can be implemented for example with the first roller 9, which ideally serves as a holding roller and simultaneously as a heating roller. This roller may also be adjusted for example in height or laterally, so that the modified geometry of the MDO route of the double layer film strip 23 results in a modified wrap angle around the first roller 9 and a modified stretching length.

(81) The same effect can also be achieved by providing another pressing roller there.

(82) In the fifth variant of the blown film installation 1, a flattening line 30 is located above the pair of haul-off rollers, and above that, a reversing unit 20 with an entry 27.

(83) A first roller 31, a second roller 32, a third roller 33 and a fourth roller 34 are provided inside the flattening line 30. From there a designated film path via a number of passive deflecting rollers 35 (exemplarily labeled) to the entry 27 into the reversing unit 20 is provided.

(84) The four rollers of the flattening line 30 are in turn provided laterally at a distance from the virtual plane 14, thus resulting in a straight route 25 for the double layer film strip 23 from the pair of haul-off rollers 2 directly to the first passive deflecting roller 35 and from there on to the reversing unit 20, if the double layer film strip 23 must not pass through the flattening system.

(85) As an alternative, the double layer film strip 23 can be guided to the first roller, here for example around the first deflector 236, from there around the roller 32, from there around the third roller 33 and finally around the fourth roller 34 until the double layer film strip 23 returns along this flattening route 37 back to the straight route 25.

(86) Respectively two of the overall four rollers of the levelling line 30 are located substantially at the same height and respectively form a pair with a low construction height. A projection onto the virtual plane 14, results in an overlap area of the first roller 31 and the second roller 32 and even a congruence of the third roller 33 and the fourth roller 34.

(87) A minor overlap is however sufficient for achieving a lower construction height as compared to a construction as shown in FIGS. 2 and 3.

(88) All four rollers of the flattening line 30 ideally have a pressing roller 38 (exemplarily labeled), which is respectively pressed onto the respective roller by way of a pressure arm 39 (exemplarily labeled).

(89) In the present exemplary embodiment pressing rollers are provided on only two of the rollers, namely on the first roller 31, which serves as a holding roller and a heating roller and on the second roller 32, which serves as an extending roller and a tempering roller.

(90) This results in an extending section 40 located between the first roller 31 and the second roller 32, wherein strong tangential forces are generated at the circumferences of the first roller 31 and the second roller 32.

(91) In contrast, the surface speeds of the third roller 33 and the fourth roller 34, configured as cooling rollers, are adjusted in accordance with the surface speed of the second roller 32 in such a manner that there is no longer an extension of the film, or even slower, so that a relaxation can occur.

(92) The described embodiment is intended to be a flattening package, i.e. usually with a maximum stretching of 1:1.05. As opposed to the MDO variants, the stretching length is relatively long.

(93) The resulting longer dwell time in the stretching section is advantageous for a wide process window.

(94) Since only a minor stretching is implemented, a low drive power is sufficient. Individual drives are also unnecessary since the film is only minimally processed. It is therefore sufficient if the holding roller and the second roller 32 are respectively driven and their speeds adjustable.

(95) According to the prototype tests of the inventors, a water heating is sufficient because the required temperature and energy level is low.

(96) In the embodiment as a flattening package, the second roller preferably has the same temperature as the first roller. The first roller serves as a heating and holding roller. The second roller serves as a stretching and tempering roller. The section between the second and the following third roller is a tempering section.

(97) In contrast, in an MDO embodiment, a stretching of 1:10 or even more is possible.

(98) The drawing length should be as short as possible in order to reduce a transversal contraction, known as necking.

(99) The control of the process is much more critical because of the very small dwell time in the very short stretching gap.

(100) Since there are more rollers whose temperature needs to be regulated, there is higher energy expenditure and a quite long tempering section is required.

(101) The drives must be quite strong in order to overcome the elastic limit of the plastic and to easily exceed the yield point.

(102) In order to allow for an individual process control, it is proposed to use individual drives.

(103) An MDO configuration is actually too big for a mere improvement of the flattening and thus normally inefficient.

(104) Since an MDO requires high temperatures, it is normally proposed to use oil heating.

