JPS61233520A - Preparation of molded product - Google Patents

Abstract

PURPOSE:To obtain excellent accuracy in the molding of optical parts, by charging a resin melted under heating in a mold held to glass transition temp. of the resin or more at the pour point of the resin or less and subsequently cooling the mold to take out a molded product. CONSTITUTION:When a top is heated to predetermined temp. by the temp. detected by the timer or temp. sensor 15 mounted in a heater temp. controller 16, the heating of a heater is stopped and a resin melted under heating is injected in a mold and passes through a sprue 10 to fill a cavity 9. Thereafter, a medium having proper temp. is flowed to medium passages 3, 4 or temp. is controlled by heaters 7, 8 to cool the temps. of tops 1, 2 to the glass transition temp. of the resin. When the tops 1, 2 are cooled to the glass transition temp. over 10sec or more after the resin was charged, a cooling medium is flowed to the medium passages 3, 4 to further cooling the tops 1, 2 and, at the point of time when the tops are cooled to certain temp., cooling is stopped to take out the molded product from the mold.

Description

[Detailed description of the invention] [Description of the field to which the invention pertains] The present invention requires high dimensional accuracy and has low internal distortion.
The present invention relates to a manufacturing method for manufacturing a molded product, and relates to a method of manufacturing a molded product that can obtain particularly excellent precision in molding optical parts such as focusing plates.

[Description of conventional technology]

Here, injection molding (including injection compression molding, etc.) will be explained as an example. Conventionally, in injection molding, a mold is set at a temperature below the glass transition point or heat distortion temperature of the resin, and while keeping the temperature constant, molten resin is injected into the mold and high pressure is applied. The method used was to transfer the shape of the wall surface of the cavity in the hanging mold, cool it, solidify it, and then take it out.

However, with this method, of the resin injected into the mold, the resin that comes into contact with the wall of the cavity inside the mold is instantly cooled and solidified to form a skin layer, and when the molten resin is forced into the upper part of the skin layer under high pressure, Additionally, shear stress is generated at the interface with the skin layer, which causes the distortion observed by birefringence. Furthermore, due to this instantaneous cooling and solidification, sufficient transfer may not be possible at that point.

Furthermore, a fairly large temperature difference occurs between the resin in contact with the mold and the molten resin inside the molded product, and as the resin in contact with the mold gradually cools down, the inside of the molded product When the inner resin cools and solidifies, it shrinks in a way that encloses the resin on the outer surface, creating a dent called a "sink mark" on the outer surface. Even if "sink" does not occur, a density distribution occurs, which becomes a distribution of residual stress, which affects the mechanical properties of molded products, and impairs the optical performance of optical parts. I was doing a lot of things.

[Purpose of the invention]

In order to eliminate the drawbacks of the above-mentioned conventional example, the present invention aims to reduce the mold temperature during molding to the resin flow temperature (=flow stop temperature) or lower.
Set the button within a relatively narrow range above the glass transition point temperature, and in that state pour in the resin, taking at least 10 seconds from the time you start pouring the resin until the mold reaches the glass transition point temperature of the resin. The resin is cooled and solidified by cooling the mold, and is then taken out as a molded product, and has high dimensional accuracy with a tolerance of 1 μm or less.
The present invention also provides a manufacturing method for manufacturing a molded product with little internal distortion.

[Description of the structure and operation of the invention]

Upon completion of the present invention, the inventors conducted detailed tests and studies on the correlation between molding conditions and the quality of molded products.

In other words, the temperature of the resin before injection into the mold, the temperature state of the mold,
As a result of changing the molding conditions, such as the pressure applied to the resin, and examining the relationship with the quality of the molded product, such as dimensional accuracy and internal distortion, we found that the temperature of the mold has the greatest effect on the quality of the molded product. It has been confirmed that it has a negative effect. The temperature condition of this mold, the initial mold temperature and dimensional accuracy when adding special resin,
The relationship between this and internal strain is shown in FIGS. 1 and 2. As is clear from these figures, the dimensional accuracy of the molded product improves as the temperature of the mold increases1, and the internal strain decreases.

