Method and apparatus for applying strip-shaped powder layer to a can body or the like, and powder-carrying can body

In order to apply a strip-shaped powder layer onto the inside of a can body to cover a welding seam of the can body in a controlled manner, the can body is moved in a predetermined direction over a spraying head transporting the powder, due to the kinetic energy imparted thereto by a moving air current, to an elongated opening of the spraying head. This elongated opening flow communicates with a spray chamber of the spraying head and is located opposite the welding seam of the can body. While the can body moves past the spraying head there is formed a strip-shaped powder layer at the welding seam without any cloud formation of the powder in the spray chamber. Advantageously, a vacuum action is applied immediately adjacent the outlet opening where the powder conveyed by the moving air current is introduced as a bundled powder jet into the spray chamber of the spraying head, so that the bundled powder jet entering this spray chamber is retained in its jet-like configuration through removal of the air current serving as carrier air for the powder particles. In this way, the powder particles are not deflected out of their intended flight path or trajectory towards the welding seam.

BACKGROUND OF THE INVENTION 
The present invention relates to a new and improved method and apparatus 
for applying a substantially strip-shaped powder layer, particularly on a 
welding seam of a container or vessel, such as a can body, and also to the 
can body itself. 
It is known to coat the welded longitudinal seams of can bodies with 
electrostatically charged powder. This method is utilized for can bodies 
in which sensitive filling materials are stored. Polymeric resins, such as 
for example epoxy resin, polyethylene and the like are frequently utilized 
as the powder for coating. The powder finely distributed in an air stream 
is supplied to the welding seam and electrostatically charged shortly 
before making contact with the seam. 
When the powder leaves the transport passage it is distributed over a 
region which is wider than the dimension of the welding seam. It also 
therefore coats such parts of the can body which must not be coated. On 
the other hand, a quantity of powder applied directly onto the welding 
seam, which often has a sharp edge, is insufficient to guarantee a 
reliable coating of the seam. 
U.S. Pat. No. 3,713,862, granted Jan. 30, 1973, discloses a method in 
accordance with which there is provided a band which covers the lateral 
regions of the seam, so that a definite small and uniform powder strip is 
applied onto the outer side of the seam. Coating of the inner side of the 
seam, however, is not possible with this method, since the covering band 
provided with slots cannot be guided through the welded or, in other 
words, closed can body. A uniform powder application over the whole cover 
region is furthermore not suitable. When there is applied a sufficient 
quantity of powder to reliably cover the welding edges of the seam, too 
much material lies in the lateral regions. Taking into consideration the 
high price of the powder, this is of course retained low, the welding seam 
becomes insufficiently coated. 
German Offenlegungsschrift No. 2,933,641, which is cognate to U.S. Pat. No. 
4,215,648, granted Aug. 5, 1980, describes a powder applying apparatus in 
which the lateral powder dissipation is prevented by partial separation of 
the air from the powder shortly before the discharge of the powder from 
the spraying head. The powder is separated from the air shortly before it 
reaches the seam by centrifugal separation, for example with a return 
track curve, then is chamber, and at that location charged by a row of 
electrodes also arranged parallel to the seam. The air stream which is 
supplied parallel to the powder stream also into the spraying chamber 
positively mixes with the powder and forms a powder-air cloud. This 
apparatus, however, does not render possible any concentrated application 
of the powder at the seam with a small quantity of powder particles 
dissipating into the surroundings. A predetermined distribution of the 
powder transverse to the seam region is therefore not possible. 
In a further development of the above-mentioned German Offenlegungschrift 
No. 2,933,641, reference being had to European patent application Ser. No. 
54,575, the spraying chamber is composed of a porous material. Gas or air 
is blown into the spraying chamber through the porous material in order to 
blow the powder particles which are brought substantially parallel to the 
welding seam, against the seam region. A predetermined application which 
covers only a very small region of the seam is excluded with this 
apparatus. This apparatus positively leads to a great powder region. 
Swiss Pat. No. 603,249 discloses a spraying head for a powder applying 
apparatus, which is provided with a plurality of bars and guiding sheets 
extending transverse to the flow direction of the powder-air mixture. The 
bars and sheets brake the powder-air stream and deflect it against a 
discharge slot which is laterally limited by flexible strips. A further 
deflecting element is provided, formed by a plurality of guiding sheets 
extending transverse to the slot. With the air of additional air blown 
through i.e. jets or nozzles, druses, the powder-air stream is 
additionally supported in the rear region of the spraying head. 
