Foam injection molding method and foam injection molding apparatus

Abstract

To provide a method and an apparatus for foam-injection molding which can regulate the retreat of a screw when a safety door is opened (when a motor power source is cut off) even in physical foam-molding and even in a motor without a brake (an electric or hydraulic plasticization metering motor), and can cope with the revision of "JISB6711".SOLUTION: In the foam injection-molding method of the present invention, in the plasticizing step, under the condition that the back pressure of the screw 20 is equal to or less than the 8MPa, after the safety door of the clamping portion 250 for taking out the molded product is opened and the power of the plasticizing and metering motor is turned off, the position of the screw 20 is confirmed before the injection filling of the next shot by closing the safety door, and the screw 20 is returned to the metering completion position.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a foam injection molding method and a foam injection molding apparatus. [Background technology]

[0002] Foam injection molding is widely used to reduce resin usage and improve dimensional accuracy by eliminating warpage and sink marks. Foam molding methods are broadly divided into chemical and physical types, with nitrogen gas, carbon dioxide gas, carbon monoxide, ammonia gas, and water vapor being the primary blowing agents. Among these, methods using physical blowing agents, such as inert gases like carbon dioxide and nitrogen, are increasingly being put to practical use due to their low environmental impact. Examples of practical applications include molding methods using high-pressure supercritical fluids and molding methods using low-pressure gases (Patent Document 1). The method described in Patent Document 1, in particular, does not require special high-pressure equipment and can produce foam molding with a low environmental impact by simply replacing the plasticizing screw and cylinder.

[0003] Meanwhile, on March 25, 2021, JIS B6711, the safety standard for injection molding machines, was revised to comply with ISO20430. The requirements of the standard include changes to safety standards for semi-automatic molding (override). As a result, tasks that were previously performed by humans, such as removing molded products from molds by opening safety doors during molding condition setting or the production cycle, must now be replaced with machinery that can perform the tasks without opening the safety doors, or special safety devices must be added.

[0004] In general, in an injection molding apparatus, a movable mold is moved relative to a fixed mold by a mold clamping unit to close the mold, resin is injected into the mold, and then the mold clamping unit is driven to open the mold and remove the molded product. Therefore, the mold clamping unit must be accessible from the outside when installing the mold in the mold clamping unit, when performing maintenance on the mold clamping unit, or when manually removing the molded product. However, because the mold clamping unit opens and closes the mold, there is a risk of an operator's hand or other object being caught in the mold. For this reason, a safety measure has traditionally been implemented, in which a safety gate is provided in the mold clamping unit to ensure that the mold clamping unit can be operated only when the safety gate is closed. The revision of "JIS B6711" can be said to have newly added requirements related to this safety gate.

[0005] When setting molding conditions, conventionally, a safety door was opened semi-automatically to remove the product. This would cut off the power to the motor that drives the molding. In the case of foam molding, the high pressure at the tip of the plasticizing screw inside the cylinder that flows the resin would cause the screw to retreat, making it impossible to set molding conditions. To avoid this, an electric motor with a brake mechanism has been installed.

[0006] Examples of electric motors with brake mechanisms are disclosed in, for example, Patent Document 2 and Patent Document 3. In particular, in the electric vertical injection molding machine disclosed in Patent Document 3, a brake is provided on the motor for the mold clamping device, and this brake serves as a safety device. In other words, the brake is activated when the power to the motor is turned off, and the frictional force of the brake restricts the rotation of the motor. For the motor's braking function, an electromagnetic brake is often used, in which a voltage is applied to an excitation coil when power is applied, causing the brake to be released. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6139038 [Patent Document 2] Patent No. 3201304 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-199449 Summary of the Invention [Problem to be solved by the invention]

[0008] In semi-automatic operation, if the power to the electric motor drops when removing the molded product through the safety door, if the mold clamping device is installed horizontally, the power to the motor on the mold clamping side will drop with the mold open, so even if there is no brake function, the movable platen and movable mold will not move, and there will be no problems with safety or the next cycle.

[0009] On the other hand, on the plasticization side, the screw stops at the metering completion position after metering is complete. In this case, the plasticization metering motor applies screw back pressure to the resin in front of the screw, but in general solid molding, the screw back pressure is relatively low at around 4 MPa or less. If the plasticization metering motor goes out in this state, the resin pressure at the tip, that is, the pressure that pushes the screw backward, is low, so even if the motor does not have a brake function, the screw's weight and the frictional force of the molten resin make it difficult for the screw to backward. This means that it is unlikely to have a negative impact on safety or the next cycle.

[0010] In contrast, in the case of physical foam molding, when the pressure of the introduced physical foaming agent is high, the pressure at the tip of the screw increases, causing the screw to retract as described above, making it difficult to determine molding conditions.

[0011] The present invention has been made in consideration of the above circumstances, and aims to provide a foam injection molding method and foam injection molding device that can regulate the retraction of the screw when the safety door is opened (when the motor power is cut off), even in physical foam molding, and even when a motor without a brake (an electric or hydraulic plasticization metering motor) is used, and that can also comply with the revision of "JIS B6711." [Means for solving the problem]

[0012] In order to solve the above problems, the present invention provides a foam injection molding method that performs a metering process by semi-automatically operating a foam injection molding device, in which a plasticization metering motor without a brake is driven to move the screw backward to a plasticization metering completion position, thereby accumulating the supplied molten resin in front of the screw and metering the molten resin, and an injection process by driving an injection motor to move the screw forward, thereby filling the molten resin accumulated in front of the screw through an injection nozzle into a cavity of a mold in a mold clamping section, In the metering process, the back pressure of the screw is 8 MPa or less, and the molten resin containing the foaming agent is plasticized and metered. After the safety door of the mold clamping section is opened to remove the molded product and the power of the plasticization and metering motor is turned off, the position of the screw is confirmed and the screw is returned to the metering completion position before the next shot of injection filling is performed by closing the safety door.

