KR101394846B1 - Molding method of thin molded article - Google Patents

Abstract

<Task>
Provided is a molding method of a thin molded article which can form a light guide plate for liquid crystal for medium or more in a thin thickness.
[Solution]
Is molded by using an injection apparatus (2) comprising a heating cylinder (5) and a screw (6). The molten resin is injected into the cavity by driving the screw 6 in the axial direction while the molds 20 and 21 are initially opened by a predetermined amount. Then, the molds 20 and 21 are closed to compress the molten resin injected. During the compression, the screw 6 is retracted for a predetermined time to lower the resin pressure of the molten resin in the cavity. Then, a driving force in the axial direction is applied to the screw 6 to apply a predetermined pressure to the molten resin. The driving of the screw 6 in the holding pressure may be controlled by speed control and then by pressure control.

Description

METHOD OF THIN MOLDED ARTICLE

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molding method for molding a thin molded article in an injection molding machine, and more particularly, to a molding method of a thin molded article preferable for molding a light guide plate used for a liquid crystal.

As known in the art, an injection molding machine is constituted by a pair of molds, a closing device for closing molds of these molds, an injection device for melting a resin material and injecting the molten resin into a mold, and the injection device includes a cylinder, And a screw driven in the rotating direction. When the pair of molds are closed by the mold closing device and the molten resin metered in the cylinder is injected by driving the screw in the axial direction, the mold is filled in the cavity formed in the mold. Waiting for cooling and solidification, the mold is opened to obtain a molded article.

In a mold for molding a so-called thin molded article having a thickness smaller than that of a light guide plate used for a liquid crystal display device, the gap or thickness of the cavity is small, and therefore the flow resistance of the molten resin to be injected is large. In this case, the molten resin injected from the injection device is difficult to sufficiently fill in the cavity, and the transcription properties also deteriorate. As a molding method for molding such a thin molded article, a so-called injection compression molding method is well known. Patent Document 1 describes an example of such an injection compression molding method.

Patent Document 1 discloses a molding method for molding a light guide plate. The mold used in this molding method is designed so that the cavity forming surface of the movable mold can be varied in its distance from the cavity forming surface of the stationary mold have. That is, the thickness of the cavity can be varied. In the method described in Patent Document 1, the mold is closed with the thickness of the cavity increased by 0.2 to 0.5 mm from the thickness of the finally obtained light guide plate. That is, the mold is slightly opened. In this state, the molten resin is injected into the cavity, and mold closing of the mold is started during injection or after injection. Closure of the mold is completed later than the end of the injection. Thereby compressing the molten resin and pushing it into the cavity. When the die is opened by waiting for cooling and solidification, a light guide plate having a desired thickness can be obtained.

In a general injection molding machine, the screw can switch between the screw speed control and the driving force control, thereby controlling the injection speed of the molten resin, or controlling the injection pressure or the resin pressure. Further, various controls such as mold closing force, screw speed, and the like can be controlled based on the screw position, or can be controlled based on time. In such an injection molding machine, when the injection compression molding method described above is carried out, the control is performed as shown in Fig. In Fig. 5, the axis of abscissa indicates the screw position or time. Initially, control is started based on the screw position. Further, the screw is controlled by the screw speed until the transfer to the holding pressure step. At the start of injection, that is, when the screw position is at the initial position 51, the target value of the screw speed, that is, the speed command 52, starts to be controlled to a relatively small value 62, and the speed of the molten resin to be injected is reduced. At this time, the injection pressure 53 is small. The mold closing force 54 is a relatively small value of 63 because the mold is kept only slightly open. When the screw position reaches the position 55, the speed command 52 is set to a high value 64 to increase the speed of the molten resin to be injected. The injection pressure 53 is increased. When the screw position reaches the position 56, the speed command 52 is set to a larger value 65, and at the same time, the closing force 54 gradually increases. In other words, gradually close the mold. Whereby the molten resin injected into the cavity is compressed and charged into the cavity. When the injection is completed, that is, when the screw position reaches the pressure changeover position 58, the control based on the screw position is switched to the control based on the elapsed time. Further, the screw is switched from screw speed control to drive force control, that is, injection pressure or resin pressure control. On the other hand, the mold closing is completed a little later than the pressure change, and the mold closing force 54 is controlled to a constant value 68 until the end of the holding pressure. After the pressure changeover, the injection pressure 53, that is, the resin pressure 53 is controlled so as to be the predetermined target value 66, but the resin pressure 53 gradually decreases after the pressure change. When the predetermined time 59 elapses, the resin pressure 53 is controlled to be the target value 67 slightly lower. After the predetermined time 61 elapses and the molten resin is solidified and cooled, the holding of the pressure is terminated.

