Abstract:
A method of forming thermoplastic panels comprises the steps of forming the thermoplastic panels by extrusion or a similar process and simultaneously or subsequently texturing or graining at least one face thereof, and utilizing such pre-textured or pre-grained panels to form a finished panel or article by disposing such panel in a thermoforming mold having at least one grained or textured mold surface against which the pre-textured or pre-grained surface of the panel is disposed and thermoforming such panel. The two-step process provides a texture or grain on a surface having greatly improved uniformity and enhanced appearance.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The invention relates generally to a method of thermoforming an article from thermoplastic panels having textured or grained surfaces and more specifically to a method of thermoforming an article in a mold having textured or grained surfaces from thermoplastic panels having textured or grained surfaces which provides improved surface appearance.  
           [0002]    The suitability of relatively large thermoplastic panels for use in various vehicular and automotive products is a direct result of both the ruggedness and stability of the material from which the panel is formed and the final characteristics of the product which include both structural and aesthetic considerations. Stated somewhat differently, the utilization of thermoplastic panels in vehicular and automotive applications has increased as the stability, strength and appearance of such products has increased.  
           [0003]    Frequently such products are utilized as covers for hatches, storage containers and pickup truck beds. As such, a thermoformed product must typically span a relatively large region and must therefore be strong and relatively immune to creep or dimensional instabilities resulting from temperature swings or short or long term loads impressed thereupon. If, as is often the case, such thermoplastic products are utilized as hatches or covers on a vehicle where they are readily visible, their appearance is critical both from a large dimension standpoint, i.e., warps and ripples, and a small dimension standpoint, i.e., surface uniformity, whether the surface is smooth or contains grain or texture.  
           [0004]    For product surfaces requiring texture or grain, a typical prior art manufacturing process begins with a smooth thermoplastic panel which is placed into a mold wherein one or both mold surfaces are textured or grained. The mold is closed and vacuum and/or pressure are applied to appropriate faces of the thermoplastic sheet to drive it into conformance with the mold and simultaneously grain or texture the surface in contact with the textured or grained mold surface.  
           [0005]    An aesthetic problem has been detected with regard to such processes wherein the grained or textured surface appears to have a ring or halo-like imperfection or variation of the texturing or graining which is typically centered about the center of the panel. The visible halo is the result of changes in reflectivity of the textured or grained surface. While the mechanism of formation of the halo is not fully understood, it is believed to result from non-uniform stretching of the thermoplastic panel along radial paths extending from the center of the panel to the edges. That is, the very center of the panel which typically contacts the mold first, and thus typically cools somewhat more quickly, undergoes very little stretching whereas points at increasing distances from the center and particularly those proximate the edges of the mold cool more slowly and may undergo significant stretching. This stretching or radial movement which occurs parallel to the surface of the mold is believed to interfere with full formation of the grain or texture as the graining or texturing surface irregularities of the mold are not filled because the movement vectors of the thermoplastic material thereacross have significant horizontal components and relatively small vertical components. By way of contrast, the vector in the center of the mold has essentially only a vertical component directed toward the surface of the mold. Because the sliding and stretching motion occurs in a radial direction substantially uniformly about the center of the mold, a visible halo-like imperfection appears on the finished product. The halo is, as noted, a result of varying reflectivity of the surface from the center of the molded panel outward to its edges.  
           [0006]    The invention disclosed and claimed herein is directed to eliminating the problem of halos and other irregularities appearing on textured or grained panels of thermoformed products.  
         SUMMARY OF THE INVENTION  
         [0007]    A method of thermoforming thermoplastic panels comprises the steps of forming a thermoplastic panel by extrusion or a similar process and simultaneously or subsequently texturing or graining at least one face thereof, and utilizing such pre-textured or pre-grained panels to form a finished panel, an article or product by disposing such panel in a thermoforming mold having at least one grained or textured mold surface against which the pre-textured or pre-grained face of the panel is disposed and thermoforming such grained or textured panel. The two-step graining process provides a texture or grain on a surface having greatly improved uniformity and thus enhanced appearance.  
           [0008]    The finished panel may be used as fabricated, may be combined with a second panel to form a hollow structure or may be combined with an in-situ foam or a rigid pre-form and a second panel to form a reinforced panel structure.  
           [0009]    Thus it is an object of the present invention to provide a method of forming panels, articles or products of thermoplastic panels which exhibit uniform texturing and graining.  
           [0010]    It is a further object of the present invention to provide a method of uniformly texturing or graining thermoplastic panels for use as covers, hatches and the like on motor vehicles.  
           [0011]    It is a still further object of the present invention to provide a method of manufacturing thermoformed products of thermoplastic panels having uniform texturing or graining and the product formed by such method.  
