JP2015193180A - Injection molding method of resin molding - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make the mechanical strength of one surface of a resin molding larger than that of the other surface of the resin molding by setting the temperature of a mold for molding the one surface of the resin molding higher than the load deflection temperature of a resin material in a filling step.SOLUTION: An injection molding method of a resin molding comprises: a heating step of heating first and second molds to first and second temperatures, respectively; a filling step of filling a molten resin material into a cavity space formed by both the heated molds; and a cooling step of cooling both the molds. In the filling step, the first and second temperatures are set so that the temperature of the first mold becomes higher than the load deflection temperature and that the temperature of the second mold becomes lower than the load deflection temperature. In the cooling step, cooling is performed so that the first and second temperatures become lower than the load deflection temperature.

Description

  The present invention relates to an injection molding method for molding a resin molded product by controlling the temperature of a mold.

  Conventionally, various methods have been proposed as an injection molding method for molding a resin molded product by controlling the temperatures of two molds that can be relatively moved. For example, Patent Document 1 discloses an injection molding method for molding a plastic lens. In the method described in Patent Document 1, a plastic lens is formed by a fixed insertion piece provided in a fixed mold and a movable insertion piece provided in a movable mold. In order to control the temperature of the resin in the cavity space formed between the fixed entrance and the movable entrance, a heater is provided close to the fixed entrance and the movable entrance, and the heating / cooling medium passage is fixed. Provided in the mold and the movable mold. In addition, a pressure cylinder is provided to compress the resin in the cavity space, and presses the movable entry frame to the fixed entry frame. In order to improve the surface accuracy of the surface layer of the plastic lens and eliminate stress cracks, the fixed entrance and the movable entrance are softened while the compressive force is applied to the movable entrance after filling the resin in the cavity space. It is heated to a temperature of 110 to 140 ° C., which is higher than the temperature, and only the surface layer of the plastic lens is melted. After that, while the compressive force is applied to the movable entrance frame, the fixed entrance frame and the movable entrance frame are cooled to a temperature of 70 to 80 ° C. which is lower than the softening temperature, and when this temperature is reached, the plastic as a molded product The lens is removed from Cavidi.

Japanese Patent Publication No. 1-334132

  By the way, it may be necessary to make the mechanical strength of one surface of the resin molded product stronger than the mechanical strength of the other surface. For example, when the housing of an electronic device that incorporates an electronic component such as a CPU is made of a resin molded product, when the external impact is applied to the housing, such as accidentally dropping the electronic device, the external impact reaches the electronic component. In order to reduce this, it is necessary to make the mechanical strength of the inner surface of the housing stronger than the mechanical strength of its outer surface.

  The method described in Patent Document 1 is effective for suppressing stress cracks on the entire surface of a resin molded product such as a plastic lens. However, in order to make the mechanical strength of one surface of the resin molded product stronger than the mechanical strength of the other surface, the method described in Patent Document 1 is not useful.

  Therefore, the present invention has been made in view of the above-described circumstances, and by making the temperature of the mold for molding one surface of the resin molded product higher than the deflection temperature under load of the resin material in the filling step, An object of the present invention is to provide an injection molding method capable of making the strength stronger than the mechanical strength of the other surface.

  In order to achieve the above object, a first aspect of the present invention according to claim 1 is a method for injection molding a resin molded article using the first and second molds, wherein the first mold is used. Is heated to the first temperature, the second mold is heated to the second temperature, and the cavity space formed by the first mold and the second mold heated in the heating step. And a cooling step for cooling the first mold and the second mold, and the temperature of the first mold is a load of the resin material in the filling step. The first temperature and the second temperature are determined so that the temperature is higher than the deflection temperature and the temperature of the second mold is lower than the load deflection temperature of the resin material in the filling step. The temperature of the first mold and the second An injection molding method in which the temperature of the mold is cooled so that a temperature lower than the deflection temperature under load of the resin material.

  In the first aspect of the invention, the number of molds for molding the resin molded product is not limited to two, and three or more molds may be used. For example, in a resin molded product, the surface other than the surfaces formed by the first and second molds may be molded by another mold. Moreover, if the 1st and 2nd metal mold | die is a structure which moves relatively, either one metal mold | die or both metal mold | die may move.

  In the first aspect of the invention, the heating step is preferably performed in a state where the first mold and the second mold are opened in order to accurately set the temperature. The process may be executed in a state where the mold and the second mold are closed.

  In the first aspect, if the temperature of the first mold is higher than the deflection temperature under load in the filling step, the first temperature may be set to a temperature lower than the deflection temperature under load. The deflection temperature under load or a temperature slightly higher than the deflection temperature under load may be set.

  In the first aspect of the invention, the first temperature and the second temperature are the occurrence of warpage of the resin molded product due to the temperature difference between the temperature of the molten resin material and the temperature of both molds, both molds It is determined based on factors such as the length of the cooling period.

  The specific aspect described in claim 2 further includes a closing step for closing the first mold and the second mold, and the heating step is performed before the closing step is performed. Is a temperature lower than the deflection temperature under load of the resin material, and the second temperature is a temperature lower than the first temperature. In this specific aspect, the first temperature may be set to a temperature within a mold temperature range suitable for molding the resin material, or may be set to a temperature higher than the mold temperature range. .

  The specific aspect of claim 3 further includes a pressure holding step of supplying the molten resin material to the cavity space in accordance with the shrinkage of the resin material in the cavity space after the filling step, and the pressure holding step In at least a part of the execution period, the temperature of the first mold is maintained at a temperature higher than the deflection temperature under load of the resin material, and the temperature of the second mold is lower than the deflection temperature under load of the resin material. This is an injection molding method maintained at a temperature. The filling step ends when the injection pressure for injecting the molten resin material from the nozzle reaches the V / P switching pressure.

