Se inicia aquí un trabajo sobre la TEORÍA DEL CIERRE, que se publica en cuatro partes o capítulos.
A.- Body of the can
B.- Can Fund
C.- Seaming terminology proper
D.- Seam tooling
3º.- GENERATION OF THE SEAM:
– Seaming roll
– 1st operation,
– 2nd operation,
– Sealing the seam
– Critical parameters.
4º.- SEAM OPERATION:
– Cylindrical can seamers
– Seam of irregular cans.
– Basic parts of the seam tooling
– Seaming roll:
– 1st operation
– 2nd operation
– Compression plate
– Cylindrical can seam
– Seam of irregular cans.
5th.- SEAM EVALUATION:
– A.- Visual inspection
– B.- External measures:
– Measurement of the depth of the countersink.
– Measurement of seaming length
– Measurement of the fall in the union
– Measurement of the seaming thickness
2nd Phase: Dismantling the seam
– Cutting the central panel of the background
– Cutting a section of the seam
– Disassembly of the seam
– Preparation of the seam for projection
3rd Phase: Internal measurements
– Measurement of body metal thickness and depth
– Determination of free space
– Measurement of hooks
– Penetration of the body hook and overlap
– Checking internal defects of the seam.
4th Phase: Leaks
– Leak detection
6º.- ACCEPTABILITY OF THE SEAM (Critical parameters):
– Tightening of the seam,
– Penetration of the body hook,
– Absence of visual defects
7º.- RECOMMENDED VALUES OF THE SEAM PARAMETERS
8º.- DIMENSIONAL VARIATIONS OF THE SEAM
9º.- INTERNAL DEFECTS IN A SEAM CAUSED BY A BAD ADJUSTMENT OF SEAMING ROLL AND THEIR CAUSES:
– 1st seaming operation.
– 2nd seaming operation:
o Short body hook
o Long body hook
o Short bottom hook
o Long bottom hook
o Excessive seaming length
o Insufficient seaming length
10º.- CLASSIFICATION OF DEFECTS IN THE SEAM
11º.- CONTROL AND DIMENSIONAL ADJUSTMENT OF THE SEAMERS
12º.- ADJUSTMENT FOR OBJECTIVES:
1º.- Comprobación del estado mecánico de la maquina cerradora
2º.- Verificación que los mandriles y rutinas son los adecuados para el tipo de envase a cerrar
3º.- Preparar la maquina para la altura de ajuste de cerrado
4º.- Ajustar la presión del plato de compresión
5º.-Comprobar la alturas de las rutinas con relación al labio del mandril
6º- Verificar las presiones de cerrado de las rulinas y ajustes finales.
7º.-Expulsores 1º.- Checking the mechanical condition of the seaming machine
2º.- Verification that the chucks and routines are suitable for the type of can to be seam
3º.- Prepare the machine for the adjustment height of seam
4º.- Adjust the pressure of the compression plate
5º.- Check the heights of the routines in relation to the chuck’s lip
6º- Verify the seaming pressures of the rollers and final adjustments.
13º.- ANALYSIS OF THE SEAMS
14º.- PROTOCOLS OF MAINTENANCE OF THE SEAMER
15º.- TECHNICAL IMPROVEMENTS OF THE SEAM AND EVOLUTION OF THE SAME (MINI SEAMS)
– Evolution of the seaming
– First operation
– Second operation
– Valuation of the “mini seaming”
– Critical parameters in the mini seam:
– Tight seam
– Tightening indicators
– Penetration of the body hook
– Influence of the tab.
– Changes in the production line
16º.- PROGRESSES IN THE TOOL DE SEAMERS
17º.- SUMMARY OF DEFECTS IN THE SEAM:
– ORIGIN AND EFFECT
– FALLOS Y SUS CAUSAS- FAULTS AND THEIR CAUSES
19º.- EQUIVALENCES OF TERMS OF SEAM: ENGLISH – SPANISH
We intend with this work to present in a clear and detailed way, both to packaging manufacturers and packers, the fundamentals of the seam theory, so that they can fully understand the characteristics and types of seam of metal cans.
