Injection molding method

Abstract

This invention suppresses the overheating of the heating device and allows the injection cylinder to be heated to a temperature suitable for injection molding without reducing productivity. [Solution] This injection molding method includes a preparation step S1 in which the injection cylinder 14 of the injection molding machine 11 is heated by an energized heating device 17 to prepare for injection molding. The preparation step S1 includes at least a first heating step S11 in which the injection cylinder 14 is heated to a first target temperature Ta1 by driving the heating device 17, a heat retention step S12 in which the temperature of the injection cylinder 14 is maintained at the first target temperature Ta1 by switching the operation of the heating device 17 after the first heating step S11, and a second heating step S13 in which the injection cylinder 14 is heated to a second target temperature Ta2 which is higher than the first target temperature Ta1 by driving the heating device 17 after the heat retention step S12.

Description

Technical Field

[0001] The present invention relates to an injection molding method, and particularly to a technique for heating an injection cylinder of an injection molding machine in a preparation process of injection molding.

Background Art

[0002] In recent years, from the viewpoint of weight reduction and thus improvement of fuel efficiency, attempts have been made to change automotive parts such as bumpers from metal to resin. Many of these resin parts are molded by injection using an injection molding machine. This injection molding machine generally includes an injection cylinder, a band heater for heating the injection cylinder, and a screw disposed inside the injection cylinder, and is configured to be able to inject while melting resin pellets by frictional heat by rotating the screw in the injection cylinder pre-heated by the band heater (see, for example, Patent Document 1). At this time, the temperature of the injection cylinder is measured by a temperature sensor such as a thermocouple attached to the injection cylinder, and the operation of a heating device such as a band heater is controlled based on the measured temperature (see, for example, Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, when performing injection molding using this type of molding machine, as mentioned above, it is necessary to preheat the injection cylinder to a predetermined temperature (the temperature at which injection molding takes place) using a heating device such as a band heater. Therefore, for example, in order to raise the temperature of the injection cylinder to a usable temperature after it has been stopped for a predetermined period of time, it is necessary to heat the injection cylinder with the heating device for a long period of time. Here, since the heating device is generally installed on the outer circumference of the injection cylinder, it is not uncommon to cover the heating device with an insulating cover (also called a jacket) to suppress heat dissipation to the surroundings (atmosphere) and save energy. However, if the heating device is heated for a long time while covered with such an insulating cover, the heating device itself tends to become hot, and if it becomes hotter than expected, there is a risk that the heating device will deteriorate due to heat and its lifespan will be reduced.

[0005] In view of the above circumstances, this specification aims to solve the following technical problem: to suppress the overheating of the heating device and to enable the injection cylinder to be heated to a temperature suitable for injection molding without causing a decrease in productivity. [Means for solving the problem]

[0006] The aforementioned problems are solved by the injection molding method according to the present invention. Specifically, this method is an injection molding method comprising a preparation step of preparing for injection molding by heating the injection cylinder of an injection molding machine with a heating device that can be driven by an electric current, wherein the preparation step comprises at least a first heating step of raising the injection cylinder to a first target temperature by driving the heating device, a heat retention step of maintaining the temperature of the injection cylinder at a first target temperature by switching control between driving and stopping the heating device after the first heating step, and a second heating step of raising the injection cylinder to a second target temperature higher than the first target temperature by driving the heating device after the heat retention step.

[0007] The inventors of this invention have diligently studied the relationship between the heating mode of the injection cylinder and the temperature rise tendency of the heating device when heating an injection cylinder using a heating device that can be driven by an electric current. As a result, they have found that when the injection cylinder is heated in stages through two or more heating processes with a heat retention process in between, compared to when the injection cylinder is heated through a single continuous heating process, the maximum temperature of the heating device during heating is lower. The present invention is based on this finding and maintains the temperature of the injection cylinder at the target temperature by switching the operation of the heating device on and off after the injection cylinder has been heated to a first target temperature lower than the final target temperature (second target temperature). By switching the operation of the heating device on and off at a level where the injection cylinder can be kept warm in this way, the heating device, which has become hot due to the first heating process, can be cooled when the heating device is stopped. Furthermore, the amount of cooling (cooling temperature) of the heating device can be easily controlled by adjusting the heat retention time by switching the heating device on and off using an electric current. Therefore, even when the injection cylinder is heated to the final target temperature (second target temperature) after the heat retention process, it is possible to avoid the heating device becoming excessively hot. Thus, even when the injection cylinder 14 is covered with an insulating cover, the injection cylinder can be heated to a temperature suitable for injection molding without any problems. Alternatively, it is possible to heat the injection cylinder in a short time using a high-output heating device. Furthermore, by heating the injection cylinder in stages without lowering its temperature, the injection cylinder can be heated to the required temperature with minimal time loss, thus maintaining productivity.

