Apparatus for applying a cover sheet to the surface of a mould

Abstract: A method and an apparatus for applying a cover sheet (12) of plastic material to the surface (18) of a mould (10), during the moulding of panels and/or of plastic pieces. The cover sheet (12) is pneumatically held and stretched by a support frame (11) provided with a sheet clamping groove (35), which peripherally extends for at least part or the edges of the mould (10); the support frame (11) is provided with an air suction chamber (26) connected to the sheet clamping groove (35) by at least an air suction slot (32, 34) which opens on the bottom of the groove (35). The pneumatic support frame (11) for the cover sheet (12) and the mould (10) are movable between each other for pneumatically adhering the cover sheet (12) to the edges and the internal surface of the mould (10).

Description: The present invention refers to an apparatus for automatically applying a plastic sheet material on a surface inside the shaping cavity of a mould, during the moulding process of panels and/or of plastic pieces according to claim 1 which are thus partially or totally provided with an aesthetic or impermeable exterior cover. PRIOR ART In the production of pieces in plastic material, for example in the production of polyurethane foam panels or paddings to be used in soundproofing and thermal protection of environments, motor vehicles, or for any other application, as well as in the production of shaped articles, it is often required the application a cover sheet of plastic material during the moulding phase; for example, in the moulding of polyurethane foam pieces, it is required a suitable proofing in order to prevent the foam from absorbing moisture or water, deteriorating with time and making its thermal and/or soundproofing features worse. In order to overcome such inconveniences, or to prevent the polyurethane material moulded pieces from deteriorating, an impermeable cover is usually provided by plastic sheets previously applied to the internal surface of the mould, during moulding. Moreover, moulding techniques by injection or thermal compression of pieces in plastic material requiring for the application of a cover sheet on at least one side of the moulded piece, are well known. Although the moulding procedures by pouring, injection and/or thermal compression of pieces in plastic material can be carried out with totally automated apparatuses, presently the step of applying a cover sheet to the internal surface of the shaping cavity of a mould is difficult to automate and therefore manual operations are often required. The conventional systems presently in use, starting from a rolled-up film, generally require for the use of a double clamping frame comprising two superimposed frame members which are arranged over and below the plastic film from which the sheet material to be applied onto the mould is cut, or require for the use of support frames comprising pins, clamping means, or co-operating with suitable chains. These conventional systems have remarkable drawbacks as they do not allow for a complete automation and for a correct stretching and positioning of the sheets; in particular they involve a complex construction and provide a low efficiency as a mechanical holding at spaced apart points not always assures a perfect stretching of the plastic sheet, or it does not assure a good sealing of the sheet against the mould edges during the vacuum forming of the same sheet against the shaping surface of the mould to be covered; in this way tearing and breaking often occur along the edges of the covering plastic sheet. A conventional clamping or holding frame, besides the above mentioned inconveniences, is moreover quite difficult to be used with several types of moulding presses, or with moulds having different shapes and dimensions. DE-A-2630021 describes a pneumatic support frame for clamping a plastic sheet suitable for shaping casting shells. According to this document the frame comprises first and second air suction slits connected to an air suction chamber and parallely arranged on the support frame. The sheet is merely clamped by pneumatic forces orthogonally arranged to the front face of the frame, which do not prevent the sheet to slip out when subjected to relevant stretching forces; pneumatic retention of the sheet and sealing action against the edges of a mould, could therefore be negatively compromised. US-A-3,954,537, US-A-4,116,736 and EP-A-0 281 724 relate to some examples of vacuum assisted moulding with cover sheets. OBJECTS OF THE INVENTION An object of the present invention is therefore to provide an apparatus for applying a cover sheet onto the surface inside the shaping cavity of a mould, in the production of panels and/or of plastic pieces provided with a covering, which overcomes the inconveniences of the traditional systems and which allows for a complete automation of the manufacturing, independently from the type of press in use and from