Plastic drying apparatus

Abstract: A plastic drying apparatus that uses microwave heating as means of heating a plastic (1) thereby allowing the surface as well as the inside of particles of the plastic to be heated simultaneously and therefore can attain to bring the plastic to a highly dried state within a short period of time. A certain amount of a plastic is fed into a plastic drying tank (2). While the plastic is stirred, the plastic is heated and dried uniformly by microwaves (21) generated by a microwave generating unit (11). The humidity is measured by a humidity measuring sensor (9) and the plastic is highly dried automatically. Dried air nozzles are provided to the wall surface of the plastic drying tank for discharging air (22) in a super-dried state in a widely flared manner so as to prevent the plastic powder, etc. from adhering.

Description: Field of the Invention The present invention relates to a plastic drying apparatus and, more particularly, to a plastic drying apparatus for bringing a plastic material used for injection molding, extrusion molding, etc. rapidly to a highly dried state. Description of the Prior art Conventionally, as plastic drying apparatuses, those are generally used which have as means of heating and drying the plastic material a dehumidifying drier that heats dried atmosphere and delivers it to a plastic drying tank. The dehumidifying drier heats and dries the plastic by a hot blast. In order to release moisture contained in the plastic by heating, first air is heated and is then sent around the plastic. Thus, the plastic is heated by the air around the plastic and at the same time the moisture in the plastic is heated, pressurized and released. Since the air for heating the plastic has a low heat conductivity and is used by blowing, the air for heating the plastic must be heated in a volume tens to hundreds times as large as the volume of the plastic. Since the heat conductivity of plastics is as low as that of heat insulating materials, the efficiency of the heat exchange is very low, the drying of the plastic starts from the surface of its particles by the hot blast and therefore it takes long time to remove the moisture completely. Accordingly it has been difficult to bring the plastic to a highly dried state and therefore the automatic drying apparatus is not satisfactury. The control of the humidity by prior art hygrometers is carried out mainly by human work in such a manner that air in a high-temperature highly dried state is continuously blown into the plastic drying tank for a period which seemed to secure the drying on the basis of the experience. That is, even if the plastic consists of the same material, the state of the material wherein the water content is high, for example, because the material is a virgin material, or a ground material, or because of the conditions under which the material is kept, must be considered. Therefore the prior art is uneconomical and takes much time. Further, the measurement of the water content of a plastic material only by a hygrometer involves many unstable factors including the temperature condition and the state of the flow of air and is only a guide. Plastic drying apparatuses can advantageously use microwave heating as means of heating the plastic material thereby allowing the surface as well as the inside of particles of the plastic to be heated simultaneously and therefore can attain to bring the plastic to a highly dried state within a short period of time. Also known is a microwave drying apparatus for plastic material (EP-A-0 312 741) wherein the wet pelletized or powdered plastic material is filled into a drying tank up to a desired level by conveying predetermined amounts of the wet material, and the temperature of the plastic material is regulated during the drying process. For an efficient and fast drying of the plastic material, it is desired that the drying process is controlled, and that the drying temperature is kept as high as possible but not too high so as to prevent the material from softening or melting. A control by measuring the absolute humidity of the drying process exhaust air, and a narrow and high margin of the temperature can be maintained by the fact that a predetermined amount of the plastic should always be present in the drying tank during drying of a batch, even if the supply becomes short for any reason. Summary of the Invention An important object of the present invention is to provide a plastic drying apparatus that can knead and dry uniformly powdery particles of plastic material when the plastic is heated by a hot blast or microwave energy, by a control of the parameters including the exhaust gas humidity for which control the amount of the material is to be kept constant for each batch. According to the invention, a plastic drying apparatus comprising a plastic raw material tank from which