FR2555496A1 - Moulding process using a lost mould - Google Patents

Abstract

The invention relates to a moulding process which consists in executing at least one model 4 of the article to be moulded, in producing at least one mould 13 from this model, in bringing a curable resin into contact with the moulding surface of the mould thus obtained, in allowing the resin to cure and then in demoulding the cured resin. It is characterised in that the mould 13 is produced by thermoforming, using a thermoplastic material, by heating the latter to its softening point, by applying a thin layer of the thermoplastic material thus softened to the model so that it adopts, in whole or in part, the shape of the latter, and then by allowing the thermoplastic material to return to its original consistency after it has taken up the shape of the model, before releasing it from the model.

Description

 The subject of the present invention is a new molding process which consists in executing at least one model of the object to be molded, in producing at least one mold from this model, in bringing a curable resin into contact with the molding surface. of the mold thus obtained, allowing the resin to harden and then removing the hardened resin from the mold.

 Known processes of this type for the production of molded objects made of plastic materials such as curable materials possibly reinforced for example with a fibrous material, are subject to numerous constraints, in particular with regard to the production of the model and the mold and the use of this mold. Indeed, the model being subjected to abrasion, torsion and / or tearing forces at the time of the model-mold separation, it must necessarily be made of a material which resists these forces, in particular if several molds must be executed from the same model. In addition, the surface of the model requires an absolute finish obtained by delicate operations of polishing cumbersome application of a varnish or enamel heat resistant and not likely to react with the material from which the mold is made. It follows that the production of the model is expensive.

 Furthermore, the production or mold, in particular from resins of the polyester or epoxy type, is also long, delicate and complex and, consequently, expensive, since it requires coating the model with a gel coat, drying it. the latter and the deposition of the resin over a largely dimensioned thickness, mechanical means also having to be provided to allow the mold-model separation which often remains laborious in spite of these means.

 On the other hand, the actual carrying out of the molding operation from the mold thus obtained requires the application of a release agent on the molding surface of the mold, the coating of a gel coat and its drying, application of the curable resin, waiting to allow the hardening of this resin and demolding, then the restoration of the mold (cleaning, repair of broken or furrows, application of a mold release agent, etc ...) before reuse.

 It thus appears that the molds used in the known methods constitute heavy, expensive, slow to develop instruments and requiring a work schedule which, even if it is extremely tight, does not make it possible to optimally manage the operations of manufacture of molded objects and the labor required to carry out these operations.

 The object of the invention is to remedy these drawbacks and to do this, it relates to a method of the type defined in the first paragraph of the present invention, which is characterized in that the mold is produced by thermoforming from '' a thermoplastic material by heating the latter up to its softening temperature, by applying a thin layer of the thermoplastic material thus softened on the model so that it matches all or part of the shape thereof, then allowing it to return to its first consistency the thermoplastic material having taken the form of the model before releasing it from the model.

 Contrary to the classic concept, the product resulting from thermoforming is no longer an end in itself, but an intermediate instrument in a chain of operations intended to lead to the production of a molded object: the thermoformed product from the model becomes mold.

 Due to the use for thermoforming of readily available, light and inexpensive thermoplastic materials, the molds obtained are inexpensive, light, of small volume and stackable, which makes their handling easy and their storage easy.

 Thanks to the thermoforming technique, it is possible to obtain molds of very different shapes and varied thicknesses by using thermoplastic material in the form of sheets available with various thicknesses. This technique also allows rapid work cycles and, therefore, obtaining a large number of molds in a minimum of time.

 Furthermore, because in accordance with the invention, the model is only covered with a thin and light layer of thermoplastic material at the time of thermoforming, the separation of model and mold is relatively easy and detachment can for example be obtained simply by blowing compressed air between the model and the mold. The model is therefore not subject to the mechanical stresses to which the models are subjected in the known methods; they can therefore be made with materials of low mechanical strength (such as wood or plaster) which can be easily worked and the production of said models is therefore quick and inexpensive. In addition, the thermoplastic material to be thermoformed is readily available under Extruded sheet form having a very good surface finish and the application of a varnish or enamel on the model before the thermoforming operation can be omitted with all the time savings that this represents.

 Given the low cost price of the molds produced in accordance with the invention and their speed of production, they can be torn if necessary at the time of demolding and the application of a release agent is unnecessary. According to a characteristic of the invention, a new mold is used for each molding; this leads to a significant saving of time compared to the methods of the prior art for molding operations, since the mold being in new condition, there is no longer the need for its reconditioning.

 Another object of the invention consists in a process for the optimal realization in time and in manpower of a series of objects and which is characterized in that it consists in making as many molds as there has objects to be molded, bringing the curable resin into contact with the molding surface of all the molds thus obtained and demolding the molded parts after they have hardened.