the moulding technology of the production plant. A further object of the invention is to provide an apparatus for applying a cover sheet, as above referred to, which makes use of a particular stretching frame for the sheet material, which has no mechanical clamping means, assuring a continuous holding of the sheet for the entire or along a specific portion of the peripheral edges; moreover damaging of sheet material is thus avoided while it is maintained in a stretched condition till a complete adhesion of the sheet against the surface of the mould. A further object of the invention is to provide for an apparatus as referred to above, which makes use of a pneumatic clamping frame of special design, allowing for a complete automation of the moulding operations and for an easy accommodation to any type of press and moulds. Therefore, the present invention is suitable for any requirements and for both cold and hot application of plastic films depending on the moulding technology. What above is achievable by an apparatus for automatically stretching and applying a plastic sheet to a shaping surface of a mould making the same sheet adhere to peripheral edges at the mould, the apparatus comprising: a sheet-clamping support frame peripherally arranged around the mould, first and second air-suction slots extending on the support frame; an air suction chamber in the support frame operatively connected to said air-suction slots; and control means to move said support frame in respect to said mould, between an advanced position, in which the support frame is positioned away from said mould to pneumatically clamp and stretch the plastic sheet, and a retracted position in which said support frame is adjacent said mould to position sealingly the plastic sheet against the peripheral edges of the mould, characterised in that said support frame comprises a front open sheet-clamping groove formed thereon; in that said first and second air-suction slots open in spaced apart positions at the bottom of the said clamping groove; said first and second air-suction slots being orthogonally arranged each others, and extending parallely to corresponding front and side walls of the support frame; and in that air-suction ducts and control valves are provided to selectively connect the air-suction chamber of the support frame, and mould to a vacuum source. BRIEF DESCRIPTION OF THE DRAWINGS These and further features and developments of the invention, will be more clear from the following description, with reference to the accompanying drawings, in which: Figs. 1 to 3 show the main steps of applying a plastic sheet with an apparatus according to the invention; Fig. 4 shows a top view of the support frame for pneumatically holding a plastic sheet, for an apparatus according to the invention; Fig. 5 is an enlarged side view of part of the front side of the support frame of Fig. 4; Fig. 6 is a further enlarged sectional view taken along line 6-6 of Fig. 5; Figs. 7 and 8 schematically show the use of the apparatus with the mould of a press opening at 90°; Figs. 9, 10 and 11 schematically show a further embodiment for the application of a plastic sheet conjointly to two mould members of a press opening at 180°, by a common pneumatic support frame; Figs. 12 to 17 schematically show the use of apparatus according to the invention for a press having a mould opening at parallel planes; Fig. 18 shows a practical embodiment of the apparatus for applying a plastic sheet to the horizontal mould member of a press opening at 90°; Fig. 19 shows a practical embodiment of the apparatus for applying a plastic sheet to the vertical mould member of a press opening at 90°. DETAILED DESCRIPTION OF THE INVENTION With reference now to Figs. 1 to 6 hereinbelow we will describe the general features of the apparatus according to the invention. Figs. 1 to 3 schematically show the sequence of the more significant operative steps of the apparatus according to the invention, to apply a cover sheet of plastic material onto the internal surface of the shaping cavity of a mould member 10, by a support frame 11 peripherally extending around the mould 10 for pneumatically clamping and stretching a sheet 12 for example of plastic material, which is unrolled and cut from a bobbin 14 of a plastic film. The frame 11 for holding or supporting the sheet 12, as hereinafter explained with reference to Figs. 4 to 6, is constituted by a special annular frame comprising tubular members provided with an air suction front slot, which is selectively connectable to a vacuum source 15 or air suction device through a respective pneumatic circuit comprising a solenoid actuated control valve 16; correspondingly, the mould 10 comprises suction ducts 17 opening on the internal surface 18 of the shaping cavity of the same mould, which are selectively connectable to the vacuum source 15 through a respective pneumatic circuit comprising a solenoid actuated control valve 19. The frame 11 is movably supported with