Japanese Patent Application Laid-Open No. 2008-302686

When molded by the so-called injection compression molding method as in the method described in Patent Document 1, the molten resin is filled in every corner of the cavity to obtain a thin molded article excellent in transferability. However, there is also a problem, and a problem arises in the case of molding a thin molded article having a very large molded article compared to the thickness. In the injection compression molding method, when the mold is closed by the mold closing, the molten resin flows in the peripheral direction from the vicinity of the central portion of the cavity. Since the flowing molten resin causes a flow resistance between the molten resin and the wall surface of the cavity, the pressure of the molten resin becomes uneven in the molded article. That is, the peripheral portion of the molten resin has a low resin pressure, while the vicinity of the central portion, i.e., the vicinity of the gate, is in a high-pressure state. As indicated by reference numeral 69 in the graph of Fig. 5, it is found that the injection pressure 53 after the pressure change is high and the resin pressure in the vicinity of the gate is high. In the case of a molded article which is very large in thickness, the deviation of the resin pressure is large. Thereby, a large residual stress remains in the obtained molded article, and the vicinity of the central portion becomes thick or deformed, and a desired quality molded article can not be obtained. The example described in Patent Document 1 poses no problem because it forms a small light guide plate for a cellular phone, but there is a problem in forming a light guide plate for a liquid crystal TV having a medium or larger size. In the case of the light guide plate, if the thickness is uneven, the optical characteristics deteriorate and can not be used. In the case of molding a light guide plate of a medium or more size, it is necessary to increase the thickness of the light guide plate in the past so as not to cause such a problem, and the weight becomes large.

It is an object of the present invention to provide a molding method of a thin molded article which solves the problems as described above. Specifically, even when a molded article which is very large in thickness is obtained, residual stress remains in the molded article, The present invention also provides a molding method capable of obtaining a thin molded article of high quality without using a mold. It is also an object of the present invention to provide a molding method capable of forming a light guide plate for a liquid crystal light-modulating plate having a medium thickness or more, which is not limited to a conventional thin plate.

In order to achieve the above object, the present invention comprises a heating cylinder, and a molding method using an injection device comprising a screw or a plunger which is axially drivable in the heating cylinder. In this molding method, the molten resin is injected into the cavity of the mold by driving the screw or the plunger in the axial direction with the mold being initially opened by a predetermined amount. Then, the mold is closed to compress the molten resin injected. During this compression, the screw or the plunger is retracted only for a predetermined time to lower the resin pressure of the molten resin in the cavity. Then, a driving force in the axial direction is applied to the screw or the plunger to apply a predetermined resin pressure to the molten resin. In other words, Thereby forming a thin molded article. On the other hand, the screw or the plunger is driven by the pressure control so that the resin pressure becomes a predetermined pressure. However, after the pressure is restored in a short time by performing the speed control for a predetermined time immediately after the start of the holding pressure, Control may be shifted to the control.

Therefore, in order to achieve the above object, the invention described in Claim 1 is characterized in that, by using a heating cylinder and an injection apparatus comprising a screw or a plunger which is axially drivable in a scavenging heating cylinder, The injection process of injecting the molten resin into the cavity of the mold by driving the screw or the plunger in the axial direction, the compression process of closing the mold and compressing the injected molten resin, And the plunger is retracted only for a predetermined time to lower the resin pressure of the molten resin in the cavity; and a pressing step of applying a driving force in the axial direction to the screw or the plunger after the pressing step, Molding of Thin-walled Molded Products Consisting of Pressure-relieving Process Applying Hydraulic Pressure It is composed of law. According to a second aspect of the present invention, in the method according to the first aspect, the compression step is configured to start before the end of the injection step. According to a third aspect of the present invention, in the method according to the first or second aspect, the pressing and draining step is constituted so as to be a time control for controlling the screw or the plunger based on the elapsed time from the end of the injection step do. According to a fourth aspect of the present invention, in the method according to any one of the first to third aspects, when the screw or the plunger is driven in the axial direction in the pressure holding step, Speed control is performed, and thereafter, the pressure is controlled to be the target hydraulic pressure. The invention described in claim 5 is constituted by a light guide plate molded by the molding method according to any one of claims 1 to 4.

