FR2918594A1 - METHOD AND APPARATUS FOR MANUFACTURING A HOLLOW BODY OF PLASTIC MATERIAL FROM TWO SHEETS - Google Patents

Abstract

A process for producing a plastic hollow body by molding two sheets of molten plastic material into a mold comprising two cavities, said method comprising the steps of: a) making two sheets of molten plastic material; b) introducing these sheets between the cavities of the mold, c) the mold is closed and the hollow body is made by conforming the two sheets to the cavities of the mold.In this process, when the mold is closed, a device secured to it automatically comes to stretch. laterally and / or smooth the leaves.Apparatus for the implementation of such a method.

Description

Method and apparatus for manufacturing a hollow body of material

  The present invention relates to a method of manufacturing a hollow body and in particular, a fuel tank, made of plastic and from two sheets. It also relates to an apparatus suitable for said method.

  Fuel systems embedded in vehicles of various natures generally comprise devices that allow the ventilation of the hydrocarbons contained in the tank. They may also include devices for fueling the engine. Such devices are the connection between elements contained in the tank (valves, fuel pump, ...) and elements disposed outside thereof (canister, filler neck, ...). The passage through the tank wall must take into account the sealing requirements imposed by the current environmental standards (LEV II, PZEV for example). For this purpose, the reduction in the number and dimensions of the openings in the tank wall is a favorable factor in reducing the evaporative losses. But this makes it more difficult to insert and position components inside the tank. The application EP 1110697 in the name of the applicant discloses a method of molding a fuel tank using a parison in several parts so as to insert the accessories into the tank during its molding. For this purpose, a tubular parison is extruded, then, at the outlet of the die, subjected to two longitudinal cuts, along two opposite generatrices. This document recommends the use of a device for guiding and spacing the two parison parts thus obtained and for the purpose of introducing accessories into the tank during its molding. Ideally, this device should also make it possible to substantially flatten and maintain the two parison parts free of wrinkles so that they can be correctly applied between the mold cavities. This can be achieved with rollers and straight cylindrical rollers, whose spacing is adjusted at will. This simple technique (and theoretically sufficient) however has the disadvantage that the sheets obtained at the exit of the rollers

  -2 are still curved and their spacing can not be well controlled. Indeed, the plastic begins to freeze right out of the die and tends to remain in the form that it has conferred. In addition, these sheets hang freely between the cavities of the mold before closing it to perform the molding RAC and therefore, folds can reform at this stage. This phenomenon is all the more marked as the thickness of the leaves is irregular and the leaves are long. It is especially important when the leaves have a thickness that has been intentionally varied in a controlled manner. This phenomenon is responsible for a significant percentage of scraps and therefore strikes the profitability of industrial processes. The application FR 0610966 in the name of the applicant proposes to solve this problem by guiding the free end of the sheets (which hangs by gravity between the mold cavities) by an edge or tongs (preferably of metal cooled or coated with PTFE for example ) to flatten them and prevent wrinkling. Such a device must, however, be initiated at each cycle, when sheets are inserted between the mold cavities. Automatic devices (robotic) can be used for this purpose but these are generally relatively complex and expensive.

  It is therefore an object of the invention to provide a method and apparatus which makes it possible to solve this problem by using a smoothing device which automatically starts when the mold is closed. To this end, the invention relates to a method for manufacturing a hollow plastic body by molding two sheets of molten plastic in a mold comprising two impressions, said method comprising the following steps: a) two sheets are made of molten plastic material; b) these sheets are introduced between the mold cavities; c) the mold is closed and the hollow body is manufactured by conformation of the two sheets mold footprints. In this process, when the mold closes, a device secured to it automatically automatically extends laterally and / or smooth the sheets (ie to stretch them, flatten them). The method according to the invention is suitable for any hollow body and in particular for any hollow body inside which it is desired to introduce at least one accessory. It applies advantageously the manufacture of tanks to

  -3 fuel (RàC). By fuel tank is meant to designate a sealed tank, able to store fuel under various and varied conditions of use and environment. An example of this tank is the one that equips motor vehicles. In the remainder of the description, the terms hollow bodies and reservoirs are used in an equivalent manner. The hollow body obtained by the method according to the invention is made with a plastic wall, generally comprising an inner face on its concave portion and an outer face on its convex portion. Plastic material is any material comprising at least one synthetic resin polymer. All types of plastics may be suitable. Well-suited plastics belong to the thermoplastics category. Thermoplastic means any thermoplastic polymer, including thermoplastic elastomers, and mixtures thereof. The term "polymer" denotes both homopolymers and copolymers (especially binary or ternary). Examples of such copolymers are, but are not limited to: random copolymers, block copolymers, block copolymers and graft copolymers.