  The specific aspect of claim 4 is an injection molding method in which the first temperature is determined such that the temperature of the first mold is lower than the deflection temperature under load of the resin material before the end of the pressure holding step. is there. In this specific aspect, the time when the temperature of the first mold becomes lower than the deflection temperature under load in the pressure holding step is determined in relation to the execution period of the cooling step.

  According to a specific aspect of the present invention, the temperature difference between the temperature of the first mold and the load deflection temperature of the resin material in the filling step is such that the temperature of the second mold and the load deflection of the resin material in the filling step. This is an injection molding method in which the first temperature and the second temperature are determined so as to be smaller than the temperature difference from the temperature. In this specific aspect, the temperature of the second mold in the filling step may be a temperature equal to or higher than the first temperature or a temperature lower than the first temperature as long as the temperature is lower than the deflection temperature under load.

The specific aspect according to claim 6 further includes a pressure holding step of supplying the molten resin material to the cavity space in accordance with the shrinkage of the resin material in the cavity space after the filling step, and at least the pressure holding pressure is provided. The first mold and the second mold are closed by fastening the first mold and moving the second mold relative to the first mold in the step. The injection molding method according to claim 5.
In this specific aspect, the first mold may be in a fixed state at least in the pressure holding step, but the first mold may be in a fixed state in all steps from the filling step to the mold opening step. It is preferable for reducing the occurrence of surface cracks in a resin molded product molded by this mold.

  In order to achieve the above object, according to a second aspect of the present invention, a resin molded product that forms a housing of an electronic device is injection-molded using the first mold and the second mold. A first mold that molds the surface of the resin molded product that becomes the inner surface of the housing to a first temperature, and second molds the surface of the resin molded product that becomes the outer surface of the housing. A heating step for heating the metal mold to a second temperature, and a filling step for filling a molten resin material into a space formed by the first mold and the second mold heated in the heating step And a cooling step for cooling the first mold and the second mold, and in the filling step, the temperature of the first mold is higher than the deflection temperature under load of the resin material in the filling step. The temperature of the second mold is the resin material The first temperature and the second temperature in the heating step are determined so as to be lower than the heavy deflection temperature, and in the cooling step, the temperature of the first mold and the temperature of the second mold are the same as those of the resin material. It is an injection molding method that is cooled so that the temperature is lower than the deflection temperature under load.

  The second aspect of the invention is also realized in various aspects, similar to the first aspect of the invention and its specific aspects. In the second aspect of the invention, the housing of the electronic device may be a housing of an image forming apparatus such as a printer, a housing of a portable terminal, or the like.

  In the first aspect of the invention described in claim 1, in the filling step, the temperature of the first mold is higher than the deflection temperature under load of the resin material, and the temperature of the second mold is equal to the load of the resin material. The temperature is lower than the deflection temperature. As a result, the residual stress of one surface molded by the first mold in the resin molded product becomes smaller than the residual stress of the other surface molded by the second mold, and the mechanical strength of the one surface Can be made stronger than the mechanical strength of the other surface.

  In a specific aspect of the present invention, the first temperature is set to a temperature lower than the load deflection temperature of the resin material, and the second temperature is set to a temperature lower than the first temperature. The heating step is performed before the closing step. As a result, in the heating step, the first mold and the second mold can be accurately heated to the first temperature and the second temperature, respectively. Thereby, in the filling step, the temperature of the first mold can be reliably raised to a temperature higher than the deflection temperature under load, and the temperature of the second mold can be reliably set to a temperature lower than the deflection temperature under load. .

  According to a specific aspect of the present invention, the temperature of the first mold is maintained at a temperature higher than the deflection temperature under load during at least a part of the execution period of the pressure holding step, and the second mold is used. Is maintained at a temperature lower than the deflection temperature under load. As a result, since the temperature of the first mold is maintained at a temperature higher than the deflection temperature under load even during at least a part of the execution period of the pressure holding step, the resin molded product is molded by the first mold. Thus, the residual stress on one surface can be reliably reduced.

  In the specific aspect of the present invention, the first temperature is determined so that the temperature of the first mold is lower than the deflection temperature under load before the pressure holding step is completed. As a result, in the cooling step, the temperature of the first mold can be quickly lowered to a temperature at which the resin molded product can be taken out, and the entire molding time can be shortened.

  In a specific aspect of the present invention, the temperature difference between the temperature of the first mold in the filling step and the deflection temperature under load is more than the temperature difference between the temperature of the second mold in the filling step and the deflection temperature under load. The first temperature and the second temperature are determined so as to decrease. As a result, in the cooling step, the temperature of the first mold can be quickly lowered to a temperature at which the resin molded product can be taken out, and the entire molding period can be shortened.

  According to a specific aspect of the present invention, in the pressure holding step, the second mold is moved in a direction in which the first mold is pressed against the first mold while the first mold is fixed. Generates mold clamping force. In the pressure-holding step, the pressure of the cavity space is maintained by the pressure of the molten resin material supplied from the nozzle to the cavity space and the clamping force corresponding to the pressure-holding, and the first mold is stationary. However, since the second mold is constantly moving, the occurrence of surface cracks on the molding surface of the resin molded product molded by the first mold is caused by the resin molding molded by the second mold. It is possible to suppress the occurrence of surface cracks on the molding surface of the product.

  In the second aspect of the invention according to claim 7, in the filling step, the temperature of the first mold is higher than the load deflection temperature of the resin material, and the temperature of the second mold is the load of the resin material. The temperature is lower than the deflection temperature. As a result, the residual stress of the inner surface molded by the first mold in the housing becomes smaller than the residual stress of the outer surface molded by the second mold, and the mechanical strength of the inner surface is increased. It can be stronger than the target strength. Thereby, the electronic device in a housing can be protected with respect to an external impact.

