JP2013043341A - Decompression drying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a decompression drying device that evenly promotes the drying of a matter to be dried in a drying container.SOLUTION: In a process of drying a resin material, the resin material is housed in a drying hopper 3 and the interior of the drying hopper 3 is decompressed. Boiling point of water content contained in the resin material is lowered by the decompression and, the resin material therefore can be dried at a low temperature. As the decompression in the drying hopper 3 is proceeded, a leak valve 30 is opened and closed so as to introduce air into the drying hopper 3 intermittently (in pulse-like pattern) while continuing the exhausting from the interior of the drying hopper 3 in order to prevent the lowering of discharge amount of water vapor from the interior of the drying hopper 3. As a result, the balance between the exhaust amount from the interior of the drying hopper 3 and the introduction amount of air into the drying hopper 3 is instantaneously lost, and the pressure in the drying hopper 3 is therefore changed suddenly and the environment in the drying hopper 3 is changed into an environment where convection flow is easily generated in the drying hopper 3.

Description

  The present invention relates to a vacuum drying apparatus for drying an object to be dried such as a resin material under reduced pressure.

  For example, a resin material (such as an engineering plastic material) has a hygroscopic property and absorbs moisture in the atmosphere. When injection molding is performed using a resin material containing a lot of moisture, the strength of the molded product may decrease due to hydrolysis of the resin in the injection molding machine, or silver stripes (silver streaks) may appear on the surface of the molded product. May occur. Therefore, prior to supplying the resin material to the injection molding machine, usually preliminary drying for removing moisture from the resin material is performed.

  A vacuum drying apparatus is known as a drying apparatus for preliminary drying. The vacuum drying apparatus includes, for example, a drying hopper that accommodates a resin material. A vacuum line extending from the vacuum pump is connected to the drying hopper. When the resin material is dried, the vacuum pump is operated to reduce the pressure in the drying hopper containing the resin material, and heat of vaporization is given to the moisture contained in the resin material. Thereby, moisture contained in the resin material is vaporized, while air containing water vapor in the drying hopper is sucked out to the vacuum line.

JP 2010-255977 A

  As pressure reduction in the drying hopper progresses, the pressure in the drying hopper reaches a state where the pressure does not drop any further, and the amount of exhaust from the drying hopper decreases. As the exhaust amount decreases, the amount of water vapor discharged from the drying hopper decreases, and the amount of water vaporized from the resin material decreases. As a result, the drying efficiency of the resin material is reduced.

  In order to prevent a decrease in the amount of water vapor discharged from the drying hopper, for example, it is conceivable to introduce gas into the drying hopper at a small flow rate while reducing the pressure in the drying hopper (while operating the vacuum pump). However, in this method, since a constant flow of gas is generated in the drying hopper and uniform convection of the gas cannot be generated in the drying hopper, drying unevenness occurs in the resin material in the drying hopper.

  An object of the present invention is to provide a reduced-pressure drying apparatus that can uniformly promote drying of an object to be dried in a drying container.

  In order to achieve the above object, a reduced-pressure drying apparatus according to the present invention includes a drying container that accommodates an object to be dried, a decompression unit that decompresses the drying container by exhausting from the drying container, and the drying container. A gas introducing means for introducing gas into the pressure, a pressure detecting means for detecting a reduced pressure value in the drying container, and a reduced pressure value detected by the pressure detecting means so as to be maintained within a predetermined reduced pressure range. Intermittent operation means for intermittently operating the gas introduction means.

  An object to be dried is accommodated in a drying container. By reducing the pressure in the drying container, the boiling point of the moisture contained in the object to be dried is lowered, so that the object to be dried can be quickly dried at a low temperature.

  When the pressure in the drying container is reduced, gas is intermittently introduced into the drying container by the gas introduction means while the exhaust from the drying container is continued to prevent a decrease in the amount of water vapor discharged from the drying container ( Introduced in a pulsed manner). As a result, the balance between the amount of exhaust from the drying container and the amount of gas introduced into the drying container is instantaneously disrupted, so the pressure in the drying container suddenly changes, and the environment in the drying container is convected in the drying container. It becomes an environment that tends to occur. As a result, uniform gas convection can be generated in the drying container. Further, the water vapor in the drying container can be discharged together with the exhaust. Therefore, the drying of the material to be dried can be promoted uniformly in the drying container.

