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— Clicking here you can access the 2nd part of this work


3rd delivery SEAM THEORY

6º.- ACCEPTABILITY OF THE SEAM (Critical parameters)

The integrity of the double seam is of crucial importance, and the minimum levels of acceptability are essentially the same for both irregular and cylindrical cans.

Inciding on what has been said before, there are two aspects of the construction of the seam, which produce a hermetic seam. The primary sealing area is that caused by the edge of the hook of the body embedded within the compound, housed inside the end hook. The secondary seal, is the area of ​​metal overlap between the body and end hooks inside the joint seam.

The critical parameters of acceptability of the seam are:

1º.-Tightening of the seaming (Pressure of 2nd operation):

The effect that the 2nd operation pressure produces on the seam is also designated as seaming tightness. The seam must have a tight fit, which ensures that the rubber, initially contained in the bottom wing, and during the same fills the empty spaces of the seam, is compressed between the hooks, leaving the body hook embedded in it.

Due to the nature of the seaming operation, the formation of wrinkles on the inside of the end hook during the first operation is inevitable, disappearing almost completely in the second operation. Those that remain can be observed with the naked eye and give an indication about the degree of tightening of the seam. The presence of pronounced wrinkles can cause slight leaks, although if they are light they do not affect practically the tightness of the seam. Therefore, an approximate judgment can be made about tightening the seam by observing residual wrinkles that result or may result in the second operation. See figure nº 42.


Figure nº 42: Evaluation of wrinkles in % of the height of the end hook


The length of the end hook that is free of wrinkles is an indicator of the tightness of the seam. This degree of tightness (or tightness of the seam) is indicated as the length of the wrinkle-free end hook, expressed as a percentage of the total length of said hook. This indicator can not be measured, it must be estimated visually, and as it is a subjective assessment, a certain experience is required to evaluate it correctly. When wrinkles do not appear on the hook, the tightening is 100% and when the wrinkle occupies the entire height of the hook it is 0%. This degree of tightening is a critical parameter of seam quality. Its minimum value must be 75%, always referring to the worst point on the end hook, and special attention must be paid to both sides of the side welding joint area. For non-round containers, A minimum value of 60% of the degree of tightening is acceptable as acceptable. Wrinkles should be soft in the form of ripples.

The proportion of wrinkles will be lower the larger the diameter of the container. This is manifested in a palpable way in the containers of rectangular shape, where no wrinkles appear on the straight sides, but their presence is very accentuated in the corners of the corners that are of small radius. Within a same format, the greater or lesser intensity of wrinkles depends on the pressure of the second operation seaming roll. The small wrinkles are filled by the rubber gasket, this being one of its main applications.

There is a clear tendency to decrease the thickness of the bottom due to unavoidable cost reduction imperatives, the ability to achieve the absence of wrinkles becomes much more difficult, and therefore complicates the task of correctly determining the tight seam evaluation. On this aspect we will return later.

There are other types of undulations or deformations on the end hook, which should not be confused with those that are related to the degree of compactness. Some of them are indicated in figures no. 43 and 44.

Thus, in drawing no. 43, the letters A, B, C and D show the typical corrugations of the seam to an acceptable degree. The protrusion E is motivated by an excessive accumulation of rubber at that point, which is not desirable. The point F has a small ridge on the cutting edge of the end hook, caused by excessive pressure of the second operation, which can lead to a lamination of the seam, a dangerous defect.

Figure nº 43: Different types of wrinkles on the end hook


In addition to these, there are other types of wrinkles that are undesirable and should be avoided, as they are a symptom of an anomaly, such as those indicated in drawing # 44.


Figure nº 44: Undesirable wrinkles and wrinkles


Large isolated crinkle: Shows that the material has not been collected uniformly.

Wrinkle in “V”: Presents an inversion in wave. The normal wrinkles are slightly concave, while the inverted ones in the form of “V”, are convex and therefore in relief on the surface of the end hook.

Folding: It supposes a step in the face of the hook of bottom, with risk of micro leaks.


Free space:

In addition to controlling the tightness of the seam by means of the evaluation of the undulations or wrinkles of the hook of the bottom, there is another way of doing it, consisting of measuring the thickness of the seam, and comparing it with the sum of the five thicknesses of metal that the seam encompasses . It will give us an idea of ​​the free space, which should be filled with rubber under normal conditions, but that can be empty if it is very large.

