Patent Publication Number: US-9415434-B2

Title: Downholder control in the manufacture of can bodies

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority of German Application No. 10 2010 019 323.2-14 filed May 3, 2010. 
     BACKGROUND OF THE INVENTION 
     The invention concerns an arrangement and a method for the manufacture of can bodies, for example, for pressure container or beverage cans. Herein a pot-shaped blank is formed by means of a deep-draw plunger into the can body. The can body includes a can bottom and a can wall consisting of the same material and extending from the can bottom without joint. At the end, opposite the can bottom, the can body is open. In order to be able to form the pot-shaped blank, it is engaged by a downholder between the downholder and a counter element. Subsequently, a drawing plunger can transform the blank into the can body, in particular, by so-called deep-draw presses. 
     WO 2009/052608 A1 discloses an arrangement or, respectively, a method whereby from a planar billet first a pot-shaped blank is formed by drawing the flat billet over a hollow-cylindrical projection. Subsequently, the bottom of the pot-like blank is pressed by a plunger into the hollow cylindrical projection whereby the blank is so-to-say inverted. 
     It is the object of the present invention, to provide a method and an apparatus for the manufacture of a can body which reduces the material stresses on the blank. 
     SUMMARY OF THE INVENTION 
     The invention concerns an arrangement ( 20 ) and a method for the manufacture of can bodies from pot-shaped blanks ( 37 ). To this end, the blank ( 37 ) is inserted into a bottom tool part ( 45 ). The blank ( 37 ) is clamped between a downholder ( 23 ) and a counter support surface ( 47 ) of the lower tool part ( 45 ). For controlling a position value α determining the position and/or position change of the downholder ( 23 ), a drive arrangement ( 22 ) is provided. The drive arrangement controls the position value in accordance with a predetermined course, so as to move the downholder ( 23 ) into the clamping position or out of the clamping position EP. As soon as the downholder ( 23 ) reaches its clamping position EP, the drive unit ( 22 ) controls a force value in accordance with a predetermined course which determines the clamping force F(t) which is applied by the downholder ( 23 ) to the blank ( 37 ). This occurs preferably by an adjustment of the motor current I to a predetermined course of the desired value I E (t). 
     In accordance with the invention, the pot-shaped blank is clamped between a downholder and a counter element. To this end the downholder is moved by a drive arrangement from a rest position to a clamping position. During the movement, the drive arrangement controls a position value which characterizes the position or the position change of the downholder, such as the rotational position of an electric motor. The counter element is generally stationary and may be, for example, part of a lower tool. After reaching the clamping position, the drive arrangement switches automatically over and controls a force value characterizing the clamping force. This may occur, for example, by controlling the motor torque of an electric motor. In the clamping position, therefore, the clamping force desired for the subsequent transformation of the blank into a can body is controlled. An excessive clamping force may result in a rupture of the material of the blank. With an insufficient clamping force on the other hand, kinks or folds may be formed in the can body. With the force-or-torque control driving the clamping of the blank by the downholder which, preferably, follows a freely programmable desired value curve, the quality of the can body produced is improved. 
     For the transformation and particularly the deep draw pressing of the blank into the can body preferably a drawing plunger arranged co-axially with the downholder is provided. In particular, the drawing plunger may extend coaxially through a tubular downholder. For actuating the drawing plunger, a plunger drive is provided which is controllable independently of the drive arrangement. 
     The position value and/or the force strength may be provided as variables. The position value and/or the force value may be provided depending on a guide value and/or depending on the time. The predetermined values are preferably freely programmable and stored, for example, in a control unit. 
     After occurrence of a certain event, it is switched between the position-or pilot control and a force-or torque control. For example, the beginning and the end of the force-or torque control on the basis of a change in a guide value, in particular, a virtual guide angle is determined. The control of the force value is, for example, terminated when the virtual guide angle has reached a predetermined threshold value. With a sine-like changing guide angle this may be the case when, since the point in time at which the downholder has reached the clamping position, a predetermined time has lapsed. The predetermined time period is adapted to the needed duration for the transformation of the blank to a can body. After the time period has passed or the predetermined guide angle value has been reached, the drive arrangement switches the control from the force or torque control to the position control and moves the downholder out of the clamping position back to its original rest position. Subsequently, the procedure begins anew. 
