EP0019669B1

EP0019669B1 - A method and apparatus for electrochemical treatment of a can body

Info

Publication number: EP0019669B1
Application number: EP79300914A
Authority: EP
Inventors: Thomas Patrick Murphy; George Bell; Fred Fidler
Original assignee: Metal Box PLC
Current assignee: Crown Packaging UK Ltd
Priority date: 1979-05-23
Filing date: 1979-05-23
Publication date: 1983-07-27
Also published as: EP0019669A1; DE2965983D1

Description

This invention relates to the electro-chemical treatment of a surface of a container and more particularly but not exclusively to a method and apparatus for electrolytically treating a container body drawn from sheet metal.
Container bodies are known which are drawn from sheet metal to have a cylindrical side wall closed at one end by an end wall. During the drawing process the surfaces of the sheet metal are subjected to frictional engagement with the drawing tools and the surface may become scratched or generally disturbed. This disturbance is more apparent on containers which have been wall ironed than on containers which have been redrawn with a positive clearance between punch and die.
The tinplates, used for the manufacture of drawn containers, comprise a layer of tin on the mild steel substrate. The surface of the tin is usually passivated by a known chemical treatment. When the passivated tinplate is drawn, in a press tool, the tin layer is subjected to frictional engagement with the tool surface and so the passivated layer is liable to be destroyed. If the degree of drawing or redrawing is severe, as could be the case in a wall ironing operation, the tin layer may be broken to leave the steel beneath, exposed and vulnerable to attack when the container is packed with certain food products, as is well known in the art. One known remedy is to use tinplate having a thicker layer of tin but this is a costly remedy.
The aluminium sheet used for manufacture of containers, is often anodised, etched or treated with chromates because such surface treatments help subsequent organic coatings to adhere to the sheet. As already explained, with reference to tinplate, these surface finishes for aluminium sheet may also be spoiled in the press tool and so with both drawn aluminium containers and drawn tinplate containers there is a need for a method and apparatus for repairing the surfaces of the containers after the metal working operations; such operations may include drawing; re-drawing with a positive clearance; redrawing with a negative clearance herein called wall ironing; flanging or beading.
Tin plate container bodies having a side seam are commonly soldered. However, certain products require that the side seam be protected after forming, the usual remedy being the application of a stripe of organic lacquer. Tinplate container bodies having a welded side seam also suffer damage to the metal surface adjacent the weld and again there is a need for a method of repairing the interrupted surfaces of the metal of the side seam.
The application of organic coating materials to containers is often impeded by the presence of lubricants. The problem is more serious with wall ironed cans and so the cans are usually washed and dried before coating. British Patent Number 1498 795 describes a method and apparatus for cleaning can bodies by means of a solvent or detergent which removes the greasy lubricants. The method described therein comprises mounting the container in a cavity in a body; the cavity having a shape corresponding approximately to that of the container, with the wall of the cavity spaced close to the surfaces of the container; and the container sub-dividing the cavity into two chambers; and passing the cleaning fluid through the chambers so that the cleaning fluid fills the chambers and flows along the surfaces of the container. The flow is preferably turbulent to obtain a good cleaning action, in contrast to the galvanic actions which are the subject of this specification.