respect to the mould 10, for example by pneumatic cylinders 20, to be moved between a retracted or lower position, where it surrounds the mould 10, and an advanced or upper position 11' where the frame 11 gets in contact with a plastic film 13 previously unrolled from the bobbin 14. More precisely, the plastic film 13 is unrolled from the bobbin 14 drawing it by a suitable gripper device 21 to slide over the pneumatic holding or support frame 11; thereafter the film 13 is locked by a locking device 22, and then it is pneumatically clamped against the support frame 11 which in the meantime has been moved toward position 11' and connected to the vacuum source 15. Successively, a plastic sheet 12 is cut by a knife 23, and then the frame 11 with the plastic sheet pneumatically held in a stretched condition, is lowered till the sheet 12 is tightly pressed against the edges of the mould 10 (Fig. 2); optionally heating of the plastic sheet 12 may be performed at a predetermined temperature by an infrared ray heating device IR, or by any other suitable heating means, depending on the nature of the plastic film used or on moulding requirements. At this point, by maintaining the air suction with the support frame 11 and therefore by maintaining the cover sheet 12 in a stretched condition, vacuum may be generated in the mould 10 by connecting the suction ducts to the vacuum source 15, till making the plastic sheet 12 completely adhere to the internal surface of the mould cavity; this condition is shown in the scheme of Fig. 3. At this point, after providing, for example, for pouring a metered quantity of a polyurethane mixture, or for the injection of a plastic material, or for layering one or more sheets of thermoforming material, the mould member 10 is closed by a cover or moved on a similar mould member, which in turn may be pre-arranged with a cover sheet by the above described method. In the closed cavity of the mould a shaped piece will be formed having the outer surface, coated on one or both sides with a layer of plastic sheet material ready for a successive use. In the example of Figs. 1 to 3, the support frame 11 is supposed to be movable toward the film 13, independently from the mould 10; however several solutions are possible, providing for example the frame 11 and the cylinder system 20 to be an integral part of the same mould, which in turn may be fixed or movable with the same frame 11 toward the cover sheet 12. The innovative features of the pneumatic support frame 11, which use is a substantial part of the method according to the invention, are hereinafter described in detail with reference to a preferred embodiment, illustrated in Figs. 4 to 6 of the accompanying drawings. As shown in said figures, the frame 11 has an annular form, for example quadrangular or closed on each side, performed by tubular sections 25 defining a suction chamber 26, annularly extending around the sides of the frame, and connectable to a vacuum source in the above mentioned mode. The tubular sections 25 preferably comprise a rectangular cross-section having the smaller side wall which is parallely extending to the main plane of the mould; the tubular sections 25 in turn are provided with a plurality of suction holes 27 on a smaller side 28 facing the plastic sheet 12, as well as a plurality of holes 29 on outer side wall 30 which is orthogonal to the previous one, the holes 27 and 29 being in a back position with respect to the corner defined by the walls 28 and 30. A bar 31 is longitudinally fixed to the narrow wall 28 of the tubular member 25, overlapping the holes 27 in a position slightly spaced apart from the same wall 20 to form a narrow slot 32 where the suction holes 27 open. Correspondingly, on the outer side wall 30, in correspondence to the holes 29 a second bar 33 is longitudinally fixed to the tubular member 25 and is similarly spaced apart to form a second slot 34 where the suction holes 29 open. The bar 31 is provided in a back position with respect to the corner of the tubular member 25, while a bar 26 extends beyond the upper side wall 27 to form together with the bar 31 a longitudinal groove 35 facing toward the plastic sheet 12 and in which slots 32 and 34 open. This particular embodiment of the pneumatic frame 11, in the example of Fig. 6 and the particular arrangement on orthogonal planes of the suction slots 32 and 34, which open toward the bottom of the groove 35, have proved to be particularly advantageous as they tend to create by suction, stretching forces for the film 13 or the plastic sheet 12 in a rearwardly oriented direction of the slots 32 and 34 for stretching and continuously holding the plastic sheet 12, thus avoiding excessive stresses or tears on the same sheet 12; this is possible as the sheet 12 is pneumatically locked for the entire periphery of the frame 11, resting at the bottom of the groove 35. In the example of Fig. 6, two slots 32 and 34 are shown, which are arranged in orthogonal planes