EP1297957A1

EP1297957A1 - Powder applying device

Info

Publication number: EP1297957A1
Application number: EP01123432A
Authority: EP
Inventors: Egil Norheim
Original assignee: Perma Press AB
Current assignee: Perma Press AB
Priority date: 2001-09-28
Filing date: 2001-09-28
Publication date: 2003-04-02

Abstract

A device for application of coating powder, comprising a conveyor (4) with an air permeable surface (5) for conveying a base sheet in a direction of travel, said base sheet being coated upstream said conveyor with at least one layer, a powder-applying device (6), arranged to apply coating powder onto the base sheet and a suction device arranged to create an under-pressure beneath said conveyor surface, forcing excess powder to pass through said surface. The suction device includes an outlet filter (24) arranged below the conveyor surface (5), through which filter (24) the flow of air (41, 42) passes, and a flow guiding element (27) arranged between the conveyor (5) and the filter (24), to prevent air that is forced through the conveyor by the suction device from directly entering the filter. This reduces the problem of the filter being clogged with powder particles.

Description

Technical Field

The present invention relates to a device for application of coating powder in a printing process. In particular, the invention relates to a device for application of adhesive powder in a screen transfer printing process.
Such a device includes a conveyor with an air permeable surface for conveying a base sheet in a direction of travel, said base sheet being coated with at least one layer upstream said conveyor, a powder-applying device, arranged to apply coating powder onto the base sheet and a suction device arranged to create an under-pressure beneath said conveyor surface, forcing excess powder to pass through said surface.

Technical Background

A conventional screen printing process includes applying one or more layers of color to an object, by pressing it through the mesh of a netting screen stretched on a frame and onto the object. A screen transfer printing process relates to producing a screen printed material (hereafter referred to as a screen transfer) that can be heat transferred to e.g. a garment or a bag. The screen transfer normally consists of several layers of colored ink having been applied to at least an area of a base sheet, i.e. a sheet of paper or cardboard. The coated area is then pulled of the sheet, and is ready to be attached to a garment or any other object of suitable material. In order to be readily attachable, an adhesive powder is applied onto the coated area after the final layer of ink has been applied. This adhesive can be adapted to be heat or pressure activated, and therefore makes it possible to attach the transfer in a simple manner. Other ways of activating the adhesive include microwave energy.
In other printing operations, for example offset printing or relief printing of Christmas cards, a printed area is often coated with different types of powders, for example a glittery powder to resemble snow.
In printing situations such as the above, a device has been developed by Perma Press AB that allows application of a coating powder (e.g. an adhesive powder) onto a base sheet at least partially covered with paint, and removal of any excess powder. The mentioned device comprises a suction device arranged to force excess powder through the conveyor so that it can be collected in a compartment below the conveyor. Reference is made to the pending US patent application 09/579 539.
Normally, such a device is fitted with a filter, for preventing the powder from leaving the compartment. A problem is that this filter easily becomes clogged with powder particles.

