Induction heating device for small scale shaped parts

Abstract

The invention relates to a mold suitable for the forming of small-size parts, comprising a box adapted to be mounted on a platen (101, 102), the box comprising a housing configured to receive an insert (350) integrated into the box, the insert comprising a housing for a punch die comprising a forming surface (121, 122) for the part; heating means for heating the punch die, the heating means comprising: a rod (131, 132) made of a material susceptible to induction heating, the rod being disposed in the insert (350) and having an end in contact with the punch die, the forming surface (121, 122) being substantially centered with respect to the rod; a coil (141, 142) made of an electrically conductive material and surrounding the rod, the coil being connected to a high-frequency current generator (151, 152) such that, when the coil is supplied with current, it generates an induced current in the rod (131, 132), the rod transferring heat to the punch die by induction heating of the rod, and wherein the outer diameter of the rod is greater than or equal to the outer diameter of a periphery (225) delimiting the forming surface (121, 122).

Description

TECHNICAL FIELD

[0001] The present invention belongs to the field of molding processes, and more particularly, to a molding process for small parts.

[0002] As a non-limiting example, the present invention applies to the molding of smartphone camera lenses.

[0003] The present invention relates to a mold suitable for the mass production of such parts, the mold having means for rapid heating and cooling. BACKGROUND

[0004] In mass production molding processes of glass or plastic parts, called organic glass, having a relatively high casting temperature, the heating and cooling times of the mold have a significant impact on the manufacturing cycle time.

[0005] Indeed, the mold must be heated to a temperature that guarantees a sufficient fluidity of the molding material to ensure a uniform filling of the cavity, and then must be cooled to a temperature at which the used material is sufficiently solidified to allow the transfer of the part produced thereby without damaging it. Depending on the molding material, the above-mentioned temperatures are defined, for example, in relation to the melting and solidification temperatures of the material and / or in relation to the glass transition temperature or crystallization temperature of the material.

[0006] Generally, the mold is huge compared to the part manufactured, and its mass is several orders of magnitude higher than the mass of the molded part.

[0007] The concept of rapid heating and cooling is therefore essentially applicable to the mass of the mold.

[0008] Induction heating techniques combined with fluid circulation cooling, such as those described in the documents US7679036 or US10232530, make it possible to concentrate the heating on the surfaces of the mold in contact with the molding material, while maintaining a uniform heating temperature on these surfaces.

[0009] These techniques of the prior art therefore make it possible to reduce the heating-cooling cycle time by limiting the volume of material to be heated and cooled, and by taking advantage of the high heating power offered by induction heating.

[0010] However, these techniques of the prior art are not applicable when the size of the part manufactured is very small, such as lenses having a diameter of less than 10 mm, and more generally lenses having a diameter of less than 2 mm, when the thickness of the part is also very thin, for example between 0.1 mm and 0.3 mm, and when the part must be manufactured with a high precision and be compatible with optical devices, even if a mold comprising several cavities is considered.

[0011] Moreover, even if these techniques in the prior art are implemented, the volume and the mass that undergo thermal cycles are important for the quality of the parts produced. SUMMARY

[0012] The present invention aims to solve the drawbacks of the prior art, for which it relates to a mold suitable for molding small-scale parts, the mold comprising a box suitable for being mounted on a press plate, the box comprising a housing configured to receive an insert; the insert being configured to be integrated into the box, the insert comprising a housing for accommodating a stamping die comprising a molding surface for the parts;

[0013] a heating device for heating the stamping die, the device comprising a rod made of a material susceptible to induction heating, the rod being disposed in the insert and having an end in contact with the stamping die, the molding surface being substantially centered with respect to the rod;

[0014] The device further comprises a coil made of an electrically conductive material and surrounding the rod, the coil being connected to a high-frequency current generator, such that when the coil is supplied with an electric current, the coil generates an induced current in the rod, the rod transferring heat to the stamping die by induction heating of the rod, and wherein the outer diameter of the rod is greater than or equal to the outer diameter of the periphery that delimits the molding surface.

[0015] Thus, the mold of the invention makes it possible to concentrate the heating on the molding surface by the combination of the rod and the coil, in order to quickly reach high temperatures on the molding surface, while reducing the electrical power used.

[0016] The invention can be implemented according to the embodiments and variants disclosed hereinafter, which can be considered individually or according to any technically operable combination.

[0017] Advantageously, the box comprises a circuit for circulating a heat transfer fluid for cooling the insert and the stamping die.

[0018] In this way, forced cooling becomes possible, which makes it possible to reduce the cycle time.

