Abstract:
A method for forming an embossed extruded plastics molding which comprises the steps of extruding a plastics molding through a die to have a required profile, and passing the molding through an embossing press the embossing part of which is unheated, and operating a heater just ahead of the embossing part of the embossing press to flash heat a first (e.g. upper) surface/side of the extruded plastics molding, whereby the embossing part of the embossing press may press into and thereby emboss the previously heated surface.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention concerns improvements in and relating to embossed boards such as moulded plastics cladding boards used in the roofing industry. The invention provides an improved method and apparatus for forming embossed extrusions without warping and enabling fine embossing to be applied to extruded profiles.  
       BACKGROUND TO THE INVENTION  
       [0002]     Traditionally the fascia boards, barge boards and other cladding used around the roofs of buildings have been of wood that is painted or otherwise treated to optimise the preservation of the wood for an adequate working life and give an attractive appearance. Inherently, however, the wood does become progressively weather-beaten and rotten. Accordingly, more and more of late, extruded plastics cladding systems have gained favour for cladding roof ends and edges. These extruded plastics fascia boards and the like may be of a PVC-u foam core co-extruded with a skin of unfoamed PVC on one face to enhance strength and integrity and give an attractive external appearance to the board. Indeed, more recently we have developed such cladding boards where the PVC foam core is provided with a skin surrounding the board, being not simply on the upper face but also on the lower face of the board as is described in our co-pending UK Patent Application No: GB-2407793.  
         [0003]     Whereas the extruded plastics cladding systems are functionally far superior to the painted wooden cladding of old, nevertheless the aesthetic of painted natural wood nonetheless remains desirable. Indeed, for period properties the property owners would generally naturally prefer to have cladding that is as close as possible in appearance to natural wood. With this in mind, a number of systems have been developed over recent years to give a wood grain effect to extruded plastics mouldings.  
         [0004]     The most basic of these is a system that simply involves applying and adhering to the extrusion a foil which carries a graphic image of wood grain.  
         [0005]     To provide an alternative and in some ways more satisfying aesthetic finish and one which is inherently more robust than a foil coated finish, it would be desirable to effectively emboss the surface of the extrusion to give contoured patterned wood grain finish. However, existing embossing systems as used in embossing wallpaper, fabrics and floor tiles and the like, generally all involve the use of a hot oil filled embossing roller or similar to emboss the pattern on the product. We have found that use of such heated rollers for pattern-embossing extruded plastics products as used in the roofing industry or similar is not generally viable since the process can compromise the original extruded shape of the boards and generally does not allow for fine detailed embossing.  
         [0006]     It is a general objective of the present invention to provide a method and system for forming embossed extruded plastics mouldings such as, for example, cladding boards for roofs, which seek to overcome the aforementioned problems of the art.  
       SUMMARY OF THE INVENTION  
       [0007]     According to a first aspect of the present invention there is provided a method for forming an embossed extruded plastics moulding which comprises the steps of: 
        a) extruding a plastics molding through a die to have a required profile and, suitably, cooling the molding; and     b) passing the molding through an embossing press the embossing part of which is unheated, and preferably is cooled, and operating a heater just ahead of the embossing part of the embossing press to flash heat a first, e.g. upper, surface/side of the extruded plastics molding whereby the comparatively cold embossing press may press into and thereby emboss the heated surface        
 
         [0010]     Thus, for example, unlike prior embossing systems that generally all use a heated embossing roller, the method and apparatus of the present invention uses a cooled embossing part (e.g. embossing roller) of an embossing press and instead it flash heats the surface of the extrusion just before it passes into the nip of the cool/cooled embossing part. This allows for fine detail embossing of a profiled extruded plastics board without distorting the profile of the board.  
         [0011]     Preferably the heater is a convection heater, suitably being a blower of heated air such as, for example, a Leister™ heater.  
         [0012]     Preferably the heater is located within about 10 mm (e.g. 5 mm to 50 mm) from the first surface and preferably a similar distance from the embossing part of the embossing press. In the context of a convection heater, the spacing is the spacing of a heated fluid (air) outlet nozzle of the heater from the leading edge of the embossing part of the press.  
         [0013]     In most cases the embossing press comprises an embossing part that is an embossing roller and which presses down onto the extrusion as supported by a support element, suitably support roller, on the other side of the extrusion. The support element/support roller preferably is shaped to the profile of the corresponding surface of the extrusion that it contacts in order to provide optimal support to the extrusion and further mitigate against any risk of change in profile of the extrusion.  
         [0014]     Preferably the heating is applied through a heater that heats via a nozzle or other heat delivery component that substantially spans the width of the part of the extrusion to be embossed to deliver the heat substantially evenly across the width of the extrusion part to be embossed.  
         [0015]     To further protect against unintentional alteration of the overall form of the extrusion by the embossing process the embossed extrusion is suitably heated on the said other side of the extrusion prior to, while or shortly after leaving the embossing press. This heating of the other side is suitably the same or closely similar in magnitude and heat delivery configuration to the heating of the first side/surface to balance any relative heat expansion of one side of the extrusion relative to the other. By ‘same configuration’ we mean same in so far as if the heat delivery part/head of the heater before the embossing press spans the width of the extrusion then so too does the heat delivery part of the heater at the exit of the embossing press.  