plastic material is supplied through a feeding side inlet into a dryer hopper, the dryer hopper for temporarily storing the plastic material and for feeding it into a drying tank, metering loader connected to said dryer hopper for metering and feeding predetermined batches of the plastic material into the plastic drying tank, the plastic drying tank in which heating and drying of the plastic material takes place, a dried air generating unit for generating dried air to be fed to said drying tank, plastic heating and drying means for heating and drying the plastic material in the plastic drying tank, a stirring means for stirring the plastic material in said plastic drying tank so as to have the plastic material uniformly heated, an air exhaust duct connected to the plastic drying tank, a sub-tank arranged subsequently to a dried plastic material outlet of the plastic drying tank, for temporarily keeping the dried plastic is characterized in that it further comprises a humidity sensor provided in the exhaust path of the moisture-laden air to be removed from said drying tank, a plastic re-supply unit for re-supplying some dried plastic from said sub-tank through a supply side inlet to said dryer hopper when the plastic raw material to be fed to the dryer hopper from said plastic raw material tank becomes short and the plastic to be loaded into the drying tank becomes short of a prescribed quantity, a flow rate adjusting unit for setting constant the dried air to be fed to said plastic drying tank supplied from said dried air generating unit, a dried air input to the sub-tank, through which input some dried air is fed to the sub-tank and a plastic drying control means including said temperature controller, to which means the measured values of said temperature sensor and said humidity sensor are inputted and which operates programs on the basis of previously set data to set a time limit and drives said heating and drying means, said stirring means, etc. By the invention, it is possible to measure stably the water absorption of the plastic material charged into the stirring tank in a certain quantity irrespective of a difference in its temperature by detecting the absolute humidity of the air in an exhaust path that has passed through the plastic, and to exploit the measuring result. To keep the certain quantity constant, the plastic re-supply unit serves for re-supplying the plastic kept in said sub-tank from the metering loader into the plastic drying tank to bring the quantity of the plastic in the plastic drying tank to a prescribed amount in the case that the amount of the plastic fed to the plastic drying tank is short of the prescribed amount. According to a particular embodiment, the plastic drying apparatus that can make the whole mass of a plastic uniform by air curtains generated by dried air nozzles to blow the plastic that adheres to the inner wall of a drying tank or rests on settling parts outside the driving region of stirring blades to bring it back into whirls of the stirred plastic. The measured value of the humidity measuring sensor is inputted into a control circuit that operates programs on the basis of previously set data to set a time limit and drives the microwave generating unit, the stirring means, etc. In this constitution, a certain amount of the plastic is metered and fed by the metering loader into the plastic drying tank and dry air dried by the dried air generating unit is fed into the plastic drying tank at a certain flow rate through the flow rate adjusting unit. By making constant the amount of the plastic in the plastic drying tank and the supply of the dried air to be fed into the plastic drying tank and by stirring uniformly the plastic in the plastic drying tank by the stirring means, the water content of the plastic can be measured indirectly from the humidity in the air discharged. In the control circuit, a time limit is set in accordance with the humidity of the air discharged from the plastic drying tank and measured by the humidity measuring sensor, and the control circuit drives the stirring means, the microwave generating unit, if any, for irradiating the plastic with microwaves to heat the plastic, etc. so that the plastic is brought to a prescribed highly dried state. The sub-tank keeps the plastic temporarily and feeds the plastic to the plastic drying tank through a plastic supply means when the quantity of the plastic to be fed into the plastic drying tank comes short of the prescribed amount. According to a preferred embodiment, in the plastic drying apparatus of the present invention the plastic material charged into the drying tank is stirred uniformly so that the plastic may be heated uniformly by a hot blast or microwaves. Such plastic drying apparatus comprises a bearing section that is fixed to the center of the bottom of the cylindrical drying tank into which