     The guide angle may follow a course of a periodic oscillation with constant frequency, in particular a sine-shaped curve. By means of the guide angle several drives of the arrangement may be synchronized with one another, for example, the drive arrangement for the downholder and the separately controlled plunger drive. 
     In a preferred embodiment of the arrangement according to the invention, the drive arrangement includes an electric motor, in particular a synchronous motor. For the position control, the desired angular position is adjusted based on the motor voltage. For the control of the clamping force, the motor torque is correspondingly controlled, for example, on the basis of the motor current. An electric motor is easily and accurately adjustable with respect to its rotational position as well as the motor torque. By way of the electric motor extremely high stroke speeds can be achieved. The arrangement operates with a stroke number in the area of 400 to 500 and, preferably, 460 min −1 . The whole cycle for the deep draw pressing to manufacture a can body from the blank takes about 120 to 150 ms, i.e., milliseconds. The stroke is in the range of 400 to 800 mm, i.e., millimeters, preferably 600 mm. 
     Alternatively, it would also be possible to provide a fluid cylinder such as a hydraulic cylinder or a pneumatic cylinder as drive device. For controlling the clamping force then the pressure in the fluid cylinder is controlled in accordance with a predetermined desired value curve. However, presently available fluid drives do not reach the stroke numbers which can be obtained with an electric motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantageous features of the invention are apparent from the drawings exemplary of the invention, in which: 
         FIG. 1  is an exemplary embodiment of an arrangement for the manufacture of can bodies in a schematic cross-sectional presentation, 
         FIG. 2A  is a representation of a crank drive of the arrangement shown in  FIG. 1  in a cross-sectional view, 
         FIG. 2B  is a schematic representation of the crank angle β of the crank shaft shown in  FIG. 2A , 
         FIG. 3  is a block diagram-like representation of an exemplary embodiment with a downholder shown in its rest position, 
         FIG. 4  is a block diagram-like representation of an exemplary embodiment of the invention with a downholder in its clamping position, 
         FIG. 5  is a block diagram like representation of the exemplary embodiment according to  FIG. 4  with a downholder in its clamping position and with a deep-draw plunger deforming the blank, 
         FIG. 6  is a block diagram of the method steps of the method according to the invention, 
         FIG. 7  is an exemplary curve indicating the position of the downholder depending on the guide angle, and 
         FIG. 8  is an exemplary curve indicating the motor torque during a torque controlled operation based on the guide angle. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
       FIGS. 1 and 2   a  show a first exemplary embodiment of an arrangement  20  for the manufacture of can bodies. The arrangement  20  includes a frame  21  on which a drive arrangement  22  for moving and applying pressure to a downholder  23  is arranged. The drive arrangement  22  includes a crank drive  24  with several crankshafts, for example, two crankshafts  26  which are rotatably supported on the frame  21  so as to be rotatable each about a crankshaft axis  25 . Each crankshaft  26  includes a crank  27  which is arranged eccentrically with respect to the crankshaft axis  25 . In each crank  27  a connecting rod  28  is supported. At the end of the connecting rod  28 , opposite the crankshaft  26 , a pressure rod  31  is pivotably connected to the connecting rod  28 . The pressure rods  31  as shown in the exemplary embodiment of  FIG. 1  are axially movably supported on the frame  21  via hollow-cylindrical guide structures  32 . The pressure rods  31  carry a carrier bracket  33  on which the downholder  23  is mounted. The two pressure rods  31  extend parallel to each other in the clamping direction  30 . The downholder  23  is arranged on the carrier bracket  33  in the center between the two pressure rods  31 . The downholder  23  has a hollow-cylindrical shape and its axis extends in the clamping direction  30 . 
     Concentrically, with the tubular downholder  23  a deep-draw plunger  36  is provided. The deep draw plunger  36  is operated via a separate plunger drive  56 . The deep draw plunger  36  is provided for the deep-draw pressing of a pot-shaped blank  37  (also called “cup”) in order to form from the blank  37  the can body. The deep-draw plunger is operated by a plunger drive  56 . The plunger drive  56  is not mechanically coupled for movement with the drive arrangement  22  for the downholder. The plunger drive  56  and the drive arrangement  22  are controllable independently of each other. 