It is a front view which shows the whole structure of the injection molding machine for enforcing the injection molding method which concerns on embodiment of this invention. It is sectional drawing which expands and shows the internal structure of the fixed metal mold | die of a injection molding machine, and a movable metal mold | die. It is explanatory drawing which shows the process step performed in the injection unit 2, and the process step performed in the mold clamping unit 3. FIG. It is explanatory drawing which shows the state from which the temperature TA of the fixed mold 32 and the temperature TB of the movable mold 33 change with elapsed time. It is explanatory drawing which shows the state from which internal pressure of cavity space CA changes with elapsed time. It is explanatory drawing which shows the relationship between the bending strength of the resin material shape | molded by resin material Styrolution PS 495F (trademark), and the highest ultimate temperature of the fixed metal mold | die 32 in filling step SB3. It is explanatory drawing which shows the relationship between the bending strength of the resin material shape | molded by resin material Terluran GP-22 (trademark), and the highest ultimate temperature of the fixed metal mold | die 32 in filling step SB3.

  Hereinafter, an embodiment in which the injection molding method of the present invention is applied to an injection molding method for molding a part of a housing of an image forming apparatus will be described. In general, an image forming apparatus such as a laser printer includes electronic components such as a CPU and a memory and optical system components such as a polygon mirror and a lens. These electronic components and optical system components are covered and protected by a housing. In the present embodiment, HIPS (High Impact Polystyrene) is used as a resin material for molding the housing. As HIPS, Styrolution PS 495F (registered trademark) is used.

[Configuration of Injection Molding Machine 1]
A configuration of an injection molding machine 1 capable of performing the injection molding method of the present embodiment will be described with reference to FIGS. 1 and 2. The injection molding machine 1 includes an injection unit 2 and a mold clamping unit 3. 1 and 2, the vertical direction and the horizontal direction are indicated by arrows.

<Injection unit 2>
In FIG. 1, the injection unit 2 mainly includes an injection cylinder 20, a screw 21, a hopper 22, a drive motor 23, and an injection hydraulic cylinder 24. A nozzle 25 is formed at the tip of the injection cylinder 20. The screw 21 has a shaft portion 21A and a screw portion 21B, and is disposed inside the injection cylinder 20 so as to be movable and rotatable in the axial direction of the shaft portion 21A. The screw portion 21B is formed in a spiral shape on the outer peripheral surface of the shaft portion 21A. The hopper 22 is fixed to the outer peripheral portion of the injection cylinder 20 and a pellet-shaped resin material is charged. The drive motor 23 has a rotating shaft connected to the shaft portion 21 </ b> A of the screw 21 and rotates the screw 23. The injection hydraulic cylinder 24 has a plunger connected to the housing of the drive motor 23 and applies pressure to the screw 21 via the housing of the drive motor 23 and the rotating shaft. A plurality of heaters 26 are disposed on the outer periphery of the injection cylinder 20 and melt the resin material supplied from the hopper 22.

  A pressure sensor 27 is attached to the cylinder 20 in order to detect the hydraulic pressure of the injection hydraulic cylinder 20. A temperature sensor 28 is attached to the injection cylinder 20 at a position close to the nozzle 25 in order to detect the temperature of the molten resin material.

<Clamping unit 3>
In FIG. 1, the mold clamping unit 3 includes a fixed platen 30, a movable platen 31, a fixed mold 32, a movable mold 33, a mounting frame 34, a mold clamping hydraulic cylinder 35, and an ejector hydraulic cylinder 36. And mainly. A pair of guide bars 37 and 38 are fixed to the fixed platen 30 and the mounting frame 34, respectively. The movable platen 31 is supported by the guide bars 37 and 38 so as to be movable in the axial direction of the both guide bars 37 and 38. The fixed mold 32 is fixed to the fixed platen 30, and the movable mold 33 is fixed to the movable platen 31. A mold clamping hydraulic cylinder 35 is attached to the attachment frame 34.

  The connecting member 39 is fixed to the movable platen 31 and is connected to the plunger 35 </ b> A of the mold clamping hydraulic cylinder 35. The ejector hydraulic cylinder 36 has a plunger 36 </ b> A shown in FIG. 2 and is attached to the movable platen 31. The mold clamping hydraulic cylinder 35 brings the movable mold 33 into contact with or away from the fixed mold 32. When the movable mold 33 is brought into contact with the fixed mold 32, pressure for pressing the movable mold 33 against the fixed mold 32, that is, mold clamping force. Is granted. In the present embodiment, the tip portion of the nozzle 25 is arranged in a state of being connected to the fixed mold 32.

  The heating / cooling device 40 is mounted on the fixed mold 32, and the heating / cooling device 41 is mounted on the movable mold 33. A pressure sensor 42 is attached to the cylinder 35 in order to detect the hydraulic pressure of the mold clamping hydraulic cylinder 35.

(Detailed configuration of molds 32, 33, etc.)
The detailed configuration of the fixed mold 32, the movable mold 33, and its peripheral devices will be described with reference to FIG. In FIG. 2, the cylinder portion of the ejector hydraulic cylinder 36 is omitted, and only its plunger 36A is shown.

  In FIG. 2, the fixed mold 32 is formed in a shape for molding the inner surface S1 of the housing part HG. The movable mold 33 is formed in a shape for molding the outer surface S2 of the housing part HG. Specifically, the fixed mold 32 includes a sprue 32A, a runner forming part 32B, a gate forming part 32C, and a cavity forming part 32D for forming the inner surface S1. The movable mold 33 includes a sprue lock forming portion 33A, a runner forming portion 33B, a gate forming portion 33C, and a cavity forming portion 33D for forming the outer surface S2. When the molds 32 and 33 are clamped, the cavity forming portions 32D and 33D form a cavity space CA, the runner forming portions 32B and 33B form a runner RN, and the gate forming portions 32C and 33C are gate GA. Form. The housing part HG molded in the cavity space CA is indicated by a two-dot chain line in FIG. A pair of positioning pins 43, 44 project from the movable mold 33 toward the fixed mold 32, and a pair of positioning holes 45, 46 can be fitted to the positioning pins 43, 44 in the fixed mold 33. It is formed.