That is not to say that movement of fluids into and out of a treatment cell prevents the use of such an arrangement in an electro-coating apparatus. In our British Patent Number 1,117,831 apparatus is described in which a tubular side seamed can body is moved to surround an electrode and define with the electrode and electrocoating cell. Electrocoating material is passed into the space between the interior of the can body and the internal electrode and application of a potential difference between the can body and the electrode causes a current to pass through the coating material so that the internal surface of the can body becomes coated. The weight of coating applied to the body is proportional to the current passed. After coating the can the electrolyte is removed from the cell and the body is removed from the apparatus. However, this apparatus is not capable of applying an external coating to the can body.
U.S. Patent No. 4,094,760 discloses an arrangement for simultaneously electrocoating both the interior and the exterior of a can body. This arrangement features a cell having a side wall and a mandrel disposed to receive a can body therebetween in close spaced relationship in such a manner that a continuous passageway exists permanently, when the can body is present, from the mandrel to the interior of the body to the exterior of the body.
In a first aspect the invention provides a method of treating a can body formed from sheet metal, said method including the steps of placing the can body on a mandrel in a cell such that the interior and exterior surfaces of the can body are in closed spaced relationship respectively with the mandrel and with a side wall of the cell, introducing an electrolyte into the cell, and applying a potential difference between the cell and the can body, characterized in that the electrolyte is directed within the cell selectively between the interior surface of the can body and the mandrel for treating the interior surface of the can body, between the exterior surface of the can body and the side wall of the cell for treating the exterior surface of the can body, or both between the interior surface of the can body and the mandrel and between the exterior surface of the can body and the side wall of the cell for treating both the interior and the exterior surfaces of the can body.
The method may including anionic or cationic treatment or treatments of the surface of the can.
In a preferred method of treating a can drawn from tinplate the surface of the can is electrocleaned and repassivated. A coating of an electrocoating material is preferablly applied to the repassivated surface.
In a preferred method of treating a can body drawn from tinplate and wall ironed to have a worked side wall, said method includes the steps of electrocleaning and electrodetinning.
The detinned surface is preferably electroplated with a coating containing chromium and thereafter electrocoated with an organic coating material.
In a second aspect the invention provides apparatus for treating a can body formed from sheet metal, said apparatus comprising a cell which has a side wall and a mandrel arranged to define a cavity within the cell and to receive therebetween in close spaced relationship within the cavity a can body, passage means for introducing an electrolyte into and conducting the electrolyte from the cell, the passage means including first passage means in communication with a first portion of the cavity for the passage of electrolyte flowing, in use, between the exterior surface of the can body and the side wall and second passage means in communication with a second portion of the cavity for the passage of electrolyte flowing, in use, between the interior surface of the can body and the mandrel, and means for applying a potential difference between the cell and the can body, characterised by means which are selectively adjustable between a first position sealing the first passage means for the second passage means and a second position permitting communication between the first passage means and the second passage means whereby the electrolyte may be selectively directed within the cell, in use, between the interior surface of the can body and the mandrel for treating the interior surface of the can body, between the exterior surface of the can body and the side wall for treating the exterior surface of the can body, or both between the interior surface of the can body and the mandrel and between the exterior surface of the can body and the side wall for treating both the interior and the exterior surfaces of the can body.
Various embodiments of the invention will now be described by way of example and with reference to the accompanying examples and drawings in which:-

