Abstract:
A sheet extrusion apparatus for producing an extruded sheet with varying thickness including one or more sheet rollers and a strip material affixed to one or more of the rollers. The strip material rotates coextensively with the cylinder it is attached to, such that it engages the plastic passing between the rollers and forms a notch in the plastic sheet at that location. In a preferred embodiment, the strip material is a length of conventional tape, such as Teflon tape, that is capable of adhering to both the cylinder and itself, having sufficient durability to last a required amount of cycles. The present invention is easily retrofitted to an existing sheet extrusion apparatus, as the tape can simply be taken off of a shelf and added to the rollers. The present invention is preferably installed after an analysis to determine which sections of the plastic sheet may structurally withstand a reduced thickness.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to thermoplastic sheet extrusion, and more particularly to an apparatus and method for varying the thickness of an extruded thermoplastic sheet.  
           [0002]    Sheet extrusion is a conventional process for converting plastic, for example, in the form of pellets or powder, into cut sheets or rolls of plastic. The typical extrusion process begins with a hopper that holds the plastic in either powder or granule form. The hopper continuously feeds the material to a conventional extruder. This extruder usually includes a container in which the material is heated and from which it is extruded through a die opening in a continuous flow. The plastic emerges from the die opening with roughly the desired thickness. The plastic is then passed between a number of parallel cylindrical rollers. The rollers further shape the plastic, pressing it into the desired final thickness. The continuous sheet emerges from the rollers and is fed, for example, by a conveyor, to a rolling or cutting station. The plastic may be cooled by fans other cooling apparatus during transfer to the rolling or cutting station to ensure that it retains the desired final thickness. The finished continuous sheet is then rolled into rolls of the desired size or cut into sheets of the desired dimensions.  
           [0003]    The thickness of the sheet is generally determined by the spacing between the parallel cylindrical rollers mentioned above. As the heated thermoplastic passes between two rollers, it is formed into a sheet that has a thickness equal to the space between the cylinders. Smooth cylindrical rollers are common and easy to produce, therefore, most sheets are formed with a single, homogeneous cross-section. The cross-sectional thickness of the sheet is usually adjusted by changing the size of the gap between the rollers, either by moving the rollers relative to each other, or by interchanging different sizes of rollers.  
           [0004]    Problems arise in sheet extrusion when it is desirable to manufacture a sheet with a variable thickness. Some applications require that a sheet have a varying thickness simply for appearance, while in other cases it may simply be a way to save money by using less thermoplastic material. While it is quite easy and well known in the art to control the thickness of a single thickness sheet, there are many times when required thickness in one area creates an excess of plastic in another. Producing a sheet with variable thickness, however, has generally been too difficult and expensive to be worthwhile. For instance, the common method for manufacturing a variable thickness sheet is to cast unique cylindrical rollers that include varying radii. The radial variations in the rollers correspond to the locations where it is desired to have a variation in sheet thickness. For example, a one inch wide section of increased radius on a cylinder creates a one inch wide notch in the extruded sheet of the same depth as the amount of increased radius. The problem, however, is that the cost of casting new rollers with varying radii is expensive, because of the extra time it takes to design the rollers and create the unique die to cast them. Further, these rollers cannot be easily altered, and are therefore only useful for one specific application. Therefore, in many cases manufacturers are left with no alternative but to manufacture a sheet of a single thickness, even in cases where it isn&#39;t needed.  
         SUMMARY OF THE INVENTION  
         [0005]    The aforementioned problems are overcome by the present invention wherein a sheet extrusion apparatus is provided with one or more sheet rollers and a strip material that is attached to one or more of the sheet rollers. The strip material rotates with the cylinder it is attached to, such that it engages the plastic passing between the rollers and forms a notch in the plastic sheet at that location. The material can be placed on the cylinder in any desired location, and is removable to accommodate any desired change in the location of the notch. The present invention enables cost effective thermoforming of a plastic sheet with variable thickness.  
           [0006]    In a preferred embodiment, the strip material is a length of conventional tape material that is capable of adhering to both the cylinder and to itself and that has sufficient durability to last a required amount of cycles, such as Teflon tape. The tape is preferably wrapped concentrically around the cylinder, with each wrap of the tape increasing the radius of the cylinder at that particular section. Functionally, the tape will contact the thermoplastic as it passes between the rollers, and form a notch in the thermoplastic at that point running the length of the sheet. The depth of the notch is directly related to the number of times the tape has been wrapped around the roller, with more wraps producing a deeper notch. If multiple notches are desired, any number of tape sections can be added. Further, each notch can have a unique depth. Alternatively, the tape may be disposed axially on the length of the cylinder, creating notches in the extruded plastic sheet at every revolution of the cylinder.  