  The ejector plate 47 is connected to the plunger 36 </ b> A of the ejector hydraulic cylinder 36. A pair of guide bars 48 and 49 are fixed to the ejector plate 47 and inserted through a pair of guide holes formed in the movable mold 33. A pair of ejector pins 50 and 51 are fixed to the ejector plate 47 and inserted into a pair of pin holes formed in the movable mold 33. The ejector plate 47 is supported by the movable mold 33 so as to be movable by both guide bars 48 and 49 so as to approach or separate from the movable mold 33. A pair of springs 52, 53 are wound around the guide bars 48, 49 between the movable mold 33 and the ejector plate 47, and elastic force is applied so that the ejector plate 47 is separated from the movable mold 33. 47 is always given. A sprue lock pin 54 is fixed to the ejector plate 47.

  A temperature sensor 55 is embedded in a portion of the fixed mold 32 facing the movable mold 33 in order to detect the temperature of the fixed mold 32. A temperature sensor 56 is embedded in a portion of the movable mold 33 facing the fixed mold 32 in order to detect the temperature of the movable mold 33. In the present embodiment, the embedded position of the temperature sensor 55 is determined in the formation region of the cavity forming portion 32D for forming the inner surface S1 of the housing part HG in the fixed mold 32. The embedded position of the temperature sensor 56 is determined in the formation region of the cavity forming portion 33D for forming the outer surface S2 of the housing part HG in the movable mold 33.

<Control unit>
The injection molding machine 1 includes a control unit (not shown). The control unit includes an input unit for inputting a set value of temperature or pressure, and controls the temperature or pressure of each part according to detection signals from various sensors and set values input from the input unit. Specifically, the control unit controls the heater 26 and the heating / cooling devices 40 and 41 in accordance with the temperature detection signals from the temperature sensors 28, 55 and 56 and the temperature set value from the input unit. Further, the control unit controls the hydraulic cylinders 24 and 35 in accordance with the pressure detection signals from the pressure sensors 27 and 42 and the pressure set value from the input unit. Further, the control unit controls the drive motor 23 and the ejector hydraulic cylinder 36.

[Injection Molding Method of Embodiment]
A series of processing steps for carrying out the injection molding method of the present embodiment will be described with reference to FIGS. The injection molding method of the present embodiment includes processing steps executed in the injection unit 2 and processing steps executed in the mold clamping unit 3.

<Processing steps executed in the injection unit 2>
The processing steps executed in the injection unit 2 include a charging step SA1, a melting step SA2, and an injection step SA3.

(Input step SA1)
Hereinafter, the charging step SA1 will be described. Here, a process in which the resin material supplied to the hopper 22 is charged into the cylinder 20 is referred to as a charging step SA1.
The pellet-shaped resin material is dried by a drying furnace (not shown). The dried resin material is supplied to the hopper 22. The amount of the resin material supplied to the hopper 22 is a sufficient amount for molding the housing part HG once. In the present embodiment, the rotation speed of the screw 21 at the time of charging and the back pressure of the screw 21 are set by the input unit of the control unit before the start of injection molding. The charging step SA1 is executed in a state where the resin material is supplied to the hopper 22, and the control unit of the injection molding machine 1 drives the drive motor 23 to rotate the screw 21 at the set rotational speed. By the rotation of the screw 21, the resin material in the hopper 22 is put into the injection cylinder 20. Further, the control unit controls the hydraulic pressure of the injection hydraulic cylinder 24 according to the pressure detection signal from the pressure sensor 27 and the set back pressure.

(Melting step SA2)
Hereinafter, the melting step SA2 will be described. Here, the process in which the resin charged in the injection cylinder 20 is heated to a set melting temperature is referred to as a melting step SA2.
The melting temperature range (Melt Temperature Range) of the resin material Styrolution PS 495F (registered trademark) is 180 to 260 ° C. In this embodiment, the melting temperature is set to 210 ° C. by the input unit of the control unit before the start of injection molding. The melting temperature 210 ° C. is a temperature that satisfies the condition that the temperature is too high and the resin material is not thermally decomposed and the condition that the temperature is too low and clogging does not occur in the piping of the injection unit 2. The control unit of the injection molding machine 1 controls the heater 26 according to the temperature detection signal from the temperature sensor 28 and the set melting temperature 210 ° C. Generally, the internal temperature of the injection cylinder 20 is lower than the melting temperature in the vicinity of the hopper 22, but as the resin material moves toward the nozzle 25 by the rotation of the screw 21, the resin material is melted while being compressed. .

(Injection step SA3)
Hereinafter, the injection step SA3 will be described. Here, the injection pressure at which the injection unit 2 injects the molten resin material after the injection unit 2 starts to inject the molten resin material from the nozzle 25 into the sprue 32A of the fixed mold 32 reaches the V / P switching pressure. The process up to this is called injection step SA3.
In this embodiment, before the injection molding is started, the injection speed is set to 80 mm / sec, the V / P switching pressure is set to 130 MPa, and the holding pressure is set to 90 MPa by the input unit of the control unit. .
The melted resin material is injected into the sprue 32A of the fixed mold 32 from the nozzle 25 by the screw 21 and the injection hydraulic cylinder 24 at a set injection speed (80 mm / sec).
After the injection is started and until the injection pressure reaches the V / P switching pressure, the control unit can inject the molten resin material into the cavity space at the set injection speed (80 mm / sec). The rotational speed of the drive motor 23 that drives the screw 21 and the hydraulic pressure of the injection hydraulic cylinder 24 are controlled.
Since the volume of the cavity space is constant in the injection step SA3, the injection pressure required for injection increases as the molten resin material is injected. Therefore, the control unit controls the hydraulic pressure of the injection hydraulic cylinder 24 so as to give the screw 21 an injection pressure that can maintain the set injection speed, in accordance with the pressure detection signal from the pressure sensor 27.
When the injection pressure reaches the V / P switching pressure, the control unit controls the rotation speed of the drive motor 23 and the hydraulic pressure of the injection hydraulic cylinder 24 so that the injection pressure of the resin material becomes the set holding pressure. Under this control, the molten resin material is continuously injected.

