This invention relates to a process for making cans made of a low carbon steel or aluminum alloy, manufactured by deep drawing, drawing and ironing or extrusion, in which the inner wall is covered with a protective coating and which are equipped with a threaded neck designed to fix any type of removable head, for example aerosol spray distribution heads, of the eco-refill type, but also for the attachment of a closing cap.
STATE OF ART
According to standard practice as described in GB 1 445 758, cans with a bottom and a cylindrical wall are manufactured starting from round and flat pieces made of low carbon steel or aluminum alloy, and using deep drawing, drawing and ironing of blanks or backward extrusion (impact extrusion) of slugs. Once formed, these cans are usually varnished on their inner surface, and painted and/or varnished on their outer surface. The open end of the can is then formed to have a cylindrical neck with a smaller diameter, and a strong plastic deformation is then applied by necking.
For practical reasons to facilitate application with a roller or a spray gun, it is better to apply the paint or varnish on the cylindrical surface before necking. The varnish on the inner surface is necessary for many types of applications, in order to prevent contact between the bare metal and the contents of the can. The metallic surface must be well protected throughout the period during which the can is being used and it is important that the inner varnish covers this surface fully and continuously. Prior art includes paints and varnish that remain undamaged during necking.
Since the metal used (low carbon steel or aluminum alloy) is an inexpensive and easy-to-recycle packaging material, it seemed a good idea to develop receptacles onto which aerosol spray distribution heads or covers could be screwed, as is the case for glass and plastic bottles. For example, the receptacle could then be sold full and a screwed cap could be fitted onto it. The user screws and unscrews the distribution head, designed for multiple use, according to his needs. The receptacle could then be refilled, or the consumer could throw it away with the rest of his waste, and it could be taken for recycling.
In order to obtain this type of metallic receptacle satisfying the “eco-refill” principle, it must be possible to make a thread, for example on the neck of the receptacle, in order to participate in the attachment of any type of head. This thread must not damage the varnish layer described above, that has been subjected to necking in order to form the neck, and which must maintain its protective properties in all cases.
In the past, the thread was usually made using internal tooling with a helical impression, acting mainly as a support and shaping mold, and an external tooling acting like one or several rollers. The European patent application EP 0 510 291 (NUSSBAUM) describes a process for making an improved thread, in which the thread(s) is(are) shaped by means of an internal tooling and an external tooling, both toolings being rotated in a coordinated manner such that sliding takes place between the neck material and each tooling. This prevents the neck metal from accumulating and folding in front of the external threading tool and being pushed in the direction of advance of the tooling.
The neck is then cut off on the same device, in other words without having removed the receptacle, leaving a bare surface, in other words unprotected and possibly with burrs, on the edge surrounding the orifice.
Problem that Arises
Even if sliding occurs between the tooling and the neck material, it is impossible to avoid further damage to the varnish. Thus, micro-crazing is observed on the varnish at the thread, both on the outer surface and the inner surface of the neck wall. This crazing makes the can more sensitive to corrosion by the packaged product.
Furthermore, with the can thus obtained, it is impossible to position the distributing head precisely with respect to the edge and the shoulder. It is difficult to control the position of the heads, which has negative consequences both on the leak tightness of the assembly and on its esthetic appearance.
Purpose of the Invention
The process according to the invention is a process for the manufacture of a metallic can comprising at least the following steps:
a) production of a can with a bottom and a cylindrical or shaped wall, for example by deep drawing, drawing and ironing, extrusion or extrusion-drawing, possibly followed by a painting or varnishing deposit on at least the inner surface of the cylindrical wall, followed by a varnish annealing treatment;
b) necking, in order to make a neck on the open end of the can, this step possibly being followed by cutting open end of the neck perpendicular to the center line of the can;
c) use of a ring with a spindle hole and threaded on its outer surface, and sleeve fitting of the said ring so that its spindle hole fits around the neck formed in the previous step, in an operation that will subsequently be called “sleeve fitting”:
and characterized in that it also includes the following step:
d) plastic expansion of the said neck, the outside diameter of the neck being expanded until it is larger than the inside diameter of the spindle hole of the ring when unstressed.
The process is characterized by the use of a ring, for example made of a plastic molded ring (but it could be made of any other material—metallic, machined or die forged, etc.), with a globally toroidal shape, with a cylindrical inner surface that we will subsequently call the spindle hole, and an outer cylindrical threaded surface. The diameter of the spindle hole is slightly greater than the outside diameter of the neck that has just been formed on the can, such that the ring can be sleeve fitted into place freely.
