Injection molding machine for foam molding

Abstract

To provide an injection molding machine to which an inert gas can be injected stably and which has a short machine length.SOLUTION: An injection molding machine (1) comprising a heating cylinder (2) and a screw (3) is sectioned into a first stage (5) on an upstream side and a second stage (6) on a downstream side. The first stage (5) is constituted of a feed section (8), a first compression section (9), and a first measuring section (10) and the second stage (6) is constituted of a starvation section (13), a second compression section (14), and a second measuring section (15). Injection parts (17, 18) of an inert gas are provided at two or more parts and provided with check valves (20, 21). A product P×N, of a gap P between the injection parts (17, 18) and the number N is set to be 0.5 H or more and 1.71 H or under with respect to a length H of the starvation section (13).SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an injection molding machine used for foam molding in which an inert gas is injected into a molten resin and the resulting foam-molded product is obtained by injection molding into a mold. [Background technology]

[0002] Molded products that contain many fine bubbles inside, that is, foam molded products, are not only lightweight but also have excellent strength, and have a wide range of applications. To obtain a foam molded product by injection molding, it is necessary to mix a foaming agent into the resin, and when a physical foaming agent is selected as the foaming agent, an inert gas such as nitrogen or carbon dioxide is used. When an inert gas is used as the foaming agent, the inert gas is injected into the molten resin in the heating cylinder, kneaded and dissolved. Then, the inert gas becomes saturated in the resin. When this is injected into a mold, the pressure is released in the resin and the inert gas becomes bubbles. When the resin is cooled and solidified, a foam molded product is obtained. When a foam molded product is molded with an inert gas, there is a method in which the inert gas is made in a high-pressure and high-temperature supercritical state and injected, but a so-called low-pressure foam molding method in which a relatively low-pressure inert gas is injected to obtain a foam molded product is also well known. When the low-pressure foam molding method is implemented, as in the injection molding machines described in Patent Documents 1 and 2, the groove depth of the screw is deepened in a specified section of the screw, thereby reducing the pressure of the molten resin in the heating cylinder. In other words, a starvation section is formed. The heating cylinder is provided with an inert gas injection section corresponding to this starvation section, and the inert gas is injected into the heating cylinder so that the inert gas permeates the molten resin in the starvation section. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-177696 A [Patent Document 2] JP 2019-18522 A Summary of the Invention [Problem to be solved by the invention]

[0004] In an injection molding machine for low pressure foam molding, the injection of inert gas can be performed at low pressure, so the inert gas injection device can be simply configured and can be provided at low cost, which is excellent. However, there are some points that need to be improved. Specifically, there is a problem that the length of the screw and the heating cylinder becomes relatively long. As described above, in an injection molding machine for low pressure foam molding, a starvation section is formed in the heating cylinder, and inert gas is injected in this section, but the screw retreats as metering progresses. In order to be able to continuously inject inert gas even when it retreats, and to prevent resin from penetrating from the inert gas injection section, it is necessary to make the starvation section sufficiently long. In other words, it is necessary to ensure a starvation section that is sufficiently longer than the length of retreat due to metering, that is, the metering stroke. Therefore, in an injection molding machine for low pressure foam molding, the screw and the heating cylinder are inevitably long, and there is a problem that the machine length becomes long. Incidentally, if an inert gas injection section is provided in two or more places in the heating cylinder separated in the axial direction, the starvation section may be shortened. That is, regardless of the forward or backward position of the screw, as long as any one of the inert gas injection parts is located in the starvation section, the injection of the inert gas can be continued, and if the other injection part is closed by a valve, the infiltration of the resin can be prevented. The injection molding machine described in Patent Document 2 has inert gas injection parts at two axially separated places in the heating cylinder, and can be said to be an injection molding machine capable of shortening the starvation section. However, Patent Document 2 does not specifically state at what intervals two or more injection parts should be provided, or how much the starvation section can be shortened. In that case, the machine length cannot be sufficiently shortened. Patent Document 1 states that the length of the starvation section is six times the screw diameter. However, the length of this starvation section is not determined on the premise that multiple inert gas injection parts are provided. Furthermore, although it is expected that the length of the starvation section is related to the metering stroke, there is no description of this relationship.In any case, the descriptions in Patent Documents 1 and 2 do not describe how the spacing between two or more injection points should be determined in relation to the starvation section and the metering stroke when two or more injection points are provided, so the starvation section cannot be shortened sufficiently, and the machine length of the injection molding machine cannot be shortened.