The careful reading of this work will help to control the quality of the seams, as well as to regulate the seaming machines. The main objective is to assist the staff that makes the decisions and determines the quality of the seam. The judgments of these people depend on whether a production line is going to continue working or, on the contrary, it has to stop
Let’s start by identifying the different parts that make up a can. Figure 1 shows a metal can and on it its different elements are annotated.
Figure 1: Elements of a metal can
It is defined as “seaming” or “double seaming”, the result of joining the end of the body of a can with its bottom or lid. The seaming is obtained by bending the wing of the bottom around the flange of the body, hooking them to each other, until producing a hermetic union. This joint therefore employs a technique of double seaming or agrafado, ie generates a double safety wall. Your goal is to obtain a totally sealed board.
Performing a good seam is an essential condition, necessary but not sufficient, to avoid bacteriological contamination, corrosion and alteration of the product. It must have sufficient mechanical characteristics to withstand, under normal conditions, the filling, handling, transport and storage process.
It is carried out in equipment called seamers. Of these machines there is a great variety of designs and capacities, from the simplest, of operation by hand, to the most complex, automatic at high production speeds.
In this document we will present how the seam is generated, the elements that compose it and the tools that make it, as well as the main problems that may arise during its execution. We will also analyze the different designs and sizes of them, recommended according to the diameters and characteristics of the cans used.
For the reader not initiated in this matter, we suggest you start by reading the TERMINOLOGY of the seam first, which follows, in order to follow without difficulty the lexicon used throughout this work. The reader with previous knowledge of this issue can skip it and move on to the next point.
Next we define the different technical terms that are used in the seam theory. Let’s start by listing the body parts of the can and the bottom that will participate in making the seam.
A.- Body of the can : Figure 2 shows these elements
– Side seam: Lateral union of the body of the can, usually achieved by electric welding, although it can also be achieved by a seaming process with the contribution of a thermoplastic cement sealant.
– Body: Cylindrical, rectangular or irregular part that forms the walls of the can.
– Tab : Flange that presents the body in each of its ends. It is formed by a flat part, almost perpendicular to the wall of the body of the can, and an arch connecting this flat part with said body
– Radius of the flange: Radius of the arch of the flange and that unites it with the body.
Figure 2: Elements of the body of the can that are part of the seam
Figure 2: Elements of the body of the can that are part of the seam
B.- Bottom of the can: Figure 3 shows the components of the bottom that participate in the seam.
– Bottom wing: External portion of the bottom that deforms during the seaming operation and becomes integrated into the seam.
– Curl edge: Final part of the curl. There is always exposed steel, which makes it sensitive to oxidation.
– Compound (or rubber) sealant: Substance formulated on the basis of synthetic rubbers, which facilitates hermetic sealing of the seam. It is applied on the inner part of the bottom wing.
– Wall of the bucket: Side partition that delimits the cavity of the bottom, that allows to lodge the chuck of seaming
– Bucket depth: (e) Distance between the top of the wing (in the case of a loose bottom), or the seam (once it has been made) up to the center panel of the bottom. It should be measured at the point where the radius of the bottom tray ends.
– Radius of the wall of the countersink : Arch that connects the bottom wall of the countersink with the central panel of the bottom
– Radius of the wing: Arch that connects the upper part of the wall of the bucket with the wing.
– Curl: Curvature towards the end of the bottom wing. During the seaming, it serves to initiate the formation of the bottom hook.
Figure nº 3: Components of the bottom for seaming
C.- Seaming terminology proper : It appears reflected in drawings No. 4 and 5.. Those elements of the seam that are physical magnitudes – measures of length – are reflected in the drawing nº 5 bounded by means of a letter.
–Adjustment of the chuck: Determines the degree of adequacy of the positioning and penetration effort of the chuck inside the bottom tray.
