The seam operation is key for the metal and packaging industry. Here we develop the necessary requirements to obtain a good seam, with interchangeable components, analyzing which are these and how they should be conditioned.
INTRODUCTION AND REQUIREMENTS
There is a common task for the metal industry and packers. It is the realization of the seam or union of the container body with its corresponding lid and bottom. To ensure that this work is carried out under appropriate conditions, and maintains uniform and safe levels of quality, a series of recommendations and standards are increasingly being applied, initially at the national level in each country, but little by little they have been internationalized to through regulatory bodies. In this article we summarize the main conclusions of these recommendations.
This work contains the recommended specifications to be applied in the purchase of containers and ends destined to products and processed foods. The data reflected here is accepted by the main metal industries worldwide.
The application of these specifications, allows to define an agreement of interchangeability, in everything related to the double seaming of the containers supplied by the manufacturers. Although the dimensions and tolerances detailed here are quite realistic, the indiscriminate use of them does not ensure identical results, applying a common adjustment. There are certain measures in which it is difficult to reach unanimity of criteria, such as the radius of the bucket of the bottoms, or the minimum diameter of the interior of the curl thereof, where differentiated values can be presented, which give rise to adjustments specific.
The containers and ends must be manufactured, packed and transported in the most suitable way for the filling, packing and distribution of the food product to which they are destined.
The containers, loose bottoms and seaming tools for the seaming machines, must be dimensioned to ensure the interchangeability of components of each manufacturer, along the equipment of filling, seaming, transporting and packaging, subject to the following limitations:
1- For a certain model of seamer, not all the diameters of containers and bottoms of any type, can be seam using chucks and common seaming routines.
2- Due to the variations in the dimensions of both components, and in the adjustments of the machine, even within the recommended standards, it is not possible to guarantee that it is not necessary to make adjustments between different supplies, even in the case of the same supplier. However, it is considered that adhering to these standards would ensure that the need for adjustments is minimized.
3- The supplier of the seaming machine, will facilitate the recommended adjustments of the machine and its setting, for any combination of containers and ends. It is advised that later, when problems arise, the set-up procedure should be used, as a basis to ensure that the adjustment of the machine is correct.
DIMENSIONAL OBJECTIVES AND TOLERANCES
In its day, the following agreements were adopted by the manufacturers, for the standards listed in this work:
– 1: ” Objectives” with their tolerance in more or less. The “objective” dimension to achieve is the value that is considered ideal for each parameter. This “target” value is defined by the nominal data. 99.7% of the containers or bottoms in any lot (see note) must be within the indicated tolerances. The average of all measures in this category should be at or close to the established “target” value. In the case of empty container height, where the defined tolerance is more or less 0.50 mm, the average for all measurements is expected to be between 0.13 mm or more from the target measure.
– 2: Established field (not “objective”). 99.7% of containers or ends in any lot (see note) must be within the established field. This field is delimited by the tolerances
– 3: Minimum value. 99.7% of the product in any lot (see note) must be at or above the minimum value established.
– 4: Field “pass / fail”. 99.7% of the product in any lot (see note), must be within the dimensions “pass / not pass” established.
– The dimensions that are indicated without tolerances, are measures of tooling included only as a reference.
Note: A lot is defined as a supply or consignment comprising not less than 50,000 containers or 200,000 ends.
CHUCKS FOR SEAMER MACHINES
The basic part of the seamers is the chuck plate, along with the seaming routines. This disc is adjusted to the interior of the basin of the bottoms in a very precise way, and acts as a support, on which the seaming is carried out by rolling, – or seaming – the material under the action of the routines.
The dimensions of the chuck are closely linked to those of the ends. Hence, to ensure that the same chuck can be used for successive supplies of them, even if they are from different suppliers, an interchangeability between them must be guaranteed, as we have already pointed out.
The chuck has two distinct parts, the upper zone by which it is fixed to the seaming machine, whose dimensioning is defined by the equipment manufacturer, and the lower or active part linked to the measurements of the ends to be used in the seaming operation. Figure 1 shows the profile of the lower area or lip of the chuck, the function of the containers to be seam.
Figure nº 1: Profile of the lip of the chuck
The appropriate measures of this profile, depending on the ends to be used, are reflected in the attached table. It really has come to unify a series of dimensions, whatever the diameter of the fund to close. These are common: The angle of inclination of the lip “D”, the radius of the same “C” and the reference point “E” on which the measurement of its diameter should be taken. Therefore, the only two variable dimensions are: The height of the lip “A” and its diameter “B” at the reference point.
Common measurements for all chuck diameters:
Height of the reference point E = 1.05 mm
Chuck radius C = 1 mm +0.00 / -0.05
Lip angle D = 4º +/- 10`
In this table, the nominal diameters of the most common straight containers, of the reduced ends – or notches – are differentiated into separate columns, since currently straight containers are found on the market with those of a fitted end.
