Method for moulding an optical element by controlling moulding parameters

Abstract

The invention relates to a method for producing at least one optical device (2) by moulding, by injecting material into a mould (1) according to moulding control parameters (Ci,k) during a moulding cycle (k). The method comprises the following steps, after the moulding of an optical device: measuring at least one parameter (mi,k) of the optical device; determining at least one modified moulding control parameter (Ci,k+d) for a subsequent moulding cycle (k + d), calculated from at least one of the moulding control parameters (Ci,k) of the moulding cycle (k), as a function of a scalar or vector error (Ek) calculated from the at least one measured parameter (mj,k) of the at least one moulded optical device and from at least one predetermined theoretical parameter (mtj).

Description

[0001] TITLE: METHOD FOR MOLDING AN OPTICAL ELEMENT BY CONTROLLING MOLDING PARAMETERS

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of manufacturing an optical element by molding.

[0004] By controlling certain manufacturing parameters by molding, the process according to the invention ensures that the manufactured optical element has interfaces which meet precise specifications, in particular in terms of shape and in terms of precision of material thickness between the interfaces.

[0005] The field of the invention is, in a non-limiting manner, that of optical control and measurement systems, in particular for the manufacture of optical elements.

[0006] STATE OF THE ART

[0007] Optical elements, such as optical assemblies or imaging lenses, generally consist of one or more lenses, possibly associated with plates or filters, and are intended to shape optical beams.

[0008] If the optical element comprises several lenses, they can be assembled in the form of a stack along a common axis, in a support such as a barrel.

[0009] Each lens surface (also called "interface" hereinafter) can be described by an optical axis, a vertex (or apex) or by the center of curvature of the surface. The optical axis is an axis of rotational symmetry in an approximation of revolution symmetry. The optical axis can be the axis passing through the apex and the center of curvature of a lens surface.

[0010] The optical performance of an optical element depends primarily on the manufacturing precision of the optical components and their assembly. During the manufacturing or handling of optical elements, it may be necessary to measure or control them to ensure that they meet the initially intended technical characteristics, since their technical characteristics determine their performance and what they are intended for (lens assembly for a smartphone camera, for example).

[0011] It may be necessary to ensure that the optical axis is in the center of the lens to avoid any centering error when assembling or positioning the lens in the optical device for implementation,

[0012] It may also be necessary to ensure that the shape of the concave or convex surface of the lens is precisely obtained, in particular so that the stacking of several lenses can ultimately correspond exactly to what is expected, or to ensure compliance with the thickness of the lens, measured at its center or on its edges.

[0013] The lenses (or optical devices), which are very precise in terms of shape and dimensions, are made by molding a material with an N index, which can be glass in certain cases.

[0014] Achieving precision in the characteristics of the molded lens requires the control of several parameters. Without control, or with poor control, the produced lenses may exhibit deformations: for example, during molding, the material may not fill the entire mold cavity, or the surface of the lens interface may wrinkle if it cools too quickly in contact with the mold: the lens will then not be optically compliant.

[0015] Conversely, too high a temperature of the internal walls of the mold leads to too long a cooling time for the material, and the manufacturing time by molding the lens is then no longer compatible with the yield required by an acceptable industrial cycle.

[0016] Similarly, a material injection pressure in the mold that is too low can lead to incomplete filling of the mold, or to significant shrinkage, i.e. a phenomenon of reduction in the volume of the lens due to cooling, or even polymerization. Finally, a material injection pressure in the mold that is, conversely, too high would lead to excessive wear of the mold due to excessively high transfer speeds of the hot material, and to plating of the material in the cavities which could stick the material to the internal surface of the mold: this leads in particular to difficulty in demolding the lens, or even to soiling of the mold impression with production stoppage if part of the material remains stuck to the impression.

[0017] To limit these drawbacks, the person skilled in the art has defined ranges of manufacturing characteristics: for example, the temperature of the wall of the mold cavity is usually included in a range of recommended temperatures, as is the injection pressure of the material into the mold.

[0018] However, even when imposing characteristic ranges for lens molding, there is still a shrinkage of the material during cooling: as the material cools from the periphery of the lens, a hard skin forms at its periphery before the core of the lens is completely cooled, which ultimately prevents the injection pressure from providing material to compensate for the shrinkage.

[0019] Devices or installations are known which make it possible to check the dimensions of the lenses produced: document EP2228620 describes a device for measuring the shape and dimensions of surfaces of objects by contact between the surface of the object and a stylus. This device makes it possible to precisely define both the shape and the thickness of the object, and these parameters can be compared with those expected, which makes it possible to determine whether or not the molded lens complies with the imposed specifications.

[0020] However, it can be annoying to have to touch the lens to check it.

[0021] The applicant has developed other measuring means which make it possible to obtain the shape and dimension characteristics of a lens without touching it: for example, patent application FR 3 114 385 describes an installation for measuring surfaces of an optical device which uses in particular a light source, an optical sensor and an interferometer to obtain the characteristics of the device. Patent application FR 3 131 955 describes another installation which makes it possible to determine information from an interface of an optical element by implementing a method which uses in particular an optical sensor and a light source.