– Hooks on the hooks: They are undulations or folds formed in a vertical sense on the bottom hook. They are undesirable, because if they have some importance, they can affect the sealing of the seam.
– Sharp edge: Sharp highlight located at the junction between the top of the seam and the wall of the countersink
– Sharp edge : Cutting edge to a lesser degree, it does not present a sharp edge but simply a right angle
– Drop in the joint: Deformation downwards of the bottom hook in the area of the lateral welding.
– Seam: It is the part of the can formed by the union of the edges of the bottom and the body, by means of hooks or folds that are interlaced and form a structure of high mechanical resistance. It has to be airtight.
– Compactness: Relationship between the five metal thicknesses of the seam and the actual thickness thereof expressed in%.
– Summit of the seaming: Upper rounded area of the seam. It is a part of the bottom wing already deformed.
– Free space: It is the difference between the thickness of the seam actually measured and the sum of the thicknesses of the five sheets that are included in the seam.
– Thickness of the seam: (h) Maximum dimension of the seam measured in the direction perpendicular to the can axis. It contains the five layers of metal of the seaming.
– False seaming : Zone in which some section of the clasp hooks are not engraved, although the appearance is of a normal seam
Figure nº 4: Terminology of the seaming
– Body hook: (d) Portion of the body flange that bends down during the formation of the seam.
– Bottom hook: (g) Part of the bottom wing that folds during seam to link with the body hook.
– Gap: (i) Empty space left between the inside of the top of the seam and the upper area of the body hook.
– Gaps: Open spaces located at the end of the phone and body hook that are filled by the compound during the seaming operation.
– Chuck trace : Marks left by the chuck around the internal periphery of the body adjacent to the radius of the countersink wall, formed by the pressure applied by the seaming routines during the same.
– Seaming length: (f) Distance between the top and the base of the seam.
– Critical parameters: Double seaming values that must be satisfied to achieve their acceptability.
– Seaming wall: The outermost part of the seam, formed by the outer face of the bottom hook. It has the shape of the profile of the seaming roll of second operation, that is to say of curve of wide radius.
– Skating: Incomplete formation of the seam, due to a displacement of the seaming roll in the second operation.
– Penetration of the body hook: (b / c) Relationship between the length of the body hook and the internal length of the seam, expressed as a percentage. Indicates the amount of the body hook that has been nailed against the bottom compound or joint.
– Peak in the assembly: Angled projection located at the base of the seam in the area of the lateral welding
– Pressure (or tightness) of the bottom hook: Height of the corrugations present on the bottom hook, measured in% of the height of the hook.
– Radius of the wall of the seam: Curve that connects the top of the seam with the wall of the same.
– Body hook radius: Bend curve in the formation of the body hook.
– Radius of the bottom hook: Curve of plague in the formation of the bottom hook.
– Seal (or seal) primary: The produced by nailing the body hook, against the rubber or composite gasket housed in the bottom hook.
– Sealing (or seal) secondary: The created by overlapping and crushing the hook of the body and the bottom hook, inside the seam.
– Overlap (or overlap): (a) Is the magnitude of the overlap generated in the seam between the body hook and the bottom hook.
. As we have said above, those elements of the seam that are physical magnitudes – measures of length – are reflected in the drawing No. 5, which appears below, bounded by a letter.
Figure nº 5: Dimensions of the measurable parts of a seam.
– Union or assembly: Part of the seam coinciding with the lateral seam of the body.
D.- Seam tooling:
The parts of the seamer that are directly linked to the seam are the tools of the same. These parts change depending on the size of the can and the type of seam we want to make. In photo # 6 some of them appear. We define the same and their main adjustments:
– Seam adjustment height: This is the path between the lower face of the seam chuck and the upper face of the compression plate. This distance is measured when the top of the cam of the first operation acts
– Base load of the compression plate: Force applied to the base of the can, through the compression plate, during the formation of the seam.