[0005] The present disclosure aims to provide an injection molding machine that forms a starvation section in a heating cylinder and injects a relatively low-pressure inert gas into molten resin to form a foam-molded product, which can inject inert gas stably and has a sufficiently short machine length.

[0006] Other objects and novel features will become apparent from the description of this specification and the accompanying drawings. [Means for solving the problem]

[0007] The present disclosure provides a foam injection molding machine with a plurality of injection sections for injecting an inert gas, and provides check valves or opening / closing mechanisms for these. The heating cylinder is divided into a first stage on the upstream side and a second stage on the downstream side according to the shape of the screw, and the first stage is composed of, from upstream to downstream, a supply section where the resin is melted, a first compression section where the resin is compressed, and a first metering section where the resin is fed, and the second stage is composed of, from upstream to downstream, a starvation section, a second compression section, and a second metering section. The present disclosure provides a product P×N of the interval P between the plurality of injection sections and the number N of the injection sections, which is 0.5H or more and 1.71H or less with respect to the length H of the starvation section. This shortens the machine length. If the length by which the screw retreats during metering is defined as the metering stroke S, then the multiple injection parts are arranged so that at least one injection part is present within a range of 0.1S or more inside from both ends of the starvation section from the start to the completion of metering. In yet another disclosure, a plurality of injection sections are provided with an opening and closing mechanism, and the opening and closing mechanism is controlled to open and close based on the position of the screw. Effect of the Invention

[0008] According to the present disclosure, the product P×N of the spacing P between multiple injection parts and the number N of injection parts is 0.5H or more and 1.71H or less with respect to the length H of the starvation section, so that even if the screw retreats during metering, at least one injection part is guaranteed to be located in the starvation section. If the length by which the screw retreats during metering is the metering stroke S, then multiple injection parts are arranged so that at least one injection part is present within a range of 0.1S or more inside from both ends of the starvation section from the start to the end of metering. Therefore, It is guaranteed that the inert gas can be stably supplied. The injection section is provided with a check valve or an opening / closing mechanism. When the screw retracts during metering, one of the multiple injection sections will be out of the starvation section, and there is a risk of resin infiltrating into such an injection section. However, since the check valve or the opening / closing mechanism is provided, it is possible to reliably prevent the resin from flowing back. Since the length of the starvation section is short, the machine length can be shortened. According to another disclosure, the length H of the starvation section is 0.7S or more and 1.2S or less with respect to the metering stroke S, which is the length by which the screw retracts during metering. Since the length H of the starvation section is short, the length of the screw is short, and the machine length of the injection molding machine is short. According to still another disclosure, the multiple injection sections are provided with opening / closing mechanisms, and the opening / closing of these opening / closing mechanisms is controlled based on the position of the screw. In this way, the screw position at which the injection section is out of the starvation section is stored, and when the screw reaches this screw position, the opening / closing mechanism is closed, thereby reliably preventing the resin from flowing back. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 shows an injection molding machine for foam molding according to an embodiment of the present invention, in which (A) and (B) are front cross-sectional views of the injection molding machine, showing the screw at its most advanced position and at its most retracted position for metering, respectively. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Specific embodiments will be described in detail below with reference to the drawings. However, the present invention is not limited to the following embodiments. In order to clarify the description, the following description and drawings are appropriately simplified. In each drawing, the same elements are given the same reference numerals, and duplicated descriptions are omitted as necessary. In addition, hatching is omitted in some parts to avoid cluttering the drawings.

[0011] An embodiment of the present invention will be described below. As shown in Fig. 1(A) and (B), an injection molding machine 1 according to an embodiment of the present invention is composed of a heating cylinder 2 and a screw 3 that is provided so as to be drivable in the rotational direction and the axial direction within the heating cylinder 2. A hopper is provided at the rear, i.e., upstream side, of the heating cylinder 2, and an injection nozzle is provided at the front, i.e., downstream side, and a plurality of band heaters are wound around the outer circumferential surface of the heating cylinder 2, but these are not shown in the drawings.

