Method of manufacturing resin molding, plastic model, resin molding, and filter

Abstract

To provide a method of manufacturing a resin molding capable of facilitating forming a skin layer all over the resin molding by adjusting a resin amount that discharges a foaming agent, and improving appearance of the resin molding.SOLUTION: A method of manufacturing a resin molding includes the steps of: generating a first mixed molten resin R2 (S3); allowing a stagnant molten resin R3 made of the first mixed molten resin R2 to stay inside a hot runner 20 (S4); discharging a foaming agent from the stagnant molten resin R3 to generate a degassed molten resin R42 (S5); generating a second mixed molten resin R2 (S6); injection-filling the degassed molten resin R42 inside a mold 40 to form a skin layer 2 (S7); and injection-filling the second mixed molten resin R2 while foaming the same inside the mold 40 to form a core layer 3 inside the skin layer 2 (S8). A resin amount of the stagnant molten resin R3 being stayed inside the hot runner 20 is adjusted, which facilitates forming the skin layer 2 all over the resin molding 1.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a method for manufacturing a resin molded body manufactured by foam injection molding, a plastic model, a resin molded body, and a filter included in an apparatus for manufacturing a resin molded body. [Background technology]

[0002] Traditionally, in the field of resin molding, foam injection molding has been widely used to reduce the amount of resin used or to improve dimensional accuracy by preventing warpage or sink marks. Resin foam molding can be broadly divided into chemical foaming and physical foaming. In these foaming methods, the blowing agents are typically nitrogen gas, carbon dioxide gas, carbon monoxide, ammonia gas, or water vapor. However, in recent years, with a view to reducing environmental impact, there has been an increase in examples of resin foaming using physical foaming, which uses inert gases such as carbon dioxide or nitrogen as the blowing agent. For example, a molding method using high-pressure supercritical fluids, as disclosed in Japanese Patent No. 2625576 (Patent Document 1), and a molding method using low-pressure gases, as disclosed in Japanese Patent No. 6139038 (Patent Document 2), have been put to practical use.

[0003] The main problems with this type of foam molding are a decrease in strength due to a decrease in the amount of resin, and a deterioration in the appearance of the resin foam molded product. The deterioration in appearance is caused by the fact that during injection molding, the gas used as the blowing agent is released from the tip of the flowing resin and becomes trapped between the mold and the flowing resin, which deteriorates the mold transferability of the surface of the resin foam molded product, resulting in the formation of irregularities on the surface of the resin foam molded product. These irregularities are generally a defect phenomenon known as swirl marks. Therefore, this type of foam molding method has traditionally been used to manufacture mechanical parts and other components that do not require good appearance.

[0004] On the other hand, methods for improving appearance are generally known, including the counterpressure method, the heat-and-cool method, and foam molding using an insulated mold. The counterpressure method involves introducing pressurized gas such as air into a mold before injecting and filling the mold with a fluid resin to prevent the gas from escaping from the tip of the fluid resin. However, the counterpressure method requires sealing gaps within the mold with a seal or other device. The heat-and-cool method involves increasing the mold temperature when injecting and filling the mold with a fluid resin to prevent the fluid resin from solidifying before mold transfer, and then cooling the mold after the fluid resin is completely filled to improve mold transferability. However, the molds used in the heat-and-cool method have complex structures, which increase costs. The foam molding method using an insulated mold involves coating the mold surface with a material with low thermal conductivity to delay the solidification of the fluid resin. However, the foam molding method using an insulated mold has problems such as a short lifespan of the insulated mold and difficulty in producing resin foam molded products with complex shapes.

[0005] International Publication No. 2013 / 129659 (Patent Document 3) discloses a foam injection molding method (a method for manufacturing foam molded articles) that can suppress the release of gasified physical blowing agent into the mold, which causes swirl marks on the surface of the molded article, and prevent the molded article from having poor appearance. In the foam injection molding method, a nozzle unit located between the plasticizing cylinder and the mold cools and solidifies the flow front of the molten resin, evacuates the physical blowing agent from the solidified flow front to reduce the physical blowing agent concentration, and then heats and melts the solidified flow front again and injects it into the mold. This suppresses the release of gasified physical blowing agent from the flow front into the mold (see paragraphs "0138," "0139," and "0141" to "0148" of the specification of Patent Document 3, Figure 10, etc.).

[0006] Japanese Patent No. 6422548 (Patent Document 4) discloses a hot runner unit capable of highly efficient discharge of gas generated from molten resin. The hot runner unit is disposed between the gate of an injection mold and an open nozzle that discharges molten resin. The hot runner unit has a nozzle connection portion to which the nozzle is connected, and a sprue portion having an internal molten resin flow path for guiding the molten resin discharged from the nozzle to the gate. A section of the molten resin flow path in the sprue portion is a vent section whose tubular wall is made of a vent member having multiple micropores sized to allow only gas to pass through but not molten resin. In addition, a small-diameter section with an inner diameter smaller than that of the vent section is provided on the gate side of the vent section of the molten resin flow path. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 2625576 [Patent Document 2] Patent No. 6139038 [Patent Document 3] International Publication No. 2013 / 129659 [Patent Document 4] Patent No. 6422548 Summary of the Invention [Problem to be solved by the invention]

[0008] However, in the foam injection molding method of Patent Document 3, the surface area of ​​the solidified resin is small, it takes time to expel the physical foaming agent from the solidified resin, and it is not easy to adjust the amount of resin at the flow front where the concentration of the physical foaming agent needs to be reduced. Therefore, there is still room for further research into more efficient expulsion of the physical foaming agent.

[0009] Furthermore, in the hot runner unit of Patent Document 4, a vent section is provided in a continuous space extending from the nozzle to the gate, making it difficult to control the amount of resin that releases gas. Furthermore, the small diameter section increases the pressure in the molten resin flow path upstream of the small diameter section, making it easier to discharge gas in the molten resin flow path to the outside through the fine holes in the vent section, but conversely, there is a risk that gas may be unnecessarily discharged from the foaming molten resin that is subsequently injected and filled into the injection mold, resulting in a problem of reduced foamability.

[0010] Therefore, an object of the present disclosure is to provide a method for manufacturing a resin molded body, a plastic model, a resin molded body, and a filter that can make it easier to form a skin layer over the entire resin molded body by adjusting the amount of resin from which the foaming agent is discharged, thereby improving the appearance of the resin molded body. [Means for solving the problem]

[0011] In order to solve the above problems, the present disclosure is configured as follows: That is, a method for manufacturing a resin molded body according to the present disclosure is a method for manufacturing a resin molded body by injection molding a resin molded body having a core layer made of a foamed resin and a skin layer covering the core layer, and includes the steps of preparing a molding machine having a nozzle, a mold, and a hot runner disposed between the molding machine and the mold, mixing a first molten resin and a first foaming agent containing a gas inside the molding machine to generate a first mixed molten resin, and flowing the first mixed molten resin from inside the molding machine into the hot runner through the nozzle, and then closing the nozzle of the molding machine. The method includes the steps of retaining a retained molten resin made of a first mixed molten resin in a retention space inside the hot runner, discharging the first foaming agent from the retained molten resin retained inside the hot runner to produce a degassed molten resin with a reduced foaming agent concentration, mixing the second molten resin and the second foaming agent containing gas inside a molding machine to produce a second mixed molten resin, injecting the degassed molten resin into a mold to form a skin layer, and injecting the second mixed molten resin into the skin layer while foaming it to form a core layer. In the retaining molten resin step, the amount of resin in the retained molten resin retained in the retention space is adjusted.