Fig. 1 is an open ended can body;
Fig. 2 is a sectioned side elevation of apparatus for treating both the inside and the outside of the can body of Fig. 1;
Fig. 3 is a diagrammatic sectioned side elevation of apparatus having a plurality of cells;
Fig. 4 is a plan view of the apparatus of Fig. 3;
Fig. 5 is a diagram of apparatus of Fig. 3 and including a transfer turret to permit a sequence of galvanic or other treatments.

The can body 1 of Fig. 1 has a concave end wall 2 and a side wall 3 extending from the periphery of the end wall to terminate in a flange 4, which defines the open end of the can body.
The can body 1 has been drawn from a single piece of sheet metal and thereafter been wall ironed so that the side wall 3 is thinner than the end wall 2. The side wall is therefore in need of repair treatment to restore the surface while the surfaces of the concave end wall are substantially unaltered from the sheet condition.
In Fig. 2, the can body 1 has been placed in apparatus which has passageways as arrowed to conduct treating fluid first across the exterior surface of the can body and then across the interior surface of the can body.
The apparatus of Fig. 2 comprises a cell 21 and a cover 22. The cell has a bottom wall 23 and a side wall 24. A mandrel 25 extends centrally upwards from the bottom wall 23 within the side wall 24 to define therewith, an annular cavity and to further define, with the cover 22, a space above the mandrel 25. An insulating locator ring 26 having triangular ribs 27 surrounds the mandrel 25 and rests on the bottom wall 23. The can body 1 is located centrally by the ribs 27 to be substantially equidistant from the side wall 24 and the mandrel 25.
A contact post 28 extends through an insulating grommet 29 to press on the end wall 2 the can body and so make good electrical contact.
In Fig. 2, the treating fluid enters the cell through the bottom wall 23 via the passage 30 and passes as arrowed, between the exterior surface of the can body 1 and the interior surface of the cell side wall 24, to leave through the passage 31 which leads the treating fluid back towards the bottom wall 23 for entry into a central passage 32 in the mandrel 25. Upon emerging from the passage 32 the fluid impinges on the interior surface of the end wall 2 and passes on between the interior surface of the side wall of the can and the mandrel 25 to leave through the bottom wall 23 via a passage 23.
The cell is given a negative polarity, so that a direct current may be passed through a fluid in the cell. Therefore, when an electrocoating material is introduced as arrowed into the cell and a current is passed through the electrocoating material, the can body becomes coated with a surface coating. After coating the can body, the spent electrocoating material is drained from the cell and the coated can body is removed from the cell.
It will be understood that the entry and exit from the cell through the bottom wall 23 permit convenient arrangements of feed pipes. Also, should it be desired to modify the cell of Fig. 2 to treat only an interior or only an exterior surface of the can, this may be achieved by movement of a plug 34 to block the passage 35 and removal of two base plugs 36 and 36A.
Then, a first electrolyte can be used to treat the interior only of the can body, and a second electrolyte can be used to treat the exterior only of the body. These treatments may be carried out simultaneously or sequentially.
Removal of the plug 36A also permits the use of a valved supply of compressed air to eject the can body from the cell 21.
It will be understood, that to fill the cavities completely and avoid residual remnants, the feed and exit conduits are preferably annular and arranged to distribute the treating fluid around the surface of the can.
It will be understood that the polarity of the can and cell in Fig. 2 may be reversed but that it is preferable to have the cell at an earthed polarity for safety.
The apparatus of Fig. 2 is particularly suitable for incorporation in a rotary machine having a plurality of such cells, such as is shown in Figs. 3 and 4. Each cell is fed with fluids from supply means in the base 38 of the machine 37.
In Fig. 3 the machine 37 can be seen to have a base plate 38 from which extends a centre post 39 which supports a drum cam 40. A turret 41 rotates on the base plate 38, about the centre post 39. The turret comprises a top plate 42, a spacer 43 and a cell block 44. The equispaced arrangement of the cells 45 around the turret 41 is best seen in Fig. 4.
Each cell 45 is closed by a cover 46 supported on a rod 47 which is urged to reciprocate by a follower 48, at the upper end, which engages with the drum cam 40, the rod being insulated from the cover, turret and cam.
An entry star wheel 49 acting with an entry guide 50 brings each can body 1 in sequence to be fitted in a cell and after treatment an exit star wheel 51, acting with an exit guide 52 removes each can from the apparatus.
A positive polarity is imparted to the cans 1 in the cells and a negative polarity is put upon the cell block 44.
As shown in Figs. 3 and 4 each cell is being used to perform a like treatment on each can body. The can body 1 is therefore placed in the cell 45A (beneath it as shown in Fig. 3) and as the turret 41 rotates about the central post 39 the co-operation of the follower 48A with the cam 40 causes the rod 47 to close the cover 46A into the cell 45A. The cell is then filled in a manner described with reference to Fig. 2. As the turret continues to rotate the rod 47A contacts the contact bar 53 which puts a positive potential onto the rod 47. The rod 47 conducts this polarity to the can body 1 in the cell so that for approximately 180° of rotation the galvanic treatment may be continued. If a cell, such as that shown in Fig. 2 is in use both the internal and external surfaces of the can body will be treated.
Various arrangements of the treatment cells are possible. A calculation of process parameters, based upon the foregoing design and involving a passivation surface treatment, similar to the known "311 treatment" given to tinplate, suggests that the process of small cans (202 x 212) is quite feasible.
For example a 40 head unit with a throughput of 1,000 can/minute could be constructed to fulfill the following requirements:-

Time of component on unit 2.4 secs.
Time of surface treatment (235° of cycle) 1.56 secs.
Type of surface treatment "311 323 coulombs/m² (30 coulombs/sq ft)

