JPH0128688B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a resin compression molding method for obtaining a resin molded product having a high degree of shape accuracy, and is particularly useful for manufacturing plastic lenses and the like.

The usefulness of plastic lenses lies in the ease of manufacturing aspherical lenses and the low cost that comes from the simple manufacturing method, but despite these advantages, the reason why no progress has been made is in ensuring shape accuracy. This is due to the fact that there are difficulties in

An example of a normal molding method will be explained based on FIGS. 1 and 2.

Conventionally, in a mold having a variable cavity volume function, there is a cavity block 3 that is guided and fixed to the stationary side mold members 1 and 2, and is slidable with the mold member 5 that is fixed to the movable side mold member 4. There is a retained cavity block 6, with both cavity blocks 3, 6 facing each other. Movable side mold member 4
A piston 8 of a cylinder 7 is fixed to a cavity lock 6. 10 is a sealing material such as an O-ring. Molten resin is filled into the cavity 9 through the sprue, runner, and gate (not shown) using the mold configured as described above, and the filled resin contracts as it cools and solidifies, and at the same time moves the piston 8 forward. As a result, the inside of the cavity 9 always maintains a pressure-retained state. It is already well understood that maintaining such a holding pressure state is necessary to obtain a precise shape, and it is used in injection compression molding or compression molding, but there are limitations for the following reasons.

(1) When the mold temperature is lowered to around room temperature, the polymer molecular chains freeze instantly, and distortions due to the stress of the holding pressure state remain in the molded product, making the optical properties unstable.

(2) The above drawbacks can be compensated for by increasing the mold temperature to a high enough temperature that the molecular chains do not freeze, but the deformation after mold release is large and it is not possible to obtain a lens that conforms to the shape of the mold.

If we consider the above phenomenon (2) further theoretically, we get
This can be explained as follows using a resin phase diagram.

Figure 2 shows a common phase diagram for amorphous polymers commonly used in plastic lenses, such as polymethyl methacrylate, polystyrene, polycarbonate, etc. If the pressure is constant, the specific volume changes with temperature. increases, and the ratio differs between the high temperature side and the low temperature side. This bending point corresponds to the glass transition point. Generally, the above-mentioned molding material used for lenses is cooled and solidified in a mold during molding from a high temperature side to a low temperature side across the glass transition point, and then taken out. Furthermore, if the temperature is constant, the specific volume decreases as the pressure increases, and the rate of change gradually decreases as the pressure increases.

Now, to explain the above phenomenon (2) in FIG. 2, point A shows the state of the molten resin just before it is compressed in the mold. Then, it is immediately pressurized and moves to point B (in this example, it is pressurized to several hundred atmospheres). Thereafter, as the resin cools and contracts due to heat exchange between the mold and the resin, the apparent specific volume decreases by moving the piston forward, reaching point C. Since the mold temperature is set near the glass transition point for the reason shown in (1), lowering the temperature below point C impairs time efficiency. Therefore, the mold is opened at this point. Then, the pressure is released, but due to the rigidity and viscosity of the resin, it slowly deforms (in this case, deformation is mainly expansion) and reaches point D, where a pressure equilibrium state of 1 atmosphere is obtained. Furthermore, the molded product is slowly cooled, and reaches room temperature with shrinkage, reaching point E.

In the above change, since point A → point C is a change under pressure inside the mold, it is acceptable to say that the outer shape of the resin is exactly the same as the mold cavity shape, but between point C → point E , complex deformation behavior occurs depending on the stress distribution and temperature distribution of the molded product and the stress relaxation properties, elasticity, and viscosity values of the polymer, so not only is it impossible to obtain a molded product with the same cavity shape at room temperature, but also This makes it impossible to even modify the cavity shape to account for the deformation of the molded product. In other words, this means that the reproducibility of the deformation behavior is that low.

The present invention provides a molding method that overcomes the drawbacks listed in (1) and (2) above.

The idea underlying the present invention will be explained using a phase diagram of resin. FIG. 3 is a state diagram similar to FIG. 2. The difference from Fig. 2 is the position of point C and the distance between point C and point D (in Fig. 2, between C and E).
This is the key point of the present invention. In other words, point C is the same as the specific volume of the molded product at the time of removal,
Both temperature and pressure are high. We have found that the temperature at point C needs to be set higher than a certain temperature. The pressure is selected depending on this temperature.

Further, between point C and point D, the specific volume is always kept constant, that is, the cavity volume is kept constant, and the heat shrinkage of the resin is converted into a pressure change based on the bulk modulus, and the point D is gradually approached. It is possible to eventually reach normal temperature and normal pressure conditions. Other transition conditions between point C and point D can be determined by providing constant conditions determined by the shape of the molded product, the temperature dependence of the stress relaxation rate of the polymer, thermal conductivity, specific heat, density, etc. (1) ), it was discovered that a lens molded product could be obtained without leaving any distortion, which is the drawback mentioned in (2), but the transition conditions mainly based on temperature changes are selected as necessary and are not the main part of the present invention. .