[0022] These measuring devices are useful for identifying characteristics that do not comply with the specifications. They allow anomalies to be detected but do not indicate how to anticipate them or even avoid them by acting on the manufacturing process of these optical devices.

[0023] The invention describes a method which not only ensures the control of certain parameters of optical devices (shapes and thicknesses of lenses in particular) obtained by molding but which also acts on the manufacturing process of optical devices to correct non-compliant characteristics, or even prevent (avoid) them.

[0024] PRESENTATION OF THE INVENTION

[0025] The invention relates to this end to a method for manufacturing at least one optical device by molding, by injecting material into a mold according to one or more molding control parameters applied to the molding cycle, parameters comprising at least one injection temperature of said material, and / or a temperature of said mold and / or an injection pressure of said material.

[0026] The method according to the invention is remarkable in that it comprises the following steps, after the molding of said at least one optical device:

[0027] - measurement of at least one parameter of said at least one molded optical device at the end of said molding cycle,

[0028] - determination of at least one modified molding control parameter for a subsequent molding cycle of d cycles of said molding cycle, said modified molding control parameter being calculated from at least one of said molding control parameters of the molding cycle from which said at least one molded optical device originates, said modified molding control parameter being a function of a calculated error, scalar or vector (if it comprises one or more terms), from said at least one measured parameter of said at least one molded optical device at the end of the molding cycle and at least one predetermined theoretical parameter.

[0029] Furthermore, according to the manufacturing method according to the invention, said at least one measured parameter of said at least one optical device comprises an optical thickness measured between at least two optical interfaces of said at least one optical device, and the error value is calculated from said measured optical thickness and a predetermined optical thickness.

[0030] The optical thickness EO is the delay experienced by an optical beam passing through a certain thickness of a material D, of group index N derived from the refractive index. If the material is homogeneous, EO = D * N.

[0031] Advantageously, the so-called optical thickness can be measured by an interferometric device, which provides it directly, without the need to explain the physical thickness or the group index derived from the refractive index.

[0032] It is then possible to determine E from EO and an expected value of EO noted EOc, which is written as

[0033] E = EO - EOc

[0034] It is also possible to determine E as follows:

[0035] E = ( Dm x Nm - De x Ne )

[0036] Or

[0037] - Dm is the physical thickness measured for said at least one optical device,

[0038] - Nm is the group index measured for said at least one optical device,

[0039] - De is the predetermined physical thickness, and

[0040] - Ne is the predetermined optical group index.

[0041] Again, we can also express E in the form:

[0042] E = (Dm - De) It will then be appropriate to express the sensitivity S in relation to the physical thickness and not the optical thickness in what follows (the sensitivity to the optical thickness SEO being N times that to the physical thickness SD).

[0043] SEO = (d(EOi,k) / d(cj,k)) SD = (d(Di.k) / d(cj,k))

[0044] As EO = D * N

[0045] SEO = (d(Di,k *N) / d(cj,k)) = N * SD

[0046] According to an advantageous embodiment, said at least one optical device is an optical lens and said measurement of said optical thickness is carried out at the center of said lens.

[0047] Preferably, said at least one measured parameter contains at least one physical or optical thickness measurement point in an active optical zone of the lens.

[0048] The term “active optical zone” shall be understood to mean an optical zone of the lens which performs the function for which the lens is made (for example, an area around the center of the lens, delimited by a circle of predetermined radius).

[0049] More preferably, said at least one measured parameter contains at least one physical or optical thickness in the so-called assembly zone.

[0050] According to a particular embodiment which may be envisaged, said at least one measured parameter comprises a minimum cycle time to be applied to enable demolding of said lens. This is carried out under conditions such that the demolded lens does not exhibit deformations likely to have consequences on its shape and therefore on the function for which it is created.

[0051] According to an implementation variant, the surface temperature of the molded optical device can be called T P(p as skin), either considered as uniform, or considering its maximum value, can be modeled according to a simplified thermal model Tp(t) where t is the time elapsed since molding. Considering a temperature Ts called solidification of said skin, the lens is considered demoldable when Tp(t) reaches Ts. This makes it possible to calculate a minimum cycle time to demold the lens equal to te.

[0052] This modeling provides a relationship between Tm and Ti, for example by keeping te constant equal to an economic value to be observed. From this relationship between Tm and Ti, the parameters Ti, Tm, and the injection pressure Pi can be modulated to observe the sensitivities according to the parameter(s) measured to calculate the sensitivity values.

[0053] Thus, a relationship is established between the mold temperature (Tm), the injection temperature (Ti), and the cycle time (te) in order to model part of the relationships between the control parameters to evaluate the sensitivities s between the control parameters and the E value(s).

