Foam injection molding machine

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 injected into a mold to obtain a foam-molded product. [Background technology]

[0002] Molded products containing numerous microscopic bubbles, or foam-molded products, are not only lightweight but also highly durable, making them suitable for a wide range of applications. To obtain foam-molded products by injection molding, a blowing agent must be mixed into the resin. When a physical blowing agent is used, an inert gas, such as nitrogen or carbon dioxide, is used. When an inert gas is used as the blowing agent, the inert gas is injected into the molten resin in a heated cylinder, kneaded, and dissolved. This process saturates the inert gas in the resin. When the resin is injected into a mold, the pressure is released within the resin, causing the inert gas to bubble. The foam-molded product is obtained when the resin cools and solidifies. While some foam-molded products are produced using inert gas by injecting the gas into a supercritical state at high pressure and high temperature, low-pressure foam-molding methods, in which a relatively low-pressure inert gas is injected to obtain a foam-molded product, are also well known. When implementing low-pressure foam-molding methods, as in the injection molding machines described in Patent Documents 1 and 2, the screw groove depth is increased in a predetermined section of the screw, thereby reducing the pressure of the molten resin in the heated cylinder. This creates a starvation section. 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 penetrates into the molten resin in the starvation section. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-177696 [Patent Document 2] Japanese Patent Application Publication No. 2019-18522 Summary of the Invention [Problem to be solved by the invention]

[0004] Injection molding machines used for low-pressure foam molding have the advantage of being able to inject inert gas at low pressure, allowing for a simple inert gas injection device and low cost. However, there are areas for improvement. Specifically, the length of the screw and heating cylinder is relatively long. As mentioned above, injection molding machines for low-pressure foam molding have a starvation zone within the heating cylinder, where inert gas is injected. However, the screw retracts as metering progresses. To ensure continuous inert gas injection even during retraction and to prevent resin from seeping through the inert gas injection port, the starvation zone must be sufficiently long. In other words, the starvation zone must be sufficiently longer than the retraction length due to metering, i.e., the metering stroke. Therefore, injection molding machines for low-pressure foam molding inevitably have long screws and heating cylinders, resulting in a long machine length. Incidentally, the starvation zone may be shortened by providing inert gas injection ports at two or more axially spaced locations on the heating cylinder. That is, regardless of the forward or reverse position of the screw, as long as one of the inert gas injection ports is located in the starvation zone, inert gas injection can continue. Closing the other injection port with a valve prevents resin from seeping in. The injection molding machine described in Patent Document 2 has two inert gas injection ports axially spaced apart on the heating cylinder, making it possible to shorten the starvation zone. However, Patent Document 2 does not specifically describe how closely two or more injection ports should be spaced apart or how short the starvation zone can be. This prevents the machine length from being sufficiently shortened. Patent Document 1 does describe the length of the starvation zone being six times the screw diameter. However, this length of the starvation zone was not determined on the assumption that multiple inert gas injection ports are provided. Furthermore, while the length of the starvation zone is expected to be 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 sections should be determined in relation to the starvation section and the metering stroke when two or more injection sections 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 an injection molding machine for foam molding, equipped with multiple injection ports for injecting inert gas, each equipped with a check valve or an opening / closing mechanism. The heating cylinder is divided into an upstream first stage and a downstream second stage based on the screw shape. The first stage, from upstream to downstream, is composed of 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. The second stage, from upstream to downstream, is composed of a starvation section, a second compression section, and a second metering section. The present disclosure provides a system where the product P × N of the spacing P between the multiple injection ports and the number N of injection ports is 0.5H or more and 1.71H or less, relative to the length H of the starvation section. This shortens the machine length. Furthermore, the multiple injection ports are arranged so that, assuming the metering stroke S is the length the screw retracts during metering, at least one injection port is located within a range of 0.1S or more from both ends of the starvation section from the start to the end of metering. Furthermore, the interval P between the plurality of injection parts is set to be 0.25S or more and 0.8S or less with respect to the metering stroke S. In yet another disclosure, an opening and closing mechanism is provided in each of the plurality of injection sections, and the opening and closing of the opening and closing mechanism is controlled based on the position of the screw. [Effects 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 even if the screw retreats during metering, at least one injection part is guaranteed to be located in the starvation section. If the distance the screw retreats during metering is the metering stroke S, then multiple injection sections are arranged so that at least one injection section exists within a range of 0.1 S or more from both ends of the starvation section from the start to the end of metering. This system ensures a stable supply of inert gas. The injection ports are equipped with check valves or opening / closing mechanisms. When the screw retracts during metering, one of the multiple injection ports deviates from the starvation zone, potentially resulting in resin infiltration. However, the check valves or opening / closing mechanisms reliably prevent resin backflow. The short starvation zone length allows for a shorter machine length. According to another disclosure, the starvation zone length H is between 0.7 seconds and 1.2 seconds relative to the metering stroke S, which is the distance the screw retracts during metering. This short starvation zone length H shortens the screw length and the overall machine length of the injection molding machine. According to yet another disclosure, the multiple injection ports are equipped with opening / closing mechanisms, and these opening / closing mechanisms are controlled based on the screw position. This allows the screw position at which the injection port deviates from the starvation zone to be memorized, and the opening / closing mechanism is closed when the screw reaches this screw position, thereby reliably preventing resin backflow. [Brief explanation of the drawings]