[0012] The plastic model according to the present disclosure is a plastic model made by foam injection molding, and includes a plurality of parts, a framework connecting the plurality of parts, and a gate portion through which a resin material is injected during foam injection molding. At least one of the plurality of parts located near the gate portion has a core layer and a skin layer covering the core layer.

[0013] The resin molded article according to the present disclosure is a resin molded article obtained by foam injection molding, and includes a core layer made of foamed resin, a skin layer covering the core layer, and a gate portion through which the resin material is injected during foam injection molding. The core layer is formed in the vicinity of the gate portion.

[0014] The resin molded article according to the present disclosure is a resin molded article obtained by foam injection molding, and has a gate portion through which a resin material is injected during foam injection molding, and the specific gravity of a portion near the gate portion is smaller than the specific gravity of a portion located farther from the gate portion than the portion near the gate portion.

[0015] The filter according to the present disclosure is used in a manufacturing apparatus for a resin molded product, the apparatus including a molding machine that mixes a molten resin with a blowing agent containing a gas to produce a mixed molten resin, and a hot runner that retains the retained molten resin made of the mixed molten resin and discharges the blowing agent from the retained molten resin to produce a degassed molten resin. The filter is disposed inside the hot runner and has a plurality of through holes for discharging the blowing agent from the retained molten resin retained inside the filter. [Effects of the Invention]

[0016] According to the method for manufacturing a resin molded body disclosed herein, by adjusting the amount of resin from which the foaming agent is discharged, it is possible to easily form a skin layer over the entire resin molded body, thereby improving the appearance of the resin molded body.

[0017] According to the plastic model and resin molded article of the present disclosure, the occurrence of sink marks near the gate portion can be suppressed, improving the appearance of the resin molded article.

[0018] According to the filter of the present disclosure, it is possible to discharge the foaming agent from the molten resin retained inside the filter. [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 1 is a flowchart of the method for producing a resin molded article according to the first embodiment. [Figure 2] FIG. 2 is a schematic diagram of a manufacturing apparatus used in the manufacturing method shown in FIG. [Figure 3] FIG. 3 is a vertical cross-sectional view of the hot runner and filter shown in FIG. [Figure 4] FIG. 4 is a cross-sectional view of the hot runner and filter shown in FIG. [Figure 5] FIG. 5 is a vertical cross-sectional view showing the filter of the first modification. [Figure 6] FIG. 6 is a vertical cross-sectional view showing a filter according to the second modification. [Figure 7] FIG. 7 is a cross-sectional view of the cold runner shown in FIG. [Figure 8] FIG. 8 is a schematic diagram showing the state of the retention step in FIG. [Figure 9] FIG. 9 is a schematic diagram showing the degassed molten resin producing step of FIG. [Figure 10] FIG. 10 is a schematic diagram showing the steps of forming the skin layer and the core layer of FIG. [Figure 11] FIG. 11 is a schematic diagram showing the steps of forming the skin layer and the core layer of FIG. [Figure 12] FIG. 12 is a schematic diagram showing the steps of forming the skin layer and the core layer of FIG. [Figure 13] FIG. 13 is a schematic diagram of a manufacturing apparatus used in the manufacturing method of the second embodiment. [Figure 14] FIG. 14 is an enlarged view of the manufacturing apparatus shown in FIG. [Figure 15] FIG. 15 is a schematic diagram of a plastic model according to this embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0020] (Configuration 1) A method for manufacturing a resin molded body according to an embodiment of the present disclosure is a method for manufacturing a resin molded body by injection molding a resin molded body having a core layer made of a foamed resin and a skin layer covering the core layer, and includes the steps of: preparing a molding machine having a nozzle, a mold, and a hot runner disposed between the molding machine and the mold; mixing a first molten resin and a first foaming agent containing a gas inside the molding machine to generate a first mixed molten resin; and flowing the first mixed molten resin from inside the molding machine into the hot runner through the nozzle, and then closing the nozzle of the molding machine. The method includes the steps of retaining a retained molten resin made of a first mixed molten resin in a retention space inside the hot runner, discharging the first foaming agent from the retained molten resin retained inside the hot runner to produce a degassed molten resin with a reduced foaming agent concentration, mixing the second molten resin and the second foaming agent containing gas inside a molding machine to produce a second mixed molten resin, injecting the degassed molten resin into a mold to form a skin layer, and injecting the second mixed molten resin into the skin layer while foaming it to form a core layer. In the retaining molten resin step, the amount of resin in the retained molten resin retained in the retention space is adjusted.

[0021] By adjusting the amount of resin in the retained molten resin in this manner and discharging the foaming agent from the retained molten resin retained in the retention space, it becomes easier to form a skin layer over the entire resin molded body depending on the size or shape of the resin molded body, thereby improving the appearance of the resin molded body.

[0022] (Configuration 2) In the method for producing a resin molded product according to Configuration 1, the second mixed molten resin may be produced at the same time as the degassed molten resin is produced. The degassed molten resin may be injected into a mold as if being extruded into the second mixed molten resin discharged from a nozzle of a molding machine, thereby forming a skin layer. The second mixed molten resin may be injected into the skin layer while foaming, following the degassed molten resin, thereby forming a core layer. This facilitates continuous and efficient production of resin molded products.

[0023] (Configuration 3) In the method for producing a resin molded product according to the first or second aspect, the hot runner may include a filter housed therein and having a plurality of through holes. In the step of retaining the retained molten resin, the retained molten resin may be retained inside the filter. In the step of producing the degassed molten resin, the first foaming agent may be discharged through the plurality of through holes. This allows the foaming agent to be discharged from the retained molten resin retained inside the filter.

[0024] (Configuration 4) In the method for producing a resin molded article according to Configuration 3, each of the plurality of through holes of the filter may have an inner diameter of 1 μm to 50 μm, thereby making it possible to suppress the inflow of the stagnant molten resin into the through holes and to discharge the foaming agent through the through holes.

[0025] (Configuration 5) In the method for producing a resin molded product according to any one of the first to fourth aspects, in the step of retaining the retained molten resin, the amount of the retained molten resin may be adjusted by changing the internal volume of the retaining space inside the hot runner or the filter, thereby adjusting the amount of the retained molten resin.