<Processing steps executed in the mold clamping unit 3>
The processing steps executed in the mold clamping unit 3 include a mold heating step SB1, a mold closing step SB2, a filling step SB3, a pressure holding step SB4, a cooling step SB5, and a mold opening step SB6. Product take-off step SB7.

(Mold heating step SB1)
Hereinafter, the mold heating step SB1 will be described. Here, the process of heating so that the temperature of the fixed mold 32 and the movable mold 33 becomes a predetermined temperature is referred to as a mold heating step SB1.
The mold temperature range (Mold Temperature Range) suitable for the resin material Styrolution PS 495F (registered trademark) is 10 to 60 ° C. In this embodiment, the preheating temperature of the fixed mold 32 is set to 70 ° C. and the preheating temperature of the movable mold 33 is set to 50 ° C. by the input unit of the control unit before the start of injection molding. Is done. That is, each preheating temperature is set by the input unit of the control unit so that the average temperature [(70 ° C. + 50 ° C.) / 2] of both molds 32 and 33 is within the mold temperature range. The control unit controls the heating / cooling device 40 so that the temperature of the fixed mold 32 becomes 70 ° C. according to the temperature detection signal from the temperature sensor 55 and the set preheating temperature 70 ° C. Further, the control unit controls the heating / cooling device 41 so that the temperature of the movable mold 33 becomes 50 ° C. according to the temperature detection signal from the temperature sensor 56 and the set preheating temperature 50 ° C.

(Mold closing step SB2)
Hereinafter, the mold closing step SB2 will be described. Here, the process of closing the fixed mold 32 and the movable mold 33 with a predetermined clamping force is referred to as a mold closing step SB2.
The mold clamping force for closing the fixed mold 32 and the movable mold 33 is set to a value at which the resin material does not leak from the gap between the molds 32 and 33. The effective value of the mold clamping force that actually acts on both dies is reduced from the set value of the mold clamping force by a value corresponding to the holding pressure set in the pressure holding step SB4 described later. In this embodiment, the mold clamping force is set to 100 tons by the input unit of the control unit before the start of injection molding. The control unit controls the hydraulic pressure of the mold clamping hydraulic cylinder 35 according to the pressure detection signal from the pressure sensor 42 and the set mold clamping force.

(Filling step SB3)
Hereinafter, the filling step SB3 will be described. Here, the process from when the molten resin material starts to be injected into the cavity space from the nozzle 25 of the injection unit 2 until the injection pressure at which the injection unit 2 injects the molten resin material reaches the V / P switching pressure, This is referred to as a filling step SB3. In the present embodiment, the execution period D1 of the injection step SA3 and the filling step SB3 is set to 2 seconds.

  FIG. 4 shows a state where the temperature TA of the fixed mold 32 and the temperature TB of the movable mold 33 change with time. The temperature TC is an average temperature of both molds 32 and 33. The temperature TA of the fixed mold 32 is a temperature detected by the temperature sensor 55, and the temperature TB of the movable mold 33 is a temperature detected by the temperature sensor 56. The temperature TA of the fixed mold 32 and the temperature TB of the movable mold 33 are set to a preheating temperature of 70 ° C. and a preheating temperature of 50 ° C. by the temperature control of the control unit before the filling step SB3 is started. Maintained. However, when the filling step SB3 is started, the temperature TA of the fixed mold 32 and the temperature TB of the movable mold 33 are rapidly increased due to the filling of the molten resin material regardless of the temperature control of the control unit. To do. In the execution period D1 of the charging step SB3, the temperature TA of the fixed mold 32 rises to a temperature 90 ° C. higher than the load deflection temperature 85 ° C. of the resin material Styrolution PS 495F (registered trademark), and the temperature of the movable mold 33 TB is lower than the deflection temperature under load of 85 ° C, but rises to a temperature of 70 ° C higher than the preheating temperature of 50 ° C. In the filling step SB3, the average temperature TC of both molds does not exceed the deflection temperature under load of 85 ° C. Generally, the temperature of both molds 32 and 33 starts to decrease from when the molten resin material is filled in all of the sprue 32A, sprue lock forming portion 33A, runner RN, gate GA, and cavity space CA.

  FIG. 5 shows a state where the internal pressure of the cavity space CA changes with the elapsed time. The internal pressure of the cavity space CA is detected by a pressure sensor (not shown) embedded at the same position as the embedded position of the temperature sensor 55. The internal pressure of the cavity space CA is about ¼ of the hydraulic pressure of the injection hydraulic cylinder 24 detected by the pressure sensor 27, but increases as the hydraulic pressure of the injection hydraulic cylinder 24 increases. As the melted resin material is filled into the cavity space CA, the internal pressure of the cavity space CA rapidly increases. When the molten resin material is filled in all of the sprue 32A, the sprue lock forming portion 33A, the runner RN, the gate GA, and the cavity space CA, the internal pressure of the cavity space CA becomes the highest. When the internal pressure becomes the highest, the control unit recognizes from the pressure detection signal from the pressure sensor 27 that the hydraulic pressure of the injection hydraulic cylinder 24 has reached the V / P switching pressure.

