Hot melt applicator and nozzle used therefor

Abstract: There is provided a hot melt applicator including (a) a nozzle opening, (b) a valve seat disposed upstream of the nozzle opening, (c) an empty chamber formed between the nozzle opening and the valve seat, (d) a valve body movable to the valve seat so that the valve seat is open or closed, and (e) a spring for biasing the valve body; and a pneumatically driven cylinder for driving the spring. The valve body is shaped to be a cone having an apex angle facing the nozzle opening, and the valve seat is formed with a tapered surface which is to make a contact with the cone. The tapered surface has an angle greater than the apex angle of the cone, and has a length of at least 1 mm. The hot melt applicator uses a pneumatically driven cylinder for driving the valve body to thereby compress the spring which in turn compresses the valve body, and thus, avoids the great resistance of hot melt with the spring used as a large capacity. By slightly changing angles of the valve body and the valve seat, the leakage which would occur when the valve is closed is prevented. By setting a length of the taper surface of the valve seat which is to make contact with the valve body to be in the range of 1 mm to 2 mm, the present invention makes it possible to prevent making of glue filament for a glue having great viscosity such as hot melt. In addition, setting a stroke of the valve body in the range of 0.3 mm to 0.5 mm ensures more effectively to prevent making of glue filament.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to an apparatus for applying thermoplastic hot melt glue, and more particularly to such an apparatus which is capable of preventing hot melt from roping from a nozzle when the nozzle is shut off. The present invention further relates to a nozzle through which viscous fluid such as glue, paint and sealing material is discharges and which is suitable for the above mentioned apparatus. DESCRIPTION OF THE PRIOR ART Boxes and bags have been conventionally made by intermittently applying a glue onto a glue margin of boxes and bags, folding the glued margins to thereby secure the margins to a predetermined areas, and thus completing boxes and bags. Glue is applied by means of a roller or a discharge nozzle. When glue is to be intermittently applied, a discharge nozzle has been widely used because of easiness of control thereof. In the case that a discharge nozzle is to be used to intermittently discharge glue, when glue discharge is shut off, glue discharge does not stop immediately, and hence glue tends to make glue filament. In order to solve this problem, Japanese Unexamined Patent Publication No.61-78460 has suggested the production of negative pressure in a nozzle when a applicator nozzle is shut off, to thereby prevent making of glue filament. Japanese Unexamined Patent Publications Nos. 50-122539 and 55-2474 have suggested the provision of an absorbing nozzle in a discharge nozzle to thereby absorb glue which remains non-discharged when the discharge nozzle is shut off. Japanese Unexamined Patent Publication No.5-97127 has suggested that when a nozzle opening is shut off, a glue discharge speed is increased and glue is blown off by discharging air through an air nozzle to thereby prevent making of glue filament. Glue is grouped into water base glue which has been generally used and is called cold glue, and thermoplastic glue which is called hot melt. With respect to general characteristics, cold glue takes time for drying until an adhesive force is generated after cold glue has been applied, whereas hot melt produces an adhesive force immediately after having been applied. Hot melt has great viscosity, and hence, when hot melt is to be applied through a nozzle, hot melt tends to make glue filament at the time a nozzle opening is shut off. The applicator disclosed in Japanese Unexamined Patent Publication No. 5-97127 is effective for cold glue to prevent making of glue filament, but could not prevent making filament of hot melt. As mentioned earlier, hot melt produces an adhesive force immediately after having been applied. Since hot melt is in a solid condition at room temperature, hot melt is heated to thereby be liquidized for applying through a discharge nozzle. However, a part of the liquid is gasified by heating, and the thus produced gas tends to adhere to a piston of a pneumatic cylinder for driving a discharge nozzle for open and close action thereof and, after cooled, fixedly secure a piston to a cylinder. There has been used a discharge nozzle for releasing the above mentioned hot melt gas to atmosphere to thereby prevent the gas from entering a piston. Fig. 1 illustrates an example of a discharge nozzle formed with an opening through which hot melt gas is to be released to atmosphere. A main body 1 of the illustrated discharge nozzle is partitioned into a glue chamber 3 and a cylinder 4 by a sealing section 2. There are provided a glue chamber cover 5 having a nozzle opening 9, and a glue inlet 11 in the glue chamber 3. Glue supplied through the glue inlet 11 is discharged through the nozzle opening 9. A piston 7 is slidably fit in the cylinder 4. A valve rod 8 extends passing through the sealing section 2, and has both a tip end 8a for opening and closing the nozzle opening 9 and a rear end at which the valve rod is connected to the piston 7. There is provided a spring 10 at the rear of the piston 7, namely at the opposite side of the valve rod 8, which spring compresses the piston 7 so