– Seamer: Machine that performs the seaming operation between bottom and body. It also receives other names such as sertidora.
– Ejector: Part of the seamer located in the center of the chuck, which pushes the seam can to extract it, once the second operation is completed. It has regulation in height and strength
– Chuck lip: Chuck plate edge in contact with the wall of the tray
– Chuck: Part of the tooling of the seamer that changes with the diameter of the can. It lodges in the bottom tray during the formation of the seam. It acts as the anvil against which the seam is constituted.
Photo nº 6: Tools of the heads of a seamer
– Profile of the seaming roll: It is the necessary contour, of the active part of the seaming roll, to obtain the correct conformed of the metal, that is required to fulfill the specifications of the hooks of body and bottom.
– Compression plate: Part of the seamer’s tool that changes with the diameter of the can. The can sits on it during the formation of the seam. It is equipped with up and down movement and acts loaded with springs. It also receives the denomination of “base dish”.
– Seaming roll: It is the part of the seamer that performs the deforming operation of the metal, to configure the body and bottom hooks and the link between them. There are two types, the first operation, which forms the hooks and binds them, and the second operation, which crushes them together. It also receives other names such as: Trucks, seaming wheels, molars, seaming rollers …
3º.- GENERATION OF THE SEAM
Another way to define the “seam” is: “the hermetic seal that is made between the body of the can and its bottom”. It is a process of deforming the metal, consisting essentially of two operations, although in cases of seams of difficult irregularly shaped cans, three operations may be necessary. In order to achieve sealing, the use of a rubber – or compound – is required, which is deposited on the internal part of the bottom wing, and after the seam operation is lodged inside the seam.
The seam is carried out as shown schematically in Figure No. 7, and is basically broken down into 3 successive steps:
– situation or placement of the bottom in position on the body, and the body of the can in the compression plate.
– first seam operation
– second seam operation
Figure nº 7: Diagram of the seam operation
The seaming is done by the action of two seaming roll or seaming rollers, which are also designated with different names according to the geographical area, (wheelbarrows, molars, wheels, sheaves …). The work of the same is divided into a first operation, followed by a second.
They have a throat, or work zone, formed with special shape silhouettes, called “seam profiles”. This profile is variable depending on whether it is first or second operation, depending on the characteristics or specifications of the wing or seam panel of the bottom of the can, and the thickness of the material to be sealed. As a general rule, the larger the size of the bottom wing and the thicker the material, the wider and deeper the profile of the seaming roll, or what is the same, is a function of the format of the can and the size of the metal employed in its components. This means that the same type of profile can be used, but its dimensions increase or decrease depending on the size of the can and the thickness of the metal.
Between the seaming seaming roll of first and second operation, the main difference consists in its profile and mission, since the wheelbarrow of the first operation has as fundamental purpose to roll the wing of the bottom around the hook of the body, at the time that it forms this. The purpose of the second operation truck is to tighten and iron the seam already started, in order to achieve airtightness. Let’s take a seamer look at these three seaming steps.
The can to be seam – body and bottom – is placed on an adjustable plate, which can be moved vertically, so that the can is retained between this plate and the seaming chuck.
The compression is performed by means of a spring, and its main mission is to prevent the relative movement of the can with respect to the seam chuck, allowing the progressive transformation of the body flange and the bottom wing during seam.
As we have already indicated, during the first seaming operation the wing of the bottom is rolled smoothly on the flange of the body of the can, at the same time as the latter is bent, forming very loose hooks, remaining space between its walls. In drawing # 8, the beginning and end of this step are represented in two sequences.
Figure nº 8: Beginning and end of a first seaming operation
As you can easily see, the correct performance of this first operation is decisive for obtaining a good seaming, and special care must be taken in its adjustment.
A cross section of the first operation should show a good overlap of the hooks or folds of the body and the bottom, and also the absence of distortions in the formation of the joint. The cube depth increases slightly progressively during the first and second operations.