Disclosure of the Invention

The present invention alleviates the above mentioned problem, by providing a device of the type mentioned by way of introduction, wherein the suction device includes an outlet filter arranged below the conveyor surface, through which filter the flow of air passes, and a flow guiding element arranged between the conveyor and the filter, to prevent air that is forced through the conveyor by the suction device from directly entering the filter.
The presence of the flow guiding element prevents the powder particles carried by the air from directly entering the filter, and instead allows for the powder particles to be disposed of, i.e. fall down to the bottom of the compartment, before the air passes through the filter. This reduces the problem of the filter being clogged with powder particles.
The filter can be elongated and extend across the width of the conveyor. This allows for the under pressure created by the suction device to be evenly distributed over the conveyor. In this case, the flow guiding element preferably extends axially along the filter, in a space between the filter and the conveyor. By this arrangement, the entire length of the filter can be covered by the flow guiding element.
The flow guiding element can further extend around at least a quarter of the filter periphery, thereby directing the air flow to pass peripherally around the filter, outside the flow guiding element, before entering the filter. A significant portion of the powder particles carried by the air are brought out of the air flow during this detour, and this effect is enhanced as the flow follows a curved path, creating a centrifugal force on the powder particles.
In a preferred embodiment, the flow guiding element has the shape of a cylinder with an axially extending opening, and is fitted around the filter in such a way that the opening is directed away from the conveyor. The cylinder shaped element acts like cyclone, forcing the air flow to follow an essentially circular path around the cylinder periphery, thereby ejecting most of the powder particles being carried by the air, before the air enters the slot shaped opening and then the filter. The ejected particles are left to fall down to the bottom of the compartment.
Note that the air flow can pass around the cylinder along two different paths, one on the upstream side, and one on the downstream side. For this reason, it can be advantageous to arrange the cylinder with the opening facing downwards.
It may be advantageous to arrange a regulating means, such as an elongated plate, in the opening of the cylinder. Such a regulating means will improve the function of the flow guiding element, by interrupting the flow of air entering the opening. By interrupting the flow and causing a certain turbulence, particles still carried by the air can be allowed to leave the air flow and to fall downwards, away from the filter. The regulating means can be adjustable, for example for forcing a larger portion of the air flow around the downstream side of the flow guiding element.
As mentioned above, the base sheet is coated with powder at the upstream side, and then conveyed on the conveyor while excess powder is sucked through the conveyor by the suction device. It is advantageous to also arrange an air knife, i.e. a device that directs a stream of air onto the conveyor and the base sheet, at the downstream side, for pushing the excess powder off the base sheet. With this arrangement, it is advantageous to arrange the filter (and therefore the flow guiding element) under the downstream side of the conveyor, as this creates a stronger under-pressure closer to the air-knife. This prevents the base sheet from moving when hit by the air from the air knife.
The filter and flow guiding element are preferably arranged in a funnel shaped compartment, formed beneath the conveyor for collecting excess coating powder forced through the conveyor. As a result, any powder that leaves the air flow from the conveyor towards the filter will be collected in the compartment. The inside of the compartment can be essentially equidistant with the outside of the flow guiding element on the downstream side of the filter. This further enhances the cyclone effect mentioned above, and improves the process of removing powder from the air stream.

Brief Description of the Drawings

These/This and other aspects of the invention will be apparent from the preferred embodiments more clearly described with reference to the appended drawings.
Fig 1 is a perspective view of a machine incorporating a device according to the present invention.
Fig 2 is a partly cut away side view of the machine from direction II in fig 1.
Fig 3 is a partly exploded perspective view of the machine in fig 1.
Fig 4 is a partly cut away side view of the machine from direction III in fig 1.