the plastic will be charged, the top of the bearing section being formed to project from the plastic, a rotating shaft that is rotated about the axis of the bearing section, one end of the rotating shaft in the drying tank being projected beyond the top of the bearing section and being provided with stirring blades, a cover in the shape of a circular cone or a pyramid provided to the one end of said rotating shaft for covering the top of the bearing section, and a motor fixed to the bottom on the outside of the drying tank for driving the rotating shaft. Alternatively the plastic drying apparatus may comprise a bearing section that is fixed to the center of the bottom of the cylindrical drying tank into which the plastic will be charged, the top of the bearing section being formed to project from the plastic, a rotating shaft that is rotated about the axis of the bearing section, one end of the rotating shaft in the drying tank being projected beyond the top of the bearing section and being provided with stirring blades, a cover in the shape of a circular cone or a pyramid provided to the one end of said rotating shaft for covering the top of the bearing section, a motor fixed to the bottom on the outside of the drying tank for driving the rotating shaft, and one or more dried air blowoff units for blowing down the dried air along the wall surface of the drying tank. In the constitution of this embodiment, the stirring blades are moved, together with the rotating shaft, relatively between the wall surface of the drying tank and the bearing shaft, there are generated a force approximately turned in the plastic by the stirring blades and a force resting the plastic due to the frictional force along the circumferential wall surface and the bearing section, the plastic performs different complicated kneading movements in positions of the particles, and a hot blast or microwave energy heats and dries the plastic in the drying tank uniformly. According to a further embodiment of the present invention, the time when the plastic material has been brought to a prescribed dried state is detected and the drying apparatus is stopped. This plastic drying apparatus again comprises the stirring means for heating and drying, the dried air feeding means including a pressure adjusting unit for adjusting the pressure of air sent out from a compressor, a dryer for bringing air discharged from a flow rate adjusting unit to a highly dried state, and a flow rate adjusting unit for sending the air in the highly dried state discharged from the dryer to a stirring tank at a certain flow rate, and further comprises a discharge port provided at an upper part of the stirring tank for keeping the pressure in the stirring tank constant, an exhaust path for constant exhaust located near the exhaust port, and a plastic drying control means connected to leads or output terminals of the temperature sensor and the humidity sensor provided in the exhaust path for calculating the absolute humidity and controlling the plastic drying apparatus. The plastic drying control means comprises an arithmetic circuit connected to the lead terminals or output terminals of the temperature sensor and the humidity sensor for calculating the absolute humidity, a humidity setting circuit whose output signal level can be arbitrarily set, a comparator which compares the output signal of the humidity setting circuit with the output signal of the comparator and operates by detecting the case when the output signal of the humidity setting circuit has become larger or smaller in level than the output signal of the arithmetic circuit, and a driving circuit that is turned on or off by receiving the operation signal of the comparator. In this constitution, a certain amount of moist plastic material is charged into the stirring tank, where it is heated and stirred. The heated and stirred plastic heats the moisture contained therein and its evaporation takes place correspondingly to the water content. By passing air in a highly dried state through the plastic in the stirring tank at a certain pressure and a certain flow rate, the air will contain moisture corresponding to the water content of the plastic, that is, will have an absolute humidity. The dried air feeding means dries the air compressed by the compressor and sends the dried air into the stirring tank at a certain pressure and a certain flow rate. The exhaust port removes the air to stabilize the pressure in the stirring tank and acts with the dried air feeding means to make the flow of the air in the stirring tank have a stabilized pressure. A small part of the air thus having a stabilized pressure and flow is sucked from the stirring tank at a certain flow rate by a suction pump. Therefore, the absolute humidity of the air passing through the exhaust path corresponds to the water content of the plastic stirred uniformly