[0026] (Configuration 6) The method for producing a resin molded product according to any one of the first to fifth aspects may include the step of preparing a cold runner to be placed between the hot runner and the mold. The cold runner may have an injection hole communicating with the interior of the mold. In the step of retaining the retained molten resin, the retained molten resin may be retained in retention spaces inside the hot runner and the cold runner by solidifying a portion of the retained molten resin inside the cold runner to close the injection hole. This method of using a cold runner to close the injection hole and retaining the retained molten resin in the retention space can be suitably used in a foam injection molding method in which foaming is performed at a relatively low pressure.

[0027] (Configuration 7) In the method for producing a resin molded article according to any one of the first to fifth aspects, the hot runner may have a discharge hole communicating with the interior of the mold and a valve for opening and closing the discharge hole. In the step of retaining the retained molten resin, the retained molten resin may be retained in a retention space inside the hot runner by closing the discharge hole of the hot runner with the valve. This method of closing the injection hole with a valve and retaining the retained molten resin in the retention space is suitable for use in a foam injection molding method in which foaming is performed at a relatively high pressure.

[0028] (Configuration 8) In the method for producing a resin molded product according to any one of aspects 1 to 7, the first or second foaming agent may be either carbon dioxide or nitrogen. Carbon dioxide and nitrogen are inert gases that have little impact on the environment, which can contribute to reducing the environmental load.

[0029] (Configuration 9) In the method for producing a resin molded article according to any one of Configurations 1 to 8, the degassed molten resin and the second mixed molten resin may be injected and filled into the interior of the mold from a plurality of locations. This method of injecting and filling the degassed molten resin and the second mixed molten resin from a plurality of locations is suitable for producing a relatively large resin molded article.

[0030] (Configuration 10) The plastic model according to this embodiment is a plastic model formed by foam injection molding, and includes a plurality of parts, a framework connecting the plurality of parts, and a gate portion through which resin material is injected during foam injection molding. At least one of the plurality of parts located near the gate portion has a core layer and a skin layer covering the core layer. A non-easy-to-form part can be located near the gate portion. This allows the core layer to be sufficiently formed inside the non-easy-to-form part, suppressing the occurrence of sink marks in the non-easy-to-form part and improving the appearance of the resin molded product. The non-easy-to-form part may be a relatively thick-walled part. The easy-to-form part may be a relatively thin-walled part.

[0031] (Configuration 11) The resin molded article according to this embodiment is a resin molded article obtained by foam injection molding, and includes a core layer made of foamed resin, a skin layer covering the core layer, and a gate portion through which the resin material is injected during foam injection molding. The core layer is formed near the gate portion. By forming the core layer near the gate portion in this manner, it is possible to suppress the occurrence of sink marks near the gate portion and improve the appearance of the resin molded article.

[0032] (Configuration 12) The resin molded article according to this embodiment has a gate portion, which is a portion into which a resin material is injected during foam injection molding. The specific gravity of the portion near the gate portion is smaller than the specific gravity of the portion located farther from the gate portion than the portion near the gate portion. By making the specific gravity of the portion near the gate portion relatively small, a core layer can be formed near the gate portion, which suppresses the occurrence of sink marks near the gate portion and improves the appearance of the resin molded article.

[0033] (Configuration 13) The filter according to the present embodiment is used in a manufacturing apparatus for resin molded articles, which includes a molding machine that mixes a molten resin with a blowing agent containing a gas to produce a mixed molten resin, and a hot runner that retains the retained molten resin made of the mixed molten resin and discharges the blowing agent from the retained molten resin to produce a degassed molten resin. The filter is disposed inside the hot runner and has a plurality of through holes for discharging the blowing agent from the retained molten resin retained inside the filter. This allows the blowing agent to be discharged from the retained molten resin retained inside the filter.

[0034] (Configuration 14) In the filter of Configuration 12, each of the plurality of through holes may have an inner diameter of 1 μm to 50 μm, which makes it possible to suppress the inflow of the mixed molten resin into the through holes and to discharge the foaming agent from the through holes.

[0035] (Configuration 15) In the filter of Configuration 13 or 14, the filter may have a cylindrical shape. The amount of resin in the mixed molten resin from which the foaming agent is discharged may be adjusted by changing the internal volume of the filter. This allows the amount of resin in the retained molten resin from which the foaming agent is discharged to be adjusted according to resin molded products of various sizes or shapes.

[0036] Hereinafter, an embodiment of the method for manufacturing a resin molded body according to the present disclosure will be specifically described with reference to Figures 1 to 14. Note that the same or corresponding components in the figures are denoted by the same reference numerals, and the same description will not be repeated. To facilitate understanding of the description, the drawings referred to below show simplified or schematic configurations, and some components are omitted.

[0037] [First embodiment] First, a method for producing a resin molded body according to a first embodiment of the present disclosure will be specifically described with reference to Figures 1 to 12. As shown in Figure 1, the method for producing a resin molded body includes a preparation step S1, a first molten resin producing step S2, a first mixed molten resin producing step S3, a retention step S4, a degassed molten resin producing step S5, a second mixed molten resin producing step S6, a skin layer forming step S7, a core layer forming step S8, and a demolding step S9.

[0038] [Preparation process S1] First, a manufacturing apparatus 100 used in the method for manufacturing a resin molded body is prepared, as shown in Fig. 2. The manufacturing apparatus 100 for a resin molded body includes a molding machine 10, a hot runner 20, a cold runner 30, and a mold 40. In Fig. 2, arrow F indicates the flow direction of molten resin, etc.

[0039] [Forming machine] The molding machine 10 has a cylindrical plasticizing cylinder 11 including a starvation zone 111, a screw 12 disposed inside the plasticizing cylinder 11, a hopper 13 for feeding resin material into the plasticizing cylinder 11, a cylinder 14 and a pressure reducing valve 15 for feeding a blowing agent into the plasticizing cylinder 11, a pressure vessel 16, a nozzle 17 located at the tip of the molding machine 10, and a valve 18 for opening and closing the nozzle 17. The valve 18 is, for example, a needle valve as shown in the figure.

[0040] [Hot runner] The hot runner 20 is disposed between the molding machine 10 and the cold runner 30. The hot runner 20 has a cylindrical shape. The hot runner 20 has a filter 21. Note that the shapes of the hot runner 20 and the filter 21 are not limited to a cylindrical shape, and may be polygonal tubular shapes, and various other shapes can be used.

[0041] 3, hot runner 20 has inlet holes 201 that communicate with the interior of nozzle 17, outlet holes 202 that communicate with the interior of cold runner 30, openings 203 formed on the outer periphery of hot runner 20 that communicate between the interior and exterior of hot runner 20, a temperature sensor 204, and a heater (not shown). The heater heats the interior of hot runner 20. Temperature sensor 204 controls the temperature inside hot runner 20.