The utilization of baffles of different shapes for braking and deflecting 
the air-powder mixture results in a very complicated construction of the 
spraying head, which also has a tendency towards clogging, for example 
powder nesting. The distribution of powder obtained by guiding and braking 
elements takes place at the cost of a very high throughflow quantity of 
powder and air, which must be withdrawn in greater part by aspiration and 
then recycled again. In addition to the cost of pressure air and suction 
air, the recycling and loss of a part of the returned powder leads to 
further losses. 
All known powder applying arrangements or apparatuses produce directly or 
indirectly in a spraying chamber a powder-air cloud which is 
electrostatically charged in the spraying chamber and then applied against 
the seam which moves over the spraying chamber, due to its charging, as 
well as due to the superpressure and the air stream in chamber. Despite 
efficient suction hoods which are located above the spraying chamber, many 
powder particles still will be deposited on the outer side of the can 
body. The greatest mass of the powder particles supplied to the spraying 
chamber travel into the aspirating device and must be cleaned in a 
recycling device. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
apparatus and a method of applying a powder layer in a manner which avoids 
the disadvantages of the prior art. 
It is a further object of the present invention to coat the sensitive 
central region of a welding seam reliably with a thick layer, and at the 
same time to coat the outwardly located regions to a properly required 
extent. 
Another object of the present invention is to bring the powder in an air 
stream or current onto the welding seam with only a small quantity of 
transporting air and to deposit in a cloudless manner and in a 
substantially strip-shaped configuration the powder particles at the 
welding seam. 
A further object of the present invention is to design the apparatus so 
that it can be located as close as possible to the welding machine. 
Yet another important object of the present invention is concerned with an 
improved apparatus and method of applying a powder layer to a welding 
seam, wherein a spray chamber of a spraying head is maintained under 
vacuum conditions at a pressure lower than the pressure of the surrounding 
atmosphere, so that the carrier gas, typically air, transporting the 
powder particles is substantially removed from the influxing bundled jet 
composed of the powder particles and the carrier gas, to thereby retain 
the bundled jet configuration of the powder jet, whereby reliable and 
accurate deposition of powder particles onto the weld seam in the form of 
a substantially strip-shaped powder layer is achieved. 
In keeping with these objects and with others which will become apparent 
hereinafter, one feature of the present invention resides, briefly stated, 
in a method in accordance with which a powder is applied onto a welding 
seam of a can body in a substantially jet-like configuration due to the 
kinetic energy of the powder particles and without the formation of a 
cloud of the infed powder and air mixture of stream. 
It is another feature of the present invention that the apparatus provides 
for at least one supply passage which opens into a substantially 
slot-shaped spraying or spray opening and is arranged so that the powder 
will be discharged from the supply passage at an angle to the welding 
seam. 
Yet another feature of the present invention resides in the distribution of 
the powder-air stream into several zones having different application 
widths. This makes it possible, in a surprising manner, to obtain a 
reliable coating of the sensitive central region of the welding seam, with 
a reduced quantity of powder. 
A further feature of the present invention is the distribution of the 
powder-air stream into several passages so that the application thickness 
in the individual regions can be varied. 
Still a further feature of the present invention is extending or bringing 
the supply passages to the vicinity or the welding seam, so that the 
apparatus can operate with a smaller air quantity and lower pressure, and 
therefore only a smaller powder surplus will need to be recycled and, in 
addition, less powder particles fling near the welding seam. A 
concentrated application of the powder onto the welding seam without 
static loading or charging is also possible. 
In accordance with yet a further feature of the present invention, two 
supply passages are arranged near one another so as to dose the powder 
layer transverse to the welding seam. 
When in accordance with another feature of the invention the supply 
passages extend helically, clogging of the apparatus is reliably 
prevented. 
An additional feature of the invention is that the aspiration is performed 
near the passage outlet opening or discharge, so that the surplus powder 
is readily removed from the application zone. As this powder has not been 
soiled, it need not be cleaned. 
Another aspect of the invention is the provision in the apparatus of a 
chamber shaped as a tapered ring, in which the powder-air stream can be 
distributed in a flow-free manner. 
A still further feature of the invention is that the apparatus can be 
provided directly on the welding machine, and because of a short transport 
path from the welding location to the apparatus, an exact positioning of 
the seam relative to the spraying head is rendered possible and the powder 
can adhere to the seam which is still hot from welding and can be exactly 
applied onto the same. 
