Patent Publication Number: US-6660118-B2

Title: Method and apparatus for manufacturing paint rollers

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation in part of U.S. patent application Ser. No. 09/024,971, filed Feb. 6, 1998 now U.S. Pat. No. 6,159,320, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a continuous process for fabricating a paint roller having an extruded plastic core and a hellically-wound absorbent fabric bound to the exterior surface of the core. 
     BACKGROUND OF THE INVENTION 
     Paint rollers are and have been commonly used to apply paints and other coating materials to surfaces for many years. Paint rollers have a unique set of required specifications due to the physical nature of the application process and due to the wide range of paints and other coating materials that the roller may be exposed to in routine use. Paint rollers must have a rigid inner core that is manufactured in a cylindrical shape with a high level of precision so that when the paint roller rotates relative to the surface to be painted, it coats evenly. The cylindrical shape of the roller should not yield, bend or deform under significant stress even when the outer fabric has absorbed paint over an extended period. Even slight deformation of the roller shape may cause uneven paint application. 
     A manufacturer of paint rollers must assume that the roller could be exposed to any of a wide range of fluid compositions. Some paints are water-based and others are oil or solvent-based. Many different pigments, solubilizing agents, surfactants, viscosifiers, emulsifiers, etc., are used in paints, stains and other surface coating compositions. Ideally, the roller core should be inert or at least resistant to all such ingredients so that its rigid cylindrical shape is maintained even after long periods of use, washing and reuse. A sturdy solvent-resistant core yields a longer effective life-time for the roller which is an important objective for those who buy and use paint rollers. 
     Paint rollers typically have an absorbent fabric material fixed to the external surface of the core. The fabric should be uniformly absorbent and bonded to the core in a manner which remains in tact when the roller is exposed to paint. The fabric must also be applied and bonded to the core in a precise and continuous configuration so that there is no overlap or gaps in the fabric which could result in a non-uniform paint application pattern. 
     Various procedures have been used by others to produce paint rollers that satisfy to some extent the specifications discussed above. However, a significant disadvantage with prior manufacturing processes is that they require multiple on and off-line procedures. For example, a desirable core material due to its water and solvent resistivity is extruded plastic such as polyethylene or polypropylene. Typically the core material is extruded, formed and cooled in one process, then put through at least a second process where the core is wrapped with fabric. Multiple on and off-line processing sequences add to manufacturing costs and manual work requirements. Thus, there is a need for a paint roller manufacturing method in which a high quality, solvent-resistant paint roller can be fabricated in a single continuous on-line process. 
     SUMMARY OF THE INVENTION 
     The invention provides a method, system and apparatus for manufacturing paint rollers through the use of an extruder employing a rotating head so that a plastic core can be extruded and rotated simultaneously while other process steps including application of an outer absorbent material are performed downstream in a single continuous process. The result is a reduction in manufacturing cost compared to prior methods, and a high quality paint roller product, in particular, a solid rigid core that is highly durable and resistant to water and solvents. 
     In a preferred embodiment of the invention, a cylindrical polypropylene core is extruded. The rotating core then translates through a vacuum sizing and cooling chamber. Next, the core passes through a winding and pulling station which is coordinated with the drive unit of the extruder. The core is subsequently plasma-treated prior to extruding an epoxy adhesive layer on the external surface of the core. Finally, the core is heated, wrapped with fabric and cut into discrete paint rollers. All of the steps are performed in a continuous time- and location-coordinated procedure with minimal if any manual involvement. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a schematic flow chart illustrating a paint roller fabrication method in accordance with a preferred embodiment of the invention. 
     FIG. 2 is a perspective view of an extruder employed in the present invention to produce a cylindrical core for a paint roller. 
     FIG. 3 is a sectional view of the output end of an extruder employing a rotatable head in accordance with a preferred embodiment of the present invention. 
     FIG. 4 is a perspective view of a vacuum sizing and cooling chamber used in the present invention. 
     FIG. 5 is a side view of a plasma discharge unit used in the present invention. 
     FIG. 6 is a side view of an extrusion assembly used to apply epoxy in a preferred process embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a system and process for producing paint rollers in a continuous process including extrusion of a highly solvent resistant core and bonding of an absorbent fabric material downstream. The continuous processing feature of the invention allows significant reduction in manufacturing costs and improvement in overall efficiency and product quality compared to prior processes. 