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

[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. For clarity of explanation, the following description and drawings have been simplified as appropriate. In each drawing, the same elements are given the same reference numerals, and duplicate explanations are omitted as necessary. Furthermore, hatching has been omitted in some areas to avoid cluttering the drawings.

[0011] An embodiment of the present invention will be described below. As shown in Figures 1(A) and 1(B), an injection molding machine 1 according to an embodiment of the present invention comprises a heating cylinder 2 and a screw 3 that is provided within the heating cylinder 2 so as to be drivable in both the rotational and axial directions. A hopper is provided at the rear, or upstream side, of the heating cylinder 2, and an injection nozzle is provided at the front, or 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 figures.

[0012] The injection molding machine 1 according to this embodiment is an injection molding machine for implementing a so-called low-pressure foam molding method, in which a foam-molded product is formed by injecting a relatively low-pressure inert gas. Therefore, the screw 3 according to this embodiment has a structure suitable for the low-pressure foam molding method. The screw 3 can be broadly divided into a first stage 5 on the upstream side and a second stage 6 on the downstream side. The first stage 5 has a supply section 8, in which the screw groove depth is large from upstream to midstream, through which resin supplied from a hopper is melted and sent forward. Downstream of this supply section 8, there is a first compression section 9, in which the screw groove depth gradually decreases. In the first compression section 9, the molten resin is compressed and sent forward. Finally, at the most downstream portion of the first stage 5, there is a first metering section 10, in which the screw groove depth is small and constant, through which the compressed resin is sent forward. In this embodiment, the first compression section 9 and the first metering section 10 have so-called barrier flights. That is, the section is composed of a main flight with a predetermined pitch and lead angle and a sub-flight with a larger pitch and lead angle than the main flight. Therefore, backflow of resin is unlikely to occur in this section. Note that these sections 9 and 10 may be composed of other types of flights, such as single flights. The second stage 6 has a starvation section 13 formed upstream. The screw groove depth in the starvation section 13 is large and constant, reducing the pressure of the molten resin. As explained below, the inert gas is supplied to the starvation section 13, melted into the resin, and penetrates and dissolves the molten resin. The injection molding machine 1 according to this embodiment is characterized by the length H of the starvation section 13, which will be explained later. A second compression section 14, in which the screw groove depth gradually decreases, is formed downstream of the starvation section 13, through which the molten resin containing the inert gas is compressed and kneaded while being fed. A second metering section 15, in which the screw groove depth is small and constant, is formed at the most downstream portion of the second stage 6, through which the molten resin permeated with the inert gas is fed 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] Injection sections 17 and 18 for injecting inert gas are provided on the heating cylinder 2. This embodiment is characterized by the fact that the first and second injection sections 17 and 18, whose number N is two, are provided at positions spaced apart in the axial direction, and by the distance P between them. 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 fully advanced. In contrast, the second injection section 18 is provided upstream of the first injection section 17, and in this embodiment, it is provided at a position corresponding to the first stage 5. More preferably, the second injection section 18 is provided downstream of the first metering section 10. Because the first and second injection sections 17 and 18 are provided in this manner, even if the screw 3 retreats during metering and the first injection section 17 moves out of the starvation section 13, as shown in FIG. 1B, the second injection section 18 enters the starvation section 13, allowing the injection of inert gas to continue. In the present invention, in order to stably inject the inert gas, the number N of the first and second injection ports 17, 18 and the spacing P between them are related to the length H of the starvation section 13, as will be explained later. The first and second injection ports 17, 18 are equipped with check valves 20, 21, respectively, and are connected to a cylinder 22 that supplies the inert gas. Note that, although the check valves 20, 21 are shown in the figure as being located at a distance from the injection ports 17, 18, in practice they may be incorporated into the injection ports 17, 18.