           [0012]    Further objects and advantages of the present invention will be apparent by reference to the following description of the preferred embodiment and appended drawings wherein like reference numbers among the several figures refer to the same component, element or feature. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a diagrammatic, perspective view of an extruding machine producing a textured or grained thermoplastic sheet;  
         [0014]    [0014]FIG. 2 is diagrammatic, perspective view showing the post production texturing or graining of one face of a thermoplastic sheet;  
         [0015]    [0015]FIG. 3 is a diagrammatic, full sectional view of an open two piece thermoforming mold in which a pre-textured or pre-grained thermoplastic panel has been disposed;  
         [0016]    [0016]FIG. 4 is a diagrammatic, full sectional view of a two piece thermoforming mold in which a pre-textured or pre-grained thermoplastic panel has been disposed;  
         [0017]    [0017]FIG. 5 is a diagrammatic, sectional view of a thermoforming mold wherein air is injected to drive the thermoplastic panel against the upper mold face;  
         [0018]    [0018]FIG. 6 is diagrammatic, sectional view of a thermoforming mold wherein air is injected and exhausted from the mold to facilitate cooling of the thermoplastic panel;  
         [0019]    [0019]FIG. 7 is a diagrammatic, sectional view of a thermoforming mold which has been opened to receive a second, lower thermoplastic panel;  
         [0020]    [0020]FIG. 8 is a diagrammatic, sectional view of a thermoforming mold wherein air is being injected into the mold to drive the second thermoplastic panel against the lower mold surface;  
         [0021]    [0021]FIG. 9 is a diagrammatic, sectional view of a thermoforming mold wherein the second, lower thermoplastic panel has been driven into intimate contact with the second, lower mold surface;  
         [0022]    [0022]FIG. 10 is a diagrammatic, sectional view of a thermoforming mold wherein air is being injected to and exhausted from the interior of the mold to cool the lower thermoplastic panel; and  
         [0023]    [0023]FIG. 11 is a full, sectional view of a product having panels formed according to the present invention with a core of rigid foam or other material. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    Referring now to FIGS. 1 and 2, a first step in the production of a thermoformed panel, component, product or article is illustrated and generally designated by the reference number  10 . An extruding machine  12  includes a supply hopper typically filled with ground, powdered or granulated thermoplastic material such as high density polyethylene (HDPE), acrylonitrile-butadiene-styrene (ABS), polypropylene, an auger or augers, polyethylene and thermoplastic olefins (TPO), and elongate extruding head (all not illustrated). A continuous sheet  14  of extruded material or extrudate exits from the extruding machine  12  at an elevated temperature. Subsequent to extrusion and preferably while the extruded continuous sheet  14  is still at an elevated temperature, it is passed between a pair of opposed rollers  16  having a texture or grain  18  which is impressed upon both the upper and lower surfaces  20  of the extruded continuous sheet  14  to produce a textured or grained surface  22 . Subsequent to the texturing or graining process step, the extruded continuous sheet  14  is cut transversely into sheets or panels  24  in a reciprocating blade cutter (not illustrated) or similar device and then cooled.  
         [0025]    As used herein, the terms “texture” and “grain” or “texturing” and “graining” refer broadly and generically to various types of surface treatments which include small scale random, irregular, semi-irregular or regular surface finishes having aesthetic significance and which mimics, for example, leather, wood or other natural or manmade surfaces, surface treatments or finishes. When used alone, the terms “texture” or “texturing” are used generically and are intended to and should be construed to include grain or graining and all other similar terms relating to small scale, aesthetic surface finishes and treatments.  
         [0026]    Generally speaking, it is necessary to texture only one of the two surfaces  20  of the extruded continuous sheet  14  or the panels  24  and thus the texture or grain  18  is necessary on only the upper or the lower one of the pair of rollers  16 . However, inasmuch as texture or grain on both the upper and lower surfaces  20  renders the extruded continuous sheet  14  and the thermoplastic panels  24  surface placement insensitive, i.e., either surface placement (up or down) may be re-grained in subsequent process steps, it may be preferable to texture or grain both the upper and lower surfaces  20  such that when subsequently used, frequently after both storage and transport, either surface placement of the extruded continuous sheet  14  and the panels  24  will result in proper placement of a textured surface  22  in a subsequent manufacturing step, such as adjacent a mold surface on what will be an outer surface of the product or article and thus achieve a desired product or article according to the present invention without requiring a check to ensure that the extruded sheet  14  or the panel  24  is properly placed prior to such manufacturing step.  