         [0016]     The heating of the embossed extrusion suitably ‘steers’ the extrusion to continue in a straight level line substantially unaltered relative to its form and path on exiting the extruder. In one embodiment a first convection heater head spans the width of the extrusion with a ‘fishtail’ nozzle to stabilize the extrusion and prevent twisting while a second heater head without a fishtail nozzle but rather a broad bore outlet directs heating fluid more centrally (i.e. more toward the median line of the extrusion) at the extrusion to help straighten the extrusion.  
         [0017]     Suitably the heating is applied to locally heat the surface to a temperature of the order of 65 to 100, and preferably 70, degrees Celsius. By contrast, the embossing part (embossing roller) of the embossing press is suitably cooled/chilled, e.g. by cold fluid such as water ducted through or suitably over the embossing part, to a temperature of the order of 5 to 15, and preferably 10 to 12, degrees Celsius. The embossing press thus immediately begins to quench the plasticization of the surface of the extrusion being embossed, giving unprecedented control over the precision of embossing and mitigating against distortion of the profile of the extruded board that otherwise occurs.  
         [0018]     According to a second aspect of the present invention there is provided an apparatus for embossing an extruded plastics molding which comprises: 
        a) a frame carrying an unheated embossing roller and having an opposing support to support the extruded plastics molding passing through the apparatus, the embossing roller and support being movable together and apart; and     b) a heater positioned just ahead of the embossing roller to flash heat a first, e.g. upper, surface/side of the extruded plastics molding whereby the embossing roller may press into and thereby emboss the heated surface and the surface is cooled as it comes into contact with the unheated embossing roller.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     A preferred embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompany drawings, wherein:  
         [0022]      FIG. 1  is a schematic general assembly diagram of an extrusion production line for extruded plastics cladding boards;  
         [0023]      FIG. 2  is a side elevation view of an embossing apparatus of the preferred embodiment of the present invention that is adapted to fit to the extrusion line, suitably between the sizing unit exit of the extruder and the haul-off equipment;  
         [0024]      FIG. 3  is a rear (i.e. from downstream) top perspective view of the embossing apparatus;  
         [0025]      FIG. 4  is a rear perspective view similar to  FIG. 3  but from below; and  
         [0026]      FIG. 5  is a frontal perspective view of the apparatus.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     Referring firstly to  FIG. 1 , this shows a substantially conventional production line for forming extruded plastics cladding boards as used for roofing. The extruder  1  has an infeed hopper  2  to receive the plastics material to be extruded and which, within the body of the extruder  1 , is heated by heating coils in the wall of the extruder  1  body while being compressed and driven forward to the outlet end of the extruder  1  by drive screw  3  within the extruder  1  body and which is powered by variable speed electric motor  4 .  
         [0028]     The temperature within the body of extruder  1  is monitored via thermocouple  5  and a vent  6  in the body of the extruder  1  vents air from within the body of extruder  1 . A breaker plate  7   a  at the outlet end of the extruder body  1  ensures a homogeneous flow of compressed molten plastics out through die adapter and die section  7   b  and thence into sizing unit  8  where the single or dual skinned extrusion is formed to the required board profile dimensions.  
         [0029]     In the preferred example the board is extruded horizontally, molded as a sandwich of a PVC-ue (i.e. expanded PVC) core enveloped between upper and lower face skins of PVC-u, or with a skin of PVC-u only on one face, and with a profile that has ridges/channels running lengthwise of the board on the upper and/or lower faces to resemble, for example, a shiplap or double shiplap board appearance. One important feature of the embossing system of the present invention is that it preserves this profiled appearance of the profiled faces when embossing details into the upper/outer in use face as will be described shortly with respect to FIGS.  2  to  5  of the accompanying drawings.  
         [0030]     Once the extrusion has exited the sizing unit  8  it is rapidly cooled to set rigid by immersion in a water bath of cold/chilled water at primary cooling unit  9  just beyond the extruder exit and is then dried by a jet of air. Downstream of this the extrusion is hauled away by haul off equipment  10  prior to being cut to required lengths by a suitable saw/cutter  11 , whereafter it is collected and stacked  12 .  
         [0031]     Turning now to FIGS.  2  to  5 , the embossing facility  13  of the present invention fits into the production line of  FIG. 1  between the exit of the sizing unit  8  and the entry of the haul off equipment  10 , being about 2 m or so downstream of the primary product cooling unit  9 .  
         [0032]     The embossing facility  13  is an embossing press that has an upper embossing roller  14 , suitably of the order of 480 mm diameter, formed with embossing surface contours to represent woodgrain or other desired embossed effect, which bears down onto the upper face  100   a  of the extruded board  100  to press into and emboss that face  100   a , the board being supported from below by a support roller  15  vertically opposed to the embossing roller  14 . Support roller  15  has a profile (here double shiplap style) that is matched to the profile of the underside  100   b  of the extruded board  100  to provide optimal uniform support to the board and help maintain the extruded shape of the board  100 .  
