Abstract:
A method for operating a four-roll calendar machine (R), according to which a plastic melt from an extrusion nozzle is homogenized first between a first and second roll (W 1 , W 2 ), and then between a third and a fourth roll (W 3 , W 4 ). The position of the third roll (W 3 ) can be modified in relation to the rolls (W 1 ) and/or (W 2 ) even during operation, in order to regulate different operating states.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a process for operating a four-roll calender in which a plastic melt from an extrusion nozzle is homogenized between a first and a second roll and then between a third and a fourth roll and also to a four-roll calender operating on the basis of the process.  
         [0002]     A conventional four-roll calender is known from and described in, for example, DE 195 44 988. Reference is specifically directed to this prior art. This prior art provides the basis for this application.  
         [0003]     In the case of conventional four-roll calenders it is disadvantageous that the dwell time of the plastic melt within the four-roll calender is too short.  
         [0004]     Since the plastic melt is a good insulator, any transfer of heat away from the plastic melt in the four-roll calender takes place insufficiently quickly with result that the production speed is limited.  
         [0005]     In the four-roll calender the plastic melt is reduced to a desired gap width, given surface treatment, cooled and forwarded to receive further treatment, such as, for example, to a roller conveyor.  
         [0006]     In that respect it is of further disadvantage that in the case of conventional four-roll calenders these can only be cleaned with difficulty, that in the event of a crash rolls are frequently damaged which is undesirable and, moreover, that it is only possible to change individual rolls of the 4-roll calender by the expenditure of considerable effort and money.  
         [0007]     DE 27 43 844 discloses a method for controlling the profile of the last roll-press gap in a calender. Here the last roll and the first roll of the calender can be moved in the vertical direction and the roll lying therebetween can be moved independently in the horizontal direction.  
         [0008]     The object of the present invention is to provide a process and also a four-roll calender which eliminate the above-mentioned disadvantages and which increase the production speed, improve the rate at which heat is removed, facilitate an automatic start-up operation and which, in the event of a crash, automatically prevent damage to the rolls.  
       SUMMARY OF THE INVENTION  
       [0009]     This object is achieved by providing a process for operating a four-roll calender (R) in which a plastic melt ( 2 ) from an extrusion nozzle ( 1 ) is homogenized between a first and a second roll (W 1 , W 2 ) and then between a third and a fourth roll (W 3 , W 4 ), characterized in that a position of the third roll (W 3 ) for setting different operating conditions relative to the rolls (W 1  and/or W 2 ) is changed even during operation. The object is further achieved by providing a four-roll calender for homogenizing a plastic melt ( 2 ) with four rolls (W 1 , W 2 , W 3 , W 4 ), where the plastic melt ( 2 ) can be homogenized between the first and second rolls (W 1 , W 2 ) by means of an extrusion nozzle ( 1 ), characterized in that even during operation, a position of the third roll (W 3 ) relative to the rolls (W 1  and/or W 2 ) can be changed in order to set different operating conditions.  
         [0010]     In the case of the present invention it has proven to be particularly advantageous to simultaneously swivel the third and fourth rolls of the 4-roll calendar which are carried in a rotary plate which swivels about an eccentric axis of rotation.  
         [0011]     In this situation, the rotary plate allows the fourth roll to be moved relative to the third roll and, independently of the movement of the rotary plate, it permits an adjustment to be made to a gap between the third and fourth rolls.  
         [0012]     In particular, the swiveling of the rotary plate permits an adjustment to be made to a gap between a second roll and the third roll while leaving a gap between the third and fourth roll unchanged and constant.  
         [0013]     Furthermore and in particular, it has proven to be advantageous in the context of the present invention that as a result of the swiveling of the rotary plate and the consequent swiveling of the third roll which is arranged below the first two rolls a start-up process is simplified in which, for example, the third roll is moved or swiveled, preferably centrally between the two upper rolls. In this way the plastic melt can be brought centrally through the first two rolls onto the third roll in order that manual wrapping of the third roll with the plastic melt can be effected when starting up the production process. This start-up procedure is considerably simplified by this approach.  
         [0014]     After the plastic melt has been wrapped around the third roll it is then possible to wrap the plastic melt around the fourth roll by opening the gap between the third and fourth rolls by moving the fourth roll, after which the plastic melt can be transferred to a roller conveyor for further processing.  