           [0007]    In another preferred embodiment, the extrusion apparatus includes three parallel cylindrical rollers. The rollers are spaced equally, at distances equal to the desired nominal sheet thickness. The plastic sheet is fed through a first gap, then around the middle roller and through a second gap. The strip material is preferably attached to the center roller, so that it contacts the plastic for the longest period of time, but may be attached to any combination of rollers.  
           [0008]    The present invention is preferably installed after an analysis to determine which sections of the plastic sheet may structurally withstand a reduced thickness. One common analysis tool is finite element analysis, which can pinpoint the structural requirements of each section of the sheet after the sheet&#39;s final application is known. Alternatively, locations of reduced thickness may be chosen simply from experience in field of thermoplastics, or for aesthetic purposes.  
           [0009]    The present invention also provides a method for extruding sheets of plastic having a variable thickness. The method generally includes the steps of (a) locating the desired sections for reduced thickness on a plastic sheet, (b) attaching a strip material to one or a number of sheet rollers of a sheet extrusion apparatus, (c) wrapping the strip around the roller until it reaches a desired thickness, and (d) pressing a heated thermoplastic material between the rollers, such that the strip material contacts the thermoplastic and forms a notch at that location.  
           [0010]    The present invention provides a simple and effective mechanism for producing an extruded sheet of varying thickness. The present invention is easily retrofitted to an existing sheet extrusion apparatus, as the strip material can be added to nearly any type of existing roller. No permanent changes to the extrusion apparatus are required. If a single thickness sheet is desired after the addition of the strip material, the strip material can simply be removed from the cylinders—a great time and cost savings over conventional methods. Likewise if additional notches are desired, or if the notch locations need to be changed, new tape can easily be added, or existing tape can be moved. In addition, the present invention is cost effective. The cost savings between casting an entire set of unique rollers to accomplish a task, as opposed to simply grabbing a roll of tape off the shelf and adding it to an existing roller is enormous.  
           [0011]    These and other objects, advantages, and features of the invention will be readily understood and appreciated by reference to the detailed description of the preferred embodiment and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a perspective view of a sheet extrusion apparatus in the preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is an enlarged perspective view of the of the present invention.  
         [0014]    [0014]FIG. 3 is a side view of the rollers, strip material, and thermoplastic, with the rest of the extrusion apparatus removed.  
         [0015]    [0015]FIG. 4 is a rear sectional view of the rollers, strip material, and thermoplastic along line A-A, with the rest of the extrusion apparatus removed.  
         [0016]    [0016]FIG. 5 is a enlarged rear sectional view of the top two rollers, strip material, and thermoplastic in the preferred embodiment.  
         [0017]    [0017]FIG. 6 is a perspective view of a roller having strip material extending in a radial direction.  
         [0018]    [0018]FIG. 7 is a sectional view showing a portion of a roller with strip material in an alternative arrangement. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]    I. Overview  
         [0020]    A thermoforming sheet extrusion system in accordance with a preferred embodiment of the present invention is shown in FIG. 1 and generally designated  10 . The sheet extrusion system  10  is preferably configured to form a thermoplastic material  12  into sheets by melting the plastic material  12  and then extruding and shaping the melted plastic material  12  so that it cools in a continuous sheet form. The sheet extrusion system  10  preferably includes a hopper  14  that receives the raw plastic; an extruder  16  that heats the plastic and extrudes it through a die opening  18 ; a set of three rollers  20   a - c  for pressing the plastic  12  to the proper thickness; a strip material  22  attached to the rollers  20   a - c  providing a variable thickness sheet; and a conveyor  24  and cutter  26  for cooling the plastic  12  and cutting it into sections. In operation, plastic  12  in the form of pellets or granules is placed in the hopper  14 . The plastic  12  is then automatically transferred into the extruder  16 . The extruder  16  heats and mixes the plastic and forces it out the die opening  18  in melted form. Once out of the die opening  18 , the plastic is fed between two of the parallel cylindrical rollers  20   a - c , with at least one of the rollers  20   a - c  having a strip material  22  attached, such that the plastic  12  is pressed to the thickness of the roller gaps  28   a - b  (shown in FIG. 3), with a reduced thickness at the location(s) of the strip material  22 . The formed plastic sheet  12  is then cooled as it is pulled down a conveyor  24  and then cut into desired lengths or rolled into a roll of the desired length.  
