JPH0114010B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a method for manufacturing a plastic lens suitable as a lens for optical equipment such as an optical head of a compact disc player or a video camera. [Field of Application of the Invention] In recent years, plastic lenses made by injection molding of thermoplastic resin have come into use as lenses for optical equipment such as optical heads of compact disc players. This is because plastic lenses in the desired shape can be easily manufactured using conventional plastic molding methods, resulting in lower manufacturing costs, and because they are much more suitable for manufacturing aspherical lenses than glass lenses. etc. However, on the other hand, it has the disadvantage that it is difficult to achieve the shape accuracy of the lens surface, and furthermore, the optical performance becomes unstable due to internal distortion generated during molding. The former causes of shape accuracy include volumetric contraction when the molten resin solidifies and volumetric contraction during the cooling process after solidification. The resin could not make sufficient contact with the mold surface, and as a result, the shape accuracy of the lens surface deteriorated. Therefore, a conventional mold structure has been adopted in which after the resin in the cavity has solidified, compressive force is applied to the mold to press only one side of the lens curvature. This method requires a pressing force and has the drawback of causing distortion within the lens. Regarding the internal distortion that occurs during the latter molding process, in addition to the one that occurs when only one side of the lens curvature surface is pressed as described above, the internal distortion that occurs in the gate part due to the difference in internal pressure between the part that passes through the gate and becomes the lens and the runner part. Resin flow may occur and this may solidify, resulting in distortion, or the large temperature difference between the molten resin and the mold land may cause the surface of the injected resin to harden before the inside. Examples include those where the internal resin is pulled by the surface layer resin and becomes distorted. [Object of the Invention] An object of the present invention is to provide a method for manufacturing a plastic lens, which eliminates the above-mentioned drawbacks of the prior art, makes it easy to achieve shape accuracy of the lens surface, and causes less internal distortion. [Summary of the Invention] In order to achieve this object, the present invention injects resin into the cavity while heating the cavity mold and core mold to the glass transition point or melting point of the resin to be injected, and then melts the resin. In this state, the cavity mold and core mold are moved to separate the runner gate, and then the cavity mold and core mold are cooled, and the compressive force applied to the cavity mold and core mold is changed according to this cooling temperature. It is characterized by the fact that [Embodiments of the Invention] Examples of the present invention will be described below with reference to the drawings. 1 to 4 are explanatory diagrams showing each step of the method for manufacturing a plastic lens according to the present invention, in which 1 is a movable mold, 2 is a runner, 3 is a fixed mold, 4 is a core drive rod, and 5 is the cavity drive rod, 6
is core type, 7 is cavity type, 8 is cavity type,
9 is resin. In these figures, the cylinder part of the movable mold 1 has a core mold 6 fixed to the tip of the core drive rod 4.
On the other hand, a cavity mold 7 fixed to the tip of a cavity drive rod 5 is slidably fitted into the cylinder portion of the fixed mold 3. These core mold 6 and cavity mold 7 form a cavity 8 when the mold is closed, and the core mold 6
The cavity mold 7 can be moved within the cylinder by applying pressure. Figure 1 shows the first step.
In the process, the mold is closed, the core mold 6, the cavity mold 7, and the surrounding cylinder portion are heated, and in this state, a resin 9 such as PMMA (polymethyl methacrylate) is injected into the cavity 8. If the heating temperature at this time is higher than the melting point of the resin 9, the resin 9 will stick to the surface of the mold or decompose, whereas if the heating temperature is lower than the glass transition point of the resin 9, A very large force is required to create the shape of the glass surface, and if you try to force the shape, it will cause cracks and large internal distortions, so the heating temperature is set to the glass transition point or melting point of the resin 9. It needs to be set between. Figure 2 shows the second process.
In the process, when the resin 9 is in a molten state, the core mold 6 and the cavity mold 7 are moved into the cylinder of the movable mold 1 or the fixed mold 3 so as not to change the volume of the cavity 8, and the resin in the cavity 8 is moved. 9
and Runner 2 are separated. This eliminates the influence of pressure from the runner 2 in subsequent steps. Figure 3 shows the third process;
In the process, the temperature of the core mold 6, cavity mold 7, and the surrounding cylinder portion is cooled. Care should be taken to ensure that the cooling rate does not cause a large difference in temperature between the surface and inside of the core mold 6, cavity mold 7, and resin 9 in the cavity 8. Although it varies depending on the resin used, a rate of 5°C/min to 20°C/min is appropriate. In addition, the volume and pressure of the resin 9 in the cavity 8 decreases as it cools, but if the resin 9 in the cavity 8 is cooled in this state until the product is removed, the resin 9 in the cavity 8 will be able to fully contact the surfaces of the core mold 6 and cavity mold 7. They cannot make contact with each other, and the precision of the shape of the lens surface deteriorates. Therefore, in order to compensate for this, either the core mold 6 or the cavity mold 7 is moved by cylinder pressure.
The resin 9 in the cavity 8 is compressed. At this time, the relationship between the compressive force applied to the resin 9 and the temperature is as shown in FIG. As the temperature inside the cavity 8 decreases, the pressure is increased, reaching a maximum pressure near the glass transition point of the resin 9, and as the temperature inside the cavity further decreases, the pressure is decreased. Figure 4 shows the fourth step.
In the process, the core mold 6 and cavity mold 7 are returned to their original positions as shown in FIG. 1, the molds are opened, the product is taken out, and the process returns to the first step. Table 1 shows specific examples using the following molding method.

【table】 The measurement results for each specific example are shown in Table 2.

[Table] The plastic lens manufactured in this way has a lens surface shape accuracy very close to that of the mold, little internal distortion, no decrease in transmittance, and stable optical performance. Obtained. Note that the resin used in the present invention is not limited to PMMA mentioned in the above examples, and other resins can also be used. However, in that case, the cavity temperature (temperature of the cavity mold and core mold) at the time of resin injection must be appropriately set in consideration of the glass transition point and melting point of the resin used. Table 3 below lists several examples of resins used, and shows the glass transition points and melting points of these resins.

[Table] *The melting point here refers to the first transition temperature at which the resin changes from crystalline to amorphous.
Refers to the melting temperature at which the fluid becomes fluid.
[Effects of the Invention] As described above, according to the present invention, it is possible to provide a method of manufacturing a plastic lens that facilitates achieving shape accuracy of the lens surface and that generates less internal distortion.

[Brief explanation of drawings]

FIGS. 1, 2, 3, and 4 are explanatory diagrams showing each step of the method for manufacturing a plastic lens according to the present invention, and FIG. 5 is an explanatory diagram showing the relationship between the internal pressure and temperature of the cavity. . 1...Movable mold, 2...Runner, 3...Fixed mold, 4...Core drive rod, 5...Cavity drive rod, 6...Core mold, 7...Cavity mold, 8...
Cavity, 9...resin.

Claims (1)

[Claims] 1. In a method of molding a plastic lens in a desired lens shape by cooling molten resin in a cavity constituted by a cavity mold and a core mold, the cavity mold and the core mold are Resin is injected into the cavity while heated to the glass transition point or melting point, and when the resin is in a molten state, the cavity mold and core mold are moved to separate the runner gate, and then the cavity mold and core mold are cooled. A method for producing a plastic lens, characterized in that the compressive force applied to the cavity mold and the core mold is changed according to the cooling temperature.