[0012] The injection molding machine 1 according to the present embodiment is an injection molding machine for implementing a so-called low-pressure foam molding method in which a foam-molded product is molded by injecting a relatively low-pressure inert gas. Therefore, the screw 3 according to the present embodiment has a structure suitable for the low-pressure foam molding method. First, the screw 3 can be roughly divided into a first stage 5 on the upstream side and a second stage 6 on the downstream side. The first stage 5 is a supply section 8 in which the groove depth of the screw is large from the upstream to the midstream, and the resin supplied from the hopper is melted and sent forward. On the downstream side of this supply section 8, a first compression section 9 in which the groove depth of the screw gradually decreases is formed. The first compression section 9 is a section in which the molten resin is compressed and sent forward. And, at the most downstream part of the first stage 5, a first metering section 10 in which the groove depth of the screw is small and constant is formed, and the compressed resin is sent forward. In this embodiment, the first compression section 9 and the first metering section 10 are formed with so-called barrier flights. That is, the section is formed of a main flight having a predetermined pitch and lead angle, and a sub-flight having a pitch and lead angle larger than those of the main flight. Therefore, in this section, the resin does not easily flow back. Note that these sections 9 and 10 may be formed of other types of flights, such as a single flight. The second stage 6 has a starvation section 13 formed on the upstream side. In the starvation section 13, the groove depth of the screw is large and constant, so that the pressure of the molten resin is reduced. As will be described next, the inert gas is supplied in this starvation section 13, melted into the resin, and penetrates and dissolves in the molten resin. The injection molding machine 1 according to this embodiment is characterized by the length H of this starvation section 13, which will be described later. A second compression section 14 in which the groove depth of the screw gradually decreases is formed downstream of this starvation section 13, so that the molten resin in which the inert gas is dissolved is sent while being compressed and kneaded. And a second metering section 15 in which the groove depth of the screw is small and constant is formed at the most downstream part of the second stage 6, so that the molten resin in which the inert gas has penetrated is sent forward. An intermediate region may be provided between the first measurement section 10 and the starvation section 13 for the purpose of adjusting pressure, etc.

[0013] The injection sections 17 and 18 for injecting the inert gas are provided on the heating cylinder 2. In this embodiment, the first and second injection sections 17 and 18, the number of which is 2, are provided at positions spaced apart in the axial direction, and the interval P between them is a feature. As shown in FIG. 1A, the first injection section 17 is provided at a position corresponding to the starvation section 13 when the screw 3 is in the most advanced state. In contrast, the second injection section 18 is provided upstream of the first injection section 17, and is provided at a position corresponding to the first stage 5 in this embodiment. More preferably, the second injection section 18 is provided downstream of the first metering section 10. Since the first and second injection sections 17 and 18 are provided in this manner, even if the screw 3 retreats during metering as shown in FIG. 1B and the first injection section 17 leaves the starvation section 13, the second injection section 18 enters the starvation section 13, and the injection of the inert gas can be continued. In the present invention, in order to stably inject the inert gas, the number N and the interval P of the first and second injection parts 17, 18 are related to the length H of the starvation section 13, as will be explained later. The first and second injection parts 17, 18 are equipped with check valves 20, 21, respectively, and are connected to a cylinder 22 that supplies the inert gas. In the figure, the check valves 20, 21 are shown to be provided at a position separated from the injection parts 17, 18, but in reality they may be incorporated into the injection parts 17, 18.

[0014] In the injection molding machine 1 according to this embodiment, the number N and the interval P of the first and second injection parts 17, 18 are defined in relation to the length H of the starvation section 13, and these are also defined in relation to the metering stroke S. Several other points are also defined. In explaining these, the design considerations in conventional injection molding machines for stably injecting inert gas into molten resin and conventional problems will be explained.