  Further, the gas is intermittently introduced into the drying container so that the pressure (reduced pressure value) in the drying container is maintained within a predetermined reduced pressure range. Thereby, it can prevent that the pressure in a drying container rises too much (a pressure reduction value falls too much), and can prevent the fall of the drying efficiency of a to-be-dried object.

  The reduced-pressure drying apparatus is divided into a plurality of small ranges, and the operation is set for each small range so that the operation time of the gas introduction means becomes longer as the small range including a relatively large reduced pressure value is included. It is preferable to further include a duty ratio storage means for storing the duty ratio of time. And it is preferable that an intermittent operation means reads the duty ratio according to the pressure reduction value in a drying container from a duty ratio memory | storage means, and operates a gas introduction means intermittently with the said duty ratio.

  Thereby, when the depressurization value in the drying container is relatively large, the amount of gas introduced into the drying container is relatively large. On the other hand, when the reduced pressure value in the drying container is relatively small, the amount of gas introduced into the drying container is relatively small. As a result, when the reduced pressure value in the drying container is relatively large, the reduced pressure value in the drying container can be reliably changed suddenly. Further, when the reduced pressure value in the drying container is relatively small, it is possible to prevent a rapid decrease in the reduced pressure value in the drying container.

  More preferably, when the gas introduction unit is intermittently operated at the duty ratio with respect to a small range including the lower limit value of the reduced pressure range, the amount of gas introduced into the drying container by the gas introduction unit is The duty ratio may be set so as to be less than the exhaust amount from the inside of the drying container by the decompression means.

  As a result, when the reduced pressure value in the drying container is reduced to near the lower limit value of the reduced pressure range, the amount of gas introduced into the drying container becomes smaller than the exhaust amount from the drying container, and the reduced pressure value in the drying container is reduced. To rise. Therefore, it can prevent that the pressure reduction value in a dry container becomes out of a pressure reduction value range. As a result, the drying efficiency of the object to be dried can be further prevented.

  It is preferable that the reduced pressure drying apparatus further includes a heating means for heating an object to be dried accommodated in the drying container.

  When the object to be dried in the drying container is heated, drying of the object to be dried can be further promoted.

  When the object to be dried in the drying container is heated in a stationary state in which almost no convection of gas is generated in the drying container, there is a risk that an overheated state that does not evaporate even when the water contained in the object to be dried reaches the boiling point may occur. is there. In this overheated state, the drying efficiency of the object to be dried is reduced.

  Due to intermittent introduction of gas into the drying container, uniform convection is generated in the drying container, so that it is possible to prevent water contained in the heated object from being overheated. As a result, it is possible to prevent the drying efficiency of the object to be dried due to the heated state from being lowered, and the object to be dried can be dried more quickly.

  In addition, since uniform convection is generated in the drying container, good convection heat transfer can be generated, and heat generated from the heating means (heat of vaporization) can be supplied to the object to be dried. As a result, drying of the material to be dried can be further promoted.

  ADVANTAGE OF THE INVENTION According to this invention, the pressure in a drying container can be changed suddenly, hold | maintaining the pressure (decompression value) in a drying container within the predetermined pressure-reduction value range. Due to a sudden change in the pressure in the drying container, the environment in the drying container becomes an environment in which airflow is likely to occur in the drying container. As a result, uniform convection of gas can be generated in the drying container without causing a decrease in the drying efficiency of the object to be dried, and drying of the object to be dried can be promoted evenly in the drying container.

FIG. 1 is a schematic cross-sectional view of a vacuum drying apparatus according to an embodiment of the present invention. FIG. 2 is a block diagram showing an electrical configuration of the vacuum drying apparatus. FIG. 3 is a flowchart (main routine) showing an overall flow of leak control executed in the step of drying the resin material. FIG. 4 is a flowchart (subroutine) showing the flow of small leak control executed in the middle of leak control. FIG. 5 is a timing chart showing the opening / closing timing of the leak valve during the leak control. FIG. 6 is a graph showing a change in gauge pressure (reduced pressure value) in the drying hopper during leak control.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a schematic cross-sectional view of a vacuum drying apparatus according to an embodiment of the present invention.

  The vacuum drying apparatus 1 is an apparatus that is connected to the injection molding machine 2 and dries (preliminarily dry) the resin material (pellets) supplied to the injection molding machine 2 before the supply. The vacuum drying apparatus 1 includes a drying hopper 3, a buffer hopper 4 that temporarily stores a resin material supplied to the drying hopper 3, and a loader hopper 5 that is supplied with a resin material dried by the drying hopper 3. .