[0054] Furthermore, the modified molding control parameter (c, k+d) is obtained from a sensitivity relationship between a deviation vector of one or more control parameter values, between the molding cycle (k) and the molding cycle u Inside r(k+d), said deviation vector having the following formula:

[0055] (tACk.d) = tCk+d - tCk),

[0056] This sensitivity relationship can be linearized.

[0057] In this case, the vector (tACk.d) is multiplied by a sensitivity matrix (S), to determine an error deviation vector '(Ek) = (Mk) - (MT)) between realization parameters (Mk) at an instant (k) and predetermined theoretical parameters (MT), according to the

[0058] Following one implementation mode, the sensitivity matrix is ​​obtained empirically.

[0059] According to an implementation variant, the sensitivity relationships are obtained by calculating the achievement of a solidification state on the surface of the optical device and its material supply, from a numerical model based on a set of elements comprising a modeling of the cooling and the viscoplastic behavior of the optical device during its solidification, under the effect of the injection pressure, and the injection temperature and mold temperature conditions.

[0060] Advantageously, said sensitivity matrix is ​​obtained by an artificial intelligence module having learned the consequences of applying molding characteristics on the formation of the molded lens, by observing the molding of other lenses and the cycle time conditions to be observed.

[0061] Furthermore, the matrix (S) is inverted into a matrix (M) to obtain several modified molding control parameters, by applying the following equation:

[0062] According to an implementation method described below, only one parameter (mi, k) of said at least one molded optical device is measured at the end of said molding cycle (k).

[0063] Preferably, the error value (E) is calculated as follows: E = ( EOm - EOc ), where

[0064] - EOm is the optical thickness measured for said at least one optical device,

[0065] - And

[0066] - EOc is the predetermined optical thickness.

[0067] Advantageously, a single modified molding control parameter is determined for a subsequent molding cycle of d cycles of said molding cycle.

[0068] Furthermore, the modified molding parameter includes the mold temperature or the injection temperature, or the injection pressure for a subsequent molding cycle, following a considered molding cycle of at least one optical device, from the following formula:

[0069] Ci,k+d = Ci,k - (1 / S) x Ek, where:

[0070] - Ek is the error value calculated for said at least one optical device of the considered molding cycle k,

[0071] - Ci,k is a control value taken from the mold temperature value, or the material injection temperature value in the mold or the injection pressure, measured or applied for the molding cycle considered k, and

[0072] - S is a sensitivity relative to the considered molding cycle k, S being calculated as follows,

[0073] S = dEk / dC k ,

[0074] Or

[0075] S = (Ek - Ek-ë) / (ci,k— ci,k- s ) Or

[0076] - Ek-s is the error value calculated for the previous molding cycle ks, and

[0077] - G, ks is the control value, measured for the previous cycle ks.

[0078] Furthermore, reaching limit values ​​of control parameters may cause a re-determination of the control parameters by modifying theoretical values ​​to be achieved, such as the molding cycle time.

[0079] The invention also relates to an installation for implementing the method as defined above.

[0080] The installation includes:

[0081] - a device for molding at least one optical device, said molding device comprising an injection mold and being associated with a central unit for controlling the operation of said injection mold,

[0082] - an optical thickness measuring device,

[0083] - a calculation module of at least one error value,

[0084] - a control module, capable of communicating with said central control unit of said at least one injection mold, said control module being capable of transmitting to said central control unit orders concerning molding control parameters during a molding cycle.

[0085] DESCRIPTION OF FIGURES

[0086] Other advantages and characteristics of the invention will appear on examining the detailed description of a non-limiting mode of implementation, and the appended drawings, in which: [Fig. 1] is a schematic representation of an installation in accordance with the invention, implementing the method in accordance with the invention,

[0087] [Fig. 2] is a schematic representation of the steps of an embodiment of the method according to the invention, illustrating the exchanges between the different elements of the installation according to the invention, and

[0088] [Fig. 3] is another schematic representation of the steps of a variant of a process in accordance with the invention, also illustrating the exchanges between the different elements of the installation in accordance with the invention.

[0089] DESCRIPTION OF ONE OR MORE EMBODIMENTS

[0090] Figure 1 shows an installation in accordance with one embodiment of the invention, which makes it possible to implement the method in accordance with the invention which will be described more particularly with reference to Figures 2 and 3.

[0091] The installation comprises a molding device 1 designed to produce optical devices 2.

[0092] The molding device 1 is an injection molding, which comprises a mold cavity into which material is injected, the cavity having the complementary shape of the optical devices that it is desired to manufacture.

[0093] The mold is movable between a closed position, allowing the injection of material into the impression to produce the optical device(s), and an open position, allowing the evacuation of the molded optical device (see arrows O and F).

[0094] We note that the mold impression, in the example presented, has four cavities, allowing four lenses 2 to be produced.

[0095] The mold cavity is connected by a channel 10 to a material injector 11 which comprises material maintained in a fluid state at an injection temperature Ti. The injector is also designed to control the injection pressure Pi of the material into the mold. The mold 1 has a mold temperature Tm, which can be controlled by circulating a fluid in the mold parts.

