Method and apparatus for manufacturing foam molded body
Patent Information
- Application Number
- PCT/JP2026/011584
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2025-03-28
- Filing Date
- 2026-03-24
- Publication Date
- 2026-10-01
Smart Images
Figure JP2026011584_01102026_PF_FP_ABST
Abstract
Description
Method and apparatus for producing a foamed molded article
[0001] The present invention relates to a method and an apparatus for producing a foamed molded article.
[0002] In recent years, as disclosed in, for example, Patent Document 1, injection foam molding methods using nitrogen or carbon dioxide in a supercritical state as a physical blowing agent have been researched and put into practical use. According to Patent Document 1, an injection foam molding method using a physical blowing agent is performed as follows. First, a physical blowing agent is introduced into a closed plasticizing cylinder, and is brought into contact with and dispersed in a plasticized and molten resin. While maintaining the inside of the plasticizing cylinder at a high pressure enough to keep the physical blowing agent in a supercritical state, the molten resin in which the physical blowing agent is dispersed is weighed and injection-filled into a mold. The supercritical fluid compatible with the molten resin is rapidly depressurized and gasified during injection filling, and the molten resin solidifies, thereby forming bubbles (foam cells) inside the molded article. In these injection foam molding methods, the physical blowing agent is metered at a pressure slightly higher than the internal pressure of the resin, and is introduced into the plasticizing cylinder after metering. Therefore, the dissolution amount of the physical blowing agent in the molten resin is determined by the introduction amount of the physical blowing agent (introduction amount control).
[0003] On the other hand, for example, Patent Document 2 discloses a method of introducing a physical blowing agent into a plasticizing cylinder by pressure control instead of introduction amount control in an injection foam molding method using a physical blowing agent. In Patent Document 2, a starvation zone where the molten resin is not fully filled is provided in the plasticizing cylinder, and a physical blowing agent at a constant pressure is introduced into the starvation zone. In the starvation zone, the molten resin in a starvation state is brought into contact with the physical blowing agent at a constant pressure, so that the physical blowing agent penetrates into the molten resin. At this time, the starvation zone is always maintained at a constant pressure of the introduced physical blowing agent.
[0004] Japanese Patent No. 2625576 Japanese Patent No. 6139038
[0005] In the injection foam molding method of Patent Document 1 described above, it was necessary to set the physical blowing agent to be introduced into the plasticizing cylinder at a high pressure and accurately meter the introduction amount. This has complicated the supply mechanism of the physical blowing agent and has been a factor increasing the initial cost of the apparatus.
[0006] In contrast, the injection foam molding method described in Patent Document 2 above does not require control over the amount of the physical foaming agent introduced into the molten resin, the introduction time, etc., because the physical foaming agent is introduced into the plasticizing cylinder by pressure control. Therefore, the injection foam molding method of Patent Document 2 can omit or simplify complex control devices and reduce equipment costs. Furthermore, the amount of dissolution (penetration) of the physical foaming agent into the molten resin can be stabilized by a simple mechanism.
[0007] However, in the aforementioned injection foam molding method in which a physical foaming agent is introduced into a plasticizing cylinder under pressure control, it is desirable to further suppress the separation of the physical foaming agent from the molten resin, which can cause, for example, enlargement and non-uniformity of the foam cell diameter, in order to stably produce high-quality foamed molded products.
[0008] The present invention has been made in view of the above circumstances, and aims to provide a manufacturing method and manufacturing apparatus that can suppress the separation of physical foaming agents from molten resin and stably produce high-quality foamed molded articles.
[0009] To solve the above problems, the present invention provides a method for manufacturing a foamed molded article, using a manufacturing apparatus that includes a plasticizing cylinder having a screw provided inside that can rotate and move back and forth, a plasticizing zone in which a thermoplastic resin is plasticized and melted to become a molten resin, and a starving zone in which the molten resin is in a starving state, and an inlet formed for introducing a physical blowing agent into the starving zone, the manufacturing method comprising: plasticizing and melting the thermoplastic resin in the plasticizing zone to become the molten resin; introducing the physical blowing agent, adjusted to a constant first pressure in the range of 1 to 20 MPa, into the starving zone, and bringing the molten resin in the starving state into contact with the physical blowing agent at the first pressure; measuring the molten resin that has been in contact with the physical blowing agent at the first pressure using a second pressure which is a screw back pressure 0 to 10 MPa higher than the first pressure; and injecting the measured molten resin from the plasticizing cylinder to form the foamed molded article. The method is characterized in that, after the metering of the molten resin is completed, the molten resin is injected from the plasticizing cylinder while the third pressure, which is the screw tip pressure after metering, is higher than the first pressure of the physical foaming agent.
[0010] The inventors discovered a phenomenon in which the pressure of the metered molten resin decreases between the completion of metering and the start of injection. They found that this pressure drop after metering is a unique phenomenon in which the first pressure in the starvation zone and the higher second pressure at the time of metering are temporarily equal, and that this causes the separation of the physical blowing agent from the molten resin. The separation of the physical blowing agent from the molten resin can also cause the foam cells in the foamed molded product to enlarge and become non-uniform, and therefore must be avoided. For this reason, in the above configuration of the present invention, after the completion of metering of the molten resin, the injection of the molten resin from the plasticizing cylinder is performed when the third pressure, which is the screw tip pressure after metering, is higher than the first pressure of the physical blowing agent, that is, before the physical blowing agent separates from the molten resin due to the pressure drop phenomenon, thereby promoting uniform mismatch.
[0011] Furthermore, in this injection method, after the metering of the molten resin is complete, the molten resin is injected from the plasticizing cylinder when the third pressure, which is the screw tip pressure after metering, is higher than the first pressure of the physical foaming agent. This eliminates the need to constantly maintain the pressure of the metered molten resin at a level higher than the first pressure from the completion of metering to the start of injection in order to prevent the separation of the physical foaming agent from the molten resin. Thus, it avoids situations where changes to the manufacturing sequence software are necessary to maintain such pressure. Moreover, the aforementioned pressure maintenance not only requires software changes, but also, in accordance with safety regulations, when opening the safety door of the mold that receives the molten resin injected from the plasticizing cylinder in semi-automatic operation, the motor power is cut off, causing the screw to retract. This creates a new problem where continuous molding becomes impossible unless the screw position is returned to its original position. However, with the above configuration of the present invention, such problems can also be avoided.