[0014] In the injection molding machine 1 according to this embodiment, the number N of the first and second injection sections 17, 18 and the spacing P between them 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, we will explain the design considerations in conventional injection molding machines for stably injecting inert gas into molten resin, as well as the problems with the conventional machines.

[0015] In conventional injection molding machines for foam molding, there is a transition zone between a resin-filled area and a resin-starved area at the beginning and end of the starvation zone, where the resin pressure changes from high to low or from low to high. Because gas cannot penetrate sufficiently in this transition zone, if the gas supply port does not have an opening / closing mechanism, molten resin will enter the gas supply port when the screw advances or retreats and the gas supply port approaches the transition zone, preventing gas supply. If this condition persists, foam molding cannot continue, requiring maintenance such as cleaning the gas supply port. In the present disclosure, by providing an opening / closing mechanism in the gas flow path, such as check valves 20 and 21 in the first and second injection ports 17 and 18, the intrusion of molten resin can be prevented even when the gas supply port approaches the transition zone. However, the inability to supply gas during this period can result in problems such as a decrease in gas dissolution rate. The length of this transition zone varies depending on the screw shape, the viscosity of the resin during metering, the screw rotation speed, and other metering conditions. Due to the existence of this transition region, simply ensuring that at least one gas inlet is always present within the starvation zone during metering does not guarantee high-quality foam-molded products. During metering, at least one gas inlet must be present within a specific length from both ends of the starvation zone. Specifically, the specific length is the distance the resin advances in 0.5 revolutions of the screw, or more preferably, the distance the resin advances in one revolution. Since the screw flight pitch is typically approximately 1D, where D is the screw diameter, this specific length is 0.5D, or more preferably, 1D. In other words, during metering, at least one gas inlet must be present within a range of 0.5D or more than 1D from both ends of the starvation zone. This prevents the gas inlet from entering the transition zone. This allows for efficient and stable supply of inert gas to the molten resin in conventional injection molding machines. 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] The injection molding machine 1 according to this embodiment solves these conventional problems by establishing several regulations. First, the product P × N of the distance P between the first and second injection sections 17, 18 and the number N of the injection sections is set to 0.5H or more and 1.71H or less relative to the length H of the starvation section 13. The distance P between the first and second injection sections 17, 18 is set to 0.25S or more and 0.8S or less relative to the metering stroke S. In this embodiment, the number N of the first and second injection sections 17, 18 is two. However, by increasing the number of injection sections and narrowing the distance P between them, the inert gas can be reliably injected. This allows the length H of the starvation section 13 to be shortened. As a result, the length H of the starvation section 13 is set to 0.7S or more and 1.2S or less relative to the metering stroke S, which is the distance the screw 3 retracts during metering. This length was not possible 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 0.5 S or more and 1.6 S or less with respect to the metering stroke S. This allows the inert gas to be supplied to the molten resin efficiently and stably, and also reduces the machine length.

