Process for producing shaped articles by starting from reinforced thermoplastic sheets

Abstract: A PROCESS FOR FABRICATING MANUFACTURED ARTICLES FROM FIBERGLASS REINFORCED THERMOPLASTIC SHEETS (1), WHICH CONSISTS OF FORMING SAID PLATES WHILE THEY ARE HEATED AT THEIR MELTING POINT AND COUPLED WITH A BARRIER FILM (2), BY EITHER BLOW-MOULDING OR THERMOFORMING.

Description: The present invention relates to a process for transforming reinforced thermoplastic sheets into manufactured articles, by heating and subsequent moulding. The technology of flow-moulding in order to transform fibreglass-reinforced sheets of thermoplastic material [polypropylene, poly-(ethylene terephthalate), polyamide, and so forth] is known; such sheets are known ad "glass mat thermoplastic" (GMT) sheet or "reinforced thermoplastic sheet" (RTS). The sheets, suitably cut to the desired size ("blanks") are heated by means of different types of heating procedure and means (irradiation, convection, contact, microwaves, etc)., until they reach the softening point of the polymer which constitutes the matrix thereof. The blanks are then charged to the hollow of the mould and, with the aid of a press, they are forced to flow and conform to said mould hollow. For that technology, high clamping pressures are required (of the order of 200 bars). The technology of blow-moulding for manufacturing box-shaped structure by starting from thermoplastic polymers, is known as well. The necessary equipment is constituted by an extruder, an extrusion head with circular cross-section, and a blowing unit (mould bearing press, with inlet system for compressed air). The thermoplastic polymer, fed to the extruder, leaves from the head with circular cross-section in tube form (parison) and is fed to the true blowing step, in which the parison is closed between two half-moulds and is conformed to the interior hollow of the mould, by means of compressed air. Also well-known is the thermoforming technology, which makes it possible shaped manufactured articles to be produced by starting from sheets (or films) of dense thermoplastic polymer, heated and shaped inside a mould, by being forced to deform by means of compressed air, or vacuum application. Blow-moulding technique, as it was first conceived and implemented in the existing facilities, is only used in order to transform polymers fed as a loose raw material, and not in order to transform semifinished articles (such as, e.g., sheets). On the contrary, the thermoforming technology is used with polymeric sheets, but these -- in order to be transformed into shaped articles by the action of compressed gas, or of vacuum, should be of dense (i.e., gas or vacuum tight) type. In the case of RTS sheets, which become porous in the molten state, the operation of fabrication by blow-moulding or vacuum-thermofoming only causes the molten mass to undergo some swelling, in that a sufficient barrier effect to pressure or vacuum does not exist. FR-A-2627127 describes a process for preparing panels comprising a self bearing body (a cloth impregnated with a plastic material) covered with a layer of a soft material, wherein an air-tight sheet is interposed between the cloth and the soft material and the sandwich structure is pressed against the mould to get the desired end form. In the process of FR-A-2627127, however, no significant deformation and/or stretching of the plastic impregnated cloth occurs since it actually lacks consistency and the pressure differential applied to it has only the effect of a better conforming to the surface of the mould. GB-A-903986 describes a process for vacuum forming thin sheets of a foamed thermoplastic material covered with a thermoplastic film, whereby the film becomes definitely bonded to the foamed thermoplastic material. The process however does not suggest any possible application to moulding of high density reinforced thermoplastic sheets. GB-A-2088281 describes a process for making flexible containers from a composite material comprising a polymer layer coupled with organic reinforcing yarns capable of changing their lengths when the material is shaped, said composite material necessitating a curing treatment at the end of the shaping step in order to set the shaped condition. No consideration is given to blow moulding or vacuum thermoforming of high density fiberglass reinforced sheets. The present Applicant found now that a process can be accomplished, which applies the technologies of blow-moulding and vacuum-thermoforming also to fiberglass reinforced thermoplastic sheets (RTS) in order to transform said sheets into hollow, or variously shaped, manufactured articles. According to the present invention there is provided a process for transforming reinforced thermoplastic sheets as defined in claim 1. Particular embodiments of the invention are the subject of the dependent claims. The process according to the present invention uses properly selected barrier films, suitable for securing air-or vacuum-lightness during the manufactured article forming step (whether carried out under pressure or depressure), and furthermore makes it possible pressure values to be used, which are considerably lower than as necessary for the technology of flow-moulding, i.e., of the order of bars (typical values are comprised within the range of from 6 to 12 bars ), instead of hundreds of bars. Such a process provides a considerable simplification of the processes known in the art, with consequent reduction in costs of machinery, of tools (moulds) used, and in operating costs of production cycles. The present invention is disclosed now by referring to the accompanying drawings, supplied for merely exemplifying, non-limitative purposes, in which: Figure 1 shows a schematic view which illustrates a first form of practical embodiment of the process for fabricating double-shell, box-shaped manufactured articles; Figure 2 shows an analogous schematic view to Figure 1, and illustrates a second form of practical embodiment of the process in order to fabricate single-shell, shaped manufactured articles. The process according to the present invention essntially requires the use of barrier films 2, to be applied to RTS sheets 1, and performing the following functions: secure tightness to air or vacuum during the blowing or intake step; preventing any mutual bonding and/or welding of the sheets in the molten state, from occurring in those zones of said sheets in which said events are undesired. In order to practice the process, a heating oven 3; a conveyor system to move the sheets from said oven to the mould; a low-operating-pressure press 4; a suitably arranged blowing/intake nozzle 6 are used. For exemplifying, non-limitative purposes, the following forms of practical embodiment are disclosed in the following: A) Case of blow-moulding Referring to Figure 1, one pair of reinforced thermoplastic sheets 1 are arranged inside a heating oven 3, mutually facing, with two films 2 being interposed between them, which display a high enough gas tightness, and have a size which is larger than the outline of mould hollow, and smaller than the outline of the sheets to be moulded. According to an alternative route, the films can be positioned on said sheets after the heating step. In any case, the heating of the sheets is carried out until the softening point of the polymer which constitutes the matrix of the sheets, or a higher temperature, is reached, so that the sheets are easily conformable but retain high enough strength in order to be drawn. At the end of the heating step, the sheets are collected by a conveyor system (not shown) and are charged between two half-moulds 5 inside the press 4, so that said barrier films 2 are facing to each other. Between said films a nozzle 6 for compressed air injection is arranged. The press is clamped, compressed air is fed and vacuum is possibly applied, through the mould 5. The sheets are forced to conform to the hollow of the mould, and at the edges, not protected by the films 2, the welding takes place. B) Case of moulding by thermoforming under air pressure, and optional vacuum application Referring to Figure 2, according to a second form of practical embodiment of the process of the invention, one single reinforced thermoplastic sheet 1, to which a barrier film 2 is juxtaposed, is charged to the heating oven. The resulting set up (i.e., the assembly consisting of the sheet 1 and of the barrier film 2) is subsequently positioned between a half-mould 5a and a counter-plate 7 with the barrier film being interposed between the latter and the sheet to be moulded. Through the centre of the counter-plate 7, a nozzle 6 injects air between the counter-plate 7 and the barrier film 2, forcing the sheet 1 to undergo the required deformation, and conform against the half-mould 5a, through which vacuum can optionally be applied. For some of polymers used (as an example, polypropylene may be mentioned), the novel technology can be used in a continuous process which starts, rather than from prefabricated RTS sheet, directly from polymer granule or powder and from fiberglass, according to a suitable formulation, then integrating, in one single on-line, continuous process, the production of the sheets and the fabrication of the manufactured articles. As said hereinabove, the barrier films perform the basic function of preventing air losses during the blowing or intake step and also of preventing the molten sheets from undergoing bonding and/or welding in those zones in which such events are undesired. Said barrier films can be of different nature: synthetic or natural polymeric films, metal films, coatings, and so forth; in particular, 50-200 microns thick poly-(ethylene terephthalate) films, the sheets of rubber latex and the siliconic rubbers with sheet thicknesses comprised within the range of from 200 to 500 microns can be reminded here. Said films must have a size which is larger than the outline of the mould, in order to be tightly sealed on mould clamping; and smaller than blanks outline, in order not to interfere with the welding of RTS sheets, also caused by press clamping. When one single blank is moulded, the barrier film can surely be re-used. On the contrary, when box-shaped, double-shell structure are manufactured, a zone in the mould must be provided in which the air injection nozzle must be installed, the free opening of which should be such as to enable the barrier films to be recovered -- which, otherways, would be lost. The advantages offered by the process according to the present invention can be better specified as follows: * Decrease in equipment costs The classic equipment for blow moulding is