[0012] In the above configuration, in order to inject the metered molten resin located at the tip of the screw from the plasticizing cylinder at a pressure higher than the first pressure of the physical foaming agent, it is preferable to inject the molten resin from the plasticizing cylinder between 0 and 10 seconds immediately after the completion of metering, and more preferably between 0 and 5 seconds. Furthermore, the screw back pressure during metering after the screw retracts may be the same as the first pressure of the physical foaming agent, or the back pressure may be temporarily increased just before the screw stops in the latter half of metering.
[0013] Furthermore, the present invention also provides a manufacturing apparatus having the features described above.
[0014] According to the present invention, the separation of the physical foaming agent from the molten resin can be suppressed, and high-quality foamed molded articles can be stably manufactured.
[0015] This is a flowchart illustrating a method for manufacturing a foamed molded article according to one embodiment of the present invention. This is a schematic diagram of a manufacturing apparatus for a foamed molded article according to one embodiment of the present invention. Figure 3(a) shows the process during the metering of molten resin, and Figure 3(b) shows the process after the metering of molten resin is complete. This is a schematic pressure waveform diagram of a conventional manufacturing process. This is a schematic time chart comparing a conventional manufacturing process with the manufacturing process according to one embodiment of the present invention. This is a schematic pressure waveform diagram of a manufacturing process according to one embodiment of the present invention.
[0016] The embodiments of the present invention will be described below with reference to the drawings. These embodiments contribute to "9. Build resilient infrastructure, including local and transboundary infrastructure, to support economic development and human well-being, with a focus on affordable and equitable access for all," which is one of the Sustainable Development Goals (SDGs) advocated by the United Nations.
[0017] The method for manufacturing a foamed molded article according to this embodiment will be described with reference to the flowchart shown in Figure 1. The method for manufacturing a foamed molded article according to this embodiment can be carried out, for example, using the manufacturing apparatus 1000 shown in Figure 2. First, the manufacturing apparatus 1000 will be described.
[0018] <Foam Molding Apparatus> The manufacturing apparatus (injection molding machine) 1000 mainly comprises a plasticizing cylinder 210 in which a screw 20 is installed, a screw drive mechanism 260 for driving the screw 20, a cylinder 100 which is a physical foaming agent supply mechanism for supplying a physical foaming agent to the plasticizing cylinder 210, a mold clamping unit 250 in which a mold 251 is provided, and a control device (not shown) for controlling the operation of the plasticizing cylinder 210, the screw drive mechanism 260, and the mold clamping unit 250.
[0019] The molten resin that has been plasticized and melted inside the plasticizing cylinder 210 flows from the right hand to the left hand in Figure 2. Therefore, inside the plasticizing cylinder 210 of this embodiment, the right hand in Figure 2 is defined as "upstream" or "rear," and the left hand is defined as "downstream" or "forward." The direction in which the plasticizing cylinder 210 and the screw 20 extend (the axial direction of the screw 20 and the flow direction of the molten resin) is defined as the "front-back direction." Furthermore, in the plasticizing cylinder 210 of this embodiment, similar to the configuration of conventionally known plasticizing cylinders, when viewed from the rear side of the plasticizing cylinder 210, rotating the screw 20 counterclockwise causes forward rotation that sends the molten resin forward, and rotating it clockwise causes reverse rotation.
[0020] The plasticizing cylinder 210 has a plasticizing zone 21 in which thermoplastic resin is plasticized and melted to become molten resin, and a starving zone 23 downstream of the plasticizing zone 21 in which the molten resin is in a starving state. A "starving state" is a state in which the molten resin does not fill the starving zone 23 and remains incomplete. Therefore, there is space in the starving zone 23 other than the portion occupied by the molten resin. An inlet 202 for introducing a physical blowing agent is formed in the starving zone 23, and an introduction rate adjustment container 300 is connected to the inlet 202. The cylinder 100 supplies the physical blowing agent to the plasticizing cylinder 210 via the introduction rate adjustment container 300.
[0021] The screw drive mechanism 260 is connected to the upstream rear end of the plasticizing cylinder 210 and includes a screw rotation drive mechanism including a screw rotation motor M2 and transmission means 262, a screw movement mechanism including a screw forward / backward motor M1 and transmission means 263, and a pressure sensor 261 such as a load cell for detecting the pressure applied to the screw 20. The screw rotation motor M2 rotates the screw 20 in both forward and reverse directions via the transmission means 262, which consists of pulleys, belts, etc. The screw forward / backward motor M1 moves the screw 20 in the forward / backward direction via the transmission means 263, which converts rotational motion of pulleys, belts, ball screw / nut mechanism, etc. into linear motion. As a result, the screw 20 can move forward from the plasticizing zone 21 towards the starvation zone 23 and backward from the starvation zone 23 towards the plasticizing zone 21.
[0022] The magnitude of the pressure on the screw 20 detected by the pressure sensor 261 is the magnitude of the pressure of the molten resin located in front of the screw 20, but if screw back pressure is applied to the screw 20, it is also the magnitude of the screw back pressure. "Screw back pressure" is the force that pushes the screw 20 from rear to front. For example, during plasticization metering of resin, that is, when the screw 20 rotates in the forward direction, the molten resin is sent to the front of the plasticization cylinder 210, and when the screw 20 retracts due to the pressure of the resin, a force pushing from rear to front (screw back pressure) is applied to the screw 20. At this time, the screw back pressure and the pressure of the molten resin located in front of the screw 20 are equal. The pressure detected by the pressure sensor 26 is the pressure of the molten resin located in front of the screw 20, and at the same time, it is the screw back pressure. In this embodiment, the screw back pressure is controlled by the screw drive mechanism 260. For the reasons described above, in this specification, the pressure of the load cell 261 during metering is specifically referred to as screw back pressure.