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

[0018] An experiment was conducted to confirm that foam molding can be stably performed in the injection molding machine 1 according to this 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: 420 mm (injection volume approximately 2,327 cm3) 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 points P: 252 mm (0.6 S) Resin used: PP resin A purge test was then conducted under the same conditions as the reference conditions described above, or with some conditions changed to confirm whether a good foaming state could be obtained. In this test, the N injection sections 17, 18, ... were arranged at equal intervals on the heating cylinder 2. The injection sections 17, 18, ... were also arranged so that when the screw 3 moved halfway through the metering stroke S, i.e., retreated 0.5 S, the center position of the starvation section 13 of the screw 3 on the heating cylinder 3 coincided with the center position of the N injection sections 17, 18, .... Examples 1 to 10, which satisfied the conditions specified in the present invention, and comparative examples 1 to 8, which did not, 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 conducted under the standard conditions, but with the spacing between the injection sections 17 and 18 changed to 0.8 s. The two injection sections 17 and 18 were positioned so that at least one was always within the starvation section 13 during metering and was located at least 0.1 s inside both ends of the starvation section 13. Specifically, one injection section 17 was positioned on the heating cylinder 2 so that it was located 0.1 s downstream of the upstream end of the starvation section 13 at the start of metering, and the other injection section 18 was positioned 0.1 s upstream of 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 sections were always positioned inside these positions. The obtained sample had uniform foam formation and was good. Example 3: Under the standard conditions, the length H of the starvation section 13 was set to 0.8 s, the number N of injection sections 17, 18, ... was set to three, spaced apart in the axial direction, and the spacing P between adjacent injection sections 17, 18, ... was set to 0.4 s, and a purge test was conducted. The length H of the starvation section 13 was shortened by 0.2 s from the standard conditions, and the machine length was shortened by 0.2 s. 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 ports 17 and 18 with check valves were simply injection holes, and a needle-type injection valve that was air-driven and opened and closed was installed in the pipe that supplied inert gas to the injection holes. A purge test was conducted under the same conditions as the standard conditions in all other respects. The injection valve was opened and closed based on the screw position. That is, the injection valve opened when the injection hole was 0.1 S or more inside both ends of the starvation section 13, and closed when it moved 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 in Example 4. However, an additional condition was added regarding the opening and closing of the injection valves, so that once metering was completed, both valves were closed until the next metering 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 was installed in the pipeline supplying inert gas to the injection ports 17 and 18, which are equipped with check valves 20 and 21, to open and close the flow path in response to an electrical signal. 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. The automatic valve opened when the injection port was more than 0.1 S inside both ends of the starvation section 13, and closed when it moved out of the starvation section 13 to prevent backflow of resin. When a purge test was conducted using this, uniform foaming was formed and a good sample was obtained. Example 7: Under the standard conditions, the check valves 20 and 21 were removed. In other words, the injection ports 17 and 18 were simply injection ports, and an automatic valve that received an electrical signal to open and close the flow path was installed in the pipe that supplied inert gas to the injection ports. 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 port was more than 0.1 S inside both ends of the starvation section 13, and closed when it moved out of the starvation section 13 to prevent backflow of resin. The obtained sample had uniform foaming and was good. Example 8: Under the standard conditions, the metering stroke S was set to 350 mm. The length H of the starvation section 13 and the distance P between the injection sections 17 and 18 were 420 mm and 252 mm, respectively, both of which were standard conditions. However, by shortening the metering stroke S, the length H of the starvation section 13 and the distance P between the injection sections 17 and 18 became 1.2 S and 0.72 S, respectively. The obtained sample had uniform foam formation and was good. Example 9: Under the standard conditions, the distance P between the injection parts 17 and 18 was set to 105 mm, or 0.25 S. The product of the distance P between the injection parts 17 and 18 and the number N was 0.5 S for the metering stroke S and 0.5 H 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: Under the standard conditions, the length H of the starvation section 13 was set to 373 mm (0.9 S), and the spacing P between the injection sections 17 and 18 was set to 189 mm (0.45 S). When a sample was obtained under these conditions, the obtained sample had good uniform foaming. 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 distance P between the injection sections 17 and 18 was set to 126 mm, i.e., 0.3 S. The obtained sample had good uniform foam formation. 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 spacing 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 1.0 S, and the spacing P between the injection sections 17, 18 was 0.3 S, which meets the conditions stipulated in the present invention. The samples obtained in the experiment showed uniform foaming and were good.

