JP6293198B2 - Thin-walled molding method - Google Patents

Description

  The present invention relates to a molding method for molding a so-called thin molded product, such as a light guide plate, which is thinner than the size of the molded product.

  Among the molded products obtained by injection molding, the thin molded product has a smaller wall thickness than the projected area of the molded product. The mold for obtaining such a thin molded product has a narrow cavity interval and a long resin flow length from the gate, and accurately fills the resin from the opposite gate side, that is, the opposite side farthest from the gate in the cavity. It ’s difficult. Filling the cavity with resin becomes more difficult as the wall becomes thinner. A light guide plate, which is a component of a liquid crystal panel, is one of the typical thin-walled molded products, and can be obtained by injection molding. However, a larger and thinner light guide plate is required due to the demand for larger and lighter liquid crystal panels. It is becoming. Therefore, it can be said that the technology is increasingly required for thin-wall molding. There are various molding methods for obtaining a thin molded article, but two methods will be described as typical methods. First, the first method is to slightly open the mold when injecting the resin and inject the cavity with a wide space, and perform the mold clamping operation during or after the injection to crush the resin and spread the entire cavity. This is a so-called compression molding method. In this compression molding method, it is necessary to control not only the injection device but also the mold clamping device with high precision, and the mold structure becomes special. The second method is a method of forming a thin molded product by adjusting the injection conditions without performing compression molding. In this case, it can be carried out only by controlling the injection device. It can be said that the latter method is a general method in injection molding. A well-known method that is conventionally performed in the latter method is as follows. First, when injecting the resin into the clamped mold, the screw is driven at a high speed so that the resin is sufficiently filled in every corner of the cavity, and the resin is filled in a short time. Since the screw is driven at a high speed, the injection pressure increases as the filling of the cavity proceeds. This is due to the elasticity of the resin, and the resin is compressed due to flow resistance when filled into a thin cavity. Therefore, at the moment when the filling is completed, the injection pressure reaches the maximum pressure, that is, the injection peak pressure, and a large pressure remains in the resin near the gate. If solidified in this state, the vicinity of the gate of the molded product becomes thick and the thickness becomes non-uniform, but the screw is switched from position control to pressure control in the pressure-holding step after the injection step. In general, in thin-wall molding, the set pressure in the pressure holding process is set low, and when the injection process is switched to pressure control of the pressure holding pressure, the pressure is reduced to a low set pressure at once. When the pressure is reduced, the screw moves backward, the resin pressure near the gate is released, the residual pressure is reduced, and a thin molded product having a relatively uniform thickness is obtained.

Japanese Patent Laying-Open No. 2015-13467

  Patent Document 1 proposes another method for forming a thin-walled molded article only by controlling an injection device without performing compression molding. In the method described in Patent Document 1, when the resin is injected into the cavity, the screw speed is changed to a low speed. Specifically, the screw speed is greatly reduced before the cavity is filled with resin. For example, the screw speed is set to zero. Thus, the pressure of the resin injected from the injection device is made substantially constant. In this way, the injection pressure is maintained for a predetermined time. Then, filling of the cavity proceeds at a predetermined pressure lower than the injection peak pressure in the conventional injection method as described above. Since injection is performed in this manner, the pressure of the resin in the vicinity of the gate is small even at the moment when the resin is completely filled in the cavity. When the filling of the cavity is completed, the process proceeds to a pressure holding process in which the screw drive is switched to pressure control. Since the pressure of the resin in the vicinity of the gate is small, only a small amount of screw retraction is required after shifting to the pressure holding process. Thereafter, a predetermined pressing force is applied to the screw to solidify the resin. In this way, a highly accurate molded product having a uniform thickness can be obtained.

  A thin molded product can be obtained only by controlling the injection device, either by a conventional injection method that does not use compression molding, or by the method described in Patent Document 1. However, there are also problems to be solved. First, the conventional method has a problem in that since the injection is performed at a high injection speed, the injection pressure becomes high, the load on the injection device and the mold is large, and the service life is shortened. Further, since the high pressure acts on the resin in the cylinder and injection nozzle, the physical properties of the resin may change. Furthermore, a high injection peak pressure is reached at the timing of completion of filling the cavity, and a large pressure difference occurs between the non-gate side and the gate side inside the cavity. Then, even after the filling of the cavity is completed, a pressure difference remains even if the pressure of the resin in the vicinity of the gate is released and the pressure difference between the gate and the gate is not completely eliminated. In this case, the obtained thin molded product is likely to have defects such as large variations in thickness, large residual stress, and large warpage.

