Although in this Web there are already extensive works dedicated to the important topic of the seaming of metallic cans, the one published here complements them. In it, in a very succinct way, it is explained how to assess the measurable parameters of a seam.
When talking about measuring seams of round cans, the dimensions should be taken in 2 points, taken at 60º on both sides of the weld of a 3-piece container or at opposite points in 2-piece cans.
In the case of cans of rectangular or irregular shape, measurements should be taken at each corner and in the center of each longer side.
The frequency of the measurement should be at regular intervals during production. Two cans of each seam head must be taken, which must be completely inspected to determine that the integrity of the double seam is acceptable, and that the minimum levels of critical parameters are being met.
The seams should be inspected:
– After any breakdown in the seamer.
– When the production begins.
– After making adjustments in the settings of the seamer.
There are 2 procedures for the evaluation of a complete seam. These are:
-1.- The method of opening .: Complete dismantling of the seam.
-2.- The method of sectioning the seam: Cutting the seam
If a section seam method is used, it should be noted that only the evaluation of the sectioned proportion is ensured. It is therefore necessary to “dismantle” the seam – pull down the entire seam – and examine it to ensure complete acceptability.
The basic dimensions of the seam that must be measured are:
- Depth of tub Pc
- Seam height Ac
– Thickness of the seam Ec
– Length hook body Lgc
– Length hook bottom Lgf
– Tin plate thickness Ehc
– Tin plate thickness Ehf
See figure # 1
Figure nº 1.- Basic dimensions of the seam
Critical parameters of the seam.
The measurements alone of the length of the body hook or lid, although they are important, do not give us conclusive data to decide the suitability of a seam. The above dimensions serve as a basis for determining the critical parameters, which define whether a seam is correct or not. The following parameters must be calculated for evaluation:
– Pressure of 2nd operation P2ªo
– Real overlap Tr
– Penetration hook body Pgc.
In addition, visual defects will be observed
As a complement, the following aspects of the seaming, must be evaluated so that it gives us added information:
– Markings on the internal wall of the seam
– Free space
Second operation pressure. (P2ªo)
The seam must be tight enough to ensure that the compound fits perfectly under the effects of pressure. Due to the nature of the process, the formation of the wrinkles of the seaming on the bottom during the 1st seaming operation is inevitable. These wrinkles are “ironed” during the 2nd operation of the seaming process.
Therefore, the seam pressure can be expressed as: the length of the bottom hook not visually wrinkled. It is evaluated, expressed as a percentage of the total length of the bottom hook. Since wrinkles can not be measured, it is a very subjective judgment, and therefore, deep experience is required to be able to judge it correctly. See figure 2
Figure nº 2.- Evolution of wrinkles
As the thickness of the bottom material is inevitably being reduced by cost reduction imperatives, the ability to achieve the absence of wrinkles becomes much more difficult and, therefore, it is much more complex to make the evaluation correctly.
The body and bottom hooks must overlap sufficiently to ensure that the compound is perfectly adhered within what is the double seam. This overlap is called overlap, corresponds to the letter “a” in Figure 3
The overlap of the body and bottom should be as great as possible, within what would be acceptable in relation to the absence of wrinkles.
The overlap varies according to the seam specification but in each case a minimum standard is set to be achieved. Its value is determined by applying the formula:
Tr = (Lgf + Lgc + 1.1Ehf) – Ac
Penetration body hook
Compare the net height of the body hook in relation to the net height of the seam, that is, discounting the thickness in both cases. To which it is the same, relates the measure “b” with the “c” of figure No. 3. This is intended to know how penetrates the body hook in the seam, with the idea of knowing the degree of collapse of the edge of the hook of the body in the compound of the cover, to ensure its tightness.
To ensure that the body hook is driven into the composite, degrees of acceptability have been set, which express the inside length of the body hook as a percentage of the inside length of the seam. Its value is determined by applying the formula:
Pgc = (Lgc – 1.1Ehc) x 100 / Ac – 1.1(2Ehf + Ehc)
A minimum acceptable standard is 70%.
No local distortion should appear on the seam. It could be mentioned as examples of visual defects: excess of inclination, cut seam, break of the flange, false seaming, skating, etc. These failures jump quickly into view and have the potential to damage the integrity of the seam.
Markings on the internal wall of the seam
The vertical wall of the cuvette should appear free of marks. The presence of them, implies defects in the seam. They can be due to several reasons such as: Lip of the broken mandrel, wear of the same, excess pressure in the second operation, etc.
This value gives us an idea of how tight the seam is, since it reflects the difference between the actual thickness of the seam and the sum of the metal thicknesses inside the seam. It is determined by the formula:
Free space = Ec – (3Ehf + 2 Ehc).
The maximum limit of free space in seams of round and irregular cans is 0.19 mm, (0.0095 “) If it exceeds this amount it is a sign that the seam is loose and therefore with risk of leakage.