that the nozzle opening 9 is closed with the tip end 8a of the valve rod 8. Within the cylinder 4 is provided an air supply inlet 16 at the side of the sealing section 2. Air under pressure is supplied into the glue chamber 3 through the air supply inlet 16 to move the piston to thereby separate the tip end 8a of the valve rod 8 from the nozzle opening 9 for discharging glue therethrough. The sealing section 2 is formed centrally with a valve rod passage 12 through which the valve rod 8 passes, and with an atmosphere releasing passage 30 which communicates the valve rod passage 12 to atmosphere. At opposite ends of the valve rod passage 12 are provided U-shaped seals 14 for preventing glue from entering the cylinder 4 from the glue chamber 3. When the valve rod 8 makes reciprocal movement, fluid glue acting as a lubricant enters the valve rod passage 12 in a small amount and is gasified. A part of the thus produced gas is released through the atmosphere releasing passage 17. As discussed earlier, even if a structure including two seals and an atmosphere releasing passage located intermediate between the two seals is used, gasified glue enters a cylinder during a discharge nozzle is used for long hours, and adheres to a slide surface of a piston, resulting in that a piston is fixedly adhered to a cylinder. In particular, when a discharge nozzle is to be used in horizontally lying condition or with a nozzle being upwardly directed, a piston frequently is fixedly adhered to a cylinder in a relatively short time. US 4 320 858, which represents the prior art as referred to in the preamble of claim 1, discloses a hot melt adhesive applicator including a dispenser head or block having a liquid chamber in one end portion and a cartridge cavity in the opposite end portion communicating through a stem passage and further comprising a replaceable needle valve cartridge including a valve body containing a pneumatically actuated valve having a valve stem depending below the valve cartridge body through the stem passage and the liquid chamber for opening and closing the orifice in the nozzle assembly at one end of the liquid chamber. SUMMARY OF THE INVENTION In view of the above mentioned problem, it is an object of the present invention as defined in claim 1 to provide an applicator which is capable of preventing hot melt from making of glue filament when a nozzle opening is shut off. There is provided a hot melt applicator including (a) a nozzle opening, (b) a valve seat disposed upstream of the nozzle opening, (c) an empty chamber formed between the nozzle opening and the valve seat, (d) a valve body movable to the valve seat so that the valve seat is open or closed, (e) a spring for biasing the valve body, and (f) a pneumatically driven cylinder for driving the spring. The valve body is shaped to be a cone having an apex angle facing the nozzle opening, and the valve seat is formed with a tapered surface which is to make a contact with the cone, the tapered surface having an angle greater than the apex angle of the cone. The tapered surface has a length of at least 1 mm. The length of the tapered surface is preferably equal to or shorter than 2 mm. The valve body is driven by the spring to thereby move towards a closed position, and is driven by the pneumatically driven cylinder for overcoming a force exerted by the spring to thereby move towards an open position. Since hot melt has great viscosity, it is necessary to provide a spring for generating a great force for closing the valve body. In order to compress the spring, there is used a pneumatically driven cylinder which is capable of producing a great force in spite of a small volume. As a cone constituting the valve body moves to the valve seat having a tapered surface which is to be in contact with the cone when a valve is to be closed, a gap between a tapered surface of the cone and the tapered surface of the valve seat is gradually decreased, and in the long run the tapered surface makes contact with the tapered surface of the valve seat. A minority of hot melt filled in the above mentioned gap escapes in a direction opposite to a direction in which the valve body is closed, whereas a majority of hot melt is compressed in a direction in which the valve body is closed. As a result, a pressure in the empty chamber formed between the valve body and the nozzle opening is increased, thereby a discharge speed of hot melt to be discharged through the nozzle opening is increased. If the valve body is designed to have the tapered surface which is 1mm long or longer, the increased discharge speed is significantly effective for prevention of making of glue filament of hot melt, but if the tapered surface is shorter than 1 mm, it is impossible to prevent hot melt from making of glue filament. A longer tapered surface of the valve seat is more effective for prevention of hot melt from making of glue filament. However, an upper limit of the tapered surface length is 2 mm, because fabrication cost of the valve seat is significantly increased for a tapered surface longer than 2 mm. In addition, by setting an angle of the tapered surface of the valve seat to be greater than an apex angle of the cone, it is ensured that the cone surely makes contact with the valve seat when the valve body is closed, to thereby be able to prevent leakage of hot melt. It is certainly possible to prevent making of glue filament of hot melt by adopting a spring which compresses the valve body with a great force and which can be used because of