         [0015]     During this process the plastic melt is uniformly cooled in each of the gaps, as a result of which it receives a desirable surface finish.  
         [0016]     By this means it is likewise possible to clean all four rolls during operation since all four rolls are freely accessible.  
         [0017]     Furthermore, in relation to the present invention it has proven to be advantageous that all the rolls are of similar construction and can be interchanged between one another. All the rolls have the same diameter, have the same drive, i.e. the same servo and gear units.  
         [0018]     Each roll can be exchanged for any another roll. This ensures that only one single roll with a single drive system need be held ready for use as a spare roll to replace either the first, second or third roll if damage is suffered. This also allows a considerable reduction in storage costs to be achieved.  
         [0019]     All in all, the present invention provides both a process and a four-roll calender with which it is possible to operate at considerably higher production and/or extrusion speeds. The rate of removal of heat from the plastic melt is also improved and this, too, permits higher throughput and/or production- and extrusion speeds. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Further advantages, features and details of the invention are to be found in the following description of preferred exemplary embodiments and with reference to the drawing, in which  
         [0021]      FIG. 1  shows a diagrammatic plan view of a four-roll calender for processing a plastic melt delivered by an extrusion nozzle;  
         [0022]      FIG. 2  shows a diagrammatic side elevation through the four-roll calender while in operation;  
         [0023]      FIG. 3  shows a diagrammatic side elevation as in  FIG. 2  but when starting up. 
     
    
     DETAILED DESCRIPTION  
       [0024]     As shown by  FIGS. 1 and 2 , a plastic melt  2  is fed to a four-roll calender R by means of a broad-slit extrusion nozzle  1  (merely indicated here). In this context the position of the extrusion nozzle  1  is fixed while the four-roll calender R can be moved relative to the background by roll drives  3  indicated here and travel to and fro in the illustrated X-direction.  
         [0025]     The essential elements of the four-roll calender R comprise parallel side walls  6  at a distance from one another. The rolls W 1  to W 4  are mounted so as to rotate between the side walls  6  and are actively driven by appropriate drive elements—not shown here in detail—and comprising gear elements and servo-motors.  
         [0026]     In addition, the temperature of each roll W 1  to W 4  can be controlled, i.e. heated or cooled.  
         [0027]     The rolls W 1  and W 2  are arranged side by side. There, an action line L 2  of a linear drive L 1  or roll W 1  leads through the one central axis M 1  of the roll W 1  and the adjacent roll W 2 .  
         [0028]     Preferably, the linear guides  5 . 1  are provided in the side walls  6  and with respect to the roll W 2  guide the roll W 1  movably along the action line L 1  to set and change a gap S 1 , even during operation. Each roll W 1  to W 4  is actively driven or capable of being actively driven.  
         [0029]     In the case of the present invention it has proven particularly advantageous not to mount the rolls W 3  and W 4  in the side walls  6  but rather within parallel rotary plates  7  at a distance from one another, which as indicated by the direction of the double arrow can be moved to or fro or swiveled about an axis of rotation D by a linear drive  4 . 2 .  
         [0030]     The linear drive  4 . 2  is supported by one side wall  6  or the housing  9  of the four-roll calender R.  
         [0031]     Preferably, a point of engagement  9  of the linear drive  4 . 2  lies far outside the axis of rotation D to facilitate a large lever arm. The rolls W 3  and W 4  are arranged one above the other between the point of engagement  9  and the axis of rotation D and are slightly offset with respect to one another.  
         [0032]     The roll W 3  is mounted solely in the rotary plate  7  and in a driven and rotating manner.  
         [0033]     The roll W 4  is arranged below the roll W 3 , where its central axis M 4  is slightly offset relative to the central axis M 3  of the roll W 3 .  
         [0034]     By means of linear drive  4 . 3  the roll W 4  can be moved to and fro in a further linear guide  5 . 2  provided in the rotary plate  7 .  
         [0035]     In that respect, the linear guide  5 . 2  or an action line L 4  is aligned approximately with the central axis M 3  of the roll W 3 .  
         [0036]     By activating the linear drive  4 . 3  it is possible to move the roll W 4  towards the roll W 3 , so that a gap S 3  can be set and changed in this manner.  