(Pressure holding step SB4)
Hereinafter, the pressure holding step SB4 will be described. Here, the process of maintaining the holding pressure at a predetermined pressure for a predetermined period after the filling step SB3 is completed is referred to as a pressure holding step SB4.
The pressure holding step SB4 is started when the hydraulic pressure of the injection hydraulic cylinder 24 reaches the V / P switching pressure. In the present embodiment, the execution period D2 of the pressure holding step SB4 is set to 7 seconds. In the present embodiment, the hydraulic pressure of the injection hydraulic cylinder 24 in the pressure holding step SB4, that is, the holding pressure is set to 90 MPa by the input unit of the control unit before the start of injection molding, as already described in the injection step SA3. Is done. In the injection step SA3 and the filling step SB3, the control unit controls the drive motor 23 so that the rotation speed of the screw 21 becomes the set injection speed, but the hydraulic pressure of the injection hydraulic cylinder 24 is V / P switching pressure. After that, that is, after the pressure holding step SB4 is started, the injection hydraulic cylinder 24 is controlled so that the hydraulic pressure of the injection hydraulic cylinder 24 becomes the set holding pressure. The holding pressure is such that the pressure is too high so that the resin material does not leak from the gap between the molds 32 and 33 and the pressure is too low to cause a large sink in the resin material. Is set.

  The resin material filled in the cavity space CA or the like contracts as the temperature of both molds 32 and 33 decreases. In order to make up for the contracted volume, in the pressure holding step SB4, under the set holding pressure, a small amount of molten resin material is discharged from the nozzle 25 through the sprue 32A, the runner RN, and the gate GA into the cavity space CA. Injected into.

  The temperature TA of the fixed mold 32 is lowered to a temperature lower than the deflection temperature of 85 ° C. before the execution period D2 of the pressure holding step SB4 ends. In the pressure holding step SB4, the control unit is configured so that the temperature of the fixed mold 32 and the temperature of the movable mold 33 gradually decrease toward the preheating temperature 70 ° C and the preheating temperature 50 ° C. The heating and cooling devices 40 and 41 are controlled. When the pressure holding step SB4 ends, the control unit controls the injection hydraulic cylinder 24 so that the application of the holding pressure to the screw 21 is stopped, and stops the application of the holding pressure. Material injection is stopped.

(Cooling step SB5)
Hereinafter, the cooling step SB5 will be described. Here, the process of cooling until the temperature of the housing part HG is lowered to a temperature at which the housing part HG can be taken out from both molds 32 and 33 after the pressure holding step SB4 is completed is referred to as a cooling step SB5.
After the pressure holding step SB4 is completed, the cooling step SB5 is executed for a predetermined cooling period. The predetermined cooling period is set to a period during which the temperature of the housing part HG is lowered to a temperature at which the housing part HG can be taken out from both the molds 32 and 33. In the present embodiment, the predetermined cooling period is set to 21 seconds by the input unit of the control unit before the start of injection molding. In the present embodiment, the temperature at which the housing part HG can be taken out is the temperature of the housing part HG when the average temperature of both molds has dropped to a temperature close to 60 ° C. In this case, the temperature of the fixed mold 32 and the temperature of the movable mold 33 are respectively lowered to the preheating temperature 70 ° C. and the preheating temperature 50 ° C. set in the mold heating step SB1. The control unit controls the heating and cooling devices 40 and 41 according to the temperature detection signals from the temperature sensors 55 and 56, the set preheating temperature 70 ° C., and the preheating temperature 50 ° C. By this control, both molds 32 and 33 are cooled for 21 seconds, respectively.

(Mold opening step SB6)
Hereinafter, the mold release step SB6 will be described. Here, the process of opening both molds 32 and 33 after the completion of the cooling step SB5 is referred to as a mold release step SB6.
When the predetermined cooling period elapses, the control unit controls the hydraulic pressure of the mold clamping hydraulic cylinder 35 to release the movable mold 33 from the fixed mold 32.

(Product removal step SB7)
Hereinafter, the product take-out step SB7 will be described. Here, the process of taking out the housing part HG from both the molds 32 and 33 after the mold release step SB6 is completed is referred to as a product removal step SB7.
After the movable mold 33 is released from the fixed mold 32, the control unit operates the ejector hydraulic cylinder 36 to move the ejector plate 47. By the movement of the ejector plate 47, the ejector pins 50 and 51 and the sprue lock pin 54 push out the housing part HG from the movable mold 33. Thereby, the housing part HG is taken out from both the molds 32 and 33.
[Effect of the embodiment]

In the present embodiment, in the mold heating step SB1, the preheating temperature of the fixed mold 32 is set to 70 ° C., and the preheating temperature of the movable mold 33 is set to 50 ° C., thereby filling step SB3. , The actual temperature of the fixed mold 32 becomes 90 ° C., which is higher than the load deflection temperature 85 ° C. of the resin material Styrolution PS 495F (registered trademark), and the actual temperature of the movable mold 33 is 85 ° of the load deflection temperature. The temperature becomes 70 ° C lower than C.
When the housing part HG is molded under the above temperature conditions, the occurrence of surface cracks can be suppressed in the inner surface S1 molded by the fixed mold 32 compared to the outer surface S2.
As a result, in the housing part HG, the bending strength of the inner surface S1 formed by the fixed mold 32 can be made larger than the bending strength of the outer surface S2 formed by the movable mold 33. In general, when an external impact is applied to the housing part HG, bending deformation occurs in which the inner surface S1 side is convex, and therefore, the edge stress degree is a compressive stress on the outer surface S1 and a tensile stress on the inner surface S1. Synthetic resin materials are generally weaker in tensile stress than compressive stress. Therefore, by increasing the bending strength of the inner surface S1, it is possible to increase the mechanical strength against bending deformation caused by an external impact and make it difficult to break.

