PRESS MOLDING METHOD OF GLASS OPTICAL ELEMENT

A press molding method of a glass optical element using a mold, the method including plural steps with pressurizing, in each of which load is imposed on a piece of glass material at a temperature above the glass transition temperature, and a step without pressurizing between two steps with pressurizing, wherein in a step without pressurizing between a first step with pressurizing and a second step with pressurizing, the second step with pressurizing being the next step with pressurizing after the first step with pressurizing, the temperature of the mold is reduced by 50 degrees centigrade or greater with respect to the temperature of the mold in the first step with pressurizing and then the mold is heated before the start of the second step with pressurizing.

TECHNICAL FIELD

The present invention relates to a press molding method of a glass optical element.

BACKGROUND ART

When optical glass elements, particularly those which are required to be shaped with a high accuracy, are formed through press molding, gas generated in an enclosed space between the mold surface and the glass material tend to affect the accuracy of the shape With this being the situation, when glass material undergoes molding by a glass mold press machine to mold an optical element, molding methods in which a step in which pressurizing is carried out and a step in which pressurizing is not carried out are alternately repeated to discharge the gas in the enclosed space between the mold surface and the glass material have been developed (Patent document 1, for example). However, for a lens of which the sag is relatively great and the radius of curvature is relatively small, a desired shape with a sufficiently high accuracy cannot be obtained through such conventional methods as described above.

Accordingly, there is a need for a press molding method of a glass optical element by which a desired shape of the element with a sufficiently high accuracy can be obtained independently of the shape of the element.

PRIOR ART DOCUMENT

Patent Document

The object of the present invention is to provide a press molding method of a glass optical element by which a desired shape of the element with a sufficiently high accuracy can be obtained independently of the shape of the element.

SUMMARY OF THE INVENTION

A press molding method of a glass optical element using a mold, the method including plural steps with pressurizing, in each of which load is imposed on a piece of glass material at a temperature above the glass transition temperature, and a step without pressurizing between two steps with pressurizing. In a step without pressurizing between a first step with pressurizing and a second step with pressurizing, the second step with pressurizing being the next step with pressurizing after the first step with pressurizing, the temperature of the mold is reduced by 50 degrees centigrade or greater with respect to the temperature of the mold in the first step with pressurizing and then the mold is heated before the start of the second step with pressurizing.

In the press molding method according to the present invention, because of a difference in thermal contraction between the piece of glass material and the mold, the thermal contraction being caused by reducing the temperature of the mold by 50 degrees centigrade or greater with respect to the temperature of the mold in the first step, the gap between both of them is widened and therefore discharge of gas in the enclosed space between both of them is facilitated in a step without pressurizing. Further, in the press molding method according to the present invention, a condition develops, in which the shape of the portion near the surface of the piece of glass material can be relatively easily altered, and therefore the piece of glass material can be easily molded into the shape of the mold cavity. As a result, a glass optical element shaped with a sufficiently high accuracy can be obtained by the press molding method according to the present invention.

In the press molding method of a glass optical element according to a first embodiment of the present invention, the temperature of the mold is reduced to a temperature below the glass transition temperature in a step without pressurizing.

In the press molding method of a glass optical element according to a second embodiment of the present invention, a value of load imposed in the second step with pressurizing is equal to or greater than a value of load imposed in the first step with pressurizing.

In the press molding method of a glass optical element according to a third embodiment of the present invention, a value of load imposed in the second step with pressurizing is greater than a value of load imposed in the first step with pressurizing.

In the press molding method of a glass optical element according to a fourth embodiment of the present invention, the temperature of the mold is reduced by an amount that is equal to or smaller than 15 degrees centigrade in a step with pressurizing before transition from the step with pressurizing to a step without pressurizing.

By reducing the temperature of the mold before transition from a step with pressurizing to a step without pressurizing, the viscosity of the piece of glass material becomes higher and therefore a possible undesirable change in shape that may take place when the load is removed can be effectively prevented.

DESCRIPTION OF EMBODIMENTS

FIG.1shows an example of a glass mold press machine with which a press molding method of a glass optical element according to the present invention is practiced. The glass mold press machine is referred to as the press machine hereinafter. The press machine100includes a mold120, an upper shaft for pressurizing111and a lower shaft for pressurizing113. The upper shaft for pressurizing111and the lower shaft for pressurizing113are referred to respectively as the upper shaft111and the lower shaft113. The mold120includes an upper mold die121, a lower mold die125and a guide123. The upper mold die121and the lower mold die125are referred to respectively as the upper die121and the lower die125herein after. The upper shaft111is immovable. The lower die125is made to move upward by moving the lower shaft113by a servomotor not shown in the drawing in order to make a piece of glass material200undergo molding using the upper die121and the lower die125.

FIG.2shows a heating apparatus and a cooling apparatus for the mold120. The mold120can be heated by a high-frequency induction heating coil131. The mold120can be cooled using nitrogen gas blown thereagainst through a nozzle133. The mold120can be heated and cooled by any other means such as an electric heater, a cooler of a water-cooling type or the like.

FIG.3illustrates sensors attached to the press machine100. Temperature of the mold120is measured by a thermocouple145. Load imposed on the upper shaft111is measured by a load cell143. Displacement of the lower shaft113is measured by an encoder141of the servomotor.

In general, when a piece of glass material undergoes molding by a glass mold press machine to mold an optical element, a step in which pressurizing is carried out and a step in which pressurizing is not carried out are alternately repeated to discharge the gas in the enclosed space between the piece of glass material and the mold surface as described above (Patent document 1, for example). In this case, the temperature of the glass material is kept above the transition temperature while a step in which pressurizing is carried out and a step in which pressurizing is not carried out are alternately repeated. Further, in general, an area of a cross section of an object to be molded, the cross section being perpendicular to the shafts for pressurizing, increases as the molding process proceeds and therefore the load imposed on the object to be molded is made to increase so as to keep a pressure acting on the object to be molded constant.

FIG.4is a flowchart describing a process of a press molding method of a glass optical element according to the present invention.

FIG.5shows a change in the shaft position of the press machine, a change in load of the press machine and a change in temperature of the mold in the process of a press molding method of a glass optical element according to the present invention. InFIG.5, the transition temperature of glass is represented by Tg. The glass material is dense lanthanum flint.

In step S1010ofFIG.4, load is imposed by the press machine100on a piece of glass material200, temperature of which is above the transition temperature, to alter the shape thereof.

At the point in time represented by t1inFIG.5, by which time the temperature of the mold120has been kept at a predetermined temperature above the transition temperature for a predetermined time period, lifting of the lower shaft113is started to start pressing. Since by the point in time t1the mold120has been kept at the predetermined temperature above the transition temperature for the predetermined time period, the temperature of the glass material200has become above the transition temperature.

After the start of pressing the load reaches a predetermined value at the point in time represented by t2inFIG.5, and after that lifting of the lower shaft113is continued while keeping the load at the predetermined value. Till the point in time represented by t3inFIG.5, by which time the position of the lower shaft113has reached a predetermined value, the load is kept at the predetermined value. At the point in time t3, lowering of the lower shaft113is started. As a result, the load becomes zero. The period of time between the point in time t1and the point in time t3corresponds to step S1010inFIG.4. Step S1010is referred to as a step with pressurizing.

In step S1020ofFIG.4, the piece of glass material200is cooled through cooling of the mold120using the nozzle133after the load has been removed. The cooling of the mold120using the nozzle133is carried out in such a way that the temperature of the mold120cooled becomes lower by a predetermined amount of temperature than the temperature of the mold120in the step with pressurizing (the temperature of the mold120at the point in time t1and at the point in time t2). In the present example, the predetermined amount of temperature is approximately 100 degrees centigrade. At the point in time t4inFIG.5, the temperature of the mold120is lower than the temperature in the step with pressurizing by approximately 100 degrees centigrade and lower than the transition temperature of the glass. The period of time between the point in time t3and the point in time t4corresponds to step S1020inFIG.4. The cooling rate in step S1020should preferably be made as great as possible from the stand point of efficiency.

As the temperature of the mold120changes, the temperature of the glass material200also changes. When the temperature of the mold120is kept for a predetermined time period, the temperature of the glass material200, at least the surface thereof, becomes equal to the temperature of the mold120. According to the findings of the inventors of the present invention, the temperature of the cooled mold120should be made lower by 50 degrees centigrade or greater than the temperature of the mold120in the step with pressurizing (the temperature of the mold120at the point in time t1and at the point in time t2) in order to obtain effects of the present invention. An amount of change in temperature described above will be described later.

The present invention can be carried out based on the temperature of a heater instead of the temperature of a mold. Even when the present invention is carried out based on the temperature of a heater, the amount of change in temperature is identical.

In the example shown inFIG.5, the mold120is allowed to cool slowly by adjusting the high-frequency induction heating coil131during the time period between the point in time t2and the point in time t3. In the example shown inFIG.5, the point in time t3at which lowering of the lower shaft113is started is determined in such away that the time period of the slow cooling is appropriate. An amount of decrease in temperature of the mold caused by the slow cooling is approximately 15 degrees centigrade. The decrease in temperature of the mold during a step with pressurizing makes the viscosity of the glass material greater and an effect of preventing a possible undesirable change in shape that may be generated when the load is removed can be achieved. The slow cooling process described above during a step with pressurizing can be omitted.

In step S1030ofFIG.4, the glass material is heated up to a temperature above the transition temperature.

At a predetermined point in time prior to the point in time t4, heating of the mold120by the high-frequency induction heating coil131is started. “A predetermined point in time prior to the point in time t4” means a point in time by which time the mold120has been cooled to a temperature that is higher by a predetermined amount than a target minimum temperature in a step without pressurizing. “A step without pressurizing” will be described later. The temperature that is higher by a predetermined amount than a target minimum temperature is determined in consideration of the heat capacity of the mold120in such a way that the temperature of the mold120will fall to the target minimum temperature. Then the temperature of the mold120is raised by the high-frequency induction heating coil131and after that the temperature of the mold120is kept at a temperature above the transition temperature for a predetermined time. The predetermined time described above is determined in such a way that at least the portion near the surface of the glass material200becomes above the transition temperature.

The period of time between the point in time t4and the point in time represented by t1′ inFIG.5corresponds to step S1030inFIG.4. The point in time t1′ is the point in time at which the succeeding step with pressurizing is started. The succeeding step with pressurizing will be described later.

During step S1020and step S1030load is not imposed on the glass material200. A set of steps S1020and S1030is referred to as a step without pressurizing.

In step S1040ofFIG.4, it is determined whether the succeeding step with pressurizing is the final one or not. If the succeeding step with pressurizing is not the final one, the process goes back to step S1010and at the point in t1′, by which time the temperature of the mold120has been kept at a predetermined temperature above the transition temperature for a predetermined time period, the succeeding step with pressurizing is started. In this way a step with pressuring and a step without pressuring are alternately repeated. If the succeeding step with pressurizing is the final one, the process goes to step S1050.

The number of repetitions of steps with pressurizing is empirically determined in advance. If the number is reached by the succeeding step with pressurizing, the succeeding step with pressurizing is regarded as the final one.

In step S1050ofFIG.4, at the point in t1′, by which time the temperature of the mold120has been kept at a predetermined temperature above the transition temperature for a predetermined time period, lifting of the lower shaft113is started to start the final step with pressurizing. Load is imposed by the press machine100on the piece of glass material200, temperature of which is above the transition temperature, to alter the shape thereof and after that a finishing process is carried out. In the finishing process, heating by the high-frequency induction heating coil131is suspended and after that the mold120is cooled by blowing nitrogen gas using the nozzle133to a temperature at which the mold120can be removed.

An amount of change in temperature between a step with pressurizing and a step without pressurizing will be described below.

FIG.6shows the piece of glass material200, the upper die121and the lower die125during a cooling period (step S1020) in a step without pressurizing in a press molding method of a glass optical element according to the present invention. The cooling period described above in a step without pressurizing continues from the point in time t3to the point in time t4shown inFIG.5. During the cooling period, the surface of the piece of glass material200is cooled and the temperature of the portion near the surface falls InFIG.6, the portion near the surface, the portion having a relatively low temperature, is represented schematically by dots less densely distributed and the portion near the center, the portion having a relatively high temperature, is represented schematically by dots more densely distributed.

FIG.10shows an example of linear expansion of glass and that of a mold. InFIG.10, the linear expansion of the glass is represented by a solid line and the linear expansion of the mold is represented by an alternate long and short dash line. The horizontal axis ofFIG.10indicates temperature and the vertical axis ofFIG.10indicates a ratio of a change in length ΔL to the original unit length L0due to a change in temperature. When the change in temperature is represented by ΔT, a coefficient of linear expansion is expressed by the following expression.

According toFIG.10, the coefficient of linear expansion of the mold is 4.4 (×10−6) and the coefficient of linear expansion of the glass at a temperature below and near the transition temperature is 110 (×10−7). Provided that the temperature of the glass material falls by 50 degrees centigrade from the transition temperature, a difference in change in length for the length of 1 millimeter between the glass and the mold due to a difference in coefficient of linear expansion between both of them is (110−44)×50=3300 (×107) millimeters, that is approximately 0.3 millimeters. The difference in change in length due to the difference in coefficient of linear expansion between both of them corresponds to the gap G1and the gap G2shown inFIG.6. Thanks to the gap, the gas in the enclosed space between both of them can be more easily discharged.

Further, according toFIG.10, the coefficient of linear expansion of the glass remarkably increases when the temperature exceeds the transition temperature.

In general, considering a difference between linear expansion of glass and that of a mold around the transition temperature, a gap between both of them that will be caused by decrease in temperature of 50 degrees centigrade from the temperature in a step with pressurizing is sufficiently great enough to discharge the gas in the enclosed space between both of them. Accordingly, an amount of decrease in temperature of the glass and the mold, the decrease in temperature being caused by cooling, should preferably be 50 degrees centigrade or greater.

FIG.7shows the piece of glass material200, the upper die121and the lower die125in a step without pressurizing in a conventional press molding method of a glass optical element. In the step without pressurizing in a conventional press molding method, the mold120is not cooled and the temperature of the mold120is maintained. Accordingly, the temperature inside piece of glass material200is uniform. InFIG.7, the state in which the temperature inside piece of glass material200is uniform is schematically represented by dots that are uniformly and densely distributed. Paragraph in Patent document 1 describing a conventional press molding method of a glass optical material describes that a high-pressure gas enclosed between the glass and the mold is discharged to the outside through gas passages between both of them. In the present invention, a difference in thermal contraction between both of them, the thermal contraction being caused by cooling, additionally widens the gap between both of them and therefore the gas in the enclosed space between both of them can be more easily discharged.

FIG.8shows the piece of glass material200, the upper die121and the lower die125at the start of step S1040with pressurizing, that is, at the point in time t1′ inFIG.5in a press molding method of a glass optical element according to the present invention. During the heating period after the point in time t4inFIG.5, the piece of glass material200is heated from the exterior and the temperature of the portion near the surface rises. InFIG.8the portion near the surface, the portion having a relatively high temperature, is represented schematically by dots more densely distributed and the portion near the center, the portion having a relatively low temperature, is represented schematically by dots less densely distributed. The temperature of the portion represented by dots more densely distributed is higher than the transition temperature.

By way of example, when the temperature of glass material rises by 50 degrees centigrade and exceeds the transition temperature, the viscosity is supposed to decrease by a factor of 0.1 to 0.01.

The viscosity of the portion represented by dots more densely distributed is lower than that of the portion represented schematically by dots less densely distributed. When load is imposed the piece of glass material200in the state shown inFIG.8, the shape of the portion near the surface represented by dots more densely distributed can be altered more easily than the portion represented by dots less densely distributed. Accordingly, the piece of glass material200can be easily molded into the shape of the mold cavity.

FIG.9shows the piece of glass material200, the upper die121and the lower die125at the start of a step with pressurizing in a conventional press molding method of a glass optical element. In a step without pressurizing in the conventional press molding method, the mold120is not cooled and the temperature of the mold120is maintained. Accordingly, the temperature inside the piece of glass material200is uniform. As a consequence, a state in which the shape of the portion near the surface of the piece of glass material200can be relatively easily altered cannot be realized. InFIG.9, the state in which the temperature inside piece of glass material200is uniform is schematically represented by dots that are uniformly and densely distributed.

Experiments in which various values of the amount of change in temperature of the mold120between a step with pressurizing and a step without pressurizing are employed for the press molding method were carried out.

Table 1 describes the results of the experiments in which various values of an amount of change in temperature of the mold120between a step with pressurizing and a step without pressurizing are employed for the press molding method.

Experiment 1 is the example described withFIG.5. The amount of change in temperature in Experiment 1 is 102 degrees centigrade. The amounts of change in temperature of Examples 2 to 4 are 62 degrees centigrade, 52 degrees centigrade and 41 degrees centigrade, respectively. By Examples 1 to 3, in each of which the amount of change in temperature was 50 degrees centigrade or greater, optical elements, each of which has a good or an acceptable shape, were obtained. By Example 4, in which the amount of change in temperature was 41 degrees centigrade, an acceptable shape was not obtained due to the residual gas.

Thus, in a press molding method according to the present invention, because of a difference in thermal contraction between the piece of glass material200and the mold120, the thermal contraction being caused by cooling, the gap between both of them is widened and therefore the gas in the enclosed space between both of them can be more easily discharged in a step without pressurizing. Further, in the press molding method according to the present invention, a condition develops, in which the shape of the portion near the surface of the piece of glass material200can be relatively easily altered, and therefore the piece of glass material200can be easily molded into the shape of the mold cavity.

Through a press molding method according to the present invention, an aspherical lens having the diameter of 1 millimeter, the sag of 0.3 millimeters and the center thickness of 1 millimeter was successfully formed with an accuracy of 0.1 micrometers in P−V value (the value indicating a difference in dimension between a designed lens and a molded lens).