As described above, since the present invention is constituted by a heating cylinder and a molding method using an injection apparatus comprising a screw or a plunger which is axially drivable in the heating cylinder, It is possible to do. An injection step of injecting a molten resin into a cavity of the mold by driving a screw or a plunger in an axial direction with a predetermined amount of the mold being opened, a compression step of compressing the molten resin injected by closing the mold, A step of pressing the screw or the plunger backward for a predetermined time in parallel with the step so as to lower the resin pressure of the molten resin in the cavity; and a step of applying a driving force in the axial direction to the screw or the plunger, So that the thin molded article is formed by the applied pressure holding step. That is, since the injection step and the compression step are provided, a thin molded article to some extent can be obtained in the same manner as in the conventional injection compression molding method. Further, in the present invention, since the pressing and dipping process in the mold is performed in parallel with the compression process, it is possible to alleviate the deviation of the pressure distribution of the molten resin in the cavity in the compression process. It is possible to prevent residual stress from remaining or uneven thickness. As a result, it is possible to form a thin molded article which can not be molded conventionally, that is, a thin molded article which is very thin compared to the size, with high precision. For example, in the case of a light guide plate of a medium liquid crystal, the thickness can be reduced by 30% compared to the conventional thickness. According to another aspect of the invention, since the compression step is configured to start before the end of the injection step, the injection step and the compression step can be performed smoothly and the molding cycle can be shortened. According to another aspect of the present invention, the pressing and draining step is constituted to be a time control for controlling the screw or the plunger based on the elapsed time from the end of the injection step. The time control can be managed by a timer, and an effect that can be easily carried out can be obtained. According to another aspect of the present invention, when the screw or the plunger is driven in the axial direction in the pressure holding step, the speed is controlled so as to be the target speed only for a predetermined time from the start of the process, and thereafter, the pressure is controlled so as to become the target hydraulic pressure. Thus, even if the molten resin is slightly returned to the heating cylinder in the pressing process, the molten resin can be quickly returned to the cavity by the speed control, and the resin pressure can be recovered in a very short time. And then the pressure is controlled to be the target resin pressure by the pressure control, so that the pressure holding step becomes stable, and a molded article with high precision can be obtained.

1 is a view schematically showing a motor-driven injection molding machine according to an embodiment of the present invention.
2 is a flowchart for explaining a molding method according to an embodiment of the present invention.
3 is a graph showing changes in screw speed, injection pressure, mold closing force and the like when the molding method according to the embodiment of the present invention is carried out.
4 is a graph for explaining a molding method according to a second embodiment of the present invention, and is a graph showing changes in screw speed, injection pressure, mold closing force, and the like in a predetermined process.
5 is a graph showing changes in screw speed, injection pressure, mold closing force, and the like when the conventional injection compression method is performed.

The molding method of the thin molded article according to the present embodiment can be carried out by a conventionally known injection molding machine. In the present embodiment, a method of molding a liquid-crystal light guide plate having a medium or more size by the motor-driven injection molding machine 1 will be described, but this motor-driven injection molding machine 1 will be schematically described. As shown in Fig. 1, the motor-driven injection molding machine 1 also includes an injection apparatus 2 and a mold-closing apparatus 3 as conventionally known. The injection apparatus 2 is composed of a heating cylinder 5 and a screw 6 provided in the heating cylinder 5. An injection nozzle 7 is provided at the tip of the heating cylinder 5. A band heater 8 is wound around the outer peripheral surface of the heating cylinder 5 and a hopper for feeding resin material into the heating cylinder 5 is provided at the rear end side. The hopper is not shown in Fig. The screw 6 can be driven in the axial direction while being able to be driven in the rotational direction by a driving mechanism (not shown). The screw 6 can also be controlled by the speed of the screw 6 when it is driven in the axial direction, As shown in FIG. Type closing device 3 also includes a fixed plate 10, a movable plate 11 that is opened and closed with respect to the fixed plate 10, a closed housing 12, and a movable plate 11, A plurality of tie bars 13 connecting the fixed plate 10 and the closed housing 12 and a toggle mechanism 13 provided between the fixed plate 10 and the closed housing 12, (15). The closed housing 12 includes a ball screw 16, a ball nut 17 threadedly engaged with the ball screw 16, a predetermined gear for driving the ball nut 17 and a servo motor 18, And the toggle mechanism 15 can be driven by the driving mechanism to open and close the mold.