  On the other hand, the method described in Patent Document 1 completes the filling of the cavity by reducing the screw speed near the completion of the filling of the resin into the cavity, or substantially holding it for a predetermined time. Thus, the injection pressure is suppressed and the injection device and the mold are not damaged. In addition, since excessive injection pressure is not applied, it is possible to reduce the variation in pressure between the resin on the opposite gate side and the gate side in the cavity, and the thickness of the thin molded product obtained becomes uniform, causing the above-mentioned defects. hard. However, the method described in Patent Document 1 has a problem to be solved. Specifically, it is unclear when to start and how long to start the process to be performed before filling the cavity, that is, to make the injection pressure substantially constant. This is a problem. In other words, in order to obtain a thin molded article with excellent quality, it is ambiguous what index should be used to determine the start timing of this process and to determine the holding time. For example, the following points have been found qualitatively regarding the start timing of this process. In other words, if the start timing is too late, this process is started after the injection pressure becomes high, so the pressure of the resin near the gate after filling is high, and molding with a uniform and accurate thickness Goods can no longer be obtained. On the other hand, if the start timing is too early, the pressure in the resin near the gate is reduced because the cavity is filled with a low injection pressure in this process, but the resin is not filled with sufficient pressure in the entire cavity, and as a result The variation in pressure between the resin on the opposite gate side and the gate side in the cavity increases, and the thickness of the resulting molded product varies. In other words, it can be said that the determination of the start timing of the process of making the injection pressure substantially constant is an important factor for obtaining a thin molded product with high accuracy. Also, the holding time of this step is not described in detail, but depending on the length, high-precision molding cannot be performed and various problems occur. The method described in Patent Document 1 does not describe a method for obtaining an optimal combination of the start timing and the holding time, which are two independent variables, for this step. The combination must be tested to obtain the optimal combination. That is, there is a problem that the cost required for the test increases. Furthermore, the flowability of the resin changes when the temperature is changed, and the flow resistance changes when the injection nozzle used is changed. That is, if the molding conditions are changed, the behavior of the resin in the cavity changes, but if the behavior changes, the optimum combination of the start timing and the holding time for making the injection pressure substantially constant also changes. Then, even if an appropriate combination of the start timing and holding time of this process is found for a predetermined temperature and a predetermined injection nozzle, the resin temperature is changed or the injection nozzle is replaced with another injection nozzle. If there is a change in the molding conditions such as, the test must be repeated from the beginning to find the optimal combination of start timing and holding time.

  An object of the present invention is to provide a thin-wall molding method that solves the above-described problems. Specifically, when a resin is injected into a cavity of a clamped mold, a predetermined holding time is provided. In the thin wall molding method in which the injection pressure is made substantially constant and the filling of the cavity is completed, it is possible to appropriately determine the start timing of the process for making the injection pressure substantially constant and the holding time, thereby improving the quality. An object of the present invention is to provide a thin-wall molding method capable of obtaining a high thin-wall molded product.

  In order to achieve the above object, the present invention achieves thin-wall molding by an injection process in which a screw is driven in the axial direction by position control to inject resin and a pressure holding process in which the screw is driven by pressure control to hold pressure. It is intended for thin-wall molding methods for obtaining products. The thin-wall molding method includes a test molding stage in which molding is performed on a trial basis and a main molding stage in which a thin-wall molded product is molded. In the test molding stage, the injection process is completed so that at least a screw speed of a predetermined size is completed, and the pressure-holding process is completed. At this time, the injection peak pressure and the injection process, which are the maximum injection pressure, are changed. The required injection process time is obtained. On the other hand, in the main molding stage, the injection process is started in the same manner as the injection process in the test molding stage, and the screw speed is switched to zero at a predetermined switching timing to be completed for a predetermined holding time. Switch to. The switching timing is assumed to be advanced in advance by the holding time so that the injection process is completed in the injection process time. There is a correlation between the injection peak pressure and the optimum holding time, which can be expressed by a linear function. The present invention provides an optimal retention time at the injection peak pressure based on a linear function.