 This process is based on a new concept which no longer apprehends the mold as a valuable instrument to be profitable at all costs by carrying out as many molds as possible, but on the contrary like a "skin" to be thrown away after use. This process allows a linear work from the beginning to the end of the execution of the molded objects and no longer a work divided into imperative cycles of application of a molding agent coating with a gel coat-drying-filling with the resin - hardening, demoulding - cleaning, as in the prior art.

The molds mentioned above may consist of several mold elements capable of jointly defining the shape of
The object to be molded.

 Thus, the invention extends in particular to the molding process in which said mold elements define the mold and the corresponding counter-mold for the part to be molded, this process being characterized in that the curable resin is brought into contact with the molding surface of these mold elements by applying said resin to the molding surface of the mold and / or the counter mold before or after bringing the mold and the counter mold closer to their molding position.

 According to the technique of applying the resin to the molding surface of the mold and the counter-mold, the method is of the injection molding or compression molding type. In the latter case and more specifically in the case of cold press molding, the mold is placed on a support conforming to the shape of the latter and, in accordance with the invention, the counter mold is adapted to a tool conforming to the latter. , brings the mold and the countermold into their molding position by displacement of the tool, applies a molding pressure via the support and / or the tool, lets the resin harden and then withdraws the said tool before demolding the mold. molded object.

 Here again, the thermoformed mold imposes itself by the advantages mentioned above and by others which will be highlighted below.

The invention is illustrated in the following description given with reference to the accompanying drawings in which
FIG. 1 is a schematic view of an embodiment of a vacuum thermoforming machine with mechanical pre-stretching of a thermoplastic sheet,
FIG. 2 represents the thermoforming machine in accordance with FIG. 1, at the time of the pre-stretching phase,
- Figure 3 shows the thermoforming machine according to Figure 1, at the time of the actual thermoforming.

FIG. 11 illustrates the succession of steps implemented to reach the molded object from the thermoformed mold obtained in accordance with FIGS. 1 to 3, and
- Figure 5 is a schematic illustration of the cold press molding process using a lost thermoformed mold and countermold.

 As shown in Figure 1, the thermoforming machine comprises in a manner known per se, a model carrier plate 1 provided at its periphery with a flange 2 on which is disposed a seal 3. A model 4 of the object to be molded rests on the plate 1 by means of a seal 5 extending all along the periphery of said plate 1, the seal 5, the plate 1 and the base of the model 4 defining a chamber 6 connected by a duct 7 on the one hand, to an inlet 8 of compressed air provided with a shut-off and closing valve 9 and on the other hand, to a vacuum circuit 10 also provided with a valve d opening and closing 11. This chamber 6 is also in communication with vents 12 made in the model 4. The assembly thus designed is also dimensioned so that the upper edge of the model is located above the defined horizontal plane by the joint 3. A sheet 13 of thermoplastic material such as polystyrene, polycarbonate, poly (vinyl chloride) etc ..., put in place in a blank holder frame 14 is arranged above the machine which has just been described, heating elements 15, for example with electrical resistances, being themselves arranged above the sheet 13. Finally, the model holder plate 1 is integral with a rod 16 capable of being moved vertically by means of a jack 17.

 The valves 9 and 11 being in the closed position and the sheet 13 being brought to its softening temperature, the model carrier plate 1 is raised by means of the jack 17 so as to pre-stretch said sheet (FIG. 2). The vacuum is then established in the chamber 6 by opening the valve 11, after which the softened sheet 13 is applied to the model 4 and conforms to the shape of the latter (FIG. 3). thermoformed (for example by blowing cold air), opens the blank frame 14 to release said thermoformed sheet, closes the valve 11 then opens the valve 9, the compressed air brought into the chamber 6 and the vents 12 helping to separation of the sheet from the model then, the sheet being released from the model, the valve 9 is closed and the model carrier plate 1 is returned to its first position by means of the jack 17, the machine then being ready for a new use. The thermoformed sheet 13 thus obtained constitutes a mold from which the object to be molded will then be made. The thickness of this mold will obviously depend on that of the sheet of thermoplastic material used and it will be chosen to obtain a self-supporting and rigid mold; usually, this thickness is between 0.5 and 5 mm depending on the nature of the thermoplastic material constituting the sheet used.

 Model 4 must withstand the temperature of thermoforming and can for example be made of plywood or epoxy resin combined with an aluminum filler. Its surface must be free from excessively large defects capable of altering the glossy surface of the thermoplastic sheet 13 used.

 In the case where a large number of molded objects have to be manufactured, as many of the molds as there are objects to be molded will be prepared by the technique described above, or by any other thermoforming technique. preferably carried out on several work chains. As they are produced, the molds are subjected to the successive operations illustrated in FIG. 4. If need be, the mold 13 is firstly placed in a support 18 made of cement or plaster for example, matching the external shape of the mold.

 The molding surface of the latter is then preferably coated with a gel coat 19 which will impart the desired surface finish to the molded object.