[0096] The molding device is associated with a central unit 3 which controls the general operation of the mold (opening, closing, ejection of the molded devices, injection of material, control of the temperature in the injector, of the injection pressure Pi, of the temperature at which the mold is maintained Tm, etc.).

[0097] The installation according to the invention also comprises a device 4 for measuring the optical thickness of the molded lenses 2 (or optical devices).

[0098] It further comprises a first module 5, called the calculation module, which is associated with the optical thickness measuring device 4, in particular to calculate an error value from the optical measurement information obtained by the measuring device 4. The calculation module 5 also performs other calculations as will be seen later.

[0099] The installation finally comprises a second module 6, called the control module, which generates orders to be transmitted to the central control unit 3 for the operation of the molding device. This involves transmitting orders making it possible to control the values ​​of the injection temperature Ti of material into the mold, the injection pressure Pi and the operating temperature of the mold 1.

[0100] By mold operating temperature, it is understood that it is independently the temperature of the mold parts (which has an impact on the temperature of the internal walls of the mold cavities) or the temperature of the internal walls of the mold cavities.

[0101] The calculation module 5 and the control module 6 may or may not be integrated into the optical thickness measuring device 4, into the control unit 3 for the operation of the molding device, or produced independently, without departing from the scope of the invention.

[0102] Reference will now be made to a method according to the invention, where only an optical measurement made on the optical device 2 makes it possible to act on a molding control parameter. In the context of the example illustrated and presented, the method according to the invention aims to ensure production of lenses which best correspond to the specifications set beforehand, with an optical thickness, at the center of the lens in accordance with the specifications.

[0103] To do this, the method according to the invention provides manufacturing steps which make it possible to modulate the shape and dimensions of the molded lenses as they are manufactured, by ensuring feedback control of the molding parameters presented above: the injection temperature, the injection pressure and / or the mold temperature.

[0104] This control is ensured by optical measurements carried out by the measuring device 4, the molding parameters being modified according to calculations carried out from the measured data.

[0105] The process makes it possible to better avoid the slow post-molding relaxation phenomena of stress release in the lenses produced.

[0106] The optical measurements carried out essentially concern an optical thickness between two optical interfaces 21 and 22 of the lens: in other words, the measured optical thickness is the optical distance between the two lens surfaces 2.

[0107] The method according to the invention preferentially uses a measurement by propagation of an optical beam 7 (figure 1) which makes it possible to reveal in the same frame of reference the relative distances of the different faces of the lens.

[0108] This is measurement by measurement by point, or optical field: the system implemented by the measuring device 4 evaluates the return delay of each optical interface 21 and 22 encountered, the interfaces 21 and 22 being spaced by a distance D whose optical medium has an optical index N (at the measurement wavelength). This delay is equal to the product D x N.

[0109] Feedback on the molding parameters can then be done preferentially by comparison on this product D x N.

[0110] The optical thickness measurement EO is carried out at the center of said lens. EOm will be called the optical thickness of the lens measured by the measuring device 4, and EOc the optical distance that should theoretically be observed (i.e. the predetermined optical distance, upstream of the molding process). In addition, Ne is the predefined group index and Nm is the group index during the measurement.

[0111] The measuring device 4 determines Nm and transmits this index, with the measured optical thickness Dm to the first calculation module 5.

[0112] The first calculation module 5, previously filled with the value of the predetermined optical thickness EOc and that of the predetermined optical group index Ne, calculates an error value E according to the following formula:

[0113] E = ( EOm - EOc )

[0114] This error value should ideally be kept at 0.

[0115] We can then, from the calculated value of E over a molding cycle k (also noted Ek), take into account the sensitivity S= dE / dTk, Tk being a mold temperature Tm or the temperature at which the material is injected into the mold: Ti.

[0116] If the observed value of E is not zero, from this calculated sensitivity S, we can modify the value of the temperature considered for the molding cycle k+d, that is to say the molding cycle distant from “d” the molding cycle k from which lens 2 comes.

[0117] The temperature Tk+d will then be determined as follows:

[0118] Tk+d = Tk - (1 / s) X Ek.

[0119] The sensitivity s can be determined by the calculation module 5 during a previous molding cycle (molding cycle k-ô), by a variation (voluntary or observed) of the temperature between the molding cycle k-ô and the molding cycle k and the observation of the variation of the error value calculated for the cycle k (Ek) and that calculated for the cycle kd (Ek-5). ô can take the value 1, or a larger value depending on the number of devices molded before knowing the measurement result and the calculation to obtain the new setpoint T. The sensitivity s is then calculated as follows: s = (Ek - Ek-8) / (Tk - Tk-ô).

[0120] It may also have been obtained previously, and considered to vary little so as not to be systematically re-evaluated.

[0121] If the error Ek+d finally obtained after molding is not exactly 0, the same method can be continued in subsequent iterations, which can compensate for deviations in the knowledge of the sensitivity s (for example, if the sensitivity sa itself has temperature dependencies). As corrections are made to the molding parameters' temperatures, the error value E is brought significantly close to 0 (apart from measurement noise and other possible fluctuations).

