FR2995551A1 - Mold for rotary molding device, has heating resistive unit that is incorporated in thickness of internal wall of each half-cavity, where temperature of internal wall is raised so as to raise internal temperature of mold - Google Patents

Abstract

The mold (1) has two half-cavities (2) that are independently attached together in a removable manner in a water-tight manner. Each of the two half-cavities is arranged with an internal wall (3), and a heating resistive unit is incorporated in a thickness of the internal wall of each half-cavity. Temperature of internal wall is raised so as to raise internal temperature of the mold. The heating resistive unit is arranged with a wire grid that is electrically conductive. Each of the cavities is provided as a hollow-envelope. An independent claim is also included for a rotary molding device.

Description

TECHNICAL FIELD The present invention relates to a mold and a rotomolding device for obtaining a reduced cycle time. PRIOR ART Rotational molding from a thermoplastic polymer material in solid form (powder or paste) or also thermosetting polymer in liquid form (to become solid during processing), also known as rotational molding, has been known for many years. many years. Known rotomolding devices or installations conventionally comprise a mold loaded with polymer as well as rotation means for rotating said mold at a speed of a few revolutions per minute around two distinct axes, one horizontal, the another perpendicular, said speed being a function in particular of the inner surface of the mold, the thickness of the workpiece and the density of the polymer. The set "rotational mold-moving means" is then placed in an oven to obtain the melting of the polymer contained in the mold. The combined movement of the two rotations of the mold allows the molten polymer to move into the mold along its inner surface and adhere thereto. When the molten polymer is homogeneously distributed over the entire inner surface of the mold to obtain the desired thickness of the workpiece to be made, the set "rotational-mold means" is placed in a specific cooling chamber for cooling the mold in order to achieve solidification of the molded piece resting on the inner surface of the mold, the cooling being carried out by a projection at the outside of the molding surfaces of air charges that can be combined with water misting. Once the cooling has been carried out, the mold is opened and the part produced is extracted. This molding technique, apparent simplicity, allows to manufacture complex and aesthetic parts, while ensuring a perfect control of the properties of the parts obtained. In addition, this technique makes it possible to produce multilayer walls. Finally, rotomolding is particularly suitable for large parts and / or thick, it can achieve at lower cost compared to the extrusion blow molding or other plastic processing techniques. However, this technique has a number of disadvantages. Firstly, given the cycle times "heating and cooling mold" high, rotomoulding is only feasible for small and medium series. In addition, for large parts requiring a large mold, it is necessary to have a large oven and a large chamber capable of accommodating said mold, said furnace and chamber then being particularly expensive facilities difficult to depreciate for small and average series.

Finally, in order to heat and cool the entire "rotation-mold means" respectively in an oven and a chamber, it is necessary to use a considerable amount of fossil energy (fuel or gas) that is particularly expensive to heat the reactor. the entire internal volume of the oven. This energy consumption is even higher than the installations (oven and chamber) are large. These three disadvantages are obstacles to the development of rotational molding, particularly in France. Thus, the object of the present invention is to provide a mold and an alternative to known rotational molding devices having a small footprint and significantly reduce cycle times. SUMMARY OF THE INVENTION and energy consumption. For this purpose, the subject of the present invention is a mold comprising two independent half-cavities capable of being secured together in a removable and substantially impervious manner, each of these two half-cavities having at least one inner wall, said mold being remarkable in that it comprises a resistive heating means incorporated in the thickness of the inner wall of each half-cavity to raise the temperature of said inner walls and therefore to raise the internal temperature of the mold. According to a preferred embodiment, the resistive heating means comprises a mesh of electrically conductive wires embedded in the thickness of the inner wall of each half-cavity, said mesh being preferably embedded in a dielectric resin. Advantageously, each of these two half-cavities is a hollow envelope having an outer wall and a hollow space located between said inner wall and the outer wall, said hollow space being arranged to receive a circulation of a coolant for cooling said mold. Each half-cavity is preferably made of a composite material whose main characteristics include a capacity to heat when it is crossed by an electric current, a very good thermal conductivity, a mechanical strength close to that of steel and a density significantly lower than that of aluminum. VIGOU1-FR-1_TEXTE DEPOSE_RG / GE According to a preferred embodiment, each half-cavity is a structure composed essentially of carbon whose cohesion is provided by a cyanate ester type resin.