[0013] According to the above-described configuration of the present invention, during the metering process, the molten resin containing the foaming agent is plasticized and metered under conditions where the screw back pressure is 8 MPa or less. Therefore, even in physical foam molding, and even when using a motor without a brake (an electric or hydraulic plasticization metering motor), the screw can be prevented from retracting after the safety door is opened and the power to the plasticization metering motor is cut off. This configuration also fully complies with the revision of "JIS B6711." Furthermore, because the screw position is confirmed and the screw is returned to the metering completion position before the safety door is closed to inject and fill the next shot, molding conditions can be reliably determined.

[0014] The present invention also provides a foam injection molding apparatus that semi-automatically performs a metering step of metering the molten resin supplied from the physical foaming agent supply mechanism by driving a plasticizing cylinder having a screw therein for causing the molten resin to flow, a physical foaming agent supply mechanism for supplying a physical foaming agent to the plasticizing cylinder, a mold clamping section in which a mold is provided and which has a safety door for removing a molded product, and a control section for controlling the operation of the screw, and a foam injection molding apparatus that semi-automatically performs a metering step of metering the molten resin supplied from the physical foaming agent supply mechanism by driving a plasticizing metering motor without a brake to move the screw backward to a plasticizing metering completion position, and an injection step of driving an injection motor to move the screw forward, thereby filling the molten resin accumulated in front of the screw through an injection nozzle of the plasticizing cylinder into a cavity of the mold in the mold clamping section, The control unit is characterized in that it cuts off the power to the plasticization metering motor when the safety door is opened during semi-automatic operation, and if it detects that the screw has further retreated from the plasticization metering completion position, it returns the screw to the metering completion position with an arbitrary back pressure before the next shot is injected and filled by closing the safety door.

[0015] According to the above-described configuration of the present invention, when the safety door is opened during semi-automatic operation, the control unit cuts off the power to the plasticization metering motor, and if it detects that the screw has further retreated from the plasticization metering completion position, it returns the screw to the metering completion position with an arbitrary back pressure before the next shot of injection filling is performed by closing the safety door, thereby making it possible to reliably set the molding conditions.

[0016] In the above configuration, the control unit preferably drives the screw backward during the metering process to plasticize and meter the molten resin containing the foaming agent under the condition that the screw back pressure is 8 MPa or less. This allows the molten resin containing the foaming agent to be plasticized and metered under the condition that the screw back pressure is 8 MPa or less. Therefore, even in physical foam molding, and even in the case of a motor without a brake (an electric or hydraulic plasticizing metering motor), the screw can be prevented from retracting after the safety door is opened and the power to the plasticizing metering motor is cut off. Furthermore, this configuration can fully comply with the revisions to "JIS B6711."

[0017] In the above configuration, "semi-automatic operation" refers to automatic operation in which human operation is partially involved, such as when a worker removes a molded product through a safety door. [Effects of the Invention]

[0018] The foam injection molding method and foam injection molding device of the present invention can regulate the retraction of the screw when the safety door is opened (when the motor power is cut off), even in physical foam molding and even when using a motor without a brake (an electric or hydraulic plasticization metering motor), and can also comply with the revision of "JIS B6711". [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a flowchart illustrating a foam injection molding method according to one embodiment of the present invention. [Figure 2] 1 is a schematic diagram of a foam injection molding apparatus according to an embodiment of the present invention. [Figure 3] FIG. 2 is a schematic diagram of a screw operating mechanism. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. This embodiment contributes to the achievement of "9. Industry, innovation and infrastructure" of the Sustainable Development Goals (SDGs) advocated by the United Nations, which states, "9.1 Develop quality, reliable, sustainable and resilient infrastructure, including regional and transborder infrastructure, to support economic development and human well-being, with a focus on affordable and equitable access for all."

[0021] The foam injection molding method of this embodiment for producing a foam molded article will be described with reference to the flowchart shown in FIG. (1) Foam injection molding equipment First, a foam injection molding apparatus for producing a foam molded article will be described. In this embodiment, a foam molded article is produced using a foam injection molding apparatus 1000 shown in FIG. 2. The apparatus 1000 mainly comprises a plasticizing cylinder 210 having a screw 20 installed therein, a cylinder 100 serving as a physical foaming agent supply mechanism for supplying a physical foaming agent to the plasticizing cylinder 210, a mold clamping unit (mold clamping section) 250 having a mold 251 installed therein, and a control unit (not shown) for controlling the operation of the plasticizing cylinder 210 and the mold clamping unit 250. The molten resin plasticized and melted in the plasticizing cylinder 210 flows from the right hand to the left hand in FIG. 2. Therefore, within the plasticizing cylinder 210 of this embodiment, the right hand in FIG. 2 is defined as "upstream" or "rear," and the left hand as "downstream" or "front."

[0022] The plasticizing cylinder 210 has a plasticizing zone 21 where the thermoplastic resin is plasticized and melted to form a molten resin, and a starvation zone 23 downstream of the plasticizing zone 21 where the molten resin is starved. The "starvation state" refers to a state in which the molten resin does not fill the starvation zone 23, leaving it underfilled. Therefore, there is space in the starvation zone 23 other than the area occupied by the molten resin. An inlet 202 is formed to introduce a physical foaming agent into the starvation zone 23, and an introduction rate adjusting container 300 is connected to the inlet 202. The cylinder 100 supplies the physical foaming agent to the plasticizing cylinder 210 via the introduction rate adjusting container 300.