[0020] Comparative Example 1: Under the standard conditions, the number of inert gas injection ports was set to one, and the metering stroke S was set to 252 mm. However, the injection port was located 42 mm (0.5D) from the start of the starvation section at the start of the metering stroke (except for Comparative Example 8, the same applies to Comparative Examples 2 and onward when there is only one injection port). The length H of the starvation section 13 was 1.7S, which does not meet the requirements of the present invention. A purge test was conducted on the obtained sample, which showed good, uniform foam formation, but the injection volume was only approximately 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 ports was reduced to one, replaced with a simple injection hole. In other words, the check valve was removed. A purge test was conducted using this. However, the sample did not foam. When the injection port was inspected, it was found to be clogged with resin and inert gas was not being supplied. This is presumably because the injection port moved outside the starvation section 13 during the metering stroke. Comparative Example 3: Under the standard conditions, the length H of the starvation section 13 was set to 672 mm, or 1.6 S. The number of inert gas injection ports N was set to one, replaced with a simple injection hole. In other words, the check valve was removed. A purge test was then carried out using this. The obtained sample showed uniform foaming and was good. Because 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 increased by 0.6 S. Comparative Example 4: Under the standard conditions, the inert gas injection ports were changed to simple injection holes without check valves. A purge test was then carried out using these ports. However, the sample did not foam. Upon inspection, it was found that two injection ports were clogged with resin. Comparative Example 5: Under the standard conditions, the distance P between the two injection ports was increased to 420 mm, or 1.0 S. A purge test was conducted, and the bubbles in the sample were found to be coarse and large. It is believed that there was a time during the metering process when both injection ports were outside the starvation zone 13, which prevented the inert gas from sufficiently penetrating the molten resin. Comparative Example 6: Under 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 under this condition, but only a few bubbles were formed in the sample. Comparative Example 7: Under 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 1.2 S. The number of injection parts N was set to 1. A purge test was carried out, but the diameter of the bubbles formed in the sample became larger. 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 located 20 mm (0.5D) from the start of the starvation section at the start 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 conducted, but the diameter of the bubbles formed in the sample increased. The experimental results are summarized in the table below.

[0021] [Table 1]

[0022] (*1) in Table 1 indicates the maximum distance that the injection ports 17, 18 were located from both ends of the starvation interval 13 during measurement. The relative position of one or more injection ports 17, 18, ... changes relative to the starvation interval 13 during measurement. Therefore, at each timing during measurement, we focus on the injection port 17, 18, ... that is located closest to the center of the starvation interval 13. At one timing during measurement, for example, one injection port 17 will be closest to the center of the starvation interval 13, and at another timing, the other injection port 18 will be closest. Thus, *1 summarizes the shortest distance from both ends of the starvation interval 13 for the injection port 17, 18, ... that is located closest to the center of the starvation interval 13 at each timing during measurement. When this number is a positive value, it means that at least one inlet 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 means that there are times during measurement when all inlets 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 measurement stroke S, the item "P / S" indicates the ratio of the injection section spacing P to the measurement stroke S, the item "P×N / S" indicates the ratio of the product P×N of the injection section spacing P and the number N to the measurement stroke S, the item "P×N / H" indicates the ratio of the product P×N of the injection section spacing P and the number 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 measurement stroke S.

[0024] As an additional test, foam molding tests were conducted using a mold as Examples 1' to 10' under conditions equivalent to those of Examples 1 to 10. Specifically, a spigot mold, which allows the cavity volume to be freely set by changing the mold opening amount, was used to mold foam-molded articles. The mold opening amount was set appropriately to match the set injection stroke, and all other conditions were the same as those of Examples 1 to 10. Under all conditions, one or more injection ports 17, 18, ... were positioned so that they were within 0.1 S or more of both ends of the starvation zone 13 during the metering stroke, allowing stable gas supply while avoiding the transition zone. As a result, the foam-molded articles obtained as Examples 1' to 10' all exhibited uniform foam formation and were of good quality.