Further details of the formation of the seam during the first operation of irregularly shaped bodies and funds will be given below; however, the principles of acceptability are the same, both for cylindrical and irregularly shaped cans.
Once the first seaming operation is carried out, a second forming seaming roll is used to compress the five metal layers (two of the body of the can and three of the bottom), to give rise to a hermetic seam. The operation of pressing and ironing the bottom and body hooks with each other is carried out progressively. Figure 9 also shows the beginning and end of this step. This operation gives the seam mechanical strength and sufficient tightness to support with guarantees, and under normal conditions of use, the various stages of the process of manufacturing and distribution of the packaging.
Figure nº 9: Beginning and end of a second seaming operation
In the seam, two sealing zones can be identified. The seal first occurs when the edge of the body of the can is nailed, ie the body hook, against the rubber or composite gasket, salting material that is inside the crease formed in the bottom wing, or bottom hook . The secondary seal is created by the overlapping and crushing of the body hook and the bottom hook, inside the seam.
The volume, type and weight of the seam rubber changes remarkably with the diameter of the cans, varying its composition according to the product that the can will contain. A lack of compatibility between the seam compound and the contents of the can can lead to product and gum contamination, significantly damaging the characteristics of the seam.
The application of this seaming compound is carried out by means of gumming machines, being able to be carried out by different techniques: tampon, shower or nozzle. Its regulation at the time of application, must be done with extreme caution to avoid problems of leakage in the can. The purpose of the compound is to occupy the free space and the existing holes within the seam.
The seam thus achieved, must have an appropriate mechanical resistance, and sufficient tightness under normal conditions of use.
Critical parameters of the seam:
The aspects or main factors in the formation of the seam, which produce a leak-free union are called critical parameters of acceptability and are:
– Tightness of the seam: Shows the degree of tightness. It guarantees that the double seaming has been under sufficient pressure.
– Body hook penetration : Reflects the embedding of the body hook within the net height of the seam. Measures the formation of the primary seal.
– Overlap (or overlap): It shows the overlap of the two hooks (body and bottom). Measure the formation of the secondary seal.
We will return to these concepts in more depth later.
4º.- SEAM OPERATION
The seaming operation of food canning cans or any other product is done in the seaming machines. In principle they can be classified into two large groups: Cylindrical can seamers and irregular can seamers.
Cylindrical can seamers
Most of the cans are cylindrical. With this shape, higher filling and seaming speeds are more easily achieved. The seamers that work with cylindrical can are divided into two types:
-1º.- What the can turns during the seaming operation. They can have one or more heads and allow great seaming speeds. In them, the can with its lid, which are fed separately to the machine, are placed between the chuck and the compression plate, the ejector keeping the lid in place while the plate goes up to place the can in the seam position , oppressed against the baboon. At that moment, the compression plate, the can and the chuck begin to rotate together, producing the first seaming operation by the action of the corresponding rollers, which come up on their arms and actuated by a cam; then, by an analogous procedure, the second operation is carried out, the same plate and the seam ends.
They are used for the manufacture of boats and also in the filling of certain products. To establish the seaming speed of a machine, factors such as: diameter of the can, product to be packed and possibilities of spillage of the product must be taken into account.
– 2º.- Those that maintain the can without rotating during the seaming. In this type of seamer the can remains motionless, while the seaming head of the machine revolves around the can. In this head, the roller bearing arms are housed.
There are also variants of seamers of this type that work under vacuum for certain products such as meat, dairy, etc. There are several ways to get a vacuum in the process of filling and seaming a can:
– Mechanical vacuum , by means of a vacuum pump, holding the can during seam in a hermetically sealed chamber.
– Vacuum by water vapor, it expels the air by injecting a jet of steam into the head space of the filled can. When the vapor condenses, vacuum is produced.
– Vacuum by hot filling , achieved by filling the can with hot product and therefore dilated, when it cools and reduces its volume an interior vacuum chamber is generated.