Advantageously, the inner wall of each half-cavity is coated with a thin layer, called gelcoat, distributed uniformly, this additional layer to give a surface appearance to the rotomoulded part. This thin layer is preferably a mixture of cyanate ester resin and a thermally conductive filler such as carbon powder or metal powder. The present invention also relates to a rotational molding device comprising a frame arranged to rest on the ground, a main frame articulated to said frame along a substantially horizontal axis XX ', a secondary frame hinged to said main frame along an axis YY' substantially horizontal and substantially perpendicular said axis X-X ', and means for respectively rotating said primary and secondary frames, said rotomolding device being remarkable in that it comprises a mold according to the invention mounted on the secondary frame. Advantageously, the rotating means comprise a motor secured to the frame rotating, by a pulley-belt assembly, a shaft disposed along an axis XX 'and formed by two sections supported and guided in rotation by bearings integral with said frame, said sections being integral and arranged on either side of the main frame. Similarly, the rotation means comprise a motor secured to the main frame driving in rotation, by a pulley-belt assembly, a shaft disposed along an axis YY 'and formed by two sections supported and guided in rotation by bearings integral with said frame, said sections being integral and arranged on either side of the secondary frame. The rotomoulding device advantageously comprises an installation comprising at least one heat transfer fluid reservoir, a channeling circuit and a circulation pump allowing the circulation of said fluid from the reservoir into each half-footprint. of the mold.

BRIEF DESCRIPTION OF THE FIGURES Other advantages and features will become more apparent from the following description of an alternative embodiment of a mold and a rotomolding device according to the invention with reference to the appended figures in which: Figure 1 is an exploded perspective view of a mold for a rotational molding device according to the invention; FIG. 2 is a partial perspective view of a half-cavity of the mold for a rotomolding device of FIG. 1; FIG. 3 is a front perspective view of a rotomolding device according to the invention; Figure 4 is a rear perspective view of the rotational molding device of Figure 3; - Figure 5 is a front view of the rotational molding device of Figure 3; FIG. 6 is a view from above of the rotomolding device of FIG. 3. Best Way of Carrying out the Technical Invention FIGS. 1 and 2 show a mold 1 intended to be used for rotational molding of parts. in synthetic material, comprising two independent half-impressions 2 30 able to be joined together substantially sealingly and arranged to receive and melt a material in solid or liquid form, preferably of the thermoplastic or thermosetting polymer type. The two half-cavities 2 are arranged to separate so as to be able to remove the mold from the workpiece. Each of these two half-cavities 2 is a hollow envelope having an inner wall 3 having a shape complementary to a part (typically about half) of the outer surface of the part to be produced by rotational molding, an outer wall 4 and a space recess 5 located between said inner and outer walls 3, 4, said hollow space 5 being arranged to receive a circulation of a coolant, such as oil or water for example, to cool said half-cavity 2 and in particular the inner wall 3 to allow demolding of said part manufactured in the mold 1. Finally, each of these two half-indents 2 is provided with at least two sleeves 6 integral with its outer wall 4 to respectively allow entry and the outlet of the coolant from the interior space 5 to create a circulation of said fluid inside each of the half-cavities 2. Said at least two sleeves 6 are arranged to be connected to an installation (not shown in the figures) comprising at least one heat transfer fluid reservoir, a piping circuit and a circulation pump for circulating said fluid from the reservoir into the interior space 5 of each half-cavity 2.

The inner wall 3 of each half-cavity 2 further comprises a resistive heating means making it possible to raise the temperature of said inner walls 3 and consequently to raise the internal temperature of the mold 1 in order to melt the material in solid form contained In said mold 1 is meant by means of resistive heating means comprising electrically conductive elements, preferably embedded in a dielectric resin, in which an electric current is passed and which emit heat by the Joule effect. According to a preferred embodiment, the heating means integral with the inner wall 3 of each half-cavity 2 comprises a mesh of conductive wires, advantageously wires or a carbon fabric, embedded in the thickness of the inner wall 3 of each half-cavity 2. The conductors are to be powered by electric current. For reasons of cost of implementation and implementation, the alternating current is preferred. Each half-cavity 2 is advantageously made of a composite material whose main characteristics are in particular an ability to heat when traversed by an electric current, a very good thermal conductivity, a very good mechanical strength advantageously similar to that of the steel and low density for obvious reasons of implementation and ease of handling. For this, each half-cavity 2 advantageously comprises a structure consisting essentially of carbon, for its good electrical and thermal conductivities, whose cohesion is provided by a thermosetting resin, preferably cyanate ester. The latter is a powerful electrical insulator resistant to high temperatures and allows almost immediate heating of each half-footprint 2 by Joule effect.