[0023] Although the apparatus 1000 has only one starvation zone 23, the apparatus 1000 used in this embodiment is not limited to this. For example, to promote the penetration of the physical foaming agent into the molten resin, the apparatus may have a plurality of starvation zones 23 and inlet ports 202 formed therein, and the physical foaming agent may be introduced into the plasticizing cylinder 210 through the plurality of inlet ports 202.

[0024] (2) Foam injection molding method First, in the plasticizing zone 21 of the plasticizing cylinder 210, a thermoplastic resin is plasticized and melted to form a molten resin (step S1 in FIG. 1). Various thermoplastic resins can be used depending on the type of molded article desired. Specifically, thermoplastic resins such as polypropylene, polymethyl methacrylate, polyamide, polycarbonate, amorphous polyolefin, polyetherimide, polyethylene terephthalate, polyether ether ketone, ABS resin (acrylonitrile-butadiene-styrene copolymer resin), polyphenylene sulfide, polyamide-imide, polylactic acid, and polycaprolactone, as well as composite materials thereof, can be used. These thermoplastic resins can be used alone or in combination. These thermoplastic resins can also be blended with various inorganic fillers, such as glass fiber, talc, and carbon fiber. It is preferable to blend the thermoplastic resin with an inorganic filler that functions as a foam nucleating agent or an additive that increases melt tension. By blending these, the foam cells can be made finer. The thermoplastic resin of this embodiment may also contain various other general-purpose additives as needed.

[0025] In this embodiment, the thermoplastic resin is plasticized and melted in a plasticizing cylinder 210 in which the screw 20 shown in Fig. 2 is installed. A band heater (not shown) is provided on the outer wall surface of the plasticizing cylinder 210, which heats the plasticizing cylinder 210. Furthermore, shear heat generated by the rotation of the screw 20 is also added, causing the thermoplastic resin to be plasticized and melted.

[0026] Next, a physical blowing agent at a constant pressure is introduced into the starvation zone 23, and the starvation zone 23 is maintained at this constant pressure (step S2 in FIG. 1). A pressurized fluid is used as the physical blowing agent. In this embodiment, "fluid" refers to any of liquid, gas, and supercritical fluid. Furthermore, from the viewpoints of cost and environmental impact, carbon dioxide, nitrogen, and the like are preferred as the physical blowing agent. Because the pressure of the physical blowing agent in this embodiment is relatively low, it is possible to use a fluid extracted from a cylinder, such as a nitrogen cylinder, carbon dioxide cylinder, or air cylinder, after reducing the pressure to a constant level using a pressure reducing valve. This eliminates the need for a pressure booster, thereby reducing the overall cost of the system. If necessary, a fluid pressurized to a predetermined pressure may be used as the physical blowing agent. For example, when nitrogen is used as the physical blowing agent, the physical blowing agent can be generated as follows. First, atmospheric air is compressed by a compressor and passed through a nitrogen separation membrane to purify the nitrogen. Next, the purified nitrogen is pressurized to a predetermined pressure using a booster pump, syringe pump, or the like to generate the physical blowing agent.

[0027] The pressure of the physical foaming agent introduced into the starvation zone 23 is constant, and the pressure in the starvation zone 23 is maintained at the same constant pressure as the introduced physical foaming agent. The pressure of this physical foaming agent is preferably 1 MPa to 15 MPa, more preferably 2 MPa to 10 MPa, and even more preferably 2 MPa to 8 MPa. While the optimal pressure varies depending on the type of molten resin, a physical foaming agent pressure of 1 MPa or higher allows the amount of physical foaming agent required for foaming to penetrate into the molten resin, while a pressure of 15 MPa or lower reduces the load on the equipment. Note that the "constant" pressure of the physical foaming agent pressurizing the molten resin means that the pressure fluctuation range relative to the predetermined pressure is preferably within ±10%, more preferably within ±5%. The pressure in the starvation zone is measured, for example, by a pressure sensor (not shown) located opposite the inlet 202 of the plasticizing cylinder 210.

[0028] In this embodiment, as shown in FIG. 2, the physical foaming agent is supplied from the cylinder 100 through the introduction rate adjusting vessel 300 and then through the inlet 202 to the starvation zone 23. The physical foaming agent is first 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 pressure increasing device or other device. In this embodiment, the amount and time of the physical foaming agent introduced into the plasticizing cylinder 210 are not controlled. Therefore, a control mechanism, such as a check valve or solenoid valve, is not required. The inlet 202 does not have a valve and is always open. In this embodiment, the physical foaming agent supplied from the cylinder 100 maintains a constant pressure from the pressure reducing valve 151 through the introduction rate adjusting vessel 300 to the starvation zone 23 in the plasticizing cylinder 210.

[0029] The physical foaming agent inlet 202 has a larger inner diameter than the physical foaming agent inlet of conventional equipment. Therefore, even a relatively low-pressure physical foaming agent can be efficiently introduced into the plasticizing cylinder 210. Even if a portion of the molten resin comes into contact with the inlet 202 and solidifies, the large inner diameter allows the inlet to function as an inlet without being completely blocked. On the other hand, if the inner diameter of the inlet 202 is too large, the molten resin may stagnate, resulting in molding defects. Furthermore, the introduction rate adjusting vessel 300 connected to the inlet 202 may become larger, increasing the cost of the entire apparatus. 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.