[0023] On the other hand, when no screw back pressure is applied, that is, when the screw back pressure is 0 (zero), the pressure sensor 261 detects only the pressure of the molten resin located in front of the screw 20. For example, if the screw back pressure is set to 0 (zero) after a predetermined amount of molten resin has been metered, the pressure detected by the pressure sensor 261 is the pressure of the metered molten resin located in front of the screw 20.
[0024] The screw 20 has a tip sealing mechanism 50, which includes a check ring 52, at its front end. The tip sealing mechanism 50 prevents the compressed resin in front of the screw 20 from flowing back to the rear.
[0025] Although the manufacturing apparatus 1000 has only one starvation zone 23, the manufacturing apparatus used in this embodiment is not limited to this. For example, in order to promote the penetration of the physical foaming agent into the molten resin, the apparatus may have multiple starvation zones 23 and inlets 202 formed therein, and the physical foaming agent may be introduced into the plasticizing cylinder 210 from multiple inlets 202.
[0026] A shut-off valve 28, which opens and closes when driven by an air cylinder, is provided at the nozzle tip 29 of the plasticizing cylinder 210, allowing the inside of the plasticizing cylinder 210 to be kept under high pressure. The mold 251 is in close contact with the nozzle tip 29, and when the shut-off valve 28 is opened, molten resin is injected and filled into the cavity 253 formed by the mold 251 from the nozzle tip 29. On the upper side of the plasticizing cylinder 210, a resin supply port 201 for supplying thermoplastic resin to the plasticizing cylinder 210 and an inlet 202 for introducing a physical foaming agent into the plasticizing cylinder 210 are formed in order from the upstream side. A resin supply hopper 211, a feeder screw 212, and an introduction speed adjustment container 300 are provided at these resin supply port 201 and inlet 202, respectively. A cylinder 100 is connected to the introduction speed adjustment container 300 by piping 154 via a pressure reducing valve 151 and a pressure gauge 152. Furthermore, a sensor (not shown) is provided within the starvation zone 23 of the plasticizing cylinder 210 to monitor the pressure in the starvation zone 23.
[0027] <Method for Manufacturing Foamed Molded Articles> (1) Plasticization and Melting of Thermoplastic Resin First, the thermoplastic resin is plasticized and melted in the plasticization zone 21 of the plasticization cylinder 210 to form a molten resin (Step S1 in Figure 1). Various resins can be used as the thermoplastic resin depending on the type of molded article to be manufactured. Specifically, for example, thermoplastic resins such as polypropylene, polymethyl methacrylate, polyamide, polycarbonate, amorphous polyolefin, polyetherimide, polyethylene terephthalate, polyetheretherketone, ABS resin (acrylonitrile-butadiene-styrene copolymer resin), polyphenylene sulfide, polyamideimide, polylactic acid, polycaprolactone, and composite materials thereof can be used. These thermoplastic resins may be used individually or in mixtures of two or more types.
[0028] Furthermore, these thermoplastic resins can also be used in which various inorganic fillers such as glass fibers, talc, and carbon fibers are kneaded together. It is preferable to mix the thermoplastic resin with inorganic fillers that function as foaming nucleating agents and additives that increase melt tension. By mixing these, the foam cells can be made finer. The thermoplastic resin of this embodiment may also contain various other general-purpose additives as needed. Moreover, the thermoplastic resin of this embodiment may also contain general-purpose chemical blowing agents. The foaming performance can be enhanced by adding a small amount of chemical blowing agent.
[0029] In this embodiment, the thermoplastic resin is plasticized and melted in a plasticizing cylinder 210, which is equipped with a screw 20 as shown in Figure 2. A band heater (not shown) is provided on the outer wall surface of the plasticizing cylinder 210, which heats the plasticizing cylinder 210. In addition, shear heat generated by the rotation of the screw 20 is added, causing the thermoplastic resin to plasticize and melt.
[0030] (2) Introduction of physical blowing agent Next, a physical blowing agent at a constant pressure (first pressure P1) is introduced into the starvation zone 23 (step S2 in Figure 1).
[0031] A pressurized fluid is used as the physical blowing agent. In this embodiment, "fluid" means either a liquid, a gas, or a supercritical fluid. Furthermore, from the viewpoint of cost and environmental impact, carbon dioxide, nitrogen, etc., are preferred as the physical blowing agent. Since the pressure of the physical blowing agent in this embodiment is relatively low, for example, a fluid can be used that has been reduced to a constant pressure by a pressure reducing valve and extracted from a cylinder that stores fluids, such as a nitrogen cylinder, carbon dioxide cylinder, or air cylinder. In this case, a pressurizing device is not required, so the cost of the entire manufacturing equipment can be reduced. Alternatively, if necessary, a fluid that has been pressurized to a predetermined pressure may be used as the physical blowing agent. For example, when using nitrogen as the physical blowing agent, the physical blowing agent can be produced by the following method. First, nitrogen is purified by compressing atmospheric air with a compressor and passing it through a nitrogen separation membrane. Next, the purified nitrogen is pressurized to a predetermined pressure using a booster pump or syringe pump, etc., to produce the physical blowing agent. Alternatively, compressed air may be used as the physical blowing agent. In this embodiment, forced shear mixing of the physical blowing agent and molten resin is not performed. Therefore, even when compressed air is used as a physical foaming agent, oxygen, which has low solubility in molten resin, does not easily dissolve in the molten resin, thus suppressing oxidative degradation of the molten resin.
[0032] The pressure of the physical blowing agent introduced into the starvation zone 23 (first pressure P1) is constant. The pressure of this physical blowing agent (first pressure P1) is preferably 1 MPa to 20 MPa, more preferably 1 MPa to 10 MPa, and even more preferably 2 MPa to 8 MPa. The optimal pressure varies depending on the type of molten resin, but by setting the pressure of the physical blowing agent to 1 MPa or higher, the amount of physical blowing agent necessary for foaming can be permeated into the molten resin, and by setting it to 15 MPa or lower, the load on the equipment can be reduced.