Depending on the product to be packaged, one type or another type of seamer is chosen, so the first case is used for dietetic products, and the last one is for meat products. The use of vacuum always reduces the speed of the cycle.
Clogger of irregular cans
With the irregular term is included all types of cans that are not cylindrical. These seamers are designed to close cans with oblong, oval, rectangular, square, or pear-shaped; they usually operate at low speeds and have a complex design. Some of these seamers have been designed to work under vacuum conditions, which further complicates the mechanical operation of the equipment.
The principle of operation is as follows: the cans filled with product reach the machine, either manually or mechanically, until the seaming platform, commonly called “compression plate”, before positioning the bottom on the can. Therefore, the placement of the can and the bottom is carried out analogously to a round can seamer.
In these machines, the seaming roll of the first operation, which are usually a pair, are placed diametrically opposite. They rotate around the chuck and the can, which are stopped, exerting on the latter a certain pressure that is regulated by the action of a cam. See drawing nº 10.
Figure nº 10: Plan view of a seaming head for rectangular cans
Next, and in a similar way, the second operation rollers come into action. Once the seam has been completed and the second operation portaruline arms are separated, the ejector acts, separating the can from the chuck, accompanying the compression plate in its downward movement.
Some irregular vacuum can sealers, fed by hand, lift the can in two phases. This means that once the can has been placed on the plate, the mechanical movement of the machine raises the can up to a point below the seaming roller. In this position the vacuum is made in the seaming chamber. This vacuum triggers the second lifting movement, which raises the can to allow the start of the seaming operation. The higher speed seamers normally work with a pre-vacuum station before seaming.
Some types of rotary star, allow to increase the speed, but seldom exceed 100 cans / minute. When it is not possible to maintain a separation between the body and its bottom before the beginning of the seaming operation, during the time that the vacuum is made, then it is required to use specially designed funds, which provide a passage to the air outlet of the inside of the can This is achieved by making notches on the bottom wing that rest on the flange of the body, slightly raising the level of the wing, and thus leaving a free space for the exit of the interior air. Without this separation, the flange of the body of the can would fit over the rubber of the bottom wing, creating a seal that would prevent the obtaining of vacuum inside the can. Normally this type of special background is denominated as background with “dimples”. Later we will return to the complexity of the seam of irregular cans.
Basic parts of the seaming tooling
Seaming roll: They are rollers of steel in the case of manufacturers of cans or stainless steel special for conserveros, with a degree of hardness very high. They are motorized on an axis or on bearings. See figure nº 11
Figure nº 11: Seaming seaming roll
The seaming roll here drawn is of the type of double guide, and in her the assembly of its axis with the corresponding bearings is appraised. The measurements bounded on the drawing refer to the dimensions suitable for a specific type of seamer – FMC. 521 -. The dimensions enseam in rectangle are basic for a correct adjustment, since they mark the distances from the point of reference in the throat of the truck to its heel (3.15 mm), and to its guiding surface (28.63 mm)
The work areas of these routines are configured with specially shaped silhouettes, called “seam profiles”. The arm where they are inserted, makes a movement of approach and separation with respect to the chuck automatically. This movement is commanded by a cam.
His mission during the seaming, and his performance during it have already been described above. The shapes and dimensions of the profiles of the routines decisively influence the tightness of the seam. The throats of the routines should have an excellent surface finish, the higher the same the results will be better.
Seaming roll of first operation: Its profile is always deep and narrow, with the curvature of the lower edge being more pronounced than that of the upper one. See figure nº 8. The profile acts as a generatrix to form the hooks of the body and bottom. Its object is to roll the metal of the wing of the bottom with the one of the flange of the body, being this introduced in that one. See figure 8. The resulting seam has a rounded outer shape and its height is lower than that of the finished seam.
Seaming roll of second operation: Its profile is shallower and higher than the first operation, presenting a more pronounced curvature in its upper edge. See Figure No. 9. As we have explained, perform the finishing operation by pressing the seam, formed in the first operation, against the chuck, so that the hooks are pressed and straight. See figure nº 9.