[0030] The introduction rate adjusting container 300 connected to the introduction port 202 has a certain volume or more, which slows the flow rate of the physical foaming agent introduced into the plasticizing cylinder 210 and ensures the physical foaming agent's residence time within the introduction rate adjusting container 300. By remaining near the heated plasticizing cylinder 210, the physical foaming agent is heated, reducing the temperature difference between the physical foaming agent and the molten resin, thereby stabilizing the amount of physical foaming agent dissolved (permeated) into the molten resin. In other words, the introduction rate adjusting container 300 functions as a buffer container. On the other hand, if the introduction rate adjusting container 300's volume is too large, the overall cost of the device increases. The volume of the introduction rate adjusting 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 adjusting container 300 within this range, the physical foaming agent's residence time can be ensured while taking costs into consideration.

[0031] As will be described later, the physical foaming agent is consumed within the plasticizing cylinder 210 by contacting and penetrating the molten resin. To maintain a constant pressure in the starvation zone 23, the amount of physical foaming agent consumed is introduced into the starvation zone 23 from the introduction rate adjusting vessel 300. If the volume of the introduction rate adjusting vessel 300 is too small, the replacement frequency of the physical foaming agent increases, which may cause the temperature of the physical foaming agent to become unstable, resulting in an unstable supply of the physical foaming agent. Therefore, it is preferable that the introduction rate adjusting vessel 300 have a volume large enough to retain the amount of physical foaming agent consumed in the plasticizing cylinder over a period of 1 to 10 minutes.

[0032] The introduction rate adjusting container 300 may be a separate container from the plasticizing cylinder 210, or may be formed integrally with the plasticizing cylinder 210 to form a part of the plasticizing cylinder 210. In this embodiment, only the physical foaming agent is introduced into the starvation zone 23, but other pressurized fluids besides the physical foaming agent may be introduced into the starvation zone 23 at the same time, as long as the effects of the present invention are not affected. In this case, the pressurized fluid containing the physical foaming agent introduced into the starvation zone 23 has the aforementioned constant pressure.

[0033] Next, the molten resin is flowed into starvation zone 23, where the molten resin is put into a starvation state (step S3 in FIG. 1). The starvation state is determined by the balance between the amount of molten resin sent from upstream of starvation zone 23 to starvation zone 23 and the amount of molten resin sent from starvation zone 23 to its downstream; if the former is smaller, the starvation state occurs.

[0034] In this embodiment, the molten resin is starved using the method described below. The plasticizing cylinder 210 used in this embodiment has a compression zone 22, located upstream of and adjacent to the starvation zone 23, where the molten resin is compressed and pressure increases. In the compression zone 22, the diameter of the screw 20 shaft is larger (thicker) than in the plasticizing zone 21 located upstream, and a large-diameter portion 20A with gradually shallower screw flights is provided. Furthermore, a ring 26 is provided at the boundary with the starvation zone 23. The ring 26 has a split structure, and the two halves are installed to cover the screw 20. Increasing the diameter of the screw shaft reduces the clearance between the inner wall of the plasticizing cylinder 210 and the screw 20, reducing the amount of resin sent downstream and increasing the flow resistance of the molten resin. Additionally, providing the ring 26 on the screw 20 similarly increases the flow resistance of the molten resin. Therefore, in this embodiment, the large diameter portion 20A and the ring 26 are a mechanism for increasing the flow resistance of the molten resin.

[0035] 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. This compresses the molten resin in the upstream compression zone 22, increasing the pressure, and leaving the downstream starvation zone 23 underfilled with molten resin (i.e., starvation state). To promote starvation of the molten resin, the screw 20 has a smaller (thinner) shaft diameter and deeper screw flights in the portion located in the starvation zone 23, i.e., downstream of the ring 26, compared to the portion located in the compression zone 22, i.e., the portion upstream of the ring 26. Furthermore, it is preferable that the screw 20 has a smaller (thinner) shaft diameter and deeper screw flights throughout the starvation zone 23, compared to the portion located in the compression zone 22. Furthermore, it is preferable that the shaft diameter and screw flight depth of the screw 20 are approximately constant throughout the starvation zone 23. This maintains a substantially constant pressure in the starvation zone 23, stabilizing the starvation state of the molten resin. In this embodiment, the starvation zone 23 is formed in the screw 20 downstream of the ring 26, as shown in FIG. 2, in a portion where the diameter of the shaft of the screw 20 and the depth of the screw flights are constant.

[0036] 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 path area through which the molten resin passes in order to limit the amount 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 of them may be used. Mechanisms for increasing flow resistance other than the large diameter portion 20A of the screw and the ring 26 include a structure in which screw flights are provided in the opposite direction to other portions, a labyrinth structure provided on the screw, etc.

[0037] The mechanism for increasing the flow resistance of the molten resin may be provided on the screw as a separate member such as a ring, or may be provided integrally with the screw as part of the screw structure. If the mechanism for increasing the flow resistance of the molten resin is provided as a separate member such as a ring, the size of the clearance portion, which is the flow path for the molten resin, can be changed by changing the ring, which has the advantage of making it easy to change the magnitude of the flow resistance of the molten resin.

[0038] In addition to mechanisms for increasing the flow resistance of the molten resin, a backflow prevention mechanism (sealing mechanism) that prevents the molten resin from flowing back from the starvation zone 23 to the upstream compression zone 22 can also be installed at the boundary between the compression zone 22 and the starvation zone 23 to starve the molten resin in the starvation zone 23. For example, a sealing mechanism such as a ring or steel ball that can move upstream due to the pressure of the physical foaming agent can be used. However, because a backflow prevention mechanism requires a drive unit, there is a risk of resin stagnation. For this reason, a mechanism for increasing flow resistance that does not have a drive unit is preferable.