adoption of a pneumatically driven cylinder, and by setting a appropriate length for the tapered surface of the valve seat. Furthermore, a stroke of the valve body between open and closed positions thereof is in the range of 0.3 mm and 0.5 mm. If a stroke of the valve body between open and closed positions (hereinafter, referred to as "the stroke") is small, an increment in the discharge speed of hot melt, which is caused by narrowing a gap between the tapered surfaces of the cone and the valve seat when the valve body is to be closed, is also small. By setting the stroke to be 0.3 mm or longer, it is possible to prevent hot melt from making of glue filament. On the other hand, if the stroke is too long, it takes much time for the valve to be closed. Thus, an upper limit of the stroke is set to be 0.5 mm. A hot melt applicator in accordance with the present invention uses a pneumatically driven cylinder for driving the valve body to thereby compress the spring which in turn compresses the valve body, and thus, avoids the great resistance of hot melt with the spring used as a large capacity. By slightly changing angles of the valve body and the valve seat, the leakage which would occur when the valve is closed is prevented. By setting a length of the taper surface of the valve seat which is to make contact with the valve body to be in the range of 1 mm to 2 mm, the present invention makes it possible to prevent making of glue filament for a glue having great viscosity such as hot melt. In addition, setting a stroke of the valve body in the range of 0.3 mm to 0.5 mm ensures more effectively to prevent making of glue filament. The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view illustrating a structure of a conventional nozzle for discharging glue therethrough. Fig. 2 is a cross-sectional view of a structure of the first embodiment in accordance with the present invention. Fig. 3 is a detailed view of a valve body, a valve seat and a nozzle opening in the first embodiment. Fig. 4 is a view showing drive forces of a pneumatically driven cylinder and an electromagnetic driving means. Fig. 5 is a view showing discharge speed of hot melt discharged through a nozzle of the embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings. Fig. 2 is a cross-sectional view illustrating a hot melt applicator made in accordance with the first embodiment. A main body 21 of the applicator is cylindrical in shape, and has two different inner diameters varying at a center of the main body: an upper part of the main body makes a cylinder 24 and a lower part makes a glue chamber 23 in Fig. 2. A partition seal 22 is provided within the stepped glue chamber 23 to thereby separate the glue chamber 23 and the cylinder 24 from each other for prevention of air leakage from the cylinder 24 to the glue chamber 23. A glue chamber cover 25 having a valve seat 29 and a nozzle opening 31 is secured to the main body 21 by means of bolts at an end of the glue chamber 23. A cylinder cover 26 is threaded into and thus fixed to the cylinder 24 at its end. In the cylinder is provided a piston 27 which is connected to a needle valve 28 passing through the partition seal 22 and the glue chamber 23 and reaching the glue chamber cover 25. A valve body 28a formed at a tip end of the needle valve 28 is a cone in shape, and is to make contact with a valve seat 29 formed at the glue chamber cover 25 and having a conically shaped tapered surface, to thereby carry out valve function. The valve seat 29 is in communication with a cylindrical empty chamber 30. A nozzle opening 31 is formed at an end of the empty chamber 30. The glue chamber 23 is formed with an glue inlet 32 through which hot melt is supplied from a device (not illustrated). Since hot melt is solidified at room temperature, hot melt is heated to thereby be liquidized when applied, and the thus liquidized hot melt is supplied through a pump. There is provided a spring 33 between the piston 27 and the cylinder cover 26 for compressing the piston 27 to thereby compress the needle valve 28 onto the valve seat 29 to shut off the valve. The cylinder cover 26 is formed with a stroke adjusting screw 34 which is fixed to a position adjusted by a nut 35. There is provided a screw receiver 36 in facing relation to the stroke adjusting screw 34 of the piston 27. A gap between the screw receiver 36 and the stroke adjusting screw 34 with the piston 27 being compressed by the spring 33 to thereby cause the needle valve 28 to be compressed onto the valve seat 29 make a stroke of the needle valve 28. Within the cylinder is provided an air passage 37 at the side of the partition seal 22. Air under pressure is supplied to and discharged from the cylinder 24 through the air passage 37. The air passage 37 is in communication with an electromagnetic directional control valve 38 which is operated with control signals transmitted from a controller (not illustrated). The electromagnetic directional control valve 38 is in communication with an air source 39 from which air under pressure is supplied. Fig. 3 illustrates an arrangement of the needle valve and the valve seat. The valve body 28a of the needle valve 28 is conical in shape, and has an apex angle  of 59 degrees. The apex angle  is preferably about 60 degrees for prevention of roping of hot melt and also for processability. The valve seat 29 includes a conical tapered surface having an apex angle of 60 degrees which is 1 degree greater than the apex angle of the valve body 28a. Thus, the valve body 