[0015] In the case of a conventional injection molding machine for foam molding, at the beginning and end of the starvation section, there is a region where the resin pressure changes from high to low or from low to high, that is, a transition region between a part filled with resin and a part in a starved state. Since gas cannot penetrate sufficiently in this transition region, if the gas supply port does not have an opening and closing mechanism, when the screw moves forward and backward and the gas supply port approaches the transition region, the molten resin will enter the inside of the gas supply port and gas supply will not be possible. If this state continues, foam molding will not be able to continue, so maintenance such as cleaning the gas supply port will be necessary. In the present disclosure, by providing an opening and closing mechanism in the gas flow path, such as the check valves 20 and 21 in the first and second injection parts 17 and 18, it is possible to prevent the intrusion of molten resin even when the gas supply port approaches the transition region. However, during that period, gas cannot be supplied, and there is a possibility of problems such as a decrease in the amount of gas dissolved. The length of this transition region varies depending on the shape of the screw, or the metering conditions such as the viscosity of the resin at the time of metering and the screw rotation speed. Due to the existence of this transition region, a high-quality foamed product cannot be obtained by simply ensuring that at least one gas supply port is always present in the starvation zone during metering. During metering, it is necessary to ensure that at least one gas supply port is present within a range of a specific length from both ends of the starvation zone. Specifically, the specific length may be the distance that the resin advances in 0.5 rotations of the screw, more preferably the distance that the resin advances in one rotation. Usually, the pitch of the screw flight is often around 1D, where D is the diameter of the screw, so this specific length is 0.5D, more preferably 1D. In other words, during metering, at least one gas inlet is present within a range of 0.5D or 1D or more from both ends of the starvation zone. This makes it possible to avoid the gas supply port approaching the transition zone. This allows the inert gas to be efficiently and stably supplied to the molten resin in a conventional injection molding machine. In other words, conventional injection molding machines are designed so that the length of the starvation section is at least 0.5D before and after the stroke S, more preferably at least 1D, i.e., at least S+1D, more preferably at least S+2D.However, with such a conventional design, the starvation section needs to be long relative to the required metering stroke S, which poses the problem of increasing the overall length of the machine.

[0016] In the injection molding machine 1 according to the present embodiment, a number of regulations are set to solve the above conventional problems. First, the product P×N of the interval P between the first and second injection parts 17, 18 and the number N of the first and second injection parts 17, 18 is set to be 0.5H or more and 1.71H or less with respect to the length H of the starvation section 13. Then, the interval P between the first and second injection parts 17, 18 is set to be 0.25S or more and 0.8S or less with respect to the metering stroke S. In this embodiment, the number N of the first and second injection parts 17, 18 is two, but by increasing the number of injection parts and narrowing the interval P, it is possible to reliably inject inert gas. Therefore, the length H of the starvation section 13 can be shortened. As a result, the length H of the starvation section 13 is set to be 0.7S or more and 1.2S or less with respect to the metering stroke S, which is the length by which the screw 3 retreats during metering. This is a length that could not be realized with conventional injection molding machines for foam molding. Furthermore, the product P×N of the distance P between the first and second injection parts 17, 18 and the number N of these parts is specified to be 0.5S or more and 1.6S or less with respect to the metering stroke S. This makes it possible to efficiently and stably supply inert gas to the molten resin and to reduce the machine length.

[0017] In this embodiment, the number N of inert gas injection parts is two, the first and second injection parts 17 and 18, but it can be three or more. However, increasing the number of injection parts too much increases costs, so it is preferable to keep it at four or less. EXAMPLES

[0018] An experiment was conducted to confirm that foam molding can be stably performed in the injection molding machine 1 according to the present embodiment. "Experimental Method" First, the standard conditions were set as follows. (The dimensions and lengths of each part are shown as a ratio of the metering stroke S.) Screw diameter D: 84mm (0.2S) Metering stroke S: 420mm (injection volume is approximately 2,327cm3) Length of starvation section 13 H:420mm (1.0S) Number of inert gas injection parts N: 2 Injection valve mechanism: check valve Distance between injection parts P: 252 mm (0.6 S) Resin used: PP resin Then, a purge test was performed under the same conditions as the above-mentioned reference conditions or by changing some of the conditions to confirm whether a good foaming state could be obtained. In this case, the N injection parts 17, 18, ... were arranged at equal pitches in the heating cylinder 2. In addition, the injection parts 17, 18, ... were arranged so that when the screw 3 moved half the metering stroke S, that is, when it retreated 0.5S, the center position of the starvation section 13 of the screw 3 in the heating cylinder 3 and the center position of the N injection parts 17, 18, ... coincided. Examples 1 to 10 that satisfy the conditions specified in the present invention and Comparative Examples 1 to 8 that do not satisfy the conditions were tested.

