Abstract:
The invention relates to a method and device for applying a material pattern by means of a sequentially operating applicator to a substrate moving relative to said applicator, by way of a control valve for liquid material, in particular a hot melt, said valve being controllable for opening and closing, which material pattern in the direction of movement is sub-divided into pattern parts with slight spacing between them, and there being at least two control valves connected to the material supply, and the respective valves being made to open and close sequentially. A considerably faster operation is achieved this way.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for applying a material pattern by means of an applicator to a substrate moving relative to said applicator, by way of a control valve for liquid material, in particular a hot melt, said valve being controllable for opening and closing and being connected to a material supply, which material pattern in the direction of movement is sub-divided into pattern parts with slight spacing between them, and to a device for carrying out said method. 
     BACKGROUND OF THE INVENTION 
     Applying a liquid material, in particular a hot melt, by means of an applicator to a substrate by way of a control valve with controllable opening and closing is a technique which is known per se. 
     So long as the speed of the material relative to the applicator remains below certain values and the space between the pattern parts is not too small, no particular problems occur, despite the fact that, particularly in the case of pneumatically operated valves, for obtaining a well-defined pattern, i.e. not ragged, large-size air supply and discharge ducts are needed in order to be able to supply and evacuate the control air in a short time. 
     However, the situation changes when the relative speed increases and the space between the pattern parts has to be small. In this case the time which elapses between quickly making the valve close and subsequently quickly making the valve open is too short to obtain a good effect. This problem occurs in particular in the case of pneumatically controlled valves, in the case of which considerable quantities of air have to be supplied or discharged through large-size ducts in order to obtain the desired rapid opening and closing of the valves. 
     The object of the invention is to provide a solution to this problem. According to the invention, for this purpose use is made of at least two control valves which are connected to the material supply and are made to open and close sequentially. 
     If, for example, there are two control valves and the first of these two is made to close at the end of its working cycle through rapid evacuation of the control air, and is not in a position to allow material through again very shortly afterwards by being made to open, the function of said first valve is taken over by the second valve, which is made to open at the correct moment; during the open period of this second valve, there is an opportunity for the situation in the first valve to recover, and said first valve is ready for the next working cycle the moment the second valve is made to close. A very rapid and error-free operation is achieved in this way. 
     Of course, the principle according to the invention can also be applied to a system with more than two control valves--for example three or even four--which are controlled sequentially. 
     It is also possible to divide these valves into two or more sets, each set comprising at least two valves, and said sets being made to open and close sequentially. For example, it is possible to make an applicator interact with four or six control valves, sub-divided into two or three sets of two or three valves each, so that even in the case of an applicator with a relatively long nozzle a good effect remains guaranteed. 
     It is pointed out that an applicator with two control valves is known per se from U.S. Pat. No. 4,735,169. In the case of this known device, however, these control valves are made to open and close simultaneously, so that the principle on which the invention is based is not known from this publication. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained with reference to the drawings in which: 
     FIGS. 1a, 1b and 1c give examples of patterns of a liquid material, in particular a hot melt, to be applied to a substrate; 
     FIG. 2 shows diagrammatically a plant with which the method according to the invention can be used; 
     FIG. 3 shows a time chart of the opening and closing times of the valves used in the plant according to FIG. 2; 
     FIG. 4 shows a perspective view of an applicator suitable for use of the method according to the invention; 
     FIG. 5 shows a diagrammatic view of a plant in which the valves are sub-divided into two sets of three valves each. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1a reference number 2a indicates the outflow nozzle of an applicator, which is known per se and is not shown in any further detail, for applying strips of liquid material, in particular a hot melt adhesive, in a fixed pattern to a substrate, which is considered to be the plane of the drawing, three of which strips are shown and are indicated by reference numbers 4a-4c. The substrate must move in the direction of the arrow 6 below the outflow nozzle 2a at a speed of, for example, 100 m per minute, thus 1,670 mm/sec, while the distance d between the respective material strips can be 3 mm. This means in fact that the time elapsing between the shutting off of the material supply to the applicator opening 2 and the reopening of said supply must be no longer than 1.8 ms. The length l 1  is approximately 30 mm, corresponding to an application time of approx. 20 milliseconds. 
     FIG. 1b relates to the situation in which narrow strips of material, indicated by 8a-8e, must be applied by means of the nozzle 2b to the substrate. The distance l 2  between the longitudinal edges of each material strip is in this case equal to the space d2 between the respective material strips. Here again, therefore, a period of only 1.8 ms is available for both the supply period of the material and the period in which the material supply is interrupted. 