Detailed description of preferred embodiments

The following description is related to a screen transfer printing line. However, it is noted that the device according to the invention may be used in connection with many other types of printing processes.
The machine shown in fig 1 can be placed downstream the screen printing process itself. Preferably, the machine is located immediately after the printing process.
Base sheets, normally made of a release coated paper or plastic material, are transported along the screen transfer line, passing different process steps, including the screen printing step, the adhesive powder application step and a drying/curing step. The sheets are transported on a moving conveyor between the steps. In the printing step, the base sheet is coated with a screen print ink, consisting of a screen print base to which colored pigmentation has been added. Depending on the specific application, each sheet can support one or several transfers.
According to a different technique, the base sheet is in fact a continuous run, carried by the conveyor from one roll to another. In this case, the transfers are printed on the run, and then rolled up on the second roll together with the run. This can be an advantage when several different colors of ink are to be printed, as the rolls can be switched and the run can be carried by the conveyer back to the first roll.
In many cases, the screen transfer is a multi-color print, demanding a multiple printing process. One way of accomplishing this is to let the base sheet pass along the mentioned printing line several times, each time applying a different colored layer of ink. If different areas of the screen transfer are printed one after the other in this way, each area is dried before the next color is applied. The final ink layer, normally being a pale shade such as white, or transparent, is however applied all over each transfer, so that the entire surface of each transfer is covered with wet paint. Thus, when a base sheet with one or several transfers printed thereon is conveyed to the machine according to the invention, the surface of each transfer is still tacky.
Returning now to fig 1, the machine 1 comprises a rigid structure 2 preferably made of welded steel profiles, supporting the different elements of the machine. In the illustrated example the structure is supported by wheels 3, making it easily movable. A conveyor 4 is arranged substantially in the horizontal plane, and has a surface which permits the passage of powder particles. For example, a conventional belt conveyor can be used, comprising a belt of a flexible and permeable material, such as a net 5 or the like, supported by two bars 7, extending across the width of the machine. Above the conveyor is arranged a powder-applying device 6 and preferably an air-knife device 8, i.e. a device for providing a jet of air directed towards the upper surface of the conveyor 4.
The powder-applying device in the illustrated example includes a hopper 9, being filled with adhesive powder which is distributed through an opening down onto the base sheet and any transfers thereon. The hopper can be arranged to vibrate, preferably in a direction transverse to the direction of travel A of the conveyor, thereby ensuring a satisfactory distribution of powder onto a base sheet 10.
At the side of the machine 1 are controls for electricity and air, in the illustrated example fitted in a hinged box 14. Behind this box 14, is the side 15 of the machine, in which an opening 13 allows access to a compartment 16, situated underneath the conveyor.
In the compartment 16 an under-pressure is generated by a suction device. The suction device comprises a fan or the like to generate the under-pressure. The fan can be fitted to one side of the machine 1, and is hidden in fig 1 by a housing 11. The housing has an outlet 12, fitted with a silencer and an outlet filter covered by a steel net cover or the like.
In fig 2 the machine 1 is shown from the downstream side, with the side of the machine partly removed. Inside the housing 11, is revealed the fan 22, sucking air through a filter 24 arranged in the compartment 16. In the illustrated example, the air flows from compartment 16, through the filter 24 to an adjacent chamber 25 connected to the fan with a protective boot 26. In order to accomplish an even distribution of the under-pressure across the conveyor, the filter preferably extends across the entire width of the compartment 16. According to the invention, a flow guiding element (shown in fig 3 and 4) is fitted around the filter.
The filter 24 can be connected to a high-pressure compression tank 28 through an air pipe 29. The compression tank is arranged to regularly clean the filter 24 with a thrust of air, and is controlled by an electronic controller in box 14.
Below the filter 24 is a removable powder collecting tray 18, connected to a cyclone 19 above the powder applying device with a powder supply line 20. Preferably the compartment 16 is tapered towards the bottom forming a large funnel shape, leading down into the narrow tray 18. The elongate tray 18 is arranged to be extractable out of the side of the machine 1, thereby enabling an operator to empty it.
Fig 3 shows the filter 24 and flow guiding element 27 in more detail. The illustrated filter 24 is essentially cylinder shaped, and comprises an outer net cage surrounding a filter body of fiber material. The fan is connected to the inner end, while the outer end is sealed, to ensure that all air is sucked through the periphery of the filter body.
The flow guiding element 27 is formed from a sheet of metal, plastic or other suitable material, which can be attached, e.g. welded or riveted, to the side walls 15 of the machine. The flow guiding element has a cylindrical shape coinciding with the shape of the filter 24, but slightly bigger. It does not enclose the entire filter, but an axial opening 31 extends along the lower side of the cylinder.
Outside the flow guiding element 27, the walls 32, 33 of the compartment 16 can be seen in fig 3. These walls are preferably comprised of metal, plastic or other suitable material, and can also be attached in the side walls 15. The downstream inner wall 32 of the compartment 16 can be shaped to follow the curvature of the guiding element 27, thereby forming a passage 37 along the outside of the element.
Two rigid bars 34, 35, extending across the compartment 16, are attached, preferably welded or screwed, to the down stream compartment wall 32, and to the flow guiding element 27, respectively. The filter 24 can be slided into place along the bars 34, 35, through the opening 13 in the side wall 15 of the machine, making it easy to change if needed.
In the preferred embodiment, the opening 31 defines an angular sector of approximately 45 degrees, and is located facing downwards, essentially in the sector between the bars 32. Depending on the filter type, distance between filter and guiding element, power of the fan, etc, the angular sector of the opening can be chosen and positioned differently.
A regulating plate 36 is fitted in the opening 31, extending along the length of the guiding element 27, and reaching closer to the inserted filter 24 than the flow guiding element 27. The purpose of this plate 36 is to disrupt the flow of air passing around the flow guiding element, and to direct it downwards, towards the tray 18. For adjustment purposes, the regulating plate 36 can turnably mounted, so that it can be operated from the outside of the compartment 16.
With reference to fig 4, the function of the guiding element will be described. In fig 4 the machine is shown from the side, offering a better view into the compartment 16, where the flow of air around the filter has been indicated.
As a sheet 10 is conveyed by the conveyer 4, excess powder is removed and sucked down through the belt 5 into the chamber 16 by the under-pressure generated by the suction device.
The air passing through the conveyor follows essentially two different paths 41, 42 around the guiding element 27. A first path 41 leads down along the upstream side of the filter 24. If the filter, as in the illustrated case, is located at the downstream side of the compartment 16, air following this first path 41 will experience a lower average under pressure. Particles 43 carried by this air will therefore tend to fall down towards the tray 18, instead of following the air.
A second path 42 leads along the downstream side of the guiding element 27, in the narrow passage 37 between the guiding element and the wall of the compartment 16. The under pressure on this side is generally higher than on the other side, and particles following this path would be likely to be sucked into the filter had the guiding element 27 not been present. In the illustrated example, however, the particles follow the flow of air around the surface of the guiding element 27, thereby being exposed to a centrifugal force, hurdling them against the wall 32. In effect, the guiding element 27 and the wall 32 act like a cyclone. By forcing the particles against the wall 32, they are brought out of the strong air flow, and instead allowed to fall down towards the tray 18.
Air following both paths is eventually sucked through the opening 31 in the guiding element. By this time, however, most of the particles have been disposed of, and the air entering the filter 24 carries little or no particles. The plate 36 further enhances this process, interrupting the air flow and creating some turbulence below the filter 24.
The powder collected by the tray 18 sucked into the tubes 20 and transported to the cyclone 9. The cyclone brings the powder to circulate in a spiral shaped path towards the hopper 9 thereby improving the powder application process, and achieving a regular distribution of powder into the hopper 9.
The invention has been described in connection to a preferred embodiment being focused on screen transfer printing. It is clear to a person skilled in the art that this embodiment does not limit the scope of the appended claims, but only serves as an example in a specific application.
For example, as is clear from the object of the invention, it is possible to implement the device and method of removing coating powder in offset or relief printing, without departing from the scope of the claims.