This gel coat must be dosed with catalysts and accelerators to avoid any chemical or thermal damage to the mold. After gelation of the gel coat, a layer of curable liquid resin 20 is applied to it. The nature of the latter will obviously depend on the properties which the molded object will have to have and mention will be made, for example, of polyester or polyepoxide resins. Given the thermoplastic nature of the mold, these resins should preferably be able to polymerize cold by the intervention of catalysts and accelerators incorporated into the liquid mass and dosed to avoid an excessively exothermic reaction; if necessary, the support 18 can be provided with cooling means. These resins can also be reinforced with fibrous materials (laminates) such as glass or carbon fibers in the form of fabrics or mats.

 The application may be carried out by conventional techniques, in particular by contact or by projection.

 As the application of the possibly reinforced resin is generally hardly possible without going beyond the mold, this involves cutting the edges of the molded object.

 In the known methods of the prior art, the cutting work is done by leveling on the upper edge of the mold; in addition, the cut can only be carried out there when the resin has reached the consistency of the leather and this consistency only, which means that there is very little time to perform this operation. In addition, the latter is delicate, because it is necessary to avoid any deterioration of the mold and often despite all the precautions that can be taken, the cut is uneven due to the differences in hardness within the resin.

 By cons, according to the method of the invention, the cut (shown schematically by the arrows 21) can be made against the walls of the mold on a resin or a laminate ranging from "still soft" to "already hard". The time during which the cutting can be carried out is therefore increased compared to known methods. To cut in support on the thermoformed mold, it will suffice to provide a model and, therefore, a mold, higher than necessary. This cutting system also has the advantage of eliminating bubbles that have arisen during application of the resin on the upper edges of the mold. Eh erfet, the mold being higher than necessary, as was specified above, the zone containing the bubbles is outside the cup.

 The cutting operation once completed, the hardening of the resin is allowed to continue and then the molded object is removed from the mold, if necessary by purely and simply tearing the mold.

 All of the operations described above can without any inconvenience be carried out linearly, they can be managed as well as the personnel called upon to carry them out.

 In the embodiments described above, the lost thermoformed mold was considered to be constituted by a single element.

However, this mold can also be made up of several elements. This case is illustrated by FIG. 5 where a thermoformed mold 22 is placed on a support 23 matching the external shape of the mold. There is then placed on the molding surface of the latter a material 24 based on glass fibers for example, then a curable resin 25. Furthermore, the counter-mold 26 corresponding to the mold 22, also obtained by thermoforming from the object to be molded is fitted onto a tool 27 conforming to this counter-mold, the latter freely adapting to the tool. The tool 27 is then lowered by means of a jack 28 actuating a rod 29 secured to the upper part of the tool, so as to press the counter-mold 26 on the fibrous material 24-resin mixture 25. Once this mixture has started to harden, the tool 27 is raised, leaving the counter-mold 26 in place on said mixture. It is then possible to carry out the cutting operation (shown diagrammatically by the arrows 30) through the counter-mold 26 while resting on the mold 22.

 This constitutes an undeniable advantage compared to the cold press molding methods of the prior art. Indeed, in these methods, the tool, which also played the role of counter-mold, could only be released when the resin-fibrous material mixture was sufficiently hard, which made cutting often impossible, the latter then being replaced by tedious grinding and / or sanding operations.

 Once the cut has been made, the hardening process is allowed to finish and then the molded object is removed from the mold, possibly sacrificing the mold and the counter-mold to facilitate this release.

Claims (5)

 1. A molding method which consists in executing at least one model of the object to be molded, in producing at least one mold from this model, in bringing a curable resin into contact with the molding surface of the mold thus obtained, allow the resin to harden and then release the hardened resin, characterized in that the mold is produced by thermoforming from a thermoplastic material by heating the latter to its softening temperature, by applying a thin layer of the material thermoplastic thus softened on the model so that it matches all or part of the shape thereof, then allowing the thermoplastic material which has taken the form of the model to return to its original consistency before releasing it from the model.  2. Method according to claim 1, for the optimal realization in time and in labor of a series of objects, characterized in that it consists in producing as many molds as there are objects to molding, bringing the curable resin into contact with the molding surface of all the molds thus obtained and demolding the molded parts after they have hardened.  3. Method according to claim 1 or 2, characterized in that the mold consists of several mold elements capable of jointly defining the shape of the object to be molded.  4. Method according to claim 3, in which said mold elements define the mold and the corresponding counter-mold for the object to be molded, characterized in that the curable resin is brought into contact with the molding surface of these elements. mold by applying said resin to the molding surface of the mold and / or the counter-mold before or after bringing the mold and the counter-mold together to their molding position.  5. Method according to claim 4 of the cold press molding type in which the mold is arranged on a support conforming to the shape of said mold, characterized in that the counter-mold is adapted to a tool conforming to the latter, brings the mold and the counter-mold into their molding position by displacement of the tool, applies molding pressure via the support and / or the tool, lets the resin harden and then withdraws the said tool before demolding the molded object.