Thus, in order to achieve the above object, the invention according to claim 1 controls the position of the screw by an injection device comprising a heating cylinder and a screw that can be driven in the rotation direction and the axial direction in the heating cylinder. An injection process for injecting resin into a mold cavity that is driven in the axial direction and clamped, and a pressure holding process for maintaining a predetermined pressure by switching the screw to pressure control after the injection process. A thin-wall molding method for obtaining a thin-walled molded article, wherein the thin-wall molding method comprises a test molding stage for performing molding on a test basis and a main molding stage for molding a thin-wall molded article, the test molding stage comprising: When the injection process is completed, the screw speed is completed so that at least a predetermined screw speed is obtained, and the pressure holding process is completed. At this time, the injection peak pressure, which is the maximum injection pressure, and the injection process The injection process time, which is the time required, is obtained. At this time, the injection peak pressure, which is the maximum injection pressure, and the injection process time, which is the time required for the injection process, are obtained. It starts in the same manner as the injection process in the test molding stage, and at a predetermined switching timing, the screw speed is switched to zero and held for a predetermined holding time to complete, and then the pressure holding process is switched. The thin-wall forming method is characterized in that it is determined in advance by the holding time so that the injection process is completed in the injection process time.
According to a second aspect of the present invention, in the method according to the first aspect, the first injection which is the injection peak pressure by performing the test molding step under a first molding condition on a predetermined mold. A peak pressure is obtained, and the main molding step is repeated a plurality of times while changing the holding time under various conditions under the first molding condition, and the holding time when the dimensional accuracy of the thin molded product obtained is the highest is the first. The test molding step is performed on the mold under the second molding condition to obtain the second injection peak pressure which is the injection peak pressure, and the second molding condition. The holding time is changed variously and the main molding step is repeated a plurality of times. The holding time when the dimensional accuracy of the thin molded product obtained is the highest is the second optimum holding time, and the first and second injections are performed. Peak pressure and optimal holding of the first and second A linear function that gives the optimum value of the holding time is determined from the injection peak pressure as a variable, and a thin-walled molded product is obtained by molding conditions different from the first and second molding conditions for the mold. The thin molding method is characterized in that the holding time is determined from the injection peak pressure obtained by performing the test molding step and the linear function.

  As described above, the thin-wall molding method of the present invention includes a test molding stage in which molding is performed on a trial basis and a main molding stage in which a thin-wall molded product is molded. In the main molding stage, the injection process is started in the same manner as the injection process in the test molding stage, and the screw speed is switched to zero at a predetermined switching timing to be completed for a predetermined holding time, and then switched to the pressure holding process. I am doing so. Thus, in the main molding stage, the screw speed is made zero during the injection process so that the injection process is completed. Therefore, the amount of resin flowing from the gate into the cavity is small after the switching timing. Since the resin injected into the cavity before the switching timing is compressed, the resin is filled in the cavity while expanding in the cavity. As a result, the pressure of the resin in the vicinity of the gate does not increase thereafter and decreases substantially or gently. As a result, the pressure of the resin in the cavity becomes uniform as a whole. This can be said to be a necessary condition for obtaining a thin-walled molded product with uniform thickness and high accuracy. By the way, in order to obtain a highly accurate thin-walled molded product, it is necessary to find the optimum switching timing and holding time. Since these are two independent variables, the number of tests to be performed for that purpose is enormous. Should be. However, according to the present invention, the test molding step is such that at the completion of the injection process, the screw speed is at least a predetermined magnitude and is switched to the pressure-holding process. The peak pressure and the injection process time, which is the time required for the injection process, are obtained. The switching timing is determined in advance by the holding time so that the injection process is completed in the injection process time. That is, according to the present invention, the switching timing can be uniquely determined once the holding time is determined. Then, the switching timing can be handled as a dependent variable of the holding time, and when determining the molding conditions for obtaining a highly accurate thin molded product, it is only necessary to determine the holding time. In other words, the number of tests to be performed to determine the optimum molding conditions is small, and the cost is low.