[0033] Furthermore, in this embodiment, only a physical blowing agent is introduced into the starvation zone 23. However, other pressurized fluids other than the physical blowing agent may be introduced into the starvation zone 23 simultaneously, to the extent that they do not affect the effects of this embodiment. In this case, the pressurized fluid containing the physical blowing agent introduced into the starvation zone 23 will have the aforementioned constant pressure.
[0034] In this embodiment, as shown in Figure 2, a physical blowing agent is supplied from the cylinder 100 to the starvation zone 23 via the introduction speed adjustment container 300 and through the inlet 202. The physical blowing agent is reduced to a predetermined pressure using the pressure reducing valve 151 and then introduced into the starvation zone 23 through the inlet 202 without passing through a pressurizing device or the like. In this embodiment, the amount of physical blowing agent introduced into the plasticizing cylinder 210, the introduction time, etc., are not controlled. Therefore, a mechanism to control these, such as a drive valve using a check valve or solenoid valve, is not required in the section from the pressure reducing valve 151 to the inlet 202. In this embodiment as well, there is no drive valve in the section from the pressure reducing valve 151 to the inlet 202, and it is always open. A check valve or solenoid valve may be provided in the section from the pressure reducing valve 151 to the inlet 202, but it should always be open for the duration of a continuous molding cycle. Furthermore, as the screw 20 moves forward and backward, the starvation zone 23 moves in the front-to-back direction within the plasticizing cylinder 210, but the inlet 202 is always positioned within the starvation zone 23.
[0035] The inlet 202 for the physical foaming agent has a larger inner diameter compared to the inlet for the physical foaming agent in conventional manufacturing equipment. Therefore, even a relatively low-pressure physical foaming agent can be efficiently introduced into the plasticizing cylinder 210. Furthermore, even if some of the molten resin comes into contact with the inlet 202 and solidifies, the large inner diameter prevents it from becoming completely blocked, allowing it to function as an inlet. For example, if the inner diameter of the plasticizing cylinder 210 is large, i.e., if the outer diameter of the plasticizing cylinder is large, it is easier to increase the inner diameter of the inlet 202. On the other hand, if the inner diameter of the inlet 202 is too large, molten resin will accumulate, causing molding defects, and the introduction speed adjustment container 300 connected to the inlet 202 will become larger, increasing the overall cost of the equipment. Specifically, the inner diameter of the inlet 202 is preferably 20% to 100% of the inner diameter of the plasticizing cylinder 210, and more preferably 30% to 80%. Alternatively, regardless of the inner diameter of the plasticizing cylinder 210, the inner diameter of the inlet 202 is preferably 3 mm to 100 mm, and more preferably 5 mm to 50 mm. Here, the inner diameter of the inlet 202 refers to the inner diameter of the opening on the inner wall of the plasticizing cylinder 210. Furthermore, the shape of the inlet 202, that is, the shape of the opening on the inner wall of the plasticizing cylinder 210, is not limited to a perfect circle, but may be an ellipse or a polygon. If the shape of the inlet 202 is an ellipse or a polygon, the diameter of a perfect circle with the same area as the inlet 202 is defined as the "inner diameter of the inlet 202".
[0036] The introduction rate adjustment container 300, connected to the inlet 202, has a volume above a certain level, which slows down the flow rate of the physical blowing agent introduced into the plasticizing cylinder 210 and ensures that the physical blowing agent can remain in the introduction rate adjustment container 300 for a sufficient amount of time. By remaining in the vicinity of the heated plasticizing cylinder 210, the physical blowing agent is heated, reducing the temperature difference between the physical blowing agent and the molten resin, and stabilizing the amount of physical blowing agent dissolved (penetrated) into the molten resin. In other words, the introduction rate adjustment container 300 functions as a buffer container. On the other hand, if the volume of the introduction rate adjustment container 300 is too large, the overall cost of the apparatus will increase. The volume of the introduction rate adjustment container 300 depends on the amount of molten resin present in the starvation zone 23, but is preferably 5 mL to 10 L, and more preferably 10 mL to 1 L. By setting the volume of the introduction rate adjustment container 300 within this range, it is possible to ensure that the physical blowing agent can remain in the container while considering costs.
[0037] Furthermore, as will be described later, the physical blowing agent is consumed within the plasticizing cylinder 210 by contacting and penetrating the molten resin. In order to maintain a constant pressure in the starvation zone 23, the amount of physical blowing agent consumed is introduced into the starvation zone 23 from the introduction rate adjustment container 300. If the volume of the introduction rate adjustment container 300 is too small, the frequency of replacement of the physical blowing agent will increase, which may cause the temperature of the physical blowing agent to become unstable, and as a result, the supply of the physical blowing agent may become unstable. Therefore, it is preferable that the introduction rate adjustment container 300 has a volume that can hold the amount of physical blowing agent consumed in the plasticizing cylinder in 1 to 10 minutes.
[0038] The introduction speed adjustment container 300 may be a separate container from the plasticizing cylinder 210, or it may be formed integrally with the plasticizing cylinder 210 and constitute a part of the plasticizing cylinder 210.
[0039] (3) Next, the molten resin is flowed into the starvation zone 23, and the molten resin in the starvation zone 23 is put into a starvation state (step S3 in Figure 1). The starvation state is determined by the balance between the amount of molten resin sent from upstream to the starvation zone 23 and the amount of molten resin sent from the starvation zone 23 downstream. If the former is less, the resin enters a starvation state.