     FIG. 1 is a schematic diagram illustrating process steps in a preferred embodiment of the invention. The first step  20  involves extrusion of a plastic cylindrical pipe or core  21 . Extruder  22  receives plastic resin, preferably polypropylene, through hopper  24 . The plastic resin melts and is extruded through rotating head  26  into a hollow cylindrical form, core  21 , that rotates around axis  28 , while translating forward at a constant velocity, under control of drive unit  29 . The size of extruder  22  is based on the plastic resin used and the desired output in pounds per hour of plastic core  21 . In a preferred embodiment of the invention, a 3½ inch 24:1 L/D air-cooled Meritt Extruder from Meritt Davis Corporation, as shown in FIG. 2, is used along with a rotating extruder head, as shown in FIG. 3, from Guill Tool &amp; Engineering Company of Rhode Island. 
     Further details of extruder  22  and head  26  are illustrated in FIGS. 2 and 3. FIG. 2 shows extruder  22  including hopper  24  at one end for receiving raw materials such as polypropylene pellets or particles for subsequent melting and extrusion through rotating head  26 , as shown in FIG. 3, which is attached to output end  30  of extruder  22 . FIG. 3 is a drawing of rotating head assembly  26  including die  31  that defines the outer diameter of extruded core  21 , and tip  32  that defines the inner diameter of extruded core  21 . The gap between die  31  and tip  32  defines the thickness of core  21 . Sprocket or gear  33  facilitates rotation of die  31  and tip  32  around rotational axis  28 . Seals  34  prevent melted material from reaching bearings  35  that are used to permit smooth rotational movement of die  31  and tip  32  relative to the outer housing. In the present invention, die  31  and tip  32  rotate at approximately between 100-140 rpms. 
     Rotating head  26  is powered by SCR drive motors in drive unit  29  to drive both die  31  and tip  32  of head  26  to allow for the extrusion to be rotated in an exact relationship to the forward movement of the extruding core. Controls are used to maintain a plus/minus ratio of 0.01-percent between extruder  22  and head  26 . This allows for maximum control of the helical angle at which the core rotates in relation to the forward motion of the core to assure a uniform seam at the fabric application station downstream. Preferably, core  21  moves 3.375 inches of lineal forward movement per 360 degrees of rotation of the core. This is required to accommodate the 2.875 inch slit width fabric used to cover the core downstream. The motors that control the rotation of head  26  are adjustable for fine tuning to assure proper butting of the seam in the fabric application step of the process. 
     In the second step  37  of the process shown in FIG. 1, plastic core  21  enters a vacuum sizing and cooling tank  38  where a vacuum is applied to the exterior of core  21  along with chilled water spray that cools core  21  down to a “Freeze Point” of about 225° F. This is the point at which full stability is achieved in the plastic. The dimensions are set with a tolerance of +/−0.005 inch to the outside and inside diameters. Typically the inside diameter of the paint roller is 1.485 inches. The wall thickness is 0.045 inch with larger walls as required by the professional market. 
     FIG. 4 shows a perspective view of vacuum sizing and cooling tank  38 . Vacuum sizing and cooling tank  38  can be procured from Extrusion Services, Inc. of Akron, Ohio. Tank  38  employs a stainless steel tunnel or chamber  39  through which the core translates after extrusion. Polypropylene core  21  is approximately 500° F. when it exits head  26  of extruder  22 . By the time the core reaches entry end  39   a  of chamber  39 , core  21  has cooled to approximately 400-450° F. Water jets or spray inside chamber  39  continue to cool core  21  so that by the time it exits output end  39   b  of chamber  39 , core  21  is approximately 200-225° F. 
     In the third step  40 , as illustrated in FIG. 1, core  21 , after traveling approximately 2-3 feet from output end  39   b  of chamber  39 , enters guide  42  made of a hollow steel mandrel that has an inside diameter of 0.010 inches larger than the outside diameter of plastic core  21 . The length of guide  42  is about 30 inches. The end of guide  42  is located within approximately 2 inches from the leading edge of winding belt  44 . Belt  44  is configured to pull and continue rotation of plastic core  21 . Winding belt  44  is controlled by drive unit  29  of extruder  22  so that winding belt  44  precisely maintains the rotation rate and translational velocity of core  21  to match the rate at which core  21  is exiting and rotating from extruder  22 . After core  21  exits winding belt  44 , it enters a second hollow steel guide  46  of the same diameters as first guide  42 . This allows core  21  to be properly aligned and positioned for the next step. Coordination of the rotational drive functions of extruder head  26  and winding belt  44  on opposite sides of cooling chamber  38  is an important feature of the invention because it allows core  21  to be rotated, in sync with fabric wrapping downstream, without deformation of the core&#39;s cylindrical shape even when core  21  is somewhat fluid as it exits extruder  22 . 