  The drying hopper 3 is made of stainless steel or the like. The upper part 6 of the drying hopper 3 has a cylindrical shape extending in the vertical direction. The lower portion 7 of the drying hopper 3 is formed continuously with the upper portion 6 and has a conical shape that tapers downward.

  A spacer 8 is provided in the drying hopper 3. The spacer 8 is made of stainless steel. The spacer 8 has a hollow substantially cylindrical shape. The spacer 8 is disposed so that the drying hoppers 3 and their central axes coincide with each other, and a constant distance is provided between the outer peripheral surface of the spacer 8 and the inner peripheral surface of the upper portion 6 of the drying hopper 3 in the circumferential direction. There is a gap. The upper portion of the spacer 8 has a conical shape that tapers upward.

  A first heater 9, a second heater 10, and a third heater 11 are provided on the outer peripheral surface of the upper portion 6, the outer peripheral surface of the lower portion 7, and the inner peripheral surface of the spacer 8, respectively. The first heater 9, the second heater 10, and the third heater 11 are band heaters, and are in close contact with each other over the entire circumference of the outer peripheral surface of the upper portion 6, the outer peripheral surface of the lower portion 7, and the inner peripheral surface of the spacer 8. ing.

  The upper end opening of the drying hopper 3 is covered with an upper lid 12 so as to be opened and closed. A vacuum line 14 extending from the vacuum pump 13 is connected to the upper lid 12. A vacuum filter 15 is interposed in the middle of the vacuum line 14 to prevent the resin material or the like from entering the vacuum pump 13.

  When the vacuum pump 13 is operated, the air in the drying hopper 3 is sucked out to the vacuum line 14, and the inside of the drying hopper 3 is decompressed to a vacuum state. In parallel with this, the first heater 9, the second heater 10, and the third heater 11 are turned on (energized), and the resin material in the drying hopper 3 is changed to the first heater 9, the second heater 10, and the third heater. Heated with heat generated from 11. Thereby, the reduced pressure drying of the resin material is achieved.

  In addition, a vacuum release valve 16 for releasing the vacuum state in the drying hopper 3 is branched and connected to the vacuum line 14.

  The vacuum state is not limited to a completely vacuum state, but includes a state close to a vacuum.

  The buffer hopper 4 is disposed above the drying hopper 3. The lower part of the buffer hopper 4 has a conical shape that tapers downward. The upper end of the connection pipe 17 is connected to the lower end portion of the buffer hopper 4. The lower end of the connecting pipe 17 is connected to the central portion of the upper lid 12 of the drying hopper 3. A supply valve 18 that opens and closes the connection pipe 17 is interposed in the connection pipe 17.

  A first branch intake line 20 extending from the selection valve 19 is connected to the buffer hopper 4. An intake line 22 extending from the transport blower 21 is connected to the selection valve 19. A transport filter 23 is interposed in the intake line 22 to prevent the resin material or the like from entering the transport blower 21.

  The buffer hopper 4 is connected to a material supply pipe (not shown).

  If the intake line 22 and the first branch intake line 20 are connected (communication) by the selection valve 19 while the transport blower 21 is operating, the air in the buffer hopper 4 is sucked into the first branch intake line 20. Then, the resin material is supplied into the buffer hopper 4 from the material supply pipe by pneumatic force. When the supply valve 18 is opened, the resin material in the buffer hopper 4 flows into the drying hopper 3 through the connection pipe 17 due to its own weight.

  The resin material flowing into the drying hopper 3 from the connection pipe 17 falls on the upper part of the spacer 8. Since the upper part of the spacer 8 is formed in a conical shape that tapers upward, the resin material that falls on the upper part of the spacer 8 is dispersed almost uniformly in the circumferential direction of the spacer 8 (drying hopper 3). Are dropped between the inner peripheral surface of the drying hopper 3 and the outer peripheral surface of the spacer 8.

  The loader hopper 5 is disposed above the injection molding machine 2. The lower part of the loader hopper 5 has a conical shape that tapers downward. The upper end of the connection pipe 24 is connected to the lower end portion of the loader hopper 5. The lower end of the connection pipe 24 is connected to the injection molding machine 2. A supply valve 25 that opens and closes the connection pipe 24 is interposed in the connection pipe 24.

  A second branch intake line 26 extending from the selection valve 19 is connected to the loader hopper 5.

  In addition, one end of a transport pipe 27 is connected to the loader hopper 5. The other end of the transport pipe 27 is connected to the lower end of the drying hopper 3 through a transport valve 28.