Chuck: It is a plate mounted on an axis, with a flange or lip that fits the bottom to close, and whose characteristics depend on the can manufacturer. See figure nº 12. Its dimensions also influence the shape and configuration of the seam, since it serves as a base for the action of the seaming roll.
The chuck shown in section in figure 12 is a typical example of a round can plate, to be used with guided type rollers. It shows a central thread of fine pitch and left, to ensure that it does not loosen with the turn of the same to right. It is equipped with four holes on its underside, for a special assembly and disassembly key. On the same side it has a recess for receiving the bottom profile.
Figure nº 12: Example of chuck for round bottom
The lip is slightly conical in shape, to facilitate the expulsion of the seam can and adapt better to the initial shape of the tray. Formerly the surface of the lip was striated to avoid sliding of the bottom during the seam, but this practice has been eliminated by generating damages in the external protection varnish of the bottom, which subsequently gave rise to oxidations in the area. Its central part has a recess, which serves to save the expansion rings and tiers of the profile of the bottom panel.
The lip of the chuck is defined by four basic dimensions that are indicated in figure no. 13
Figure nº13: Profile of the lip of a chuck
– Lip height S1. The value of S1 is linked to the depth of the tray (A) of the bottom. SEFEL proposes the following formula for its determination:
o S1 = A + 0.13 for all round bottoms equal to or greater than 60 mm in diameter
o S1 = A + 0.13 for all round bottoms equal to or greater than 60 mm in diameter
or S1 = A + 10 for diameters less than 60 mm.
– Lip angle of inclination: Its normal value is 4º. Amount admitted by SEFEL
– Radius of the base of the lip P1. Its normal value is 1 mm. Amount admitted by SEFEL.
– Diameter at reference point N2: Diameter N2 is obtained from the diameter of the punch block of the bottom punch (B1), that is to say from the part that defines the diameter of the bottom trough. SEFEL recommends the following formula:
o N2 = B1 + 0.12
As previously discussed, it serves as a support for the can, centering it and transmitting the pressure of the spring on the chuck. Its shape depends on the end of the can. In the case of seamers for the manufacture of cans, it has a profile copying from that of the tab of the body of the can; for filling sealers has channels on its surface, which coincides with the seam of its base, to fix the boat in the correct position. See figure nº 14. In the same the profile of a plate for seaming of manufacture of cans is presented, and the approximate formulas for the determination of their measures are given, according to the format to be seam.
Figure nº 14: Profile of the compression plate for seamers of packaging manufacturers
It is mounted on an axis. At rest it is at a distance from the upper chuck to that of the can with the bottom positioned at the moment of seaming. The pressure is regulated by acting on this plate, later we will return to it when dealing with the adjustment of a seamer.
The centering of the plate also has influence during seaming. It must be perfectly centered with respect to the axis and as parallel as possible to the chuck.
All the automatic seaming machines are equipped with ejection mechanisms, which at the same time contribute to maintaining the correct position of the bottom on the can, before reaching the seaming position. In addition, the ejector separates the seam can from the chuck, once the seam has been completed.
This ejector is an axis, loaded with a spring, with a small plate at its end, which is housed in a hole made in the center of the chuck plate. It is endowed with an alternative movement (ascending-descending) acting in the center of the background. See figure No. 15
Figure nº 15: Seaming head for domes and bottoms of aerosols with ejector
The head shown in Figure No. 15, is that of a seamer with horizontal heads for the seaming of domes and funds for aerosol cans. The upper part of the drawing presents the section of a head for domes, in this case the extractor is shorter to leave space at the highest height of the dome. The bottom part of the drawing shows the case of funds, which have a concave shape.
In the seam machines equipped with steam injection to achieve vacuum, a complete synchronization of the movements of the compression plate and the ejector is necessary. When increasing the speed, the adjustment of the ejector is more critical, and it must be more accurate the faster you work.