The higher the mold temperature is raised, the better the quality of the molded product will be, but even if the mold temperature is raised above the flow temperature (= flow stop temperature), the effect will not be as good, and on the other hand, the molding cycle will be longer. At the same time, the problem of Paris arose,
Secondary processing becomes necessary, resulting in inferior productivity. Therefore, from the point of view of high precision, it is recommended that the mold temperature when charging the resin be within a relatively narrow range of above the glass transition point temperature of the resin and below the flow temperature (=flow stop temperature).
We have found that it is also useful from a productivity standpoint. In particular, when the required accuracy is strict, it is desirable to raise the temperature to the very limit of the flow temperature (=flow stop temperature).

In this way, even if the mold is not raised above the busy flow temperature (= flow stop temperature), if the temperature is above the glass transition point, the resin in contact with the cavity wall will not flow macroscopically. Since the inside is in a microscopic fluid state, the cavity shape can be well transferred if sufficient processing pressure is applied above that temperature, and the strain inside the molded product can be alleviated if it is sufficiently maintained above this temperature. This is because it is possible.

When molten resin is poured into a mold set at a temperature within the above-mentioned range, the resin in contact with the mold will immediately drop to around the mold temperature, and the temperature inside the molded product will still remain. It is in a molten high state. However, since the mold temperature is above the glass transition temperature of the resin, pressure is applied after filling in this state to transfer the cavity shape, and the mold temperature is controlled until the mold temperature reaches the glass transition temperature of the resin. By doing this, the temperature of the resin in the molded product becomes uniform, and until this becomes uniform, the mold is cooled so that the temperature of the resin in the entire molded product is below the glass transition temperature. By doing so, a highly accurate molded product can be obtained while maintaining the initial transfer accuracy without causing uneven shrinkage due to uneven temperature distribution within the molded product.

Here, the cooling time for the mold temperature to drop from the high temperature at the time of resin injection to the glass transition point temperature is determined by the wall thickness of the molded product and must be controlled arbitrarily, but we conducted various tests and studies. As a result, as shown in Figure 4, in order to achieve high dimensional accuracy with a tolerance of 1 μm or less, no matter how thin the molded product is, the mold must be cooled to the resin's glass transition temperature from the moment the resin is introduced. It was confirmed that it is necessary to take at least 10 seconds to Note that the flow temperature in the above refers to 1500 psi for 2 minutes using a Lodge Peaks flow tester.
1nch Flowing temperature is expressed as A8TM: D569-
59. Also MOLDFLOW PTY
, LTD, No-FLOW in the resin database in the provided Mold flow 80 program.
The temperature (flow stop temperature) is determined by applying a pressure of 50 MPa to the resin in the barrel using a high shear viscometer, increasing the barrel temperature by 10° C., and setting the temperature immediately before the resin begins to flow.

The following is a configuration diagram (Fig. 5) showing a specific embodiment of the present invention.
Let me explain.

FIG. 5 shows the main parts of a lens mold for injection molding, where l is a fixed side insert piece provided in the fixed side mold plate 13 and a movable side insert piece provided in the 2-piece movable side mold plate 14. , 1 and 2 form a capacity 9.

7.8 is a heater that heats the input pieces on the fixed side and the movable side, respectively, and the heater-temperature controller 16 is connected via the temperature sensor 15.
It seems to be controlled by. Furthermore, a timer is also provided in the heater temperature controller, and it is also possible to control the heaters 7 and 8 using the timer.

3 and 4 are heating and cooling medium passages for adjusting the temperature of the fixed side and movable side input pieces, respectively, and are sealed with O-rings 5 and 6 and controlled by a medium temperature regulator 17 having a built-in solenoid valve. It looks like this. 3A and 4A indicate connection paths between the heating/cooling medium passage 3.4 and the temperature regulator 17. Further, a heater temperature controller 16 and a medium temperature controller 1
7 can be made possible by sending control signals to each other to control their respective operations.

The molding operation and molding method of the injection mold configured as above will be explained.

Before the resin heated and melted in the cylinder (not shown) of an injection molding machine (not shown) is injected into the mold, pieces 1 and 2 are heated by heaters 7 and 8 while the mold is clamped in advance. be done.