  If the intake line 22 and the second branch intake line 26 are connected (communication) by the selection valve 19 in a state where the transport blower 21 is activated and the transport valve 28 is opened, the air in the loader hopper 5 is second The resin material that has been sucked into the branch intake line 26 and is dry in the dry hopper 3 is supplied into the loader hopper 5 through the transport pipe 27 by pneumatic force. When the supply valve 25 is opened, the resin material in the loader hopper 5 flows into the injection molding machine 2 through the connection pipe 24 due to its own weight.

  One end of a leak line 29 is connected to the middle portion of the transport pipe 27. The other end of the leak line 29 is open to the atmosphere. A leak valve 30 and a leak filter 31 are interposed in the leak line 29. The leak valve 30 is composed of, for example, an electromagnetic valve, and opens and closes the leak line 29. The leak filter 31 is interposed upstream of the leak valve 30 in the air flow direction (open end side of the leak line 29), and prevents entry of dust and the like into the leak line 29.

  Note that one end of the leak line 29 may be connected to the lower end of the drying hopper 3.

  FIG. 2 is a block diagram showing an electrical configuration of the vacuum drying apparatus.

  The reduced pressure drying apparatus 1 includes a control unit 41 including a microcomputer including a CPU 41A and a memory 41B.

  Further, the vacuum drying apparatus 1 is provided with a first temperature sensor 42, a second temperature sensor 43, and a third temperature sensor 44 in association with the first heater 9, the second heater 10, and the third heater 11, respectively. Yes. The first temperature sensor 42, the second temperature sensor 43, and the third temperature sensor 44 detect the temperatures (heat generation temperatures) of the first heater 9, the second heater 10, and the third heater 11, respectively. Detection signals from the first temperature sensor 42, the second temperature sensor 43, and the third temperature sensor 44 are input to the control unit 41.

  Further, the reduced pressure drying apparatus 1 is provided with a pressure sensor 45 that detects the gauge pressure (reduced pressure value) PM of the vacuum line 14. Since the gauge pressure PM in the vacuum line 14 is equal to the gauge pressure in the dry hopper 3, detecting the gauge pressure PM in the vacuum line 14 is the same as detecting the gauge pressure in the dry hopper 3. A detection signal of the pressure sensor 45 is input to the control unit 41.

  The controller 41 includes a first heater 9, a second heater 10, a third heater 11, a vacuum pump 13, a vacuum release valve 16, supply valves 18, 25, a selection valve 19, a transport blower 21, a transport valve 28, and a leak valve. 30 is connected as a control target.

  In the vacuum drying apparatus 1, the supply valve 18, the selection valve 19, and the transport blower 21 are controlled by the control unit 41, and the resin material to be dried is supplied to the drying hopper 3. Specifically, the transport blower 21 is activated. Further, the selection valve 19 is switched, and the intake line 22 and the first branch intake line 20 are connected (communication). Thereby, the resin material is supplied into the buffer hopper 4 from a material supply pipe (not shown). When the supply of the resin material to the buffer hopper 4 is completed, the transport blower 21 is stopped. Thereafter, the supply valve 18 is opened, and the resin material is supplied from the buffer hopper 4 into the drying hopper 3. At this time, the transport valve 28 and the leak valve 30 are closed.

  When a predetermined amount of resin material is supplied to the drying hopper 3, the supply valve 18 is closed by the control unit 41. Thereafter, the vacuum pump 13 is operated by the control unit 41. In addition, the first heater 9, the second heater 10, and the third heater 11 are turned on (energized) by the control unit 41, and the resin material in the drying hopper 3 is changed to the first heater 9, the second heater 10, and the third heater. Heated with heat generated from 11. Thereby, heat of vaporization is given to the moisture contained in the resin material, and the moisture contained in the resin material is vaporized. On the other hand, air containing water vapor in the drying hopper 3 is sucked out to the vacuum line 14.

  Then, when a predetermined time elapses after the supply valve 18 is closed, the step of drying the resin material is completed, and the vacuum pump 13 is stopped by the control unit 41, and the first heater 9, the second heater 10, and The third heater 11 is turned off.

  Thereafter, the control unit 41 controls the selection valve 19, the transport blower 21, and the transport valve 28 to transport the dried resin material from the dry hopper 3 to the loader hopper 5. Specifically, the transport blower 21 is activated. In addition, the transport valve 28 is opened. Further, the selection valve 19 is switched to connect (communicate) the intake line 22 and the second branch intake line 26. Thereby, the dried resin material in the dry hopper 3 is supplied into the loader hopper 5 through the transport pipe 27. When the transport of the resin material to the loader hopper 5 is completed, the transport valve 28 is closed and the transport blower 21 is stopped. At this time, the leak valve 30 is closed.