         [0021]    II. Structure  
         [0022]    As noted above, the system  10  preferably includes a hopper  14  and extruder  16  for accumulating, melting and extruding the thermoplastic material. In the embodiment illustrated in FIG. 1, the hopper  14  and extruder  16  are generally conventional. As shown in FIG. 1, the hopper  14  is generally a basket with an open top  30  and a funnel shaped bottom  32  that forms a tube  34  leading into the extruder  16 . The extruder  16  is generally barrel shaped and contains complex inner mechanisms (not shown) for heating and mixing the plastic and forcing it out the die opening  18 . The hopper  14  and extruder  16  are generally conventional and therefore will not be discussed in further detail here. Although the present invention is described in connection with a conventional hopper and extruder, the present invention is well suited for use with other conventional apparatus for collecting, heating, and extruding thermoplastic.  
         [0023]    As noted above, the system  10  the extruder  16  extrudes the melted plastic into a die opening  18 . The die opening  18  provides the melted thermoplastic with the desired initial shape. In the illustrated embodiment, the die opening  18  is connected to the extruder  16  by a tube  36 . The die opening  18  is generally conventional, and therefore will not be discussed in detail here. Briefly, the die opening  18  has a shape that is approximate to the desired cross section of the plastic sheet  12 . The melted plastic  12  is forced from the extruder  16  through the tube  36 , then through the die opening  18 , wherein it begins to take the form of a plastic sheet. The die opening  18  may include a plurality of conventional adjustment screws that permit fine control over the size and shape of the opening through which the melted plastic is extruded, and hence over the initial shape of the extruded sheet.  
         [0024]    In the illustrated embodiment, a plurality of sheet rollers  20   a - c  are positioned adjacent to the die opening  18  to further shape the extruded sheet as it passes from the die opening  18 . In the illustrated embodiment, the present invention includes three parallel cylindrical rollers  20   a - c , which press the plastic  12  and provide the desired sheet thickness. The cylindrical rollers  20   a - c  are typically, and preferably, cast (or otherwise formed) from stainless steel, but may be made from any appropriate material. Referring to FIGS. 1 and 2, the rollers  20   a - c  are supported on a conventional roll stand  38  by conventional axles (not shown). Shown in FIG. 3, rollers  20   a  and  b  are vertically disposed, such that roller  20   a  is positioned directly above roller  20   b . Roller  20   c  is offset from rollers  20   a  and  b , such that it is diagonally lower and in front of roller  20   b . Referring now to FIG. 2, rollers  20   b  and  c  are disposed such that the gap  28   b  between them is directly below the die opening  18 . The rollers  20   a - c  are preferably spaced apart evenly, forming two roller gaps  28   a  and  b , including a lower gap  28   b  between rollers  20   b  and  20   c , and an upper gap  28   a  between rollers  20   a  and  20   b . The size of the gaps  28   a  and  b  is determined by the desired nominal thickness of the plastic sheet  12 . Conventional drive motors (not shown) provide the rollers  20   a - c  with constant and uniform rotation, in order to continually pull the thermoplastic  12  through the roller gaps  28   a  and  b . Although the present invention is described in connection with a system having three sheet rollers, the present invention may include a different number of rollers as desired in the specific application. For example, in some applications only two rollers are required to provide sheet of acceptable quality, while in other applications, four or more rollers may be included to provide a sheet meeting more precise tolerances.  