However, Fig. 5 shows diagrammatically how a machine 60 having eight cells may be fitted with two sets of four different treatment cells 61, 62, 63, 64. A transfer turret 72 is used to recycle each can from exit star wheel 71 back into the entry star wheel 73 for delivery to the next cell until the four stage treatment is completed. A finished coated can may be detected by an electrical conductivity test to prevent continuous cycling through the apparatus.
Examples of treatments which may be carried out, according to the method of the invention, are tabulated below together with the duration of time in seconds expressed as an order of magnitude: for example 3 seconds in the table means a range of from 1 to 9 seconds.
It is possible to fill or empty each cell, of the treatment fluid (such as would be used for a can 76.2 mm (3") diameter by 76.2 mm (3") deep) in about t second. Therefore the turret of Figs. 3 and 4 would rotate at about 10 revolutions per minute when working on a single repair treatment.
The dissolution treatments such as tin stripping or cleaning may be accelerated if desired by applying ultrasonic vibration to the can body. The transducer for such vibration may be situated in the mandrel or in the wall of each cell.
The apparatus of Figs. 2 to 5 may also be used to carry out a sequence of repair and metal finishing treatments as already described and examples of such sequences follow:-

Example 1

In the production of plain or lacquered drawn and redrawn cans from tinplate, the following steps are used:-ELECTROCLEANING -+ SURFACE TREATMENT〉̶ ELECTROCOAT
Electrocleaning as a separate first stage improves the efficiency and uniformity of treatment of the subsequent stages. The surface treatment chosen could be either the equivalent of the known "311" electrochemical process of passivation or the known "300" chemical immersion process of passivation. The choice depending upon the specific application and need for electrocoating.

Example 2

In the production of a drawn and wall ironed beverage container, the following steps may be used:-
In which case, after electrocleaning the tin is stripped from the can by anodic dissolution. Thereafter the additional surface treatment step 1 would be used to replace the tin coating by a film approximately equivalent to that on known steels bearing chromium/chromium oxide layers. The second surface treatment (2) could, if desired, be a passivation treatment. The final stage would be electrocoating with an organic lacquer or the like.
The coating composition used for electrocoating may be a water-dispersed coating composition, such as a partially neutralized acrylic interpolymer and an amine-aldehyde condensation product of polyepoxide or both. Examples of such interpolymers are found listed in patent issued to Donald P. Hart, U.S. Patent No. 3,403,088, and assigned to P.P.G. Industries, Inc.
It will be understood that these protective coatings have high di-electric strength, coat metallic articles completely, have efficient electro-depositing qualities, and result in cured films which are clear, glossy and have attractive appearance and good durability.
The apparatus of the invention may alternatively be used as a means to examine the quality of the electrochemical treatment or as a separate apparatus for testing coatings applied by conventional means.
Whilst the invention has been described in terms of cans formed from tin plate and aluminium it is not limited thereto, for example the cans may be built up from components made of blackplate or other mild steel sheets. Furthermore the cans may be drawn from an uncoated steel so that the method and apparatus described provide the all or part of the can finishing.

Claims

1. A method of treating a can body formed from sheet metal, said method including the steps of placing the can body on a mandrel in a cell such that the interior and exterior surfaces of the can body are in close spaced relationship respectively with the mandrel and with a side wall of the cell, introducing an electrolyte into the cell, and applying a potential difference between the cell and the can body, characterized in that the electrolyte is directed within the cell (21) selectively between the interior surface of the can body (1) and the mandrel (25) for treating the interior surface of the can body (1), between the exterior surface of the can body (1) and the side wall (24) of the cell (21) for treating the exterior surface of the can body (1), or both between the interior surface of the can body (1) and the mandrel (25) and between the exterior surface of the can body (1) and the side wall (24) of the cell (21) for treating both the interior and the exterior surfaces of the can body (1 ).

2. A method according to claim 1, characterized in that the electrolyte is drained from the cells (21) and thereafter a second electrolyte is introduced into the cell (21) and a second potential difference is applied between the cell (21) and the can body (1 ).

3. A method according to claim 2 characterized in that one of the electrolytes is passed between the interior surface of the can body (1) and the mandrel (25) and the other of the electrolytes is passed between the exterior surface of the can body (1) and the side wall (24).

4. A method according to claim 1, characterized in that after treatment of the can body the electrolyte is drained from the cell (21), and the can body (1) is expelled from the cell (21).

5. A method according to claim 1, characterized in that a supply of compressed air is provided to the cell (21) for expelling the can body (1).

6. A method according to any preceding claim characterized in that the treatment comprises anionic treatment of the can body (1).