[0019] Example 1: An injection molding machine under the same conditions as the reference conditions was prepared, and a purge test was carried out using the reference condition metering stroke S. The obtained sample had good uniform foaming. Example 2: A purge test was carried out under the standard conditions, with the distance between the injection parts 17 and 18 changed to 0.8 S. The two injection parts 17 and 18 were arranged so that at least one of them was always in the starvation section 13 during metering and was located 0.1 S or more inside both ends of the starvation section 13. Specifically, one injection part 17 was arranged on the heating cylinder 2 so that it was located 0.1 S downstream from the upstream end of the starvation section 13 at the start of metering, and the other injection part 18 was arranged so that it was located 0.1 S upstream from the downstream end of the starvation section 13 at the completion of metering. From the start to the completion of the metering stroke, the injection parts were always located inside these positions. The obtained sample had uniform foaming and was good. Example 3: Under the standard conditions, the length H of the starvation section 13 was set to 0.8S, the number N of injection parts 17, 18, ... was set to 3, spaced apart in the axial direction, and the interval P between adjacent injection parts 17, 18, ... was set to 0.4S, and a purge test was performed. The length H of the starvation section 13 was shortened by 0.2S from the standard conditions, and the machine length was shortened by 0.2S. The obtained sample had uniform foaming and was good. Example 4: Under the standard conditions, the check valves 20 and 21 were replaced with other valves. That is, the injection parts 17 and 18 with check valves were simply injection holes, and a needle-type injection valve that was opened and closed by air was installed in the pipe that supplied inert gas to this injection hole. A purge test was carried out under the same conditions as the standard conditions in other respects. The opening and closing of the injection valve was switched based on the screw position. That is, the injection valve was opened when the injection hole was 0.1 S or more inside both ends of the starvation section 13, and closed when it was outside the starvation section 13 to prevent backflow of resin. The obtained sample had uniform foaming and was good. Example 5: The conditions were the same as those of Example 4. However, an additional condition was added for the opening and closing of the injection valve, so that when a measurement was completed, both valves were closed until the next measurement began. When a purge test was carried out, uniform foaming was formed and a good sample was obtained. Example 6: In addition to the standard conditions, an automatic valve that receives an electric signal and opens and closes the flow path was installed in the pipeline that supplies inert gas to the injection parts 17 and 18 equipped with check valves 20 and 21. In other words, an opening and closing mechanism was provided. The opening and closing of this automatic valve was switched based on the screw position, and the automatic valve opened when the injection hole was 0.1 S or more inside both ends of the starvation section 13, and closed when it was outside the starvation section 13 to prevent backflow of resin. When a purge test was carried out using this, uniform foaming was formed and good samples were obtained. Example 7: Under the standard conditions, the check valves 20, 21 were removed. That is, the injection parts 17, 18 were simply injection holes, and an automatic valve that received an electric signal to open and close the flow path was installed in the pipe that supplied inert gas to the injection holes. In other words, an opening and closing mechanism was provided. The opening and closing of this automatic valve was switched based on the screw position, and the automatic valve opened when the injection hole was 0.1 S or more inside both ends of the starvation section 13, and closed when it was outside the starvation section 13 to prevent backflow of resin. The obtained sample had uniform foaming and was good. Example 8: Among the standard conditions, the metering stroke S was set to 350 mm. The length H of the starvation section 13 and the interval P between the injection sections 17 and 18 were 420 mm and 252 mm, respectively, both of which were standard conditions, but by shortening the metering stroke S, the length H of the starvation section 13 and the interval P between the injection sections 17 and 18 became 1.2 S and 0.72 S, respectively. The obtained sample had uniform foaming and was in good condition. Example 9: Among the standard conditions, the distance P between the injection parts 17, 18 was set to 105 mm, that is, 0.25S. The product of the distance P between the injection parts 17, 18 and the number N was 0.5S for the metering stroke S, and 0.5H for the length H of the starvation section 13. When a sample was obtained under these conditions, the obtained sample had uniform foaming and was good. Example 10: Among the standard conditions, the length H of the starvation section 13 was set to 373 mm, i.e., 0.9 S, and the interval P between the injection parts 17 and 18 was set to 189 mm, i.e., 0.45 S. When a sample was obtained under these conditions, the obtained sample had uniform foaming and was in good condition. Example 11: Among the standard conditions, the length H of the starvation section 13 was set to 294 mm, i.e., 0.7 S, and the interval P between the injection parts 17, 18 was set to 126 mm, i.e., 0.3 S. The obtained sample had uniform foaming and was excellent. Example 12: The screw diameter D was 40 mm, the metering stroke S was 180 mm, the length H of the starvation section 13 was 180 mm, the interval P between the injection sections 17, 18 was 120 mm, and the number N of the injection sections 17, 18 was 2. That is, the length H of the starvation section 13 was 1.0S, and the interval P between the injection sections 17, 18 was 0.3S, which satisfied the conditions stipulated in the present invention. The samples obtained by the experiment showed uniform foaming and were good.