[0039] In this embodiment, the amount of thermoplastic resin supplied to the plasticizing cylinder 210 may be controlled to stabilize the starvation state of the molten resin in the starvation zone 23. This is because if the amount of thermoplastic resin supplied is too large, it becomes difficult to maintain the starvation state. For example, the amount of thermoplastic resin supplied is controlled using a general-purpose feeder screw.

[0040] In this embodiment, the length of the starvation zone 23 in the flow direction of the molten resin is preferably long to ensure the contact area and contact time between the molten resin and the physical foaming agent. However, if the length is too long, it can adversely affect the molding cycle and screw length. Therefore, the length of the starvation zone 23 is preferably 2 to 12 times, and more preferably 4 to 10 times, the inner diameter of the plasticizing cylinder 210. Furthermore, the length of the starvation zone 23 preferably covers the entire range of the metering stroke in injection molding. That is, the length of the starvation zone 23 in the flow direction of the molten resin is preferably equal to or longer than the length of the metering stroke in injection molding. Although the screw 20 moves forward and backward as the molten resin is plasticized, metered, and injected, by making the length of the starvation zone 23 equal to or longer than the length of the metering stroke, the inlet 202 can always be positioned (formed) within the starvation zone 23 during the production of a foamed molded article. In other words, even if the screw 20 moves forward and backward during the production of a foamed molded article, no zone other than the starvation zone 23 will come to the position of the inlet 202. As a result, the physical blowing agent introduced through the inlet 202 is always introduced into the starvation zone 23 during the production of a foamed molded article. By providing a starvation zone of sufficient and appropriate size (length) in this way and introducing a constant pressure of physical blowing agent therein, it becomes easier to maintain a constant pressure in the starvation zone 23. In this embodiment, the length of the starvation zone 23 is approximately the same as the length of the portion of the screw 20 downstream of the ring 26 where the shaft diameter of the screw 20 and the screw flight depth are constant, as shown in FIG. 2.

[0041] Next, while the starvation zone 23 is maintained at a constant pressure, the starved molten resin is brought into contact with the physical foaming agent at a constant pressure in the starvation zone 23 (step S4 in FIG. 1). That is, in the starvation zone 23, the molten resin is pressurized at a constant pressure by the physical foaming agent. Because the starvation zone 23 is not filled with molten resin (starved state) and has space in which the physical foaming agent can exist, the physical foaming agent and the molten resin can come into efficient contact. The physical foaming agent that comes into contact with the molten resin penetrates into the molten resin and is consumed. When the physical foaming agent is consumed, the physical foaming agent remaining in the introduction rate adjusting vessel 300 is supplied to the starvation zone 23. As a result, the pressure in the starvation zone 23 is maintained at a constant pressure, and the molten resin continues to come into contact with the physical foaming agent at a constant pressure.

[0042] In conventional foam molding using a physical blowing agent, a predetermined amount of high-pressure physical blowing agent is forcibly introduced into the plasticizing cylinder within a predetermined time. Therefore, the physical blowing agent must be pressurized to a high pressure and the amount and time of introduction into the molten resin must be precisely controlled, resulting in only a short contact time between the physical blowing agent and the molten resin. In contrast, in this embodiment, instead of forcibly introducing the physical blowing agent into the plasticizing cylinder 210, a constant pressure of the physical blowing agent is continuously supplied into the plasticizing cylinder to maintain a constant pressure in the starvation zone 23, thereby continuously bringing the physical blowing agent into contact with the molten resin. This stabilizes the amount of physical blowing agent dissolved (penetrated) into the molten resin, which is determined by temperature and pressure. Furthermore, because the physical blowing agent in this embodiment is constantly in contact with the molten resin, a necessary and sufficient amount of physical blowing agent can penetrate into the molten resin. As a result, the foam molded article produced in this embodiment has finer foam cells than conventional molding methods using a physical blowing agent, despite the use of a lower-pressure physical blowing agent.

[0043] Furthermore, since the manufacturing method of this embodiment does not require control of the amount and time of introduction of the physical foaming agent, it does not require drive valves such as check valves and solenoid valves, nor any control mechanisms for controlling these, thereby reducing equipment costs. Furthermore, the physical foaming agent used in this embodiment has a lower pressure than conventional physical foaming agents, so the load on the equipment is also smaller.

[0044] In this embodiment, the starvation zone 23 is maintained at a constant pressure throughout the production of a foam molded article. That is, all steps of the foam molded article production method are carried out while continuously supplying the physical foaming agent at the constant pressure to replenish the physical foaming agent consumed in the plasticizing cylinder. Furthermore, in this embodiment, for example, when performing continuous multiple-shot injection molding, the next shot of molten resin is prepared in the plasticizing cylinder during the injection, cooling, and ejection steps. This means that in continuous multiple-shot injection molding, the molten resin and the physical foaming agent at a constant pressure are always present and in contact within the plasticizing cylinder. That is, the molten resin is constantly pressurized by the physical foaming agent within the plasticizing cylinder at a constant pressure during one injection molding cycle, including the plasticization metering step, injection step, cooling, and ejection steps. Similarly, when continuous molding such as extrusion molding is performed, molding is performed in a state where the molten resin and the physical foaming agent at a constant pressure are always present and in contact with each other in the plasticizing cylinder, that is, the molten resin is always pressurized at a constant pressure by the physical foaming agent in the plasticizing cylinder.