It is also possible to heat the welding seam or to maintain its heat so 
that the powder can be applied with a low charge or even without any 
charging. In many cases friction charging of the powder in the supply 
passages is sufficient so that the powder will adhere to the welding seam. 
Yet a further important aspect of the present invention contemplates 
applying a vacuum action immediately adjacent the outlet opening or 
discharge of the at least one supply passage which infeeds the powder-air 
stream or mixture by virtue of the kinetic energy of the powder particles 
against the weld seam. The vacuum which is thus applied enables a 
predetermined or defined withdrawal of the air from the influxing bundled 
jet-like powder-air stream or mixture. Consequently, the powder particles 
are maintained in their desired flight path or trajectory extending 
towards the welding seam and are not deflected out of such desired flight 
path or trajectory. By sucking off or removing the carrier gas, namely the 
air of the air stream, in a direction essentially parallel to but opposite 
the flight path of the powder particles towards the weld seam there is 
enhanced the reliable deposition of the powder particles at the welding 
seam in a defined cross-sectional configuration of the deposited powder. 
Furthermore, establishment of the vacuum conditions within the spray 
chamber of the spraying head prevents undesirable flow of the powder 
particles laterally of the strip-shaped powder layer which has been 
deposited onto the weld or welding seam, whereby there can be beneficially 
avoided the use of heretofore otherwise required lateral limiting or 
sealing brushes along the spray opening of the spray chamber.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The treating, recycling and melting of powder into a homogeneous layer on a 
welding or weld seam are not an object of the present invention, and they 
will be described only to the extent which is needed for understanding the 
present invention. 
Turning now specifically to FIG. 1, there is schematically shown a can body 
seam welding machine 1 which has electrode rollers 2 and 3. Several newly 
welded containers or vessels, here can bodies 4 are shown, as well as a 
powder applying device 5 with a spraying or spray head 6, an outer 
aspirating device 7, a combined treating and recycling device 8, and a 
melting device 9 of well-known construction for melting the powder on the 
seam. An electrical device 10 for generating the high voltage for charging 
the powder is also shown schematically in FIG. 1. 
A large scale longitudinal section of the spraying or spray head 6 is shown 
in FIG. 2. A can body 4 is located on the spraying head 6, and such can 
bodies 4 passes the spraying head 6 from the left to the right in the 
direction of arrow A. An upper section 11 of the can body 4 is taken 
through the welding seam 12 of the can body 4, and lies above a spraying 
opening 13. Several supply passages 15 for a powder-air mixture are 
inclined to the horizontal and open into a spray chamber 14 flow 
communicating with the spraying or spray opening 13. A further aspiration 
passage or passageway 16 is connected to an aspirating device that leads 
to the treating device 8. 
The supply passages 15 begin in a chamber 18 that is formed as a tapered 
ring, in which the powder-air mixture that has been received from a 
passage 19 is distributed into the supply passages 15. Needle-shaped 
electrodes 20 can extend into the chamber 18. These electrodes 20 are 
located coaxially relative to the supply passages 15 that extend from the 
chamber 18. The electrodes 20 are connected to a high voltage device 10 by 
means of an annular copper electrode 21 and a conductor 22. 
Depending on the diameter of the can body 4, there may be insufficient 
space for all the passages 15 to be arranged in the lower half of the 
spraying or spray head 6 parallel to one another. When the diameter of the 
can body 4 is less than 65 mm, the passages 15 extend advantageously 
helically from the chamber 18, as shown in FIG. 2 of the drawing. 
The spraying or spray opening 13 can be provided with lateral sealing 
elements 23 in the form of rubber strips or bristle strips. These sealing 
elements 23 serve for sealing or laterally limiting the powder discharged 
from the can body 4. A head chamber 17 is closed upwardly from the can 
body 4 by a cover 24. 
It is possible, however, to have the passage or passageway 16 generate a 
pressure in the spray or spraying chamber 14 which is lower than the 
pressure of the surrounding atmosphere. In this way there can be 
beneficially eliminated the need for using the lateral sealing elements 
23, since the thus established vacuum conditions within the spray chamber 
14 tend to retain the influxing bundled jet-like powder-air mixture in a 
bundled jet-like configuration having a defined or predetermined 
cross-section. This is so because by virtue of the vacuum conditions 
prevailing in the spray chamber 14 the air of the infed powder-air mixture 
is substantially removed therefrom, which, as stated, promotes the 
retention of the jet-like configuration of the powder particles propelled 
through their kinetic energy in the direction of the weld seam and there 
is not formed any spray cloud of the powder particles in the spray chamber 
14. Also, the vacuum conditions established within the spray chamber 14 
tend to draw in air from the surrounding atmosphere and such indrawn air 
moves along the outer skin of the bundled jet-like powder-air mixture 
introduced into the spray chamber 14, to thus beneficially retain the 
bundled jet-like configuration thereof. 