[0042] The filter 21 has a cylindrical shape and is housed inside the hot runner 20 near its inner circumferential surface. As shown in FIGS. 3 and 4 , the filter 21 has a plurality of through holes 211. The through holes 211 connect the interior of the hot runner 20 to the opening 203. The ends of the filter 21 on the molding machine 10 side and the mold 40 side are supported by protrusions 205, 205 provided on the inner circumferential surface of the hot runner 20 at the ends of the filter 21 on the molding machine 10 side and the mold 40 side. This forms a gap between the outer circumferential surface of the filter 21, including the through holes 211, and the inner circumferential surface of the hot runner 20, including the opening 203. The gap allows communication between the through holes 211 and the opening 203.

[0043] The through hole 211 is a two-stage through hole having a large diameter portion 211a and a small diameter portion 211b. The large diameter portion 211a is provided to open to the outer peripheral surface of the filter 21. The small diameter portion 211b is provided to open to the inner peripheral surface of the filter 21 and is provided between the inner peripheral surface of the filter 21 and the large diameter portion 211a. As described below, the mixed molten resin R2 is retained in retention spaces inside the filter 21, i.e., inside the hot runner 20 and inside the cold runner 30 described below, and the foaming agent containing gas from the mixed molten resin R2 is discharged through the through hole 211 and the opening 203. The provision of the small diameter portion 211b prevents the resin components of the mixed molten resin R2 from flowing into the through hole 211, allowing the foaming agent to be appropriately discharged. The provision of the large diameter portion 211a allows a larger amount of gas to be discharged. When the inner diameter of the small diameter portion 211b becomes smaller, it becomes more difficult for the resin component to flow in but also more difficult for degassing. Conversely, when the inner diameter of the small diameter portion 211b becomes larger, it becomes easier for the resin component to flow in but also more easily for the degassing. From this viewpoint, the inner diameter of the small diameter portion 211b is, for example, 1 μm or more, preferably 5 μm or more, and 50 μm or less, preferably 30 μm or less, and more preferably 20 μm or less. In other words, the inner diameter of the small diameter portion 211b is 1 to 50 μm, preferably 5 to 30 μm, and more preferably 5 to 20 μm. The multiple through holes 211 can be provided, for example, at intervals of 200 μm. The radial length of the small diameter portion 211b, i.e., the length in the direction perpendicular to the flow direction F, can be, for example, 1 mm. The inner diameter of the large diameter portion 211a can be, for example, 100 μm. The inner peripheral shape of each of the through-holes 211, i.e., the large-diameter portion 211a and the small-diameter portion 211b, is not particularly limited and may be, for example, circular, elliptical, or other polygonal when viewed radially. A filter 21 having a large number of through-holes 211 can be relatively easily produced using a 3D printer. The length of the small-diameter portion 211b in contact with the retained resin in a direction perpendicular to the flow direction F is preferably 5 mm or less, more preferably 1 mm or less, and even more preferably 0.5 mm or less, and is preferably relatively short. The length of the large-diameter portion 211a in a direction perpendicular to the flow direction F is not particularly limited.

[0044] (Variation 1) As shown in Fig. 5, the filter 21 may be formed to be thicker than the filter 21 shown in Fig. 3. That is, the inner diameter of the filter 21 may be smaller than that of the filter 21 shown in Fig. 3. In this way, by preparing a plurality of filters 21 having different inner diameters and replacing the filters 21 having different inner diameters, i.e., different internal volumes, according to resin molded bodies having various shapes and sizes, it is possible to adjust the amount of resin of the mixed molten resin R2 remaining inside the filter 21. In this case, in the plurality of through holes 211, the radial length of the large diameter portion 211a, i.e., the length in the direction perpendicular to the flow direction F, can be made larger than the length of the small diameter portion 211b.

[0045] (Variation 2) 6, the filter 21 may have through holes 211 whose inner diameter gradually increases from the inner peripheral surface toward the outer peripheral surface. That is, the inner peripheral surface of the through holes 211 may be formed as a tapered surface whose inner diameter gradually increases. This makes it possible to prevent the resin component of the mixed molten resin R2 from flowing into the through holes 211, and to increase the amount of gas discharged, just as in the case where the through holes 211 are two-stage through holes.

[0046] (Variation 3) Although not shown, the filter 21 may not be disposed inside the hot runner 20, but may be provided on the outer periphery of the hot runner 20 with multiple through-holes 211 connecting the inside and outside of the hot runner 20. In this case, it is not necessary to provide the opening 203 on the outer periphery of the hot runner 20. Conversely, the filter 21 may function as the hot runner 20. For example, instead of providing the hot runner 20 shown in FIG. 5, the filter 21 may be provided with inlet holes 201 and outlet holes 202 at the end of the filter 21 facing the molding machine 10 and the end of the filter 21 facing the mold 40 in the flow direction F. In this case, a heater and a temperature sensor 204 (not shown) may be provided on the outer periphery of the filter 21. Note that if the filter 21 is not disposed inside the hot runner 20, the amount of resin in the mixed molten resin R2 retained in the retention space inside the hot runner 20 may be adjusted by preparing multiple hot runners 20 with different inner diameters and replacing the hot runners 20 with different inner diameters, i.e., different internal volumes, depending on the shape and size of the resin molded products.

[0047] [Cold runner] As shown in FIG. 2, the cold runner 30 is disposed between the hot runner 20 and the mold 40. As shown in FIG. 7, the cold runner 30 has an internal space (retention space) including an inlet hole 301 and an injection hole 302. The inlet hole 301 communicates with the interior of the hot runner 20. The injection hole 302 communicates with the interior of the mold 40. The cold runner 30 has a smaller internal volume than the hot runner 20. The inner circumferential surface of the cold runner 30 has an expanded diameter portion 30a and a reduced diameter portion 30b. The expanded diameter portion 30a is disposed between the inlet hole 301 and the reduced diameter portion 30b, and gradually expands in diameter from the inlet hole 301 toward the reduced diameter portion 30b, i.e., along the flow direction F. The reduced diameter section 30b is disposed between the injection hole 302 and the expanded diameter section 30a, and gradually reduces in diameter from the expanded diameter section 30a toward the injection hole 302, i.e., along the flow direction F. In the flow direction F, the length of the reduced diameter section 30b is shorter than the length of the expanded diameter section 30a. The inclination angle of the reduced diameter section 30b relative to the flow direction F is greater than the inclination angle of the expanded diameter section 30a. As will be described later, the molten resin inside the cold runner 30 solidifies slightly near the injection hole 302, thereby sealing off the interior of the hot runner 20. Although not specifically shown, a flow path may be formed in the cold runner 30 to allow a cooling medium such as a coolant to flow through.