  According to another invention, a test molding step is performed on a given mold under a first molding condition to obtain a first injection peak pressure, which is an injection peak pressure, and held under the first molding condition. The molding process is repeated several times at various times, and the holding time when the dimensional accuracy of the thin molded product obtained is the highest is the first optimum holding time, and the mold is subjected to the second molding conditions. A test molding step is performed to obtain a second injection peak pressure which is an injection peak pressure, and the molding step is repeated a plurality of times while changing the holding time under the second molding conditions. The holding time when the dimensional accuracy is the highest is the second optimum holding time, and the optimum value of the holding time using the injection peak pressure as a variable from the first and second injection peak pressures and the first and second optimum holding times. Determine the linear function that gives the first When you get a thin molded article by a different molding conditions and 2 of the molding condition, it is configured to determine the retention time of the test molding step from an injection peak pressure and the linear function obtained by implementation. That is, for a given mold, the optimum value of the holding time in the main molding stage can be given as a linear function of the injection peak pressure obtained in the test molding stage, so once the linear function is determined, Thereafter, if the test molding step is carried out once and only the injection peak pressure is measured, the optimum holding time can be determined without any special test. When the holding time is determined, the switching timing is inevitably determined by the above-described invention. Therefore, even if the molding conditions are changed, specifically, even if the injection nozzle is changed or the temperature of the resin is changed, and as a result the behavior of the resin changes, the test molding stage is performed and the injection peak pressure is And the injection process time, the switching timing and holding time in the main molding stage can be immediately determined, and the cost required to determine the optimal combination of switching timing and holding time is substantially zero. . According to the present invention, even if the molding conditions change and the flowability and behavior of the resin change, the optimum combination of switching timing and holding time can be determined. It can be guaranteed that it will be obtained.

It is a front view which shows the injection molding machine which concerns on embodiment of this invention in a cross section. It is a graph which shows the relationship between the screw speed in each of the test shaping | molding stage and this shaping | molding stage, and injection pressure about the thin-wall shaping | molding method which concerns on this Embodiment. The (a) and (b) are graphs showing experimental results obtained by carrying out the thin-wall molding method according to the present embodiment.

  The thin-wall molding method according to the present embodiment can be implemented by any injection molding machine as long as it is a general injection molding machine, and no special device is required. Therefore, the injection molding machine 1 according to the present embodiment will be briefly described with reference to FIG. An injection molding machine 1 according to the present embodiment includes an injection device 2 and a mold clamping device (not shown). The injection device 2 includes a heating cylinder 4 and a screw 5 provided in the heating cylinder 4. An injection nozzle 7 is provided at the tip of the heating cylinder 4. The injection nozzle 7 can be replaced in the same manner as the conventional injection apparatus 2. When the injection nozzle 7 is replaced with an injection nozzle 7 having a different inner diameter, the flow resistance changes and the behavior of the resin in the cavity changes. A hopper 8 is provided near the rear end of the heating cylinder 4, and a band heater that heats the heating cylinder 4 is provided on the outer peripheral surface, although not shown in the drawing. The screw 5 can be driven in the rotational direction and the axial direction by the screw driving mechanism 10. The mold clamping device of the injection molding machine 1 may be a toggle type mold clamping device, a direct pressure type mold clamping device, or another type of mold clamping device. A mold 11 for obtaining a thin molded product according to the present embodiment is provided in a mold clamping device so as to be opened and closed. The injection nozzle 7 of the injection device 2 is in contact with such a sprue of the mold 11, and the resin injected from the injection device 2 is filled from the gate 13 into the cavity 14 in the mold 11. ing.

  The thin-wall molding method according to the present embodiment for molding a thin-wall molded product by the injection molding machine 1 according to the present embodiment is the same as the conventional molding method for molding a thin-wall molded product only by controlling the injection device 2. In addition, the mold 11 is clamped by an injection process for injecting resin into the cavity 14 and a pressure holding process for applying pressure to the resin to prevent sink marks after the injection process. The screw 5 is controlled by the screw drive device 10 in the same manner as in the prior art in the position control, in other words, the speed control. That is, a target position or target speed is given to the screw 5, and the position of the screw 5 is detected and feedback controlled. In the pressure holding process, pressure control, that is, torque control is performed. That is, when the screw 5 is driven in the axial direction, a target torque is applied, and the torque is detected to perform feedback control. However, in the thin-wall molding method according to the present embodiment, characteristic control is performed from the middle to the completion of the injection process. That is, the thin-wall forming method according to the present embodiment reduces the screw speed to substantially zero during the injection process in which the screw is driven at a predetermined speed. In this state, the injection process is completed by holding for a predetermined holding time. After the injection process is completed, the screw is switched to pressure control, that is, torque control, as in the conventional case.