         [0027]    [0027]FIG. 1 illustrates texturing, graining or other surface treatment of the extruded continuous sheet  14  of material immediately subsequent to extrusion and, as noted, while the extruded continuous sheet  14  is still at an elevated temperature. In situations where immediate, post-extrusion texturing or graining is unavailable or undesirable from either machinery or manufacturing standpoints, the extruded thermoplastic panels  24  may be subsequently textured or grained at any convenient time. As illustrated in FIG. 2, the extruded thermoplastic panels  24  are translated through opposed heaters, such as gas or electric infrared or convection heating assemblies  26 , which elevate at least the temperature of the surface and typically the entirety of the thermoplastic panels  24  prior to their passing between opposed rollers  16  and  16 ′. Texturing of only a single surface  20  of the thermoplastic panels  24  by the upper roller  16  having surface texture  18  and a lower roller  16 ′ having a smooth, i.e., untextured or ungrained, surface  18 ′ is achieved. As noted above, since the majority of applications and uses for the panels  24  require only one textured or grained surface  22 , the use of a single textured or grained roller  16  is sufficient. As also noted above, however, such single surface texturing or graining renders the panels  24  placement sensitive.  
         [0028]    It should be understood that the immediate post-extrusion texturing or graining step illustrated in FIG. 1 may, as noted previously, be utilized to texture or grain only a single surface  20  of the panels  24  and similarly that the post-production texturing or graining illustrated in FIG. 2 may be utilized to texture or grain both the upper and lower surfaces of the thermoplastic panels  24 .  
         [0029]    Referring now to FIG. 3, a thermoforming molding machine  30  suitable for practicing the present invention is illustrated and includes a first or upper platen  32  to which is secured a first or upper mold section  34 . The upper platen  32  is disposed upon suitable vertical rods or guides (not illustrated) for bi-directional vertical translation. The upper mold section  34  includes a textured or grained upper mold surface  36  having texturing, graining or other surface treatment which is desired to be molded into an outer surface of a thermoformed article or product. Preferably, the pattern of texturing or graining of the upper mold surface  36  will be the same as that on the surface  22  of the thermoplastic panel  24 . About the periphery of the upper mold section  34  is a first or upper planar, continuous edge  38 .  
         [0030]    In accurate vertical alignment with the upper platen  32  and the upper mold section  34  is a lower platen  42  and a lower mold section  44 . The lower mold section  44  includes a textured or grained lower mold surface  46  as well as a second or lower planar continuous edge  48  which aligns with and is co-extensive with the continuous edge  38  of the upper mold section  34 . When the upper and lower mold sections  34  and  44  are brought together, they define a mold cavity  50  and the continuous edges  38  and  48  align and grip a pre-textured or pre-grained thermoplastic panel  24 . The lower mold section  44  includes a plurality of oblique channels or passageways  52  which receive axially movable air tubes or air injectors  54 . Preferably, three air tubes or air injectors  54  are disposed along both the left and right sides of the lower mold section  44 .  
         [0031]    A first or upper panel  24 A is first heated to a suitable temperature sufficient to render the particular thermoplastic material plastic or pliable when subjected to molding in the mold cavity  50 . Such heating can be accomplished by the electric or gas convection or infrared ovens or heaters  26  illustrated in FIG. 2. Alternatively, the panels  24  may be provided to the molding machine  30  immediately after extrusion and texturing as illustrated in FIG. 1.  
         [0032]    When the upper platen  32  and the upper mold section  34  are raised or separated from the lower platen  42  and the lower mold section  44 , to the position illustrated in FIG. 3, the single first or upper panel  24 A of thermoplastic material is inserted into the space between the mold sections  34  and  44  with a textured or grained surface  22  facing the upper mold section  34 . Again, if the first panel  24 A is textured on both surfaces  22 , it may be disposed in the upper mold section  34  with either surface  22  up. If it is textured only on one surface  20 , the first panel  24 A must be checked to ensure that the texturing is facing up, toward the upper mold surface  36 .  
         [0033]    Referring now to FIG. 4, the upper platen assembly  32  and upper mold section  34  are lowered such that the upper continuous edge  38  engages and grips the upper surface of the thermoplastic panel  24 A and the lower continuous edge  48  engages and grips the lower surface of the panel  24 A such that the thermoplastic panel  24 A is tightly and sealingly held within the mold cavity  50  between the upper and lower mold sections  34  and  44 .  
         [0034]    Referring now to FIG. 5, the plurality of air tubes or air injectors  54  are advanced into the mold cavity  50  and a flow of air under pressure is delivered into the mold cavity  50  through the air tubes or injectors  54  thereby driving the thermoplastic panel  24 A upward into intimate contact with the textured or grained mold surface  36  of the upper mold section  34 . Typically, air provided to the mold cavity  50  is at about 50 p.s.i. and, depending on operating variables, may be in the range of 40 to 80 p.s.i.  