Claims

A device for application of coating powder, comprising:

a conveyor (4) with an air permeable surface (5) for conveying a base sheet in a direction of travel, said base sheet being coated upstream said conveyor with at least one layer,

a powder-applying device (6), arranged to apply coating powder onto the base sheet,

a suction device (22) arranged to create an under-pressure beneath said conveyor surface, forcing excess powder to pass through said surface,

said suction device being characterized by:

an outlet filter (24) arranged below the conveyor surface, through which filter the flow of air (41, 42) passes, and

a flow guiding element (27) arranged between the conveyor (4) and the filter (24), to prevent air that is forced through the conveyor by the suction device from directly entering the filter.
The device according to claim 1, wherein the filter (24) is elongated and extends axially across essentially the width of the conveyor.
The device according to claim 2, wherein the flow guiding element (27) extends axially along the filter (24).
The device according to claim 3, wherein the flow guiding element (27) extends around at least a quarter of the filter periphery.
The device according to claim 4, wherein the flow guiding element (27) has the shape of a cylinder with an axially extending opening (31), and is fitted around the filter in such a way that the opening is directed away from the conveyor (4).
The device according to claim 5, wherein the cylinder is arranged with the opening (31) facing essentially downwards.
The device according to claim 5 or 6, said suction device further comprising a regulating means (26), arranged in said opening of said cylinder, for controlling the flow of air around the flow guiding element.
The device according to any of the preceding claims, wherein the filter (24) is arranged under the downstream side of the conveyor.
The device according to any of the preceding claims, wherein the filter (24) is arranged in a funnel shaped compartment (16), formed beneath the conveyor (4) for collecting excess coating powder forced through the conveyor.
The device according to claim 9, wherein the inside of the compartment (32, 33) is essentially equidistant with the outside of the flow guiding element (27) at least on the downstream side of the filter (24).