Furthermore, it is advantageous if the vacuum conditions are established at 
the floor or base of the spray chamber 14 and, specifically, near to the 
outlet opening or discharge of the related passage 14 from which effluxes 
the jet-like powder-air mixture. The carrier air which is removed from the 
powder-air mixture is sucked off by the vacuum action prevailing in the 
spray chamber 14 in a direction opposite to the intended trajectory or 
flight path of the introduced bundled jet-like powder-air mixture, so that 
the powder particles are not deflected out of their intended flight path 
or trajectory which carries them in a defined manner towards the weld 
seam. The powder is thus propelled by its kinetic energy, imparted to the 
powder particles by the carrier gas, namely here the air, in a most 
reliable, efficient manner and along a defined trajectory towards the weld 
seam. 
Continuing and as can be seen from FIG. 3, the spraying or spray opening 13 
in the spraying or spray head 6 has two slot zones 25 and 26 of different 
widths. The slot zone 25 which is located at the inlet side, as considered 
in the transporting direction A of the can body 4, is smaller than the 
slot zone 26 located at the outlet side. The central part of the can body 
4 shown in FIG. 3, represents the welding or weld seam 12. Recesses 27 are 
shown immediately adjacent thereto which are free from lacquer, and a 
protective coating, for instance a lacquer coating 28 is applied in flat 
condition onto the sheet material of the can body 4 outside the recesses 
27. 
The approximate widths of the welding seam, the neighboring regions 27 
which in some cases can also be coated, and the slot zones 25 and 26 can 
be recognized from FIG. 3 of the drawing. The small zone 25 is 
insignificantly wider than the seam 12, whereas the wider zone 26 exceeds 
the entire width of the seam 12 and the recesses 27. 
In accordance with a modification shown in FIG. 4, the spraying opening 13 
continuously increases from the inlet side to the outlet side of the 
spraying head 6. The supply of the powder-air mixture, however, continues 
to be distributed into a plurality of the supply passages 15, as had been 
shown in the embodiment of FIG. 2. 
It is to be understood that instead of four passages 15 provided in the 
shown example, a smaller number or a greater number of these passages can 
also be provided. 
In accordance with a further modification of the invention shown in FIGS. 5 
and 6, supply passages 115 which guide and supply the powder-air stream 
extend substantially perpendicular to the outer surface of the can body 4 
or to the welding seam 12 thereof, and at a very small distance thereto. 
The distance between the ends of the passages 115 and the welding seam 12 
lie in the region of between 2 mm and 5 mm, advantageously approximately 
2.5 mm. The cross section of the discharge mouths or outlet openings of 
the passages 115 can be round or cornered. The passages 115 can extend in 
pairs near one another, particularly in the wider zone 26 of the opening 
13. Advantageously, easily exchangeable fittable end pieces 315 are 
arranged at the passages 115. In addition to the suction or aspiration 
passage 16 which opens into the head chamber 17, further suction or 
aspiration passages 116 can be provided either before and/or after each 
passage 115. These suction or aspiration passages or passageways 116 
advantageously open into the bottom of funnel-shaped depressions 117 
located between the supply passages 115. The suction or aspiration 
passages or passageways 116 are connected by way of the suction passage 16 
with the treating device 8. Again, the aforedescribed vacuum conditions 
may be advantageously established within the spray chamber by the action 
of the suction or aspiration passages or passageways 16 and 116 located at 
the floor of the spray or spraying chamber and laterally of the related 
supply passages 115, specifically the outlet openings or discharge mouths 
thereof. 
The sealing elements 23 or synthetic belts which run together can be 
provided laterally of the spraying opening 13, to protect the regions 
located near the welding seam 12 from deposit of any powder particles. In 
the embodiment shown in FIGS. 5 and 6 these protective elements can be 
dispensed with, especially when the passages 115 open very close to the 
welding seam 12 and as a result of this the powder-air mixture can exit 
with a very low speed so as to cover the remaining free path up to the 
seam 12. Powder particles which have not reached the seam, or are loose, 
are removed by way of the passages 116 from the spraying opening. 