Free space = E – (2 Gc + 3 Gf)

E being the actual thickness of the seam, Gf the thickness of the bottom metal and Gc the metal thickness of the body.

The “compactness” is an index, which is also used to express the degree of contact of the layers of tin that form the seam. Therefore, it is intimately related to free space. It is expressed by:

Compactness = C = (2 Gc + 3 Gf) x 100

Another way to express free space would be:

Free space = (100 – C) x E

A tight seam will have a low headroom and a high compactness. In view of this, the seam can be classified as:

–      Very good……………………… C> 85%
–      Good ……………………………. 75% < C < 85%
–      Dangerous……………………… C <75%

This measure of compactness is valid only for round containers, and not of the type used in carbonated drinks or beer, where the internal pressure is high and requires higher compactness values. In the case of rectangular and oval shaped containers, which have a somewhat higher composite weight, up to a minimum compactness value of 60% can be accepted.

Another valid criterion that can be used is that the limit of the value of this free space in seams of round and irregular containers is 0.19 mm (0.0075 “).

Therefore another way of expressing this concept is:

Actual seaming thickness < (2 Gc + 3 Gf) + 0.19


2nd.- Penetration of the body hook

The length of the body hook in relation to the internal length of the seam must be sufficient to ensure that it is well sunk in the seam rubber. This guarantees that the first seal is well insured. This value is indicated by the percentage of penetration of the body hook, and measures the relationship between the internal length of the hook and the internal length of the seam, expressed as a percentage.   (See figure #45)

Figure nº 45: Critical parameters of the seaming


There are two ways to determine it:

1ª.- Starting from a “scrapped” seaming and measuring the hooks by means of a hook or micrometer.

In this case, as we can not measure the internal lengths of the body hook and the seam, we will have to apply the following formula:

% Penetration hook body = (LGc – 1.1 Gc) x 100

Lc – 1.1 (2Gf + Gc)


–          LGc = Body hook length
–          LGf = End hook length
–          Gc = Metal body thickness
–          Gf = Thick metal background
–          Lc = Length of the seaming

Experience has shown that to ensure a hermetic seal, at least 70% penetration is required for canning containers and 80% for beer and carbonated beverage containers.

2nd.- Starting from a sectioned seam. In this case the penetration of the body hook can be measured directly in the seam projector, determining by means of the mobile rules the measures c and b, as shown in figure 46.



The penetration of the body hook would be determined by the formula

% Penetration of the body hook = a = b x 100

It can also be measured directly on the projector screen using an abacus, as shown in figure 47. To do this, open the mobile rules as much as possible, and place the abacus so that it is visible on the screen, taking it in that the reference lines of the same appear parallel to the body hook. Adjust the position of the abacus so that the zero is located inside the radius of the body hook. The penetration reading in percentage will be the value that marks the line that coincides with the end of the body hook.


3º.- Overlap (or overlap)

The body and end hooks must overlap sufficiently to ensure that the seam compound is under compression between them with a correct seaming thickness. See figure 45

The overlap of the hooks should be as large as possible, within what is assumable in relation to the absence of wrinkles. The overlap varies according to the specification of the type of seam used, but in each case a minimum standard is set to be achieved.

It can be measured directly in the seaming projector or by the formula

Overlap = LGc + LGf + 1.1 Gf – Lc


If the formula is applied, the calculated overlap will be less accurate than that obtained by direct measurement of a cut on the projector, but can be considered accurate enough to evaluate the seam.

The minimum acceptable overlap dimension for a conventional seam, whether by the section of cut or by calculation, can be evaluated in 1.1 mm both for two pieces and three in a conventional seam, although this value is a function of the type of seam used .


4th.- Absence of visual defects

Finally a good seam must be free of visible defects, that is, it is not desirable that there is any local distortion in it. It is difficult to make a complete list of possible defects, but among others the following can be mentioned:

–          Excess of tilt
–          Seam cut.
–          Break the tab.
–          False seam.
–          Skating.
–          Marks on the wall of the bucket.
–          Damage to the coating.
–          Varnish damaged by mechanical coding in the background panel
–          Etc.