     Finally, FIG. 1c shows by way of example how a regular pattern of rows of material points 12a . . . 12d, each with a length l 3  and a space d3 between them of approx. 3 mm can be applied to a substrate with a single applicator nozzle 2c, provided with a number of obstructions 10a-10c. Here again, only 1.8 ms is available as the time in which the material supply takes place or is interrupted. 
     This cannot be achieved with the device according to the prior art, but it can with the measures according to the invention. The principle of the invention is explained with reference to FIGS. 2 and 3. 
     FIG. 2 shows diagrammatically an applicator 14, the nozzle 16 of which lies a short distance above the substrate 18 moving relative thereto and at right angles to the plane of the drawing. The space 20 inside the applicator nozzle 22 is connected by means of two pneumatically controlled control valves 24, 26 to the common material supply line 28, through which the material, in particular a hot melt, is supplied under the influence of the pressure pump 30, under pressure from a source 32. The control valve 24 is pneumatically controlled by means of the line 34 by the shuttle valve 36, the connection 36a of which is in communication by means of the line 38 with a pressure medium source 40, and the connection 36b of which opens out into the atmosphere, or can be connected to an air vent. The control valve 26 is controlled by means of the line 42 by the shuttle valve 44, the connection 44a of which is in communication with the line 38, and thus with the pressure medium source 40, while the connection 44b opens out into the atmosphere. 
     The shuttle valve 36 and the shuttle valve 44 are controlled electrically by means of control lines (46, 48 respectively) by a central control unit 50, which supplies the shuttle valves 36 and 44 with the current pulses which are necessary for the control thereof. 
     The course of said control pulses as a function of time is indicated in FIG. 3, in which figure the line 3a relates to the current pulses supplied to the shuttle valve 36, the line 3b relates to the current pulses supplied to the shuttle valve 44, and the line 3c is the time axis. It is assumed that during the current pulses applied to the shuttle valves 36 or 44 the latter are controlled in such a way that the control valve 24 or 26 controlled thereby is open. The chart relates to the situation shown diagrammatically in FIG. 1a. 
     The control valve 24 thus opens at the moment t1 and closes at the moment t2; the time interval ΔT1 corresponds to the length l 1  of the material strip 4a in FIG. 1a. During this period of time, which can be, for example, 20 ms, the material flows out of the nozzle aperture 2a. 
     At the moment t3, ΔT2 after t2, for example 1.8 ms after t2, the control valve 26 is opened. This situation continues until the moment t4, ΔT1 after t3, and during this period the material is now supplied by means of the control valve 26 to the outflow aperture 16 of the nozzle 22, resulting in the material strip 4b. The function of the material supply is then taken over again by the control valve 24, which opens at the moment t5 and remains open until the moment t6. The control valve 26 then takes over the function of the control valve 24. 
     It is clear that with such a method of operation of the applicator nozzle the two pneumatically operated control valves 24 and 26 after closure have plenty of time to move into a stable closed position in which transitional phenomena have died out, and the valves are in a position in which they can be opened reliably again in order to ensure the material supply. 
     FIG. 4 shows how the control valves 24 and 26 can be combined with the applicator nozzle 22 to form a constructional unit. The various supply and control lines are not shown in this figure. 
     Finally, FIG. 5 shows diagrammatically how, if use is made of a relatively long applicator nozzle 60 lying above the substrate 62, a uniformly distributed material supply can be achieved through the use of more than two control valves. In the case shown there are six of such control valves, sub-divided into two sets which are indicated by 64a, 64b, 64c and 66a, 66b, 66c respectively. The control valves 64a 64b, 64c are operated simultaneously by means of the common control line 68, and the control valves 66a, 66b, 66c are operated simultaneously by means of the control line 70. The control line 68 corresponds, for example, to the control line 34 in FIG. 2, and the control line 70 corresponds to the control line 42 in FIG. 2. Line 68 is connected to the shuttle valve 72, the functioning of which corresponds to that of the shuttle valve 36 in FIG. 2, while line 70 is connected to the shuttle valve 74, the functioning of which corresponds to that of the shuttle valve 44 in FIG. 2. For the sake of clarity, the remaining connections of said shuttle valves are not shown. 
     It is clear that in the case in which the length l 2 , l 3  of the material strips is considerably smaller than the length l 1  of the material strips shown in FIG. 1a the time duration ΔT1 of the respective control pulses will be correspondingly shorter, but even then it remains long enough to ensure good functioning.