The seaming operation in a rectangular container has characteristics that make it different from a round one, since it presents a greater degree of complexity. The following study explains this complication and gives guidelines to solve it.
The seaming operation consists of solidly joining the bottom or lid to the body of the container. This is achieved by means of a seaming of the edge of the lid – called “wing” – with the edge of the body – called “flange” -. The basic tools of the seam are:
– Chuck : Plate that is housed in the lid bucket and serves as a firm support for the operation.
– Seaming roll of seam : Rolls endowed of a throat with a special profile for each case. There are two kinds of routines, first and second operation. The first, have the mission to curl and roll the edge or wing of the lid on the flange of the body, thus initiating the seaming. The second, solidly crush the whole. These routines according to the piases and zones of the same, they receive different names as they are: wheelbarrows, moletas, rollers, slices, etc.
Figure No. 1 schematically represents the development of a seam. In it 1) indicates the seaming chuck, 2) first operation seaming roll, 3) container body, 4) cover. 5) seaming roll of second operation and 6) zone to close: wing and flange.
Sequence a) is the starting position, b) is the action of the seaming roll of first operation and c) that of the second.
Figure No. 1: Sequence of a seam
For more details on the technique of a good seaming it is recommended to read the following works published in this Web:
– “Seam Control”
– “Seam measures”
It is not the object of this article to dwell more on general issues about double seam, we will focus on the particular case of non-round containers, especially in rectangular ones. They are the most difficult case of a seam.
The reason for this difficulty originates because two extreme and opposite situations come together in the same seam. On all four sides of the rectangle, the seam follows a straight path, we could say that it describes an arc of circumference of infinite radius. On the other hand, in the four rounded corners, it describes an arc with a very small radius – it usually oscillates between 15 and 30 mm. Combining these two situations, so that the seam is appropriate throughout its perimeter, involves reaching a compromise between different elements.
Of course, in order to achieve a good seam in a rectangular container, it is not enough just to act on the seamer’s settings. It is necessary to adequately prepare all the elements that participate in it. The most important are:
– Wing of the lid
– Body eyelash
– Seaming tooling
– State of the seamer
Of these four components, we will focus on this writing in the first three, because they are more specific to this topic. The condition of the seamer, is solved with the application of a good maintenance system of the same.
Going into the detail of these elements, we will attend in a concrete way to certain factors of them, such as:
– Design and measurements of the lid wing and curl thereof.
– Application of compound in the wing
– Shape and dimensioning of the body flange
– Profile of the grooves of the first and second operation routines.
– Seaming measures
From the precise definition of each of them, the achievement of a good seaming in a rectangular container depends substantially.
DESIGN AND MEASUREMENTS OF THE WING OF A RECTANGULAR COVER
We can define the wing as the outer area of the lid, located between the wall of the bucket and the external perimeter. During the seaming operation it transforms into the lid hook . There are two types of wing : flat and curly.
The flat wing is completely smooth and ends in the sharp edge defined by the cutting of the material. In other words, its outer edge is the cutting edge. Currently its use is very limited because it is an obsolete technology.
In the curled wing, the outer edge is folded inward, so that the cutting edge is tucked. This facilitates the seaming operation and the stackability of the lids. Within this type there are two versions.
- The one that presents its flat top. See figure nº 2.
- The one that has it curved. This second case favors more the formation of the seam, but its manufacturing tooling is more complex, especially if it is a rectangular lid as is our theme.
Figure nº 2: curly wing versions
In general, for simplicity, we will consider in our following comments that the wing is of type a).
To determine the width of the wing of a cover, it will be measured externally, if it is of the curled type, the curl will be straightened and in the case of rectangular caps it will be measured in the straight parts.
During the seaming operation, when the wing of the lid is tucked to form the lid hook, it happens that in the straight areas this action is a simple folding of the metal that presents no difficulties, but in the corners the material behaves as if let’s try to bend the rods of a fan. The metal located on the periphery of the wing must be folded over a radius lower than the original, that is, it must be stitched to a smaller volume than the original. This causes it to flow towards the free part, that is towards the cutting edge, or it deforms in the form of wrinkles or creases. The second option is very negative, so we have to choose the first option.
Therefore, to facilitate this fluency of the material, it is necessary to ensure that its hardness is moderate. This is the first conclusion: Tinplate – or TFS – of medium or low temper for rectangular caps should be used, the lower the radius of the corners. The temper T 3 or T 57 gives satisfactory results.
On the other hand, if the width of the wing of the cover is uniform throughout its perimeter, it will happen that when generating this flow of its material in the corners, the same will result in an increase in its width at the time of seaming in these areas. This would result in excessive hooks on the spokes, leaving the seam unbalanced. To avoid this defect, it is necessary to reduce the size of the wing in the four corners, this must be done progressively, starting from the existing dimension in the straight part and reaching the minimum at 45º in the arch. In this way, the smallest initial dimension in the curved areas of the wing will be compensated with its increase in length during the process, resulting in a uniformity of the lid hook in the entire perimeter of the seam.