         [0025]    The present invention includes a strip material  22  that is affixed to one or more of the sheet rollers  20   a - c  to provide local variations in the thickness of the extruded sheet. The strip material  22  is preferably attached to one or more of the rollers  20   a - c . The strip material  22  is preferably a conventional thin, flexible material that can be easily affixed to a sheet roller, for example, by wrapping the material around the roller. The strip material may be a thermoplastic material that adheres to itself in response to the heat and pressure of the sheet-forming process. The strip material may also be a tape, such as Teflon tape, having an adhesive material on one side to facilitate attachment to one or more of the rollers. In the illustrated embodiment, the tape  22  is wound circumferentially around the center roller  20   b , but it may alternatively be attached to any of the rollers  20   a - c  in any desired combination. As the tape  22  is wound around one of the rollers  20   a - c  it adheres to the roller  20   a - c  and itself and continually increases the thickness of the roller  20   a - c . FIG. 5, a close-up view of the tape  22 , plastic  12 , and rollers  20   a  and  b , shows a strip material  22  that has been wound 6 times as an example of how the desired thickness is achieved. The tape  22  may be wound as many times as needed to achieve the desired thickness. The tape  22  is also removable, such that it may be unwound from the roller  20   a - c  after a length of time. FIGS. 1 and 2 show examples of the present invention including a strip material  22  at two locations, whereas FIGS.  4 - 5  show a strip material at only one location. A preferred embodiment of the present invention may include any number of locations of strip material, wrapped to any thickness, depending on the desired thickness of the sheet  12 . Although the strip material is preferably wrapped circumferentially around the surface of the roller, it may alternatively be affixed to the surface in other orientations. For example, FIG. 6 shows two segments of radially-extending strip material  22   a ′and  22   b ′ affixed to the surface of the roller  20   b ′ In this alternative embodiment, the reduced thickness regions will occur at even intervals in the sheet depending on the diameter of the roller. As an additional alternative, strip materials of different widths can be employed to provide notches of different shapes. In some applications, strip materials of different widths can be wrapped around a roller at the same location to provide a “stepped” notch. For example, a three inch wide tape  22   a ″ can be wrapped around the roller  20   b ″ to a height of one quarter inch and then a two inch wide tape  22   b ″ can be wrapped around the roller  20   b ″ atop the three inch tape to a height of one half inch (See FIG. 7). The strip material may also be applied to a pair of opposed rollers so that notches are formed in both sides of the resulting sheet.  
         [0026]    In one embodiment, the roll stand  38  is connected to a table  42  for feeding the continuous sheet away as it emerges from the roller assembly. The table  42  preferably includes a plurality of horizontally disposed feed rollers (not shown). The table  42  is preferably adjacent to roller  20   a  and set to a height that is even with the top of the roller  20   a , such that the sheet  12  coming off the roller  20   a  will travel directly onto the table  42 . A pull roller  46  and cutter head  26  are located on the table  42  at a distance from the roll stand  38 . These mechanisms are generally conventional in the art of sheet extrusion, and therefore will not be discussed in detail here. Briefly, the pull roller  44  is a cylindrical roller disposed vertically above one of the feed rollers, provided with rotation from a conventional drive motor (not shown). The cutter  48  is generally a blade that sits above the table  42 , spanning the width of the table  42  and provided with actuation such that it can translate down to contact the table  42  and cut the plastic sheet  12 . In some applications, the sheet material may be rolled into rolls of desired length, rather than cut into separate sheets. A variety of conventional apparatus is available for rolling the sheet material into rolls.  
         [0027]    III. Operation  
         [0028]    In operation, the first step is to determine the desired locations for reduced thickness along the length of the plastic sheet  12 . Preferably, this consists of performing an analysis, such as finite element analysis, of a thermoplastic sheet in its final application. The analysis breaks the sheet down into small sections, and simulates the forces that will be exerted on each section. If the results of the analysis show that a certain section of the sheet will receive relatively low stress, then it will be desirable to have a reduced thickness at that section. The amount of reduced thickness will be determined accordingly. For applications in which the strip material is wrapped concentrically about the sheet rollers, the analysis will preferably disclose a line where reduced thickness is permissible along a line passing fully across the sheet in the direction of extrusion.  
         [0029]    Once the locations of reduced thickness have been determined, plastic material  12  in pellet or granule form is loaded into the upper end  30  of the hopper  14 . The plastic  12  falls through the hopper funnel  32  and tube  34  into the extruder  16 . Inside the extruder  16 , the plastic  12  is heated above its glass transition temperature and mixed into a molten form. The molten plastic  12  is then forced through the extruder tube  36  and die opening  18 . At the die opening  18 , the plastic  12  takes the shape of the die  18 , forming a sheet of plastic  12  with an upper surface  54  and opposing lower surface (not shown).  
         [0030]    After exiting the die opening  18 , the plastic  12  enters the lower roller gap  28   b , wherein the rotation of the rollers  20   b  and  20   c  pull the material  12  through the gap  28   b  to further shape the cross section of the sheet  12 . The center roller  20   b  includes a strip material  22  that has been wrapped around the roller  20   b  a predetermined number of times according to the results of the aforementioned analysis, increasing the radius of the roller  20   b  by a desired amount in that location. The strip material  22  engages the upper surface of the plastic  54  as it contacts the roller  20   b  and passes through the gap  28   b , forming a notch in the upper surface  54  at the location of the strip material  22 . The depth of the notch is equal to the thickness of the strip material  22 .  