These defects quickly jump into view and have the potential to impair the integrity of the double seal. On some of these defects we have already dealt with before

In summary, the critical parameters of acceptability of the seam require preferential attention. A typical example of dimensions of these parameters for bodies and bottom of tinplate are: 1.1 mm of effective overlap, a minimum of 70% of penetration of body hook and a minimum of 75% of degree of compactness. However, with aluminum as a material, the degree of compactness would be at least 90%.

An external visual examination of the seam should be carried out regularly, analyzing cans from each seaming station, once every 15 minutes. To evaluate the cans in terms of the total quality of the seam, the seam should be opened by complete “scrapping” of the same in each seaming station and register its dimensions. The frequency of the evaluation in theory would be hourly, however, the amount of personnel of the plant can limit this frequency of check.

The statistical control of the process, provides a stable degree of confidence of the maintenance under control of the double seaming, based on the analysis of the resulting tendencies. Assuming that the manufacturer of the components meets the specifications of the body and bottom, it is unlikely that the acceptability of the seam will decline rapidly. It is more common to observe the trend over a period of days, since aspects such as tooling or adjustment of the seamer have a great effect on the quality of the seam. Typically, trend analyzes denote failures in the dimensions of the seam progressively, before the critical parameters of acceptability are exceeded.



Previously we have given some indicative data of the critical values ​​in general terms, now they are not valid for all the containers. Depending on their form these values ​​can be more or less demanding. We have already discussed the case of non-round containers.

But what most influences the amounts of these values ​​is the type of seam chosen. This type depends on the dimensions of the container and especially the thickness and hardness of the metal used in the bottom and body. Later, when we discuss the evolution of the seams, we will study in more detail how the characteristics of the metal influence the base parameters (amount of metal to be seamd and flange). Depending on these dimensions, the critical values ​​of the seams are defined.

Base parameters:

a.- It is defined as metal to seam the area of ​​the bottom that is incorporated to the seam of the container, that is to say its wing. Its approximate value is determined by the formula indicated at the bottom of the following summary table.

b.- The other basic parameter, or starting parameter for the seaming, is the tab length, which we have already defined in the seaming terminology.

SEFEL classifies the seams in six different types or sizes, the first three fall within what could be called “mini seams” and the three remaining for conventional seams. There is a seventh type for large seams that we do not include in the summary table.

In the box are reflected:

1º.- The SEFEL types of seams
2º.- Its starting or base parameters
3º.- The recommended magnitudes of the fundamental parameters of the seam itself
4º.- The values ​​of the critical parameters of the seams
5º.- The empirical formulas for the approximate determination of:

o      Overlap the seam

o      Metal to seam from the bottom



It is inevitable that small variations occur in the seam. These variations are mainly due to the following factors:

a.- The thickness of the metal used depends on the type and size of the container. In addition to the normal manufacturing tolerances of tinplate or TFS, there may be a variation in the ductility and tempering of the metal.

b.- The type of seamer used, since each machine has different characteristics, such as number of revolutions of the chuck, diameter of seaming roll, speed of seaming, etc.

c.- Surface characteristics of the tinplate or TFS. The presence of different types of varnish or lithography affects the behavior of the metal during the formation of the seam.

d.- Temperature at which the seam is made.

In order to minimize the effects that these factors have on the dimensions of the seam, it is imperative a strict discipline in the adjustment and regulation of the seaming machine, as well as in its management.



We will start from the following hypotheses:

1º.- We assume that both the funds and the bodies – or containers – supplied to the seamer are correct.
2º.- The tools installed in the machine are adequate and in perfect condition.

Therefore, the defects that can appear during the seaming operation are only attributable to a bad adjustment of the equipment.

If the tools and component to seam present problems, the range of possible causes would be opened strongly. Therefore, when a defect occurs, the first thing that we have to verify is that the funds, packaging and tools are good, and thus discard these factors and focus exclusively on the work of the adjustment.



After the first seaming operation, the ideal appearance of the section of the same is analogous to the one shown in Figure No. 48.

A.- If there has been an insufficient formation of body and end hooks, that is to say if the seaming roll is too loose, as shown in figure 49, this can cause the following defects:

1.- Short end hook.
2.- Excessive seaming length.
3.- Folds formation in the end hook.
4.- Large body hook
5.- Small overlap
6.- Peaks at the base of the seam

Defects 2 and 6 are easily seen without the need to disassemble the seam and have already been treated beforehand. Defects 1, 3, 4 and 5 it is necessary to open the seam for analysis.