It is understood that the mold 1 according to the invention is particularly interesting because it allows a temperature control of each half-cavity 2, said control can be achieved easily by the introduction, for example between two folds of carbon fabrics, thermocouples not shown in the figures, the latter then being connected to a control cabinet not shown via rotary joints fixed on each half-cavity 2. Furthermore, each half-cavity 2 may, without departing from the scope of the present invention , comprising a vent, not shown in the figures, for creating a hot air flow inside the mold 1 to accelerate the temperature rise of said mold 1. In FIG. 3 to 6, the rotomolding device 10 comprises a frame 11 arranged to rest on the ground, a main frame 12 hinged to said frame 11 along a substantially horizontal axis XX ' ntal, a secondary frame 13 on which is mounted the mold 1 and articulated to said main frame 12 along an axis YY 'substantially horizontal and substantially perpendicular to said axis X-X', and means 14 and 15 for rotating respectively said frames primary and secondary 12,13. These are advantageously metal tubular structures made from steel square section tubes. The rotating means 14 comprise a motor 141 integral with the frame 11 driving in rotation, by a pulley-belt assembly 142, a shaft 143 disposed along an axis X-X '. The shaft 143 is formed by two sections 144 supported and guided in rotation by bearings 145 secured to said frame 11, said sections 144 being integral and arranged on either side of the main frame 12. The rotating means 15 comprise a motor 151 integral with the main frame 12 driving in rotation, by a pulley-belt assembly 152, a shaft 153 disposed along an axis Y-Y '. The shaft 153 is formed by two sections 154 supported and guided in rotation by bearings 155 integral with said frame 11, said sections 154 being integral and arranged on either side of the secondary frame 13. Thus, with such a configuration, the mold 1 can simultaneously rotate about the axis XX 'in the direction of the arrow R1, as well as a rotation around the axis YY' in the direction of the arrow R2. It is understood that this gives a perfect homogenization of the thermoplastic or thermosetting polymer blend. VIGOU1-FR-1_TEXTE DEPOSE_RG / GE - 9 The rotomoulding device 10 further comprises an installation (not shown in the figures) comprising at least one heat transfer fluid reservoir, a pipe circuit and a circulation pump for the circulation of said fluid. from the reservoir into the interior space 5 of each half-cavity 2 of the mold 1. It is understood that with the mold 1 and the rotomoulding device 10 according to the invention it is no longer necessary to have an oven and a cooling chamber of large dimensions and high power to perform the cycles "heating and cooling of the mold 1". In addition, it is clearly understood that the energy consumption is significantly reduced since the means 15 for heating and cooling the mold 1 are arranged on and / or near the inner walls 3 of the half-impressions 2 of said mold 1, and therefore at the immediate periphery of the molding. For the same reasons, the cycle times are reduced. This reduction in cycle time is accentuated, on the one hand, by the material used to make the half-impressions 2 of said mold 1 and, on the other hand, for the type of heating means used.

DESCRIPTION OF OTHER EMBODIMENTS According to an alternative embodiment not shown, the rotomolding device 10 comprises an intermediate frame which is interposed for example between said main and secondary frames 12, 13. Said intermediate frame 30 is then articulated to said main frame 12 along a substantially vertical axis and substantially perpendicular to said axis X-X ', the secondary frame 13 on which is mounted the mold 1 then being articulated to said intermediate frame along the axis YY' substantially 35 and substantially perpendicular to said axis X-X '. Said rotational molding device 10 further comprises means for rotating said intermediate frame similar to the means 15 for rotating the frame VIGOU1-FR-1_TEXTE DEPOSE_RG / GE - 10 - secondary 13 previously described. According to another embodiment not shown, the rotomolding device 10 comprises a mold whose half-impressions are not hollow envelopes, the cooling of the mold then being provided by the flow of air blown. In this variant, the rotomoulding device 10 then comprises fans advantageously placed on the secondary frame in the immediate vicinity of the mold. However, this variant is less rapid. Finally, according to a last variant embodiment not shown, the inner wall of each half-cavity of the mold is coated with a thin layer, called gel-coat, distributed uniformly, said thin layer being advantageously a mixture of cyanate ester resin and a heat-conductive filler such as, for example, carbon powder or metal powder. This additional layer is intended to give a surface appearance to the rotomolded part, for example a smooth appearance or, on the contrary, textured if necessary. Industrial Applicability As previously described, molds 1 and rotomolding device 10 according to the invention are intended to be used for rotational molding from a thermoplastic polymeric material in solid form (powder or paste) or also thermosetting polymer in liquid form (to become solid during the implementation). Thus, the embodiment of the mold 1 may for example be resumed the manufacture of ultra-thin radiator or a device for heating various fluids. Finally, it is clear that the present invention is not limited to the sole embodiment of these mold 1 and rotomolding device 10; it encompasses, on the contrary, all variants of implementation and application respecting the same principle. VIGOU1-FR-1_TEXT DEPOSE_RG / GE