         [0037]     In the present invention it is important that independently of the setting of the gap S 3  it is possible to set or adjust the gap S 2  between the rolls W 2  and W 3  by swiveling the rotary plate  7  even during operation.  
         [0038]     Consequently there is no coupling between any movement or adjustment of all the gaps S 1 , S 2  and S 3 , any of which actions can be effected independently even during operation.  
         [0039]     However, it is decisive for the present invention and particularly so for the start-up process as shown, for example, in  FIG. 3  that when starting-up, the plastic melt  2  passes directly between the rolls W 1  and W 2  onto the roll W 3  if this is driven about the axis of rotation D by the rotary plate  7  into an approximately central position between the two rolls W 1  and W 2 .  
         [0040]     Then by appropriate turning of the roll W 3  the plastic melt  2  can be taken up automatically without manual help and wrapped around the roll W 3 .  
         [0041]     After the melt  3  has been accepted by the roll W 3  it is conceivable that the rotary plate D should sink still further in order to transfer the plastic melt  2  to the roll W 4 .  
         [0042]     Once the plastic melt  2  has passed between the rolls W 3  and W 4  and wraps round these then, as illustrated in  FIG. 2 , the plastic melt  2  is then transferred to a roller conveyor  10 —which is only indicated here—for further processing.  
         [0043]     Even when an emergency shutdown has taken place or during servicing and cleaning operations it is possible by swiveling the rotary plate  7  and hence the rolls W 3  and W 4  easily to carry out cleaning or an exchange of roll drives, etc.  
         [0044]     In this context, it is only necessary that the linear drive  4 . 3  should drive the roll W 4  backwards and thereby enlarge the gap S 3  between roll W 4  and W 3 .  
         [0045]     By simultaneous swiveling of the rotary plate  7  about the axis of rotation D it is also possible to open the gap S 2  between roll W 2  and roll W 3  in order that here, too, servicing and cleaning work can be undertaken without needing to dismantle the four-roll calender R or to remove individual rolls.  
         [0046]     Furthermore, it has proven to be particularly advantageous that each of the linear drives  4 . 1 ,  4 . 2  and  4 . 3  should be aligned towards the center of the bearing arrangements or the central bearing position of the rolls W 1 , W 2  or the center of the bearing of the axis of rotation D to prevent or avoid bending of the rolls W 1  to W 4 . This is also intended to come within the framework of the present invention.  
         [0047]     Furthermore, it has also proven to be particularly advantageous in the context of the present invention that all the rolls W 1  to W 4  are of similar construction and dimensions and are provided with similar drives, servomotors and/or gear elements.  
         [0048]     As a result of this the individual rolls W 1  to W 4  are of universal application or they can be exchanged with one another.  
         [0049]     Consequently it is only necessary that for example, only one roll W 1  to W 4  with gear elements, drive elements or the like need be kept ready for use and can serve if required as a replacement for the rolls W 1 , W 2 , W 3 , W 4 . In particular, this makes it possible to hold a low-cost stock level of spare parts.  
         [0050]     In the four-roll calender the plastic melt is reduced to a desired gap width, given surface treatment, cooled and forwarded to receive further treatment, such as, for example, to a roller conveyor.  
         [0051]     In that respect it is of further disadvantage that in the case of conventional four-roll calenders these can only be cleaned with difficulty, that in the event of a crash rolls are frequently damaged which is undesirable and, moreover, that it is only possible to change individual rolls of the 4-roll calender by the expenditure of considerable effort and money.  
         [0052]     DE 27 43 844 discloses a method for controlling the profile of the last roll-press gap in a calender. Here the last roll and the first roll of the calender can be moved in the vertical direction and the roll lying therebetween can be moved independently in the horizontal direction.  
         [0053]     The object of the present invention is to provide a process and also a four-roll calender which eliminate the above-mentioned disadvantages and which increase the production speed, improve the rate at which heat is removed, facilitate an automatic start-up operation and which, in the event of a crash, automatically prevent damage to the rolls.  
         [0054]     This object is achieved by the characteristic features of the patent claims  1  and  16 .  
         [0055]     In the case of the present invention it has proven to be particularly advantageous to simultaneously swivel the third and fourth rolls of the 4-roll calendar which are carried in