between each other; however other solutions or arrangements or the use of one or more suction slots are possible, depending on the requirements and features of the plastic sheet 12 to be pneumatically stretched and held. In order to accurately control the thickness of the two suction slots 32 and 34, suitable spacers (not shown) are provided between the bars 31, 33 and corresponding side walls 28 and 30 of the tubular member 25; for example said spacers may be provided in correspondence to the fastening screws 36 and 37 screwing in corresponding threaded holes of the tubular section 25, while a weld bead 38, 39 provides for the necessary fastening on the side of the slots 32, 34 opposed to the groove 35. So far the fundamental features of the apparatus according to the invention for applying plastic sheets to the surface of a mould, in the production of panels, polyurethane foam or plastic pieces with an impermeable protective cover have been described, as well as a preferred embodiment of the pneumatic support frame. The apparatus has been described in the production of polyurethane foam pieces and in its general features, with reference to a generic mould where the pneumatic frame could be a separate or an integral part with the mould itself; however the same apparatus may be applied to other moulding processes, as initially referred. Several specific solutions are also possible, which may be suitable for moulds of different type and for the opening system of the moulding press. For example in the previous figures, the frame 11 had an annular flat form, closed on each side; however, it is possible to differently make the support frame, for example with an open side, or with a three-dimensional shape, providing for suitable air sucking means for the cover sheet 12 on one front face of the same frame, for pneumatically holding the plastic sheet 12 along a continuous sucking line extending for the entire peripheral outline of the frame or for a part of the same. In this way it is possible to retain the cover sheet 12 in a stretched condition along all or few of its sides, in the desired positions, by an extremely simple, reliable and easily automated solution for any type of press. This is shown, as an example, in Figs. 7 and 8 for the application of the plastic sheet 12 to the two mould members of a press opening at 90°. In Figs. 7 and 8 the same reference numbers have been used for each similar or corresponding parts with respect to the previous figures. Fig. 7 shows in particular the application of a plastic sheet 12 to the horizontal mould member 10.1, while Fig. 8 shows the application of the plastic sheet 12 to a vertical mould member 10.2. The operating mode of the apparatus of Figs. 7 and 8 is quite similar to the previous one, illustrated with reference to Figs. 1 to 3; therefore the film 13 of plastic material will be unrolled from the bobbin 14, laid down in front of the mould, possibly heated at the desired temperature by an infrared ray heater IR, pneumatically taken by the support frame 11, cut and transferred in a stretched condition against the edges of the horizontal mould 10.1, or respectively of the vertical mould 10.2, where it will be made adhere to the internal surface at the mould by simple suction, as previously described. Figs. 9 to 11 show a further embodiment of the apparatus which makes use of a common pneumatic frame for both the mould members 10.3 and 10.4 in a press opening at 180°; for all the rest, the operating mode of the apparatus is at all the same of the previously illustrated one, with the only difference that now the frame 11 is to be removed or supported by a suitable system to move for example in the directions of the two arrows indicated in Fig. 9, in such a way that the frame 11 can be moved between a holding position for the plastic sheet 12, or a position for applying the sheet to the mould, and back or spaced apart position in which allows for filling in the mould cavity with a metered quantity of polyurethane mixture before closing the same mould; of course each process phase may occur in the same position on a fixed press, or in different positions of a general plant of revolving type. Therefore in Figs. 9, 10 and 11 the same reference numbers have been used for similar or corresponding parts with respect to the previous examples. Figs. 12 to 17 show a further embodiment particularly suitable for presses opening parallely to the closing plane of the moulds, in which the use of an auxiliary double pneumatic frame is suggested to move the plastic sheets 12' and 12" to the support frames 11 for the upper mould member 10.5 and respectively for the lower mould member 10.6. As shown in the figures, the apparatus comprises an auxiliary double frame 40, in an intermediate position between the support frames 11 in the open condition of the mould; the auxiliary frame 40 comprises two tubular frame members 25' and 25", of the type described in Fig. 6, which are side by side arranged and