The invention relates to a method and a device for heating a mold. The invention is more particularly but not exclusively suitable for heating a plastic injection mold. Such a mold comprises a molding cavity into which is injected the molten plastic. Said cavity is defined by the molding surfaces, the molding reproduces the shape. Ledites molding surfaces are carried by at least two separable shells from each other, so opening the mold and demolding the solidified part.

During the molding operation, the molding cavity is subjected to a thermal cycle so that the temperature of said cavity is sufficient for the injected material retains its fluidity and fill correctly. Then the temperature is lowered, if necessary by forced cooling, so as to solidify the workpiece, until the opening of the mold and demoulding of the room, where the temperature of the molding surfaces fall before being reheated and repeat the cycle. Thus, cycle time, particularly critical parameter in extreme series, is dictated by the time of heating and cooling of the molding cavity. The quality of the parts obtained, notably their appearance depends on the ability to obtain a uniform temperature distribution on the molding surfaces of the cavity, and in certain circumstances the structural quality of the parts obtained depends on the heating and cooling rates of the molded material to the contact surfaces of the molding. The induction heating technique is particularly suitable for the answer to these needs.

The EP1924415 discloses an induction heating device of the molding cavity of a plastic injection mold, wherein the inductors run in the dies on the molding surfaces. However, the energy efficiency achieved when using a mold made of an aluminum alloy, current situation with regard to plastic injection, leads to the installation of a large electric power.

The EP2861399 describes a method and apparatus for preheating a plastic injection mold. Said device essentially comprises two heating means to achieve a most direct possible heating faces

molding of the cavity. In this device of the prior art, one of the molding surfaces is heated by placing the die on said molding face, facing an electrically conductive ring which is electrically insulated so that the molding surface of said matrix is ​​one of the faces of an air gap with said core. The entire core matrix and is placed inside an induction circuit. The molding surface is heated by the circulation of induced currents on the faces of the gap. The other molding surface carried by the other die constituting the mold is heated by radiation or by conduction by bringing the latter in contact either vis-à-vis exposure of a core previously heated. This prior art solution requires the opening of the mold is sufficient to insert the core between the two matrix. In all cases there is only a preheating solution, not allowing the temperature control of the molding cavity once the closed cavity.

These solutions of the prior art are satisfactory ,; However, they require the facilities of a large electric power, the power required to heat one of the matrices commonly state of around 100 kW. Where the manufacturing site has multiple installations of this type, the power of the corresponding electrical installation becomes a disadvantage.

The 14772 DE1020141 discloses a plastic injection mold including a very localized area is heated by approaching a heated member of the molding cavity, the wall épaissuer is reduced in the application area of ​​this element, in order in particular remove the burr at the jojnt plan mold. Thus, this device only heats the area concerned after or while the piece moiulée is cooled to separate the flash from the remainder of the room.

The invention aims to solve disadvantages of the prior art relates to this end a mold, in particular for injection molding, including:

at. a shell defining a cavity defining a molding surface; b. a heat accumulator;

c. means for heating by induction, configured to heat the heat accumulator;

d. comprising means for exposing a mask and shell surface, said receiving surface other than the molding surface, the

heat of the heat accumulator, so as to carry the entirety of the molding surface at a temperature suitable for the injection of the material into said cavity.

Thus, after bringing the heat accumulator to appropriate températuure, it is simply maintained at that temperature which requires a reduced power.

The invention is advantageously implemented according to the embodiments and variants described below, which are to be considered individually or in any technically operative combination.

Advantageously, the shell includes a circuit for circulating a coolant for cooling of the molding surface. Thus, the mold object of the invention allows to implement forced cooling of the molding cavity without affecting the temperature of the heat accumulator.

According to a variant embodiment, the heat accumulator is a graphite block. This embodiment allows particularly preferred radiant heating of the shell.

According to another embodiment, compatible with the previous, the heat accumulator includes a phase change material. This embodiment allows to store thermal energy in the latent heat of phase change of said material.

The invention also relates to a method for heating the surface of a mold of any of the embodiments of the invention, which method comprises the steps of:

i. heating the heat accumulator;

ii. exposing the shell to heat the heat accumulator;

iii. inject the material into the cavity after step ii).