[0122] Similarly, during dynamic feedback, the previous calculation can be maintained and a parameter modified at each cycle in order to compensate for various continuously varying parameters, such as, for example, the temperature of the location where the mold 1 is located, which would vary and create a residual variation on the injection temperatures Ti and mold temperatures Tm linked to molding, or a variation linked to a replenishment of material from the injector 11; with a material of slightly different initial temperature or composition, or transient effects linked to this replenishment.

[0123] If the error value E is however equal to 0, then no modification action will be taken (see reference 8 in Figure 2).

[0124] It should be noted that it is better to calculate E as previously indicated, rather than as (Dm - De) or as (Nm - Ne). Indeed, there are at least two reasons:

[0125] The optical deflection effect - also known as optical power - of a lens is equal to the difference of the integral of the optical index encountered multiplied by each distance traveled, and this in differential between the different zones encountered by the optical beam whose shape we are trying to modify. In other words, the optical power is a function of the spatial difference of the product D x N, between the different parts of the lens. Thus, it is very advantageous to create feedback on this product D x N: in fact, the process of shrinking the lens, at the moment it occurs, first involves a skin effect on the surface, which then causes a slight decrease in the density of the material internally, because of the solidification of the surface skin which prevents the maintenance of external pressure in the core of the lens.

[0126] Thus, the optical index Nm of the molded material is not necessarily uniform along each path of each ray of light. In other words, the product of the physical thickness times the realized optical index of the material is not exactly equal to the integral of the distance traveled multiplied by the optical index encountered locally by the beam.

[0127] Feedback on only the thickness D can produce a variation in the effective optical index obtained.

[0128] However, it may be sufficient if the thickness to be obtained is recalculated as a function of the average index obtained (preferably obtained on the lens itself, and at its center), but this amounts to formulating differently the consideration of the observed optical thickness which is in question in the method according to the invention.

[0129] The method according to the invention has another advantage: most of the shrinking takes place during the cooling of the molded lens 2, in the few tens of seconds to a few minutes following molding and ejection of the lens.

[0130] The shrinkage that has not yet taken place in lens 2 occurs very slowly: it has been found that it can last up to 1 month. Indeed, in the core of lens 2, the material can continue to reorganize because of the negative or lower stress than at the surface resulting from the lower pressure at the core of the material after the appearance of the surface skin and the shrinkage that continues in the still hot material at the core of the lens. Thus, the shrinkage that occurs slowly after molding can modify the effective thickness of the lens over the month following molding. A thickness measurement taken just after molding (say within a few minutes to a few hours) does not then provide a stable parameter of the lens, whereas the product of the optical thickness Dm by the optical index Nm will vary less because at first glance each optical beam will continue to encounter the same quantity of material, whether reorganized or not.

[0131] In summary, as what is counted on the optical delay is in the first approach the density of atoms encountered by the light, it is more accurate to evaluate the product D x N after injection because it evolves less, or not at all during the month which follows the design of the lens 2 by injection in a mold 1.

[0132] Figure 3 shows another embodiment of the method according to the invention, according to which the method comprises several measurements of optical thicknesses of the molded lens 2.

[0133] According to this implementation variant, we can measure the optical thickness at the center of the lens, and on the edges (figure 3). And even take into consideration a vector of optical elements relative to the molded optical device, to act on one or more molding parameters simultaneously.

[0134] This offers the advantage of evaluating the optical thickness Dm1 at the center, but also of providing an indicator of a thickness Dm2 on the peripheral parts of the optical zone, by continuity of variation between the support zone and the peripheral zones of the lens 2.

[0135] Dm2 is preferably measured at least on two points located in the vicinity of the edge of the optical part of lens 2 (see figure 3 - Dm2 and Dm2'), in order to provide an indicator of optical thickness in the vicinity of the edge of said optical device.

[0136] From these measurements Dm1, Dm2 and Dm2', several parameters can be deduced, such as the difference between design and realization between the gradient of D x N between the center and the periphery of the lens 2. It is then possible to control not only the thickness of the lens, but also partly its shape, for the following reasons:

[0137] The temperature during injection is broken down at least between the temperature in the injection piston (Ti) and the average temperature of the mold (Tm), or even the temperature of each cavity of the mold because it is common to inject several cavities simultaneously to make several lenses in each molding operation, as illustrated elsewhere in figure 1.

[0138] The feedback control of the injection temperature Ti and the mold temperature Tm can be done by maintaining or reducing the total cycle time, while modifying the average optical thickness EO of the lens: for example by increasing the injection temperature Ti, by decreasing the mold temperature Tm, and this by controlling the product Dm per Nm obtained with the optical thickness measuring device 4.