in the stirring tank. Then, by providing a temperature sensor and a humidity sensor on the exhaust path, the temperature and the absolute humidity of the air can be measured. The measurement values of the temperature sensor and the humidity sensor are inputted into the dried air control means, where the absolute humidity is calculated and the time is detected when the absolute humidity has reached a prescribed absolute humidity to stop the drying operation of the plastic in the stirring tank, so that the plastic is made ready for injection molding or extrusion molding. According to a further embodiment of the present invention, the mentioned air type wiping means is provided for preventing a plastic powder or the like from adhering to the wall surface of the plastic drying tank of the plastic drying apparatus of the present invention, which air type wiping means comprises a dried air generating unit for sending air into the drying tank where the plastic is heated and stirred and dried air nozzles provided at the upper part of the wall surface of the drying tank for discharging the air in a super-dried state generated by the dried air generating unit along the inner wall of the drying tank in a widely flared manner. The dried air nozzles are arranged at the upper part of the wall surface of the plastic drying tank to discharge the dried air along the inner wall downward slantly in a widely flared manner. In this constitution, by discharging and blowing down the dried air from the dried air nozzles along the inner wall of the drying tank in a widely flared manner, the plastic adhering to the inner wall is blown, and the flows of the air along the inner wall form air curtains of compressed air layers to prevent the powdery plastic from adhering. The air in a super-dried state discharged from the dried air nozzles forms air curtains widely along the inner wall, so that the air is discharged in a widely flared manner. By directing downward slantly the discharge direction of the air, the areas of the air curtains can be made large. In this manner, the plastic in the drying tank can be heated and dried uniformly. Brief Description of the Drawings Fig. 1 is a flow sheet illustrating a plastic drying apparatus showing a first embodiment according to the present invention. Fig. 2 shows an axial section through a stirring means according to an embodiment of the present invention. Fig. 3 is a diagram illustrating an embodiment of the control and energizing means of the plastic drying control apparatus. Fig. 4 shows sectional views of embodiments of sensors of the plastic drying control means shown in Fig. 3. Fig. 5 is a first block diagram of the plastic drying control means shown in Fig. 3. Fig. 6 is a second block diagram of the plastic drying control means shown in Fig. 3. Fig. 7 is a diagram illustrating a plastic drying tank equipped with an air type wiping means showing a further embodiment according to the present invention. Fig. 8 is a diagram illustrating a first blowing-down state made by dried air nozzles of the plastic drying tank equipped with the air type wiping means shown in Fig. 7. Fig. 9 is a diagram illustrating a second blowing-down state made by dried air nozzles of the plastic drying apparatus equipped with the air type wiping means shown in Fig. 7. Description of the Preferred Embodiments Now embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a flow chart illustrating a plastic drying apparatus showing a first embodiment according to the present invention. In the drawing, reference numeral 1 indicates a plastic in the form of pellets, for example, of nylon, polypropylene, or polyethylene terephthalate, which will be used as material for extrusion molding or injection molding. The plastic 1 is fed into a plastic drying tank 2 from a dryer hopper 17 through a metering loader 4 in such a manner that each batch consists of a certain amount thereof. The metering loader 4 is composed of the dryer hopper 17 in which the plastic 1 is prepared and stored, a shutter 28, and a level sensor 34 provided in the plastic drying tank 2 and supplies the plastic 1 into the plastic drying tank 2 by opening the shutter 28 until the level sensor 34 detects the upper limit of the plastic 1. The plastic drying tank 2 is provided with, in addition to the metering loader 4 and the dryer hopper 17, a stirring means 3, a microwave generating unit 11, an exhaust port 12, a drying tank side charging port 29 for dried air 22, and a discharge gate 16. The stirring means 3 is made up of a motor 31 that is attached outside of the plastic drying tank 2 and stirring blades 32 that are located in the plastic drying tank 2 and are driven by the motor 31 to stir the plastic 1 uniformly. Therefore, even although microwaves 21 are high in directivity, the temperature of the plastic 1 in the large plastic drying tank 2 is elevated uniformly. The