  In the thin-wall molding method according to the present embodiment, the screw speed is substantially zero from the middle to the completion of the injection process as described above. The behavior of the resin in the cavity 14 when the screw speed is switched to zero is as follows. It becomes as follows. That is, at the timing when the screw speed is switched to zero, that is, at the switching timing, a space not filled with resin remains in the cavity 14. However, since the resin is injected at a screw speed of a predetermined size until just before this, the resin receives a strong flow resistance in the thin cavity 14 and is compressed, and a pressure of a predetermined size is generated in the vicinity of the gate 13. A pressure equivalent to this pressure is also generated in the resin at the tip of the screw 5 in the heating cylinder 4, and this resin is also compressed. Therefore, after the switching timing, the resin in the heating cylinder 4 expands and the pressure is supplied little by little from the gate 13 while the pressure is substantially constant or slightly decreased, and the resin in the cavity 14 also expands. Thus, the cavity 14 is filled. That is, after the switching timing in the injection process, the cavity 14 is completely filled in a state where the pressure is substantially constant or slightly reduced, so that the injection pressure of the resin is suppressed to be relatively low. When the screw 5 is switched to pressure control, that is, torque control, and the process proceeds to the pressure holding process, the screw 5 is slightly retracted as necessary to release the resin pressure in the vicinity of the gate 13 and then apply a predetermined pressure. Become. Since the thin-wall molding method according to the present embodiment performs the injection process and the pressure-holding process in this manner, the pressure variation of the resin in the cavity 14 is small, and the thin-wall molded product with high accuracy and uniform thickness. Is obtained.

  By the way, in the thin-wall forming method according to the above-described embodiment, how to determine the switching timing and how to determine the holding time becomes a problem. Therefore, in order to determine these, in the thin-wall molding method according to the present embodiment, a test molding stage in which a test molding is first performed is performed. After that, the main forming step, which is the above thin-wall forming method, is carried out.

  The test molding stage will be described. In the test molding step, a conventional molding method is carried out. That is, in the injection process, as shown by the dotted line graph 21 in FIG. 2, the speed of the screw 5 is kept constant, and the injection process is completed at this speed. As a result, the pressure of the resin, that is, the injection pressure, increases in the injection process as indicated by the dotted line graph 22 and reaches the maximum value 23 at the completion. The injection molding machine 1 records this maximum value 23 as the injection peak pressure 23. Similarly, in the test molding stage, the time required to reach the injection peak pressure 23, that is, the time required for the injection process is recorded as the injection process time 24. When the screw 5 is switched to pressure control after the injection process is completed in the test molding stage and the process proceeds to the pressure holding process, the screw 5 is driven in the opposite direction, and the pressure of the resin in the vicinity of the gate 13 is rapidly released, whereby the injection pressure is reduced. It drops rapidly from the injection peak pressure 23. A constant torque is applied to the screw 5 so that the injection pressure, that is, the resin pressure becomes constant.

  In the main forming stage of the thin wall forming method according to the present embodiment, the switching timing 28 is treated as a dependent variable of the holding time 25 and is determined by the holding time 25. First, the holding time 25 for setting the screw speed to zero is determined at an arbitrary time. The switching timing 28 is set to a timing that is advanced by the holding time 25 from the injection process time 24 obtained in the test molding stage so that the injection process is completed at the injection process time 24. That is, the injection process is completed in the injection process time 24 in the main molding stage. The state of injection molding in the main molding stage thus performed is shown by a solid line graph 26 in FIG. The main molding stage starts in the same manner as the test molding stage, and at the switching timing 28, the speed of the screw 5 is made substantially zero. In this state, the holding time is 25. The injection process is completed at the injection process time 24. At this time, the injection pressure in the injection process increases in the same manner as in the test molding stage until the switching timing 28 is reached, and becomes constant or slightly decreases after the switching timing 28. When the injection process time 24 is reached and the injection process is completed, the screw 5 is switched to pressure control, that is, torque control, and the pressure holding process is performed. Then, the screw 5 is slightly retracted as necessary, and the injection pressure is reduced. Thereafter, a constant injection pressure, that is, a resin pressure is applied by the screw 5 to complete the pressure holding process. When the injection process in the main molding stage is completed, the pressure of the resin near the gate 13 is not high. Therefore, even if the screw 5 is retracted in the pressure holding process, the retraction amount is small as compared with the test molding stage. Alternatively, the screw 5 may not retreat.