[0048] (Variation 4) Although not specifically shown, the discharge hole 202 of the hot runner 20 shown in FIG. 3 may be provided with a valve for closing the discharge hole 202, without providing the cold runner 30. The valve may be, for example, a needle valve. By providing a valve in this manner, the discharge hole 202 can be more reliably closed. The valve may be suitably used, for example, in a foam injection molding method in which foaming is performed at a relatively high pressure. The cold runner 30 may be suitably used, for example, in a foam injection molding method in which foaming is performed at a relatively low pressure. However, as long as the discharge hole 202 can be opened and closed freely, the cold runner 30 or valve is not the only option. Therefore, the cold runner 30 or valve may also be referred to as a closing portion for the discharge hole 202 of the hot runner 20.

[0049] [Mold] As shown in Fig. 2, the mold 40 has a cavity 41 and a core 42. A resin molded body is manufactured by cooling and solidifying molten resin injected into the mold 40 from the inside of the cold runner 30. The internal shape of the mold 40 is not particularly limited and can be changed according to resin molded bodies having various shapes or sizes.

[0050] [First molten resin generation step S2] Next, as shown in FIG. 2, a resin material (resin pellets) (not shown) is fed from a hopper 13 of a molding machine 10 into a plasticizing cylinder 11, and the interior of the plasticizing cylinder 11 is heated to produce a molten resin (first molten resin) R1. The resin material is not particularly limited as long as it is a thermoplastic resin. That is, the thermoplastic resin may be any of general-purpose plastics, engineering plastics, and super-engineering plastics. General-purpose plastics are thermoplastic resins having a deflection temperature under load of less than 100°C, such as polyethylene (PE) or polypropylene (PP). Engineering plastics are thermoplastic resins having a deflection temperature under load of 100°C or higher, such as polycarbonate resin (PC), modified polyphenylene ether (m-PPE), syndiotactic polystyrene (SPS), polyamide (PA), and polybutyl terephthalate (PBT). Super engineering plastics are thermoplastic resins that have a deflection temperature under load of 150°C or higher, such as polyphenylsulfone (PPSU), polysulfone (PSU), polyarylate (PAR), polyetherimide (PEI), polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyethersulfone (PES), polyamideimide (PAI), polyvinylidene fluoride (PVDF), tetrafluoroethylene perfluoroalkylvinyl copolymer (PFA), etc.

[0051] The resin material may also contain various types of additives to the extent that they do not significantly affect the effects of the present disclosure. The types of additives are not particularly limited, but include, for example, bubble nucleating agents, crystal nucleating agents, lubricants, surfactants, tension modifiers, shrinkage inhibitors, flow modifiers, impact modifiers, fillers, reinforcing agents, antioxidants, heat stabilizers, light stabilizers, UV absorbers, plasticizers, lubricants, mold release agents, antistatic agents, colorants (pigments, dyes, etc.), surface effect additives, infrared absorbers, radiation stabilizers, drip-proofing agents, and antioxidants. The amount of additive added can be selected appropriately as long as it does not impair bubble formation, and the amount used in molding conventional thermoplastic resins can be used. Reinforcing fillers such as glass fiber, carbon fiber, or organic fiber, or inorganic fillers such as talc or calcium carbonate, can also be mixed.

[0052] [First mixed molten resin production step S3] Next, as shown in FIG. 2, the screw 12 is rotated to flow the molten resin R1 in the flow direction F. A blowing agent (first blowing agent) is injected into the starvation zone 111 to produce a mixed molten resin (first mixed molten resin) R2, in which the blowing agent is kneaded and dissolved in the molten resin R1. At this time, the nozzle 17 of the molding machine 10 is closed by the valve 18. The blowing agent may be either a chemical blowing agent or a physical blowing agent, as long as it contains a gas. From the perspective of contributing to reducing environmental impact, a physical blowing agent, such as an inert gas such as carbon dioxide or nitrogen, is preferably used as the blowing agent. The blowing agent is injected into the plasticizing cylinder 11 from a cylinder 14 via a pressure reducing valve 15 and a pressure vessel 16. The pressure reducing valve 15 reduces the nitrogen gas pressure to a constant level. The mixed molten resin R2 can be produced, for example, by a low-pressure gas physical foam molding method described in Japanese Patent Publication No. 6139038. In this case, the gas pressure introduced into the cylinder 14 is, for example, 6 MPa.

[0053] The foaming agent can be selected appropriately depending on the internal volume of the hot runner 20 or the filter 21, the concentration or pressure of the foaming agent, or the amount of resin in the retained molten resin R3 retained in the retaining step S4 (described later). In other words, the amount of resin in the retained molten resin R3 may be determined depending on the concentration or pressure of the foaming agent. For example, in the case of chemical foam molding using a chemical foaming agent or physical foam molding using a low-pressure gas, even if the foaming agent is discharged from the retained molten resin R3 in an amount less than the amount of resin that forms the skin layer 2 in the skin layer forming step S7 (described later), the amount of foaming agent mixed into the foamed resin R5 that forms the core layer 3 in the core layer forming step S8 (described later) is reduced, thereby minimizing adverse effects on the skin layer 2 and allowing for the production of a foam injection-molded part with excellent appearance.

[0054] [Retention process S4] Next, the nozzle 17 of the molding machine 10 is opened, and the mixed molten resin R2 flows inside the hot runner 20, which in this embodiment is inside the filter 21. As shown in FIG. 8, a retained molten resin R3 made of the mixed molten resin R2 is retained inside the filter 21. At the same time, a retained molten resin R3 made of the mixed molten resin R2 also retains inside the cold runner 30. In this disclosure, the space inside the hot runner 20 (filter 21) and the cold runner 30 where the retained molten resin R3 retains may be referred to as the "retention space." At this time, a portion of the retained molten resin R3 solidifies slightly near the injection hole 302 of the cold runner 30. This closes the injection hole 302. After the flow of the mixed molten resin R2 into the filter 21 is complete, the nozzle 17 of the molding machine 10 is closed by the valve 18. The amount of resin of the retained molten resin R3 retained in the retention space is calculated in advance according to resin molded bodies having various shapes and sizes, and can be adjusted by changing the inner diameter of the filter 21 or the shape of the inner peripheral surface of the filter 21, as described above. Alternatively, the amount of resin of the retained molten resin R3 may be adjusted by closing the nozzle 17 of the molding machine 10 after a predetermined amount of resin of the retained molten resin R3 has retained inside the retention space.