[0008] Furthermore, in the injection molding method according to the present invention, during the heat retention step, the heating device may be switched on and off to maintain the temperature of the injection cylinder at the first target temperature until the temperature of the heating device falls below the second target temperature, more preferably below the first target temperature.

[0009] By maintaining the injection cylinder temperature at the first target temperature until the heating device temperature falls below at least the second target temperature, it is possible to more reliably avoid the heating device exceeding the expected temperature.

[0010] Furthermore, in the injection molding method according to the present invention, the operation and stopping of the heating device may be switched on and off in the first heating step and the heat retention step based on the difference between the temperature measurement value of the injection cylinder and the first target temperature.

[0011] Since the heating device can be driven by the application of electricity, the temperature of the injection cylinder can be proportionally controlled based on the difference between the temperature measurement of the injection cylinder and the first target temperature. This allows for highly accurate control of the injection cylinder temperature even with a heating device that switches between driving and stopping by the application of electricity. Furthermore, the temperature of the injection cylinder can be efficiently brought to the first target temperature in a short amount of time.

[0012] Furthermore, in the injection molding method according to the present invention, in the second heating step, the heating device may be switched on and off based on the difference between the temperature measurement value of the injection cylinder and the second target temperature.

[0013] In this way, by proportionally controlling the temperature of the injection cylinder during the second heating process, the final temperature of the injection cylinder can be controlled with high precision. Furthermore, since the temperature of the injection cylinder can be efficiently brought to the target temperature in a short time, the overall preparation time can be reduced. [Effects of the Invention]

[0014] As described above, the injection molding method according to the present invention makes it possible to suppress the overheating of the heating device and to raise the temperature of the injection cylinder to a temperature suitable for injection molding without causing a decrease in productivity. [Brief explanation of the drawing]

[0015] [Figure 1] This diagram conceptually shows one example of the configuration of an injection molding apparatus for carrying out an injection molding method according to one embodiment of the present invention. [Figure 2] This flowchart shows the flow of an injection molding method that can be performed using the apparatus shown in Figure 1. [Figure 3] It is a flowchart showing the flow of the preparation process shown in FIG. 2. [Figure 4] It is a flowchart showing the detailed flow of the first temperature-raising process shown in FIG. 3. [Figure 5] It is a flowchart showing the detailed flow of the heat preservation process shown in FIG. 3. [Figure 6] It is a flowchart showing the detailed flow of the second temperature-raising process shown in FIG. 3. [Figure 7] It is a graph respectively showing an example of the temperature history of the injection cylinder and the temperature history of the heating device in the preparation process when the injection molding method according to the present embodiment is implemented.

Mode for Carrying Out the Invention

[0016] Hereinafter, the content of the injection molding method according to an embodiment of the present invention will be described based on the drawings.

[0017] FIG. 1 shows the configuration of an injection molding apparatus 10 according to an embodiment of the present invention. As shown in FIG. 1, the injection molding apparatus 10 according to the present embodiment includes an injection molding machine 11, a control device 12, and a temperature sensor 13. Further, the injection molding machine 11 includes an injection cylinder 14, a hopper 15, a screw 16, and a heating device 17.

[0018] The heating device 17 is, for example, a band heater, and in the example shown in this figure, it is provided at a plurality of locations in the longitudinal direction of the injection cylinder 14. Thereby, the injection cylinder 14 can be heated (temperature-raised) over a wide area in the longitudinal direction.

[0019] Further, the heating device 17 has a structure that can be driven (generate heat) by energization. Thereby, it continuously generates heat in the energized state and the drive (heat generation) stops in the non-energized state. The control mode of the heating device 17 will be described in detail when the control device 12 is described.

[0020] The temperature sensor 13 is composed of, for example, a thermocouple and is capable of measuring the temperature of the injection cylinder 14. As shown in the illustrated example, if multiple temperature sensors 13 are provided, it may be possible to measure the temperature at multiple locations in the longitudinal direction of the injection cylinder 14. Alternatively, it may be possible to measure the temperature at different locations in the thickness direction (radial direction) of the injection cylinder 14.

[0021] The control device 12 is capable of controlling the drive of the heating device 17, and is configured to control the drive of the heating device 17 based on the temperature measurement value of the injection cylinder 14 measured by the temperature sensor 13.