[0040] In this embodiment, a compression zone 22 where the molten resin is compressed and the pressure increases is provided upstream of the starvation zone 23, thereby starving the molten resin in the starvation zone 23. The compression zone 22 has a large-diameter section 20A where the diameter of the screw 20 shaft is larger (thicker) than that of the plasticizing zone 21 located upstream, and the screw flight is made progressively shallower. Furthermore, a ring 26 is provided adjacent to the downstream side of the large-diameter section 20A, at the boundary with the starvation zone 23. The ring 26 has a split structure and is installed by dividing it into two parts and placing them over the screw 20. The large-diameter section 20A and the ring 26 reduce the clearance between the inner wall of the plasticizing cylinder 210 and the screw 20 by increasing the diameter of the screw 20 shaft, thereby reducing the amount of resin supplied downstream and increasing the flow resistance of the molten resin. Therefore, in this embodiment, the large-diameter section 20A and the ring 26 are mechanisms that increase the flow resistance of the molten resin. Furthermore, the ring 26 also has the effect of suppressing the backflow of the physical foaming agent, that is, the movement of the physical foaming agent from the downstream side to the upstream side of the seal portion 26.
[0041] The presence of the large-diameter portion 20A and the ring 26 reduces the resin flow rate supplied from the compression zone 22 to the starvation zone 23. In the upstream compression zone 22, the molten resin is compressed and the pressure increases, while in the downstream starvation zone 23, the molten resin remains incomplete (starvation state). To promote the starvation state of the molten resin, the screw 20 has a smaller (thinner) shaft diameter in the portion located in the starvation zone 23 compared to the portion located in the compression zone 22, and the screw flight is deeper.
[0042] The mechanism for increasing the flow resistance of the molten resin provided in the compression zone 22 is not particularly limited as long as it is a mechanism that temporarily reduces the flow area of the molten resin in order to limit the flow rate of resin supplied from the compression zone 22 to the starvation zone 23. In this embodiment, both the large-diameter portion 20A of the screw and the ring 26 are used, but only one may be used. Other mechanisms for increasing flow resistance besides the large-diameter portion 20A of the screw and the ring 26 include a structure in which the screw flight is provided in the opposite direction to the other parts, and a labyrinth structure provided on the screw.
[0043] The mechanism for increasing the flow resistance of molten resin may be provided on the screw as a ring or the like which is a separate member from the screw, or may be provided integrally with the screw as a part of the structure of the screw. If the mechanism for increasing the flow resistance of molten resin is provided as a ring or the like which is a separate member from the screw, the size of the clearance that serves as the flow path for the molten resin can be changed by changing the ring, which has the advantage that the magnitude of the flow resistance of the molten resin can be easily adjusted.
[0044] In addition to the mechanism for increasing the flow resistance of molten resin, a backflow prevention mechanism (sealing mechanism) for preventing backflow of molten resin from the starvation zone 23 to the upstream compression zone 22 may be provided between the compression zone 22 and the starvation zone 23, which can also maintain the molten resin in a starved state in the starvation zone 23. Examples of such sealing mechanism include a ring movable toward the upstream side under the pressure of the physical blowing agent, a steel ball, and the like. However, since the backflow prevention mechanism requires a driving part, there is a risk of resin retention. For this reason, a mechanism that increases flow resistance without having a driving part is preferable.
[0045] In the present embodiment, in order to stabilize the starved state of the molten resin in the starvation zone 23, the supply amount of the thermoplastic resin supplied to the plasticizing cylinder 210 may be controlled. This is because it becomes difficult to maintain the starved state if the supply amount of the thermoplastic resin is too large. In the present embodiment, a general-purpose feeder screw 212 is used to control the supply amount of the thermoplastic resin. By limiting the supply amount of the thermoplastic resin, the metering speed of the molten resin in the starvation zone 23 becomes higher than the plasticizing speed in the compression zone 22. As a result, the density of the molten resin in the starvation zone 23 is stably reduced, and the permeation of the physical blowing agent into the molten resin is promoted.
[0046] In this embodiment, the length of the starvation zone 23 in the front-rear direction is preferably long in order to ensure sufficient contact area and contact time between the molten resin and the physical foaming agent. However, if it is too long, it will result in the disadvantage of longer molding cycles and screw length. For this reason, the length of the starvation zone 23 is preferably 2 to 12 times the inner diameter of the plasticizing cylinder 210, and more preferably 4 to 10 times. Furthermore, it is preferable that the length of the starvation zone 23 covers the entire range of the metering stroke in injection molding. That is, it is preferable that the length of the starvation zone 23 in the flow direction of the molten resin is greater than or equal to the length of the metering stroke in injection molding. As the resin is plasticized, metered, and injected, the screw 20 moves forward and backward, but by making the length of the starvation zone 23 greater than or equal to the length of the metering stroke, the inlet 202 can always be positioned (formed) within the starvation zone 23 during the production of the foamed molded product. In other words, even if the screw 20 moves forward and backward during the manufacturing of the foamed molded product, zones other than the starvation zone 23 will never be located at the position of the inlet 202. As a result, the physical foaming agent introduced from the inlet 202 is always introduced into the starvation zone 23 during the manufacturing of the foamed molded product. The length of the starvation zone 23 in this embodiment is, as shown in Figure 2, the length from downstream of the ring 26 on the screw 20 to upstream of the recompression zone 24, which will be described later, where the molten resin is compressed and the pressure increases. In the starvation zone 23 of this embodiment, the diameter of the screw 20 shaft and the depth of the screw flight are constant.
[0047] (4) Contact between molten resin and physicoblasting agent Next, in the starvation zone 23, the molten resin in a starvation state is brought into contact with a physicoblasting agent at a constant pressure (first pressure P1) (step S4 in Figure 1). That is, in the starvation zone 23, the molten resin is pressurized at a constant pressure by the physicoblasting agent. Since the starvation zone 23 is not filled with molten resin (starvation state) and there is space for the physicoblasting agent to exist, the physicoblasting agent and the molten resin can be brought into contact efficiently. The physicoblasting agent that comes into contact with the molten resin is absorbed into the molten resin and consumed. When the physicoblasting agent is consumed, the physicoblasting agent remaining in the introduction rate adjustment container 300 is supplied to the starvation zone 23, and the molten resin continues to be in contact with the physicoblasting agent at a constant pressure. The starvation zone 23 is always maintained at a constant pressure (first pressure P1).
[0048] Note that, when the pressure in the starvation zone 23 (the first pressure P1) is "constant", it means that the fluctuation range of the pressure relative to the predetermined pressure is preferably within ±10%, more preferably within ±5%. The pressure in the starvation zone 23 is measured, for example, by a pressure sensor (not shown) provided in the starvation zone 23, such as at a position facing the introduction port 202 of the plasticizing cylinder 210.