[0055] The resin amount of the retaining molten resin R3 is preferably 10% by volume to 85% by volume, and more preferably 10% by volume to 50% by volume, based on the resin material of the entire resin molded body. Therefore, the resin amount of the retaining molten resin R3 is preferably 10% by volume or more, preferably 85% by volume or less, and more preferably 50% by volume or less, based on the resin material of the entire resin molded body. As described below, the retaining molten resin R3 is degassed and then used to form the skin layer of the resin molded body. If the resin amount of the retaining molten resin R3 is too small, it becomes difficult to form a skin layer throughout the entire resin molded body. On the other hand, if the resin amount of the retaining molten resin R3 is too large, it may take a long time for the foaming agent to be exhausted, making it difficult to suppress the occurrence of swirl marks due to foaming when the core layer (described below) is formed. However, when the mixed molten resin R2 is foamed only inside the mold 40 near the injection hole 302, the resin amount of the retaining molten resin R3 may be 10% by volume to 20% by volume, based on the resin material of the entire resin molded body. As explained in the above-mentioned variant example 4, when the discharge hole 202 of the hot runner 20 is closed by a valve or the like without providing a cold runner 30, the amount of resin of the mixed molten resin R2 to be retained as described above is the amount to be retained inside the hot runner 20 (filter 21), and the inside of the hot runner 20 (filter 21) becomes the retention space.

[0056] [Degassed molten resin production process S5] 9, the foaming agent is discharged from the retained molten resin R3 inside the hot runner 20 and the cold runner 30 through the opening 203 of the hot runner 20 and the through-hole 211 of the filter 21, producing degassed molten resins R42 and R41 with reduced foaming agent concentrations. At this time, to expedite the discharge of the foaming agent, a vacuum pump (not shown) may be provided outside the hot runner 20 to suck the foaming agent.

[0057] [Second mixed molten resin generation step S6] While the above-mentioned degassed molten resin producing step S5 is being carried out, as described above, a new molten resin (second molten resin) R1 and a foaming agent (second foaming agent) are mixed together inside the plasticizing cylinder 11 of the molding machine 10 to produce a downstream mixed molten resin (second mixed molten resin) R2. Note that although the second mixed molten resin R2 is produced separately from the above-mentioned first mixed molten resin R2, the produced second mixed molten resin R2 is basically the same as the first mixed molten resin, and therefore will be referred to as the "downstream mixed molten resin R2" in the following description.

[0058] [Skin layer forming step S7] Next, as shown in FIG. 10 , the nozzle 17 of the molding machine 10 is opened, causing the downstream mixed molten resin R2 generated in the degassed molten resin generating step S5 to flow toward the inside of the hot runner 20. Accordingly, the degassed molten resin R41 in the cold runner 30 and the degassed molten resin R42 in the hot runner 20 are extruded by the downstream mixed molten resin R2 and injected into the mold 40 through the injection hole 302. The extruded degassed molten resin R41 and degassed molten resin R42 flow along the inner wall of the mold 40 as shown in FIG. 11 . As shown in FIG. 12 , the degassed molten resin R41 and degassed molten resin R42 are cooled and solidified to form a skin layer 2 made of the degassed molten resin R41 and degassed molten resin R42, each having a low foaming agent concentration. When the cold runner 30 is not provided as described above, the degassed molten resin R42 is injected into the mold 40 through the discharge hole 202 of the hot runner 20. Therefore, if the cold runner 30 is not provided, the discharge hole 202 may be read as an injection hole.

[0059] [Core layer forming step S8] Next, returning to FIG. 10 , the downstream mixed molten resin R2 passes through the interior of the hot runner 20 and the interior of the cold runner 30 without being retained in the retention space. As a result, the downstream mixed molten resin R2 foams around the time it enters the interior of the cold runner 30, forming foamed resin R5. As shown in FIG. 11 , when the downstream mixed molten resin R2 is further extruded from the molding machine 10, foamed resin R5 is injected into the interior of the mold 40 and fills the interior of the degassed molten resin R41 and degassed molten resin R42. As shown in FIG. 12 , by further injecting and filling the interior of the mold 40 with foamed resin R5 and then cooling and solidifying the foamed resin R5, a core layer 3 made of foamed resin R5 covered with a skin layer 2 can be formed. At this time, the degassed molten resin R41 and degassed molten resin R42 are pushed outward by the foaming of the foamed resin R5 and come into close contact with the inner surface of the mold 40. This prevents swirl marks from forming on the outer surface of the resin molded product.

[0060] [Mold release process S9] Finally, the cooled and solidified resin molded body 1 (not shown in detail) having the skin layer 2 and core layer 3 is removed from the mold 40. In this way, it is possible to manufacture a resin molded body 1 that is easy to form the skin layer 2 over the entire resin molded body, suppresses the occurrence of swirl marks, and improves appearance.

[0061] In the above-described skin layer forming step S7 and core layer forming step S8, while the degassed molten resin R41, degassed molten resin R42, and foamed resin R5 inside the mold 40 are being cooled and solidified, a new mixed molten resin (first mixed molten resin) R2 is retained in the retention spaces inside the hot runner 20 and the cold runner 30, as shown in FIG. 12, and the above-described degassed molten resin producing step S5 can be performed with the valve 18 of the molding machine 10 closed. During this time, a second mixed molten resin producing step S6 can be performed simultaneously with the degassed molten resin producing step S5, thereby newly producing a downstream mixed molten resin (second mixed molten resin) R2 inside the plasticizing cylinder 11. After that, when the above-described mold releasing step S9 is completed, the degassed molten resin R41 and degassed molten resin R42 newly produced in the degassed molten resin producing step S5 and the newly produced downstream mixed molten resin R2 are injected and filled into the mold 40 in a single step. That is, newly produced downstream mixed molten resin R2 is discharged from nozzle 17 of molding machine 10, and degassed molten resin R42 and degassed molten resin R41 are injected and filled into mold 40 as if being extruded by the downstream mixed molten resin R2 to form skin layer 2, and the downstream mixed molten resin R2 is foamed continuously with degassed molten resin R42 and degassed molten resin R41 and injected and filled into skin layer 2 to form core layer 3. In this series of cycles, resin molded articles 1 can be produced continuously and efficiently.

[0062] [Second embodiment] Next, a method for manufacturing a resin molded body according to the second embodiment will be specifically described with reference to Figures 13 and 14. Descriptions of the same components as those in the method for manufacturing a resin molded body according to the first embodiment will be omitted, and basically, components that differ from those in the method for manufacturing a resin molded body according to the first embodiment will be described.

[0063] In the method for manufacturing a resin molded product according to the second embodiment, a plurality of hot runners 20 are provided, as shown in FIG. 13 . No cold runner 30 is provided between the hot runners 20 and the mold 40. The hot runners 20 do not have openings 203, and a filter 21 having a plurality of through holes 211 is provided on the outer periphery. The discharge holes 202 of each of the plurality of hot runners 20 communicate with the interior of the mold 40 and can be freely opened and closed by a valve 52 connected to an air cylinder 51. In this embodiment, the discharge holes 202 are provided in two locations. Note that the hot runners 20, filters 21, and cold runners 30 may be combined with any of the above-described modifications 1 to 4 and applied to the method for manufacturing a resin molded product according to the second embodiment.