[0022] In this embodiment, the control device 12 is configured to drive the heating device 17 based on the difference between the temperature measurement of the injection cylinder 14 and the target value (the first target temperature Ta1 and the second target temperature Ta2, described later), or to switch between driving and stopping the device at a predetermined time ratio as needed, using feedback control (proportional control). Details of the control mode will be described later.

[0023] The control device 12 may control both the heating device 17 and the injection molding machines 11 other than the heating device 17, or it may control only the heating device 17. In the latter case, the injection molding machines 11 other than the heating device 17 are controlled by a separate control device (not shown in the figure) from the control device 12.

[0024] The injection molding method according to this embodiment is performed using the injection molding apparatus 10 with the above configuration, and as shown in Figure 2, comprises a preparation step S1 in which the injection cylinder 14 is heated to prepare for injection molding, and an injection molding step S2 in which injection molding is performed while the injection cylinder 14 is kept at a predetermined temperature by the preparation step S1.

[0025] Furthermore, as shown in Figure 3, the preparation step S1 includes a first heating step S11 to raise the injection cylinder 14 to a first target temperature Ta1, a heat retention step S12 to maintain the injection cylinder 14 at the first target temperature Ta1, and a second heating step S13 to raise the injection cylinder 14 to a second target temperature Ta2. The second target temperature Ta2 is set to a temperature at which injection molding is possible in the injection molding step S2. The details of each step S11 to S13 will be explained in order below.

[0026] (S11) First heating process First, in the first heating step S11, as shown in Figure 4, the temperature of the injection cylinder 14 is measured by the temperature sensor 13 (step S111). Then, the drive of the heating device 17 is controlled based on the temperature measurement of the injection cylinder 14. Specifically, the control device 12 determines the control mode of the heating device 17 according to the magnitude of the difference between a predetermined first target temperature Ta1 and the temperature measurement (step S112). That is, if the above difference is greater than or equal to a predetermined value (sufficiently large) and it is determined that continuous heating of the injection cylinder 14 is necessary, the control device 12 controls the heating device 17 to drive continuously (step S113). As a result, the heating device 17 generates heat, the adjacent injection cylinder 14 is heated, and the injection cylinder 14 begins to heat up. In this case, the heating device 17 heats up at a constant gradient from the beginning, as shown by the solid line in Figure 7, while the injection cylinder 14 heats up by gradually increasing the heating gradient to reach a constant gradient, as shown by the dashed line in Figure 7.

[0027] By repeatedly executing steps S111 to S113 described above at predetermined time intervals, the injection cylinder 14 heats up toward the first target temperature Ta1. Then, in step S112, when the difference between the first target temperature Ta1 and the measured temperature falls below a predetermined value, that is, when the temperature of the injection cylinder 14 approaches the first target temperature Ta1, the control device 12 controls the operation of the heating device 17 to switch between driving and stopping based on the magnitude of the difference (step S114). Specifically, if the difference is relatively large, the operation time of the heating device 17 is controlled to be longer than the subsequent stopping time, and if the difference is relatively small, the operation time of the heating device 17 is controlled to be shorter than the subsequent stopping time. By controlling in this way, the slope of the temperature curve of the injection cylinder 14 gradually decreases and converges toward the first target temperature Ta1 (see the right-hand portion of the dashed line in Figure 7 for the first heating step S11). Then, when the temperature measurement of the injection cylinder 14 reaches the first target temperature Ta1 (step S115), this process S11 is terminated and the process proceeds to the next process, the heat retention process S12.

[0028] During the execution of process S11, the heating device 17 continues to heat up at a constant gradient until it reaches the first maximum temperature Tm1, after which it gradually cools down (see the right-hand portion of the solid line in Figure 7 representing the first heating process S11). This is presumed to be because the heating device 17 switches to a control mode that repeatedly switches between driving and stopping, and a temperature decrease occurs in the heating device 17 when it stops.

[0029] The first target temperature Ta1 is set in a range of 40% or more and less than 70% of the second target temperature Ta2, which will be described later, but is preferably set in a range of 50% or more and less than 60%. Furthermore, the change from continuous drive control of the heating device 17 to switching control between drive and stop may be triggered when the difference between the first target temperature Ta1 and the measured temperature falls below a predetermined value (for example, 10% of the first target temperature Ta1), as described above. Alternatively, the change may be triggered when the difference between the first target temperature Ta1 and the measured temperature falls below a certain value (for example, 15% of the first target temperature Ta1) and the heating gradient (heating rate) of the injection cylinder 14 begins to decrease from a constant state. The conditions for the control change in step S132 of the second heating process S13 can be set in the same way.