[0019] According to one particular embodiment, the insert comprises a circuit for circulating a heat transfer fluid, the circuit being connected to the fluid circulation circuit of the box.

[0020] In addition to being able to cool the stamping die more quickly and to shorten the cycle time, the circuit also allows thermal regulation by simultaneous use of induction heating and fluid circulation cooling, to meet applications that can require such thermal regulation.

[0021] According to a particular embodiment, the box and the insert have separate cooling circuits. This embodiment allows different heat transfer fluids to circulate in the box and in the insert, said fluids being adapted to the temperatures they respectively reach during the forming operation.

[0022] According to one embodiment, the cooling circuit of the insert comprises a barrier. Such a barrier promotes the turbulent flow of the cooling fluid in the duct and the convective exchange with the duct wall.

[0023] Advantageously, the mould comprises a thermocouple housed in the stem, the hot junction of which is close to the end of the stem.

[0024] The information given by the thermocouple makes it possible to control heating and cooling, thus optimizing the manufacturing cycle.

[0025] According to one preferred embodiment, the outer diameter of the periphery delimiting the periphery of the forming surface is less than 5 mm, preferably less than 2 mm.

[0026] According to one advantageous embodiment, the coil generating the induction is made of copper and comprises an annular portion surrounding the second portion of the stem and a handle for connecting a high-frequency alternating current source, the cross-section of the handle and of the annular portion being less than or equal to 10 mm 2 . The cross-section of the coil is thus reduced, which limits the electrical resistance of the coil and its heating under the effect of the alternating current, and therefore does not require the provision of forced cooling means for the coil.

[0027] According to one preferred embodiment, the stamping die comprises a forming surface, a forming portion made of a steel susceptible to induction heating and a technological portion made of a material having a high thermal diffusivity. This embodiment promotes rapid heating and rapid cooling.

[0028] Advantageously, the forming surface of the forming portion of the stamping die is made of nickel. This embodiment is particularly suitable for the forming of optical parts.

[0029] This mould is therefore advantageously used for a plastic injection machine for the mass production of micro-lenses for cameras.

[0030] This use of the mould is particularly suitable for the manufacture of lenses having a thickness of between 0.1 mm and 0.3 mm. BRIEF DESCRIPTION OF DRAWINGS

[0031] The application is implemented according to the following preferred embodiments, with reference to the attached Figure 1 to the attached Figure 5 figures, and without the application being limited thereto.

[0032] Figure 1 a front view of an injection-moulded assembly implemented according to the mould of the application is shown;

[0033] Figure 2An exploded side perspective view of a rod and a punch of an exemplary embodiment of a mold according to the present application is shown;

[0034] Figure 3 A partial cross-sectional view of an exemplary embodiment of a portion of a mold according to the present application is shown;

[0035] Figure 4 A perspective view of an exemplary embodiment of a barrier is shown. Figure 3 An exemplary embodiment of a partial A-A cross-section of a duct defined in accordance with the present application, provided with a barrier for the circulation cooling of a heat transfer fluid in an insert;

[0036] Figure 5 A perspective view of an exemplary embodiment of a barrier is shown. DETAILED DESCRIPTION

[0037] According to an exemplary embodiment, as shown in Figure 1 The device implementing the mold of the present application comprises a plastic injection machine, two opposite portions (111, 112) of the mold being assembled on two platens (101, 102) respectively.

[0038] One of the platens (101) is movable towards the other platen (102) and movable away from the other platen. Thus, when the two portions (111, 112) of the mold are brought together and held against each other by the opening / closing mechanism of the injection machine, they define a plurality of sealed cavities into which the molding material is injected in liquid or paste state.

[0039] The molding material conforms to the shaping surfaces of the sealed cavities and then, upon cooling of the mold, solidifies.

[0040] The mold is then opened by moving the platens of the injection machine away from each other, the part thus produced being removed from the mold.

[0041] Thus, according to this exemplary embodiment, the first portion of the mold (111) carries a plurality of punches comprising convex shaping surfaces (121), while the facing mold portion (112) carries a plurality of punches comprising concave shaping surfaces (122).

[0042] When the mold is closed and the two parts (111, 112) are brought against each other, the shaping cavities are delimited by the shaping surfaces (121, 122) of the two portions.

[0043] The punches carrying these shaping surfaces (121, 122) are mounted in contact with rods (131, 132) held in each portion of the mold.

[0044] These rods are made of a material susceptible to induction heating, such as a ferromagnetic alloy, for example a martensitic tool steel.