The stationary-side mold 20 and the movable-side mold 21 according to the present embodiment are attached to the stationary plate 10 and the movable plate 11, respectively. The molds 20 and 21 are molds for forming a light guide plate. A recessed portion is formed in the stationary-side mold 20 and a core corresponding to the recessed portion is formed in the movable- When the movable mold 21 is closed with respect to the stationary mold 20, the core is inserted into the concave portion to form the light guide plate molding cavity. However, when the molds 20 and 21 are slightly opened, So that it becomes thick in correspondence with the opening amount of the mold. A hydraulic cylinder 22 is embedded in the upper surface of the movable mold 21 and opposed to the fixed mold 20. The piston rod 23 of the hydraulic cylinder 22 is in contact with the stationary-side mold 20 in a state in which the molds 20 and 21 are slightly opened. In the present embodiment, the hydraulic cylinder 22 is driven to maintain the degree of parallelism of the molds 20 and 21 with high accuracy. Whereby the molds 20 and 21 can be closed without being tilted.

A cutting groove 24 into which the injection nozzle 7 is inserted is formed in the fixing plate 10 of the mold closing device 3. The injection apparatus 2 is inserted from the back face of the fixed plate 10 and the injection nozzle 7 touches the spool of the fixed side mold 20. [ Although not shown in Fig. 1, the electric injection molding machine 1 according to the present embodiment also has a controller, and each of the devices is controlled by a controller.

A molding method according to the first embodiment for molding the light guide plate by the motor-driven injection molding machine 1 will be described with reference to Figs. 2 and 3. Fig. In the injection apparatus 2, the heating cylinder 5 is heated by the band heaters 8, 8, ..., and the screw 6 is rotated to supply the resin material from the hopper. Then, the resin material is melted by the heat by the band heaters 8, 8, ..., and the heat by the rotation shear of the screw 6, and is metered to the tip of the heating cylinder 5. When the screw 6 is retracted by the predetermined length by the molten resin to be weighed, the metering is completed (step S1).

Type closing device 3 to drive the molds 20 and 21 in the mold closing direction. The molds 20 and 21 are slightly opened (step S2). Since the molds 20 and 21 are not closed, the mold closing force 28 is a relatively low value 41 as shown in the graph of Fig. The screw 6 is driven in the axial direction to start the injection process (step S3). During the injection process, the screw 6 is controlled by its speed, that is, the screw speed. The screw speed command, that is, the speed command 27 is set based on the position of the screw 6. That is, based on the screw position. The speed command 27 is set to the low speed 38 while the molten resin after the initiation of injection reaches the gate, that is, between the start position 30 and the position 31 of the screw 6. Therefore, the molten resin is injected at a low speed. After the molten resin reaches the gate, it is injected at an intermediate speed according to the speed command 27 set to the medium speed 39. [ The injection pressure 26 gradually increases from the start of the injection process, but the mold closing force 28 is maintained at a substantially constant value 41. [ At this time, although the molten resin is filled in the cavities of the molds 20 and 21, it is a short shot state as shown in Fig.

When the screw 6 reaches the predetermined position 32, the compression process is started (step S4). That is, the mold closing device 3 is driven to gradually close the mold. Thereby compressing and diffusing the molten resin in the cavity. Type closure force 28 rises. On the other hand, in the present embodiment, after the screw 6 reaches the position 32, the speed command 27 is set to the high speed 40, and the molten resin is injected at a high speed. That is, the injection continues even after the start of the compression process. When the screw 6 reaches the pressure change position 33, the injection process is terminated (step S5). That is, the speed command 27 of the screw 6 is set to zero, and the injection of the molten resin is stopped and the injection process is terminated. Thereafter, the screw 6 is controlled based on the elapsed time from the end of the injection process. That is, shifts to time control.