[0030] (S12) Heat retention process In step S12, first, as shown in Figure 5, the temperature of the injection cylinder 14 is measured by the temperature sensor 13 (step S121). Then, the control device 12 controls the heating device 17 to switch between driving and stopping so that the temperature measurement value of the injection cylinder 14 is maintained at the first target temperature Ta1 (step S122).

[0031] For example, if the difference between the first target temperature Ta1 and the measured temperature is a positive value, that is, if the actual temperature of the injection cylinder 14 is lower than the first target temperature Ta1, the control device 12 adjusts the ratio of the operating time and stopping time of the heating device 17 so that the difference approaches zero. For example, if the period from the time after step S121 is performed until the time S121 is performed again is considered a unit period, the control device 12 controls the ratio of the operating time and stopping time of the heating device 17 so that the operating time (energized time) of the heating device 17 per unit period is longer than the stopping time (unenergized time).

[0032] On the other hand, if the difference between the first target temperature Ta1 and the measured temperature is a negative value, that is, if the actual temperature of the injection cylinder 14 is higher than the first target temperature Ta1, the control device 12 adjusts the ratio of the operating time and stopping time of the heating device 17 so that the difference approaches zero. For example, the control device 12 controls the ratio of the operating time and stopping time of the heating device 17 so that the operating time of the heating device 17 per unit period is shorter than the stopping time.

[0033] By repeatedly performing steps S121 and S122 described above at predetermined time intervals, the injection cylinder 14 is maintained at the first target temperature Ta1, while the temperature of the heating device 17 continuously decreases (see Figure 7). Then, after a predetermined time has elapsed since the start of the heat retention process S12 and the temperature of the heating device 17 has decreased to the temperature that was previously expected (step S123), this process S12 is terminated and the process proceeds to the next process, the second heating process S13.

[0034] In step S123, the "predetermined time" is the period from the start time t1 to the end time t2 of the heat retention process S12, and is set as the time required for the heating device 17 to decrease to a pre-determined temperature. Here, it is set as the time required for the heating device 17 to decrease to below the second target temperature Ta2, and preferably as the time required to decrease to the first target temperature Ta1.

[0035] (S13) Second heating process In the second heating step S13, as shown in Figure 6, the temperature of the injection cylinder 14 is measured by the temperature sensor 13 (step S131). Based on the temperature measurement of the injection cylinder 14, the drive of the heating device 17 is controlled. Specifically, the control device 12 determines the control mode of the heating device 17 according to the magnitude of the difference between the preset second target temperature Ta2 and the temperature measurement (step S132). That is, if the above difference is greater than or equal to a predetermined value (sufficiently large) and it is determined that continuous heating of the injection cylinder 14 is necessary, the control device 12 controls the heating device 17 to drive continuously (step S133). As a result, the heating device 17 generates heat, the adjacent injection cylinder 14 is heated, and the injection cylinder 14 begins to rise further from the first target temperature Ta1. In this case as well, the heating device 17 heats up at a constant gradient from the beginning, as shown by the solid line in Figure 7, while the injection cylinder 14 heats up by gradually increasing the heating gradient until it reaches a constant gradient, as shown by the dashed line in Figure 7.

[0036] By repeatedly executing steps S131 to S133 described above at predetermined time intervals, the injection cylinder 14 heats up toward the second target temperature Ta2. Then, in step S132, when the difference between the second target temperature Ta2 and the measured temperature falls below a predetermined value, that is, when the temperature of the injection cylinder 14 approaches the second target temperature Ta2, the control device 12 controls the operation of the heating device 17 to switch between driving and stopping based on the magnitude of the difference (step S134). Specifically, if the difference is relatively large, the operation time of the heating device 17 is controlled to be longer than the subsequent stopping time, and if the difference is relatively small, the operation time of the heating device 17 is controlled to be shorter than the subsequent stopping time. By controlling in this way, the slope of the temperature curve of the injection cylinder 14 gradually decreases and converges toward the second target temperature Ta2 (see the right-hand portion of the dashed line in Figure 7, corresponding to the second heating step S13). Then, when the temperature measurement of the injection cylinder 14 reaches the second target temperature Ta2 (step S135), this process S13 is terminated and the process proceeds to the next process, injection molding process S2.

[0037] In this process S13 as well, during its execution period, the heating device 17 continues to raise the temperature at a constant gradient until it reaches the second highest temperature Tm2, ​​after which it gradually cools down (see the right-hand portion of the solid line in Figure 7 representing the second heating process S13).

