GB2300044A

GB2300044A - Melting hot melt adhesives using electrothermal heater

Info

Publication number: GB2300044A
Application number: GB9507760A
Authority: GB
Inventors: Takeo Yamada; Katsuhiko Koike; Norihiro Kuzuu
Original assignee: Nireco Corp
Current assignee: Nireco Corp
Priority date: 1994-04-25
Filing date: 1995-04-13
Publication date: 1996-10-23
Anticipated expiration: 2015-04-13
Also published as: DE19515007A1; DE19515007C2; JP2812197B2; JPH07289968A; GB9507760D0; GB2300044B

Description

1 2300044 Apparatus for Heating and Melting Hot Melt Adhesives and Method

for Operating the Same

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for heating and melting thermoplastic hot melt adhesives to thereby provide to an adhesive applicator in the case of making envelopes, slips and boxes.

Glue is usually used for securing parts to each other in making boxes, packages and bags. A method for securing parts using glue includes a cold glue process in which a cold adhesive is to be used, and a hot melt process in which pelletized solid phase adhesive is heated and thus liquidized, and dispensed through a nozzle, and then cooled to thereby solidify. The cold glue process takes much time for drying adhesives, but is cheap in cost, while the hot melt process can rapidly dry adhesives, but is expensive.

Fig. 1 illustrates a conventional apparatus for heating and melting hot melt adhesives together with an applicator of hot melt adhesives. The reference numeral 1 represents molten hot melt adhesives, the reference numeral 2 represents a tank which has a thick wall and composed of aluminum and which has an electrothermal heater 3 and a temperature sensor 4 both embedded in a bottom of the tank, both of the heater 3 and the sensor 4 comprising a device for heating and melting hot melt adhesives. The reference numeral 5 represents a melt pump for sucking up the hot melt adhesives 1, the reference numeral 6 represents a filter, the reference numeral 7 represents a horse with a heater, and the reference numeral 8 represents an application nozzle of the hot melt adhesives. The parts 5 to 8 comprises an applicator.

The above mentioned conventional apparatus for heating and melting hot melt adhesives has problems as follows.

1. Heating efficiency is low.

The electrothermal heater 3 embedded in a bottom of the tank 2 heats a 1 2 bottom and a side-wall of the tank to thereby heat the hot melt adhesives 1 in contact with them. Since a surface of the tank which is not in contact with the hot melt adhesives 1 is also heated, much of heat is released through the surface with the result of low heating efficiency. Excessive heating for compensating for such release of heat further deteriorates the heating efficiency.

2. It takes too much time to melt hot melt adhesives.

Heat transfers from the bottom and side-wall of the tank to the hot melt adhesives. Since the hot melt adhesives have a low thermal conductivity, and have a high viscosity which does not allow convection in the hot melt adhesives, it takes too much time for pelletized solid phase hot melt adhesives to melt.

3. The hot melt adhesives tend to be scorched.

Since much of heat is released through the walls of the tank, excessive heating tends to be carried out for compensating for the released heat. Thus, the hot melt adhesives are likely to be scorched at an interface between the molten hot melt adhesives and the walls. The scorched hot melt adhesives are carbonized, peeled off from the wall, and then mixed with the hot melt adhesives. Thus, the scorched hot melt adhesives clogs the filter 6 and/or the application nozzle 8. Even if the scorched hot melt adhesives are discharged through the application nozzle 8, they gives rise to unapporopriate adhesion. Accordingly, it is necessary to periodically clean the tank 2, the filter 6, the horse 7 and the application nozzle 8 with the result of low serviceability.

4. The hot melt adhesives tend to be degraded.

The molten hot melt adhesives have a high viscosity, and hence a part of the hot melt adhesives tends to locally stay in the tank 2. Such part of the hot melt adhesives are heated long and hence tend to be degrades.

