METHOD OF AND APPARATUS FOR THERMOFORMING SEMIFINISHED PRODUCTS EXTRUDED IN FORM OF CONTINUOUS, ALVEOLATE SHEETS OR PANELS, IN PARTICULAR OF POLYPROPYLENE

Abstract: A method of thermoforming extruded semifinished products (1) in form of continuous, alveolate sheets or panels, in particular of polypropylene is disclosed, wherein during the extrusion process the thermoforming is effected at an elasto-plastic state of the extruded semifinished product. Such a state is reached by means of heat generators able to soften the material of the semifinished product, the edges of which are then shaped by forming means so as to avoid stress and deformation of the sheet, thus obtaining a correct thermoforming of the edges of the continuous semifinished product (1). An apparatus for carrying out said method is further disclosed.

Description: This invention relates to a method of and an apparatus for thermoforming semifinished products extruded in form of continuous, alveolate sheets or panels, in particular of polypropylene, comprising the steps and the devices, respectively, of extruding, feeding, treating and finishing the products which are interacting and correlated to one another and the product obtained therefrom as well as the phases and the means for cutting, shaping and thermoforming the semifinished product and parts thereof obtained from the above mentioned processing steps. Under the term semifinished product of extruded, alveolate polipropylene the products are intended which are obtained from the homonymous plastomer and provided in the form of sheets or plates or panels having a partially hollow structure and formed by deforming said starting material at the elasto-plastic state and by shaping and settling the material after the stress generated by said deformation is disappeared. At the present status of art, as well known, the extruded, alveolate sheets cut as requested have hollow inner sections communicating with the outside. Such a structural form causes many troubles and drawbacks which affect the withstand of said semifinished products, in particular due to the inlet and deposit of powders generally suctioned by thermal convection between hollow cells and peripheral parts as well as to the growth of mildews caused by the inlet of spore-generating organisms. Another problem is caused by the roughness of the sharp edges which make the semifinished products awkward. According to the status of art the troubles due to the open cavities are avoided by closing such cavities or socalled "canes" by heat treatment with several thermoforming rollers. Even if good results have been achieved in the industrial production, a lot of several disadvantages due to the necessity of peripheral "out-of-machining" forming, i.e. a processing separate from the main extrusion step have been noticed. Moreover the deformation of the semifinished product as for the general flatness as well as the straightness of the edges has been detected. Furthermore the production rate is reduced and the matching of the forming rollers to the different thickness and type of material is hard with relevant loss of time in the starting phase and in the production of sheets according to the status of art as well. The negative factors causing an onerous industrial production are due to the operative discontinuity, the withstand to the movement, the uneven distribution of the heat necessary for correct peripheral forming and the time needed for the fitting of the above mentioned members according to the different forms. This invention relates, according to a first aspect thereof, a method of manufacturing a semifinished product in such a way as to avoid the above mentioned problems, wherein the forming is carried out directly during the step of extrusion of the sheet and the step of longitudinal and cross-cutting. The method essentially provide the steps of: - transmitting heat, in particular by convection and radiation, which is developed by heat generators on line with the extrusion means and is irradiated by suitable members non-contacting the parts of the semifinished products involved; - forming said parts to which heat has been transmitted during the preceding step so as to close the peripheral edges of the socalled "canes" cut by suitable trimming means preferably separated from said heat generators. According to an advantageous embodiment of the invention the radiating members of said heat generators have reduced size so as to be fed from corresponding heat generators of the same size. Furthermore the forming units are able to operate on the peripheral edges of the semifinished products or on other areas and parts involved at the elasto-plastic state of the material. In case high thermal levels are reached, it is possible to carry out a stabilization, i.e. a cooling of the machined parts or the heating members by preferably air flows of suitable temperature. This invention, according to a second aspect thereof, relates to an apparatus of structural thermoforming of an extruded