[0025] As can be seen from Table 1, the screw 3 of the injection molding machine 1 used in the test had only two screw diameters D: 84 mm and 40 mm. In other words, tests were not conducted for other screw diameters D. However, even if the screw diameter D is different, the effects of the present disclosure can be achieved as long as two or more gas injection ports are provided, each of these multiple gas injection ports is equipped with a check valve or an opening / closing mechanism, and the product P × N of the injection port spacing P and the number N of injection ports is 0.5H or more and 1.71H or less relative to the length H of the starvation section 13. Generally, when designing a screw with a specific shape to achieve a specific effect in an injection molding machine, the lengths of each portion (e.g., screw length L, length H of the starvation section 13, lengths L1, L2, etc.) are determined based on the ratio to 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 the respective ratio by the screw diameter D. This is because doing so produces the same effect. In other words, it is common technical knowledge that the same effect will be produced if the ratio of the length of each part to the screw diameter D is the same. 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. Therefore, the ratios of these items in Table 1 can essentially be shown based on the screw diameter D. From the above, it can be seen that even if a screw does not have a screw diameter D listed in Table 1, determining the length of each part based on the ratio of these items can produce the same effect as the screw 3 according to the present disclosure.

[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, when 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 will approach the transition region at the start and end of the starvation section 13, which will impede the supply of inert gas and adversely affect the quality of the molded product.

[0027] The injection molding machine 1 according to this embodiment can be modified in various ways. For example, as implemented in the above experiments, the injection sections 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 pipeline supplying inert gas to the injection sections 17 and 18. The opening / closing mechanism can be controlled by a controller and opens and closes depending on the position of the screw 3. In this way, the injection sections 17 and 18 can be opened when they are in the starvation section 13 and closed when they leave the starvation section 13, reliably preventing backflow of resin from the injection sections 17 and 18. Other modifications are possible, such as providing three or more injection sections 17 and 18, as described above. Alternatively, check valves and opening / closing mechanisms may be provided simultaneously. Furthermore, three or more injection sections may be arranged at different pitches, or the N injection sections may be arranged off-center upstream or downstream of the starvation section 13. Furthermore, the flight of the screw 3 can be modified and can be selected from a variety of flights such as single flight and double flight.

[0028] The invention made by the inventor has been specifically described above based on the embodiments, but it goes without saying that the present invention is not limited to the above-described embodiments and various modifications are possible without departing from the spirit of the invention. The multiple examples described above can also be implemented in appropriate combinations. [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 portions provided at a plurality of axially spaced locations on 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 being 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 being composed of an upstream starvation section where the pressure of the resin is reduced, a downstream second compression section where the molten resin is compressed, and a downstream second metering section, the inert gas from the plurality of injection ports being supplied 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 the interval P between the plurality of injection parts and the number N of the injection parts is 0.5H or more and 1.71H or less, where H is the length of the starvation section; the plurality of injection parts are arranged so that, when a length by which the screw retreats during metering is defined as a metering stroke S, at least one injection part is present 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; an injection molding machine for foam molding, wherein the interval P between the plurality of injection parts is 0.25 S or more and 0.8 S or less with respect to the metering stroke S;

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

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

4. A heating cylinder; a screw contained in the heating cylinder; a plurality of injection portions provided at a plurality of axially spaced locations on 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 being 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 being composed of an upstream starvation section where the pressure of the resin is reduced, a downstream second compression section where the molten resin is compressed, and a downstream second metering section, the inert gas from the plurality of injection ports being supplied 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 the interval P between the plurality of injection parts and the number N of the injection parts is 0.5H or more and 1.71H or less, where H is the length of the starvation section; the plurality of injection parts are arranged so that, when a length by which the screw retreats during metering is defined as a metering stroke S, at least one injection part is present 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; an injection molding machine for foam molding, wherein the interval P between the plurality of injection parts is 0.25 S or more and 0.8 S or less with respect to the metering stroke S;

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