The thread formed on the outer surface of the ring is preferably a standard thread, for example with a
triangular or trapezoidal section, more suitable for precise positioning of the distributor head with respect to the metallic can. The thread obtained in prior art, in other words directly by rolling on the neck, was rounded and consequently imprecise. Furthermore, since rolling is no longer necessary to form the thread, there is no additional damage to the varnish on the inner surface of the neck of the can.
Finally, the choice of a plastic ring together with a distribution head fitted with an attachment skirt also made of plastic, improves the sealing conditions when the said distribution head is put into position by screwing.
A first end of the ring is brought into position facing the can neck that was obtained by necking during the previous step b) and the said neck is then inserted inside the spindle hole in the said ring. This is a relative movement; this sleeve fitting operation may also be made by moving the ring and keeping the can motionless. In this case, this step can be carried out in the same way as the previous and next steps, during the same can clamping phase, in other words these operations may be carried out on the same machine, the cans being placed on a circular rotating table for which the step by step rotation brings them in front of different tools in sequence, each adapted to one of these steps, and themselves installed on a circular tool holder plate. A device of this type has already been described, for example in FR 1 434 177 (LECHNER).
Since precise positioning of the heads to be fixed on the can is required, it is desirable to place the ring precisely on the neck and to create a stop system that gives good positioning at the end of sleeve fitting. The shape of the first end of the ring can be designed so that it is recessed and matches the shoulder of the can. It would also be possible, and preferable, to form a small shoulder on the neck, at a certain distance from the edge of the neck. This distance can be very precise when the shoulder is made at the same time as the end of the neck is cut off (optionally) in step b). The ring also has a surface that stops in contact with the shoulder formed on the neck. For example, this surface may be obtained by making a shoulder in the spindle hole. The shoulder made on the neck must be sufficiently “high” (radial height) so that it can act as a stop for the corresponding surface on the ring while it is being put into position, in other words before expansion of the neck.
The height of the neck above the spindle hole is controlled by this stop system acting when the ring is fully sleeve fitted. It is advantageous to include a subsequent step in which this part projecting beyond the ring is rolled outwards, so that it traps it and prevents any axial backwards movement. Therefore, the projection distance is chosen such that a rolled edge can be made, for example by stamping. It depends on the outside diameter of the plastic ring and the inside diameter of the neck.
Once the ring has been sleeve fitted and is in contact with the neck, the neck and the ring surrounding it are expanded, for example by stamping, the neck being expanded in the range of plastic deformations (in other words irreversible), the final diameter after elastic recovery being slightly greater than the diameter of the spindle hole of the ring at rest, in other words in the initial state free of any mechanical stress.
The neck is preferably expanded over its entire length, so as to obtain the largest possible contact surface between the neck and the ring after expansion.
By carrying out this type of deformation and ensuring that the elastic recovery effect of the spindle hole of the ring is greater than the elastic recovery effect of the neck, which is easy to ensure when the ring is made of a plastic material, a strong bond is obtained between the ring and the neck over the entire contact surface. The bond force between the ring and the neck depends on the amount of the expansion and the magnitude of their contact areas. The amount of the expansion is limited by the ductility of the neck material. The contact surface, which depends primarily on purely geometric conditions, is an easier parameter to control.
Expansion consists of applying a plastic deformation by expanding a metal that has already been strongly deformed during shaping of the cylindrical can (deformation which is particularly high close to the free edge of the can) and then strong necking. Consequently, the metal is in a very work hardened state, characterized by high mechanical properties but low residual ductility.
The applicant was surprised to observe that this residual ductility was actually greater than expected, due to the particular deformation history imposed on the metal; necking followed by circumferential expansion, the principal deformation axes remaining parallel to the axial, radial and ortho-radial directions at all times. However, although the metal is more ductile than expected, it is desirable to allow for a sufficiently large gap between the diameter of the ring hole and the initial diameter of the neck formed in step b) to facilitate sleeve fitting of the ring over the neck, but sufficiently small so that expansion will not cause necking, or even rupture of the metal.
The limiting value that must not be exceeded depends on the nature of the metal and the geometry of the can to be obtained, and can be determined experimentally using simulation tests reproducing the thermo-mechanical conditions of the various steps in the shaping process considered, on the metal considered. Preferably, expansion is carried out such that it results in an incremental plastic deformation exceeding 2% on the inner surface of the neck. The upper limit of this incremental plastic deformation varies as a function of the ductility of the alloy chosen, for which the work hardened state after necking is very favorable for good ductility in circumferential expansion.