  FIG. 6 shows the relationship between the bending strength of the resin material molded by the resin material Styrolution PS 495F (registered trademark) and the maximum temperature reached by the fixed mold 32 in the filling step SB3. In FIG. 6, when the maximum temperature reached is a temperature 69.0 ° C lower than the deflection temperature under load 85 ° C and a temperature 79.0 ° C, the bending strength is 34.6 MPa and 34.9 MPa. . When the maximum temperature reached is 84.5 ° C, which is substantially the same as the deflection temperature under load of 85 ° C, the temperature is higher than the deflection temperature under load of 85 ° C, 89.7 ° C, and 95.0 ° C, the bending strength is , 35.7 MPa, 36.1 MPa, and 36.2 MPa. The bending strength increases relatively greatly when the maximum temperature reaches a deflection temperature under load of 85 ° C or higher. However, if the maximum temperature reaches too high, the temperature of the fixed mold 32 may not become lower than the deflection temperature of 85 ° C. by the end of the pressure holding step SB4. In this case, if the stationary mold 32 is rapidly cooled in the cooling step SB, the bending strength of the inner surface S1 of the housing part HG is lowered. Since the maximum temperature 95.0 ° C. shown in FIG. 6 is too high, in this embodiment, the preheating temperature of the fixed mold 32 is set so that the maximum temperature of the fixed mold 32 is 90 ° C. Is set to 70 ° C. As a result, in this embodiment, the bending strength of the housing part HG can be increased without increasing the cooling period.

  Generally, the mold temperature range set for injection molding of resin material is determined according to the type of resin material, and the maximum temperature of this mold temperature range is lower than the load deflection temperature of resin material. It is. For example, when the resin material is Styrolution PS 495F (registered trademark), the temperature range of the mold is 10 ° C. to 60 ° C., and the deflection temperature under load is 85 ° C. In this embodiment, the preheating temperature of the fixed mold 32 is set to 70 ° C., and the preheating temperature of the movable mold 33 is set to 50 ° C., but the average temperature 60 of the preheating temperatures of both molds is set. ° C is within the temperature range of the mold. As a result, while setting the average temperature of the preheating temperatures of both molds within the temperature range of the mold set in the resin material Styrolution PS 495F (registered trademark), up to a temperature exceeding the deflection temperature under load in the filling step SB3. The temperature of the fixed mold 32 can be raised, and the bending strength of the inner surface S1 of the housing part HG molded by the fixed mold 32 can be increased.

  In the present embodiment, in the filling step SB3, the maximum ultimate temperature of the fixed mold 32 is 90 ° C, which is 5 ° C higher than the deflection temperature under load 85 ° C, and the maximum ultimate temperature of the movable die 33 is the deflection under load. The preheating temperatures of both molds are set to 70 ° C. and 50 ° C. so that the temperature is 70 ° C., which is 15 ° C. lower than the temperature 85 ° C. As a result, since the temperature difference between the maximum temperature reached by the fixed mold 32 and the deflection temperature under load is smaller than the temperature difference between the deflection temperature under load and the maximum temperature achieved by the movable mold 33, the average temperature of both molds is the load. The deflection temperature is not exceeded, the cooling period can be made relatively short, and the entire one molding period can be shortened.

  In the present embodiment, the fixed mold 32 is fixed to the fixed platen 30 and hardly deformed by the mold clamping force from the mold clamping cylinder 35, so that the resin molded product molded by the first mold is used. The occurrence of surface cracks on the inner surface S1 can be suppressed from the occurrence of surface cracks on the outer surface S2 of the resin molded product molded by the second mold.

[Correspondence between embodiment and invention]
The fixed mold 32, the movable mold 33, and the cavity space CA are examples of the first mold, the second mold, and the cavity space of the present invention. The housing component HG is an example of the resin molded product of the present invention or the resin molded product of the housing, and the inner surface S1 and the outer surface S2 of the housing component HG are examples of the inner surface and the outer surface of the housing of the present invention. It is. The mold heating step SB1, the filling step SB3, and the cooling step SB5 are examples of the heating step, the filling step, and the cooling step of the present invention. A temperature of 70 ° C. and a temperature of 50 ° C. are examples of the first temperature and the second temperature in the heating step of the present invention. The temperature 90 ° C. is an example of a temperature higher than the deflection temperature under load in the filling step of the present invention, and the temperature 70 ° C. is an example of a temperature lower than the deflection temperature under load in the filling step of the present invention.

[Modification]
The present invention is not limited to this embodiment, and various modifications can be made without departing from the spirit of the present invention. An example of the modification will be described below.

(1) The injection molding method of the present embodiment is a method of molding a housing part of an image forming apparatus with a resin material HIPS, but is not limited to this method. A method of molding a housing part of an electronic device such as a personal computer or a portable terminal with a thermoplastic resin material other than the resin material HIPS may be used. Moreover, it may replace with a thermoplastic resin material and the method of shape | molding with a thermosetting resin material may be sufficient. For example, as another thermoplastic resin material, a resin product can be molded using an ABS (Acrylonitrile Butadiene Styrene) resin. As the ABS resin, Terluran GP-22 (registered trademark) can be used. Since the melting temperature, the deflection temperature under load, and the mold temperature of Terluran GP-22 (registered trademark) are higher than those of Styroluth PS 495F (registered trademark) used in this embodiment, the fixed mold 32 and the movable mold By setting the preheating temperature of the mold 33 to a temperature higher than the preheating temperature in the present embodiment, the mechanical strength of the portion formed by the fixed mold 32 can be increased. FIG. 7 shows the relationship between the bending strength of the resin material molded from the resin material Terluran GP-22 (registered trademark) and the maximum temperature reached by the fixed mold 32 in the filling step SB3. In FIG. 7, when the maximum temperature reached is 89.0 ° C lower than the deflection temperature under load of 94 ° C, the bending strength is 60.4 MPa. When the maximum temperature reached is 95.6 ° C, 101.5 ° C, 105.8 ° C higher than the deflection temperature under load of 94 ° C, the bending strength is 60.9 MPa, 61.5 MPa, 61. 3 MPa. However, since the maximum reached temperature of 105.8 ° C. is too high, the cooling period becomes long, so that it cannot be adopted as the maximum reached temperature of the fixed mold 32.

