KR910007452B1 - Method of molding plastic and injection compression molding apparatus using the method - Google Patents

Abstract

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Description

Injection Compression Molding and Injection Compression Molding Equipment

1 is a schematic side view of an injection molding machine according to the present invention.

2 is a graph illustrating the relationship between the elongation of a tie bar and the stress applied to the tie bar of the device of the present invention.

3 is a cross-sectional view of one of four tie bars embedded in the device shown in FIG.

4 is a configuration diagram showing a state slightly opened by the secondary pressure of the resin injected into the cavity.

* Explanation of symbols for main parts of the drawings

1: Hydraulic Cylinder 2: Clamp Screw

3: movable platen 4: movable mold

5: fixed mold 6: front fixed platen

7: cavity 8: tie bar

9: injection nozzle 10: fixed back platen

11: second member 12: first member

The present invention relates to a method for molding plastics very accurately and an injection compression molding machine for molding plastics with high dimensional accuracy.

In recent years, in injection molding of plastics, lenses and optical disk substrates have been required to be made in very precise shapes of units of 1 micron or less.

In order to accurately mold the article, the following process has typically been used. First, the mold is clamped at high pressure, and molten resin is injected into the cavity of the mold through the nozzle. Immediately after the cavity of the mold was filled with molten resin, a molding method was generally applied to give the mold a holding force large enough to prevent the mold from being opened, so as to obtain a molded article as faithful as possible to the mold shape. In this case, it is necessary to set the holding force to be equal to or less than the clamping force and to perform molding under the condition that the mold is not opened.

Therefore, in this method, by adjusting the temperature and pressure, the specific volume of the resin present in the mold in the molten state becomes close to the specific volume in the normal temperature state to reduce the sink mark or shrink mark generated in the molded article during cooling. . However, to achieve this condition actually requires a very large force. Under high pressure, the components and molds of the molding machine with tie bars must have sufficient rigidity to prevent the tie bars from deforming. For this purpose, molds and molding machines had to be of large volume. This often became a huge molding system.

It is an object of the present invention to provide a method for molding high precision plastics.

It is another object of the present invention to provide an injection compression molding apparatus which is capable of forming plastics quickly, reliably, stably, and accurately while also having a simple structure.

The concept of the present invention is very different from the prior art, and the molding apparatus has a very small portion of rigidity. Specifically, the tie bar of the injection molding machine is configured to yield to a specific length calculated by the relationship between the volume coefficient of shrinkage of the plastic material and the volume of the cavity by yielding to the high pressure of the plastic material injected just before the completion of the injection. Therefore, in the mold, the cavity is slightly opened by the pressure of the injected material. In practice, the plastic material is shaped so that when cooling the molded material, the restoring force of the elongated tie bar due to the elastic deformation acts in the direction of reducing the cavity in consideration of the length, thickness and elastic force of the tie bar material, and The high pressure energy generated during the storage is stored in the form of a tie bar. That is, if the inlet of the mold is completely sealed, and at the same time, the molded material is cooled, the tie bar extended in the thickness direction of the molded article exhibits a restoring force due to elastic deformation. The shrinkage volume coefficient of the plastic material can be calculated in advance from the state equations of pressure, temperature and specific volume, which are generally P-V-T curves.

The method according to the present invention is a kind of injection compression molding method, which is characterized by using the physical deformation of the tie bar as the source of the compressive force. When cooling the molten resin in the mold, the tie bar shrinks in response to the shrinkage of the resin. do. As a result, the shape of the mold is accurately transmitted to the molded article. As a specific embodiment of the injection compression molding machine, each tie bar is composed of a first member having a small modulus of elasticity and a second member having a modulus of elasticity larger than the first member. In such a dual structure, when the amount of elongation is small, only the first member having a small modulus of elasticity acts, and when the amount of elongation reaches a specific amount ΔI ₁ , the two members are connected in parallel. In other words, the elastic modulus increases rapidly. At the time of injection, the high-pressure energy applied to the mold cavity by the injection device is stored in the form of elongation ΔL in the rib bar in which the two members are connected in parallel. When the molten resin in the mold is cooled and hardened, the molten resin contracts. At this time, the tie bar stretched by ΔL exerts a compressive force on the resin in the mold.