The smaller the radius of the arc at the corners, the greater this decrease in material must be. Figure 3 summarizes the recommended values of this reduction based on the nominal width of the wing and the values of the spokes.
Figure nº 3: Recommended values of the wing in the corners
These values are taken at 45º, at their minimum point of length. The design of the lid die has to incorporate this reduction of the wing in the spokes. On the other hand, the height of the curl should be kept more or less uniform, to facilitate the seaming of the lid hook formation. The latter leads to having to reduce the width of the wing channel in areas where there is less material, ie in the four corners. This originates in the die of the lid, that the centers of the radii of the outer face of matrix center and of the inner face of the punch, do not coincide with the center of the other interior pieces to them. In addition, in these two pieces, these corners are configured with composite spokes. See figure nº 4. For the determination of the values of these radios there is recourse to define them by geometrical tracing, later corroborated in a practical way.
Figure nº 4: Rectangular lid with reduced width wing at the corners
. Figure 4 shows a cover format RR 76 x 49 with a radius of 12 mm. In its design, the above indications have been taken into account. In it, it can be seen that in section 2-2, corresponding to the corner of the lid, the channel width is 4.2 mm, while in the others it is 4.8, that is, the material reduction is assumed to be mostly in this dimension.
Subsequent to its die-cutting, the edge is curled at about 55º. This operation can be done:
– 1º: On the two major sides. It is the classic and simple procedure.
– 2º: In all its perimeter: It requires a more complex equipment, but it is the best solution for the seam.
We avoid entering into details for not deviating from our subject. Suffice it to say that either of the two can give a good seam, although if possible it is better to opt for the second option.
APPLICATION OF COMPOUND ON THE WING OF A RECTANGULAR COVER
In gumming rectangular caps can be made by two different techniques:
– By buffer. It is the traditional procedure and even the most widespread.
– By shower. It provides a better quality.
In both cases, if the lid has been curled only on the larger sides, the amount of compound used in smaller on these two sides, since the access channel for the application is narrower. This causes a lack of uniformity in the volume of rubber applied along the perimeter of the wing.
Another desirable condition is that the compound is applied as close as possible to the outer edge of the wing, as this will ensure its location in the seam in the area of overlap of hooks of cover and body. This is difficult to achieve on the sides with curl initiated, but it is necessary to approximate as much as possible to this objective.
At the corners, as the channel width of the wing is narrower than on the sides without curl initiated, the volume of rubber deposited will also be reduced. In this zone, the composite trace will decrease from the point of union with the unrigid sides until the curl starts.
Figure 5 shows the shape of the gumming pad for a rectangular lid, with the larger sides with curl initiated. The dimensions of it are linked to the wing width of the lid.
Figure nº 5: Dimensioning of a gumming tampon
To get a good seal it is very important that the rubber:
– is located at the intended site and does not contaminate other areas of the wing
– that elastic to fit into the interstices of the seam.
– It is well polymerized and adhered to the metal
The amount of compound to be applied to achieve a good seam depends on the wing width of the lid. It is controlled by weighing the lid before and after gumming. To calculate the theoretical weight that must be applied, the basic data to know previously is the volume that once dry must occupy on the wing. Once this is defined, it is sufficient to apply the specific weight of the compound used once it is dry and its percentage in solids – both data to be provided by the manufacturer – to calculate the wet weight of the compound to be deposited on the wing.
The following table defines the appropriate theoretical volume to be deposited in the wing expressed in mm3 per linear meter of wing. Knowing this data and the perimeter of the lid, by a simple ratio the volume per lid is determined and starting from it to develop the theoretical but humid calculation
Wing width of the lid Volume in mm3 / m linear. ( V )
6 mm 180
6.7 “ 200
7 “ 210
8 “ 250
Formulas: p= V x L x d /1000 P = 100 x p/E
p = theoretical weight of dry compound per fund expressed in mgrs.
P = theoretical weight of liquid compound per fund in mgrs.
d = specific weight of the compound
E =% solids of the compound
V = Volume of compound per linear meter in mm3 / m
L = Development of the perimeter of the lid in mm.
FORM AND DIMENSION OF THE BODY’S TAB
In general, the influence of the bodies is less significant than the caps to obtain a good seam. In a summary way we can say that they should:
– have enough stiffness and firmness to properly support the closed operation.
– be dimensioned so that the lid fits smoothly over your mouth.
– the tab of them has a suitable radius and width.
We will only look at the third point because it is perhaps the most important. The first is linked to choosing the right metal and the second is based on the dimensions of the center of the die matrix of the lid.
The execution of the tab in a rectangular container, is usually done in three different ways:
– By folding – or also called by “squeezer” -. The flange is formed when the edge of the body is folded over outer jaws that embrace it, pushed by expanding blades that open. See figure 6.