[0038] As described above, according to the injection molding method of this embodiment, after raising the temperature of the injection cylinder 14 to a first target temperature Ta1 which is lower than the final target temperature (second target temperature Ta2), the temperature of the injection cylinder 14 is maintained at the target temperature Ta1 by switching the operation and stopping of the heating device 17. At the same time, by switching the operation and stopping of the heating device 17 to a level that can keep the injection cylinder 14 warm, the heating device 17, which has become hot due to the first heating step S11, can be cooled when the heating device 17 is stopped. Furthermore, by adjusting the warming time by switching the heating device 17 by energizing it, the amount of cooling (cooling temperature) of the heating device 17 can be easily controlled. Therefore, even when the injection cylinder 14 is raised to the final target temperature (second target temperature Ta2) after the warming step S12, it is possible to avoid the heating device 17 becoming hotter than expected. In other words, the maximum temperatures Tm1 and Tm2 of the heating device 17 in each heating step S11 and S13 can be kept low. Furthermore, by gradually heating the injection cylinder 14 without lowering its temperature, the injection cylinder 14 can be heated to the required temperature (second target temperature Ta2) with minimal time loss, thus maintaining productivity.

[0039] Furthermore, in the injection molding method according to this embodiment, the period t2-t1 of the heat retention step S12 is set so that the temperature of the injection cylinder 14 is maintained at the first target temperature until the temperature of the heating device 17 reaches the first target temperature Ta1 (step S123), thereby more reliably preventing the heating device 17 from exceeding the expected temperature.

[0040] Furthermore, in the injection molding method according to this embodiment, the temperature of the injection cylinder 14 is proportionally controlled by switching the operation of the heating device 17 on and off based on the difference between the temperature measurement of the injection cylinder 14 and the first target temperature Ta1 and the difference between the temperature measurement of the injection cylinder 14 and the second target temperature Ta2 during the first heating step S11 and the heat retention step S12. Therefore, even with a heating device 17 that switches between operation and stop by energizing, the temperature of the injection cylinder 14 can be controlled with high precision. In addition, the temperature of the injection cylinder 14 can be efficiently brought to the first target temperature Ta1 and the second target temperature Ta2 in a short time. As a result, even if the heating step of the injection cylinder 14 is multi-stage and a heat retention step S12 is provided in between, it is possible to prevent as much as possible the increase in the time required to carry out the preparation step S1 and maintain production efficiency.

[0041] Although one embodiment of the present invention has been described above, the injection molding method according to the present invention may also adopt configurations other than those described above, without departing from the spirit of the invention.

[0042] For example, in the above embodiment, an example was given in which the duration of the heat retention process S12 is set by determining whether a predetermined time has elapsed from the start time t1 of the heat retention process S12 in step S123 of the heat retention process S12, but of course, it is not limited to this. For example, although not shown in the figures, the operation of the heating device 17 may be switched on and off while measuring the actual temperature of the heating device 17 so that the injection cylinder 14 is maintained at a first target temperature Ta1 (steps S121, S122), and the heat retention process S12 may be terminated using whether or not the temperature measurement value of the heating device 17 has fallen to or below the first target temperature Ta1 as a trigger.

[0043] Furthermore, in the above embodiment, an example was given in which the injection cylinder 14 is heated in two stages (first heating step S11 and second heating step S13) with one heat retention step S12 in between. However, the preparation step S1 according to the present invention can also take other heating modes. For example, the injection cylinder 14 may be heated in three or four or more stages, with heat retention steps provided between each heating step. [Explanation of Symbols]

[0044] 10 Injection molding equipment 11 Injection molding machine 12 Control device 13 Temperature sensor 14 Injection Cylinder 15 Hoppa 16 Screw 17 Heating device S1 Preparation process S11 First heating process S12 Heat retention process S13 Second heating process S2 Injection molding process Ta1 First target temperature Ta2 Second target temperature Tm1,Tm2 Maximum temperature

Claims

1. An injection molding method comprising a preparation step of heating the injection cylinder of an injection molding machine with a heating device that can be driven by an electric current to prepare for injection molding, The aforementioned preparation process is, A first heating step involves raising the temperature of the injection cylinder to a first target temperature by driving the heating device, After the first heating step, a heat retention step is performed to maintain the temperature of the injection cylinder at the first target temperature by switching the operation and stopping of the heating device, An injection molding method comprising at least the following steps: a first heating step followed by a second heating step of driving the heating device to raise the temperature of the injection cylinder to a second target temperature higher than the first target temperature.

2. The injection molding method according to claim 1, wherein in the heat retention step, the heating device is controlled to maintain the temperature of the injection cylinder at the first target temperature until the temperature of the heating device falls below the second target temperature.

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