[0045] Next, the molten resin contacted with the physical foaming agent is molded into a foamed molded article (step S5 in FIG. 1). The plasticizing cylinder 210 used in this embodiment has a recompression zone 24 located downstream of and adjacent to the starvation zone 23, where the molten resin is compressed and the pressure increases. First, the rotation of the plasticizing screw 20 causes the molten resin in the starvation zone 23 to flow into the recompression zone 24. The molten resin containing the physical foaming agent is pressure-adjusted in the recompression zone 24 and extruded forward of the plasticizing screw 20 to be metered. At this time, the internal pressure of the molten resin extruded forward of the plasticizing screw 20 is controlled as screw back pressure by a hydraulic motor or electric motor (not shown) connected rearward of the plasticizing screw 20. In this embodiment, in order to homogenously dissolve the physical foaming agent in the molten resin without separating it and to stabilize the resin density, it is preferable to control the internal pressure of the molten resin extruded forward of the plasticizing screw 20, i.e., the screw back pressure, to be about 1 to 4 MPa higher than the pressure in the starvation zone 23, which is maintained constant. In this embodiment, a check ring 50 is provided at the tip of the screw 20 to prevent the compressed resin in front of the screw 20 from flowing back upstream. This ensures that the pressure in the starvation zone 23 is not affected by the resin pressure in front of the screw 20 during metering.

[0046] In this embodiment, injection foam molding is performed by injecting a measured amount of molten resin from a plasticizing cylinder 210 shown in Figure 2 into a cavity 253 in a mold 251 to fill it. For injection foam molding, a short-shot method may be used in which the mold cavity 253 is filled with molten resin to a filling volume of 75% to 95% of the mold cavity volume, and the mold cavity is filled as the bubbles expand. Alternatively, a core-back method may be used in which the mold cavity is filled with molten resin to fill 100% of its volume, and then the cavity volume is expanded and foamed. The foamed molded article obtained has foam cells inside, which suppresses shrinkage of the thermoplastic resin during cooling, reducing sink marks and warpage, and resulting in a molded article with a low specific gravity.

[0047] Next, the operating mechanism of the screw 20, which moves forward and backward in association with the plasticization, metering and injection of the molten resin, will be described. In injection molding, the forward rotation of the screw 20 sends molten resin to the front of the screw 20, increasing the resin pressure (density), and the screw 20 retreats to the plasticization metering completion position, thereby metering the molten resin. Specifically, as shown in FIG. 3, a ball screw shaft 121 is rotatably supported by the frame 114, and one end of the ball screw shaft 121 is connected to a plasticization movement motor 122. The ball screw shaft 121 is threadedly engaged with a ball screw nut 123, and the ball screw nut 123 is connected to the actuation mechanism 11 via a spring 124 and a bracket 125. Therefore, when the motor 122 is driven in the forward or reverse direction, the rotational motion of the plasticization movement motor 122 is converted into linear motion by the combination of the ball screw shaft 121 and the ball screw nut 123, i.e., by the screw device 91, and this linear motion is transmitted to the bracket 125. Then, the bracket 125 is moved in the direction of arrow A along the guide 81, and the operating mechanism 11 is moved forward and backward.

[0048] Here, the operation of the operating mechanism 11 will be explained. First, in the plasticization metering step described above, the metering servo motor 83 serving as the plasticization metering motor is driven to rotate the screw 20 via the timing belt 84, causing the screw 20 to retreat to a predetermined position (move to the right in the drawing). At this time, the resin supplied from the hopper 211 is heated and melted in the cylinder 210, and is accumulated in front of the screw 20 as the screw 20 retreats.

[0049] Next, in the injection process, the nozzle tip 29 is pressed against the mold 251, and the injection motor (servo motor) 86 is driven to rotate the ball screw shaft 85 via the timing belt 87. At this time, the support member 82 is moved in conjunction with the rotation of the ball screw shaft 85, causing the screw 20 to move forward (to the left in the drawing). As a result, the resin accumulated in front of the screw 20 is injected from the nozzle tip 29 and fills the cavity 253 of the mold 251. The above operations are performed under the control of the control unit of the device 1000 described above. [Example]

[0050] The present invention will be further described below using examples and comparative examples, but the present invention is not limited to the examples and comparative examples described below.

[0051] In the expansion molding of the present invention, any method can be used as long as it uses a blowing agent gas that can set the screw back pressure to less than 8 MPa, and examples of such methods include chemical foaming, physical foaming using foam beads, and physical foaming using an inert gas. Physical foaming methods using high-pressure gas require high settings of the blowing agent pressure and back pressure, at 10 to 15 MPa or higher. This increases the risk of the screw 20 retracting again after metering is complete, making this method unsuitable from the standpoints of safety and machine wear.

[0052] (1) Foam injection molding equipment In this example, the apparatus 1000 shown in Fig. 2 used in the above-described embodiment is used. Details of the apparatus 1000 will be described. As described above, the apparatus 1000 is an injection molding apparatus, and includes a plasticizing cylinder 210, a cylinder 100 which is a physical foaming agent supply mechanism that supplies a physical foaming agent to the plasticizing cylinder 210, a mold clamping unit (mold clamping section) 250 provided with a mold 251, and a control section (not shown) for controlling the operation of the plasticizing cylinder 210 and the mold clamping unit 250.

[0053] A shutoff valve 28, which opens and closes by driving an air cylinder, is provided at the nozzle tip 29 of the plasticizing cylinder 210, allowing the interior of the plasticizing cylinder 210 to be maintained at high pressure. A mold 251 is attached to the nozzle tip 29, and molten resin is injected from the nozzle tip 29 to fill a cavity 253 formed by the mold 251. On the upper side of the plasticizing cylinder 210, there are formed, in order from the upstream side, 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. A resin supply hopper 211 and an introduction rate adjusting container 300 are respectively provided at the resin supply port 201 and the introduction rate adjusting container 300. The cylinder 100 is connected to the introduction rate adjusting container 300 by piping 154 via a buffer tank 153, a pressure reducing valve 151, and a pressure gauge 152. Furthermore, a sensor (not shown) for monitoring pressure is provided at a position facing the inlet 202 of the plasticizing cylinder 210.