  Any type of thermoplastic polymer or copolymer whose melting point is below the decomposition temperature is suitable. Synthetic thermoplastics having a melting range spread over at least 10 degrees Celsius are particularly suitable. Examples of such materials are those having a polydispersion of their molecular weight. In particular, it is possible to use polyolefins, thermoplastic polyesters, polyketones, polyamides and their copolymers. A mixture of polymers or copolymers may also be used, as well as a mixture of polymeric materials with inorganic, organic and / or natural fillers such as, for example, but not limited to: carbon, salts and other inorganic derivatives, natural or polymeric fibers. It is also possible to use multilayer structures consisting of stacked and solid layers comprising at least one of the polymers or copolymers described above.

  -4 A polymer often used for fuel tanks is polyethylene. Excellent results have been obtained with high density polyethylene (HDPE). Preferably, the hollow body for which the method according to the invention is intended comprises a multilayer structure comprising at least one layer of thermoplastic material and at least one additional layer which may, advantageously, consist of a liquid-barrier material and / or gases. Preferably, the nature and the thickness of the barrier layer are chosen so as to limit as much as possible the permeability of the liquids and gases in contact with the wall of the hollow body. Preferably, in the case of a fuel tank, this layer is based on a barrier material i.e. a fuel-impermeable resin such as EVOH for example (partially hydrolysed ethylene vinyl acetate copolymer). Alternatively, the tank may be subjected to a surface treatment (fluorination or sulfonation) for the purpose of rendering it impervious to fuel. According to the invention, a reservoir is molded from two sheets of molten plastic material. These sheets may have been obtained by extrusion in two dies flat preferably just before molding the hollow body (to avoid having to heat / melt). However, very particularly preferably, these sheets are obtained by cutting a parison. Parison means an extruded preform of any shape, generally substantially cylindrical or tubular, which is intended to form the wall of the reservoir after molding.

  By extrusion, it is meant that the parison is derived from the melting and / or plasticizing of the plastic material in an extruder, then from its expulsion thereof through an extrusion head, which generally gives it a cylindrical shape. . By extrusion head is meant a set of metal blocks and a core comprising a passage for at least one stream of molten plastic material exiting an extruder. Such an assembly generally comprises at least one block for the distribution of the material in the form of an annular flow (or distributor), and in the case of a coextrusion head, it generally comprises at least one distributor per layer of material. The dispenser or distributors, if any, are traversed through and through by an orifice whose outlet end is generally substantially annular and which determines with the core, an annular outlet passage section for the molten plastic material. The stream of molten plastic that is fed from the extruder to the extrusion head is generally a cylindrical stream full of pressurized plastic material. In the case of a coextrusion head (intended for the extrusion of multilayer structures), there are generally as many feed or flow holes of cylindrical material. Generally, a die is attached to the extrusion head to give the parison the shape and thickness desired. According to this variant of the invention, the tubular parison must be transformed into two sheets, ie it must undergo at least one transverse cut and two longitudinal cuts. The transverse cut can be made by relative movement of the core and the mantle of the die, so as to temporarily interrupt the flow of material and thus, to cut the parison. Alternatively, it is possible to deposit under the die (or possibly integrate on its lower surface), tongs or knives to perform this cutting. As for the longitudinal cuts, they are usually made either with two expensive which may or may not be secured (fixed on) the die, or be made within the same die. This last variant is preferred because it makes it possible to automate a maximum the process and to facilitate the stops / starts of production campaigns. In this variant, preferably, the parison is first cut using flow dividers integrated in the die and then, the two cut parison portions are progressively flattened thanks to a progressive modification of the internal passage of the die to through which flow or flow of plastic material from the parison. This variant is the subject of the aforementioned application FR 0610966. Preferably, the parison has an adjustable thickness (ie it can be varied in a longitudinally controlled manner (according to a generatrix) and / or transversely (on the same section)) and this with the aid of at least one known device such as WDS (vertically movable core), PWDS (deformable ring), SFDR (shape punch or variable profile core, machined) or die slide (part inserted locally in the die: see US Pat. No. 5,057,267 to name of the applicant), integrated into the sector. Compared to the molding of a parison whose thickness is constant, this way of proceeding makes it possible to take into account the reduction in thickness which takes place during the molding (and in particular, the blowing) in certain parts of the parison, following non-constant deformation rates of the material in the mold. When sheets already cut and partially flattened are obtained at the outlet of the die, handling and transfer to the mold are much easier than in the processes of the prior art. It is therefore possible to reduce the necessary height between the outlet of the die and the impressions of the mold. This will decrease the residence time of the extruded material in the ambient air and thereby increase the temperature of the sheets, which will facilitate the subsequent molding process, particularly when it includes a component (s) attachment step (acessories). (s)) inside the parison, on its inner side, before final molding of the hollow body. Such attachment of components, which is advantageous in the context of the present invention, is for example described in applications WO 2006/008308, WO 2006/032672 and WO 2007/000454 in the name of the applicant.