However, by virtue of the vacuum conditions which can be established in the 
spray chamber and for the reasons enunciated more fully previously, it is 
equally possible to dispense with the use of the sealing elements or 
ledges 23. 
A tiltable flap 215 (FIG. 5) may be provided in the passage 19, and the 
tilting position of the flap 215 may be adjusted for dosing the powder 
flow. 
When the spraying head 6 is formed with only two passages 15, 115 the 
distribution of the powder-air stream at the end of the supply passage 19 
can also be carried out through a Y-shaped splitting of the passage 19. 
The electrodes 20 can lie in this case in the legs of the Y-shaped 
splitting. 
Particularly during application of the powder in which the greater part of 
the transport or carrier air flows essentially parallel to the powder 
stream, the shape of the powder stream during discharge from the powder 
supply passages 15, 115 is important. The schematic cross-sections shown 
in FIGS. 7, 8 and 9, and the orifices shown in FIG. 10, illustrate the 
shapes of the passages 15, 115 which make possible the discharge of the 
powder as a band-like or strip-like stream normally or at least at an 
obtuse angle to the welding or weld seam. The initially round 
cross-section of the powder supply passages 15, 115, starting at the 
bottom-side part of the passages 15, 115 below the outlet opening, have a 
rectangular cross section extending to the outlet opening. The powder 
particles flowing in the air stream or current in the horizontally located 
portion of the passages 15, 115 are distributed on the flat outer radius 
and glide there upwardly, where they discharge from the passages 15, 115 
as a small band transversely to the seam. 
Operation 
The can body 4, the seam of which had been welded between the welding 
rollers 2 and 3, moves from the welding machine 1, by means of a transport 
system 29, to the which coats only the welding seam 12 is applied from the 
smaller zone 25 of the spraying opening 13. During passage of the wider 
zone 26 the powder application takes place in a region which includes not 
only the welding seam 12, but also the neighboring region 27. Thus the 
already coated seam region 12 is again coated with powder. 
The adherence of the powder to the can body 4 is obtained in a known manner 
in that the powder particles are electrostatically charged either on the 
electrodes 20 or by friction in the supply passages 15, whereas the can 
body 4 has an opposite charge. The adherence can also be obtained by 
gluing on or fusing to the welding seam 12 which is still hot or retained 
heated by a heat source (not shown) to a temperature exceeding the melting 
temperature of the powder. 
The supply of the powder into the air stream is performed through the 
passage 19. After the distribution in the chamber 18 having the shape of a 
tapered ring or in the Y-shaped end of the passage 19 into one or several 
supply passages 15 and the static charging, the powder-air stream is 
guided directly and without further baffles through the spray opening 13 
onto the seam 12 of the can body 4, and into the region or recess 27. With 
the aid of the sealing elements 23, or in the case where it is desired to 
avoid the use of such sealing elements 23 by applying the aforedescribed 
vacuum conditions within the spray chamber and preferably at the region of 
the outlet opening or discharge mouth of each provided supply passage, the 
application of the powder to the weld seam can be limited exactly to the 
width of the slot opening 13. The powder particles which have not adhered 
to the can body 4, and also the transport air are removed through the 
suction or aspiration passage 16 from the head chamber 17 and/or the 
funnel-shaped depressions 117. With the inventive measures there is also 
avoided the formation of any spray cloud of powder particles within the 
spray or spraying chamber, which otherwise would result in an uncontrolled 
deposition of the powder particles on the weld seam and neighboring 
regions thereof. 
The powder particles discharged between the successive can bodies 4 are 
removed by the aspiration device 7. They are supplied again to the 
treating device 8. After application of powder through the spraying head 
6, the can body 4 is displaced along a heat source or heating element 9, 
so that the powder is melted and forms a coating that firmly adheres to 
the can body 4. 
It is to be understood that the above-described method and apparatus for 
coating of the inwardly located seam can be utilized analogously for the 
outer seam coating. It is also to be understood that with the 
above-described method, application of powder onto a seam located below 
can also be performed. 
The invention is not limited to the details shown since various 
modifications and structural changes are possible without departing in any 
way from the spirit and teachings of the present invention. 
While there are shown and described present preferred embodiments of the 
invention, it is to be distinctly understood that the invention is not 
limited thereto, but may be otherwise variously embodied and practiced 
within the scope of the following claims. Accordingly,