B.- If excessive pressure is applied in the first operation, as indicated in figure 50, this could cause the following seaming defects:

1.- Short body hook.
2.- Excessively long end hook.
3.- Insufficient length of the seam.
4.- Insufficient tightening of the lower part of the seam

Except for defect 3, it is necessary to disassemble the seam for detection.



The analysis of the results obtained after applying the 2nd operation in a seam, can not be simplified as much as in the case of the 1st operation (loose or tight seaming roll), since there can be a series of adverse effects, whose origins can be several Therefore we will do the analysis backwards, we will start from the result to define the possible causes.

A.- Short body hook

If the seaming section has a body hook that is too short, as shown in figure 51,

The causes of this defect can be in:

1.- Insufficient pressure in the compression plate.
2.- Seaming rolls of first operation too tight.
3.- Seaming rolls of second operation little tight.
4.- Incorrect adjustment height of seamd, ie chuck too high in relation to the compression plate.

B.- Long body hook

If, on the other hand, the section of the seam has a too long body hook, as shown in figure 52, the causes in this case may be the following:

Figure nº 52: Long body hook

1.- Too much pressure in the compression plate.
2.- Incorrect adjustment height of seamd.
3.- Seaming rolls of 1st operation little tight.
4.- Seaming rolls of 2nd operation too tight.


C.- Short end hook

The seaming section in this case is presented as in figure 53. Its probable causes are:

1.- Seaming rolls of first operation very little tight.
2.- Too much pressure in the compression plate
3.- Compression plate too high.
4.- Seaming roll of first operation high respect to the mandril
5.- Seaming roll of second operation high respect to the mandril

Figure nº 53: Short end hook


D.- Long end hook

See figure nº 54. Possible causes:

1.- Seaming rolls of first operation too tight

Figure nº 54: Long end hook


E.- Excessive length of the seam

See figure nº 55. Probable causes:

1.- Seaming rolls of 1st operation very little tight.
2.- Seaming rolls of 2nd operation too tight.
3.- Seaming roll of first operation low respect to the mandril
4.- Seaming roll of second operation low with respect to the mandril

            Figure nº 55: Excessive seaming length


F.- Insufficient seaming length

See figure nº 56. Probable causes:

1.- Seaming rolls of 1st operation too tight
2.- Seaming rolls of 2nd operation very little tight
3.- Seaming roll of first operation high respect to the mandril


Figure nº 56: Insufficient seaming length



Defects in seam do not all have the same importance. Following the classic division of “critics”, “majors” and “minors”, there are those that should always be framed in only one of the categories, because they persistently have the same type of incidence, but there are also those that depend on the magnitude When they are presented, they can be included in two or even in the three categories.

Taking into account the different types of defects we have seen so far, in general terms the following classification can be made:



The quality of the seam is a true reflection of the capacity of the seaming machine to function properly.

It is easy to understand, particularly with irregular cans, that if, for example, the seaming rollers do not follow precisely the seaming chuck, the degree of adjustment of the seaming roll will not reach perfect results. Similarly, if the chuck of the bottom is not correct, or the profiles of the seaming seaming rolls do not have the optimal form to control and form the same, a good result in the dimensions will not be obtained.

A good design in the seamer, allows the machine to produce a uniform seaming quality, with minimal stops aimed at repairing mechanical faults or making adjustments.

It is essential that the first operation is checked frequently, registering the height and width of the same. This provides the necessary information to determine when to change the seaming roll. In recent decades significant improvements have been made in the seam tools, introducing materials resistant to corrosion with surfaces that, due to their hardness, have a longer service life without the need for replacement. These improvements, to a certain extent, are linked to the introduction of materials for the thinnest and hardest or even new containers such as the TFS.

Certainly, the change of tinplate by tin-free steel (TFS), provoke a dramatic reduction in the half-life of the seaming roll that comes in contact with the chromic oxide coating of the bottom, with respect to the tinier bottoms, much softer. The change to TFS, despite everything, improves the cost-effectiveness ratio; however, the cost of the tool is a remarkable part of the economy associated with the introduction of TFS.