  FIG. 3 is a flowchart showing the overall flow of leak control executed in the step of drying the resin material. FIG. 4 is a flowchart showing a flow of small leak control executed in the middle of leak control. FIG. 5 is a timing chart showing the opening / closing timing of the leak valve during the leak control. FIG. 6 is a graph showing a change in gauge pressure (reduced pressure value) in the drying hopper during leak control.

  In the step of drying the resin material, the inside of the drying hopper 3 is depressurized by the operation of the vacuum pump 13. When the pressure in the drying hopper 3 is reduced and the inside of the drying hopper 3 is in a vacuum state, the pressure in the drying hopper 3 does not decrease any more. If this state continues, the amount of gas exhausted from the drying hopper 3 to the vacuum line 14 is small, and the water vapor in the drying hopper 3 cannot be sufficiently discharged. Therefore, the drying efficiency of the resin material (the amount of water vaporized in the resin material) Decreases.

  Therefore, the vacuum drying apparatus 1 performs the opening / closing control (leak control) of the leak valve 30 described below.

  In the step of drying the resin material, the control unit 41 acquires the detection signal of the pressure sensor 45 at a constant period. After the operation of the vacuum pump 13, the control unit 41 first acquires the gauge pressure (the gauge pressure of the vacuum line 14) PM in the drying hopper 3 from the detection signal of the pressure sensor 45 as shown in FIG. 3. Then, it is determined whether or not the gauge pressure PM is larger than an upper limit value PH of a predetermined reduced pressure range R (PH-PL) (step S1).

  Until the gauge pressure PM exceeds the upper limit PH (NO in step S1), the leak valve 30 remains closed.

  When the gauge pressure PM exceeds the upper limit PH (YES in step S1), as shown in FIG. 5, the controller 41 starts large leak control (step S2). In the large leak control, the leak valve 30 is opened / closed (ON / OFF) at a predetermined large leak duty ratio DH. The large leak duty ratio DH is set to 75%, for example, and is stored in the memory 41B of the control unit 41 in association with the range OR exceeding the upper limit PH of the decompression value range R.

  When the leak valve 30 is opened, air (gas) flows into the drying hopper 3 from the leak line 29 due to a pressure difference between the inside and outside of the drying hopper 3. Since the leak valve 30 is opened intermittently, air is introduced into the drying hopper 3 from the leak line 29 in a pulsed manner. Thereby, the pressure in the drying hopper 3 changes suddenly, and the environment in the drying hopper 3 becomes an environment in which convection easily occurs in the drying hopper 3. Further, when the leak valve 30 is opened and closed with a large leak duty ratio DH, a relatively large amount of air flows into the dry hopper 3, and the gauge pressure PM in the dry hopper 3 is compared as shown in FIG. Decline rapidly.

  After the start of the large leak control, as shown in FIG. 3, it is repeatedly checked whether or not the gauge pressure PM has dropped below the upper limit PH of the reduced pressure range R (step S3).

  Until the gauge pressure PM drops below the upper limit PH (NO in step S3), the control unit 41 continues the large leak control.

  When the gauge pressure PM falls below the upper limit PH (YES in step S3), the large leak control is terminated (step S4).

  Subsequently, the control unit 41 starts small leak control (step S5).

  For small leak control, the reduced pressure range R is divided into, for example, three small ranges R1 (PH-PC1), R2 (PC1-PC2), and R3 (PC2-PL), and the memory 41B of the control unit 41 Stores small leak duty ratios DL1, DL2, and DL3, respectively, corresponding to the small ranges R1, R2, and R3.

  The small leak duty ratio DL1 is smaller than the large leak duty ratio DH. Further, the small leak duty ratio DL2 is smaller than the small leak duty ratio DL1. Further, the small leak duty ratio DL3 is smaller than the small leak duty ratio DL2. That is, the small leak duty ratios DL1, DL2, and DL3 are set so that the inequality DH> DL1> DL2> DL3 is satisfied. The small leak duty ratios DL1, DL2, and DL3 are set to 60%, 45%, and 30%, respectively, for example.