[0139] Thus, in accordance with the steps of the method illustrated in Figure 3, the first calculation module 5 determines a first error value Ec of an optical thickness at the center of the lens 2 and at least a second error value Ep of an optical thickness in the vicinity of the edge (at the periphery) of the lens 2. At least a third error value Ea can be calculated from measurements in the so-called assembly zone of the lens, optionally. This assembly zone is a location of the device in contact with other lenses, or with an assembly barrel which holds several lenses or optical devices to form an optical objective.

[0140] According to an implementation variant, the surface temperature of the molded optical device, called T p (p as skin) considered as uniform, or at its maximum value, can be modeled as:

[0141] Tp(t) = (Ti-Tm). exp(-kt) + Tm,

[0142] Where k is a constant related to the thermal conductivity in the material, and also to its shape, exp() the exponential function, and ln() the logarithmic function, inverse of the exponential.

[0143] We introduce a constant Ts called solidification temperature of said skin which depends on the material. The lens is considered to be demouldable when Tp(t) reaches Ts. This makes it possible to calculate a minimum cycle time to demould the lens equal to td = -(1 / k) * ln( (Ts-Tm) / (Ti-Tm)). This makes it possible to calculate the cycle time te by adding the other moulding times (mould opening, mould closing, injection) to the cooling time.

[0144] This modeling provides a relationship between Tm and Ti, for example by keeping te constant equal to an economic value to be observed. The constant k can be obtained from the observation of molding lenses of equivalent shape and material, or approximate. From this relationship between Tm and Ti, Ti, Tm, and the injection pressure Pi can be modulated to observe the sensitivities of the shape of the lens at its measurement point(s) to calculate the sensitivity parameters s, in particular for example for the error values ​​Ec, Ep previously defined.

[0145] This is how the shape profile of lens 2 can be modulated, by controlling the narrowing between the center and the peripheral zones differently.

[0146] On a convex lens, the reduction of the injection temperature Ti and the increase of the mold temperature Tm, at cycle k+1, while maintaining the cooling cycle time makes it possible to reduce the abruptness of skin formation, which, by modification (for example increase) of the injection pressure Pi, leads to a different replenishment of material from the core of the lens (for example greater) during cooling (as long as the periphery of the optical device is sufficiently plastic), thus modifying the differential shrinkage between center and periphery (in the sense of a limitation in the sense of the case of the example).

[0147] Thus the "effective shape", or rather the differential slowing power of light D x N between center and periphery is modified. The lens 2 produced can thus be better approached to a profile determined during design, for example, or controlled in relation to the optical quality obtained on the assembled lens.

[0148] In summary, the feedback control of Ti and Tm from optical thickness measurements at the center and periphery of the produced lens 2 offers the possibility of slightly modulating the optical efficiency profile of the lens according to its radius. This can make it possible to best adjust the shape to be obtained, without retouching the mold. This process is thus advantageous in terms of development time before moving on to mass production of lenses 2.

[0149] Advantageously, according to an efficient embodiment, several measured values ​​mi,k from the measurement of the optical device, at different locations i, can be considered at the molding cycle k.

[0150] Indeed, not only can cooling shrinkage modify the optical (and physical) thickness at the center of the lens, but also the modulation of the injection conditions can also modify the shape of the resulting optical device in addition to its thickness at the center. Thus, one can add thicknesses (optical or physical) measured at different positions of the optical part of the device.

[0151] Furthermore, the injection cycle time, which is an important economic parameter, could also be modified by thickness control, for example at the center of the optical device. Thus, the cycle time to be respected is one of the molding parameters that can be modified by the method according to the invention.

[0152] We can define the vector Mk at the cycle k with coordinates (rm,k; r .k; ... m a ,k) for example, if we consider “a” values ​​for a cycle k.

[0153] We can also preferably include among the quantities to be obtained, the total duration of the injection cycle tek, therefore including the duration before demolding, time defined between the injection time of a cycle k and the injection time of the following cycle k+1 (we could also consider the waiting time between the end of the step of injecting material into the mold and the opening of the mold, which, ultimately, would amount to the same thing). This cycle time is the one necessary to obtain demolding in good conditions, that is to say without risk that the shape of the optical device is modified during this operation. It is not a "measurement" of the optical device, but depending on the case, an observation that with the current control parameters, this cycle time (minimum) is necessary to obtain good demolding, or preferably a calculation from a model including at least the injection and mold temperatures, and the thickness profile of the optical device.This model can be established in different ways, empirically by previous observations, or by numerical calculation of a model modeling the diffusion of heat in the material of the injected device.

[0154] The vector Mk is then formally written (mi,k ; m2,k ; ... ; m a -i,k ; tek) (stopping the numbering of the measured parameters at a-1 to leave a parameters to be maintained around the device, the last of which is its manufacturing cycle time).

[0155] We can define the vector Ek (i.e. a vector of error values ​​to be calculated) as Mk-MT, where MT is the vector formed with the expected values ​​of the quantities mTj, called theoretical values, hence the letter T.

[0156] Ek = (Mk - MT)

[0157] It is also possible to simultaneously consider the control of several values ​​among the mold temperature Tm, the injection temperature Ti, the injection pressure Pi and the injection time.