considerably reduced: neither the extruder, nor the circular-cross-section extrusion head are used any longer. Furthermore, a high-pressure press is not required, which, on the contrary is necessary for flow-moulding. This is so, because according to the present novel technology, the press is only requested to counteract low blowing pressures (typical values comprised within the range of from 6 to 12 bars, or even lower) and, for double-shell, box-shaped structures, must be capable of securing the welding of the peripheral edges of the shaped body to take place. * Decrease in mould costs Thanks to the low pressure values used, the moulds can be made, instead of steel, also from aluminum, plastics or wood. Furthermore, heating the mould is no longer necessary: the cold-moulding technique is used. * Short production cycles The production cycles are the same as provided for flow-moulding: from 30 to 60 seconds. * Possibility of different typologies of shapes Are as already mentioned hereinabove: double-shell, box-like shapes; asymmetrical-drawing, double-shell, box-like shapes; single-shell contoured shapes; keeping in mind that in those zone in which maximal stretch is expected, a supplementary blank can be charged in order to make it possible a higher wall thickness to be obtained in the end manufactured article. * Different thicknesses or different compositions of the two used layers Sheets with different sheet thicknesses can be used (such as, e.g.: a double thickness for prefabricated sheet and a reduced thickness for non-drawn, flat portion). Furthermore, there can be used, e.g., a sheet with continuous filament reinforcement for low drawing percentages (and high mechanical properties), and a chopped-filament reinforced sheet for higher drawing rates, or a sheet with a cloth-type reinforcer agent (which will not be drawn), together with a chopped-filament reinforced sheet. Obviously, double-shell, box-shaped structures with different percentage contents of fibreglass in the two constituting layers, are possible. Also sheets of different polymeric materials can be used, with the proviso that mutual compatibility should exist in the welding area. * Possibility of applying external coating films (between the mould and the blanket) By pre-arranging a coating film inside the interior of the mould, finished articles can be obtained with a desired aesthetical or functional finish (e.g., a scratch-resistant, soil-resistant, antistatic finish). Furthermore, coating films can be used, which are heated together with the blank and undergo the drawing during the moulding step, with finished articles being thereby obtained with desired end characteristics (e.g.; a soft outer feel, by coating with soft polypropylene films). If the mould is embossed, pieces with soft feel, embossed surface finish and variable fiberglass content can be obtained as well.

Claims: A process for transforming reinforced thermoplastic sheets (RTS) which become porous in the molten state into hollow manufactured articles, by heating and subsequently moulding the sheets (1), said process comprises the following steps: i) endowing at least one fiberglass reinforced thermoplastic sheet (1) with gas tightness characteristics by means of a barrier film (2) put into contact with said sheet (1) ii) heating the sheet (1) to the softening point of the material which constitutes the matrix of the sheet or to a higher temperature so that the sheet is easily conformable but retain enough strength in order to be drawn; iii) positioning said gas-tight, heated sheet on a half mould (5;5a) provided with at least one hollow; iv) clamping the mould (5;5a,7) and subjecting the heated, gas-tight sheet to a pressure differential between the two opposite faces of the sheet (1) in the range of from 6 to 12 bars, sufficient to draw the sheet to conform to the concavity of the half-mould; v) remove the article from the mould and detach the barrier film (2) from the moulded article. A process according to claim 1, whereby the process comprises the following steps: a) positioning one single gas-tight, heated reinforced thermoplastic sheet (1) between a half mould (5a) and the counter plate (7) with the barrier film (2) being interposed between the counter plate (7) and the sheet (1) to be moulded; b) clamping the mould and forcing the sheet (1) to conform the concavity of the half mould (5a) by injecting compressed air between the counter plate (7) and the barrier film (2). A process according to claim 1, whereby the process comprises the following steps: a) positioning two gas-tight heated reinforced thermoplastic sheets (1) between two half-moulds (5) with the sheets (1) facing each other and having the barrier films (2) between the reinforced thermoplastic sheets; b) clamping the mould (5) and forcing the sheets (1) to conform against said half-moulds by using compressed air which is fed between said barrier films (2), allowing the sheets (1) to bond or weld to each other at the edges not protected by the films (2), whereby to obtain a double-shell box-shaped manufactured article.

Inventors: ADDEO ANTONIO DR, BONARI ROBERTO, VITALI MARIO ENG, ADDEO, ANTONIO, DR., BONARI, ROBERTO, VITALI, MARIO, ENG.
Applicants: HIMONT ITALIA, MONTELL ITALIA S.P.A.
Application year: 1992
Main classification: B29C51
IPC codes: B29C51, B29C49, B29L22