  In this way, since the switching timing 28 is inevitably determined only by determining the holding time 25, the main forming step can be easily performed. However, it is unclear whether the cavity 14 can be properly filled with resin. Although the injection process in the main molding stage is completed at an injection pressure lower than that in the test molding stage, the injection process is performed only for the same injection process time 24 as the injection process in the test molding stage, so that the cavity 14 may not be sufficiently filled with the resin. This is because a short shot may occur. However, as is apparent from the experiments described below, it has been found that no short shot occurs in the main forming stage according to the present embodiment. By the way, even if there is no short shot problem, it is still unclear how to determine the optimum time for the holding time 25. However, the inventors have found that the optimum holding time 25 can be uniquely determined by the injection peak pressure 23. Specifically, it has been found that the optimum holding time 25 can be determined by a linear expression using the injection peak pressure 23 as a variable. The following experiment was conducted to confirm these points.

Apparatus, mold, and resin As shown in FIG. 1, the first to third injection nozzles 7A, 7B, and 7C are used as exchangeable injection nozzles 7 using the injection molding machine 1 according to the present embodiment. Prepared. As the mold 11, a mold for forming a light guide plate having a thickness of 430 μm and a size of 12.5 cm × 7 cm was used. Polycarbonate resin was used as the resin.
Description of Experiment (1) The first injection nozzle 7A was attached to the injection molding machine 1, and a test molding step was performed to mold a light guide plate. The injection peak pressure obtained at this time was 410 MPa. This is the first injection peak pressure. No short shot was seen on the molded light guide plate. It was 447 micrometers when the thickness of the gate 13 vicinity of a light-guide plate was measured.
(2) Next, using the first injection nozzle 7A, the holding time was set to 0.02 seconds, 0.03 seconds, 0.04 seconds, and 0.05 seconds, and this molding step was performed four times. When the light guide plates molded with the respective holding times were examined, no short shot was observed, but the thickness in the vicinity of the gate 13 was 439 μm, 437 μm, 435 μm, and 443 μm, respectively.
(3) Data of the test molding stage and the main molding stage by the first injection nozzle 7A are shown as a graph 31 in FIG. It was found that the thickness in the vicinity of the gate 13 was closest to the ideal thickness of 430 μm of the light guide plate was the holding time of 0.04 seconds. Thus, it was found that the optimum holding time when using the first injection nozzle 7A, that is, the first optimum holding time is 0.04 seconds. It was also confirmed that no short shot occurred even if the holding time was changed from 0.02 to 0.05 seconds and the main forming step of the thin wall forming method according to the present embodiment was performed.
(4) Next, the second injection nozzle 7B was attached to the injection molding machine 1 and the test molding stage was carried out, and the main molding stage was carried out four times while changing the holding time, whereby a graph 32 was obtained. The injection peak pressure obtained in the test molding stage by the second injection nozzle 7B, that is, the second injection peak pressure was 405 MPa. It was found that the optimum holding time, that is, the second optimum holding time, was 0.03 seconds when the thickness near the gate 13 was closest to the ideal thickness of 430 μm. In addition, it was confirmed that the light guide plate obtained in the test molding stage and the four main molding stages did not generate short shots.
(5) Finally, the third injection nozzle 7C was attached to the injection molding machine 1 and the test molding stage was performed, and the main molding stage was performed three times with different holding times to obtain a graph 33. The injection peak pressure obtained in the test molding stage by the third injection nozzle 7B, that is, the third injection peak pressure was 390 MPa. It was found that the optimum holding time, that is, the third optimum holding time was 0.02 seconds when the thickness near the gate 13 was closest to the ideal thickness of 430 μm. In addition, it was confirmed that the light guide plate obtained in the test molding stage and the three main molding stages did not generate short shots.
(6) The first to third injection peak pressures obtained for each of the first to third injection nozzles 7A, 7B, and 7C and the first to third optimum holding times are plotted, and FIG. The graph 35 was obtained. It was confirmed that the graph 35 is a linear function.