  The transfer of the sheets to the mold can take place in any known manner. However, according to a preferred variant, the mold cavities are disposed below the die and the parison (leaves) is continuously extruded between the mold cavities, which are then closed on said leaves just before their transverse cutting and their molding.

  In the method according to the invention, a device integral with the mold stretches laterally and / or smooth the sheets automatically when the mold is closed (ie its footprints are closer to finally being contiguous on their periphery by trapping the edges of the leaves). By automatically, it is meant that it is the closure of the mold which initiates the device and that no further intervention is necessary for this purpose. By solidarity, is meant in fact that the device is either integrated in the mold (integrally with its imprints and / or its core, if any (see below), or fixed on it), or is directly actuated by that i.e. that the closing movement of the mold causes the actuation (priming) of the device In order to reduce the number of parts and to simplify the system, the device is preferably integrated in the mold. This device is preferably located in a part of the mold outside the molding zone and which is generally removed (deburred) in the form of scraps (manufacturing waste). In a first variant, the shape of the internal surface of the cavities and / or the external surface of a core inserted between the cavities of the mold in

  Outside the molding zone (generally in the lower part of the mold) is such that these parts themselves perform the smoothing. For this purpose, these parts may be provided with reliefs such as corresponding curved portions (concave and convex respectively) and / or wear jaws, clamps ... or other surface (s) having a mechanical attachment. In a second variant, the mold also comprises a core (part which is by definition intended to be inserted between the cavities of the mold and whose function will be explained later) and the cavities and the core are provided with complementary parts intended to cooperate when the mold closes to stretch / smooth the leaves. Preferably, these parts are located in the lower part of the mold outside the molding zone. Most preferably, they are located on the underside of the imprints and the core. They are preferably 8 in number, or 2 pieces on each footprint (4 pieces in total) intended to cooperate each with a complementary piece on the core in front of it (or 4 complementary parts on the core also). The 4 pairs of pieces are preferably each located facing a lower corner of the sheets so that each sheet sees its lower edge stretched by 2 pairs of pieces. One could also use 8 pairs of pieces (4 at the top and 4 at the bottom of the mold / core set). These stretching parts (smoothing) are preferably moving parts by rotation, by pivoting about at least one axis and / or by translation. The pivot axis - where applicable - generally passes through a point of attachment of the workpiece to the mold. Generally, a return device (spring for example) to return these parts in the initial position, when reopening the mold / core. Preferably, it is articulated arms (consisting of several articulated parts as are some windshield wipers for example) terminated by a kind of pad (smooth part or not), tip system / tip (or cone), toothed jaws, claws ... or any other part (s) having a mechanical attachment. It can also be wheels with toothed perimeter, knurled ... Note that these two variants (form of imprints / core and moving parts) can be combined. In the process according to the invention, the molding of the tank is carried out in one piece (in a single step at the end of which a tank is obtained in one piece, without resorting to an additional step of assembly of separate shells) and this usually by welding the sheets on their periphery when closing the mold. In particular, the reservoir is advantageously molded: by blowing, ie by expanding and plating the cut parison on the mold cavities by means of a fluid under pressure (as described in EP 1110697); • by thermoforming the parison ie by plating it on the impressions of the mold for example by sucking (evacuating) behind them.

  Preferably, the reservoir is blow molded. Indeed, thermoforming generally involves heating the mold at an elevated temperature to be able to achieve the deep deformations (corners of the tank for example, where there is a significant stretch of the parison). This results in longer cycle times than with blowing, where this constraint does not exist.

  The method according to the invention may involve the use of a core which is used during a first closure of the mold to fix accessories on the inner surface of the sheets. In this variant, there is preferably a device preventing the edges of the sheets from being welded during this first mold closure (when the parison is pressed against the impressions to fix the accessories on the parison). This device is advantageously integrated into the core. Therefore, it is preferably an insert having a shape and size appropriate to be partially (usually on at least a part of its periphery) inserted between the sheets to be welded. To facilitate welding, mold cavities are advantageously provided with a thermal control device for heating the welding zone during the steps concerned by the first closure of the mold (to fix one or more components), if necessary. The core may also include such a device (thermal regulation) in its / her zone (s) of contact with the parison, and in particular in the aforementioned welding zone. This variant could further improve the quality of the weld of the tank (by decreasing the internal bead and thus, by improving the impact resistance of the tank). Such apparatus is for example described in the application FR 04.13407 in the name of the applicant.