microwave generating unit 11 consists of a magnetron for generating the microwaves 21, a transmitting antenna for transmitting the microwaves 21, and a waveguide for guiding the microwaves 21 into the plastic drying tank 2, which are not shown, and the molecules constituting the plastic 1 irradiated with the microwaves 21 are caused to rotate and vibrate, so that heat is generated inside and outside the molecules due to the frictional heat between the molecules and the plastic 1 is dried in a short period of time. The drying tank air charging port 29 serves as an inflow port for allowing the dried air 22, that has been compressed and dried through a compressor 15 that takes in air 23 and a dried air generating unit 8, to be flowed into the plastic drying tank 2 with the flow rate of the dried air 22 being kept constant by a flow rate adjusting unit 7, and the dried air 22 catches the moisture evaporated from the plastic 1 in the plastic drying tank 2 and is discharged via the exhaust port 12. The air discharged from the exhaust port 12 is released into the atmosphere through a filter 13. Part of the air discharged via the exhaust port 12 is sucked by a suction pump 14 and its humidity is measured by a humidity measuring sensor 9. The discharge gate 16 sends the plastic 1 brought to a prescribed highly dried state into a sub-tank 5. The discharge gate 16 may be driven by a motor, an air cylinder, or the like. The sub-tank 5 holds temporarily the plastic 1 dried in the plastic drying tank 2, and air-delivers the plastic 1 from a molder side outlet 25 toward a hopper of a molder, and also air-redelivers some dried plastic 1 to a supply side inlet 26 from a supply side outlet 24 through a plastic re-supply unit 6 if the plastic 1 to be fed to the dryer hopper 17 from a plastic raw material tank 30 through a feeding side inlet 27 becomes short and the plastic 1 to be loaded into the plastic drying tank 2 becomes short of a prescribed quantity. Reference numeral 18 indicates a sub-tank air charging inlet through which some of the dried air 22 is fed to the sub-tank 5 thereby preventing the plastic 1 therein from reabsorbing moisture, and also serving as a delivering air when air-delivering is effected. The plastic resupply unit 6 supplies the plastic 1 from the sub-tank 5 to the dryer hopper 17 by feedback and stops when a hopper level sensor 33 provided in the dryer hopper 17 detects the upper limit of supply of the plastic 1. The drying tank level sensor 34 provided in the plastic drying tank 2 is a sensor for detecting the plastic 1 of one batch and detects the upper limit of the plastic 1 then closing the shutter 28 to stop the further supply of the plastic 1. The humidity of the air discharged from the exhaust port 12 measured by the humidity measuring sensor 9 indicates indirectly the water content of the plastic drying tank 2 as described above. The humidity measuring sensor 9 takes the dew point temperature of the air discharged from the exhaust port 12; the dew point sensor for measuring the water content of the plastic 1 comprises an absolute humidity sensor or the like consisting of a humidity sensor and a temperature sensor enclosed in a dry gas, and the time limit of the output of the humidity measuring sensor 9 is set by a plastic drying control means 10 that operates programs on the basis of previously inputted data. Time t timed by the plastic drying control means 10 drives the stirring means 3, the microwave generating unit 11, etc. to bring the plastic 1 to a prescribed highly dried state, so that the plastic 1 is not hydrolyzed when it is molded thereby allowing the molding to be done highly accurately. Fig. 2 shows a stirring means according to a particular embodiment of the present invention. Again, reference numeral 1 indicates the plastic in the form of powdery particles, and reference numeral 2 indicates the drying tank 2 in the shape of a cylinder for stirring the plastic 1. A bearing section 35 is integrally fixed to the drying tank 2 coaxially therewith. The bearing section 35 projects into the central part in the drying tank 2 and is formed not to be buried in the plastic 1. Reference numeral 36 indicates a rotating shaft powered by the motor 31 to be rotated with it and borne by the bearing section 35. The rotating shaft 36 is provided with the stirring blades 32 through fixed arms 40 at the upper end thereof. The stirring blades 32 are rotated and moved relatively between a wall surface 39 formed on the cylinder inner surface of the drying tank 2 and the bearing section 35 to stir the plastic 1. The plastic 1 is kneaded uniformly by the rotation of the stirring blades 32 and the stirring action caused by the difference of the circumferential speed of the stirring blades 32 and the difference of the relative speed between the bearing section 35 and the wall surface 39. Reference numeral 37 