The height “A” of the chuck lip, remains constant in many diameters, with the exception of the small ones and bigger ones. In the latter case – diameter 153 – two different heights still survive on the market, which has repercussions on two different values of the “B” diameter.
DIMENSIONS OF THE ENDS
As we have already said, to allow a unification of seam tools for the same format, and obtain a quality seam, whoever is the container supplier, it is necessary to maintain an equality of measures in all those levels that affect the realization of the seaming. This has been working for many years between manufacturers, trade associations, organizations, etc. and today has achieved a good degree of standardization.
In drawing no. 2, the measurements for round bottoms of canning containers, of the conventional type, are included, they do not include the easy opening with ring covers. Respecting these measures, it is possible to use the previous chucks with good results.
Figure nº 2: Basic measures of a fund related to the seaming
It is important to clarify that the seams are classified into seven different types, depending on the dimensions of their different components (body and bottom hooks, seaming height, overlap, …). The least of them is the OIII and the largest is the IV. The first (OIII) would be a mini seam for very small and low-responsibility containers and the last one (IV) for large-size brass. The most used types are OI, I and II, which cover the normal sizes of containers. It happens that the same format can be found in the market with one or another type of seam. For example diameter 73 in types OI and I.
Keep in mind that by changing the type of seam, the dimensions of the ends linked to it and the body’s tab are modified. Therefore, each user must be very clear about what type of seam he wants to use and link to it the packaging supplies.
The following table shows the measures of conventional ends, related to their seam, standardized to achieve interchangeability of components. In order not to make the table very complex, only the measurements for the most usual type of seam in each case have been reflected, which is indicated in the column “Type of recommended seam” .
As in the case of the chucks, we have differentiated the diameters of straight containers and the values for notched ends. Therefore the “notched” column does not reflect packaging measures but rather reduced ends.
The diameter “F”, which is the outside, defines the amount of material available to make the bottom hook in the seam, and influences the dimensioning of many pieces of the seamer (feeder of ends, stars, etc.). It is a basic measure of the lid. There are a series of formulas that link the different dimensions of the elements of the seam, ie the measurements of the chuck with those of the lid or flange. For example, the diameter “F” is related to that of the chuck “B” through the following expressions:
For seam type OI: F = B + 8.76
For seaming type I: F = B + 9.36
For type II seam: F = B + 9.88
For seam type III: F = B + 10.92
In the case of the diameter ” K” , which is the chuck adjustment, and therefore the fundamental for the seam, the maximum and minimum values have been expressed, which correspond to the “pass”, “does not pass” of the Quality control gauges for this measure. The tolerances reflected in the columns “passes” and “does not pass”, are those of construction of the corresponding gauges. The formula that links is mediated with the diameter of the chuck at the reference point is as follows:
Diameter of ends up to 99: “Pass” K = B – 0.12 / “It does not pass” K = B – 0.05
Diameter of ends greater than 99: “Pass” K = B – 0.17 / “Does not pass” K = B – 0.05
The “J” dimension , the diameter of the inside of the curl, has a relative importance, and its purpose is to prevent interference with the body flange. That’s why only its minimum value is reflected. The formula for determining the value of “J” according to the diameter “B” of the chuck varies in relation to the type of seam selected, and is as follows:
For seam type OI: J = B + 6.40
For seam type I: J = B + 7.00
For sean type II: J = B + 7.60
For seam type III: J = B + 8.40
This is so because when increasing the type of seam, the tab does so and there is therefore more room for its location within the lid’s wing.
The height or depth of bucket “ H ” should be measured at three points equidistant from the bottom and find the average of the three readings. This mean is also a basic dimension and is linked to the height of the mandril in the following terms:
Diameters of ends 49 and 52: A = H + 0.12
Other diameters: A = H + 0.15
The dimension “L”, height of curl, is not indicated in millimeters, but in the amount of ends that can fit in 2 “(50.8 mm) and is measured with a gauge. In another article, complementary to this one, the design, dimensioning and use of the necessary gauges to control these dimensions will be detailed, as well as others that we will comment next, although already in the work:
– Dimensional control of ends can be seen to a large extent this material
In the ends there are also two measures that remain constant in all diameters, these are:
Height of reference point “E” = 1.05 mm
Internal radius of cuvette “G” = 0.95 mm + 0.05 / – 0.00
The position or height of the reference point “E” is the same on the bottom as on its chuck.
The internal radius of the bucket “G” is equal to that of the center of the punch of the die, and its value is 0.05 mm smaller than that of the seaming chuck “C”, but with the tolerance changed.