         [0033]     The embossing roller  14  and support roller  15  are held within a framework  16  that has a crank shaft arrangement  16   a  that allows for adjustment of separation between the rollers  14 ,  15 , i.e. here allows for the upper, embossing, roller  14  to be moved upwardly and downwardly relative to the board  100  supported on support roller  15  to control the engagement, disengagement and depth/pressing force of the embossing process.  
         [0034]     The framework  16  of the embossing press also has a crank shaft arrangement  16   b  that allows for lateral adjustment of the support roller  15  relative to the embossing roller  14  within the framework  16 . In other words the support roller  15  can move somewhat (e.g. up to about 300 mm) from side to side relative to the longitudinal axis of the production line in order to fine tune alignment therewith or to serve as a means of finely adjusting the position of the embossed pattern if required.  
         [0035]     At its upstream end the framework  16  of the press carries a heater arrangement  17  configured to direct heated air onto the upper face  100   a  of the extruded board  100  just ahead of the nip between the embossing roller  14  and support roller  15 . For heating the two parallel planar surfaces of the double shiplap profile upper side  100   a  of the board heater arrangement  17  suitably comprises a parallel pair of 3.7 kW Leister™ heaters each with a respective fishtail nozzle  17   a , 17   b  that is thin in the direction of the production line but spans the width of the respective planar surface (e.g. 150-170 mm wide exit to each nozzle to correspond to each of the two 150-170 mm wide planar surfaces on upper side of board  100  that are to be embossed). The nozzles  17   a  and  17   b  are held by an adjustable gantry arm  18  to be adjustably positioned so that their exits are about 10 mm from the pinch of the rollers  14 ,  15  and also about 10 mm from the surface  100   a  of the extruded board.  
         [0036]     The heaters  17  supply air that exits the nozzles  17   a ,  17   b  at a temperature of about 250 to 280 degrees Celsius to serve in use to instantly heat the planar faces of the upper surface  100   a  of the board to about 70 degrees Celsius sufficient to plasticize just the surface  100   a  for embossing by the embossing roller  14 . The application of the heat can be described as flash heating since the heat is applied instantaneously to the board surface  100   a  just before it is embossed and the temperature of the surface  100   a  directly cooled by the embossing roller  14 . The heat has no time to penetrate deep enough into the board  100  to cause significant distortion to the profile and shape of the board.  
         [0037]     The embossing roller  14  is cooled to about 10 degrees to 12 degrees Celsius by a supply of chilled water that streams over the outside of the roller  14  or, more preferably, that is ducted through the interior of the roller  14  via a coiled heat exchange duct and enters and exits via the spindle shaft of the roller  14 .  
         [0038]     Just at the exit of the embossing press the framework  16  carries a further set of convection heaters  19 , the purpose of which is to reheat the extruded board  100  at the underside  100   b  to counterbalance any bowing of the board that may have occurred as a result of the heating and pressing by the embossing press and to steer the board  100  straight. A first convection heater head  19   a  spans the width of the extrusion with a ‘fishtail’ nozzle to stabilize the extrusion and prevent twisting while a second heater head  19   b  without a fishtail nozzle but rather a broad bore outlet directs hot air more centrally (i.e. more toward the median line of the extrusion) at the extrusion to help straighten the extrusion.  
         [0000]     Example Operational procedure  
         [0039]     Preliminary checks should firstly be carried out to check the machine operation and adjustments including checking that guarding is in place and secure, checking heater settings and thermocouples, checking top  14  and bottom  15  rollers for free rotary movement and top roller  14  for upward and downward adjustment, all before starting the embossing process.  
         [0040]     Next the extruding machine  1  is started up and run to begin extruding to a satisfactory standard. When pulling through the extruded profile  100  the top roller  14  is adjusted well away from the profile  100  to ensure a clean pull through.  
         [0041]     The profile  100  should run on the bottom roller  15  and the extruder should be operated to give a flat—in specification product. It should be ensured that the blowers that dry the water cooled extrusion exiting primary product cooling unit  9  are operational. The bottom roller  15  should be adjusted laterally into alignment with the extruded product  100  using the crank shaft  16   b  and the product should fit exactly to the bottom roller  15 .  
         [0042]     Once product is running to specification the top roller  14  is adjusted down until it just touches the profile  100  but not forced down to its embossing position yet. The Leister heater  17 ,  19  settings should be checked and they should then be switched on and the top roller  14  then lowered carefully until an initial light print is achieved. The machine is let to run  10  meters of product and the top roller  14  lowered further to give a more in-depth print. Once the roller  14  is producing a deeper embossed product the machine setting is corrected and print is checked.  
         [0043]     To shut down the embossing facility the top roller  14  is first moved upwards away from the product  100  and the Leister heaters  17 ,  19  are then turned off.