If the seamers are maintained in an appropriate way, and the seaming tools (chucks and seaming rolls) in good condition, the analysis of the same will only provide information that will confirm that the process is under control. To ensure that the seamers are not subject to continuous adjustments, a process called “adjustment by objectives” has been introduced universally in the canning and metal processing industry, which ensures that the seam is carried out correctly.



Each seaming machine must be adjusted and regulated according to its particular specifications, according to the type of funds / covers and bodies with which it will work, metal gauges and diameters or packaging formats. In general, the following procedure should be followed.


1º.- Checking the mechanical condition of the seaming machine

To begin a process of adjustment by objectives, it is first necessary to establish the mechanical state of the seaming machine. For its greater simplicity we will describe the adjustment of a multiple-use seamer for cylindrical cans, of the rotating container type.

The first step is to determine the alignment between the axis of the chuck plate and the assembly of the compression plate or lifting plate. To do this, one of these sets of compression plates must be removed from its support bearing. This alignment can now be measured exactly, and record its assessment, placing a probe with magnetic support, on the axis of the chuck. Going up and down the axis of the compression plate, we will see the variation of the needle. In theory, the reading should be less than 0.005 “(0.16 mm), however it is only imperative to make an adjustment if it is 0.012” (0.30 mm). Each machine has its own adjustment system, which can vary from the use of suitable keys to more complex systems. Sometimes it is a difficult task, but to achieve a good quality of seam the alignment is essential.

Once the alignment is established, the vertical and lateral clearance of the chuck shaft bearings must be measured and must be less than 0.002 “(0.05 mm). An excessive value of the vertical game, for example of 0.005 “(0.13 mm), will require an additional examination and its replacement if necessary. Next, the bearings and axes of the seaming routines should be examined, in terms of their lateral and vertical play. In no case should values ​​that exceed 0.002 “(0.05 mm) in lateral play and 0.005” (0.13 mm) in vertical be accepted.

Finally it goes to the compression plates. To ensure a good quality of the seam, both the parallelism and alignment losses with the chuck and the excessive play with the flange must be corrected if an empty container is to be seamd, as with the lid tray already in place, in case of seaming a full container.

Once established the good condition of the key points of the seamer, which determine the quality of the seam, you can start the adjustment exercise by objectives.


2º.- Verification that the chucks and routines are suitable for the type of container to be seam

The first objective is to check the seat between the chuck and the bottom. It is useless to adjust the seamer with precision, if the bottom is too loose or tight in the chuck. In the case of covers that are not of the easy opening type, a positive effort must be required to lodge the bottom in the chuck. See figure 57. The degree of interference between chuck and bottom should be 0.10 mm. This value is very difficult to measure, but in principle, with the chuck deposited on the bottom, a gap of 0.10 mm must remain between the end of the radius of the chuck lip and the bottom depth of the tray. Thus when the chuck is pushed firmly on the bottom, and comes to rest on the base of the bucket, an interference of this value (0.10 mm) will overcome. The design of the chuck lip is a critical point to take care for seaming operation.


Figure nº 57: Chuck seat on the bottom


The profiles of the routines are a function of the type of seam to be used, which in turn is linked to the dimensions of the bottom wing, the body flange and the metal thicknesses used. They are also linked to the round or irregular format of the container.


3º.- Prepare the machine for the adjustment height of seam

Assuming that the setting between chuck and bottom is correct, then the “Adjustment height of seamd” (Hc) must be adjusted. It is the distance between the upper face of the compression plate and the lower face of the seaming chuck.

In order to carry out this task properly, it will be necessary to have available the specifications of the container and funds supplier.

Adjusting the seaming height well on the seamer is important for two reasons:

–          From a suitable height of adjustment of seamd, it depends to obtain a correct dimension of the hook of body in the seaming.
–          In the filling plants, the proper position of the body flange in relation to the steam flow change parts, must be achieved and maintained

The formula to calculate this dimension is:

– Unseamd can height (M) minus the height of the chuck lip (A) plus a constant that takes a value of 0.043 “(1.09 mm)”

Hc = M  – A  +1.09

The value 0.043 “(1.09 mm) represents the elevation of the compression plate during the action cycle of the first seaming operation, which is 0.022” (0.56 mm), plus the reduction in height of the container this cycle, which is 0.021 “(0.53 mm). The “seamd adjustment height” must be measured during the first operation. See figure no. 58. The height gauge must be adjusted, when the cam follower of the first seaming operation coincides with the top or maximum point of the cam.