indicates a cover that is placed on the upper end of the bearing section 35 and is in the shape of a circular cone or a pyramid. The cover 37 prevents the plastic 1 from piling up and being heated locally when the plastic 1 is kneaded or charged into the drying tank 2. One or more dried air blow-in units 38 cause the dried air 22 to flow down along the wall surface 39 and form a film of dried air on the wall surface 39 which prevents the plastic 1 from being charged with electricity and adhering to the wall surface 39 in the course of the drying. Fig. 3 is a diagram illustrating an embodiment of the control and energizing means of the plastic drying apparatus. Items 5, 6, 17, 24, 26 and others are omitted in Fig. 3, for representation simplicity. Reference numeral 1 indicates the plastic in the form of a powder. A metered certain amount of the plastic 1 is charged into the drying tank 2, where the plastic 1 is stirred uniformly by the stirring blades 32 driven by motor 31 provided at the drying tank 2. At the same time, the plastic 1 is heated and dried directly by the microwave generating unit 11 provided at the drying tank 2 and/or indirectly with highly dried air heated by a heater 42 provided in the dried air generating unit 8. In Fig. 3, although both the microwave generating unit 11 and the heater 42 are shown, it is possible that there is only one of them. The compressor 15 supplies air into the drying tank 2 by first feeding air to a dried air feed means 60 where the air is brought to a highly dried state and then the dried air is fed into the drying tank 2, with the pressure and the flow rate of the dried air kept constant. The dried air feed means 60 consists of a pressure adjusting unit 43a for keeping the pressure of the air from the compressor 15 constant and the flow rate adjusting unit 7 for feeding the air in a highly dried state to the drying tank 2 at a certain flow rate through the dried air generating unit 8 that brings the air fed from the pressure adjusting unit 43a to a highly dried state. The exhaust port 12 is provided at an upper part of the drying tank 2, so that the air in the drying tank 2 is removed to stabilize the pressure in the drying tank 2. The air flows from the dried air feed means 60 through the drying tank 2 into the exhaust port 12. Thus, since the air feed to and the air removal from the drying tank 2 are stabilized, the pressure and the flow rate of the air in the drying tank 2 are stabilized as well. Since evaporation from the plastic 1 uniformly stirred and heated in the drying tank 2 takes place in conformity with the moisture content of the plastic 1, the absolute humidity of the air discharged from the exhaust port 12 shows a value corresponding to the moisture content of the plastic 1. An exhaust path 41 is provided adjacent to the exhaust port 12, and through it the air in the drying tank 2 may be released into the atmosphere, for example, little by little so that the flow of the air in the drying bank 2 may not change, or may be removed at a constant flow rate by providing the suction pump 14 to the terminal of the exhaust path 41 as shown in Fig. 3. It being assumed that the air removed from the exhaust path 41 is the same as the air removed from the exhaust port 12, the absolute humidity of the air from the exhaust path 41 indicates a value corresponding to the moisture content of the plastic in the drying tank 2. A temperature sensor 9b and a humidity sensor 9a are arranged in the exhaust path 41 with them integrated or separated adjacently thereby measuring the temperature and the humidity of the air passing through the exhaust path 41. The data of the measurements at the temperature sensor 9b and the humidity sensor 9a are inputted in the plastic drying control means 10 where the absolute humidity is measured and an arbitrary humidity is programmed to stop the operation of the apparatus needed for the drying including the microwave generating unit 11, the heater 42, and the motor 31. Fig. 4 shows sectional views of embodiments of the sensors used in the present invention: Fig. 4a shows the humidity sensor 9a and Fig. 4b shows the temperature sensor 9b. The humidity sensor 9a is formed with a thermistor bead 44a housed in a case 45a and the thermistor bead 44a is connected to a pair of terminals 47a through lead wires 46a of platinum. The thermistor bead 44a comprises an electronic part whose resistance value changes linearly in accordance with the temperature, such as a posistor. The case 45a has a vent hole 48 so that the humidity outside the case 45a may be the same as that inside the case 45a. The temperature sensor 9b has a structure approximately similar to that of the humidity sensor 9a, in the reference numerals "a" having been exchanged by "b" to indicate equal parts, but does not have the vent hole 48 in the case 45a so as to cut the gas passage to and