         [0031]    After the plastic exits the lower gap  28   b , the upper surface  54  remains in contact with the center roller  20   b  and strip material  22 , and the plastic  12  is carried around roller  20   b  and into the upper gap  28   a . The plastic  12  then passes through the upper gap  28   a , and is carried around the upper cylinder  20   c  to the table  42 . FIG. 5 shows a view of the plastic  12  at this stage, wherein it passes between upper rollers  20   a  and  b , continuing to engage the strip material  22 . Once the sheet  12  has reached the table  42 , the notch(es) on the upper face  54  of sheet  12  are facing vertically upward, away from table  42 . The plastic  12 , now in sheet form with the desired cross-sectional dimensions, cools and hardens as it is fed down the table  42  on small rollers  44  with the upper surface  54  including the notch(es) exposed. Finally, the sheet  12  passes under the cutter blade  48 , which cuts the plastic  12  manually or automatically into desired lengths.  
         [0032]    IV. First Alternative Embodiment  
         [0033]    In an alternative embodiment of the present invention, the strip material  22  is added to rollers  20   a  and  20   c , such that all three rollers  20   a - c  are provided with a strip material  22 . The sheet extrusion method is the same as in the preferred embodiment, except that the plastic  12  now engages strip material  22  from each roller  20   a - c  that it contacts. The result of having strip material  22  on all three rollers  20   a - c  is that the both the upper  54  and lower surfaces engage the plastic  12 , forming notches in both surfaces at the locations of the strip material  22 .  
         [0034]    V. Second Alternative Embodiment  
         [0035]    In another alternative embodiment, the strip material  22  is attached lengthwise on one or more of the cylindrical rollers  20   a - c . The result of this placement is that a notch the entire length of the strip material  22  is formed in the plastic sheet  12  for each revolution of the cylinder(s). The frequency of the notches depends upon the size of the rollers  20   a - c , for instance, a smaller roller produces more revolutions and therefore more closely spaced notches. Multiple segments of strip material can also be added to one or more of the rollers  20   a - c  to provide separate notches in the extruded sheet.  
         [0036]    As an alternative option, the strip material can be configured to provide raised or recessed lettering, logos, instructions, insignia or other graphics in the manufactured sheet. For example, the strip material may be die cut, laser cut or otherwise shaped to provide a finished sheet with a customer&#39;s logo or trademark. In one embodiment, a first strip material can be used to build up a base portion on the roller. This first material is preferably a relatively thin, flexible, tape material, such as the Teflon tape discussed above, that is layered repeatedly over the roller to build a raised portion that is equal to the desired variation in the overall thickness in the sheet at that location minus the desired thickness of the raised or recessed graphics. A second strip material having a thickness that corresponds to the desired height or depth of the raised or recessed graphics can then be layered on the roller atop the first strip material. The second strip material is preferably die or laser cut to define the desired graphics and then secured to the first strip material. For example, the second strip material may be cut to form discrete letters that are secured atop the first strip material by an adhesive. During manufacture, the raised letters define corresponding recessed letters in the sheet material. As an alternative example, the second strip material may be laser cut to define voids in the shape of the desired graphics. The second strip material is preferably secured to the first strip material, for example, by adhesive. During manufacture, the voids define corresponding raised graphics. As an alternative to using a single, relatively thick layer of strip material to define the raised or recessed graphics, a plurality of layers of a relatively thin strip material may be used where each layer is pre-cut or otherwise pre-shaped to define the desired graphics. For example, this can be achieved with a strip material in which the desired cut-outs are repeated along the length of the strip material In applications where the strip material is wrapped around the roller, the spacing of the cut-outs along the length of the strip is preferably set as a function of the diameter of the roller and the thickness of the strip material so that the graphics properly align with each successive layer. Although the strip material is preferably pre-cut or otherwise pre-shaped to provide the desired raised or recessed graphics, the strip material may alternatively be cut after application to the roller.  
         [0037]    The above description is that of the preferred embodiments of the invention. Various alterations and changes can be made without departing from the spirit and broader aspects of the invention as defined in the appended claims, which are to be interpreted in accordance with the principles of patent law including the doctrine of equivalents. Any reference to claim elements in the singular, for example, using the articles “a,” “an,” “the” or “said,” is not to be construed as limiting the element to the singular.