The predetermined temperature (
11 When the piece is heated to a temperature below the flow temperature of the resin and above the glass transition point temperature, the heating with the heater is stopped and the melted resin is injected into the mold, passes through the sprue 10, and enters the cavity 9. Filled. Thereafter, a medium at an appropriate temperature is flowed through the medium passages provided in 3 and 4, or the temperature of pieces 1 and 2 is cooled down to the glass transition temperature of the resin while controlling the temperature of pieces 1 and 2 using heaters 7 and 8. .

When the piece has been cooled to the glass transition point temperature of the resin, cooling medium is poured into the medium passages 3 and 4 to further cool the piece. When the piece has cooled to a certain temperature, cooling is stopped, the molded product is removed from the mold, and the mold is clamped. After this, pieces 1 and 2 are heated again by heaters 7 and 8.

Temperature control when the mold (piece) is cooled down to the glass transition point temperature of the resin after the resin is injected may be performed by sequentially detecting the temperature inside the cylinder with a temperature sensor 15, or may be controlled by a timer. This also applies to the process of cooling the mold (piece) from the glass transition temperature of the resin to the temperature at which the molded product is taken out.

When sequentially controlling the temperature of the mold (piece) using the temperature detected by the temperature sensor 15, a cooling temperature gradient is set in advance, and heating and cooling are controlled by performing comparative calculations so that the detected temperature follows the curve. As shown, in that case, a new computing unit is required.

Next, a specific example will be described.

(Description of Examples) Using polymethyl methacrylate (PMMA) resin, a convex lens having a diameter of 43 mm and a center wall thickness of 9 mrn was injection molded under the following molding conditions.

Resin temperature before injection: 250℃ mold temperature
During injection at 130°C 1 Time from 80°C injection until the mold temperature reaches the resin's glass transition point temperature 4 minutes Also, the mold temperature was kept constant at 80°C and other conditions were the same as above. Lenses were also manufactured by injection molding using conventional methods.

28 mentioned above! Among the lenses, those molded at a mold temperature of 130°C before resin injection were optically excellent lenses, but those molded at a constant mold temperature of 80°C did not meet the requirements for lenses. The quality could not be met. The flow temperature (=flow stop temperature) of this resin is 1
The temperature was 50°C, and the glass transition temperature was 105°C.

Now, it is possible to start cooling the mold immediately after pouring the resin into the mold, but in the case of thick-walled molded products or molded products with strict precision requirements, the temperature of the mold should be adjusted as soon as the resin is poured. It is necessary to hold the mold for several seconds and apply sufficient processing pressure during this time to sufficiently transfer the cavity shape and also to alleviate the internal distortion of the molded product.

This manufacturing method can be applied to resin manufacturing methods such as injection molding and injection compression molding, and is a useful method for manufacturing high-precision molded products.

[Explanation of effects]

As explained above, according to the present invention, by specifying the mold temperature, which has the greatest effect on the precision and internal strain of the molded product, as described above, it is possible to achieve high dimensional accuracy with a tolerance of 1 μm or less and to reduce internal strain. We were able to obtain a small number of molded products.

[Brief explanation of the drawings] Figure 1 is a graph showing the relationship between mold temperature and dimensional accuracy during resin injection in injection molding, and Figure 2 shows the relationship between mold temperature and internal strain during resin injection. The graph diagram shown in Figure 3 shows the characteristics necessary to achieve high dimensional accuracy with a tolerance of 1 μm or less.
A graph showing the relationship between the thickness of the molded product and the cooling time from when the resin is introduced until the mold cools down to the glass transition temperature of the resin.
FIG. 4 is a graph showing how the mold temperature changes in a specific embodiment of the present invention, and FIG. 5 is a cross section around the inner piece of the lens molding die in a specific embodiment of the present invention. It is a block diagram including a figure.

Claims (2)

[Claims] (1) Heat and melt the resin at a temperature below the flow temperature of the resin,
A method for manufacturing a molded article, which comprises placing the molded article into a mold set at a temperature equal to or higher than the glass transition temperature, and then cooling the mold and taking out the molded article. (2) The resin is poured into the mold, and the mold is cooled for at least 10 seconds after the resin is started to be poured until the mold reaches the glass transition point temperature of the resin. A method for manufacturing a molded article according to claim 1.