U. S. Patents Nos. 4,474,311, 4,485,941 and 4,485,942 has suggested an apparatus for melting and dispensing thermoplastic material. U. S. Patent No. 4,535,919 has suggested a hot melt adhesive system. U. S. Patent No. 4,456,151 has suggested a housing for apparatus for melting and dispensing thermoplastic material. They are hereby incorporated by reference to extent that they are 3 consistent herewith. However, all of them do not resolve the problems as aforementioned.

SUMMARY OF THE INVENTION

In view of the above mentioned problems, it is an object of the present invention to provide an apparatus for heating and melting hot melt adhesives and also a method for operating such an apparatus in order to enhance a higher heating efficiency, reduce a period of time for melting and prevent the hot melt adhesives from getting scorched and being degraded.

In one aspect, the invention provides an apparatus for heating and melting thermoplastic hot melt adhesives, comprising a tank containing hot melt adhesives therein, characterized by that an internal electrothermal heater disposed in the tank, and means for driving the internal electrothermal heater in vertically reciprocating movement.

In a preferred embodiment, the internal electrothermal heater comprises a plate having a plurality of through-holes.

In another preferred embodiment, the apparatus further includes a bottom surface electrothermal heater disposed at an external surface of a bottom of the tank.

In still another preferred embodiment, the apparatus further includes a side surface electrothermal heater disposed at an external surface of a side wall of the tank.

In yet another preferred embodiment, the side surface electrothermal heater is designed to have a lower temperature than that of the internal electrothermal heater and the bottom surface electrothermal heater.

In another aspect, the invention provides a method for operating an apparatus comprising an internal electrothermal heater movable vertically and reciprocatively in a tank, and a driver for driving the internal electrothermal heater, the method including the steps of (a) moving the internal electrothermal 4 heater onto a bottom surface and supplying solid phase hot melt adhesives on the internal electrothermal heater in the tank to an upper level of the tank, and then starting to apply an electrical current to the internal electrothermal heater, (b) applying an electrical current to the internal electrothermal heater in a certain period of time and repeating a step of raising the internal electrothermal heater to a level at which the solid phase hot melt adhesives are in molten condition and lowering the internal electrothermal. heater to the bottom surface of the tank, and (c) moving the internal electrothermal heater reciprocatively between a middle level and a bottom of the tank after the internal electrothermal heater has reached the upper level of the tank.

In a preferred embodiment, the internal electrothermal heater is formed with a plurality of through-holes so that molten hot melt adhesives are fed through the internal electrothermal heater from upward to downward of the internal electrothermal heater.

In another preferred embodiment, the method further includes the step of heating a side wall of the tank at a lower temperature than both a temperature of the internal electrothermal heater and a temperature of a bottom surface of the tank.

The advantages obtained by the aforementioned present invention will be described hereinbelow.

Since the internal electrothermal heater is disposed in the tank and hence is surrounded by the hot melt adhesives, the heater is able to efficiently transfer heat to the hot melt adhesives therefrom. In addition, the vertical and reciprocative movement of the internal electrothermal heater stirs the hot melt adhesives to thereby enhance heat transmission. Thus, the heating efficiency is considerably enhanced, a period of time for melting is reduced, and the hot melt adhesives are prevented from getting scorched since it is no longer necessary to excessively heat the hot melt adhesives. In addition, the mobility of the hot melt adhesives are also enhanced to thereby avoid the degradation of the hot melt adhesives.

By designing the internal electrothermal heater to be a plate and providing the heater with a plurality of through-holes, molten hot melt adhesives move downward through the through-holes. When the internal electrothermal. heater moves upwardly, an upper surface of the internal electrothermal. heater has a greater number of chances to contact with solid phase hot melt adhesives with the result that a period of time for melting is reduced.

By providing the tank with electrothermal heaters at external surfaces of a bottom and a side-wall of the tank, it is possible to keep the molten hot melt adhesives to be warm to thereby prevent solidification of the hot melt adhesives.