connected each other; the auxiliary frame 40 is greater than the pneumatic frames 11 of the mould, as schematically shown in Fig. 13 to allow the removal during foaming steps. The double pneumatic frame 40 is arranged in an intermediate position between the two plastic sheets 12' and 12" which have been unrolled by suitable bobbins, not shown, and is suitable for pneumatically holding the same for the time necessary to allow the transfer of the same sheets to the two support frames 11 of the two mould members, which in the meantime were approached for pneumatically clamping the sheets 12' and 12" retained on the auxiliary frame 45. When the sheets 12' and 12" have been released by the auxiliary frame 40, the same sheets are transferred by the frames 11 against the moulds members 10.5 and 10.6, Fig. 14, and are made adhere to the internal surface of the mould, Fig. 15, as previously described; successively, a metered quantity of a polyurethane mixture is fed, in the lower mould member 10.6 as shown in Fig. 16, and the mould is then closed to allow the polyurethane mixture to foam in the cavity of the mould and polymerise. The use of a double pneumatic frame 40 having the same functional features of the pneumatic frames 11 for the single mould members, is advantageous as it allows the adaptation of the apparatus to this type of press. Figs. 18 and 19 show a practical embodiment of a press opening at 90° embodying the apparatus for applying cover sheets of plastic material, according to the examples of the previous Figs. 7 and 8. In the shown case, the press constitutes a part of a general plant of revolving type, which has been partially shown to illustrate the station for applying the plastic sheets. In particular, in Fig. 18 the press is shown in an open condition in a station for applying the cover sheet to the horizontal mould member, while in Fig. 19 it has been shown in correspondence of a station for applying the cover sheet to the vertical mould member. The press is supported by a revolving table 41 to be moved along a circular path provided with several working stations. The press comprises a lower plane 42 for supporting the mould member 43, and an upper plane 44 for supporting the mould member 45; the upper plane 44 is hinged at 46 to the lower plane 42. Reference number 47 in Fig. 19 indicates a pneumatic cylinder which drives the rotating movement of the mobile plane 44, as well as operates a latching device 48 for the mould. Reference number 49 in Fig. 18 lastly indicates the station for applying a plastic sheet 12 to the lower mould member 43. The station 49 always comprises a structure 50 provided with a horizontal guide 51 along which a trolley 52 runs; the trolley 52 is provided with grippers 52' for holding and unrolling the plastic film 13 from the bobbin 14. The trolley 52 is moved along the guide 51 by any drive device, for example by a pneumatic cylinder or a chain drive device, not shown, while reference number 53 in the same figure indicates a infrared ray heating device, movable along the horizontal guides 51. Reference number 54 moreover indicates a locking device for the film 13 while 55 indicates a cutting device for cutting the sheet 12; lastly, reference number 11 indicates again the pneumatic support frame for the cover sheet 12, connected to the cylinders 20 according to the previous examples. Fig. 19 of the accompanying drawings, on the contrary, shows an apparatus for applying a plastic sheet to the vertical mould member of the open press; in this case the entire unit for unrolling the plastic film, comprising the support for the bobbin 14 and the vertical guide 56 for the trolley 57 of the grippers 58, respectively for sliding the infrared ray heating device 59, is in turn supported by a carriage 60 which is movable along a horizontal guide 61 fixed to the structure 50 of the whole plant. From what said and shown in the accompanying drawings it is therefore evident that a completely automated apparatus have been provided for applying cover sheets to the internal surface of moulds, in the production of panels and/or articles in plastic material, comprising the use of a special frame provided with pneumatically actuated gripping means for gripping and stretching a cover sheet, provided with an air sucking slot peripherally extending to the mould; in this way an apparatus has been provided having a high degree of reliability and adaptability to any type of press. The proposed solution overcomes the inconveniences of the conventional holding systems; in particular it allows for holding and perfectly stretching the plastic sheet on each side, or some of them, both with planar support frames of closed or open type, and with frames having any required three-dimensional shape. However, it is to be intended that what above said and shown with reference to the accompanying drawings has been given only as an example of the general features of the apparatus according to the scope of the appended claims.