  In the small leak control, as shown in FIG. 4, the control unit 41 first determines whether or not the gauge pressure PM is larger than the first threshold value PC1 that is the upper limit value of the small range R2 (step). S51). That is, it is determined whether or not the inequality PC1 <PM ≦ PH is satisfied.

  Immediately after the gauge pressure PM drops below the upper limit value PH of the reduced pressure range R, the gauge pressure PM is greater than the first threshold value PC1, so is the gauge pressure PM normally greater than the first threshold value PC1? The determination of whether or not is affirmed (YES in step S51). Then, the small leak duty ratio DL1 stored in association with the small range R1 is read from the memory 41B, and the control unit 41 uses the small leak duty ratio DL1 to leak the leak valve 30 as shown in FIG. Control for opening and closing is started (step S52).

  Also in this control, since the leak valve 30 is intermittently opened, air is introduced into the drying hopper 3 from the leak line 29 in a pulsed manner. Further, since the small leak duty ratio DL1 is smaller than the large leak duty ratio DH, in this control, the amount of air flowing into the drying hopper 3 is reduced as compared with the large leak control, as shown in FIG. As described above, the gauge pressure PM in the drying hopper 3 gradually decreases.

  After the control for opening and closing the leak valve 30 with the small leak duty ratio DL1 is started, the subroutine returns to the main routine shown in FIG. 3 from the subroutine shown in FIG. It is determined whether or not the value has fallen below the lower limit value PL (step S6).

  At this point, since the gauge pressure PM has not normally decreased below the lower limit PL (NO in step S6), it is then determined whether or not the process of drying the resin material by the control unit 41 is completed. It is determined whether or not a predetermined time has elapsed since the supply valve 18 was closed (step S7).

  If the process of drying the resin material is not completed (NO in step S7), the process proceeds to a subroutine shown in FIG. 4 (step S5), and the small leak control is continued.

  While the steps S5 to S7 are repeated and the small league control is continued, if the gauge pressure PM falls below the first threshold value PC1 (NO in step S51), the control unit 41 causes the gauge pressure PM to be small. It is determined whether or not it is greater than a second threshold value PC2 that is the upper limit value of the range R3 (step S53). That is, it is determined whether or not the inequality PC2 <PM ≦ PC1 is satisfied.

  Immediately after the gauge pressure PM has dropped below the first threshold value PC1, the gauge pressure PM is greater than the second threshold value PC2, and therefore whether or not the gauge pressure PM is usually greater than the second threshold value PC2 or not. Is affirmed (YES in step S53). Then, the small leak duty ratio DL2 stored in association with the small range R2 is read from the memory 41B, and the control unit 41 uses the small leak duty ratio DL2 to leak the leak valve 30 as shown in FIG. Control for opening and closing is started (step S54).

  Also in this control, since the leak valve 30 is intermittently opened, air is introduced into the drying hopper 3 from the leak line 29 in a pulsed manner. Further, since the small leak duty ratio DL2 is smaller than the small leak duty ratio DL1, in this control, the amount of air flowing into the drying hopper 3 is reduced as compared with the control at the small leak duty ratio DL1. As shown in FIG. 6, the gauge pressure PM in the drying hopper 3 gradually decreases.

  After the control for opening and closing the leak valve 30 with the small leak duty ratio DL2 is started, the control unit 41 returns to the main routine shown in FIG. 3 again from the subroutine shown in FIG. It is determined whether or not the value has fallen below the lower limit value PL of R (step S6).

  At this point in time, the gauge pressure PM has not normally decreased below the lower limit value PL (NO in step S6), so the control unit 41 determines whether or not the process of drying the resin material is complete. (Step S7).

  If the process of drying the resin material is not completed (NO in step S7), the process proceeds to a subroutine shown in FIG. 4 (step S5), and the small leak control is continued.

  When steps S5 to S7 are repeated and the small league control is continued, if the gauge pressure PM drops below the second threshold value PC2 (NO in step S53), the memory 41B stores it in association with the small range R3. The small leak duty ratio DL3 thus read is read out. Then, as shown in FIG. 5, the control unit 41 starts control to open and close the leak valve 30 with the small leak duty ratio DL3 (step S55).

  Also in this control, since the leak valve 30 is intermittently opened, air is introduced into the drying hopper 3 from the leak line 29 in a pulsed manner. The small leak duty ratio DL3 is set to a duty ratio that is smaller than the small leak duty ratio DL2 so that the amount of air introduced into the dry hopper 3 is less than the exhaust amount from the dry hopper 3. Therefore, as shown in FIG. 6, normally, immediately after this control is started, the gauge pressure PM in the dry hopper 3 slightly decreases. However, as the control proceeds, the gauge pressure PM in the dry hopper 3 gradually decreases. Begins to rise.