By setting a temperature of the side surface electrothermal heater disposed at a side wall of the tank to be lower than a temperature of the internal and bottom surface electrothermal heaters, it is possible to prevent the hot melt adhesives from getting scorched at a side wall of the tank. The hot melt adhesives are less likely to be scorched at a bottom surface of the tank due to the reciprocative motion of the internal electrothermal. heater, while more likely to be scorched at a side wall of the tank. This is because that the internal electrothermal heater does not move above the uppermost level of the molten hot melt adhesives in order to prevent entrance of air into the molten hot melt adhesives, and that an oxide layer formed at the uppermost level of the molten hot melt adhesives is also raised and lowered keeping in contact with the side wall. Thus, a temperature of the side surface electrothermal heater is adjusted.

The method for operating an apparatus for heating and melting in accordance with the invention has the steps of situating the internal electrothermal heater at a bottom of the tank, supplying solid phase hot melt adhesives on the internal electrothermal heater, vertically moving the internal electrothermal. heater to thereby melt the hot melt adhesives until the hot melt adhesives at an uppermost level thereof are molten, and reciprocatively moving the internal electrothermal heater between a middle level and a bottom of the tank. Thus, it is possible to rapidly melt all the hot melt adhesives and prevent entrance of air into the molten hot melt adhesives which would be caused due to the reciprocative 6 motion of the internal electrothermal heater. In addition, the hot melt adhesives are always being stirred, stagnation of the hot melt adhesives does not occur to thereby prevent degradation of the hot melt adhesives.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a cross-sectional view showing a structure of the conventional apparatus.

Fig. 2 is a cross-sectional view showing a structure of the first embodiment.

Fig. 3A is a plan view and Fig.3B is a side view of the electrothermal heater plate.

Fig. 4 shows the operation of the electrothermal heater plate.

Fig. 5 shows the operation of the thorough-holes formed in the electrothermal heater plate.

Fig. 6 shows the comparison of performance between the first embodiment and the conventional apparatus.

Fig. 7 is a cross-sectional view showing a structure of the second embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments in accordance with the present invention win be explained hereinbelow with reference to drawings.

Fig. 2 is a cross-sectional view illustrating a first embodiment. Fig. 3A is a plan view, and Fig. 3B is a side view of an electrothermal heater plate. Parts that correspond to those of Fig. 1 have been provided with the same reference numerals.

In a tank 10 is disposed an electrothermal heater plate 9 in which the electrothermal heater 3 and the temperature sensor 4 are embedded. The tank 10 is composed of aluminum with a thinner side wall than that of the tank 2 illustrated in Fig. 1, and is provided at an external surface of a side wall and a bottom thereof 7 with a heat insulator 11. At the top of the tank 10 is disposed a support member 12 covering a center of the tank. On the support member 12 is formed a cylinder support 13 at the center of the tank. Above the cylinder support 13 is vertically disposed a pneumatic cylinder 15 with a rod extending downward. A connection rod 14 is connected to the rod at its end, and transverse run-out of the connection rod 14 is prevented by a guide port 12a formed at the center of the support member 12. The connection rod 14 is secured at its lower end to an electrothermal heater 9.

An air source 16 provides air under pressure to the pneumatic cylinder 15 through a pressure control valve 17 and a direction changing valve 18. The direction changing valve 18 is controlled by a sequence circuit 19.

As illustrated in Figs. 3A,313, in the electrothermal heater plate 9 are embedded the three electrothermal heaters 3 and the temperature sensor 4 in the vicinity of the electrothermal heaters 3. Wirings of the electrothermal heaters 3 and the temperature sensor 4 extend to a wiring outlet 14a through the inside of the connection rod 14. The electrothermal heater plate 9 is formed with a plurality of through-holes 9a. The reference numeral 9b is a cut-out thorough which a suction pipe of the melt pump 5 extends.