1, an injection compression molding apparatus embodying the present invention is shown. This injection compression molding machine has a cavity between the clamping device 30 composed of the hydraulic cylinder 1 and the clamp screw 2, the movable platen 3, the front fixed platen 6, the rear fixed platen 10, and a cavity therebetween. The plastic resin melted by heating consists of a movable mold (4) and a stationary mold (5), a tie bar (8) (there are four tie bars), and an injection nozzle (9) forming the mold (7). It is injected into the cavity 7 through the nozzle 9 to form the disc 7a.

The clamp fixture 30 is fixed to the rear fixed platen 10 supported at each of the four corners by one end of the tie bar 8, respectively, and the front fixed platen 6 has its own tie bar 8, respectively. It is fixed to each of the four corners by the other end of. The stationary mold 5 is fixed to the front stationary platen 6, and the movable mold 4 is fixed to the movable platen 3 so as to be movable by the movement of the movable platen 3. In order to close the pair of molds 4 and 5, the movable platen 3 is guided to the tie bar 8 by pushing the movable platen 3 toward the front fixed platen 6 with the clamping device 30 ( 3) moves the movable mold 4 until the separating surfaces 16 of the molds 4 and 5 contact each other. At this time, each of the tie bars 8 is extended to a specific length DELTA L ₁ (total length L ₀ ) so that the separating surfaces 16 of the molds 4 and 5 are in precise contact with each other. In general, the platens 3 and 6 do not correspond exactly to each other because there are several tens of microns of position error even when the relative positions of the platens 3 and 6 are precisely adjusted. The separation surfaces of (4) (5) do not correspond exactly to each other. Therefore, if the tie bar is sensitive, the movable mold 4 will no longer move towards the stationary mold 5 after a part of the separating surface of the movable mold 4 is in contact with a part of the separating surface of the stationary mold 5. This means that the molding surfaces of the molds 4 and 5 do not exactly correspond to each other. However, since each of the tie bars 8 of the present invention has a small modulus of elasticity until it is elongated to a specific length ΔL ₁ , each tie bar is subjected to the force of the clamping device 30 and elongated to ΔL ₁ . The specific extension length Δl ₁ of each tie bar is the total surface of the separation surface 16 of the molds 4 and 5 when the tie bar 8 extends their respective specific extension length Δl ₁ degree. Are selected to contact each other. Therefore, the molding surfaces of the molds 4 and 5 correspond exactly to each other.

After the separated surfaces of the molds 4 and 5 are completely in contact with each other in the same manner as above, the molten resin is injected into the cavity 7 at a high pressure from the nozzle 9. At this time, the tie bar 8 is further increased to a length ΔL of the micron unit due to the high pressure, as a result, the separation surface of the mold half is separated into ΔL as shown in FIG. Each tie bar 8 is configured such that its elastic modulus fluctuates to a large value when its elongation rate exceeds the length Δl ₁ , and thus, each tie bar that extends further from Δl ₁ to ΔL has a cavity ( It generates the restoring force which appears as compressive force on the resin in 7). Immediately after the resin is injected, the resin in the gate 31 of the stationary mold 5 is cooled and hardened to seal the gate 31. After the gate sealing is completed, cooling of the resin in the cavity 7 applies the compressive force generated by the tie bar 8. Therefore, the final molded product is exactly the same shape as the shape partitioned by the molding surface of the molds 4 and 5.

2 shows the relationship between the elongation of each tie bar and the stress applied thereto. The tie bar is stretched with a small stress along the line a until it is stretched to the length Δl ₁ , but after the stretched length exceeds Δl ₁ it is required to be given a large stress as shown by the line b. That is, the elongation-stress characteristic curve suddenly becomes very inclined when the elongation rate exceeds a specific length Δ1 ₁ . The tie bar is stretched along the line a by the force of the clamping device until the elongation reaches Δl ₁ , and the elongation is increased when the separating surfaces of the movable and stationary molds come into complete contact with each other (primary clamping step). It is further stretched along the line b by the high pressure of the injected resin until it reaches ΔL. It would be ideal to use a material having a characteristic as shown in FIG. 2, but since such material does not exist, as a preferred structure, each tie bar is, for example, a double as shown in FIG. It is formed to have a structure.

