Abstract:
A method for correcting the conicity of a tread strip is described. The method includes the steps of extruding a tread profile having a right hand side, a left hand side and a chimney; measuring the thickness of the tread profile at the right hand side and the left hand side; calculating the mass of the tread on the left hand side and the right hand side; and adjusting the location of the chimney incrementally towards the side which is least in mass.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates in general to extrusion, and more particularly to extrusion of elastomeric or rubber components, particularly treads for tires. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is known in the art of tire manufacturing to form tire components by extrusion. Typically, a strip of elastomeric or rubber material enters an extruder in solid pellet or strip form. The extruder typically has one or more internal screws in a heated barrel which perform work on the elastomer until it has reached a desired consistency. The elastomer exits the extruder and typically enters a flow channel comprised of one or more passages or channels that direct the plasticized material through the extruder head to an outlet or discharge die that forms the material into the proper predetermined cross-sectional profile. For example, if the material is a tread component, it is important that the formed profile of the tread be uniform in size and corresponding to the desired specified green tread contour. 
         [0003]    It is a common practice in the rubber industry to use a single flow channel to extrude tire treads. Imbalances in the mass and velocity flow may occur, resulting in an uneven tread profile. These imbalances are typically correctible by adjusting the contours in the flow channel and adjusting the performer and die dimensions. Dual tread extrusion has proven to be more difficult to manufacture two precise tread profiles at the same time where the contours of both tread extrudates match each other and match the green tread specification. The dividing of the rubber flow into two flow channels has the disadvantage of causing a more severe mass and velocity imbalance, which varies with the types and viscosity characteristics of rubber compounds selected. This problem may be partially addressed in the proper design of the dual cavity flow channel and allowing for proper flow channel lengths to allow disturbances to settle, before the compound reaches the die preformer. 
         [0004]    In processing compounds for tire treads, there is batch-to-batch and day to day variations in compound viscosity. In treads processed on a properly designed single cavity tread extruder line, this typically results in treads being thicker or thinner than nominal specification at the center area of the tread, with very little net conicity variation. In the case of dual cavity tread extrusion, batch-to-batch and day to day variations in compound viscosity typically causes one tread profile to have positive conicity variation (too much mass) while the other tread cavity has negative tread conicity variation (too little mass). 
         [0005]    It is, therefore desired to provide a simple way of balancing the flow among one or more flow channels so that the proper side to side mass balance of the dual cavity treads is achieved. It is also desired to provide an adjustable means to compensate for mass variation and conicity variation within a profiled component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a side cross-sectional view of multiple extruder flow heads being fed into a profiling die for forming a multi-compound tread profile. 
           [0007]      FIG. 2  is a cross-sectional view of an elastomeric strip of a tread profile having a chimney in the center. 
           [0008]      FIG. 3  shows a top plan view of an extruder flow head connected to an extruder on the upstream side of the material flow and to a profiling die on the downstream side of the channel flow, and having a flow splitter in the flow channel. 
           [0009]      FIGS. 4   a  and  4   b  illustrate dual extruded tread profiles, wherein each inner portion of the tread profiles are heavier than the outer portions. 
           [0010]      FIGS. 5   a  and  5   b  illustrate dual extruded tread profiles, wherein each tread profile has a chimney in the outside portion. 
           [0011]      FIGS. 6   a  and  6   b  illustrate two improperly balanced tread profiles, having more mass on the outer portion than the inner portion. 
           [0012]      FIGS. 7   a  and  7   b  illustrate two properly balanced tread profiles. 
           [0013]      FIG. 8  illustrates a laser vision system; 
           [0014]      FIG. 9  illustrates a flow chart describing a first embodiment of a conicity closed loop correction system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    With reference to  FIG. 1 , a multiple extruder system  10  is partially shown having multiple screw extruders  12 ,  14 ,  16 ,  18 , each capable of extruding different compounds. 
         [0016]    The extruder system  10  can be used to extrude two tread profiles simultaneously, or a single tread profile, as desired. For example as shown in  FIG. 2 , a first extruder can extrude a tread base compound  24 , a second extruder can extrude the tread cap  26 , and optionally two extruders can extrude the end caps  28 ,  30 . The multiple extruder system further comprises at least one smaller extruder  20  for a single tread extrusion, and two smaller extruders  20 ,  22  for a dual tread extrusion. The one or more smaller extruders  20 ,  22  are used to extrude a chimney  32  in a tread profile. A chimney is an electrostatic strip of carbon black rubber which provides a conductive path for electrical discharge of electricity from the cap to the base. Chimneys are typically needed when non-conductive rubber is used to form the cap. The chimney is typically extruded on the centerline of the tread. The width of the chimney is typically very thin, on the order of 0.005 inches. Because each of the extruders  20 , 22  is dedicated to only one tread profile, the width of the chimney is adjustable by varying the speed of the extruder. The gauge and location of the chimney can be used to correct the conicity of the tread profile, as described in more detail, below. 