(105) In the fifth implementation variant according to FIG. 6, it is also proposed to provide a transversely oriented processing line above the pair of haul-off rollers. This line is concretely horizontally oriented with four rollers, wherein a cooling roller is disposed further upwards and would allow for a straight guidance of the double layer film strip above the pair of haul-off rollers.

(106) Two rollers are movable inside the horizontally oriented roller line, namely a second roller that is disposed so as to be pivotable around the first roller and a fourth roller that is disposed so as to be displaceable or pivotable and disposed on the other side of the virtual plane 14.

(107) Otherwise the fifth variant according to FIG. 6 is usable in the blown film installation in the exact same manner as the previously described variants.

(108) The fundamental behavior of the films to be processed herein is graphically explained in the diagram of FIG. 7.

(109) The abscissa 41 shows the longitudinal extension c of the film, whereas the ordinate 42 shows the longitudinal tension within the film, i.e. a quantity that is proportional to the longitudinal tension inside the film. The longitudinal tension is labeled .

(110) Starting from a zero point 43, the film usually shows an increasing longitudinal extension c in a linear tension increase field 44. From a certain longitudinal extension *, respectively the corresponding tension * onward, the film leaves the area of a linear increase in tension and the tension curve levels off, i.e. has a lesser increase relative to the abscissa 41.

(111) After the longitudinal extension *, the longitudinal extension is irreversible.

(112) The tension has then reached a first maximum 45. The so-called plastic flow of the film begins at this point. The corresponding longitudinal extension .sup.Streck is referred to as the elastic limit. A flow area 46 extends from the first maximum 45 of the longitudinal tension , labelled .sup.Streck to the area of the again increasing branch 47, where the longitudinal tension again reaches the tension .sup.Streck.

(113) From there, the longitudinal tension a increases again continuously with the increasing longitudinal extension s until a sudden failure in form of a tear 48 of the film.

(114) The MDO operating range 49 extends from the return to the tension .sup.Streck to the tear of the film.

(115) In contrast, the operating range of a flattening package lies in the area beyond the linear tension increase field 44, but below the elastic limit .sup.Streck. In the linear tension increase field 44, i.e. up to the longitudinal extension .sup.Streck, the film behaves elastically.

(116) To put it simply, an extension of a flattening package takes place between * and the local maximum. In contrast, the stretching of an MDO occurs after the return to .sup.Streck.

(117) With regard to its construction with five rollers, the processing roller line 50 in FIG. 8 corresponds in principle to the second variant according to FIG. 2 and to the third variant from FIG. 3, but has three meshing rollers, the first roller 51, the third roller 52 and the fifth roller 53, whereas its second roller 54 and its fourth roller 55 are designed to be stationary and rotative.

(118) In order to thread the double layer film strip at the beginning of a blowing process, the three meshing rollers, i.e. the first roller 52, the third roller 52 and the fifth roller 53 are moved out of their meshed position, i.e. in FIG. 8 toward the left of the virtual plane 14, so that the double layer film strip can simply be moved vertically upward from a nip 56 of the pair of haul-off rollers 57 to a deflecting roller 58. The deflecting roller 58 is the first roller that lies beyond the processing roller line 50. After the deflecting roller 58 the double layer film strip is guided transversely to an entry 59 into the reversing unit 60.

(119) During meshing, the first roller 51 can not only be displaced into the plane of the stationary rollers, i.e. the second roller 54 and the fourth roller 55 (shown in FIG. 8 by a first contour 61 of the meshed first roller 51); the first roller 51 can also be displaced through this plane, so that the central axis of the first roller 51 moves beyond the plane formed by the central axes of the second roller 54 and the fourth roller 55. The first roller 51 can thus take up a position, in which it has passed through the plane, for the blowing operation (shown in figure by a second contour 62 of the first roller 51).

(120) During prototype tests, it turned out that a preferably continuous adjustability of the meshing depth, i.e. more specifically with a meshing depth through the plane of the stationary rollers, can be advantageous for the safety of the process and the resulting quality of the film.

(121) The processing roller line 63 according to the seventh variant in FIG. 9 has a somewhat different construction:

(122) The processing roller line 63 is located above the pair of squeezing roller 64 in a vertical orientation.