[0049] In foam molding using conventional physical blowing agents, a predetermined amount of high-pressure physical blowing agent is forcibly introduced into a plasticizing cylinder within a predetermined period of time. Therefore, it is necessary to boost the pressure of the physical blowing agent to a high pressure and accurately control the introduction amount, introduction time, etc. of the physical blowing agent into the molten resin, and the physical blowing agent only contacts the molten resin for a short introduction time. In contrast, in the present embodiment, instead of forcibly introducing the physical blowing agent into the plasticizing cylinder 210, the physical blowing agent at a constant pressure is continuously supplied into the plasticizing cylinder, and the physical blowing agent is continuously brought into contact with the molten resin. This stabilizes the dissolved amount (permeation amount) of the physical blowing agent into the molten resin, which is determined by temperature and pressure. In addition, since the physical blowing agent of the present embodiment is always in contact with the molten resin, a necessary and sufficient amount of the physical blowing agent can permeate into the molten resin. As a result, the foam molded article produced in the present embodiment has fine foam cells, even though a lower-pressure physical blowing agent is used compared with conventional molding methods using physical blowing agents.
[0050] Furthermore, since the manufacturing method of the present embodiment does not require controlling the introduction amount, introduction time, etc. of the physical blowing agent, driving valves such as check valves and solenoid valves, as well as control mechanisms for controlling these valves, are eliminated, enabling reduction in equipment cost. In addition, since the physical blowing agent used in the present embodiment has a lower pressure than conventional physical blowing agents, the load on the equipment is also reduced.
[0051] In this embodiment, all steps of the manufacturing method for the foamed molded body are carried out while continuously supplying the physical foaming agent at a constant pressure to replenish the physical foaming agent consumed in the plasticizing cylinder. Furthermore, in this embodiment, for example, when performing injection molding of multiple shots in succession, even while the injection process, the cooling process of the molded body, and the removal process of the molded body are being carried out, the molten resin for the next shot is prepared in the plasticizing cylinder, and the molten resin for the next shot is pressurized at a constant pressure by the physical foaming agent. In other words, in injection molding of multiple shots performed in succession, the molten resin and the physical foaming agent at a constant pressure are always present in contact with each other in the plasticizing cylinder, that is, the molten resin is constantly pressurized at a constant pressure by the physical foaming agent in the plasticizing cylinder, and one cycle of injection molding, including the plasticizing metering process, injection process, cooling process of the molded body, and removal process, is carried out in this state.
[0052] (5) Measuring the molten resin Next, a predetermined amount of molten resin that has come into contact with the physical foaming agent is measured (step S5 in Figure 1). Figure 3(a) shows the state during the measurement of the molten resin, and Figure 3(b) shows the state when the measurement of the molten resin is completed. The plasticizing cylinder 210 used in this embodiment is located downstream of the starvation zone 23 and adjacent to the starvation zone 23, and has a recompression zone 24 in which the molten resin is compressed and its pressure increases. First, the molten resin in the starvation zone 23 is made to flow into the recompression zone 24 by the forward rotation of the screw 20. The molten resin containing the physical foaming agent is pressure-regulated in the recompression zone 24. By further forward rotation of the screw 20, the molten resin is sent forward to the plasticizing cylinder 210, and the screw 20 retracts due to the pressure of the resin. As shown in Figures 3(a) and (b), when the screw 20 retracts, a metering zone 25 is formed in front of the screw 20, and a predetermined amount of molten resin (molten resin for one shot) is metered into the metering zone 25.
[0053] In this embodiment, from the start of plasticization of the thermoplastic resin until the completion of metering of the molten resin (during resin plasticization and metering), a screw back pressure (second pressure P2) higher than the constant pressure (first pressure P1) in the starvation zone 23 is applied to the screw 20. When screw back pressure is applied, the pressure in the metering zone 25 (pressure of the molten resin in front of the screw 20) is equal to the screw back pressure. Therefore, until the completion of metering of the molten resin, the pressure in the metering zone 25 is maintained at a pressure higher than the constant pressure in the starvation zone 23 (second pressure P2). This suppresses the separation of the physical foaming agent from the molten resin and promotes uniform miscibility. On the other hand, if the screw back pressure during plasticization and metering is too high, the pressure in the metering zone 25 becomes too high. This makes it difficult to send the molten resin in front of the tip sealing mechanism 50, and there is a risk that the molten resin will vent up from the inlet 202 of the starvation zone 23. From these perspectives, the screw back pressure (second pressure P2) until the metering of the molten resin is completed is, for example, 0.4 MPa to 4 MPa higher, preferably 0.5 MPa to 4 MPa higher, and more preferably 0.5 MPa to 3 MPa higher, than the constant pressure (first pressure P1) in the starvation zone 23. Alternatively, the metering back pressure may be increased in multiple stages just before metering. For example, it can be increased to 0 to 4 MPa up to a distance of 1 to 4 mm before the metering completion position, and to 0.5 to 10 MPa from 1 to 4 mm before the metering completion position to the metering completion position. By keeping the back pressure as low as possible to suppress vent-up and maintain the kneadability of the blowing agent just before the metering completion position, and then increasing the back pressure just before metering, it becomes easier to suppress the separation of the physical blowing agent.
[0054] (6) Foam molding Next, the molten resin measured by contacting it with a physical foaming agent is molded into a foamed molded body (step S6 in Figure 1). For injection foam molding, a short shot method may be used in which molten resin filling to 75% to 95% of the mold cavity volume is filled into the mold cavity at a predetermined injection pressure, and the mold cavity is filled while bubbles expand. Alternatively, a core-back method may be used in which molten resin filling to 100% of the mold cavity volume is used, and then the cavity volume is expanded to cause foaming. The resulting foamed molded body has foam cells inside, so shrinkage during cooling of the thermoplastic resin is suppressed, reducing sink marks and warping, and a low specific gravity molded body can be obtained.