from the outside atmosphere and an inert gas or air in a highly dried state is confined in the case 45b so that the temperature of the air in a constant and highly dried state all the time may be measured. Variety types of humidity sensors and temperature sensors are on the market and as the humidity sensor 9a and the temperature sensor 9b used in the present invention any ones can be used if they carry out the measurement electrically, and a dew sensor also operates similarly in principle. Fig. 5 is a first block diagram illustrating a control circuit for the present plastic drying control means. The terminals 47a and 47b of the humidity sensor 9a and the temperature sensor 9b provided in the exhaust path 41 are connected to an arithmetic circuit 49 forming a bridge circuit to output a signal corresponding to the absolute humidity. Reference numeral 50 indicates a humidity setting circuit where an arbitrary humidity is set, which is inputted together with the output of the arithmetic circuit 49 to a comparator 51. The comparator 51 detects that the output of the arithmetic circuit 49 corresponding to the absolute humidity has become smaller or larger than the output corresponding to the set humidity of the humidity setting circuit 50 and generates an output signal, which is inputted to a driving circuit 52 for driving the microwave generating unit 11, the heater 42, the motor 31, etc. shown in Fig. 3. When the absolute humidity has become lower than the set humidity, the apparatus for heating and stirring the plastic 1 including the microwave generating unit 11, the heater 42, the motor 31, etc. is stopped through the output of the driving circuit 52. Therefore, when the output of the arithmetic circuit 49 is larger or smaller in level than the output of the humidity setting circuit 50, the measures can be taken in accordance with the operation conditions of the comparator 51 and the driving circuit 52 located downstream thereof, and it is enough to keep in mind that the operation may be regulated only by the results. It is also possible that the driving circuit 52 uses a circuit employing a PNP transistor or NPN transistor, thyristor, GTO, etc. to allow the output signal of the comparator 51 to become a pulse signal. Fig. 6 is a second block diagram of the control circuit for the present plastic drying apparatus. In Fig. 6, after a certain period of time after receiving the output signal of the comparator 51, a time limit setting circuit 53 generates a continuous signal or a pulse signal and by the operation of the time limit setting circuit 53, the driving circuit 52 is turned on or off. Fig. 7 is a diagram illustrating an embodiment of the plastic drying tank. The drying tank 2 has an inner wall 54, and dried air nozzles 55 discharge the air 22 toward the inner wall 54 of the drying tank 2 in a widely flared manner, causing air curtains 56. Fig. 8 is a diagrammatic sectional view illustrating a first blowing-down state made by the dried air nozzles 55 of the drying tank 2 equipped with the air type wiping means of Fig. 7. Each of the dried air nozzles 55 is made up of an air inflow port 57a located outside the drying tank 2, an air discharge port 57b located in the drying tank 2 for blowing down the super-dried air in such a manner that the air may be widely flared along the inner wall 54, and an attaching section 58 that is attached to the wall surface 39 by screws or the like. The super-dried air discharged from each air discharge port 57b is blown directly downward along the inner wall 54 to form the air curtain 56. Each air curtain 56 is a layer of compressed super-dried air that flows in the state of a widely spread thin film and acts to blow and scatter the plastic 1 that is floated lightly by stirring of the stirring blades 32 so that the plastic 1 may be prevented from collecting at parts where it is liable to stay or from adhering to the inner wall 54 due to the static electricity or the like. As a result, the plastic 1 can be prevented from staying on the surface of the inner wall 54. Fig. 9 is a diagram illustrating a second blowing-down state made by the dried air nozzles 55. In Fig. 9, the air in the super-dried state discharged from the air discharge ports 57b of the dried air nozzles 55 is blown downward slantly along the inner wall 54 to form each air curtain 56. Between the air curtains 56 are formed respective air curtain-unformed sections 59 each in the shape of an inverted triangle. To reduce the area of the air curtain-unformed sections 59, it is at first of course considered to use many dried air nozzles 55; and by blowing the curtains 56 slantly downward along the inner wall 54 as shown in Fig. 9, the area of each of the air curtain-unformed sections 59 between the air curtains 56 can be decreased. Therefore, it is not required to increase the number of the dried air nozzles 55 excessively.