[0158] We can define the vector Ck of control parameters, Ck = (Tmk ; Tik ; Pik ), or more generally, Ck = (ci,k ; ...Cb,k) with b the integer number of control parameters.

[0159] For example, c1, k may correspond to a mold temperature setpoint for cycle k; c2, k may correspond to an injection temperature setpoint for cycle k; and c3, k may correspond to an injection pressure setpoint for cycle k.

[0160] Finally, if we consider a linear or linearized behavior between the control parameters and the parameters of the obtained optical device, we can define by the matrix (S) called sensitivity matrix, of "a" lines and "b" columns.

[0161] As we seek to cancel or reduce the set of values ​​of 'Ek+d,

[0162] Then we obtain the following relation to obtain the control parameters 'Ck+d at cycle k+d, seeking to obtain 'Ek+d = 0:

[0163] We also write it 'ACk.d = -M * ('Ek)

[0164] WHERE ACk.d — Ck+d - Ck

[0165] The notation 'X' denotes the transposition of vectors to form column vectors, in order to be able to calculate the matrix product defined by the symbol '*' and the sums in the previous relation.

[0166] The matrix M is derived by calculating the inversion of the sensitivity matrix S which contains the individual sensitivities between the control parameters and the measured values, i.e. the partial derivatives d(mi,k) / d(k).

[0167] S = ( d(mi.k) / d(cj,k) ) matrix of a row by b columns, i varying from 1 to a, j varying from 1 to b.

[0168] For large variations in the molding control parameters Ci, it is possible that the coefficients of S evolve. In this case, the matrix S can be indexed by k.

[0169] The matrix S can be obtained in different ways:

[0170] - It can be obtained from a model of the behavior of the plastic mass during cooling. This model can be obtained by solving the heat diffusion equation on a finite element model describing the volume of the plastic material, the boundary and initial conditions of which are defined by the mold temperature, for example uniform, the initial condition linked to the injection temperature, and the calculation of the initial cooling of the plastic mass, linked to its travel time in the still empty mold, time itself obtained from the injection pressure and the effective diameter of the injection nozzle when it is open. - It can be measured empirically by small variations in the control values ​​and measurements of their effects on the controlled parameters.

[0171] - It can be obtained from an observation base based on experience from similar devices. This observation base can be created from an artificial intelligence module that enriches the base with scenarios that relate actions on control parameters in parallel with the consequences on the molded lenses.

[0172] From the matrix S, there are techniques for searching for an inverse matrix M, in order to calculate the value of the conditions to be applied to the time index k+d such that:

[0173] The coefficient relating to the variation of the cycle time (tck-tct) of the mold of the vector Ek will for example be kept at zero in the calculation above, to only modify the values ​​mi,k of the device to be controlled for i from 1 to a-1.

[0174] In a development phase, it may be appropriate to modify this cycle time if the calculation of the control parameters Ck+d leads to obtaining parameters at the limit or outside the limits previously mentioned, such as an injection pressure that is too high which would produce too much wear on the mold, or an injection temperature beyond the consistency of the plastic material (possible degradation of the plastic), or a curve mixing the two quantities, etc. Similarly, one can take into account the lower limits such as a risk of wrinkling of the surface of the plastic, or of non-filling, or of significant deformation after cooling, if the mold temperature becomes too low, or if the temperature difference Tm - Ti becomes too negative, which leads to similar risks, or even a limit curve between the two parameters, to avoid these same risks.

[0175] Preferably, the measuring step comprises the measurement of at least one optical thickness (named for example EOm, or mi) is carried out at the center of said optical device. Preferably, it is completed by an optical thickness at at least two points located in the optical zone of the lens (for example named m2 and m3), for example in the vicinity of the edge of said optical device, in order to provide an indicator of optical thickness in the vicinity of the edge of said optical device.

[0176] More preferably, at least one physical thickness of the optical device is measured in a so-called assembly zone (i.e. a measurement called r), preferably 4 for example, equally distributed angularly (r; ms; me; m?).

[0177] These values ​​of the device can still be supplemented by others, such as measurements at more points, or even such as the refractive index obtained.

[0178] It is understood from the preceding description how the method according to the invention makes it possible to act on the molding parameters to produce optical devices with characteristics which best correspond to the characteristics defined before the design.

[0179] It should be noted that the process according to the invention saves production time, compared to that which would be necessary to redesign an injection mold.

[0180] It should be understood that the invention is not limited to the implementation examples specifically described and shown and that it extends to the implementation of equivalent means.