  In this variant of the method according to the invention, during the first closing of the mold, the sheets are first pressed onto the cavities of the mold (by blowing through the core and suction behind the cavities) and then, devices on the core (cylinders for example) come fix accessories on the plated sheets. Then, the mold is reopened, the core is removed and the mold is closed to weld the pre-molded sheets and make the tank by a final blow. The present invention also relates to an apparatus for implementing the method as described above. This apparatus consists of a mold comprising: two indentations each capable of molding a plastic sheet; and a device integral with the mold which, when it closes, is capable of stretching and / or smoothing the sheets automatically before molding.

  All the preferred features described above in the part of the description relating to the process are applicable to this apparatus (presence of a core, nature of the smoothing device, etc.). Figures 1 to 6 are intended to illustrate some concrete aspects of the invention, without wishing to limit the scope. They illustrate schematic views of the underside of apparatus according to certain variants of the invention respectively at the beginning of closure of the mold (odd figures) and at the end of closing of the mold (even figures). Figures 1 and 2 illustrate the apparatus in full; Figures 3 to 6 illustrate only one of its corners.

  In these figures, identical n denote identical elements, namely: 1. the impressions of the mold 2. its core 3. 4 pairs of moving parts fixed on the underside of the cavities (1) and the core (2) 4. the lower edge of the molding sheets which hangs by gravity between the indentations and the core of the mold. The closing direction of the mold is illustrated by vertical arrows on the two figs and the smoothing direction is indicated by horizontal arrows in Figure 2.

  Figures 1 and 2 illustrate a variant where the moving parts (3) are articulated arms integral with pads. When the impressions of the mold (1) close on the sheets (4) and on the core (2), the pads of each pair of arms (3) pinch the lower corners of the sheets (4). If the mold continues to close, the pads (3) slide outwardly and in doing so, smooth the lower edge of these sheets (4) thus eliminating any wrinkles that would have formed there. Figures 3 and 4 illustrate a variant where the movable parts (3) comprise complementary jaws (5) integral with one, a slider in translation (6) and the other, an articulated arm (7). In Figure 4, arrows indicate the respective movement of the pad (6) and the arm (7) and the manner in which this movement brings the jaws (5) closer to smooth the sheets (4). Finally, Figures 5 and 6 illustrate a variant where the moving parts (3) comprise eccentric wheels (8) provided with a return spring (9). In Figure 6, arrows indicate the direction of rotation of the wheels (8) and can be seen the smoothing effect that these wheels (8) exert on the sheets (4). These wheels have a perimeter toothed or knurled and they are based on PTFE.

Claims (10)

  For this purpose, the invention relates to a process for producing a plastic hollow body by molding two sheets of molten plastic material into a mold comprising two cavities, said method comprising the following steps: manufactures two sheets of molten plastic material; b) these sheets are introduced between the mold cavities; c) the mold is closed and the hollow body is manufactured by conforming the two sheets to the mold impressions, and being characterized in that, when the mold is closed, a device secured to it automatically automatically extends laterally and / or smooth sheets.   2. - Method according to the preceding claim, characterized in that the sheets are obtained by cutting a parison which has a variable thickness.   3. Process according to any one of the preceding claims, characterized in that the stretching / smoothing device is integrated in the mold.   4. - Process according to the preceding claim, characterized in that characterized in that the stretching / smoothing device is located outside the molding area of the mold.   5. Method according to the preceding claim, characterized in that the stretching / smoothing is carried out at least in part via an appropriate form of the internal surface of the impressions and / or the outer surface of a core inserted between them. , outside the molding area.   6. - Method according to any one of the preceding claims, characterized in that the mold comprises a core intended to be inserted between the cavities of the mold and in that the cavities and the core are provided with complementary parts intended to cooperate when the mold closes to stretch / smooth the leaves. - 12-   7. Process according to the preceding claim, characterized in that the complementary parts are located on the underside of the cavities and the core.   8. ù Process according to claim 6 or 7, characterized in that the complementary parts are 8 in number, or 2 parts on each footprint (4 pieces in total) intended to cooperate each with a complementary piece on the core in front of it (ie 4 complementary pieces on the core as well) and in that these 8 pieces constitute 4 pairs of pieces each located opposite a lower corner of the leaves so that each sheet sees its lower edge stretched by 2 pairs of pieces .   9. Process according to any one of claims 6 to 8, characterized in that the complementary parts are integral with a return device.   10. Apparatus for carrying out a method according to any one of the preceding claims and consisting of a mold comprising: two indentations each capable of molding a plastic sheet; and a device integral with the mold which, when it closes, is capable of automatically stretching and / or smoothing the sheets before molding.