semifinished product in form of continuous sheets or panels, comprising: - a heat transmission assembly of suitable size mounted on an adjustable platform near the parts of the semifinished product to be treated; - a forming assembly placed downstream of the heat transmission assembly; - a cutting unit longitudinally acting on the sheet produced by the extrusion machine as well as a crosswise acting cutting unit; - an air or other fluid treatment means associated to the thermoforming assemblies, which is able to produce a thermal variation in the material. This invention solves the above mentioned problems with a method and an apparatus providing a structural thermoforming of semifinished products, in particular of extruded, alveolate polypropylene, such that the troubles and drawbacks which can be ascribed to the semifinished products of the prior art cannot be ascribed to those resulting from the present invention. The advantages obtained with this invention essentially consist in the integration in a sole manufacturing process of all operations which were previously carried out in different processing steps. According to the prior art the extruded alveolate sheet (with the term alveolate sheet it is intended a sheet formed of longitudinal alveoli, socalled "canes", provided by the extrusion machine and consisting of a plurality of longitudinal cavities separated by horizontal and vertical walls) is produced and cut in the longitudinal direction by suitable rotating or stationary blades and in the cross direction by an intermittent blades, and only in a subsequent step the sheet is subjected to the action of thermoforming assemblies. According to the method and the apparatus of this invention the thermoforming assemblies are on line with the extrusion machine and provide first the softening of the peripheral edges which have been suitably cut and trimmed and then the junction thereof, thus forming the peripheral edges of a plurality of sheets subjected to said thermoforming action. The advantages of the method and the apparatus of this invention are self evident, in particular considering the continuos thermoforming of the semifinished products simultaneously with the process of extrusion of the sheets. The method and the apparatus of this invention for the thermoforming of semifinished products, in particular of extruded polypropylene having an alveolate structure, fully achieve the prefixed purpose with a high efficiency due to the correlation of the apparatus with the extrusion machine as well as to the treatment of the edges of the sheet without impairing the flatness thereof, the evenness of the thermal exchange between the areas involved, and the accuracy of the forming action. The method and the apparatus of structural thermoforming of semifinished products, in particular of extruded, alveolate polypropylene, according to this invention will be now described in detail with reference to a preferred, non-limitative embodiment shown in the annexed drawings, wherein: Figg. 1A and 1B are schematic views of a production line of continuous, alveolate sheets according to the prior art; Fig. 2 shows a side view of the thermoforming assembly according to the present invention; Fig. 3 shows a front view of the cutting assembly; Fig. 4 shows a schematic plan view of the thermoforming assemblies located at the sides of the peripheral edges of the continuous, alveolate sheet; Fig. 5 shows a schematic side view of the continuous, alveolate sheet with the cutting blades; Fig. 6 shows a cross section of the heat generating unit of the thermoforming assembly; Fig. 7 shows a cross section of the forming unit of the thermoforming assembly; Fig. 8 shows a section of another embodiment of the forming unit; Fig. 9 shows a section along the line X-X of Fig. 10 of the platform supporting the sheet; Fig. 10 shows a partial cross section along the line II-II of Fig. 2 of the support platform; Fig. 11 shows a perspective view of the station for the deflection of the continuous, alveolate sheets; Fig. 12 shows a schematic side view of another embodiment of the station for the deflection of the continuous, alveolate sheets; and Fig. 13 shows a perspective view of the embodiment of the station of Fig. 12. Figures 1 to 13 in reduced scale show the preferred embodiment of the apparatus carrying out the method for the structural thermoforming of semifinished products, in particular of extruded, alveolate polypropylene, and essentially comprising the units for the extrusion of the continuous, alveolate sheets, the cutting and the thermoforming thereof. Figures 1A and 1B show a side view of the units of the production line for the continuous, alveolate sheets, in particular of extruded polypropylene, according to the prior art comprising a conventional press 18 fed at one end by the charging hopper 19 and provided at the other end with the forming matrix 20 having an extrusion head able to produce the continuous, alveolate sheet 1 formed of longitudinal canes with horizontal and vertical