[0020] Comparative Example 1: Among the standard conditions, the number of inert gas injection parts was one, and the metering stroke S was 252 mm. However, the injection part was provided at a position 42 mm (0.5D) from the start position of the starvation section at the start point of the metering stroke (except for Comparative Example 8, the same applies to Comparative Example 2 and onwards when the number of injection parts is one). The length H of the starvation section 13 was 1.7S, which does not satisfy the provisions of the present invention. A purge test was performed on the obtained sample, which was uniformly foamed and good, but the injection volume was only about 931 cm3. In other words, a sufficient injection amount was not obtained. Comparative Example 2: Under the standard conditions, the number of inert gas injection parts was set to one, which was replaced by a simple injection hole. In other words, the check valve was removed. A purge test was carried out using this. However, the sample did not foam. When the injection hole was examined, it was found to be clogged with resin and inert gas was not being supplied. This is presumably because the injection part moves outside the starvation section 13 during the metering stroke. Comparative Example 3: Among the standard conditions, the length H of the starvation section 13 was set to 672 mm, or 1.6 S. Also, the number N of inert gas injection parts was set to 1, which was replaced with a simple injection hole. In other words, the check valve was removed. A purge test was carried out using this. The obtained sample had uniform foaming and was good. Since the length H of the starvation section 13 was long, it was guaranteed that the injection hole would always be within the starvation section 13, but the machine length was longer by 0.6 S. Comparative Example 4: Among the standard conditions, the inert gas injection parts were changed to simple injection holes without check valves. A purge test was carried out using these. However, the sample did not foam. When examined, it was found that the two injection holes were clogged with resin. Comparative Example 5: Among the standard conditions, the interval P between the two injection parts was increased to 420 mm, i.e., 1.0 S. When a purge test was carried out, the bubbles in the sample were coarse and large. It is expected that there was a timing during the metering process when both of the two injection parts were out of the starvation section 13, and as a result, the inert gas did not sufficiently penetrate into the molten resin. Comparative Example 6: Among the standard conditions, the length H of the starvation section 13 was set to 168 mm, that is, 0.4 S. A purge test was carried out using this, but only a few air bubbles were formed in the sample. Comparative Example 7: Among the standard conditions, the metering stroke S was set to 350 mm. As a result, the length H of the starvation section 13 was set to 1.2S. The number of injection parts N was set to 1. A purge test was performed, but the diameter of the bubbles formed in the sample became large. Comparative Example 8: The screw diameter D was 40 mm, the metering stroke S was 180 mm, and the length H of the starvation section 13 was 180 mm. The injection section was provided at a position 20 mm (0.5D) from the start position of the starvation section at the start point of the metering stroke. In other words, the length H of the starvation section 13 was 1.0S, which satisfies the conditions stipulated in the present invention. However, the number N of injection sections was 1. A purge test was performed, but the bubbles formed in the sample had a large diameter. The experimental results are summarized in the table below.