7. A method according to any of claims 1 to 5 characterized in that the treatment comprises cationic treatment of the can body (1).

8. A method, according to any of claims 1 to 7, of treating a can body drawn from tinplate, characterized in that the treatment includes the steps of electrocleaning and repassivation of the can body (1).

9. A method according to claim 8 characterized in that the treatment includes the further step of electrocoating the repassivated can body (1 ).

10. A method, according to any of claims 1 to 7 of treating a can body drawn from tinplate and wall ironed to have a worked side wall, characterized in that the treatment includes the steps of electrocleaning and electrodetinning the can body (1).

11. A method according to claim 10 characterized in that the treatment includes the further step of electroplating the detinned can body (1) with a coating containing chromium.

12. A method according to claim 11 characterized in that the treatment includes the further step of electrocoating the chromium containing coating with an organic lacquer.

13. Apparatus for treating a can body formed from sheet metal, said apparatus comprising a cell which has a side wall and a mandrel arranged to define a cavity within the cell and to receive therebetween in close spaced relationship within the cavity a can body, passage means for introducing an electrolyte into and conducting the electrolyte from the cell, the passage means including first passage means in communication with a first portion of the cavity for the passage of electrolyte flowing, in use, between the exterior surface of the can body and the side wall and second passage means in communication with a second portion of the cavity for the passage of electrolyte flowing, in use, between the interior surface of the can body and the mandrel, and means for applying a potential difference between the cell and the can body, characterized by means (34, 35) which are selectively adjustable between a first position sealing the first passage means (30, 31) from the second passage means (32, 33) and a second position permitting communication between the first passage means (30, 31) and the second passage means (32, 33) whereby the electrolyte may be selectively directed within the cell (21), in use, between the interior surface of the can body (1) and the mandrel (25) for treating the interior surface of the can body (1), between the exterior surface of the can body (1) and the side wall (24) for treating the exterior surface of the can body (1), or both between the interior surface of the can body (1) and the mandrel (25) and between the exterior surface of the can body (1) and the side wall (24) for treating both the interior and the exterior surfaces of the can body (1 ).

14. Apparatus according to claim 13, characterized in that adjustable closure means (34, 36, 36A) are provided for blocking the passage means (30, 31, 32, 33, 35) for selective direction of the electrolyte.

15. Apparatus according to claim 14, characterized in that the closure means (34, 36, 36A) comprise removable plugs.

16. Apparatus according to any of claims 13 to 15, characterized in that the first passage means (30, 31) comprise inlet and outlet passages (30, 31) in the side wall (24) of the cell (21) for directing the electrolyte, in use, between the exterior surface of the can body (1) and the side wall (24), the second passage means (32, 33) comprise inlet and outlet passages (32, 33) in the mandrel (25) for directing the electrolyte, in use, between the interior surface of the can body (1) and the mandrel (25), and there is provided a connection passage (35) joining the outlet passage (31) in the side wall (24) and the inlet passage (32) in the mandrel (25), which connection passage (35) is closable for separating the outlet passage (31) in the side wall (24) from the inlet passage (32) in the mandrel (25).

17. Apparatus according to claim 16, characterized in that the inlet and outlet passages (30, 31 ) in the side wall (24) and the inlet and outlet passages (32, 33) in the mandrel (25) all communicate with the exterior of the cell (21 ) through a bottom wall (23) thereof.

18. Apparatus according to any of claims 13 to 17 characterized in that an electrode (28) of the cell (21) is arranged to press the can body (1) into engagement with a spacer ring (26) for locating the can body (1) in close spaced proximity to the mandrel (25) and the side wall (24).

19. Apparatus according to claim 16 or 17 characterized in that means are provided for directing compressed air into the cell for expelling the can body (1) from the cell (21) after treatment.

20. Apparatus according to any of claims 13 to 19 characterized by means to pass a first electrolyte between the exterior surface of the can body (1) and the side wall (24) and a second electrolyte between the interior surface of the can body (1) and the mandrel (25), so that the interior and exterior surfaces of the can body (1) may be treated simultaneously or sequentially.

21. Apparatus according to any of claims 13 to 20 characterized in that the cell (21) is mounted with a plurality of similar cells on a turntable (41).