[0045] According to one embodiment, the forming surface is made of nickel, polished according to optical quality polishing. The nickel can be deposited on the forming surface by any electroplating or coating technique.

[0046] Each rod is surrounded by a coil (141, 142) made of an electrically conductive material, such as copper, without mechanical contact.

[0047] The coil is connected to a high frequency current generator (151, 152), the high frequency being generally between 10 KHz and 200 KHz.

[0048] When the coil is supplied with high frequency current, the current induced in the rod, due to the fact that the rod is made of a material with high magnetic permeability, heats the rod, which transfers its heat by conduction to the punch.

[0049] According to Figure 1 As shown in the cross-sectional view, each die part (101, 102) comprises two punch dies, one at each end of the two rods, the rods, punch dies and coils of each group being identical for the same die part (101, 102), this particular arrangement not being limiting.

[0050] According to alternative embodiments, the punch dies are formed integrally with the rods, by machining at the end of the rods, or are assembled on one end of the rods by mechanical assembly, welding or brazing, or the punch dies are formed directly at the end of the rods by additive manufacturing.

[0051] According to the present embodiment, a thermocouple (161, 162) is installed in each rod, which makes it possible to measure the temperature of the end of the rod close to the forming surface.

[0052] Each die part comprises a duct (171, 172) for circulating a heat transfer fluid.

[0053] According to one embodiment, the heat transfer fluid is water, the duct being connected to a pumping and cooling unit (175) which circulates the heat transfer fluid in the ducts (171, 172) of the die, in order to cool the heat transfer fluid.

[0054] According to other exemplary embodiments, the heat transfer fluid is an oil or a gas, depending on the forming temperature, the mass of the die and the formed part, the circulation of said fluid being carried out in a closed circuit or in an open circuit.

[0055] The system of the application advantageously comprises a control room (190) which in particular controls the opening and closing of the injection molding machine, the power supply of the high frequency generator, the coils and the circulation of the heat transfer fluid, in particular as a function of the measurements produced by the thermocouples installed in the rods.

[0056] Thus, under control, the system generates repeatable injection and temperature cycles.

[0057] As shown in Figure 2 According to an exemplary embodiment, the rod (131, 132) essentially comprises three functional parts.

[0058] According to one exemplary embodiment, the first part (231) enables the rod to be centered and mounted in the mold.

[0059] Advantageously, the mold comprises an insert mounted in the box. According to this embodiment, the rod (131, 132) is then mounted in the insert and the first part (231) enables the rod to be positioned in the insert.

[0060] The substantially cylindrical second part (232) continuous with the first part (231) and integral therewith forms the zone mainly subjected to electromagnetic induction.

[0061] To this end, the second part (232) of the rod is made of a material susceptible to induction heating, preferably of a material of high magnetic permeability, such as ferromagnetic steel.

[0062] According to another embodiment, the part of the rod subjected to the inductive magnetic field of the coil comprises a coating susceptible to induction heating. As a non-limiting example, the rod is made of copper or a copper alloy, or more generally, the rod is made of a material of high thermal conductivity and the rod has a coating, such as a nickel coating, disposed in the zone subjected to the magnetic field generated by the coil. The thickness of the coating is chosen according to the penetration depth of the induced current.

[0063] The induction is generated by a coil (141, 142) comprising a loop portion (241) surrounding the second part (232) of the rod and a handle (242) for connecting the coil to a high-frequency alternating current source.

[0064] As a non-limiting example, the high-frequency current source is a 25 KW generator generating alternating current at a frequency of between 35 KHz and 70 KHz. This type of generator can power up to 4 devices of the invention.

[0065] The handle (242) is in electrical contact with the loop portion (241), according to the embodiment, the handle (242) is integral with the loop portion (241) and is made of an electrically conductive material, such as copper.

[0066] Advantageously, the cross-section (245) of the coil is less than 10 mm 2 in order to limit the heating of the rod when the high-frequency alternating current generated by the generator flows through the rod.

[0067] According to an advantageous embodiment, the coil is insulated from the stem by an insulating ring (243) made of an insulating material, transparent with respect to the magnetic field, and resistant to the temperature generated by the coil (141) in the second portion of the stem.

[0068] This temperature can reach 450°C, however, the insulating ring, the stem and the coil are affected by this temperature only for a very short time (much less than 1 second).

[0069] As a non-limiting example, the insulating ring (243) is made of ceramic, or the ring is made of reinforced or non-reinforced plastic material and is able to resist such temperatures for a very short time.

[0070] The insulating ring (243) ensures the centering and fixing of the coil with respect to the stem.