Claims: A plastic drying apparatus comprising a plastic raw material tank (30) from which plastic material is supplied through a feeding side inlet (27) into a dryer hopper (17); the dryer hopper (17) for temporarily storing the plastic material (1) and for feeding it into a drying tank (2); a metering loader (4) connected to said dryer hopper (17) for metering and feeding predetermined batches of the plastic material (1) into the plastic drying tank (2); the plastic drying tank (2) in which heating and drying of the plastic material takes place; a dried air generating unit (8) for generating dried air (22) to be fed to said drying tank (2); plastic heating and drying means (11,22) for heating and drying the plastic material (1) in the plastic drying tank; a stirring means (3) for stirring the plastic material (1) in said plastic drying tank (2) so as to have the plastic material (1) uniformly heated; an air exhaust duct (12) connected to the plastic drying tank (2); a sub-tank (5) arranged subsequently to a dried plastic material outlet (16) of the plastic drying tank (2), for temporarily keeping the dried plastic (1); characterized in that it further comprises a humidity sensor (9a) provided in the exhaust path of the moisture-laden air to be removed from said drying tank (2); a plastic re-supply unit (6) for re-supplying some dried plastic (1) from said sub-tank (5) through a supply side inlet (26) to said dryer hopper (17) when the plastic raw material to be fed to the dryer hopper (17) from said plastic raw material tank (30) becomes short and the plastic (1) to be loaded into the drying tank (2) becomes short of a prescribed quantity; a flow rate adjusting unit (7) for setting constant the dried air (22) to be fed to said plastic drying tank (2) supplied from said dried air generating unit (8); a dried air input (18) to the sub-tank (5), through which input some dried air (22) is fed to the sub-tank (5); and a plastic drying control means (10) including a temperature controller, to which means (10) the measured values said humidity sensor (9a) are inputted and which operates programs on the basis of previously set data to set a time limit and drives said heating and drying means (11), said stirring means (3), etc. A plastic drying apparatus as claimed in claim 1, characterized in that said heating and drying means is a means of heating and drying by microwaves (21) generated by a microwave generating unit (11). A plastic drying apparatus as claimed in claims 1 or 2, characterized in that said stirring means (3) comprises a bearing section (35) that is fixed to the center of the bottom of the cylindrical drying tank (2) into which the plastic (1) will be charged, the top of the bearing section being formed to project from the plastic, a rotating shaft (36) that is rotated about the axis of said bearing section (35), one end of the rotating shaft (36) in said drying tank (2) being projected beyond the top of said bearing section and being provided with stirring blades (32) for stirring the plastic, a cover (37) in the shape of a circular cone or a pyramid for allowing said one end of said rotating shaft to cover the top of said bearing section, and a motor (31) fixed to the bottom on the outside of said drying tank (2) for driving said rotating shaft (36). A plastic drying apparatus as claimed in any of claims 1 to 3, characterized in that it comprises a temperature sensor (9b) for sensing the exhaust air temperature and a temperature controller (10) to which the measured values of said temperature sensor (9b) and of said humidity sensor (9a) are inputted and which controls the temperature in said drying tank (2). A plastic drying apparatus as claimed in claim 4, characterized in that said plastic drying control means (10) comprises an arithmetic circuit (49) that is connected to lead terminals of said temperature sensor (9b) and said humidity sensor (9a) to calculate absolute humidity, a humidity setting circuit (50) whose output signal level can be set arbitrarily, a comparator (51) that operates by detecting the output signal of said humidity setting circuit (50) and the case when said output signal becomes larger or smaller in level than the output signal of said arithmetic circuit (49), and a driving circuit (52) that is turned on or off by receiving an operation signal of said comparator (51), and when it is detected that the absolute humidity of the air in the exhaust path has become a prescribed dry state and less, the drying operation of the plastic drying apparatus is automatically stopped. A plastic drying apparatus as claimed in any of claims 1 to 5, characterized in that an air wiping means comprises dried air nozzles (55) provided on the wall surface of said plastic drying tank (2) for discharging the air (22) in a super-dried state generated by the dried air generating apparatus (8) along the inner wall (54) of said drying tank (2) in a widely flared manner to form air curtains (56) along the inner wall (54) of the drying tank (2). A plastic drying apparatus as claimed in claim 6, characterized in that said dried air nozzles (55) are arranged at the upper part of the wall surface of the plastic drying tank (2) for discharging the air (22) along the inner surface (54) of the plastic drying tank (2) downward slantly in a widely flared manner to form air curtains (56) along the inner wall of said drying tank (Fig. 9).

Inventors: NAKAGOMI SHOJI, NAKAGOMI, SHOJI
Applicants: NAKAGOMI SHOJI, NAKAGOMI, SHOJI
Application year: 1991
Main classification: B29B13
IPC codes: F26B3, B29B13, F26B11