         [0035]    Referring now to FIG. 6, after the passage of a suitable period of time during which the first thermoplastic panel  24 A partially cools, the flow of air under pressure into the mold cavity  50  is modified such that air flows in through the plurality of air tubes or injectors  54  on one side of the lower mold  44  and out through the plurality of air tubes or injectors  54  on the opposite side of the lower mold  44 . So configured and operated, a certain pressure is maintained within the mold cavity  50  while at the same time a significant flow of cooling air is provided which rapidly reduces the temperature of the first thermoplastic panel  24 A, thereby ensuring both a reasonable process cycle time and that the desired position of the panel  24 A against the textured mold surface  36  of the upper mold section  34  and thus the desired shape is maintained.  
         [0036]    Referring now to FIG. 7, the upper platen assembly  32  and the upper mold section  34  as well as the textured or grained first thermoplastic panel  24 A are raised. If the desired product is simply a molded thermoplastic panel having improved texturing or graining uniformity, the product is complete and may be removed from the upper mold section  34 . If the final product includes two panels, a second or lower thermoplastic panel  24 B is placed in position within the lower mold section  44 . Upon proper disposition of the second or lower thermoplastic panel  24 B within the lower mold section  44 , such that its textured or grained surface  22  faces down, toward the textured surface  46  of the lower mold section  44 . The upper platen  32  and the upper mold section  34  as well as the grained or textured first thermoplastic panel  24 A are lowered so that the continuous edges  38  and  48  securely and sealingly engage the thermoplastic panels  24 A and  24 B positioned between the mold sections  34  and  44 .  
         [0037]    Referring now to FIG. 8, the air tubes or injectors  54  are now once again extended into the mold cavity  50 , through the second thermoplastic panel  24 B and a flow of air under pressure is once again provided through them such that the second thermoplastic panel  24 B is forced into intimate contact with the lower mold surface  46  to both conform the shape of the second thermoplastic panel  24 B thereto and impart the graining or texturing on the surface of the lower mold surface  48  onto the lower surface of the second thermoplastic panel  24 B. The completion of this step and the intimate contact of the second thermoplastic panel  24 B with the textured surface  46  of the lower mold section  44  is illustrated in FIG. 9.  
         [0038]    Referring now to FIG. 10, the supply of air into the mold cavity  50  through the air tubes or injectors  54  is once again modified such that air under pressure is provided to the mold cavity  50  through the plurality of the air tubes or injectors  54  on one side of the lower mold section  44  and is removed or released from the mold cavity  50  through the plurality of air tubes or injectors  54  on the opposite side of the lower mold section  44 . The flow of cooling air under pressure through the mold cavity  50  thus ensures that the lower thermoplastic panel  24 B will remain in intimate contact with the lower mold surface  46  as it cools to provide both the desired overall shape to the panel  24 B as well as the textured surface.  
         [0039]    It should be appreciated that quite commonly only one of the two outer surfaces, that is, the upper surface of the first thermoplastic panel  24 A or the lower surface of the second thermoplastic panel  24 B need be textured or grained as typically only one of such surfaces will be an appearance surface, the other being disposed within a vehicle, vehicle compartment or in some other location where it is not generally visible. Thus while the foregoing disclosure has taught the texturing or graining of both exposed (outer) surfaces of a product or article, it should be appreciated that quite commonly only one outer surface (of only one panel) will require texturing or graining according to the present invention as only one surface of a final article or product will be visible, i.e., will be an appearance surface.  
         [0040]    At the completion of the forming and cooling cycle, the upper platen assembly  32  and upper mold section  34  may be raised and a completed product or part  60  comprising the first or upper thermoplastic panel  24 A and second or lower thermoplastic panel  24 B which are now sealed about their peripheries due to the pressure applied by the continuous edges  38  and  48  of the upper mold section  34  and lower mold section  44 , respectively, may be removed from the machine  30 . Excess material or flash  62  about the periphery of the product or part  60  may then be removed by conventional processes.  
         [0041]    Referring now to FIG. 11, it will be appreciated that the product  60  will typically not be hollow but will be filled with foam  64  or other relatively lightweight and rigid reinforcing material which is either in-situ molded or inserted as a rigid preform during the step illustrated in FIG. 7. Preforms of other rigid or relatively rigid and lightweight materials such as metal honeycomb, balsa wood or other rigid and lightweight materials may also be used to fill the interior of the product  60 , if desired.  
         [0042]    The foregoing disclosure is the best mode devised by the inventors for practicing this invention. It is apparent, however, that apparatus and methods incorporating modifications and variations will be obvious to one skilled in the art of thermoforming processes. Inasmuch as the foregoing disclosure presents the best mode contemplated by the inventors for carrying out the invention and is intended to enable any person skilled in the pertinent art to practice this invention, it should not be construed to be limited thereby but should be construed to include such aforementioned obvious variations and be limited only by the spirit and scope of the following claims.