  After the control to open / close the leak valve 30 with the small leak duty ratio DL3 is started, the subroutine returns to the main routine shown in FIG. 3 again from the subroutine shown in FIG. It is determined whether or not the value has fallen below the lower limit value PL of R (step S6).

  If the gauge pressure PM has not dropped below the lower limit PL (NO in step S6), the control unit 41 determines whether or not the process of drying the resin material is complete (step S7).

  If the process of drying the resin material is not completed (NO in step S7), the process proceeds to a subroutine shown in FIG. 4 (step S5), and the small leak control is continued.

  When the steps S5 to S7 are repeated and the control with the small leak duty ratio DL3 proceeds, the gauge pressure PM normally becomes larger than the second threshold value PC2. Then, the determination whether the gauge pressure PM is greater than the second threshold value PC2 is affirmed (YES in step S53). Then, the small leak duty ratio DL2 stored in association with the small range R2 is read from the memory 41B, and control for opening and closing the leak valve 30 with the small leak duty ratio DL2 is started again by the control unit 41. (Step S54).

  Thus, the small leak control is performed after the large leak control, whereby the gauge pressure PM in the drying hopper 3 is maintained within the reduced pressure range R.

  When the step of drying the resin material is completed, that is, when a predetermined time has elapsed after the supply valve 18 is closed (YES in step S7), the leak valve 30 is closed (step S8). Open / close control is terminated.

  If for some reason the gauge pressure PM drops below the lower limit PL for some reason before the process of drying the resin material is completed (YES in step S6), the leak valve 30 is closed (step S9). Then, the leak valve 30 is kept closed until the gauge pressure PM exceeds the upper limit PH (NO in step S1).

  As described above, in the step of drying the resin material, the resin material that is the object to be dried is accommodated in the drying hopper 3 and the inside of the drying hopper 3 is decompressed. Since the boiling point of the moisture contained in the resin material is reduced by this reduced pressure, the resin material can be quickly dried at a low temperature.

  When the pressure reduction in the drying hopper 3 proceeds, the vacuum valve 30 is configured to continue the exhaust from the drying hopper 3 in the vacuum drying apparatus 1 to prevent a decrease in the amount of water vapor discharged from the drying hopper 3. The air is opened and closed, and air is intermittently (pulsed) introduced into the drying hopper 3. As a result, the balance between the amount of exhaust from the drying hopper 3 and the amount of air introduced into the drying hopper 3 is instantaneously broken, so that the pressure in the drying hopper 3 changes suddenly, and the environment in the drying hopper 3 changes to the drying hopper 3. 3 is an environment in which convection is likely to occur. As a result, uniform convection of gas can be generated in the drying hopper 3. Further, it is possible to satisfactorily discharged to the vacuum line 14 together with the exhaust steam in the drying hopper 3. Therefore, the drying of the resin material can be promoted uniformly in the drying hopper 3.

  Further, air is intermittently introduced into the drying hopper 3 so that the gauge pressure (reduced pressure value) in the drying hopper 3 is maintained within a predetermined reduced pressure range R. Thereby, it can prevent that the pressure in the drying hopper 3 goes up too much (a gauge pressure falls too much), and can prevent the fall of the drying efficiency of a resin material.

  In the reduced pressure drying apparatus 1, the reduced pressure value range R is divided into a plurality of small ranges R1, R2, and R3, and the memory 41B of the control unit 41 is associated with the small ranges R1, R2, and R3, and has a small leak duty. The ratios DL1, DL2 and DL3 are stored. The small leak duty ratios DL1, DL2, and DL3 are set so that the one-time opening time of the leak valve 30 becomes longer in the small ranges R1, R2, and R3 including a relatively large gauge pressure. Then, the small leak duty ratios DL1, DL2, DL3 corresponding to the gauge pressure PM in the drying hopper 3 are read from the memory 41B, and the leak valve 30 is operated with the read small leak duty ratios DL1, DL2, DL3. Opened and closed.

  Thereby, when the gauge pressure PM in the dry hopper 3 is relatively large, the amount of air introduced into the dry hopper 3 is relatively large. On the other hand, when the gauge pressure PM in the drying hopper 3 is relatively small, the amount of air introduced into the drying hopper 3 is relatively small. As a result, when the gauge pressure PM in the drying hopper 3 is relatively large, the gauge pressure PM in the drying hopper 3 can be surely suddenly changed. Further, when the gauge pressure PM in the drying hopper 3 is relatively small, it is possible to prevent a rapid decrease in the gauge pressure PM in the drying hopper 3.