[0064] The manufacturing apparatus 100 used in the manufacturing method for a resin molded body of the second embodiment further includes a manifold 50. The manifold 50 is disposed between the molding machine 10 and the hot runner 20. The manifold 50 has a flow path 53 that branches into multiple paths. The mixed molten resin R2 flows into the flow path 53 from the nozzle 17 of the molding machine 10 through a filter 21. The filter 21 is formed in the flow path 53. In this way, a plurality of filters 21 may be provided in the flow path 53 interposed between the nozzle 17 of the molding machine 10 and the interior of the mold 40.

[0065] This manufacturing method for producing a resin molded body by providing multiple discharge holes 202 (or injection holes 302) is particularly suitable for producing relatively large resin molded bodies. In addition, in the manufacturing method for a resin molded body according to the present disclosure, the degassed molten resin R41 and the degassed molten resin 42 are injected and filled into the interior of the mold 40, and then the foamed resin 5 is injected and filled. Therefore, a core layer 3 is formed near the discharge holes 202 (or injection holes 302), and the specific gravity of the area near the discharge holes 202 (or injection holes 302) tends to be low. By injecting and filling the degassed molten resin R41, the degassed molten resin R42, and the foamed resin R5 through the multiple discharge holes 202, the specific gravity of each area of ​​the resin molded body can be made relatively uniform. Meanwhile, by selecting the position of the discharge holes 202, i.e., the position within the mold 40 where the core layer 3 is to be injected and filled, the position in the resin molded body 1 where the core layer 3 is to be formed can be appropriately selected.

[0066] [Plastic model] The methods for manufacturing a resin molded body according to the above-described embodiments can be suitably used to manufacture a plastic model 200, as shown in FIG. 15 . Although not specifically illustrated, the plastic model 200 includes multiple parts 210 for forming the assembled model and a framework 220 for connecting the multiple parts 210 to one another. The multiple parts 210 include a difficult-to-form part 210a and an easy-to-form part 210b. The difficult-to-form part 210a is a relatively thick part that is prone to sink marks and low shape accuracy. The easy-to-form part 210b is a relatively thin part that is less prone to sink marks and easier to maintain shape accuracy. This is because it is difficult to supply a sufficient amount of mixed molten resin R2 to the interior of a mold 40 used to form the difficult-to-form part 210a, particularly to the edges of the mold 40.

[0067] 15, during the above-described injection molding, the hard-to-form part 210a is placed near the injection hole 302 (or the discharge hole 202), and the easy-to-form part 210b is placed farther from the injection hole 302 than the hard-to-form part 210a. That is, in the plastic model 200, the hard-to-form part 210a is placed near the gate 230, which is the portion through which the resin material is injected from the injection hole 302, and the easy-to-form part 210b is placed farther from the gate 230 than the hard-to-form part 210a. According to the manufacturing method for a resin molded body disclosed herein, first, the degassed molten resin R41 and the degassed molten resin R42 are injected and filled into the mold 40. The amount of resin injected and filled with the degassed molten resin R41 and the degassed molten resin R42, i.e., the amount of resin of the retained molten resin R3 retained in the retention space, is adjusted. The amounts of degassed molten resin R41 and degassed molten resin R42 should be sufficient to form the distant easy-to-form part 210b and the skin layer 2 of the nearby difficult-to-form part 210a. The downstream mixed molten resin R2 is then injected to fill the skin layer 2 of the nearby difficult-to-form part 210a, generating foamed resin R5 and forming the core layer 3. By forming the difficult-to-form part 210a in this manner while positioning it near the injection hole 302 (or discharge hole 202), sink marks that may occur in the difficult-to-form part 210a can be suppressed.

[0068] [Resin molded body] The resin molded body 1 manufactured by the above-described manufacturing method has, like the example of the plastic model 200 described above, a gate portion 230, a core layer 3 made of foamed resin, and a skin layer 2 covering the core layer 3. The core layer 3 may be formed throughout the entire interior of the resin molded body 1, or may be formed in the vicinity of the gate portion 230.

[0069] Furthermore, the specific gravity of the resin molding 1 in the vicinity of the gate 230 can be made smaller than the specific gravity of the region farther from the gate 230 than the vicinity thereof.

[0070] The resin molded body 1 manufactured by the manufacturing method according to this embodiment is foam-molded, which allows for a reduction in the amount of resin used. As a result, the resin molded body 1 can contribute to improving resource utilization efficiency, easing transportation burdens, reducing energy consumption, and reducing CO2 emissions. By providing the resin molded body 1 to society, it is possible to contribute to the achievement of Goal 7 (Affordable and Clean Energy), Goal 9 (Industry, Innovation and Infrastructure), and Goal 11 (Sustainable Cities and Communities) of the 17 Sustainable Development Goals (SDGs) established by the United Nations. Furthermore, since the resin molded body 1 according to this embodiment can be melted and reused, it can contribute to the achievement of Goal 12 (Responsible Consumption and Production).

[0071] Although the embodiments have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0072] (Example 1 and Comparative Example 1) A resin molded body of Example 1 manufactured by the manufacturing method according to the first embodiment described above (see Figures 1 to 12) and a resin molded body of Comparative Example 1 consisting only of a non-foamed resin were produced, and their appearances, etc. were compared.

[0073] The manufacturing conditions for the resin molded article of Example 1 are as follows. Resin material: Polycarbonate resin (Sumitomo Chemical Co., Ltd. SF5301V, 30% glass fiber, specific gravity 1.42) Foaming agent: Nitrogen, Nitrogen cylinder gas pressure: 6MPa Mold conditions: Flat cavity with a center direct gate as shown in Figure 1 (Width 20cm, height 40cm, thickness 3mm, volume 240cm 3 ) Hot runner: internal volume 75cm 3 , heating temperature 260℃ Filter: Cylindrical, length in flow direction 9 cm, inner diameter 3 cm, internal volume 63 cm 3 Cold runner: internal volume 10cm 3 , cooling temperature 30℃ Resin material volume: Total 240cm 3 , degassed mixed molten resin 85cm 3 (approximately 35% of the total)

[0074] In the resin molded body of Example 1, a core layer was formed up to 5 mm from the vicinity of the injection-filled area. Furthermore, the weight of the resin molded body of Example 1 was 250 g, a 26% weight reduction compared to the resin molded body of Comparative Example 1. When the tip of the resin molded body of Example 1 was pressed, the amount of warping on the surface opposite the pressed surface was approximately 0.2 mm, which was less than the warping amount of approximately 1.5 mm of the resin molded body of Comparative Example 1. Furthermore, the surface roughness Ra of the resin molded body of Example 1 was approximately 1 μm. However, this surface roughness Ra value was due to the floating of glass fibers contained in the resin material, and in reality, no swirl marks due to foam molding were observed.

[0075] (Example 2 and Comparative Example 2) A resin molded body of Example 2 manufactured by the manufacturing method according to the second embodiment described above (see Figures 13 and 14) and a resin molded body of Comparative Example 2 consisting only of non-foamed resin were produced, and their appearances, etc. were compared.