Figure nº 58: Adjustment of the distance between the chuck and the compression plate.


4º.- Adjust the pressure of the compression plate

The next step is to adjust the force of the compression plate, ie the pressure of the plate spring. It is a factor that has a great influence on the formation of a correct seam. During the seaming cycle, and until it has been formed, the height of the body of the container is reduced, to achieve the correct formation of the hook of the body and the bottom, for this it is necessary to exert a constant and controlled pressure on the body From the can.

The load of the compression plate is related to the thickness of the material of the body of the can, that is to say to equal format of container, the thicker the material of the body, the greater the pressure must be. It is also linked to the size of the container. For a typical body thickness of 0.18 mm it can be adjusted to a value of 200 pounds, with a deflection of 0.022 “(0.56 mm). See figure # 59.

Figure nº 59: Adjustment of the compression plate


A special dynamometer is needed to adjust the compression plate to this pre-set load. Most manufacturers of seamers offer this equipment in their catalog, and give information on the appropriate values ​​of this pressure depending on the container to be seamd.

If during the final analysis of the seam, it is necessary to increase or decrease the length of the body hook to reach the desired objectives, this must be done by increasing or decreasing the load and never altering the seamd adjustment height.

In the article stability of the compression plate pressure in a seamer this work is developed in more detail.


5º.- Check the distances of the seaming rolls in relation to the chuck’s lip

Seaming roll of 1st operation:

During the adjustment of the seaming rolls of first operation should avoid a direct contact, without control, between the profile of the same and the lip of the chuck, as this profile could be damaged by the action of the edge of the lip of said chuck . To position the seaming roll properly, this adjustment must be made when the top of the cam that controls the movement of the arm of these routines is acting, ie in its position of maximum approach to the chuck.

Once in the protrusion of the first operation cam, the distance of the seaming roll to the lip of the chuck is first adjusted, for this it is necessary to know the value of the thickness of the 1st seaming operation, which must be indicated in the specifications of the seaming. For its adjustment, wire gauges or calibrated rods are used.

Then adjust the seaming roll in the height of the seaming roll in relation to the lip of the chuck. This value is normally 0.003 “(0.07 mm) for the 1st first operation above the lip of the chuck. See figure nº 60.


Figure nº 60: Adjustment in height of seaming rollers


The regulation with precision of these heights can be achieved by using a set of thickness gauges. For non-round containers, this seaming roll should be regulated as low as possible to avoid laminations at the top of the seam.

One way to simplify the adjustment in height of the seaming rolls in relation to the chuck, is to use guiding trucks of “double guidance”. In them a channel is made that fits a heel or protrusion that has the upper part of the chuck. See figure No. 61.


Figure nº 61: Double guide ruler


In this way, the seaming roll of the 1st operation that during its work has a tendency to rise, will make butt with the inferior side of the heel of the chuck. The seaming roll of 2nd operation presents the opposite case.


Seaming roll of 2nd operation:

The same procedure as described for the 1st operation seaming roll, is repeated for the second operation. The adjustment is also made in the highest part of the 2nd operation cam.

The height in relation to the upper edge of the lip of the chuck in this case is 0.005 “(0.13 mm) See figure no. 60. In the case of non-round containers, this seaming roll must be regulated somewhat higher than in cylindrical containers.


6º.- Verify the seaming pressures of the rollers and final adjustments.

After the adjustment of the first operation, the machine can be operated with cans and bottoms, measuring the dimensions of this operation and analyzing its cross sections. If it is correct, the procedure is repeated for the second operation rollers. Otherwise, it is necessary to review the adjustment of the seaming rolll pressure again. The resulting cuvette depth after seaming must also be verified.

In this way the seam is already prepared for evaluation, either by the method of “scrapping” or by projection of its section, as we have previously commented. See figure nº 62.


Figure nº 62: Methods of seaming evaluation


7º.- Expulsores

This issue has already been addressed in the section “Seamd operation” – “Basic parts of the tooling”

– Clicking here you can access the 4th part of this work




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