  For the small range R3 including the lower limit PL of the decompression value range R, the small leak duty ratio DL3 is set so that the amount of air introduced into the drying hopper 3 is less than the amount of exhaust from the drying hopper 3. Has been.

  As a result, when the gauge pressure PM in the drying hopper 3 decreases to near the lower limit value of the reduced pressure range R, the amount of air introduced into the drying hopper 3 becomes smaller than the amount of exhaust from the drying hopper 3, and the drying hopper 3 gauge pressure PM rises. Therefore, the gauge pressure PM in the drying hopper 3 can be prevented from being outside the reduced pressure range R. As a result, it is possible to further prevent the drying efficiency of the resin material from decreasing.

  The drying hopper 3 is provided with a first heater 9, a second heater 10, and a third heater 11. In the step of drying the resin material, the first heater 9, the second heater 10, and the third heater 11 are turned on.

  Thereby, since the resin material in the drying hopper 3 is heated, drying of the resin material can be further promoted.

  When the resin material in the drying hopper 3 is heated in a stationary state in which almost no convection of gas occurs in the drying hopper 3, there is a possibility that an overheating state in which the water contained in the resin material does not evaporate even when the water reaches the boiling point may occur. is there. In this overheated state, the drying efficiency of the resin material decreases.

  Due to the intermittent introduction of air into the drying hopper 3, uniform convection is generated in the drying hopper 3, so that it is possible to prevent the water contained in the article to be heated from being overheated. As a result, a decrease in the drying efficiency of the resin material due to the heated state can be prevented, and the resin material can be dried more quickly.

  Further, since uniform convection is generated in the dry hopper 3, good convection heat transfer can be generated, and heat generated from the heating means (heat of vaporization) can be satisfactorily supplied to the resin material. As a result, drying of the resin material can be further promoted.

  As mentioned above, although one Embodiment of this invention was described, this invention can also be implemented with another form.

  For example, a configuration in which air (atmosphere) is introduced into the dry hopper 3 through the leak line 29 is taken as an example, but the gas introduced into the dry hopper 3 is not limited to air, and dry nitrogen gas, etc. Various gases that do not adversely affect the resin material can be used.

  Further, the object to be heated is not limited to the resin material, and may be a granular material other than the resin material, or may be a fluid such as air or water.

  In addition, various design changes can be made to the above-described configuration within the scope of the matters described in the claims.

1 Vacuum dryer 3 Drying hopper (drying container)
9 First heater (heating means)
10 Second heater (heating means)
11 Third heater (heating means)
13 Vacuum pump (pressure reduction means)
14 Vacuum line (pressure reduction means)
29 Leak line (gas introduction means)
30 Leak valve (gas introduction means)
41 Control unit (intermittent operation means)
41A CPU (intermittent operation means)
41B memory (duty ratio storage means)
45 Pressure sensor (pressure detection means)

Claims (4)

A drying container for storing the object to be dried;
Decompression means for decompressing the inside of the drying container by exhausting from the inside of the drying container;
Gas introduction means for introducing gas into the drying container;
Pressure detecting means for detecting a reduced pressure value in the drying container;
A reduced pressure drying apparatus comprising: intermittent operation means for intermittently operating the gas introduction means so that the reduced pressure value detected by the pressure detection means is maintained within a predetermined reduced pressure value range. The operation time set for each of the small ranges so that the operation time of the gas introduction means becomes longer for the small range in which the pressure reduction value range is divided into a plurality of small ranges and a relatively large pressure reduction value is included. A duty ratio storage means for storing the duty ratio of
The intermittent operation means reads a duty ratio according to a reduced pressure value detected by the pressure detection means from the duty ratio storage means, and intermittently operates the gas introduction means at the duty ratio. Vacuum drying equipment.   The amount of gas introduced into the drying container by the gas introduction means when the gas introduction means is intermittently operated at the duty ratio with respect to the small range including the lower limit value of the reduced pressure range. The reduced-pressure drying apparatus according to claim 2, wherein a duty ratio is set so that the pressure is less than an exhaust amount from the inside of the drying container by the decompression unit.   The vacuum drying apparatus according to any one of claims 1 to 3, further comprising a heating means for heating an object to be dried accommodated in the drying container.

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