[0076] The manufacturing conditions for the resin molded body of Example 2 differ from those of Example 1 in the following respects. Mold: U-shaped cross section cavity (Width 400mm, height 800mm, plate thickness 4mm, internal volume 1600cm 3 ) Resin material volume: Total 1600cm 3 , degassed mixed molten resin 480cm 3 (approximately 30% of the total) Inner volume: 480cm 3 (Total internal volume of multiple filters and flow channels) Filter: Cylindrical, length in flow direction 9 cm, inner diameter 4 cm, internal volume (per filter) 113 cm 3

[0077] In the resin molded body of Example 2, a core layer was formed up to 10 mm from the injection-filled area. Furthermore, the weight of the resin molded body of Example 1 was 1,600 g, a 28% weight reduction compared to the resin molded body of Comparative Example 2. When the tip of the resin molded body of Example 2 was pressed, the amount of warping on the surface opposite the pressed surface was approximately 0.3 mm, which was less than the warping amount of approximately 5 mm for the resin molded body of Comparative Example 2. Furthermore, the surface roughness Ra of the resin molded body of Example 2 was similar to that of the resin molded body of Comparative Example 2, and no swirl marks caused by foam molding were observed. It was confirmed that even if the size of the resin molded body was increased, by providing multiple discharge holes (injection holes), the core layer could be formed to a certain degree of uniformity, and the occurrence of warping and swirl marks could be suppressed. [Explanation of symbols]

[0078] S1 preparation process, S2 first molten resin generation process, S3 first mixed molten resin generation process, S4 retention process, S5 degassed molten resin generation process, S6 second mixed molten resin generation process, S7 skin layer formation process, S8 core layer formation process, S9 demolding process, 100 resin molded body manufacturing apparatus, 10 molding machine, 20 hot runner, 203 opening, 21 filter, 211 through hole, 30 cold runner, 40 mold, 50 manifold, R1 molten resin, R2 mixed molten resin, R3 retained molten resin, R41 R42 degassed molten resin, R4 foamed resin, 200 plastic model, 210 part, 210a non-easy to form part, 210b easy to form part, framework 220, gate portion 230, 1 resin molded body, 2 skin layer, 3 core layer

Claims

1. A method for manufacturing a resin molded article by injection molding a resin molded article having a core layer made of a foamed resin and a skin layer covering the core layer, providing a molding machine having a nozzle, a mold, and a hot runner disposed between the molding machine and the mold; mixing a first molten resin and a first blowing agent containing a gas inside the molding machine to produce a first mixed molten resin; a step of causing the first mixed molten resin to flow from the inside of the molding machine into the inside of the hot runner through the nozzle, and then closing the nozzle of the molding machine to retain the retained molten resin made of the first mixed molten resin in a retention space inside the hot runner; discharging the first foaming agent from the retained molten resin to produce a degassed molten resin having a reduced foaming agent concentration; mixing a second molten resin and a second blowing agent containing a gas inside the molding machine to produce a second mixed molten resin; a step of injecting and filling the degassed molten resin into the mold to form the skin layer; and injecting and filling the second mixed molten resin into the skin layer while foaming the second mixed molten resin to form the core layer, In the step of retaining the retained molten resin, the amount of the retained molten resin retained in the retention space is adjusted.

2. A method for producing the resin molded article according to claim 1, the second mixed molten resin is generated at the same time as the degassed molten resin is generated, the degassed molten resin is injected and filled into the mold so as to be extruded into the second mixed molten resin discharged from the nozzle of the molding machine, thereby forming the skin layer; The method for manufacturing a resin molded body, wherein the second mixed molten resin is injected and filled while foaming into the inside of the skin layer continuously with the degassed molten resin to form the core layer.

3. A method for producing the resin molded article according to claim 1, the hot runner has a filter housed inside the hot runner and having a plurality of through holes; In the step of retaining the retained molten resin, the retained molten resin is retained inside the filter, In the step of producing the degassed molten resin, the first foaming agent is discharged through the plurality of through holes.

4. The method for producing the resin molded article according to claim 3, The method for producing a resin molded article, wherein each of the plurality of through holes of the filter has an inner diameter of 1 μm to 50 μm.

5. A method for producing the resin molded article according to claim 1, A method for manufacturing a resin molded body, wherein in the step of retaining the retained molten resin, the amount of resin in the retained molten resin is adjusted by changing the internal volume of a retention space inside the hot runner.

6. The method for producing a resin molded article according to claim 1, further comprising: providing a cold runner disposed between the hot runner and the mold; the cold runner has an injection hole communicating with the interior of the mold, a retaining molten resin in the retaining space inside the hot runner and the cold runner by solidifying a portion of the retaining molten resin inside the cold runner and closing the injection hole in the retaining molten resin process.

7. A method for producing the resin molded article according to claim 1, the hot runner has a discharge hole communicating with the interior of the mold and a valve for freely opening and closing the discharge hole; In the step of retaining the retained molten resin, the retained molten resin is retained in a retention space inside the hot runner by closing a discharge hole of the hot runner with the valve.

8. A method for producing the resin molded article according to claim 1, The method for producing a resin molded body, wherein the first foaming agent is either carbon dioxide or nitrogen.

9. A method for producing the resin molded article according to any one of claims 1 to 8, The degassed molten resin and the second mixed molten resin are injected into the mold from a plurality of locations to fill the interior of the mold.

10. A plastic model made by foam injection molding, Multiple parts and a framework for connecting the plurality of parts; a gate portion through which a resin material is injected during foam injection molding; A plastic model, wherein at least one of the plurality of parts that is located near the gate portion has a core layer and a skin layer that covers the core layer.

11. A resin molded body obtained by foam injection molding, a core layer made of a foamed resin; a skin layer covering the core layer; a gate portion through which a resin material is injected during foam injection molding; The core layer is formed in the vicinity of the gate portion.

12. A resin molded body obtained by foam injection molding, It has a gate portion through which the resin material is injected during foam injection molding, A resin molded body, wherein a specific gravity of a portion in the vicinity of the gate portion is smaller than a specific gravity of a portion located farther from the gate portion than the portion in the vicinity.

13. A filter used in a manufacturing device for a resin molded product, the filter including: a molding machine that mixes a molten resin with a foaming agent containing a gas to produce a mixed molten resin; and a hot runner that retains a retained molten resin made of the mixed molten resin and discharges the foaming agent from the retained molten resin to produce a degassed molten resin, The filter is disposed inside the hot runner and has a plurality of through holes for discharging a foaming agent of the retained molten resin retained inside the filter.

14. 14. The filter of claim 13, A filter, wherein each of the plurality of through holes has an inner diameter of 1 μm to 50 μm.

15. 14. The filter of claim 13, The filter has a cylindrical shape, A filter that adjusts the amount of resin in the retained molten resin from which the foaming agent is discharged by changing the internal volume of the filter.

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