(2) The injection molding method of the present embodiment is a method in which the mold heating step SB1 is executed before the mold closing step SB2, but is not limited to this method. For example, the mold heating step SB1 may be executed in parallel with the mold closing step SB2, or may be executed after the mold closing step SB2. In a modified example in which the mold heating step SB1 is performed after the mold closing step SB2, a configuration in which a material having low thermal conductivity is coated on the contact surface between the fixed mold and the movable mold is conceivable.

(3) In the injection molding method of the present embodiment, the fixed mold 32 molds the inner surface S1 of the housing part HG of the image forming apparatus, and the movable mold 33 forms the outer surface S2 of the housing part HG. However, the method is not limited to this method. For example, in a member to which a bending load is applied, regardless of the inner and outer surfaces of the member, one surface on which tensile stress is applied by the bending load is formed by a fixed mold, and the other surface on which compressive stress is applied is movable. A method of molding with a mold may be used.

(4) In the injection molding method of the present embodiment, the fixed mold is set such that the maximum temperature 60 ° C in the mold temperature range 10 ° C to 60 ° C suitable for molding the resin material HIPS is the average temperature. In this method, the preheating temperature of 32 ° C. and the preheating temperature of the movable mold 33 are set to 50 ° C. However, the present invention is not limited to this method. For example, when the preheating temperature of the fixed mold 32 for increasing the bending strength is equal to or higher than a specific temperature lower than the maximum temperature in the mold temperature range, the preheating temperature of the fixed mold 32 is the specific temperature. Alternatively, a method may be used in which the preheating temperature of the movable mold 33 is set to a temperature lower than a specific temperature. On the other hand, when the preheating temperature of the fixed mold 32 is equal to or higher than the specific temperature and is set to a temperature higher than the maximum temperature, the temperature of the fixed mold 32 is loaded before the end of the pressure holding step SB4. The preheating temperature of the fixed mold 32 needs to be set so that the temperature is lower than the deflection temperature.

(5) In this embodiment, in order to take out the housing part HG from the movable mold 33, members such as the ejector hydraulic cylinder 36 and the ejector plate 47 are mounted on the movable platen 31, but the present invention is not limited to this configuration. . For example, when the housing part HG is taken out from the fixed mold 32, members such as the ejector hydraulic cylinder 36 and the ejector plate 47 may be mounted on the fixed mold 32.

1 Injection Molding Machine 32 Fixed Mold 33 Movable Mold SB1 Mold Heating Step SB2 Mold Closing Step SB3 Filling Step SB4 Pressure Holding Step SB5 Cooling Step SB6 Mold Opening Step CA Cavity Space HG Housing Parts S1 Inner Surface S2 Outer Surface

Claims (7)

A method of injection-molding a resin molded article using first and second molds,
A heating step of heating the first mold to a first temperature and heating the second mold to a second temperature;
A filling step of filling a molten resin material into a cavity space formed by the first mold and the second mold heated in the heating step;
A cooling step for cooling the first mold and the second mold,
In the filling step, the temperature of the first mold is higher than the deflection temperature under load of the resin material, and the temperature of the second mold is lower than the deflection temperature under load of the resin material in the filling step. A temperature of 1 and a second temperature are defined;
An injection molding method in which, in the cooling step, cooling is performed so that the temperature of the first mold and the temperature of the second mold are lower than the load deflection temperature of the resin material. A closing step of closing the first mold and the second mold;
The heating step is performed before the closing step,
The injection molding method according to claim 1, wherein the first temperature is set to a temperature lower than a load deflection temperature of the resin material, and the second temperature is set to a temperature lower than the first temperature. A pressure holding step of supplying the molten resin material to the cavity space in response to the shrinkage of the resin material in the cavity space after the filling step;
During at least a part of the execution time of the pressure holding step, the temperature of the first mold is maintained at a temperature higher than the deflection temperature under load of the resin material, and the temperature of the second mold is deflected under load of the resin material. The injection molding method according to claim 2, wherein the injection molding method is maintained at a temperature lower than the temperature.   The injection molding method according to claim 3, wherein the first temperature is determined so that the temperature of the first mold is lower than the deflection temperature under load of the resin material before the pressure holding step is finished.   The temperature difference between the temperature of the first mold in the filling step and the deflection temperature under load of the resin material is smaller than the temperature difference between the temperature of the second mold in the filling step and the deflection temperature under load of the resin material. The injection molding method according to any one of claims 1 to 4, wherein the first temperature and the second temperature are determined. A pressure holding step of supplying the molten resin material to the cavity space in response to the shrinkage of the resin material in the cavity space after the filling step;
At least in the pressure holding step, the first mold is fixed, and the second mold is moved with respect to the first mold to close the first mold and the second mold. The injection molding method according to any one of claims 1 to 5. A method of injection-molding a resin molded product that forms a housing of an electronic device using a first mold and a second mold,
The first mold that molds the surface of the resin molded product that becomes the inner surface of the housing is heated to a first temperature, and the second mold that molds the surface of the resin molded product that becomes the outer surface of the housing is the second. A heating step for heating to a temperature of
A filling step of filling a molten resin material into a space formed by the first mold and the second mold heated in the heating step;
A cooling step for cooling the first mold and the second mold,
Heating is performed so that the temperature of the first mold is higher than the deflection temperature under load of the resin material in the filling step, and the temperature of the second mold is lower than the deflection temperature under load of the resin material in the filling step. A first temperature and a second temperature in the step are defined;
An injection molding method in which, in the cooling step, cooling is performed so that the temperature of the first mold and the temperature of the second mold are lower than the load deflection temperature of the resin material.

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