Claims

CLAIMS 1. Method for manufacturing at least one optical device (2) by molding, by injecting material into a mold (1) according to one or more molding control parameters (ci, k) applied to the molding cycle (k), parameters comprising at least one injection temperature (Ti) of said material, and / or a temperature of said mold (Tm) and / or an injection pressure of said material (Pi), said method being characterized in that it comprises the following steps, after molding said at least one optical device: - measurement of at least one parameter (mi, k) of said at least one molded optical device at the end of said molding cycle (k), - determination of at least one modified molding control parameter (ci, k+d) for a subsequent molding cycle (k+d) of d cycles of said molding cycle (k), said modified molding control parameter being calculated from at least one of said molding control parameters (ci, k) of the molding cycle (k) from which said at least one molded optical device originates, said modified molding control parameter (ci, k+d) being a function of a scalar or vector error (Ek) calculated from said at least one measured parameter (mj, k) of said at least one molded optical device at the end of the molding cycle (k) and at least one predetermined theoretical parameter (mtj).

2. Manufacturing method according to claim 1, characterized in that said at least one measured parameter (mi, k) of said at least one optical device (2) comprises an optical thickness (EOm) measured between at least two optical interfaces (21, 22), of said at least one optical device (2), and in that the error value (Ek) is calculated from said measured optical thickness and a predetermined optical thickness (EOc).

3. Method according to claim 2, characterized in that said at least one optical device (2) is an optical lens and in that said measurement of said optical thickness (EOm) is carried out at the center of said lens.

4. Method according to one of the preceding claims, characterized in that said at least one measured parameter (mi, k) contains at least one physical or optical thickness measurement point in an active optical zone of the lens.

5. Method according to one of the preceding claims, characterized in that said at least one measured parameter (mi, k) contains at least one physical or optical thickness in the so-called assembly zone.

6. Method according to one of the preceding claims, characterized in that said at least one measured parameter (mi, k) comprises a minimum cycle time to be applied to allow demolding of said lens.

7. Method according to one of the preceding claims, characterized in that a relationship is established between the mold temperature (Tm), the injection temperature (Ti), and the cycle time (te) in order to model part of the relationships between the control parameters to evaluate the sensitivities (s).

8. Method according to one of the preceding claims, characterized in that the modified molding control parameter (G, k+d) is obtained from a sensitivity relationship between a deviation vector of one or more control parameter values, between the molding cycle (k) and the subsequent molding cycle (k+d), said deviation vector corresponding to the following formula: (tACk.d) = (tCk+d - tCk), 9. Method according to one of the preceding claims, characterized in that the sensitivity relationship is linearized, the vector (tACk.d) being multiplied by a sensitivity matrix (S), to determine an error deviation vector '(E) = '((Mk) - (MT)) between realization parameters (Mk) at an instant (k) and predetermined theoretical parameters (MT), according to 10. Method according to claim 9, characterized in that said sensitivity matrix is ​​obtained empirically.

11. Method according to claim 9, characterized in that the sensitivity relationships are obtained by calculating the achievement of a solidification state on the surface of the optical device and its supply of material, from a digital model based on a set of elements comprising a modeling of the cooling and the viscoplastic behavior of the optical device during its solidification, under the effect injection pressure, and injection temperature and mold temperature conditions.

12. Method according to claim 9, characterized in that said sensitivity matrix is ​​obtained by an artificial intelligence module having learned the consequences of applying molding characteristics on the formation of the molded lens, by observing the molding of other lenses and the cycle time conditions to be observed.

13. Method according to one of the preceding claims, characterized in that the matrix (S) is inverted into a matrix (M) to obtain several modified molding control parameters, by applying the following equation:

14. Method according to one of the preceding claims, characterized in that only one parameter (mi, k) of said at least one molded optical device is measured at the end of said molding cycle (k).

15. Method according to one of the preceding claims, characterized in that a single modified molding control parameter (ci,k+d) is determined for a subsequent molding cycle (k+d) of d cycles of said molding cycle (k).

16. Method according to claim 14, characterized in that the modified molding parameter (ci, k+d) comprises the mold temperature (Tm, k+d) or the injection temperature (Ti, k+d) or the injection pressure (Pi, k+d) for a following molding cycle (k+d), following a considered molding cycle (k) of at least one optical device, from the following formula: Ci, k+d = Ci,k - SX Ek, where: - Ek is the error value calculated for said at least one optical device of the considered molding cycle k, - Ci,k is a control value taken from the mold temperature value, or the material injection temperature value in the mold, or the injection pressure, measured or applied for the molding cycle considered k and - S is a sensitivity relative to the considered molding cycle k, S being calculated as follows, S = dEk I dci,k, Or S = (Ek - Ek-g) / (Ci,k— Ci,ks) WHERE - Ek-s is the error value calculated for the previous molding cycle k-5, and - Ci,ks is the control value, measured for the previous cycle k-§.

17. Installation for implementing the method according to any one of the preceding claims, said installation comprising: - a molding device (1) for at least one optical device (2), said molding device (1) comprising an injection mold (1) and being associated with a central control unit (3) for operating said injection mold (1) - an optical thickness measuring device (4), - a calculation module (5) of at least one error value, - a control module (6), capable of communicating with said central control unit (3) of said at least one injection mold (1), said control module (6) being capable of transmitting to said central control unit (5) orders concerning molding control parameters (ci, k) during a molding cycle (k, k+d).

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