In our example applicable to a particular geometry of a can made of a 1050 A alloy (inside diameter of the neck of the order of 15 mm), the clearance between the hole of the ring and the neck remains approximately one tenth of a millimeter, and the inside diameter of the neck after expansion is increased by about 0.3 mm, corresponding to an incremental plastic deformation of about 2% at the inner surface of the neck.
Therefore at the end of expansion, there is a strong bond over the entire contact surface between the neck and the hole of the ring. This bond can be quantified by measuring the untightening torque necessary to separate the ring from the neck, that we will subsequently call the sliding torque. In our example, it is found that the sliding torque exceeds 20 Nm, in other words that it is far greater than torques necessary to screw and unscrew the removable head. Standard NF H 35103 for glass rings can be used to estimate the order of magnitude of these tightening torques.
The process is advantageously followed by the following steps:
e) rolling, for example by stamping, the end of the neck
f) smoothing, which consists of moving a roller bearing on the edge of the neck formed by rolling in the previous step, and designed to improve the surface condition of the varnish layer.
The edge is rolled preferably outwards, since in this case the ring can be fixed in position axially, preventing any axial backwards movement. However, the principal function of the rolled edge is to improve the leak tightness of the assembly, since it forms a rounded edge covered with varnish, in other words a toroidal shape with a circular section which is much more suitable to produce a leak tight joint than the as-cut edge, bare and flat edge, on which burrs may be present, used in prior art. Furthermore, this geometry prevents contact between the packaged product and the metal edge that is not necessarily protected by varnish.
There is no doubt that the varnished layer is damaged once again during rolling, resulting in a disturbed surface condition on which there are sharp edges and micro-cracks, fairly similar to what is observed on threads obtained by rolling directly. The next smoothing operation is designed to improve this surface condition, by closing crazing and leveling of sharp edges.
These two additional operations can thus give particularly satisfactory leak tightness of the can + head assembly.
If the ring is made of plastic, it is recommended that the stress relaxation that inevitably takes place in this type of material should be taken into account, and which has the consequence of reducing the tightness a few hours after expansion. Taking account of geometric manufacturing tolerances, this loosening may be significant, in other words sufficient so that the ring can no longer resist the head screw untightening torque, which would trap it on the can without providing leak tightness. In this situation, the can would then be unsuitable for use.
Longitudinal grooves can be made in the hole of the plastic ring in order to prevent the unwanted effects of this loosening. The applicant has observed that this type of relief on the surface of the hole, which is easy to produce while the ring is being molded, improves the long term tightness between the ring and the neck, no doubt because it modifies the distribution and intensity of stresses and therefore the effect of stress relaxation.
Additional operations can also be carried out on the neck before sleeve fitting the ring; for example sanding, scratching, deformation by rolling, machining, allowing at least one pin to project during machining of the shoulder that acts as a ring penetration stop. An inverse thread can also be made on the neck and on the ring, to be sure that it is always possible to unscrew the head even if the ring is completely loose.
Thus, it is possible to make a receptacle according to the invention with a metal can fitted with the neck, designed to be fixed to any type of removable head (of the eco-refill type), the head being fitted with fastening means such as a thread, a rim or a click fit groove, characterized in that it comprises a ring fitted with additional fastening means other than those on the head, the said ring bonding to the neck of the metallic can with a sufficiently strong bond to resist the removable head separation torque.
The distribution head is usually covered by a protective cap that is fitted with a cylindrical skirt that extends close to the shoulder of the can. For the purposes of this invention, this inner end of the skirt and the lower end of the ring can be fitted with means of preventing the receptacle from being opened, for example a radial click fit locking system (vertical attachment), or notches inside a breakable locking strip (plastic rings with unbreakable notches and multi-notches).
Due to the presence of the ring used in this invention, it is possible to have eco-refill type metal cans equipped with reliable locking systems. The rolling means used in prior art resulted in a rounded and imprecise thread, but could not be used to make a sufficiently sharp relief capable of trapping a locking strip. However, with the ring according to this invention, this sharp relief is easy to form during molding, for example by increasing the outside diameter at the first end of the ring.
This end thus acts as a mating ring, the edge of which can trap a locking strip, for example connected to the inner end of a cap skirt by several breakable bridges, like the ring described in EP 0 107 680. This ring can also be fitted with ratchet teeth, and the locking means described in FR 2 665 142 can be reproduced. This latter system has the advantage that a large torque is not necessary to separate the strip.
The process according to the invention will be better understood after reading the detailed description of a particular embodiment, presented as a non-limitative example.