[0054] The screw 20 is disposed in the plasticizing cylinder 210 so as to be able to rotate and move back and forth freely in order to promote the plasticization and melting of the thermoplastic resin and to measure and inject the molten resin. As described above, the screw 20 is provided with a half-split ring 26 and a large diameter portion 20A of the screw 20 as a mechanism for increasing the flow resistance of the molten resin.

[0055] In the plasticizing cylinder 210, thermoplastic resin is supplied into the plasticizing cylinder 210 from the resin supply port 201, and the thermoplastic resin is plasticized by a band heater (not shown) to become molten resin, which is then sent downstream as the screw 20 rotates forward. Due to the presence of the ring 26 and the large diameter portion 20A provided on the screw 20, the molten resin is compressed and pressure increases upstream of the ring 26, and the molten resin is not filled (starved) downstream of the ring 26. The molten resin sent further downstream is recompressed near the tip of the plasticizing cylinder 210 and metered before injection.

[0056] As a result, within the plasticizing cylinder 210, from the upstream side, there are formed a plasticization zone 21 where the thermoplastic resin is plasticized and melted, a compression zone 22 where the molten resin is compressed and the pressure increases, a starvation zone 23 where the molten resin is not filled, and a recompression zone 24 where the molten resin decompressed in the starvation zone is compressed again. A ring 26 provided on the screw 20 is located at the boundary between the compression zone 22 and the starvation zone 23. An inlet 202 through which a physical foaming agent is introduced is provided in the starvation zone 23.

[0057] In this embodiment, semi-automatic operation using the foam injection molding apparatus 1000 involves two steps: a metering step in which the supplied molten resin is accumulated in front of the screw 20 by driving the plasticization metering motor 83, which does not involve a brake, to move the screw 20 back to a plasticization metering completion position, and a injection step in which the injection motor 86 is driven to move the screw 20 forward, thereby filling the molten resin accumulated in front of the screw 20 from the nozzle tip (injection nozzle) 29 into the cavity 253 of the mold 251 in the mold clamping unit 250. In the metering step, the molten resin containing the foaming agent is plasticized and metered under the condition that the back pressure of the screw 20 is 8 MPa or less. After the safety door (not shown) of the mold clamping unit 250 is opened to remove the molded product and power to the plasticization metering motor 83 is cut off, the position of the screw 20 is confirmed, and the screw 20 is returned to the metering completion position before the safety door is closed to perform the next injection filling shot. Specifically, the control unit of the apparatus 1000 cuts off the power to the plasticization metering motor 83 when the safety door is opened during semi-automatic operation, and if it detects, for example, by a position detection sensor (not shown) that the screw 20 has further retreated from the plasticization metering completion position at that time, it returns the screw 20 to the metering completion position with an arbitrary back pressure before the next shot of injection filling is performed by closing the safety door. Also, in the metering process, the control unit drives the screw 20 backward under the condition that the back pressure of the screw 20 is 8 MPa or less, to plasticize and meter the molten resin containing a foaming agent.

[0058] (2) Example 1 In this example, foam molding was carried out as follows using semi-automatic operation of the device 1000. That is, talc-containing PP resin (4700G polypropylene containing 20% ​​talc, manufactured by Idemitsu Lion Composites) was used as the resin, and a flat molded product of 100 x 200 x 2 mm with a center direct gate was used as the mold 251. The temperature of the mold 251 was 40°C, and the resin temperature was 200°C.

[0059] First, during plasticization metering, the screw 20 built into the cylinder 210 plasticized the resin pellets dropped from the hopper 211 at an arbitrary rotation speed, and then dissolved nitrogen, a physical foaming agent, into the molten resin on the starvation flight 23. When metering was completed, the molten resin with the dissolved foaming agent accumulated at the tip of the screw 20. The foaming agent was introduced by constantly pressurizing the cylinder 210 along with the screw 20 with nitrogen decompressed to a constant pressure from a nitrogen cylinder. As a result, the foaming agent was metered while being mixed and dissolved into the molten resin on the screw 20 on the starvation flight 23. In this example, the metering rotation speed was 100 rpm, the gas pressure introduced from the nitrogen cylinder was 4 MPa, and the back pressure of the screw 20 was 6 MPa.

[0060] After that, injection filling was performed without applying dwell pressure, resulting in a foam-molded product that was 10% lighter. After cooling was complete, the mold was opened and the product was ejected. The metering process for the next shot was performed after injection filling and was completed before cooling was complete.

[0061] The metering completion position was shortened and the shot volume was reduced for the first shot only, and the metering conditions were set to the preset conditions from the second shot onwards. After cooling, the safety door was opened and the product (molded item) was removed manually. With the safety door open, the power to all drive motors, including metering motor 83, was turned off. As a result, the position of the screw 20 was retracted by 0.3 to 0.5 mm during the 10 to 15 seconds until the injection filling step of the next shot.

[0062] After closing the safety door, the power to all drive motors was turned on, and the back pressure of the plasticization metering motor 83 caused the screw 20 to move forward to the set position for the metering completion position. The back pressure at that time was set to 8 MPa, 2 MPa higher than the back pressure during metering. When the safety door was opened, the screw 20 moved backward, and the pressure at the tip of the screw 20 was reduced, causing the foaming agent dissolved at 4 MPa to separate. In order to re-dissolve the foaming agent in a short time, the screw back pressure may be increased when returning the screw 20 to its original position.