[0055] In the manufacturing method of this embodiment described above, there is no need to control the amount and time of introduction of the physical blowing agent into the molten resin, so complex control devices can be omitted or simplified, and equipment costs can be reduced. Furthermore, in the manufacturing method of the foamed molded article of this embodiment, the molten resin in a starved state and the physical blowing agent at a constant pressure are brought into contact in the starved zone 23. This allows the amount of dissolution (penetration) of the physical blowing agent into the molten resin to be stabilized by a simple mechanism.
[0056] The present inventors have discovered a phenomenon in a method for manufacturing a foamed molded article (injection foam molding method) using the molding apparatus 1000 shown in Figure 2 used in this embodiment, in which the pressure of the metered molten resin decreases between the completion of metering the molten resin and the start of injection.
[0057] Figure 4 shows an example of the change in pressure (pressure waveform) in the metering zone 25 when this phenomenon occurs. Figure 4 shows a pressure line L1 that indicates the change in pressure in the metering zone 25 from the injection process of the previous shot and the start of plasticization to the start of injection of the following shot, as detected by the pressure sensor 261 (load cell) of the screw drive mechanism 260. Figure 4 also shows a pressure line L2 that indicates the pressure in the starvation zone 23, i.e., the constant first pressure P1 of the physical blowing agent. As mentioned above, screw back pressure (second pressure P2) refers specifically to the pressure of the load cell 261 during metering, but after metering is complete, the pressure of the same load cell 261 is specifically referred to as screw tip pressure (third pressure P3) in this specification.
[0058] As shown in the diagram, in the conventional process, after closing the mold 251 and clamping it (shown as "mold closing and clamping" in Figure 4), injection molding is performed at a predetermined injection pressure (shown as "injection holding pressure" in Figure 4), and immediately thereafter the metering process (shown as "metering" in Figure 4) begins (pressure line L1 becomes screw back pressure (second pressure) P2). Therefore, there is a certain time between the completion of metering and the subsequent injection (the next "mold closing and clamping" and "injection holding pressure" for the metered molten resin). As time passes (during which time the molded body from the previous shot is removed from the mold 251, as shown in Figure 4 as "mold opening and product removal"), the third pressure P3, which is the screw tip pressure, drops to the same pressure as the first pressure P1 after metering is complete (see line L11). When injection starts at the predetermined injection pressure, the shut-off valve 28 opens, causing the screw tip pressure (third pressure P3) just before injection to decrease (i.e., the pressure relationship becomes P3 ≤ P1 ≤ P2).
[0059] Specifically, as shown in the time chart in Figure 5, in this conventional process, the safety door of the mold 251 is closed (during the semi-automatic operation described above) (step S10), and after the clamping of the mold 251 is completed (step S21), the shut-off valve 28 is opened with a slight delay while the mold is still clamped (step S22) (step S31). As a result, the injected molten resin is filled into the cavity 253 of the mold 251 (step S41), and then the shut-off valve 28 is closed while the mold is being held under pressure (step S42) (step S32). After the holding pressure is maintained, with a predetermined delay, the metering process (step S51) is performed after the shut-off valve 28 is closed, as shown as "conventional" in Figure 5 (during this time, the molded body of the mold 251 is cooled (step S71)). After metering is complete, the metering position of the screw 20 is maintained (step S52). However, a certain amount of time elapses before the metered molten resin is injected, due to processes such as mold opening (step S23), opening of the safety door due to motor power loss (step S11), and removal of the molded body (product) from the mold 251 (step S12). As a result, a decrease in screw tip pressure PD (see Figure 5) occurs, as shown in line L11 of Figure 4. Furthermore, because a large amount of metered and pressurized resin accumulates at the screw tip, the screw 20 is pushed backward by the resin pressure when the motor power that maintains the position of the screw 20 is lost, making it impossible to continue molding.
[0060] Therefore, in order to address these problems, in this embodiment, after the metering of the molten resin is completed, the injection of the molten resin from the plasticizing cylinder 210 is performed when the third pressure P3, which is the screw tip pressure after metering (transitioned from the screw back pressure), is higher than the first pressure P1 of the physical foaming agent, that is, before the physical foaming agent separates from the molten resin due to the pressure drop phenomenon. Specifically, the metering process, which was started immediately after injection in the conventional process described above, is delayed (in this case, the screw 20 waits in the forward position), and the metering process is started after a predetermined delay time T, so that the metering process is completed just before the start of injection. More preferably, the screw back pressure is increased just before the metering completion position. More specifically, as shown in the time chart of Figure 5, the shut-off valve 28 is opened (step S31), the molten resin from the previous shot is filled into the cavity 253 of the mold 251 (step S41), and then, after the shut-off valve 28 is closed while the pressure is held (step S42) (step S32), with a predetermined delay T, the metering process (step S61) is performed immediately before the start of injection (without giving time for the screw back pressure (second pressure) P2 during metering to decrease), as shown as "the present invention" in Figure 5. Here, as an example, metering is performed during mold opening (step S23) in conjunction with filling the mold 251. As shown in Figure 6, which is a pressure waveform diagram corresponding to Figure 4 mentioned above, the screw back pressure P2 is increased just before the metering is complete. Injection (step S41) is started within 1 to 2 seconds after the third pressure P3, which is the screw tip pressure after metering, begins to decrease (with a certain time lag, and of course at a pressure higher than the first pressure P1 of the physical foaming agent). As a result, as shown in line L12 in Figure 6, the molten resin can be injected at a pressure P3 just before the start of injection, which is almost the same as the second pressure P2 at the time of metering (i.e., the pressure relationship is P1 ≤ P3 ≤ P2). In this way, by increasing the screw back pressure just before metering, the difference between P2 and P1 can be further increased (for comparison, line L12 just before injection is also shown in Figure 4). Furthermore, because the screw 20 moves to its furthest forward position, reducing the amount of resin that accumulates in front, the screw 20 is less likely to retract due to resin pressure even if the power supply is cut off.