[0021] [Table 1]

[0022] (*1) in Table 1 indicates the maximum distance of the injection ports 17, 18 from both ends of the starvation interval 13 during measurement. During measurement, the relative position of one or more injection ports 17, 18, ... changes with respect to the starvation interval 13. Therefore, at each timing during measurement, attention is focused on one injection port 17, 18, ... located closest to the center of the starvation interval 13. At one timing during measurement, for example, one injection port 17 is closest to the center of the starvation interval 13, and at another timing, the other injection port 18 is closest. In this way, the shortest distance from both ends of the starvation interval 13 for one injection port 17, 18, ... located closest to the center of the starvation interval 13 at each timing during measurement is summarized in *1. When this number is a positive value, it means that at least one of the injection ports 17, 18, ... is within the starvation zone 13 during measurement, and is always within the positive value from either end of the starvation zone 13. On the other hand, when it is a negative value, it indicates that there are times during measurement when all of the injection ports 17, 18, ... are outside the starvation zone 13.

[0023] In Table 1, the item "H / S" indicates the ratio of the length H of the starvation section 13 to the metering stroke S, the item "P / S" indicates the ratio of the injection section spacing P to the metering stroke S, the item "P×N / S" indicates the ratio of the product P×N of the injection section spacing P and the number of pieces N to the metering stroke S, the item "P×N / H" indicates the ratio of the product P×N of the injection section spacing P and the number of pieces N to the length H of the starvation section 13, and the item "A / S" indicates the ratio of the length of *1 to the metering stroke S.

[0024] As an additional test, a foam molding test was carried out as Examples 1' to 10' under the same conditions as Examples 1 to 10. That is, a molded foam molding test was carried out by using a spigot mold in which the cavity volume can be freely set by the mold opening amount, and an appropriate mold opening amount was set to match the set injection stroke, and all other conditions were the same as Examples 1 to 10 to mold a foam molded product. In all conditions, the injection parts 17, 18, ... were arranged so that one or more injection parts 17, 18, ... were within a range of 0.1 S or more inside from both ends of the starvation section 13 during the metering stroke, and gas was supplied stably while avoiding the transition section. As a result, the foam molded products obtained as Examples 1' to 10' were all uniformly foamed and were good.

[0025] As can be seen from Table 1, the screw 3 of the injection molding machine 1 used in the test has only two types of screw diameters D, 84 mm and 40 mm. In other words, tests for other screw diameters D were not conducted. However, even if the screw diameter D is different, the effect of the present disclosure can be achieved as long as two or more gas injection parts are provided, each of these multiple gas injection parts is provided with a check valve or an opening and closing mechanism, and the product P×N of the interval P of the injection parts and the number N of the injection parts is 0.5H or more and 1.71H or less with respect to the length H of the starvation section 13. Generally, when designing a screw having a specific shape to achieve a specific effect in an injection molding machine, the length of each part, for example, the screw length L, the length H of the starvation section 13, the lengths L1, L2, ..., etc. of the other parts are determined by the ratio with the screw diameter D. When scaling up or down by increasing or decreasing the screw diameter D, the length of each part is determined by multiplying each ratio by the screw diameter D. This is because the same effect is achieved in this way. In other words, it is common technical knowledge that if the ratio of the length of each part to the screw diameter D is the same, the same effect is achieved. In Table 1 above, the items "H / S", "P / S", "P×N / S", and "A / S" are all ratios based on the metering stroke S, not the screw diameter D. However, the metering stroke S itself can be said to be a length that can be expressed as a ratio based on the screw diameter D. In that case, the ratios of these items in Table 1 can be shown substantially based on the screw diameter D. From the above, it can be seen that even if the screw has a screw diameter D not in Table 1, if the length of each part is determined based on the ratio of these items, the same effect as the screw 3 according to the present disclosure can be achieved.

[0026] Incidentally, in Examples 1 to 12 and Comparative Examples 1 to 8, the metering stroke S is not necessarily the maximum metering stroke of the injection molding machine 1. However, in designing the injection molding machine 1, the metering stroke S shown in Examples 1 to 12 should be taken into consideration as the maximum metering stroke. If the maximum metering stroke exceeds the metering stroke S, the gas supply port approaches the transition region at the start and end of the starvation section 13, and the supply of the inert gas is hindered, which adversely affects the quality of the molded product.