In other words, the gist of the present invention is to obtain compression molded products of excellent quality by varying the cavity volume or controlling it to a constant value in accordance with the progress of cooling of the resin in a single molding process. A mold device for this purpose includes a pair of cavity blocks disposed opposite to each other, a driving means for operating one of the blocks, a first computing unit for specifying the operation of the cavity blocks in advance, and the detection unit. and a second arithmetic unit that compares the signal of the first arithmetic unit with a signal of the first arithmetic unit and generates a control signal, and a control unit that controls the driving means of the cavity block based on the signal of the second arithmetic unit. By knowing the positional relationship of the cavity blocks at all times, the cavity volume is known, and therefore, the control means can be used to continuously provide the intended volume, thereby achieving the above-mentioned object.

FIG. 4 is a sectional view of a mold showing an embodiment of the present invention. A cavity block 13 is fixed to the stationary mold members 11 and 12. Sprues, runners, and gates are provided on the mold members 11 and 12 and the cavity block 13 as necessary, but they are omitted here.

The movable side includes a spacer block 15 fixed to the movable mold member 14, a template 16 fixed to the spacer block 15, a cylinder 17 fixed to the movable mold member 14, and a cavity fixed to the piston 18 of the cylinder 17. The block 19 and the rod 20 constitute a movable mold, and the cavity block 19 is slidably held on the mold plate 16.

An actuating transformer 21 is provided on the constant fixed side mold member 12, and an actuating piece 22 of the actuating transformer is provided so as to come into contact with the rod 20 provided on the movable side during mold clamping, and is used to detect the cavity volume. It constitutes means 33.

The operation of the mold having the above configuration is generally the same as that shown in FIG. 1, but the rod 20 and the actuating piece 22 move as the cavity block 19 advances in accordance with the cooling contraction of the resin in the cavity 23.
The displacement of the cavity block 19, that is, the change in the volume of the cavity 23, can be electrically detected by the actuating transformer 21.

FIG. 5 shows that due to the detection signal of the actuating transformer 21,
2 is a block diagram showing the manner in which the operation of the fluid cylinder 17 is controlled. FIG.

The resin temperature, mold temperature, and pressure of the resin filled in the cavity at the start of one molding process are detected, and the respective signals are input to the first computing unit 31. This first calculator 31 calculates an ideal specific volume-time profile from the state change diagram of the resin and the expected cooling rate. The second computing unit 32 compares the molding start signal and the signal obtained from the operating transformer via the amplifier with the specific volume-time profile computed by the computing unit 31, and controls the fluid control valve so as to draw an ideal profile. do. The controlled fluid is injected into cylinder 17 in FIG.

Further, the means for detecting the cavity volume is not limited to the embodiments, and means such as an encoder, potentiometer, laser interference, etc. can also be used.

Although not shown in the mold drawings of the examples, it is effective to actively control the mold temperature in implementing the present invention, and for this purpose water holes and steam holes used for normal mold temperature control are effective. Holes, oil holes, heaters, thermo pipes, etc. are provided as appropriate, but it does not matter which one is selected.

Further, it is desirable for precision to take out the molded product at room temperature, but since the molding cycle becomes very long, taking out the molded product at a temperature higher than room temperature is also an effective means from a cost standpoint. In this case, the temperature is selected to be below the heat distortion temperature, and the wider the tolerance range in terms of accuracy, the higher the temperature can be taken than room temperature. Naturally, the cavity internal pressure is set in accordance with this target temperature at the time of extraction.

One guideline for the temperature at which the cavity volume begins to remain constant is the glass transition point. Since the glass transition point shifts to the high temperature side as the pressure increases, it is selected with consideration to the cavity internal pressure.

Another guideline is Poisson's ratio. The molten state of plastic has properties similar to rubber, and the decrease in the pressurized deformation volume in the direction of pressure is equal to the increase in the perpendicular direction. In other words, Poisson's ratio is
0.5, but this value decreases below a certain temperature. For example, 130℃ for polymethyl methacrylate.
For polystyrene, this temperature corresponds to 100°C.
At temperatures below this temperature, pressurization causes a density change and deformation of the resin inside the cavity at the same time, which tends to leave distortions in the molded product.

As described above, the feature of the present invention is that in injection compression molding or compression molding, the cavity volume is controlled to draw an ideal profile in relation to other conditions, and this has a great effect on the shape accuracy of plastic lenses. be.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a conventional mold, Figure 2 is a graph showing changes in the state of the resin in the conventional molding method,
Fig. 3 is a graph showing changes in the state of the resin as intended by the present invention, Fig. 4 is a sectional view of a mold device showing an embodiment of the present invention, and Fig. 5 is a circuit controlling the operation of the mold device. FIG. 13, 19... Cavity block, 17...
Cylinder, 31...first computing unit, 32...second
Arithmetic unit, 33...detection means.

Claims (1)

[Claims] 1. A resin compression molding method in which resin is compressed and molded into a predetermined shape, comprising a pair of cavity blocks configured to make the relative distance variable, and means for detecting this relative distance. At least one of the cavity blocks is driven so that the pressure of the molten resin confined in the cavity surrounded by the cavity block and a template provided around the cavity block becomes a predetermined value. After that, the thrust force of the cavity block is kept constant and the cavity block is advanced according to the volume contraction caused by the cooling of the resin in the cavity, and the cavity volume is reduced by a predetermined amount. After a certain amount of time has elapsed according to the conditions, the control is switched so that the amount of displacement in the relative distance between the pair of cavity blocks becomes zero, and the volumetric contraction caused by the cooling of the resin in the cavity is reduced to A resin compression molding method in which the thrust force of the cavity block is controlled so as to offset the volumetric expansion caused by the decrease in the volume.