[0071] The third portion (233) of the stem is in contact with the stamping die, more specifically with the forming portion (221) of the stamping die.

[0072] According to this exemplary embodiment, the stamping die comprises a forming portion (221) comprising a forming surface for manufacturing the component, integral with the second portion (232) of the stem, and a technological portion (222) suitable to be assembled on the forming portion (221) and located at its center.

[0073] According to alternative embodiments, the forming portion (221) is connected to the stem by mechanical assembly, welding or brazing.

[0074] The forming surface of the stamping die is fitted within a circle (225) having a diameter smaller than the diameter (235) of the second portion (232) of the stem.

[0075] Such a disposition ensures that the forming surface (221) is heated quickly and uniformly even if the inductive heating, which is reduced in power, is limited to the second portion (232) of the stem.

[0076] As a result, such a device is particularly suitable for manufacturing small components, the forming surface having an outer footprint diameter (225) smaller than 5 mm, preferably smaller than 2 mm, and having a recess depth or a protrusion height of the same order of magnitude. The die of the present invention is therefore more particularly, but not exclusively, applied to the forming of small optical lenses, such as the lenses on a mobile phone camera, a webcam or a micro-optical sighting device.

[0077] The technological portion (222) of the stamping die is particularly used to implement the parting surface of the forming cavity and provides a sealing function, a function of bringing the material into said forming cavity, and facilitates the cooling of the forming portion (221) by diffusing the heat to the cooling means of the die.

[0078] According to one exemplary embodiment, the shaped portion (221) is made of tool steel susceptible to induction heating. Advantageously, the shaped surface of the shaped portion of the die is made of nickel and is polished.

[0079] Thus, when an alternating current at high frequency is supplied to the coil (141, 142), a certain heating effect is generated on the die. However, the configuration of the coil, with its annular portion (241) surrounding the second portion (232) of the stem, means that most of the heating effect is concentrated on said second portion of the stem, the heat then being transmitted over a short distance to the die by conduction.

[0080] According to this exemplary embodiment, the stem advantageously comprises one or more housings (260) extending over all or part of its length, in order to introduce thermocouples therein.

[0081] These thermocouples advantageously make it possible to measure the temperature closest to the shaped portion (221).

[0082] According to this exemplary embodiment, the process portion (222) is made of the same tool steel as the shaped portion, but according to other variants, it is made of a steel or other material not susceptible to induction heating but having a high thermal diffusivity, such as a copper alloy resistant to the shaping temperature of the material.

[0083] The thermal diffusivity of a material is defined by the ratio λ / pc, where λ is the thermal conductivity of the material, p is the density of the material and c is the specific heat capacity of the material. In the present case, a high thermal diffusivity is considered to be greater than 50 x 10 -6 m 2 / s, preferably greater than 100 x 10 -6 m 2 / s.

[0084] This last embodiment makes it possible to cool the die more quickly after injection of the material.

[0085] According to one exemplary embodiment, as Figure 3 illustrated, the die of the application comprises a box (310) capable of receiving one or more inserts (350), each insert carrying its own die and its own heating device, which can be connected to the high-frequency alternator by appropriate connection means (340).

[0086] Thus, the insert carries the die composed of the shaped portion (221) and the process portion, the stem, the heating device comprising the coil and its connection means to the high-frequency alternator, and the cooling device in the form of a conduit for circulating a heat transfer fluid.

[0087] Thus, the die of the application is modular, the shaped portion being interchangeable without having to rework the box or the insert, it being sufficient to replace the stem.

[0088] Advantageously, the cartridge (310) comprises a circuit for circulating a heat transfer fluid to cool the cartridge, the insert and the punch.

[0089] Thus, according to one embodiment, one or more first circuits allow circulation between the inlet (371) and the outlet (372) to circulate the heat transfer fluid in the cartridge around the insert.

[0090] One or more other circuits comprise an inlet (375) and an outlet (376) for circulating the heat transfer fluid through the cartridge (310) and as close as possible to the stem through the insert (350). According to this exemplary embodiment, the insert (350) comprises an internal circuit for circulating the heat transfer fluid, which is connected to the circuit of the cartridge when the insert is mounted therein. To this end, the insert comprises suitable sealing means (not shown in the figures) for securing this connection.

[0091] According to other embodiments, the insert comprises its own cooling circuit, which optionally uses a different heat transfer fluid than in the cartridge.

[0092] Thus, the cartridge is cooled, for example, by water circulation, while the insert, and optionally the stem, is cooled by gas circulation, for example, air, argon, nitrogen, carbon dioxide or helium, without the gas being limited to these examples.