[0061] The present invention will be further described below with reference to examples and comparative examples. However, the present invention is not limited to the examples and comparative examples described below.
[0062] [Example] In this example, the shut-off valve 28 is, for example, a needle valve, and a pressure gauge is placed directly below the pressure vessel 300 into which the nitrogen pressure, which is the foaming agent, is introduced, to monitor the pressure inside the plasticizing cylinder 210. Since the molten resin is in a starved state, the nitrogen pressure is approximately the same as that of the pressure sensor. The pressure reducing valve 151 is used to adjust the first pressure P1 of the foaming agent to 6 MPa, and the pressure gauge 152 is set to constantly display 6 ± 0.5 MPa. PP containing 17% talc was used as the resin. In this example, in fully automatic operation, the metering delay time was set so that the shut-off valve 28 is opened and injection occurs within 1 second after metering is completed. The product was weighed at a rotation speed of 100 rpm and a pressure of 6 MPa, the same as the foaming agent, up to 2 mm before the weighing position. The remaining 2 mm was then weighed under back pressure conditions of 12 MPa, 6 MPa higher than the foaming agent, after the cooling time was complete and during mold opening. The product was removed, and the weighing delay time T was adjusted so that weighing was completed 1 to 1.5 seconds after the mold clamping was complete.
[0063] The simple flat plate shape was injected into a center-direct gate mold 251 with a thickness of 2 mm, a width of 100 mm, and a length of 200 mm, with the mold temperature adjusted to 40°C. The screw tip pressure P3, monitored by a load cell 261 just before the start of injection, temporarily decreased due to the release of the shut-off but remained in the range of 8-9 MPa, which is higher than the pressure of the foaming agent, and the injection peak pressure was 80 MPa. Injection foam molding was performed with zero holding pressure and a primary filling speed of 80 mm / s.
[0064] The weight reduction rate compared to solid molding was 15%, and the foamed cells near the end of the flow, where the cell diameter tends to increase due to pressure loss, were evaluated. The average cell diameter 5 mm inward from the end was measured by CT scan. The cell size in a 2 mm cube per field of view was analyzed using image analysis. In this example, the average cell diameter was approximately 50 μm, and no large variation was observed. After cutting out the molded product and preparing evaluation test pieces, the flexural modulus was measured. It was 13% lower than that of solid, which was equivalent to the weight reduction rate.
[0065] [Comparative Example] Weighing was performed immediately after injection filling under the same conditions as in the example. Three seconds after the completion of weighing, the pressure at the screw tip decreased to 6 MPa, the same pressure as the foaming agent. Thereafter, injection filling was performed in the same manner as in the example. Immediately before the start of injection, the pressure at the screw tip decreased to 2 MPa. Observation of the foamed cells at the end of the flow revealed that the average cell diameter had enlarged to 190 μm, and the maximum was approximately 500 μm, indicating cell unification. It is presumed that the nitrogen gas, which is the foaming agent, separated from the resin and foamed before injection, causing the cells in the molded product to enlarge. When the weight reduction ratio was 15%, the flexural modulus decreased by 20% compared to the solid, and it was considered that the rate of strength reduction worsened due to the enlargement of the cell diameter.
[0066] It should be noted that the present invention is not limited to the embodiments described above, and can be implemented in various ways without departing from its spirit. For example, in the present invention, the configuration of the manufacturing apparatus, etc., is not limited to the embodiments described above. Furthermore, without departing from the spirit of the present invention, some or all of the embodiments described above may be combined, or some of the configurations of one of the embodiments described above may be omitted.
[0067] 20 Screw 21 Plasticizing Zone 23 Starvation Zone 100 Cylinder (Physical Foaming Agent Supply Mechanism) 202 Inlet 210 Plasticizing Cylinder 1000 Manufacturing Equipment
Claims
1. A method for manufacturing a foamed molded article, comprising a manufacturing apparatus including a plasticizing cylinder having a screw provided inside that can rotate and move back and forth, a plasticizing zone in which a thermoplastic resin is plasticized and melted to become a molten resin, and a starving zone in which the molten resin is in a starving state, and an inlet formed for introducing a physical blowing agent into the starving zone, the manufacturing method comprising: plasticizing and melting the thermoplastic resin in the plasticizing zone to become the molten resin; introducing the physical blowing agent, adjusted to a constant first pressure in the range of 1 to 20 MPa, into the starving zone, and bringing the molten resin in the starving state into contact with the physical blowing agent at the first pressure; measuring the molten resin that has been in contact with the physical blowing agent at the first pressure using a second pressure which is a screw back pressure 0 to 10 MPa higher than the first pressure; and injecting the measured molten resin from the plasticizing cylinder to form the foamed molded article, A manufacturing method characterized in that, after the metering of the molten resin is completed, the molten resin is injected from the plasticizing cylinder while the third pressure, which is the screw tip pressure after metering, is higher than the first pressure of the physical foaming agent.
2. The manufacturing method according to claim 1, characterized in that the molten resin is injected from the plasticizing cylinder within 0 to 10 seconds immediately after the completion of metering of the molten resin.
3. A manufacturing apparatus for producing a foamed molded article, comprising: a plasticizing cylinder having a screw provided inside that can rotate and move back and forth, a plasticizing zone in which a thermoplastic resin is plasticized and melted to become molten resin, and a starving zone in which the molten resin is in a starving state, and which measures a certain amount of the molten resin and injects it to the outside; and a physical blowing agent supply mechanism that supplies a physical blowing agent adjusted to a constant first pressure in the range of 1 to 20 MPa to the starving zone of the plasticizing cylinder through an inlet formed in the starving zone, wherein the molten resin that has come into contact with the physical blowing agent at the first pressure is measured at a second pressure which is a screw back pressure 0 to 10 MPa higher than the first pressure, and the measured molten resin is injected from the plasticizing cylinder when the third pressure which is the screw tip pressure after measuring is higher than the first pressure of the physical blowing agent.
4. The manufacturing apparatus according to claim 3, characterized in that the molten resin is injected from the plasticizing cylinder within 0 to 10 seconds immediately after the completion of metering of the molten resin.