After the completion of the injection process, the screw 6 is retracted at a predetermined speed only for a short time during the compression process. That is, a pressing process (step S6). When the molten resin is pressurized and diffused into the cavity in the compression process, the central portion, that is, the molten resin in the vicinity of the gate, becomes higher in pressure than in the periphery, and a significant deviation occurs in the pressure distribution. However, The pressure of the molten resin is lowered. Whereby the unevenness of the pressure distribution of the molten resin is alleviated. The graph of FIG. 3 shows a state in which the injection pressure 26 is sharply lowered, as indicated by reference numeral 43. The compression process ends later than the end of the compression process (step S7). That is, the mold closing is completed, and the mold closing force 28 reaches the maximum value 42. [ The molten resin is filled up to every corner of the cavity. After completion of the pressing process or after completion of the compression process, the pressure holding process is started (step S8). That is, when the molten resin in the cavity is cooled, the screw 6 is driven by a predetermined back pressure to apply a predetermined resin pressure 44 to prevent a sink mark. When the predetermined time 34 elapses and the molten resin cools to some extent, the pressure of the molten resin is lowered as indicated by reference numeral 45. [ When the cooling time 35 elapses and the resin is solidified and cooled, the pressure holding step is terminated.

Generally, in the pressure holding process, the driving force of the screw 6 is controlled to control the pressure to be the target resin pressure 44. [ That is, the servo motor for driving the screw 6 is subjected to torque control. When the amount of resin returned to the heating cylinder 5 by the retraction of the screw 6 in the pressing process is proper, there is no problem because the target resin pressure quickly reaches the target pressure in the holding process. However, when the amount of resin returned to the heating cylinder 5 is large in the pressing process, the resin pressure is not increased unless the screw 6 is driven to some extent in the axial direction to press the molten resin into the cavity. In this case, when the screw 6 is driven only by the pressure control, the responsiveness is deteriorated, and it takes time to obtain the desired resin pressure, which affects the quality of the molded product. A molding method according to a second embodiment capable of avoiding this phenomenon will be described with reference to Fig. Fig. 4 is a graph in which the section indicated by symbol A in the graph of Fig. 3 is enlarged in the horizontal axis direction. The forming method according to the second embodiment is different from the forming method according to the first embodiment already described only in the pressing step. Therefore, only the holding process will be described. In the forming method according to the second embodiment, the speed of the screw 6 is controlled only for the short time in which the screw 6 is set. In Fig. 4, the speed command 27 'of the screw 6 is set, as indicated by reference numeral 47. Fig. The screw 6 is driven in the axial direction so that the set target speed is achieved, and the resin pressure becomes high in a very short time. Thereafter, the drive of the screw 6 is shifted to the pressure control. Then, the target hydraulic pressure 44 is obtained in a short time.

The molding method according to the present embodiment can be carried out in other injection molding machines. For example, a hydraulic injection molding machine, and the injection apparatus may be constituted by a heating cylinder and a plunger driven in the axial direction in the heating cylinder. The molding method according to the present embodiment can be modified in various ways. For example, in the present embodiment, although the compression process is described as being started before the injection process is completed, the compression process may be started after the injection process is completed. In the injection process, the speed command is set to three stages, and the pressure process is set to the two stages. However, these may be set to one stage only or to multiple stages.

1: Electric injection molding machine
2: Injection device
3: Closing device
5: Heating cylinder
6: Screw
7: injection nozzle
10: Fixed plate
11: movable plate
12: Closed housing
13: Taiba
15: toggle mechanism
20: Fixed side mold
21: movable side mold

Claims (5)

Using a heating cylinder and an injection apparatus comprising a screw or a plunger which is axially drivable in the heating cylinder,
An injection step of injecting molten resin into the cavity of the mold by driving the screw or the plunger in the axial direction with the mold being opened by a predetermined amount,
A compression step of closing the mold and compressing the molten resin injected,
A compressing step of retracting the screw or the plunger for a predetermined time in parallel with the compressing step to lower the resin pressure of the molten resin in the cavity;
And applying a predetermined hydraulic pressure to the molten resin by applying an axial driving force to the screw or the plunger after the pressing process. The method according to claim 1,
Wherein the compression step starts before the end of the injection step. The method according to claim 1,
Wherein the pressing step is a time control for controlling the screw or the plunger based on the elapsed time from the end of the injection step. The method according to claim 1,
Characterized in that, when the screw or the plunger is driven in the axial direction in the pressure holding step, the speed is controlled so as to be the target speed only for a predetermined time from the start of the process, and thereafter the pressure is controlled so as to become the target hydraulic pressure. Way. delete

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