         [0017]    Each extruder  12 ,  14 ,  16 ,  18 ,  20 ,  22  typically includes an extruder screw  2  having an extruder tip  4  enclosed in an extruder barrel  3 , as shown in  FIG. 3 . Attached to the extruder barrel is an extruder head  5 . The extruder head  5  includes an internal flow channel  40  which has an inlet opening  42  for receiving plasticized material preferably elastomeric material from the extruder outlet. The flow channel  40  is divided into first and second flow passages  41 , 43  by a flow dam  45 . The flow dam  45  is a triangular shaped flow splitter. The flow dam separates the first flow passage  41  from the second flow passage  43 . The flow dam can be used to extrude two rubber treads or other components, at the same time. The rubber flow from flow passage  41 , 43  is each fed into its own preformer and die, commonly referred to as a profile die  46 . The function of the flow channel  40  is to ensure that the elastomeric material is uniform in velocity and mass to ensure a uniform rubber strip exits the die. For dual tread extrusion, there is a tendency for the rubber flow to remain higher near the extruder head centerline of the flow channel than further away from the centerline of the channel. This results in each tread having a heavier portion on the “inside” or part of the tread closest to the centerline of the channel, as shown in  FIGS. 4   a  and  4 B. In order to correct for the mass imbalance, the chimney  32  is moved to the side of the tread that has the least amount of mass, as shown in  FIGS. 5A and 5B . The width of the chimney  32  may also be increased in the range of about 5 to about 20 thousands, or 0.005 to 0.002 inches wide. The chimney compound may also be selected to be made from a heavier rubber compound. 
         [0018]      FIGS. 6A and 6B  illustrate dual tread extrusions  298 ,  306  wherein the outer portions  300 ,  310  of each tread are heavier than the inner portions  302 ,  308  of the tread.  FIGS. 7A  and  7 B illustrate the conicity corrections of the tread profiles, wherein the chimney  304 ,  312  for each tread has been shifted to the “inside” portion of the tread. The width of each chimney  304 ,  312  has also been increased in order to offset the mass imbalance. 
         [0019]      FIG. 8  illustrates a continuous strip of tread  132  having an exemplary cross-sectional shape of a tread profile exiting the extruder system  10 .  FIG. 8  further illustrates a laser scanning system  100  for monitoring the tread profile characteristics. The laser scanning system  100  includes a laser  134  having a light beam that is dispersed by lens  135  into a sheet of light  122 . The light sheet  122  has a field depth  133  and a field width  137 . Reflected light  136  off of the exemplary tread profile strip  132  is reflected upward through a lens  138  and to a detector  140 . The detector  140  functions to interpret in three dimensions the reflected light and generate data indicative of the dimensions including thickness and conicity of the target strip. From the scanning procedure, the dimensions (i.e., shape) and thickness of the strip is fed into a controller  146  and compared with the specifications within a tolerance range on a continuous basis. 
         [0020]    Next, the controller calculates the mass of the left hand side of the tread and the mass of the right hand side of the tread. If the mass on the left hand side and the right hand side of the strip falls outside the range of acceptable tolerances, then the controller will determine which side has a greater mass. If for example, the right hand side of the strip has a greater mass than the left hand side of the strip as shown in  FIG. 4   a , then the controller will direct the operator to change out the preformer dies so that the chimney may be extruded on the left hand side of the tread, as shown in  FIG. 5   a . The controller may also slow the speed of the chimney extruder in order to increase the width of the chimney, thereby increasing the mass. The chimney width is sized to offset the tread mass imbalance. If however the left hand side of the strip has a greater mass than the right hand side, the controller will direct the operator to change out the preformer dies so that the chimney may be extruded on the right hand side of the tread to offset the mass imbalance.  FIG. 10  illustrates a flow chart of the closed loop process for correcting the tread conicity, as described above. The tread measurements are taken on specific time intervals, and the controller performs the needed calculations. A set of preformer die plates are preferably used with the invention. A first preformer die allows for the chimney to be extruded on the tread centerline. A second preformer die allows for the chimney the left hand side. A third preformer die allows for the chimney to be extruded on the right hand side of the tread. The location and width of the chimney is used to offset the mass imbalance of the tread. 
         [0021]      FIG. 10  is a flow chart outlining the process steps to correct the tread conicity, or mass imbalance. This process applies for a single tread extrusion or a dual tread extrusion. The first step  200  is to start the extrusion process. The second step  220  is to extrude the various rubber compositions through the extruder system  10 , wherein each flow channel has been split via a flow dam into two channels for extruding two tread profiles, side by side. 
         [0022]    The next step  230  is to measure for each tread, the tread thicknesses at several locations using a laser system or other system known to those skilled in the art. The next step  240  is to calculate the mass for each half of the tread profile, i.e., the left hand side (LHS) and the right hand side (RHS). Next the controller compares the calculated mass of each half with the predetermined tolerance or specification. If both sides are in specification (step  250 ), then the process continues with no changes. If the left hand side (LHS) mass is greater than the right hand side (RHS) in step  260 , then the chimney will be extruded on the right hand side and preferably the width of the chimney increased. If the right hand side mass is greater than the left hand side, then the chimney will be extruded on the left side of the tread and preferably the width of the chimney increased. Preferably the chimney width is increased incrementally until the left hand side and right hand side are in specification. The chimney width is increased by increasing the extruder screw speed wherein more mass of compound is pumped to form the chimney Thus by increasing the width of the chimney, the mass is increased a sufficient amount to correct the conicity. The above process is performed for each tread in a dual tread extrusion system.