Figure nº 6: Defaced by “squeezer”
– By buffer. In this case, the flange is formed by pressing the body against a blinking die that has its shape. This type of tooling usually has some stops in the straight parts and the angle of the flange varies in the spokes. See figure # 7.
Figure nº 7: Tabulated by buffer
– By means of a mixed tooling, which combines the technique of folding – expansion – by sheets on the four straight sides of the container and of tampon in the four corners. This makes the tab not even in your profile. In the straight parts of the same, the radius is small and the flange tends towards the horizontal, while in the corners the radius is greater and the flange is smaller and with a certain inclination. This represents rather a problem of appearance than of quality. If the tab is well dimensioned, these shape variations along the perimeter do not transcend the seam. Of the three systems this is the best.
The value of the tab is related to the size of the wing. In straight areas its value is slightly higher than in radii. As a general rule this difference is 0.15 mm. This is due in part to the design of the tooling and partly to the elasticity of the material. In this way there is a better correlation between the wing in the spokes and the tab in the same area. The indicative values of the same are given below.
Wing width of the cover Body flange Body flange on the spokes
6 2.55 2.40
6.7 2.85 2.70
7 3.00 2.85
8 3.70 3.50
The flange width will be evaluated externally and results from the average obtained by measurement in different points, since it is difficult to get a uniform flange around the perimeter.
Within the seam tooling, the most critical part is the seaming routines. The proper choice of the profile of the same, especially the first operation, depends on the achievement of a good quality in it. In our case, rectangular containers have special characteristics as we have seen in the different sections. These can be summed up in a lack of uniformity in the wing of the lid and in the body flange.
Therefore, the profiles of routines used in round containers, with a theoretical value of wing and flange equal to the rectangular container that we intend to close (for example wing 7 mm and flange 3), do not usually give a fully satisfactory result. The problem is usually presented in the radios, where often appear “peaks” in the base of the seam and wrinkles in the lid hook.
We insist again that when this happens, the best thing is not to waste time trying different types of profiles of routines until you find some that go well, this does not always work. We must be sure that the wing and the flange, especially in the radios, meet the requirements described above and the covers are of good quality, otherwise we will lose time “fighting” with the seamer. Now, if we start with adequate components, it is time to enter into the study of routines.
Seaming roll 1st operation : It is critical for a good seaming. Defines the correct formation of the hooks.
Experience has shown that good results are usually obtained by using a profile that meets the following conditions:
– Throat height : Which would correspond to a round container seam with a smaller wing than the rectangular container in question. For example, if the rectangular lid has a wing 6.7, a seaming roll of throat height should be taken which corresponds to a 6 mm wing seam.
– Depth of throat : In this case you should increase by 015 mm which would correspond to a round container seam with the smallest wing. Following the previous example, the throat depth would be taken for a 6 mm wing and 0.15 mm more depth would be added.
In summary, it would be a matter of choosing a seaming roll that would be suitable for seaming a round container with a wing lower than the rectangular one that we want to close, deepening its throat 0.15 mm.
These values have been reached in a practical way, but it has its logic. Taking into account the reduction of measures in the radii of the components of the seam, it is a question of looking for an intermediate throat profile to the values that would correspond separately to the straight parts and to the radii of the container. Figure 8 shows a profile that works well for rectangular containers with a nominal wing of approximately 7 mm.
Figure nº 8: Seaming roll profile of 1st operation
Seaming roll 2nd operation : Not as important as the previous one. It pursues the correct ironing of the seam The equivalent to a seaming of round containers of equal parameters can be used, although there are also with slight adjustments to improve its behavior. In figure 9 an example is drawn that gives good results; It is also designed for a 7mm wing.
Figure nº 9: Profile of seaming roll of 2nd operation
There are other components that must be taken care of, such as the chuck plate and the compression plate. As well as its settings: height gauge, compression spring, etc … But this is already part of what is the seam technique, which is not the subject of this work.
The steps to follow, to achieve a good seam in a rectangular container, are the generic ones for this operation and we have already referred the reader to some work related to them, so we will not expand on it.
Just for orientation, we show the standard values of a seam in a rectangular 3 mm flange container and 7 mm cap flange, which is usually the most general case. In figure No. 10 they are reflected.
Figure nº 10: Measures of a rectangular seam
Although the following observations should be taken into account:
– The double seam in the corners present slight dimensional differences in relation to the straight parts.
– The thickness of the seam increases in the order of 0.1 mm in the corners.
– If the pressure is increased to try to equalize the thickness around the perimeter, it will generate an excess seaming height in the equines.
– To ensure uniform seaming results, it is convenient that the body is always positioned on the plate in the same position, in relation to the lateral seam of the same.
– In seamers that use more than one roller of each operation, it is essential that their profiles are identical and their adjustment equal.