Referring to FIG. 3, each tie bar includes a first member 12 having a small modulus of elasticity and a second member 11 having a large modulus of elasticity. The second member 11 is in the shape of a hollow cylinder, one end of which is fixed to the front fixed platen 6 by a mounting screw 17. The other end 13 of the second member 11 is not fixed but is loosely fitted to the rear fixed platen 10 like the free end. The first member 12 inserted into the hollow inner surface of the second hollow member 11 is formed of two parts 12a and 12b coupled together, and the two parts 12a and 12b have different thicknesses. It is. One end 19 of the thin portion 12a is fixed to the front fixed platen 6 by a screw 21, and the other end 20 of the thick portion 12b is rear fixed platen by a screw 22. It is fixed to (10). The elastic modulus of the thin portion 12a is less than the elastic modulus of the second member. Preferably, the thick portion 12b has an elastic modulus equal to or greater than the elastic modulus of the second member 11. After the elongation rate of the first member reaches Δl ₁ , the two members 11 and 12 are connected in parallel by joining the screws 15 and 14 attached to the members 11 and 12, respectively. As a result, the apparent elastic modulus of the tie bar suddenly increases. Value △ l ₁ can be set by adjusting the gap △ l ₁ between the adjusting screw (14), (15). The screw 14 is mounted to the thick portion 12b of the first member 12 therein, and the screw 15 is mounted to the inner wall of the second member 11 of the outer side.

4 shows that when each tie bar 8 is further elongated to ΔL in the elongation state of Δl ₁ , the separation surface 16 of the mold 4 and 5 causes the injection nozzle 9 to contain the resin. Figure 7 shows the state in which ΔL is also separated after injection. As the resin in the cavity cools, the strain (extension) of each tiebar is reduced so that ΔL approaches zero. As a result, accurate injection compression molding varies.

In the new molding apparatus of the present invention, each tie bar is composed of two parts having different elastic modulus. During the first clamping step of closing the mold, the low elastic modulus member extends into the region a of the stress-elongation characteristic line shown in FIG. Therefore, even when four tiebars increase in different amounts during closing of the mold, the stress or compressive force does not differ significantly between the four tiebars. For this reason, the pressures applied to the four bars are approximately the same. Therefore, approximately the same pressure is applied to the platen at four corners when the mold is closed.

After the cavity key of the mold is filled with the resin, the opening force generated by the injection pressure on the inner surface of the cavity opens the mold. During this process, the stress-elongation line of each tiebar proceeds to region b shown in FIG. 2, where a large amount of stress can be stored even when the elongation is small. That is, the elongation energy stored in the tie bar member having a large modulus of elasticity is released as a force compressing the resin in the cavity after completion of the injection. Thus, as soon as the injection is completed, the compression force acts on the resin on the inner surface of the mold, thereby obtaining an ideal compression molding. Strictly speaking, when the compression stroke is started, the force applied to the resin is equal to the pressure divided by a factor that depends on the elastic modulus of the tie bar.

In general, the platen in the molding apparatus cannot be expected to exhibit the high degree of parallelism as described above. Typically, even with very careful adjustment, deviations of tens of microns occur in parallelism. Therefore, when the melt is squeezed by the extension of the tie bar after the injection is completed, the four tie bars must be stretched to the same length, and the compression force of the same size must act on the four corners of the bar, otherwise the mold remains parallel. It can't be compressed.

In view of the foregoing, the present invention has been made. The new forming apparatus shown schematically in FIG. 1 is characterized in that each tie bar 8 has a double structure consisting of an outer second member 11 and an inner first member 12 (third) Degree).

The first clamping process of mold closing is performed before injection, and for this purpose, the hydraulic cylinder 1 is operated to increase the first member 12 inside each tie bar by Δ1. Opposite ends of the first member 12 having a small modulus of elasticity are fastened to the stationary platens 10 and 6, and at this time, the adjustment screw 14 on the first member 12 having a small modulus of elasticity has a large elastic modulus. The stop screw 15 mounted on the second member 11 is in contact with the stop screw 15. When starting the injection process, the injection nozzle 9 generates a high pressure inside the cavity 7, which pressure is received by two members 11 and 12 having different elastic modulus. As a result, the apparent modulus of elasticity significantly increases. The inflection point switched to another place where the elastic modulus is large can be set at will by adjusting the gap Δ 1 ₁ with the screws 14 and 15 mounted on the members 12 and 11, respectively.