[0027] The injection molding machine 1 according to the present embodiment can be modified in various ways. For example, as implemented in the above experiment, the injection parts 17 and 18 may be provided with an opening / closing mechanism consisting of an injection valve as described in Examples 4 and 5, instead of the check valves 20 and 21. Alternatively, as described in Examples 6 and 7, an opening / closing mechanism consisting of an automatic valve may be provided in the pipe line supplying the inert gas to the injection parts 17 and 18. The opening / closing mechanism is configured so that it can be controlled to open and close from a controller, and opens and closes according to the position of the screw 3. In this way, the injection parts 17 and 18 can be opened when they are in the starvation section 13, and closed when they leave the starvation section 13, thereby reliably preventing the backflow of resin from the injection parts 17 and 18. Other modifications are possible, and as described above, three or more injection parts 17 and 18 may be provided. Alternatively, a check valve and an opening / closing mechanism may be provided at the same time. In addition, three or more injection parts may be arranged at different pitches, or the arrangement of N injection parts may be offset upstream or downstream with respect to the starvation section 13. Furthermore, the flight of the screw 3 can also be modified, and can be selected from various flights such as a single flight and a double flight.

[0028] Although the invention made by the present inventor has been specifically described above based on the embodiment, it goes without saying that the present invention is not limited to the embodiment already described, and various modifications are possible without departing from the gist of the invention. The above-described examples may be implemented in appropriate combination. [Explanation of symbols]

[0029] 1 Injection molding machine 2 Heating cylinder 3 Screw 5 First stage 6 Second Stage 8 Supply Section 9 First compression section 10 First measurement section 13 Starvation section 14 Second compression section 15 Second metering section 17 Injection section 18 Injection part 20 Check valve 21 Check valve

Claims

1. A heating cylinder; a screw contained in the heating cylinder; a plurality of injection parts provided at a plurality of positions spaced apart in the axial direction of the heating cylinder for injecting an inert gas; The inside of the heating cylinder is divided into an upstream first stage and a downstream second stage according to the shape of the screw, the first stage is composed of an upstream supply section where the resin is melted, a downstream first compression section where the molten resin is compressed, and a downstream first metering section, the second stage is composed of an upstream starvation section where the pressure of the resin decreases, a downstream second compression section where the molten resin is compressed, and a downstream second metering section, the inert gas is supplied from the plurality of injection ports in the starvation section, Each of the plurality of injection parts is provided with a check valve or an opening / closing mechanism, a product P×N of an interval P between the multiple injection parts and the number N of the injection parts is 0.5H or more and 1.71H or less with respect to a length H of the starvation section, an injection molding machine for foam molding, in which the length by which the screw retreats during metering is defined as a metering stroke S, and the multiple injection parts are arranged so that at least one injection part is located within a range of 0.1 S or more inside from both ends of the starvation section from the start to the completion of metering.

2. 2. The injection molding machine for foam molding according to claim 1, wherein a distance P between said plurality of injection parts is set to be 0.25 S or more and 0.8 S or less with respect to said metering stroke S.

3. 3. The injection molding machine for foam molding according to claim 1, wherein a product P×N of a distance P between the plurality of injection parts and a number N of the injection parts is 0.5 S or more and 1.6 S or less with respect to the metering stroke S.

4. 4. The injection molding machine for foam molding according to claim 1, wherein the number N of the plurality of injection parts is 2 or more and 4 or less.

5. A heating cylinder; a screw contained in the heating cylinder; a plurality of injection parts provided at a plurality of positions spaced apart in the axial direction of the heating cylinder for injecting an inert gas; The inside of the heating cylinder is divided into an upstream first stage and a downstream second stage according to the shape of the screw, the first stage is composed of an upstream supply section where the resin is melted, a downstream first compression section where the molten resin is compressed, and a downstream first metering section, the second stage is composed of an upstream starvation section where the pressure of the resin decreases, a downstream second compression section where the molten resin is compressed, and a downstream second metering section, the inert gas is supplied from the plurality of injection ports in the starvation section, an opening / closing mechanism is provided in each of the plurality of injection parts, and opening / closing of the opening / closing mechanism is controlled based on the position of the screw; a product P×N of an interval P between the multiple injection parts and the number N of the injection parts is 0.5H or more and 1.71H or less with respect to a length H of the starvation section, an injection molding machine for foam molding, in which the length by which the screw retreats during metering is defined as a metering stroke S, and the multiple injection parts are arranged so that at least one injection part is located within a range of 0.1 S or more inside from both ends of the starvation section from the start to the completion of metering.

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