     For rotating the crankshafts  26  about the crankshaft axes  25  each of the crankshafts  26  has a crankshaft gear  40  mounted thereon. Each crankshaft gear  40  is in engagement with a driven gear  41  which is supported on the frame  21 . In order to synchronize the movement of the two crankshafts  26  also the two driven gears  41  are in engagement with each other, one of the driven gears  41  is driven by a drive gear  42  by a motor, for example, an electric motor  43  in the form of a synchronous motor. 
     The arrangement  20  further includes a lower tool part  45  which is shown in the figure schematically as a single part. It is to be understood that the lower tool part  45  may also consist of an arrangement of several separate parts. 
     The lower tool part  45  comprises a counter element  46  which is stationary with respect to the frame  21  and which cooperates with the downholder  23 . The counter element  46  is arranged at a fixed location. The counter element  46  is, for example, in the form of an annular counter support surface area  47  disposed on the lower tool part  45 . The lower tool part  45  is provided with a cylindrical cavity  48 . The cavity  48  is annularly surrounded by the counter support surface  47 . The axes of the cavity  48  of the downholder  23  and of the draw plunger  36  coincide and form a common longitudinal axis L. 
     For the manufacture of the can body, the downholder  23  and the drawing plunger  36  are first removed from the lower tool part  45 . To start a pot-shaped blank  37  is supplied by a supply system which is not shown. The counter support surface  47  is partially limited by a structure  38  which forms an abutment area for the blank  37  for forming a positioning reference for the blank  37  coaxially with the longitudinal axis L. The downholder  23  is in its rest position FP which is spaced from the lower tool part  45  sufficiently for permitting the insertion of the blank  37 . Subsequently, the downholder  23  is moved by the drive arrangement  22  to its clamping position EP in which it engages the blank  37  and rests on the bottom  37   a  of the blank  37  so that the blank  37  is engaged between the downholder  23  and the counter support surface  47 . The downholder  23  is at least partially surrounded by the cylindrical wall  37   b  of the blank  37 . The front face of the downholder  23  presses herein onto an annular area of the bottom  37   a  next to the cylindrical wall  37   b . During movement of the downholder  23  from the rest position FP to the clamping position EP, the drive arrangement  22  controls a position value which determines the position or the position change, for example, the speed of the downholder  23 . As soon as the clamping position EP has been reached, the drive arrangement  22  controls, instead of a position value, a force value so that the clamping force F(t) assumes a certain value or follows a certain curve. Subsequently, the deep draw plunger  36  is moved through the hollow cylindrical downholder  23  into the cavity  48  wherein the blank  37  is pulled completely into the cavity  48 . The blank  37  is pulled out between the downholder  23  and the counter support surface  47  while overcoming the engagement force F(t) whereby the can body is formed. 
     For careful treatment of the material of the blank and to avoid the formation of fractures of folds in the can body manufactured, the control the pressure application or respectively the movement of the downholder  23  by the drive arrangement  22  is important. An exemplary embodiment of such a control by the drive arrangement  22  is shown in  FIG. 6 . The drive arrangement  22  may include a control unit  55  for controlling the downholder  23  or it may be controlled by a control unit  55 . As control unit, for example, a microprocessor may be used. 
     As guide value for operating condition changes of the drive unit  22  a virtual guide angle W(t) is used which has, for example, a time-based sine-shaped course with constant circle frequency w:
 
 W ( t )=sine(ω t ).
 
     It is assumed that at the beginning of the procedure is disposed in its rest position FP remote from the lower tool part  45 , ( FIG. 3 ). The electric motor  43  is then in its start-out angular position α F . After start-up of the procedure, in a first step S 1 , the position value for the adjustment of the position of the downholder  23  is controlled. This occurs by the control of the angular position α(t) or respectively α(W(t)) of the electric motor  43 . To this end, the electric motor  43  is operated until it has reached an angular position α E  corresponding to the clamping position EP. The plus or minus sign of the voltage U indicates the direction of rotation of the electric motor  43 . The reaching of the clamping position EP is evaluated in a second step S 2 . As long as the rotational position α E  corresponding to the clamping position EP has not been reached, the electric motor continues to be operated in the first step S 1 . The rotational angle α(t) of the electric motor  43  can change in accordance with a predetermined curve as it is shown, for example, in  FIG. 7 . The first deduction (inclination) of the rotational angle indicates the angular speed of the electric motor  43 . The second time based deduction of the rotational angle indicates the angular acceleration. The rotational angle α(t) is dependent on the guide angle W(t) in such a way that a shock-free stopping of the downholder  23  occurs in the area of the exit position FP and, particular, in the entrance position EP. To this end, the course of the rotational angle α(t) is so defined that the angular acceleration includes no jumps. 