DIMENSIONS OF THE CONTAINERS
There are few measures of the container that are related to the seam, in fact only three:
–Height: It must be uniform throughout the lot, to obtain an analogous compression effort in all the containers during the seaming process.
– Tab: Its dimension defines the measurements of the body hook, therefore it is a very important value to control.
– Inner diameter : In its extreme zone, the one corresponding to the flange, requires the adjustment between the bottom to be seam and the container.
Figure nº 3: Measures of a container with incidence on the seam
Figura nº 3: Medidas de un envase con incidencia en el cierre
In the following table we indicate the values of these measures, but first it is convenient to make the following comments:
1º.- There is so much diversity in packaging heights in the market that it is not possible to list them, we only indicate the tolerance applicable to them. The height must be measured with adequate equipment – see gauges for control – and in an area away from the side seam.
2º.- The tab width varies with the type of seam, therefore its value is linked to it. It must be measured from the reference point, located 2.5 mm from the edge. The average of two measurements at 180º, at least 10 mm away from the lateral seam, shall be taken as value.
3º.- The internal diameter of the container is not a critical measure in relation to the seam, although it is in terms of the capacity of the container. Therefore, in this work, centered on the first factor, we will indicate the value of the diameter as a reference, but without reflecting tolerances. As a general criterion, the bottom should adjust slightly to pressure on the body. An easy test is that once the bottom is placed on it, and pressed lightly, if the set is turned over, the bottom should not fall.
In the case of embedded packaging, the criteria to be followed are the same as for three-piece cans.
It is defined as “seam” or “double seam” to the union resulting from interlacing the end of the body of a container with its bottom or lid. This connection is made by a double seaming or agrafting procedure, that is, with a double safety wall. It must be perfectly hermetic.
This section recommends the operating standards for double seaming, for different diameters of containers and bottoms, as well as the details of the parameters and the points at which they should be measured.
These parameters are divided into two categories:
Critics : These are certain double seaming measures whose values must be reached to ensure the suitability of the same.
Others: They are the parameters that serve as a means to reach the critics, but allow a certain flexibility in their measure without detriment to the quality of the seam.
When it comes to acquiring a new seaming machine, the buyer must fix the seaming standard that he wants to implement in the new equipment.
There may be some differences of opinion when defining which are the important measures or values to be controlled in a double seam, including “critical” and “other”. We lean towards those reflected in the form of dimensions in drawing no. 4.
Figure nº 4: Measures of a seaming
The values of the parameters of the seam will be obtained, in the case of containers of three pieces, as an average of two measurements in the clockwise position of 2 o’clock and 10 o’clock, taking 12 o’clock as the location of the side seam. For sausage packaging the two measurements will be made at 180º.
In addition to the bounded measures, there are two values to be determined by calculation, both considered critical, which are:
A.- Seam thickness ratio : Also designated as “seaming tight”. Measures the compactness of the seam, comparing the theoretical thickness with the real, taking into account the contribution to it of the thickness of the composite. For this, the thickness of the seam was measured at the most unfavorable point of its perimeter. To make the calculation of the ratio, three complementary criteria can be used:
a.- Applying the following formula:
(3ef + 2ec) / true thickness> 0.8
In which “ef” is the thickness of the bottom, and “ec” the thickness of the body wall.
This formula can be stated: “The thickness of the five walls of the seam, divided by the actual thickness must be greater than the ratio 0.7”.
b.- Applying this other formula:
Real thickness = (3 ef + 2ec + 0.15) +/- 0.05
0.15 millimeters is the value of “free space”, which must fill the rubber gasket.
c.- Evaluation of wrinkles of the bottom hook.
In the generation of the seam always produce wrinkles in the hooks, especially in the bottom hook that is the one that more material tucks towards the interior. The height – also called length – of the bottom 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 bottom 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%. Its minimum value must be 75%, always referring to the worst point on the bottom hook, and special attention must be paid on both sides of the side welding union area.
Any of the three procedures is valid, although the most traditional and used is the evaluation of the wrinkles inside the bottom hook. In the first two cases the measurements were made in millimeters.
B.- Penetration of the body hook . Determine the relationship between the length of the hook of the body “b” and that of the interior of the seam “c”, according to the expression
Penetration = b / c> 0.7
The following table summarizes the recommended values of these parameters, divided into “critical ” and ” other” , for the types of seam mentioned in this work.
To control these measures it is necessary to use a series of specific gauges, which will be the object of another article complementary to this.
In order to carry out this work, the following material has been taken into account:
– Standards “SEFEL” (European Secretariat for Manufacturers of Light Metal Containers)
– “Recommended Industry Specifications for Open Top Processed Food Cans” prepared by the MPMA (Metal Packaging Manufacturers Association)