Cylindrical canister seams
Currently, the seam equipment for cylindrical cans, both standing and rotating canisters, produce uniform and high quality seams, with few problems in terms of metal shaping. However, the changes that have been introduced in the thickness of the material of the bottom, with sensible reductions of the same, have forced to a redesign of the wing of the bottoms with the end to maintain a high quality of the seaming. We will come back to this topic later.
The frequent use of sausage packaging in different sectors, such as fish and meat, has significantly reduced the risk of lack of quality in the seam, which would be reflected in alterations due to leaks. If three-piece type cans are used, with the side seam electrically welded, there is always a lower quality area in the overlap of the bottom and body hooks, coinciding with the region of the side seam, due to an increase in thickness Body hook, originated by welding. This effect is accentuated to a point that can be dangerous when it comes to cans with the side seam snapped – or engargolada – with contribution of thermoplastic cement as sealing material. Of course, the limitations of using this type of packaging to products without process, substantially reduce the risk of leaks. In any case, the rules of acceptability of the seam that are applied must guarantee that adequate integrity is maintained in that high risk area of the seam.
If you compare the quality of seam that is now achieved, with the widespread use of three electrically welded packages or sausages, which formerly were obtained with engargolados and soldered tin-lead solder, the results have improved significantly , since the seam remains uniform throughout the perimeter, without presenting areas of high risk, although there is always a point of lower quality in the area of the seam electrically welded, as we have already indicated.
Irregular can seams
There are a number of markets that use irregularly shaped cans, such as: meat, fish and edible oils. However, the need to achieve high quality seams is more difficult if these body cans and irregular bottoms are used, compared to the results obtained with cylindrical lats.
This will be better understood, if we recall again how the seam of these cans is done. Once the bottom is deposited on the body of the can, the assembly is placed on the compression plate. This elevates and houses the bottom tray in the chuck chuck. The carrying arms of the seaming seaming roll initiate their movement inwards – we remember that normally there is a diametrically opposed pair of seaming roll of each operation – and they contact with the wing of the bottom. Once this contact is made, the deforming operation of the metal begins. The seaming roll of the first operation move progressively inward, as they turn the can over. The deformation operation creates a metal wave in front of the profile of the seaming roll by the action of the same. In cylindrical cans this wave is distributed evenly during the cycle of the first seaming operation. In the irregular cans this forming operation works in the same way, with the exception that as the seaming roll of the first operation goes around the can, the wave of material that runs in front of it when arriving at each corner is deposited in it . This lack of control of the metal at the corners often produces creases and wrinkles in the bottom hook of the first seaming operation.
When the cycle of the second seaming operation begins, also usually by means of two roller bearing arms, the joint is compressed to produce a hermetic seal with the help of the rubber gasket, maintained with sufficient pressure to produce a seal leak free. Logically, the smaller the radius of the corner of the irregular can, the greater the risk of forming strong wrinkles that distort the seam. Various methods are used to reduce the risk of crease and crease formation in corners. The design of the background plays a crucial role in solving these problems.
If the material of the bottom wing is reduced at each corner, the metal wave deposited at this point during the first seaming operation will be reduced in size. This produces a more even bottom hook on the straight sides and corners. Obviously, it is necessary to define minimum critical parameters, and from them calculate the reduction in wing width, in order to balance the material that will be deposited in each corner during the seam. For more information, we recommend reading the work:
The machinery manufacturers, in conjunction with the metal, have studied the design of the seam operations to properly control the material in the corners of irregular cans. The reduction of wing material in the spokes is made progressively, starting before the end of the straight section of the wing. The seaming roll of first operation usually also begin their movement in a gradual curve, a little before the end of the straight side, and before reaching the radius of the can. This way, the formation of the hook on the spokes is better controlled.
Another way to improve the seam, is that the first operation of the same, is carried out in two turns around the can instead of one and fraction, as it happens in the round cans. This allows to shape the metal of the wing more smoothly and progressively, with which the risk of folds and wrinkles decreases.