     If in the second step S 2 , it is determined that the electric motor  43  has reached the predetermined rotational position α E  and the downholder is in the engagement position EP, the procedure is continued in a third step S 3 . This is the case when the guide angle W(t) has reached a first predetermined guide angle value W 1 . The reaching of the engagement position EP can alternatively or additionally to the evaluation of the guide angle W(t) occur also by rotational position switches at the electric motor  43 . 
     In the third step S 3 , the control unit  55  switches the drive arrangement  22  from a position control to a force or torque control. The drive arrangement  22  then controls the motor current I to a desired current value I E (t) depending on the guide value W or, respectively, depending on the time t whereby the torque M of the electric motor  43  assumes the desired torque value M E (t). An exemplary course for the desired torque value M E (t) is shown in  FIG. 8 . The desired torque value M E (t) has, after the clamping position EP has been reached at the first guide angle value W 1 , an amount which is greater than the size of an upper threshold value MO. In the further time-based course, the desired torque value M E (t) drops below the upper desired threshold value MO only after the deep-draw plunger  36  has reached the bottom of the blank  37 . In this way, the engagement force F of the downholder  23  is within a certain period after the deep-draw plunger  36  has reached the bottom  37   a , sufficiently large so that the plunger  36  can start with the deep-draw procedure. 
     Subsequently, the desired torque value M E (t) is lowered to a value which is below a lower threshold value. Before the upper rim of the wall  37   b  of the blank  37  is pulled through between the downholder  23  and the counter support surface  47  the desired torque value M E (t) is again increased until it exceeds the lower threshold value MU, so that it has a value between the lower threshold value MU and the upper threshold value MO. 
     During the control of the torque M E (t) of the electric motor  43  for reaching a predetermined clamping force F(t) the conversion of the torque M to a clamping force F is to be considered. The clamping force F(t) generated by the downholder  23  is at the same torque M(t) of the electric motor  43  dependent on the crank  27  with respect to the crankshaft axis  25  ( FIG. 2 b   ). This non-linearity is well known. 
     In order to achieve a fast and reasonable control of the clamping force F(t) by controlling the motor torque M(t) as crank angle β and, consequently as rotational angle α E  of the electric motor  43  which corresponds to the clamping position EP of the downholder  23 , a value in the range of 165° to 175° is predetermined. In this range, the change of the rotational angle α or respectively of the crank angle results in a particularly large change of the clamping force F(t). However, small changes of the motor current I(t) of the electric motor  43  are sufficient for the force control. This is very advantageous since the arrangement operates with very short cycle times of 120 to 150 ms, so that the electric motor  43  needs to be controlled very rapidly for the adjustment of the course of the desired motor torque M E (t). 
     As soon as the guide angle W(t) has reached a second predetermined guide angle value W 2  (corresponding, for example, to the end of a predetermined period since reaching the clamping position EP) the control of the motor current I E (t) determining the force value is terminated. The downholder  23  is moved back from the engagement position EP to its rest position FP. To this end, in a fourth step S 4  it is questioned whether the second predetermined guide angle value W 2  has already been reached. If this is not the case, the motor current I of the electric motor  43  is controlled in a third step S 3  to the desired current value I E (t) in order to obtain the desired torque M E (t). Otherwise, the method is continued in a fifth step S 5  and the angular position α(t) of the electric motor  43  is changed in a direction opposite to that of the first step S 1 . In the process, the electric motor  43  is moved from rotational position α E  corresponding to its clamping position EP back to the start out rotational position α F  corresponding to the start-up position. Preferably, the rotational speed and/or the rotational acceleration of the electric motor  43  during movement of the downholder  23  out of the clamping position EP to the start-out or rest position FP is less than during movement of the downholder  23  out of the rest position FP to the clamping position EP. In  FIG. 7  the curves are mapped to show the values of the guide angle W larger than the second guide angle value W 2  and flatter than for values of the guide angle W smaller than the first guide angle value W 1 . 