As illustrated in Fig. 2, the cylinder support 13 is provided therein with limit switches 21a, 21b and 21c in order to detect whether the electrothermal heater 9 reaches a bottom level (LH), a middle level (H) or an upper level (HH). Each of the limit switches 21 is operated by a contact rod 22 disposed at a distal end of the rod of the pneumatic cylinder 15. The limit switches 21 transmit signals to the sequence circuit 19 through not illustrated wirings.

An operation will be explained hereinbelow.

Fig. 4 shows a reciprocative sequence of the electrothermal heater plate 9. The indications LH, H and HH represent an elevational position of the electrothermal heater plate 9 in the tank 10, as mentioned earlier. Appliance of a current to the electrothermal heater plate 9 and the reciprocative motion of the pneumatic cylinder 15 are controlled by the sequence circuit 19. The operation mode includes A zone in which hot melt adhesives are to be molten until an 8 uppermost level of the hot melt adhesives are molten and B zone in which the molten hot melt adhesives are to be stirred. Prior to appliance of a current to the electrothermal heater plate, the electrothermal heater plate 9 is situated at LH level, that is, at a bottom of the tank, and then pelletized hot melt adhesives are filled in the tank so that the molten hot melt adhesives would reach the HH level of the tank.

Then, appliance of a current to the electrothermal heater plate 9 is started to thereby raise the pneumatic cylinder 15. Thus, the electrothermal heater plate 9 is raised to a level at which the hot melt adhesives are molten. A command for lowering the electrothermal heater plate is transmitted from a timer when a period of time tl has passed after the start of raise of the pneumatic cylinder 15. The lowering movement of the electrothermal heater plate 9 advantageously stirs the molten hot melt adhesives existing under the electrothermal heater plate 9. A force for raising the electrothermal. heater plate 9 is determined in such a manner that the electrothermal heater plate 9 is able to raise through the molten hot melt adhesives, and also that the force is smaller than a force necessary for pushing up the solid phase hot melt adhesives. Pneumatic drive system is suitable to such determination of the force. If a motor is to be used for driving the electrothermal heater plate, excessive current may break down the motor when rotation of the motor is intended to stop in accordance with loads. When the electrothermal heater plate 9 is lowered and thus the limit switch 21c detects the level LH, the electrothermal heater plate 9 begins to raise. In this way, the electrothermal heater plate repeats raising in a period of time U, lowering and raising again at the level LH. After several repetition, the electrothermal heater plate 9 reaches the upper level HH, which in turn operates the limit switch 21a, thereby the electrothermal heater plate is lowered. The above mentioned steps are the operation of A zone for melting the hot melt adhesives.

In B zone, the limit switch 21b detects the level H located between the uppermost level HH and the lowermost level LH of the electrothermal heater plate 9, and thus reciprocatively moves the electrothermal heater plate 9 between the 9 level H and the level LR This is to avoid the entrance of air to the molten hot melt adhesives which would occur, if the electrothermal heater plate 9 is raised near the uppermost level of the molten hot melt adhesives and then lowered. The above mentioned operation in A zone and B zone enables to efficiently melt and stir the hot melt adhesives.

The above mentioned operation is to be applied to a case in which hot melt adhesives are newly supplied, however, the same can be applied to a case in which heating the hot melt adhesives in the tank 10 is once stopped to thereby solidify the hot melt adhesives, and then heating the hot melt adhesives is started again to melt the hot melt adhesives. It is necessary to situate the electrothermal heater plate 9 at the level LH if heating the hot melt adhesives is stopped.

Fig. 5 shows the operation of the through-holes 9a formed in the electrothermal heater plate 9. A hatching area A located above the electrothermal heater plate 9 represents solid phase hot melt adhesives, and an area B represents molten hot melt adhesives. The electrothermal heater plate 9 is adapted to have a raising speed nearly equal to a melting speed of hot melt adhesives so that the molten hot melt adhesives are able to pass through the through-holes 9a into the area B. Thus, an upper surface of the electrothermal heater plate is kept in contact with the solid phase hot melt adhesives with the result that the hot melt adhesives are facilitated to be molten.