Advantageously, step ii) comprises heating the heat accumulator. This embodiment makes it possible in particular to regulate the temperature of the molding cavity.

According to one embodiment, step ii) is performed by moving the shell side to the heat accumulator. Thus, the heat accumulator, high temperature remains fixed scope. This embodiment enables, including exposing

a plurality of molds according to an oscillating cycle, one of the molds being heated so that another cooled by limiting the installed power to that required to heat a single thermal reservoir.

According to this embodiment, the heat transfer from the heat accumulator to the shell is preferably achieved by radiation.

Additionally, the heat transfer from the heat accumulator to the shell comprises a share of forced convection of a gas. Thus the heat transfer is faster.

According to another embodiment, step ii) is carried out by the heat storage in contact with a surface of the shell. This embodiment is particularly, but not exclusively, suitable for producing autonomous heating mold, enjoying the benefits of the invention.

According to this latter embodiment, step ii) is performed by the thermal expansion and the thermal accumulator. Thus, exposure of the receiving surface of the shell does not require the implementation of a displacement mechanism.

The invention is explained below according to its preferred embodiments, in no way limiting, and with reference to Figures 1 to 4, whereinthe:

- Figure 1 shows in a view from cutting the shells of a mold according to an exemplary embodiment of the mold object of the invention;

- Figure 2 schematically illustrates an injection system employing the mold of Figure 1

- Figure 3 shows another embodiment of a shell of a mold according to the invention, Figure 3A except during heating of the molding surfaces, and Figure 3B during heating of the molding surface;

- and Figure 4 shows in a partial view, an alternative embodiment of the shell of Figure 3, Figure 4A, except heating period of the molding surface and 4B during heating of the molding surface.

Figure 1, according to an exemplary embodiment, the mold object of the invention comprises two shells (11 1, 112) each carrying a plurality of indentations (121, 122) each cavity corresponding to the molding surfaces for the realization of a piece by injection of material into the closed cavity formed by each pair (121, 122) prints when the two shells (1 11: 1 12) are contacted to each other, that is to say the mold is closed. Said shells are made of a thermally conductive material, preferably a metallic material such as aluminum or a tool steel alloy. Each half shell has a structural function of resistance to the injection pressure without deformation of the molding surfaces so that their thickness is dimensioned accordingly. According to this exemplary embodiment, each shell includes conduits (131, 132) for circulation of a coolant in liquid or gas phase, used to cool said shell and more particularly the molding surfaces and the material in contact with these . Advantageously, said ducts (131, 132) include turbulators (not shown) for improving the exchange by convection between the heat transfer fluid and the shell.

According to this exemplary embodiment, each shell (1 1 1, 112) comprises a surface, said receiving surface (141, 142) opposite to the tray according to this embodiment. Still according to this embodiment, the receiving surface has a coating encouraging the absorption of infrared radiation. As non-limiting examples, said coating comprises amorphous carbon deposited physically vapor or PVD, acronym for "Physical Vapor Deposition" on said receiving surface, or is obtained by chemical treatment of said burnishing said surface, or by the electrolytic deposition of a black chromium plating. Exposure of the receiving surface of each half-shell heat accumulator, whether conduction, radiation, convection or a combination of these modes of heat transfer, allows to bring the molding surfaces of the impressions to a temperature adapted to the injection of the molded material, to ensure a uniform and complete filling of the cavities. The heat transfer from the receiving surfaces (141, 142) to the molding surfaces (121, 122) takes place by conduction through the thickness of the half shells, which ensures a uniform distribution of the temperature on the molding surface without the blemishes on the parts obtained by means of the mold object of the invention.

2A, according to an example of plant carrying out the mold object of the invention, said plant comprises, for example, two molds (201, 202) used alternately as a pendulum automation. To this end, said

installation comprises two stations (291, 292) of discharge, an injection station comprising an injection head (250) capable of injecting a plastic material into the mold cavities (201, 202). The installation further comprises a mechanism (not shown) for transferring the molds (201, 202) of their unloading station to the injection station. Alternatively the plant comprises more than two unloading stations placed on a carousel.

The injection station consists of two heat accumulators (241, 242) constituted by examples of graphite blocks. According to a first embodiment each heat accumulator is heated by an induction circuit, for example in each placing them in a coil traversed by high frequency alternating current, for example of 10 KHz and 100 KHz, so that the wear at high temperature, for example at a temperature between 700 ° C and 1200 ° C.