  Originally, if the injection nozzle 7 is replaced, or if the molding conditions are changed such as changing the type of resin or changing the resin temperature, the wall thickness is increased even in molding using the same mold. It is necessary to repeat the test in order to examine the optimal combination of the switching timing 28 and the holding time 25 in order to obtain a uniform and highly accurate thin molded product. However, in the thin-wall molding method according to the present invention, after obtaining the linear function as shown in the graph 35 by the experiment as described above, the molding conditions are changed and thereby injected into the cavity 14. Even if the behavior of the resin changes, the optimal combination of the switching timing 28 and the holding time 25 can be easily determined. That is, the test molding stage is performed under the molding conditions, and the injection peak pressure 23 and the injection process time 24 are obtained. The optimum holding time 25 is obtained by substituting the injection peak pressure 23 for the linear function of the graph 35. The optimum switching timing 28 is a timing in which the determined holding time 25 is advanced from the injection process time 24. In addition, in order to obtain the linear expression of the graph 35, in the above experiment, it experimented using the 1st-3rd injection nozzles 7A, 7B, and 7C. That is, the experiment was performed under three types of molding conditions with different injection peak pressures. However, to determine the linear function, it is sufficient to perform the test under two types of molding conditions with different injection peak pressures.

  The thin wall molding method according to the present embodiment can be variously modified. For example, in the above description, the injection molding process is performed in the test molding stage with the speed of the screw 5 being constant, and the speed of the screw 5 is also constant in the main molding stage until the switching timing 28. However, the speed of the screw 5 may be changed. For example, in the test molding stage, the speed of the screw 5 may be changed to two stages and changed midway. In this case, in the injection process at the test molding stage, the screw 5 may be completed at a non-zero predetermined speed. At this time, the speed of the screw 5 is changed to three stages in the injection process in the main molding stage. That is, in the injection process in the main molding stage, the speed of the screw 5 is substantially zero at the switching timing 28 after the speed of the screw 5 is changed to two stages in the same manner as the injection process in the test molding stage. This is because the speed of 5 is changed to 3 stages.

DESCRIPTION OF SYMBOLS 1 Injection molding machine 2 Injection apparatus 4 Heating cylinder 5 Screw 7 Injection nozzle 11 Mold 13 Gate 14 Cavity 23 Injection peak pressure 24 Injection process time 25 Holding time 28 Switching timing

Claims (2)

Resin is injected into the cavity of the mold that is clamped by driving the screw in the axial direction by position control by an injection device comprising a heating cylinder and a screw that can be driven in the rotational direction and the axial direction within the heating cylinder. A thin-wall molding method for obtaining a thin-walled molded article by performing an injection step of injecting, and a pressure-holding step of maintaining a predetermined pressure by switching the screw to pressure control after the injection step,
The thin-wall molding method comprises a test molding stage for performing molding on a trial basis and a main molding stage for molding a thin-wall molded product,
In the test molding step, the injection process is completed so that the screw speed is at least a predetermined magnitude at the time of completion, and the pressure holding process is completed. At this time, the injection peak pressure that is the maximum injection pressure and the injection process are performed. The injection process time, which is the time required for
In the main molding stage, the injection process is started in the same manner as the injection process in the test molding stage, and the screw speed is switched to zero at a predetermined switching timing to be completed for a predetermined holding time. To switch to
The switching timing is determined in advance by the holding time so that the injection process is completed in the injection process time. The method of claim 1, wherein
The test molding step is performed on a predetermined mold under a first molding condition to obtain a first injection peak pressure which is the injection peak pressure, and the holding time is set under the first molding condition. The main molding stage is repeated several times with various changes, and the holding time when the dimensional accuracy of the thin molded product obtained is the highest is the first optimum holding time,
The test molding step is performed on the mold under a second molding condition to obtain a second injection peak pressure that is the injection peak pressure, and the holding time is varied under the second molding condition. The main molding step is repeated a plurality of times, and the holding time when the dimensional accuracy of the thin molded product obtained is the highest is the second optimum holding time,
From the first and second injection peak pressures and the first and second optimum holding times, determine a linear function that gives the optimum value of the holding time using the injection peak pressure as a variable,
When a thin molded product is obtained for the mold by molding conditions different from the first and second molding conditions, the holding time is calculated from the injection peak pressure and the linear function obtained by performing the test molding step. A thin-wall molding method characterized by determining the thickness.

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