[0093] Thus, according to Figure 4 According to the exemplary embodiment shown, the insert (350) and optionally the stem comprise conduits (450) for circulating a gas under pressure to ensure gas cooling, these conduits advantageously comprising baffles (445) or turbulators to promote convective exchange.

[0094] Figure 5 An exemplary embodiment of such a baffle is shown, which is straight (4451) or twisted (4452), the baffle being made of steel, bronze or a polymer material, without the material of the baffle being limited to these examples.

[0095] The advantageous effect of these baffles is not limited to the case of gas circulation, but also promotes convective exchange in the case of a liquid heat transfer fluid, for example, water or oil, thus having a cooling effect.

[0096] The partial induction heating of the stem and the fluid circulation circuit makes it possible to achieve fast heating and cooling cycles suitable for mass production.

[0097] As a non-limiting example, for a typical cycle of forming a cell phone camera lens, the forming surface is heated from a temperature of 130°C, corresponding to the temperature of the previous part-removing punch die, to a temperature of 260°C in less than 5 seconds, more typically less than 2 seconds, with an inductive power of less than 5 KW. Using a 5 mm diameter conduit, the temperature is cooled to a part-removing temperature of less than 190°C in 10 seconds, with a water flow of 2.5 liters per minute per cooling circuit, enabling a cycle between two removals to be completed in less than one minute.

[0098] The foregoing description and exemplary embodiments demonstrate that the present application achieves the intended objectives, in particular, the mold of the present application allows for rapid heating and cooling of the punch die and the part contained therein at reduced power, and is particularly suitable for the production of small scale, thin parts, such as optical lenses.

Claims

1. A mold suitable for the molding of small components, comprising a box (310) suitable for being mounted on a platen (101, 102), the box comprising a housing configured to receive an insert (350); said insert (350) being configured to be integrated into said box (310), said insert (350) comprising a housing for a stamping die comprising a molding surface (121, 122) for said components; heating means for heating said stamping die; characterized in that, said heating means comprise: a rod (131, 132) made of a material susceptible to induction heating, said rod being disposed in said insert (350) and having an end in contact with said stamping die, said molding surface (121, 122) being substantially centered with respect to said rod; and a coil (141, 142) made of an electrically conductive material and surrounding said rod, said coil being connected to a high-frequency current generator (151, 152) such that, when current is supplied to said coil, said coil generates an induced current in said rod (131, 132), said rod transferring heat to said stamping die by induction heating of said rod, and in that the outer diameter (235) of said rod is greater than or equal to the outer diameter of a periphery (225) that delimits said molding surface (121, 122).

2. The mold of claim 1, wherein, said box (310) comprises at least one circuit (171, 172) for circulating a heat transfer fluid for cooling said insert (350) and said stamping die.

3. The mold of claim 2, wherein, said box (310) and said insert (350) comprise separate cooling circuits.

4. The mold of claim 3, wherein, said cooling circuit of said insert comprises a duct (440) suitable for circulation of a heat transfer fluid.

5. The mold of claim 4, wherein, said duct (440) for circulating a heat transfer fluid in said insert comprises a barrier (445, 4451, 4452).

6. The mold according to claim 1, comprising a thermocouple (161, 162) housed inside said rod (131, 132), the hot junction of said thermocouple being proximate to the end of said rod.

7. The mold of claim 1, wherein, the outer diameter of said periphery (225) delimiting the outer edge of said molding surface (121, 122) is less than 5 mm.

8. The mold of claim 7, wherein, the outer diameter of said periphery (225) delimiting the outer edge of said molding surface (121, 122) is less than 2 mm.

9. The mold of claim 7, wherein, Said coil (141, 142) is made of copper and comprises an annular portion (241) surrounding the second portion (232) of the stem and a handle (242) for connection to a high-frequency alternating current power supply, the cross section (245) of said handle and of said annular portion being less than or equal to 10 mm 2 .

10. The mold of claim 1, wherein, said stamping die comprises a molding portion (221) made of a steel susceptible to induction heating and carrying said molding surface (121, 122) and a process portion (222) made of a material with high thermal diffusivity.

11. The mold of claim 10, wherein, the molding surface (121, 122) of the molding portion of said stamping die is made of nickel.

12. Use of a mold as claimed in claim 11 on a plastic injection machine for mass production of micro-lenses for cameras.

13. Use according to claim 12, wherein, the thickness of said lenses is between 0.1 mm and 0.3 mm.

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