     In the fourth step  50  of the process illustrated in FIG. 1, the external surface of core  21  is treated with high voltage electrical plasma in order to attract and accommodate adhesive applied in the next step of the process. In a preferred embodiment a surface treater obtained from Intercon Industries Corporation of Wisconsin is used. The surface treater employs a corona discharge head including two electrodes that generate an air blown electrical arc to form a treatment plasma. The corona discharge electrodes are positioned approximately ¼ inch away from the external surface of rotating core  21 . The plasma treatment increases the surface energy and tension on the outer surface of plastic core  21  which allows easier application and improved adhesion of epoxy in the next step. As shown in FIG. 5, surface treater  52  employs corona discharge head  53  which includes electrodes  54  and  56 . Electrode  54  generates a plasma treatment area  57  that is elliptical in shape on the external surface of core  21  as it rotates and translates past surface treater  52 . Similarly, electrode  56  generates a plasma treatment pattern  58  on the external surface of core  21  adjacent plasma treatment pattern  57 . Other numbers of heads, electrodes and combinations of treatment patterns can be used. The important thing is that, given the rates of forward movement and rotation of core  21 , the overall treatment should totally cover the external surface of core  21 . 
     Between plasma treatment step  50  and the next step  60 , as shown in FIG. 1, core  21  should have at least about 3-4 seconds to react before application of epoxy in step  60 . A thin layer of epoxy is applied to the surface in step  60 . This is accomplished by use of gear pumps for both the “a” and “b” resins, driven by an SCR-type motor which extrudes a thin film of epoxy onto the surface of plastic core  21 . Adhesive resins which work well for this application are sold under the trademarks MASTER 5200A and 5200B, and MASTER GRIP 5200A and 5300B, which are available from Fielco Industries of Huntingdon Valley, Pa. 
     FIG. 6 illustrates an epoxy extrusion unit for dispensing a thin layer of epoxy resin, parts A and B, on the external surface of core  21 . A cross-section of core  21  is seen in FIG. 6 with its axis of rotation perpendicular to the page. Core  21  is held against V-block  22 , in part, by the fabric wrapping unit downstream. A doctor blade or knife bar  23  is positioned near the external surface of core  21  for the purpose of metering the thickness of the adhesive layer being applied to core  21 . The thickness of the extruder adhesive layer is preferably approximately 3-5 thousandths of an inch. Subparts of the adhesive are combined and mixed in dispenser  24  prior to dispensing the adhesive through tip  25  near the edge of doctor blade  23 . 
     Once the film is applied, it is heated to 300° F. by use of a parabolic infrared heater in step  70 . This allows the epoxy film to rapidly set so the continuous core can be cut in a short amount of time. 
     Fabric is introduced in step  80  at the proper helical angle to match the angle of core rotation originally established by extruder  22  and rotating head  26  at the beginning of the process, and as perpetuated by winding belt  44  in the third step  40  of the process. The fabric is guided onto the core by use of a fabric stand and tensioning device. The tension on the fabric is preferably about 5-7 pounds to assure that the fabric is embedded into the epoxy film. Core  21  travels approximately 30 feet after fabric winding step  80  to accommodate completion of epoxy curing. 
     Finally, in step  90  fabric-covered core  21  enters traveling cut-off saw  92  which is programmed to cut the core into pre-selected lengths. The cut cores are discharged onto an accumulation table. The core lengths are then ready to be processed into finished lengths and packaged. 
     An experiment was performed to measure how much the core shrinks as it cools after exiting the extruder head. In accordance with the methods described above, the outer diameter of the core as it left the extruder head was 2.010-inches. The core translated from the extruder head to the cooling chamber without any internal chuck or support structure. When the core entered the cooling chamber approximately 4-inches downstream from the extruder head, the outer diameter of the core was approximately 1.690-inches. Upon exiting the cooling chamber, 16-inches further downstream, the outer diameter was 1.595-inches. The temperature transition over this distance was approximately 470° F. to 90° F. The significant amount of contraction which occurred as the core cooled demonstrates the importance of enabling extrusion of a rotating core into free space, without any internal chuck, support structure, or any other type of molding device. 
     Although the invention has been disclosed in its preferred forms, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense, because numerous variations are possible. Applicants regard the subject matter of their invention to include all novel and non-obvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein.