JPH08244085A - Method for injection molding - Google Patents

Abstract

PURPOSE: To obtain an optical part with high accuracy and a little birefringence in a short cycle time. CONSTITUTION: After a molten resin is injected and filled into a cavity part 8 formed by a fixed mold 1, a fixed insert 3 of a movable mold 1 and a movable insert 4 in injection molding, it is kept cooling by the time when the resin temp. becomes at least the glass transition point and uniform. Thereafter, the temp. of the fixed and movable inserts 3 and 4 is lowered in such a way that the increment (the changing ratio to the time) in the amt. of birefringence of the resin becomes const.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an injection molding method capable of obtaining an optical component having a high accuracy and a small internal strain (amount of birefringence) with a short cycle time.

[0002]

2. Description of the Related Art Conventionally, as a method for molding a molded article having a highly accurate transfer property, there is a method disclosed in Japanese Patent Laid-Open No. 64-36421. In this method, a mold heated to a temperature exceeding the glass transition point is filled with resin, and then the mold is kept outside the molding machine at a constant cooling rate (1.3 ° C./min in the example of the above publication). After cooling, the molded product is taken out when the temperature falls below a certain temperature.

That is, according to the above conventional method, as shown in FIG.
As shown by the broken line B in (a), the cooling is performed so that the resin temperature linearly decreases with the passage of time. For this reason, as indicated by a broken line D in FIG. 1B, the birefringence amount changes so as to draw a gentle curve with the passage of time, and reaches a constant value over a considerably long time. Become.

[0004]

However, the above-mentioned conventional molding method has the following problems. In general, when cooling a plastic resin, internal stress remains when it is cooled rapidly.
A method is adopted in which no internal stress is left. However, internal stress is not likely to remain uniformly in all cooling processes (resin temperature). For example, if the resin is cooled near the glass transition point, internal stress tends to remain, but if the temperature drops to a certain extent and the resin is completely solidified, internal stress is unlikely to remain even if it is cooled to some extent. Tend.

That is, when the cooling process is simply performed at a constant cooling rate as in the conventional method, the cooling rate must be slowed down more than necessary in order to suppress the generation of internal stress. As a result, there was a problem that the cycle time had to be extended.

The present invention has been made in view of the above conventional problems. The invention according to claim 1 is an injection molding method capable of obtaining an optical component with high accuracy and a small amount of birefringence in a short cycle time. The purpose is to provide. In addition to the above object, an object of the invention according to claim 2 is to provide an injection molding method that does not require a complicated structure of the injection molding machine. In addition to the above object, an object of the invention according to claim 3 is to provide an injection molding method capable of controlling the mold temperature with higher accuracy.

[0007]

In order to solve the above-mentioned problems, the invention according to claim 1 is, in an injection molding method, at least a resin temperature is glass transition after injection filling a cavity of a mold with a molten resin. Cooling was maintained until the temperature was above the point and became uniform, and then the mold temperature was lowered so that the increment of the birefringence amount of the resin became constant. The invention according to claim 2 is characterized in that, in the invention according to claim 1, before the molding is started, a rate of change of the birefringence amount with respect to the temperature of the resin is calculated in advance, and the mold temperature is controlled based on the data. And The invention according to claim 3 is characterized in that, in the invention according to claim 1, the mold temperature is controlled while actually measuring the birefringence amount of the resin during molding.

[0008]

After the molten resin is injected and filled into the cavity of the mold, if the resin is kept cold at least until the resin temperature is equal to or higher than the glass transition point and becomes uniform, the resin has no stress inside. Here, the time to keep cool depends on the wall thickness of the molded product, and if the wall thickness is thick, it takes a long time to reach a uniform temperature, so it is necessary to lengthen the cool keeping time. On the other hand, thin ones can be kept cool for a short time.

A solid line C in FIG. 1B shows a state where there is no internal stress (state where there is no birefringence) due to keeping cold.
As shown by, the mold temperature is lowered so that the increment of the birefringence amount (the rate of change of the birefringence amount with the passage of time) becomes constant. That is, as indicated by the solid line A in FIG. 1A, the mold temperature is slowly decreased in the resin temperature region where the birefringence is likely to increase, and the mold cooling rate is increased in the resin temperature region where the birefringence is unlikely to increase. To In this way, the rate of decrease in mold temperature is optimally controlled according to the degree of influence on the increase in internal stress (increase in birefringence), so that the product can be cooled in the shortest time. FIG.
As can be seen from (a) and (b), according to the present invention,
The amount of time required to reach the same small amount of birefringence and the same resin temperature as in the conventional technique is greatly shortened.

[0010]

【Example】

[Embodiment 1] FIG. 2 shows the shape of a lens molded in this embodiment. The lens 30 has an outer diameter of 80 mm, a center thickness of 3 mm, and an edge thickness of 10 mm. The material of the lens 30 is a polyolefin resin (Nippon Zeon Co., Ltd.).
Manufactured by ZEONEX 280S). The glass transition point of this polyolefin resin at atmospheric pressure is 140 ° C.

FIG. 3 shows a mold used in this embodiment, and a fixed insert 3 is fixed inside the fixed mold 1. In addition, inside the movable-side mold 2, the movable-side insert 4
Has been inserted. Furthermore, inside each nest 3, 4,
Temperature control paths 5 and 6 are formed, and both temperature control paths 5 and 6 are connected to an external heat cycle temperature controller 7 for communication. A cavity 8 having a shape corresponding to the lens 30 shown in FIG. 2 is formed between the fixed side insert 3 and the movable side insert 4. Further, inside the fixed side insert 3, a thermocouple 9 for measuring the mold temperature is fixed.

The lens 30 is molded by the injection molding die having the above structure. The molding method will be described below. First, the fixed side mold 1 and the movable side insert 2 are preheated to 155 ° C.
It was heated to. Molten resin at about 250 ° C. was injection-filled into the cavity 8 of the mold in that state. After filling, it was kept cold for 5 minutes. During the cold keeping, the temperature of the resin reached the uniform temperature of 155 ° C. which was the same as that of the inserts 1 and 2. afterwards,
The heat cycle temperature controller 7 was operated and the mold temperature was cooled to 120 ° C. so that the increment of the birefringence amount of the molded product was constant, and then the molded product was taken out and naturally cooled in air to normal temperature. .

Specifically, the mold temperature is from 155 ° C. to 14
It cooled at a rate of about 5 ° C./minute until it reached 6 ° C. Also,
About 2 ℃ / when the mold temperature goes from 146 ℃ to 142 ℃
Minute ratio, about 0.6 from 142 ℃ to 138 ℃
° C / min rate, from 138 ° C to 135 ° C about 1.5 ° C / min, from 135 ° C to 130 ° C about 4 ° C / min, from 130 ° C to 120 ° C Was cooled at a rate of 10 ° C./min. Molding was carried out for a total cooling time of about 20 minutes.

As a result, the amount of birefringence is equivalent to that in the case where the conventional cooling is performed at a constant rate of 1.0 ° C./min from 155 ° C. to 120 ° C. (cooling time of about 40 minutes). The lens was molded. FIG. 4 shows a change pattern of the mold temperature and a change pattern of the birefringence amount in this example. As can be seen from FIG. 4, the amount of birefringence increases almost linearly with the cooling time.

The mold temperature change pattern used in this example so that the change in birefringence amount was constant was obtained as follows. First, the actual molded product is heated to a temperature at which the internal stress is completely relaxed (155 ° C.). Then, it is cooled at a rate of 0.2 ° C./minute. During the cooling process, the molded product is taken out at several temperature points, soaked in water or the like, and rapidly cooled. The birefringence amount at each temperature point is measured, and the temperature dependence of the birefringence amount change (difference in birefringence amount / temperature difference) is calculated from the difference in birefringence amount between the temperature points. Then, the cooling gradient at each temperature is determined so as to be inversely proportional to the degree of temperature dependence.

In this embodiment, an example in which a polyolefin resin is used as the injection molding material is shown, but the same effect as in this embodiment can be obtained by using a polycarbonate resin having a large amount of intrinsic birefringence. You can Further, the shape of the molded product is not limited to the concave lens, and a convex lens, a prism, an F-θ lens or the like can be used for molding by the same method. Further, in this embodiment, the molten resin is injected and filled into the mold heated above the glass transition point (155 ° C.).
(° C) injection-filled into the mold, then rapidly reheat to a temperature (above the glass transition point) at which birefringence disappears, and then lower the mold temperature so that the increment of birefringence of the molded product becomes constant. You may do it.

According to this embodiment, the rate of decrease of the mold temperature is optimally controlled according to the degree of influence on the increase in birefringence, so that the product can be cooled in the shortest time. That is, a lens having an equivalent level of birefringence could be molded in about half the cooling time compared to molding with a constant temperature cooling gradient.

[Embodiment 2] FIG. 5 shows a mold used in this embodiment, and only points different from Embodiment 1 will be described below. In this embodiment, a birefringence measuring device for directly measuring the change in birefringence of the die cavity is incorporated in the die. This birefringence measuring device is configured inside a fixed-side insert 13 and a movable-side insert 14 that form a cavity 12. The cavity 12 is provided inside the stationary insert 13.
Strain-free glass 15, sensitive color plate 16, polarizing plate 1 in this order from the side
7 is fixed. In addition, a white light source 18 is fixedly provided inside the stationary insert 13, and the white light source 18 is fixed in the cavity 12.
It is possible to generate light in any direction.
On the other hand, inside the movable-side insert 14, an unstrained glass 19 and a polarizing plate 20 are fixedly installed in order from the cavity portion 12 side. In addition, inside the movable side insert 14, the CCD camera 2
1 is provided so that an image in the direction of the cavity 12 can be displayed on the monitor 22 outside the mold. In addition, the fixed plate 11 and the movable plate 10 are independently provided to the fixed side insert 13 and the movable side insert 14, respectively.
A temperature control pipe 23 is arranged in the vicinity of.

Using the injection molding die having such a structure,
After the molten resin was injected and filled into the cavity 12 inside the mold heated above the glass transition point, the resin was kept cold until the resin reached a uniform temperature. After that, the heat cycle temperature controller 7 is operated in the same manner as in Example 1, and the mold temperature is adjusted by the temperature control tube 23 so that the increment of the birefringence amount of the molded product becomes constant. Lowered.

At this time, the rate of decrease of the mold temperature was controlled while actually measuring the birefringence amount of the resin inside the cavity 12 output to the monitor 22. That is, the white light source 18 fixed inside the fixed-side insert 13 is caused to emit light,
The linearly polarized light generated by passing through the sensitive color plate 16 and the polarizing plate 17 passes through the inside of the cavity portion 12 (inside the resin), and further passes through the polarizing plate 20 inside the movable side insert 14,
The temperature was controlled while observing the light with the CCD camera 21. When the resin is not contained in the cavity 12, it looks as a sensitive color (red-purple), but when the resin that has birefringence is in the cavity 12, an optical path difference occurs in the light, and
An interference fringe pattern (colors are formed in the order of red purple → reddish brown → cyan → sky blue → light green → light yellow green → white) is seen every 5 nm. By utilizing this, the change amount (increment) of the birefringence amount can be measured by measuring the change speed of the color of the same portion. The strain-free glasses 15 and 19 are attached to form the cavity portion 12, and since they are strain-free, they do not affect the measurement of the birefringence amount.

According to this embodiment, since the birefringence amount can be measured in real time, it is possible to control with high accuracy the rate of decrease of the mold temperature at which the increment of the birefringence amount becomes constant. As a result, a molded product can be obtained in a more optimal cooling time, that is, in the shortest time.

[0022]

As described above, according to the first aspect of the present invention, the mold temperature lowering speed can be optimally controlled according to the degree of influence on the increase in birefringence amount. The product can be cooled with high accuracy in a short time and with a reduced amount of birefringence. According to the invention of claim 2, in addition to the above effects, it is not necessary to complicate the configuration of the injection molding machine in molding. According to the invention of claim 3, in addition to the above effect, the mold temperature can be controlled with higher accuracy.

[Brief description of drawings]

1A is a graph showing a relationship between a cooling time and a resin temperature, and FIG. 1B is a graph showing a relationship between a cooling time and a birefringence amount.

2 is a side view showing the lens molded in Example 1. FIG.

FIG. 3 is a cross-sectional view showing an injection mold used in Example 1.

FIG. 4 is a graph showing the relationship between molding time, mold temperature and birefringence amount in Example 1.

5 is a cross-sectional view showing an injection molding die used in Example 2. FIG.

[Explanation of symbols]

 1 Fixed Side Mold 2 Movable Side Mold 3,13 Fixed Side Insert 4,14 Movable Side Insert 5,6 Temperature Control Path 7 Heat Cycle Temperature Controller 8,12 Cavity 9 Thermocouple 16 Sensitive Color Plate 17 , 20 Polarizer 21 CCD camera 22 Monitor

Claims (3)

[Claims] 1. After injection-filling the molten resin into the cavity of the mold, the temperature is kept cold until at least the resin temperature is equal to or higher than the glass transition point and becomes uniform, and then the increment of the birefringence amount of the resin is kept constant. Injection molding method characterized by lowering the mold temperature so that 2. The injection molding method according to claim 1, wherein the rate of change of the birefringence amount with respect to the temperature of the resin is calculated in advance before the molding is started, and the mold temperature is controlled based on the data. 3. The injection molding method according to claim 1, wherein the mold temperature is controlled while actually measuring the birefringence amount of the resin during molding.