Alternatively the heat accumulators (241, 242) consist of ferromagnetic material and comprise, at least on one of their faces, a coating to improve their thermal emissivity.

According to an alternative embodiment of the induction circuit, the heat accumulators (241, 242) are heated by the inductors placed in trenches dug in said accumulators.

2B, according to an exemplary performed by a pendulum automation, when one of the molds (201) is in the injecting station, the other mold (202) is in its unloading station (292). On arriving in the injection station, the mold (201) is subjected to the radiation of heat accumulators (241, 242) on its receiving surfaces. For example, the heat flux emitted by radiation by a graphite heat accumulator heated to 1000 ° C reaches values of the order of 150.10 3 Wm "2 . Advantageously, a device (not shown) can be blown on the mold a gas heated in contact with said thermal battery (241, 242) to cause a heat exchange by forced convection with the mold surfaces. Advantageously, the injecting station comprises a chamber (251) filled with a neutral gas preserving mold and heat accumulators oxidation.

Subjected to such heat flow, the mold (201) rapidly heated until the temperature suitable for the injection is reached in its molding cavities. The injection is then performed. Once the injection performed the mold (201) is transferred from the injection station to the unloading station which has the effect of bringing the other mold (202) in the injection station and to submit it to radiation heat accumulators. The unloading station advantageously includes means for circulating a heat transfer fluid in the conduits of the mold, so as to accelerate cooling. Said heat transfer fluid is, for example, water, oil or gas. According to one embodiment, said heat transfer fluid circulates in a closed circuit comprising a refrigerating unit.

Beyond the initial heating phase of the heat accumulators (241,

242), which is performed on a time sufficient to reduce the power demand, the energy consumed at the maintenance temperature of said heat accumulators which requires a smaller power demand than the direct heating of cold mold by induction. The use of induction heating for the heat accumulator, however, can continue to be heated when they transfer their heat to the mold by radiation, convection or conduction.

Figure 3A according to another exemplary embodiment of a half-shell (310) of a mold according to the invention, it comprises two parts (31 1, 312) for example made of an aluminum alloy. One of the two parts (31 1) has a cavity (320) forming a molding surface, and conduits (330) for circulation of a cooling fluid of said molding surface Said first part (31 1) comprises a receiving surface (341).

The second portion (312) of the half shell, attached to the first, comprises intestines which extend inductors (360). According to a particular embodiment, said second portion is made of a nonmetallic refractory material, eg ceramic or concrete, transparent to the magnetic field. Inducers are, for example, constituted with copper or copper braids son. They realize an induction circuit. A heat accumulator (340) is inserted between the two parts (31 1, 312) of the half shell. Said heat accumulator is for example made of a ferromagnetic steel high Curie point, for example an iron based alloy (Fe) and silicon (Si) or iron (Fe) and cobalt (Co). It is preferably thermally insulated from the second portion (312) of the half shell. Said inductors (360) are connected to a high frequency generator (not shown).

When heated, via the inductors (360) at a known temperature maintains said accumulator does not come into contact with the receiving surface (341) of the first part (31 1) of the shell. The contact resistance between the accumulator (340) and the receiving surface is high and the heat transfer between the heat accumulator (340) and the portion (31 1) of the shell bearing the imprint (320) is reduced.

3B, to heat the first part of the half shell, the temperature of the heat accumulator is increased by means of inductors, said accumulator then expands and comes into intimate contact with the receiving surface (341). The contact resistance lower, and the heat accumulator transmits its heat to the portion (31 1) of the half-shell bearing the imprint (320). Advantageously, the receiving surface (341) of the first portion of the shell comprises an interface layer (342) consisting of a thin strip of thermally conductive but malleable or crushable material, and soldered or welded on the receiving surface. In way of nonlimiting examples, said strip is made of copper or a copper alloy, nickel or graphite. Thus, the entry into contact with the heat accumulator (340) with said strip deforms thereof, to compensate for slight differences of shapes between the heat accumulator and the receiving surface, and thereby ensure optimum heat transfer between the two. Thus, the heat accumulator is maintained at a reduced holding temperature of 50 ° C to 100 ° C compared to its heating phase temperature. This retention requires the implementation of a reduced electric power, and the extra power required during the heating phase is also reduced due to the preheating of the accumulator.