When forming an optical disk substrate having a diameter of 130 cm and a thickness of 1.2 cm, tie bars work in the following manner. Each of the first members 12 of the tie bar is designed to increase by about 100 microns when subjected to a ton of 10 tons. The first clamping step of mold closing is performed with a force of 3 tons, wherein the screws 14 and the set screws 15 on the four tie bars are set so that ΔL becomes zero. Then, the nozzle 9 injects 320 degreeC molten polycarbonate resin at the pressure of 240 kg / cm <2>, and the separation surface 16 of the metal mold | die 4, 5 falls about 30 microns from each other. That is, the mold is opened. At this time, the injection energy is stored in the form of elongation of the members 11 and 12 of the tie bar.

After that, when the gate 31 is sealed and the disk 7a formed on the inner surface of the cavity 7 is cooled, the extrusion force due to the elongated tie bar members 11 and 12 is cooled in the cavity. Is applied to make DELTA L zero. The molded optical disk 7a has spiral grooves having a depth of 0.01 microns and a width of 0.8 microns. The double refractive index of an optical disc is smaller than the frequency of 12 mm in which information is recorded on the disc. The depth of the grooves is 0.098 to 0.099 microns and the disc substrate is formed very accurately.

As described above, an important feature of the present invention is the use of tiebars in which the elastic modulus rapidly increases after reaching a constant elongation, which maintains the parallelism of the cavity when the platen and the mold have a somewhat low degree of parallelism. On the other hand, by applying a compressive force to the cavity it can be satisfactorily molded optical disks and lenses that need to be manufactured with less than 1 micron erroneous error. Since the elastic modulus of the tie bar and its inflection point can be set arbitrarily, the elastic modulus and thickness of the two members 11 and 12 of each tie bar even when molding a portion having a different thickness or requiring a different shrinkage amount. By appropriately selecting, the opening amount ΔL and the compression force of the mold can be accommodated. Therefore, the injection molding apparatus is very simple in structure without the need for complicated and expensive hydraulic control circuits. As a result, the apparatus can form the plastic part stably and reliably.

The component material of the molding apparatus is not limited to steel only. For example, the first member 11 of each tie bar can be made of plastic having a small elastic modulus and a large elastic limit. In addition, the structure of each tie bar is not limited to being composed of a hollow member and a second member inserted into the hollow member. For example, the tie bar may be composed of two parallel cylinders or spectroscopic members. .

Claims (7)

Using an injection molding apparatus, the molten plastic material is injected at high pressure through the cavity gate between the pair of molds; Sealing the gate; In the plastic molding method comprising the step of applying a compressive force to the plastic material in the cavity while cooling the injected molten plastic material, the high-pressure energy during the injection process is in the form of an elastic believer over a specific extension amount to the tie bar of the injection molding apparatus Wherein each tie bar has a greater modulus of elasticity when stretched beyond a certain length than when stretched to a particular length and the compressive force during the cooling process is generated by the restoring force of the stretched tie bar. 2. The injection compression molding method according to claim 1, wherein the mold is clamped to the mold, and the tie bar extends to the specific length during the clamping process. A pair of stationary platens 6 and 10 connected to each other by tie bars 8; A movable platen 3 capable of moving along the tie bar; A fixed mold (5) mounted on one (6) of the stationary platen and a movable mold (4) mounted on the movable platen, a pair of fixings forming a cavity (7) in which molding is performed between them; Movable molds 5 and 4; An injection nozzle 9 for injecting molten resin into the cavity; And a clamping means (30) for clamping the movable mold, wherein each of the tie bars (8) has a small modulus of elasticity when elongated to a specific length and a large modulus of elasticity when elongated over a certain length. Injection compression molding apparatus, characterized in that. 4. The injection compression molding apparatus according to claim 3, wherein the length, thickness and large elastic modulus of the tie bar are determined from the relationship between the volume coefficient of shrinkage of the plastic material to be molded and the volume of the cavity. 5. The tie bar (8) according to claim 3 or 4, wherein each tie bar (8) comprises: a first member (12) having opposing ends mounted on a pair of stationary platens, each having a small elastic modulus; A second member 11, one end of which is mounted on one of the pair of fixed platens, and having a high elastic modulus; And means (15,14) for increasing the elastic modulus of each tie bar by connecting the first member and the second member when the first member is elongated to a specific length. 6. The injection compression molding apparatus according to claim 5, wherein the second member (11) is made of a cavity member, and the first member (12) is mounted on a cavity inner surface of the cavity member. 6. The injection compression molding apparatus as claimed in claim 5, wherein the connecting means (15, 14) are formed on the first and second members, respectively, so as to be adjacent to each other when the first member is extended to a specific length. .

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