walls as shown in Fig. 5. The thickness of sheet 1 is successively stabilized by calibration units 21 and 22 in synergic action with the compressed air flows emitted by nozzles associated to matrix 20 and forming the alveolate structure, i. e. the cavities 1b of sheet 1 which is fed by opposed belt conveyors 23 and 24. From belt conveyors 23 and 24 sheet 1 is guided between the opposed units 25 and 26 of an apparatus for the electroerosion of surfaces 2 and 2′ of sheet 1 which are thus prepared for the printing. After the electroerosion units 25 and 26 sheet 1 is conveyed by belt conveyor 27 provided with blades for cutting the selvedge and then fed to the shearing machine 28 where it is crosswise cut and deposited on the board 29 to form the pile 30. The device 31 between units 25, 26 and belt conveyor 27 is able to effect the flexographic printing of surface 2′ of sheet 1. As mentioned before, in order to avoid that the alveolate cavities of sheet 1 communicate with the outside the socalled "canes" should be closed by a thermoforming process acting on the side edges of the extruded sheet. In Figg. 2, 3 and 11 there is shown a cutting and thermoforming station for the side edges of alveolate sheet 1. Sheet 1 is cut by blades 34 and 34′ at the entrance into the cutting and thermoforming station. Blade 34 longitudinally cuts sheet 1 along an axis thereof, and blades 34′, as shown also in Fig. 5, cut and trim the edges of sheet 1, thus eliminating the selvedge which is deformed by thermal effect upon the extrusion, in order to provide two correctly aligned side edges which will be closed by the thermoforming assemblies. Sheet 1 is then passed about first deflecting rollers 32. The two halves of sheet 1 longitudinally cut by blade 34 are conveyed upwards and downwards, respectively, and passed about second deflecting rollers 33. Afterwards the cut edges of sheet 1 are subjected to a thermal action by assembly 6 which through electrical resistance 8 protected by insulator 9 produces heat which is irradiated through flanges 10 and 10′ and softens edges 4 and 4′ of sheet 1 guided between skids 11 and 11′ in which cooling water fed through conduits 41 is circulated. Sheet 1 is then passed along forming assembly 12, better shown in Fig. 7, which by means of its shaped sections 13 and 13′ joins by contact softened edges 4 and 4′, thus closing the side edge of alveolate sheet 1. The alternative embodiment of Fig.8 shows assembly 12 provided with ducts 14 and 15 conveying cooling air from duct 35 towards sheet 1 so that the heating action of assembly 6 does not impair the evenness of sheet surfaces and the alveolate cavities adjacent to the outer edge to be treated. According to this embodiment assembly 12 is formed of two specular halves 12a and 12b, the distance of which can be adjusted in order to adapt forming assembly 12 to the different thickness of the semifinished product 1. Fig. 4 shows the multiple arrangement of thermoforming assemblies 6 and 12 sidewise facing the edges of alveolate sheet 1. Thermoforming assemblies 6, 12 are mounted through guides 17 on a basement 37 fastened to a carriage driven by motor 36 effecting the fine adjustment of the gap between thermoforming assemblies 6, 12 and sheet 1. The fine adjustement serves to keep the gap between side edges 4, 4′ cut by blade 34 and thermoforming assemblies 6, 12 essentially constant. To this end support basement 37 is driven by an alternative step motor 36 responsive to a signal which is generated by a sensor (not shown) of said gap. Motor 36 draws near or move away assemblies 6,12 from sheet 1 so that the thermoforming action can be carried out under steady condition. The transmission of movement is effected by means of a bellow junction box 38 and an endless screw 39 engaging the threaded end of shaft 40 on which basement 37 supporting thermoforming assemblies 6, 12 is mounted (Fig. 10). In Fig. 9 there is shown the section along line X-X of Fig. 10 in which basement 37 supporting thermoforming assemblies 6, 12 through guides 17 and shaft 40 driven by step motor 36 are illustrated. In Fig. 12 and 13 another embodiment of deflection station for alveolate sheet 1 is shown. This arrangement allows a pair of deflection rollers to be eliminated by arranging thermoforming assemblies 6, 12 along an oblique path of sheet 1 which after being cut is deflected by rollers 32′ and conveyed at an angle upwards and downwards. Sheet 1 is subjected to thermoforming treatment along the oblique path as above described and then deflected again by rollers 33′ and 32′. The multiple thermoforming of alveolate sheets is thus achieved by the arrangement of several thermoforming assemblies able to close the alveolate structures of semifinished products at the edges of sheet 1. The present invention has been described on the base of a preferred embodiment thereof. It is obvious to the skilled in the art to modify such an embodiment without departing from the scope of this invention according to the appended claims.