28a can certainly sit on the valve seat 29 to thereby prevent leakage of hot melt. The tapered surface of the valve seat 29 has a length L in the range of 1 mm to 2 mm. The valve body 28a is influenced by the spring 33 and air under pressure to thereby move between positions indicated with solid and broken lines, and thus makes open and close movement. When the valve body 28a is compressed, a minority of hot melt present between the tapered surfaces of the valve seat 29 and the valve body 28a indicated with a broken line is forced to return to the glue chamber 23, whereas a majority of hot melt is forced to be discharged through the nozzle opening 31 in an increased speed. Since hot melt is incompressible and highly viscous liquid and the glue chamber 23 is filled with hot melt, when the valve body 28a is compressed to thereby move, only a part of hot melt is returned to the glue chamber 23 and most of hot melt is forced to move into the empty chamber 30. The length L of the tapered surface of the valve seat 29 significantly influences on the discharge speed of hot melt when the valve is closed. If the length L is smaller than 1 mm, making of glue filament of hot melt can scarcely be prevented, whereas if the length L is equal to or longer than 1 mm, making of glue filament of hot melt can be prevented almost without failure. By setting the length L longer and longer, it would be possible to prevent making of glue filament of hot melt, but the cost for fabrication of the valve seat 29 would also be increased. Hence, an upper limit of the length L is about 2 mm. The stroke of the needle valve 28 is set in the range of 0.3 mm to 0.5 mm by means of the stroke adjusting screw 34. If the stroke is set small, an amount of hot melt discharged into the empty chamber 30 by the valve body 28a is not sufficient when the valve is closed, resulting in that an increment in the discharge speed is small and that making of glue filament of hot melt cannot be sufficiently prevented. If the stroke is set to be 0.3 mm or longer, it is possible to substantially certainly prevent hot melt from making of glue filament. The longer stroke would ensure a greater increment in the discharge speed of hot melt. However, it is no longer necessary to increase the discharge speed of hot melt any more, if hot melt does no longer make glue filament. Since the longer stroke would make a time for closing the valve longer, an upper limit of the stroke is 0.5 mm. Fig. 4 shows comparison in a driving force between a pneumatically driven cylinder and an electromagnetically driving means including a solenoid. An axis of abscissa indicates a stroke S of the needle valve, and an axis of ordinate indicates a force to be produced. A force P produced by a pneumatically driven cylinder is uniform to the stroke S, whereas a force Q produced by the electromagnetic driving means including a solenoid rapidly decreases with an increase of the stroke S. Since hot melt has great viscosity, the needle valve 28 receives great resistance when closed with the result that the spring 33 compresses with greater resilient force. For the above mentioned reason, a pneumatically driven cylinder which is capable of continuously producing great force is suitable as a means for compressing the spring 33. Hereinbelow is explained the performance of the hot melt applicator having the above mentioned structure. Fig. 5 shows the discharge speed of hot melt to be discharged through the nozzle opening 31. An axis of abscissa indicates time, and an axis of ordinate indicates a discharge speed of hot melt to be discharged through the nozzle opening 31. That is, an axis of ordinate indicates a pressure in the empty chamber 30. If the valve starts its close action, the discharge speed increases, and the discharge speed at the time when the valve is fully closed is greater than the normal discharge speed. Thus, it is possible to prevent hot melt from making of glue filament when the valve is closed. Fig. 5 is the same as Fig. 8 of Japanese Unexamined Patent Publication No. 5-97127 which relates to cold glue, but shows that the present invention can prevent making of glue filament of hot melt as well as cold glue.

Claims: A hot melt applicator comprising: (a) a nozzle opening (31); (b) a valve seat (29) disposed upstream of said nozzle opening (31); (c) an empty chamber (30) formed between said nozzle opening and said valve seat (29); (d) a valve body (28) movable to said valve seat (29) so that said valve seat (29) is open or closed; and (e) a spring (33) for biasing said valve body (28); and (f) a pneumatically driven cylinder (27) for driving said spring (33), said valve body (28) being shaped to be a cone (28a) having an apex angle facing said nozzle opening (31), said valve seat (29) being formed with a tapered surface which is to make a contact with said cone (28a), said tapered surface having an angle greater than said apex angle of said cone, characterized in that said tapered surface having a length of at least 1 mm and wherein a stroke of said valve body between open and closed positions thereof is in the range of 0.3 mm to 0.5 mm. The hot melt applicator as set forth in claim 1, wherein said tapered surface has a length equal to or shorter then 2 mm.

Inventors: KOIKE KATSUHIKO, KUZUU NORIHIRO, ONO ISAO, YAMADA TAKEO, YODO SHOZO, KOIKE, KATSUHIKO, KUZUU, NORIHIRO, ONO, ISAO, YAMADA, TAKEO, YODO, SHOZO
Applicants: NIRECO CORP, NIRECO CORPORATION
Application year: 1997
Main classification: B05C5
IPC codes: B05C5, B05B7, B05B1