(123) A first roller 65 is designed as a heating roller and at the same time as a holding roller. It is displaceable so as to mesh through the virtual plane 14. The meshing has already been described repeatedly above. It is assumed that its advantages and the possibility of a meshing through the plane of the stationary rollers are now known.

(124) A second roller 66 is provided with a significantly smaller diameter than the first roller 65.

(125) A third roller 67 is also provided with a lesser diameter than the first roller 65, preferably with the same diameter as the second roller 66, as shown here.

(126) The second roller 66 and the third roller 67 form a stretching station, in which the third roller 67 is configured to mesh. Due to the continuous adjustability of at least one of the two rollers 66, 67 of the stretching station, the stretching length can be continuously adjusted, which turned out to be very advantageous during prototype tests.

(127) The second roller 66, which is at the same time a first roller of the stretching station, is preferably driven with the same circumferential speed as the first roller 65, i.e. as the big holding roller. In consideration of the function of the holding station, the second roller 66 should be attributed to the holding station rather than the stretching station, namely together with the first roller 65. In consideration only of its diameter, it is also possible to attribute the second roller 66 together with the third roller 67 to the stretching station.

(128) The two small rollers, i.e. the second roller 66 and third roller 67 are not heated but driven. This makes it possible to design the second roller 66 and the third roller 67 with a very small diameter.

(129) However, the third roller 67 is driven at a higher circumferential speed than the second roller 66. Thus a stretching section for the double layer film strip is formed between the second roller 66 and the third roller 67.

(130) Assuming that with the given geometric conditions there is a static friction for instance covering 70 around the circumference of the faster driven third roller 67, and the diameters of the big rollers lie between 250 mm and 290 mm and the diameters of the small rollers lie between 100 mm and 140 mm, the stretching length of the stretching section amounts to between 15 cm and 19 cm. During a prototype test it amounted to about 17 cm.

(131) After the faster driven third roller 67, the processing roller line 63 is provided with a first tempering roller 68 and a second tempering roller 69, wherein the latter can also be brought into its meshed operating position.

(132) The two tempering rollers 68, 69 are followed by a cooling roller 70; in the present example with a slightly increased diameter. The cooling roller 70 has a contact roller 71.

(133) Together with its contact roller 71, the cooling roller 70 forms the last station of the processing roller line 63. From there, the double layer film strip is guided to the entrance to the reversing unit.

(134) The rollers of the processing roller line 63 are disposed quite close to each other, with a clearance in a vertical arrangement of only about 10 mm to 30 mm, in order to achieve a construction height that is as small as possible.

(135) Several or all roller surfaces of the processing roller line 63 preferably have a rough, non-slip surface, ideally with embedded silicone.

LIST OF REFERENCE NUMBERS

(136) 1 blown film installations 2 pair of haul-off rollers 3 first haul-off roller 4 first holder 5 second haul-off roller 6 adjusting cylinder 7 sliding bearing support 8 processing roller line 9 first roller 10 second roller 11 third roller 12 fourth roller 13 fifth roller 14 virtual plane 15 central axis 16 first side 17 opposite second side 18 gap 19 deflecting roller 20 reversing unit 21 machine frame 22 temper devices 23 double layer film strip 24 collar 25 straight route 26 straight route deflecting roller 27 entry 28 MDO route 29 other deflecting roller 30 flattening line 31 first roller 32 second roller 33 third roller 34 fourth roller 35 passive deflecting roller 36 first deflector 37 flattening route 38 pressing roller 39 pressure arm 40 flattening extension section 41 abscissa 42 ordinate 43 zero point 44 linear tension increase field 45 first maximum 46 flow area 47 ascending branch 48 film tear 49 MDO operating range 50 processing roller line 51 first roller 52 third roller 53 fifth roller 54 second roller 55 fourth roller 56 nip 57 pair of haul-off rollers 58 deflecting roller 59 entry 60 reversing unit 61 first contour 62 second contour 63 processing roller line 64 pair of squeezing rollers 65 first roller 66 second roller 67 third roller 68 first tempering roller 69 second tempering roller 70 cooling roller 71 contact roller