Claims: 1. Method of thermoforming semifinished products extruded in form of continuous, alveolate sheets or panels, in particular of polypropylene, wherein the sheet formed of a plurality of tubular sidewise joined members is extruded and shaped by means of a matrix, characterized by following steps: - introducing the continuous, extruded sheet into a cutting and thermoforming station; - longitudinally cutting the continuous sheet along the edges for the trimming thereof and along at least one longitudinal axis for obtaining at least two continuous sheets; - deflecting the at least two sheets after their longitudinal cutting along different paths slanted with respect to the original feeding direction of the sheet; - thermoforming the continuous sheets which are subjected first to a thermal action to soften the cut side edges and afterwards to a shaping action to close said side edges together; - cooling said longitudinal side edges of the sheets to avoid their deformation due to the thermal action; - deflecting the thermoformed sheets to the same original feeding direction; - cross-cutting the continuous thermoformed sheets in predetermined lengths. 2. The method of claim 1 characterized in that during the thermoforming step the gap between the longitudinal side edges of the continuous sheets and the thermoforming means is kept essentially constant. 3. The method of claims 1 and 2, characterized in that said gap between the longitudinal side edges of the continuous sheets and the thermoforming assemblies is kept essentially constant by a transversal movement of the thermoforming assemblies with respect to the continuous sheet, which is controlled by a motor responsive to a gap variation signal generated by gap sensing means. 4. Apparatus for thermoforming extruded semifinished products in form of continuous, alveolate sheets or panels, in particular of polypropylene, comprising an extrusion station for a semifinished product in form of an alveolate sheet formed of a plurality of tubular sidewise joined members shaped by a matrix, a thickness adjustement station, a sheet conveying station and an electroerosion and printing station, characterized in that there is provided means for cutting the continuous sheet both along the longitudinal side edges for trimming the selvedge of the sheet and along at least one longitudinal axis, means for deflecting the longitudinally cut sheets along paths slanted with respect to the original feeding direction of the sheet, thermoforming means able to subject the longitudinal side edges of the continuous sheet to a thermal action to soften said edges and then to a shaping action to close said edges together, means for cooling the longitudinal side edges and means for deflecting the continuous sheet to the same original feeding direction. 5. The apparatus of claim 4, characterized in that said deflecting means consists of deflection and guide rollers about which the longitudinally cut sections of the continuous sheet are guided and deflected to at least two directions differently slanted with respect to the original feeding direction of the sheet. 6. The apparatus of claim 4, characterized in that the thermoforming means comprises a heating assembly provided with an electrical resistance directed to the longitudinal side edges of the continuous sheet and with heating flanges conveying heat and softening said edges as well as a forming assembly formed of concave die shaping by contact said softened edges and closing them together. 7. The apparatus of claim 4, characterized in that the cooling means comprises an air or other fluid source and a duct conveying said fluid to the longitudinal edge of the continuous sheet. 8. The apparatus of claim 4, characterized in that it further comprises means for maintaining the gap between the thermoforming means and the longitudinal side edges of the continuos sheet essentially constant. 9. The apparatus of claims 4 and 8, characterized in that said means for maintaining said gap essentially constant comprises gap sensing means able to feed a signal in response to the variations of said gap, step motor means responsive to said signal to drive the thermoforming assembly to a transversal direction with respect to the feeding direction of the continuous sheet. 10. Product obtained by the thermoforming method and apparatus according to claim 1 to 9 as essentially described and illustrated in the annexed drawing.

Inventors: BRESSAN, MATTEO
Applicants: KARTON S.P.A.
Application year: 1988
Main classification: B29C51
IPC codes: B29C57, B29C67, B29C35