     Finally, in a sixth step S 6  it is examined whether the start-out rotational position α F  of the electric motor  43  was reached. To this end, it is interrogated whether the guide angle W(t) has reached a third predetermined guide angle value W 3 . Alternatively or additionally, a rotary position switch at the electric motor may be used. As long as this is not the case, the rotational position α(t) of the electric motor  43  is further changed in the fifth step S 5 . When the electric motor  43  has reached the desired rotational rest position α F  corresponding to the rest position FP of the downholder  23 , the motor voltage is switched off and the procedure is terminated. The procedure described in  FIG. 6  is performed cyclically for the processing of each blank  37 . 
     During the third step S 3 , the deep draw plunger  36 , which pulls the blank  37  into the cavity  48  is activated as long as the motor current I(t) of the electric motor  43  is controlled for the setting of the engagement force F(t). Herein the blank  37  is pulled out from between the downholder  23  and the counter support surface  47  while the respective value of the clamping force F(t) is maintained. It is essential herein that the clamping force is maintained at the desired course. In this way, it is ensured that the blank  37  does not rupture (which would happen with an excessive clamping force F) and also pleat formation in the finished can body is avoided (which would happen with an excessively low engagement force F). 
     Instead of an electric motor  43  also other servo-drives, for example, fluid cylinders, may be used for operating the downholder  23 . The crank drive  24  then is not needed. As force value then the pressure P in the fluid cylinder is used. As position value the fluid volume V supplied to the fluid cylinder or the volume flow into or respectively out of the fluid cylinder may be used. 
     As it is shown schematically in  FIG. 5 , the control unit  55  may at the same time be used for controlling the plunger drive  56  for the movement of the deep-draw plunger  36 . In this way, the plunger drive  56  for the deep draw plunger  36  and the drive arrangement  22  for the downholder  23  can be coordinated in a simple manner. Via the control unit  55 , the two drives  22 ,  56  can be jointly controlled in a predetermined interrelation as, for example, provided by the guide angle W(t). 
     The invention concerns an arrangement  20  and a method for the manufacture of can bodies from pot-shaped blanks  37 . To this end, the blank  37  is inserted into a bottom tool part  45 . The blank  37  is clamped between a downholder  23  and a counter support surface  47  of the lower tool part  45 . For controlling a position value α determining the position and/or position change of the downholder  23 , a drive arrangement  22  is provided. The drive arrangement controls the position value in accordance with a predetermined course, so as to move the downholder  23  into the clamping position or out of the clamping position EP. As soon as the downholder  23  reaches its clamping position EP, the drive unit  22  controls a force value in accordance with a predetermined course which determines the clamping force F(t) which is applied by the downholder  23  to the blank  37 . This occurs preferably by an adjustment of the motor current I to a predetermined course of the desired value I E (t). 
     LISTING OF THE REFERENCE NUMERALS 
     
         
         
           
               20  arrangement 
               21  frame 
               22  drive arrangement 
               23  downholder 
               24  crank drive 
               25  crankshaft axis 
               26  crankshaft 
               27  crank 
               28  connecting rod 
               30  clamping arrangement 
               31  pressure rod 
               32  guide 
               33  carrier bracket 
               36  deep-draw plunger 
               37  blank 
               38  structure 
               40  crankshaft gear 
               41  driven gear 
               42  drive gear 
               43  motor 
               45  lower tool part 
               46  counter element 
               47  counter support surface 
               48  cavity 
               55  control unit 
               56  plunger drive 
             EP clamping position 
             F(t) clamping force 
             FP rest position 
             I motor current 
             I E (t) desired current value 
             L longitudinal axis 
             M torque 
             M E (t) desired torque 
             W(t) guide angle 
             W 1  first guide angle value 
             W 2  second guide angle value 
             W 3  third guide angle value 
             S 1 -S 6  steps  1  to  6   
             α(t) rotational position 
             β crank angle