Said heat accumulator (340) has no structural function in the mold. Its incorporation are thus chosen to optimize its response to induction heating and its ability to transfer its heat to the first portion (311) of the half shell and thence to the molding surface. According to a particular embodiment, detail Z, said accumulator is of cellular structure, each cell (345) being filled with a phase change material having a latent heat of transition. Advantageously, the phase change material is selected such that its transition temperature is close to the holding temperature of the heat accumulator. For example, if the holding temperature is in the range of 200 ° C the phase change material is for example an organic material such as a polyol. If the holding temperature is higher, for example of the order of 400 ° C or more, the phase change material is for example a salt. According to these examples, the phase change material changes from the solid state at low temperature, the higher temperature liquid state by absorbing a latent heat of transition. Passing the high temperature phase to the low temperature phase of the phase change material solidifies and restores said latent heat of transition. The combination of the honeycomb structure and the presence of a phase change material increases the apparent thermal inertia of the heat accumulator (340) for maintaining it at the holding temperature, any retaining ability rapid heating to the heating temperature.

Cooling the footprint is realized by the circulation of heat transfer fluid in the conduits (330) of the first portion (311) of the shell. Advantageously, the second portion (312) of the shell comprises channels (332) for the conveyance of a heat transfer fluid around the heat accumulator (340) so as to accelerate its cooling to its holding temperature after stage heating and maintaining the temperature of the mold cavity (320).

Fig 4 according to a variant of the embodiment shown in Figure 3, the interface between the first portion (41 1) of the shell is the heat accumulator (440) is not flat but present complementary profiles. This embodiment makes it possible to increase the potential contact area between said first portion (411) of the shell carrying the print, and the heat accumulator (440). 4A outside the heating period of the first portion (41 1), the two profiles are separate at the receiving surface. 4B, in a situation of heating, the thermal expansion of the heat accumulator (440) due to its temperature rise door profile in contact with the receiving surface of the first portion (41 1) of the shell thereby reducing the resistance thermal contact between the two and promoting the heat transfer by conduction.

The above description and the exemplary embodiments show that the invention achieves the intended aim, and allows to enjoy the advantages of induction heating to heat the molding cavity of a mold consisting of a non-ferromagnetic material, example an aluminum alloy, while reducing the power demand required to achieve this heating and thus maintain a reasonable dimensioning of the electric supply circuit.

CLAIMS
Mold, in particular for injection molding, comprising:
at. a shell (11 1, 112, 310) defining a cavity defining a molding surface (121, 122, 320);

b. a heat accumulator (241, 242, 340, 440);

c. induction heating means (360) configured to heat the heat accumulator;

d. characterized in that it comprises means for exposing and mask shell surface, said receiving surface (131, 132, 330) other than the molding surface, the heat of the heat accumulator (241, 242, 340 , 440), so as to carry the entirety of the molding surface at a temperature suitable for the injection of the material into said cavity.

The mold of claim 1, wherein the shell comprises a circuit (131, 132, 330) for circulating a coolant for cooling of the molding surface.

The mold of claim 1, wherein the heat accumulator (241, 242) is a graphite block.

The mold of claim 1, wherein the heat accumulator (340) comprises a phase change material.

Method for heating the surface of a mold according to claim 1 characterized in that it comprises the steps of:

i. heating the heat accumulator (440);

ii. exposing the shell (21, 1 212, 310) to heat the heat accumulator, to bring the molding surface at a temperature suitable for injection;

iii. inject the material into the cavity after step ii).

6. The method of claim 5, wherein step ii) comprises heating the heat accumulator.

7. The method of claim 5, wherein step ii) is performed by moving the shell (11 1, 1 12) facing the heat accumulator (241, 242).

8. The method of claim 7, wherein the heat transfer from the heat accumulator to the shell is achieved by radiation

9. The method of claim 8 wherein the heat transfer between the thermal accumulator to the shell comprises a share of forced convection of a gas.

10. The method of claim 5, wherein step ii) is carried out by the heat accumulator (340, 440) in contact with a surface of the shell.

11. The method of claim 10, wherein step ii) is performed by the thermal expansion of the heat accumulator (440, 340).