Fig. 6 shows the comparison of the above mentioned embodiment to the conventional apparatus illustrated in Fig. 1. The axis of ordinate indicates a temperature at a center of a height and a cross-section of the tank, and the axis of abscissa indicates a period of time for heating. The temperature T in the axis of ordinate indicates a temperature at which a central portion of hot melt adhesives are sufficiently molten. The sequence circuit 19 operates so that the temperature T is maintained. A line A indicates the above mentioned embodiment, while a line B indicates the conventional apparatus illustrated in Fig. 1. It is shown that the embodiment takes a one-fourth period of time for melting relative to the conventional apparatus. The heating ability of a heater used in the embodiment is 50%, and electrical power necessary for melting is one-eighth both relative to the conventional apparatus.

Fig. 7 illustrates a second embodiment in accordance with the invention. Parts that correspond to those of Fig. 2 have been provided with the same reference numerals. In comparison with the first embodiment illustrated in Fig. 2, the second embodiment is further formed with flexible plateshaped heaters 24 at external surfaces of a side wall and a bottom of the tank 10 for keeping the tank 10 warm. A temperature of the side wall heater is set to be about 10 degrees lower than a temperature of the bottom heater and the electrothermal heater plate 9 to thereby prevent the hot melt adhesives form getting scorched at a side wall of the tank.

As is obvious from the foregoing description, the present invention raises and lowers the electrothermal heater plate in the tank to thereby enhance the thermal conductivity, shorten a period of time for melting and prevent degradation of the hot melt adhesives. In addition, it is possible to avoid the hot melt adhesives from getting scorched by adjusting a temperature of the heaters situated at a side wall of the tank.

Claims

CLAIMS.

11 1 - An apparatus for heating and melting thermoplastic hot melt adhesives, the apparatus comprising a tank containing hot melt adhesives therein, characterized by that an internal electrothermal heater (3) disposed in the tank (10), and means (14-19) for driving the internal electrothermal heater (3) in vertically reciprocating movement.
2. The apparatus as recited in claim 1, wherein the internal electrothermal heater (3) comprises a plate (9) having a plurality of through-holes (9a).
3. The apparatus as recited in claim 1 or 2 further comprising a bottom surface electrothermal heater (24) disposed at an external surface of a bottom of the tank (10).
4. The apparatus as recited in claim 3 further comprising a side surface electrothermal heater (24) disposed at an external surface of a side wall of the tank (10).
5. The apparatus as recited in claim 4, wherein the side surface electrothermal heater (24) is designed to have a lower temperature than that of the internal electrothermal heater (3) and the bottom surface electrothermal heater (24).
6 A method for operating an apparatus comprising an internal electrothermal heater (3) movable vertically and reciprocatively in a tank (10), and a driver (14-19) for driving the internal electrothermal heater (3), the method comprising the steps of:

moving the internal electrothermal heater (3) onto a bottom surface and supplying solid phase hot melt adhesives (1) on the internal electrothermal heater (3) in the tank (10) to an upper level of the tank (10), and then starting to apply an 12 electrical current to the internal electrothermal heater (3); applying an electrical current to the internal electrothermal heater (3) in a certain period of time and repeating a step of raising the internal electrothermal heater (3) to a level at which the solid phase hot melt adhesives (1) are in molten condition and lowering the internal electrothermal heater (3) to the bottom surface of the tank (10); and moving the internal electrothermal heater (3) reciprocatively between a middle level and a bottom of the tank (10) after the internal electrothermal heater (3) has reached the upper level of the tank (10).
7. The method as recited in claim 6, wherein the internal electrothermal heater (3) is formed with a plurality of through-holes so that molten hot melt adhesives (1) are fed through the internal electrothermal heater (3) from upward to downward of the internal electrothermal heater (3).
8. The method as recited in claim 6 or 7 further comprising the step of heating a side wall of the tank (10) at a lower temperature than both a temperature of the internal electrothermal heater (3) and a temperature of a bottom surface of the tank (10).