Claims (12)

1 - Mold (1) comprising two independent half-indentations (2) capable of being secured together removably and substantially sealed, each of these two half-cavities (2) having at least one inner wall (3), said mold ( 1) being characterized in that it comprises a resistive heating means incorporated in the thickness of the inner wall (3) of each half-cavity (2) for raising the temperature of said inner walls (3) and consequently to raise the internal temperature of the mold (1). 2 - mold (1) according to claim 1 characterized in that the resistive heating means comprises a mesh of electrically conductive son embedded in the thickness of the inner wall (3) of each half-cavity (2). 3 - mold (1) according to claim 2 characterized in that said mesh is embedded in a dielectric resin. 20 4 - Mold (1) according to any one of claims 1 to 3 characterized in that each of these two half-cavities (2) is a hollow envelope having an outer wall (4) and a hollow space (5) located between Said inner wall (3) and the outer wall (3, 4), said hollow space (5) being arranged to receive a circulation of a coolant for cooling said mold (1). 30 5 - mold (1) according to any one of claims 1 to 4 characterized in that each half-cavity (2) is made of a composite material whose main characteristics are in particular an ability to heat when it is crossed by a VIGOU1-FR-1_TEXTE DEPOSE_RG / GE- 12 - current, a very good thermal conductivity, a mechanical strength close to that of steel and a significantly lower density than that of aluminum. 6 - Mold (1) according to claim 5 characterized in that each half-cavity (2) is a structure consisting essentially of carbon whose cohesion is provided by a thermosetting resin cyanate type ester. 7 - Mold (1) according to any one of claims 1 to 6 characterized in that the inner wall (3) of each half-cavity (2) is coated with a thin layer, called gel coat, distributed so uniform, this additional layer to give a surface appearance to the rotomoulded part. 8 - Mold (1) according to claim 7 characterized in that said thin layer being a mixture of cyanate ester resin and a heat-conductive filler such as carbon powder or metal powder. 9 - Rotational molding device (10) comprising a frame (11) arranged to rest on the ground, a main frame (12) hinged to said frame (11) along a substantially horizontal axis XX ', a secondary frame (13) articulated to said main frame (12) along an axis YY 'substantially horizontal and substantially perpendicular to said axis X-X', and means (14) and (15) for rotating respectively said primary and secondary frames (12, 13), said rotomolding device (10) being characterized in that it comprises a mold (1) according to any one of claims 1 to 8, said mold (1) being mounted on the secondary frame (13). 35 10 - Rotational molding device (10) according to claim 9 characterized in that the means of setting VIGOU1-FR-1_TEXTE DEPOSE_RG / GE- 13 - rotation (14) comprise a motor (141) integral with the frame (11) driving in rotating, by a pulley-belt assembly (142), a shaft (143) disposed along an axis XX 'and formed by two sections (144) supported and guided in rotation by bearings (145) integral with said frame (11); ), said sections (144) being integral and arranged on either side of the main frame (12). 11 - Rotational molding device (10) according to any one of claims 9 or 10 characterized in that the rotating means (15) comprise a motor (151) integral with the main frame (12) driving in rotation, by a pulley-belt assembly (152), a shaft (153) disposed along an axis YY 'and formed by two sections (154) supported and guided in rotation by bearings (155) integral with said frame (11), said sections (154) being integral and arranged on either side of the secondary frame (13). 12 - Rotational molding device (10) according to any one of claims 9 to 11 characterized in that it comprises an installation comprising at least one heat transfer fluid reservoir, a channel circuit and a circulation pump for the circulation of said fluid from the reservoir into each half-cavity (2) of the mold (1). VIGOU1-FR-1_TEXT DEPOSE_RG / GE