Claims: An apparatus for automatically stretching and applying a plastic sheet (12) to a shaping surface of a mould (10) making the same sheet (12) adhere to peripheral edges at the mould (10), the apparatus comprising: a sheet-clamping support frame (11) peripherally arranged around the mould (10), first and second air-suction slots (32, 34) extending on the support frame (11); an air suction chamber (26) in the support frame (11) operatively connected to said air-suction slots (32, 34); and control means (20) to move said support frame (11) in respect to said mould (10), between an advanced position, in which the support frame (11) is positioned away from said mould (10) to pneumatically clamp and stretch the plastic sheet (12), and a retracted position in which said support frame (11) is adjacent said mould (10) to position sealingly the plastic sheet (12) against the peripheral edges of the mould (10), characterised in that said support frame (11) comprises a front open sheet-clamping groove (35) formed thereon; in that said first and second air-suction slots (32, 34) open in spaced apart positions at the bottom of the said clamping groove (35); said first and second air-suction slots (32, 34) being orthogonally arranged each others, and extending parallely to corresponding front and side walls (28, 30) of the support frame (11); and in that air-suction ducts and control valves (16, 19) are provided to selectively connect the air-suction chamber (26) of the support frame (11), and mould (10) to a vacuum source (15). An apparatus according to claim 1, characterized in that said support frame (11) comprises at least one tubular frame member (25) defining the air suction chamber (26) and provided with suction holes (27, 29) spaced apart on a side wall (28, 30), and a bar member (31, 33) longitudinally extending over said wall (28, 30), said bar member (31, 33) being slightly spaced apart from the side wall of the tubular member (25) to define one of said air-suction slot (32, 34), said air-suction slot (32, 34) being connected to the air-suction chamber (26) by said suction holes (27, 29). An apparatus according to claim 1, characterised in that said support frame (11) and the control means (20) for moving the support frame (11) between the advanced and retracted positions are provided in the form of integrated part of the same mould (10). An apparatus according to claim 1, in particular for press opening at 90° characterised in that separated pneumatic support frames (11) for clamping plastic sheets (12) are provided for both mould members of the mould (10). An apparatus according to claim 1 in particular for press having mould-supporting planes opening at 180°, characterized in that a single pneumatic support frame (11) for clamping plastic sheets (12) is provided for both mould members (10.3, 10.4) of the mould (10). An apparatus according to claim 1 in particular for a press having upper and lower mould members (10.1, 10.2) opening on parallel planes characterized in that each mould member (10.5, 10.6) is provided with a respective sheet-clamping support frame (11) for clamping a cover sheet (12), and in that an auxiliary double pneumatic frame (40) is provided between both support frames (11) in the open condition of the mould (10), for holding a plastic sheet (12) on each side and for clamping each plastic sheet (12) to a respective support frame (11) of the mould (10).

Inventors: SBRANA LOREDANO, SBRANA, LOREDANO
Applicants: TECNOS SPA, TECNOS S.P.A.
Application year: 1997
Main classification: B29C51
IPC codes: B29C33, B29C44, B29C35, B29C51