[0063] This semi-automatic operation produced good products from the eighth shot onwards. Continuous molding was carried out up to 55 shots. The amount of warpage was reduced to an average of approximately 0.2 mm, compared to an average of 1 mm for the solid, and no variation in warpage was observed. Starting from the 10th shot, samples were taken every 5 shots for 10 shots. The standard deviation σ divided by the average weight ave. (σ / ave.) was 0.25, which was almost the same as the 0.23 obtained when samples were taken continuously in automatic operation mode using a take-out machine without opening the safety door. The average cell diameter was 80-100 μm, and the variation in cell diameter between samples was the same as when using automatic operation.

[0064] (3) Example 2 In this example, molding was performed in a semi-automatic manner similar to Example 1, except that the pressure of the nitrogen physical blowing agent was set to 6 MPa, the back pressure during metering was set to 8 MPa, and the back pressure of the screw 20 after the safety door was re-closed was set to 10 MPa. The retraction amount of the screw 20 when the safety door was opened was 1.0 to 1.5 mm. As in Example 1, good products were obtained from the 8th shot onwards. Continuous molding was carried out up to 55 shots. The amount of warpage was reduced to an average of approximately 0.15 mm compared to an average of 1 mm for the solid, and no variation was observed.

[0065] Starting from the 10th shot, 10 shots were sampled every 5 shots. The value of σ / ave., calculated by dividing the standard deviation σ by the average weight ave., was 0.32, which was no greater than the 0.25 obtained when samples were continuously collected in automatic operation mode without opening the safety door and using the extraction machine without changing the weighing conditions. The average cell diameter was 60-80 μm, and the variation between samples was the same as in automatic operation.

[0066] (4) Comparative Example In this comparative example, the back pressure during metering was set to 10 MPa, and molding was carried out in the same semi-automatic operation as in Examples 1 and 2. When the safety door was opened, a loud noise was heard and the screw retreated again by 30 mm or more. This comparative example revealed that safe semi-automatic operation was not possible when the back pressure of the screw 20 was higher than 8 MPa. Furthermore, as in Examples 1 and 2, continuous molding was possible by returning the screw to the metering completion position at a back pressure of 12 MPa, but the warpage varied greatly, from 0.1 to 0.2 mm. This is thought to be due to insufficient re-dissolution of the separated blowing agent.

[0067] The standard deviation σ of the samples taken every 5 shots from the 10th shot onwards, divided by the average weight ave., was 0.60 (σ / ave.), a value significantly worse than the 0.27 obtained when samples were continuously taken in automatic operation mode without opening the safety door and using the take-out machine without changing the weighing conditions. The average cell diameter was 100-250μm, larger than the 30-60μm observed during continuous operation, and the variance was also greater.

[0068] As can be seen from the above, if the back pressure of the screw 20 is 8 MPa or less, it is possible to restrict the retraction of the screw 20 after the safety door is opened and the power to the plasticization metering motor is turned off. Furthermore, by checking the position of the screw 20 and returning the screw 20 to the metering completion position before the next shot of injection filling is performed after the safety door is closed, it becomes possible to reliably determine the molding conditions.

[0069] The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention. Furthermore, some or all of the above-described embodiments may be combined, or part of the configuration of one of the above-described embodiments may be omitted, without departing from the spirit of the present invention. [Explanation of symbols]

[0070] 20 screws 83 Plasticizing metering motor 86 Injection motor 100 Cylinder (Physical foaming agent supply mechanism) 210 Plasticizing Cylinder 250 Mold clamping unit (mold clamping section) 1000 Foam injection molding equipment

Claims

1. A foam injection molding method using a foam injection molding device in semi-automatic operation, which performs a metering process in which a plasticization metering motor without a brake is driven to move a screw backward to a plasticization metering completion position, thereby accumulating the supplied molten resin in front of the screw and metering the molten resin, and an injection process in which an injection motor is driven to move the screw forward, thereby filling the molten resin accumulated in front of the screw through an injection nozzle into a cavity of a mold in a mold clamping section, This foam injection molding method is characterized in that, in the metering process, the molten resin containing a foaming agent is plasticized and metered under conditions where the back pressure of the screw is 8 MPa or less, and after the safety door of the mold clamping section is opened to remove the molded product and the power of the plasticization metering motor is turned off, the position of the screw is confirmed and the screw is returned to the metering completion position before the next shot of injection filling is performed by closing the safety door.

2. a plasticizing cylinder having a screw therein for causing molten resin to flow; a physical foaming agent supply mechanism for supplying a physical foaming agent to the plasticizing cylinder; a mold clamping section having a mold provided therein and a safety door for removing a molded product; and a control section for controlling the operation of the screw, the foaming injection molding apparatus semi-automatically performing a metering step of metering the molten resin supplied from the physical foaming agent supply mechanism by driving a plasticizing metering motor without a brake to move the screw back to a plasticizing metering completion position, and an injection step of filling the molten resin accumulated in front of the screw through an injection nozzle of the plasticizing cylinder into a cavity of the mold in the mold clamping section by driving an injection motor to move the screw forward; The foam injection molding apparatus is characterized in that the control unit cuts off the power to the plasticization metering motor when the safety door is opened during semi-automatic operation, and if it detects that the screw has further retreated from the plasticization metering completion position, it returns the screw to the metering completion position with an arbitrary back pressure before the next shot of injection filling is performed by closing the safety door.

3. The foam injection molding apparatus according to claim 2, characterized in that, in the metering process, the control unit drives the screw backward to plasticize and meter the molten resin containing a foaming agent under conditions where the back pressure of the screw is 8 MPa or less.

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