5th.- SEAM EVALUATION
The evaluation of seam is the methodology used to judge its acceptability. It is related, therefore, with the specifications and requirements indicated by the manufacturer. These vary for each format and type of container. Therefore, in order to deal with the evaluation of a seam, it must first be provided with the table of complete specifications that it must comply with.
It often happens that a seemingly normal seam and within specifications in its external measurements, presents a more complete analysis, a series of defects that make it incorrect.
By means of the evaluation or adequate examination, it is possible to guarantee or not the hermeticity of it. The examination of a seam includes four phases, each of which provides a wealth of qualitative and quantitative information. These are:
1st Phase: Visual Examination and External Measures.
2nd Phase: Disassembly and / or Sectioning of the seam
3rd Phase: Direct and indirect Internal Measures and Internal Defects
4th Phase: Leak detection.
To open a seam, a series of basic tools are needed, which are:
1º.- Bacteriological Opener: Consisting of a rod with a handle, which ends in a bent tip. Along the rod slides a sharp blade that allows to cut the metal. See figure no. 16. If it is not available, it is possible to use metal cutting scissors with curved tips.
Figure nº 16: Basic material for opening a seam
2º.- Tongs. See figure No. 16. They are small and manageable wide-mouth pliers, which allow maneuvering effectively.
3º.- Seaming control rule. It is the simplest element of measurement. See figure No. 16. It does not allow exact readings, but it has some minimum elements such as:
-Regula graduated in mm. with the “zero” at the end to measure, by direct support of the tip, different parameters such as: diameter of the container, length of the seam, etc.
– Set of calibrated notches for reading the thickness of the seam.
– 2 “housing (50.8 mm) to evaluate the number of funds that can fit in this size.
– Calibrated heel to check the depth of the tray.
4th.- Micrometer. It is a micrometer of special design for the measurement of the parts of the seam.
5th.- Gauge with cover to measure the depth of the tray.
6º.- Gauge with cover to measure seaming thickness
7º.- Special saw for the seaming of the seaming
8º.- Thickness micrometer for metal thickness
9º.- Projector of seams
In addition to adjust the seamer it is necessary:
10º.- Set of gauges of thickness (sheets) for height of seaming roll
11º.- Set of rod gauges for the distance of seaming roll
12º.- Interior micrometer for base plate height
13º.- Dynamometer for pressure of the base plate
Later he will go describing each of these elements. Any evaluation of a seam will require the use of this instrument, which must, in turn, be calibrated, so that the zero is correctly adjusted.
Generally, two methods of analysis are used. The first is to open the seam completely. This requires disassembling the double seam, measuring the components, and recording their dimensions and observations detached from their examination. The second method is the sectioning of the seam. To perform this type of analysis, the seam is cut perpendicularly, and the transverse surface of the resultant itself is projected onto a screen, or placed in a microscope, to allow accurate measurement of the body hook, the bottom hook, effective overlap , and with the help of a scale, the penetration of the body hook.
The method of sectioning the seam can be carried out quickly in the cylindrical cans. However, the vulnerability of the corners of the irregular cans, requires cutting each corner to determine the general quality of these areas. This is a long task and is often considered to take more time than the full opening method.
One of the main disadvantages of the evaluation of the quality of the seam by the sectioning and projection method, is that it does not reveal the ratio or degree of compactness or the level of wrinkles of the lid hook.
To check the quality of a seam, the normal way forward is to unfold each of the phases listed above, in a series of steps that are the following:
1st Phase: Visual inspection and exterior measurements
A.- Visual inspection
1.- Visually inspect in it the possible presence of: Skating, cutting edge, false seaming, “peak” in the assembly, peaks in the seam, crushed eyelash, crushed crimp, sunken bottom, seam jump, striped seam, mandrel broken, expulsion of compound, lamination at the base of the seam, irregularities in the seam, etc.
2.- Depth of the bucket
3.- Length of the seaming
4.- Fall in the union
5.- Seaming thickness. Compactness
2nd Phase: Dismantling the seam
6.- Cut and remove the center of the bottom of the container
7.- Sections of the seaming
8.- Separate the seam, if you have opted for the “scrapping” procedure or
disassembly (1st procedure).
9.- Preparation of the seaming for its projection. (2nd procedure)
3rd Phase: Internal measurements
10.- Carry out the following measures:
– thickness or caliber of the body metal
– thickness or gauge of the bottom metal
11.- Tightness of the seam . Determine the free space – (Critical parameter)
– length of the body hook
– bottom hook length
– penetration of the body hook (critical parameter)
– overlap (Critical parameter)
13.- Check internal defects of the seam
4th Phase: Leaks
14.- Leak detection.
Let’s see each of these steps:
1st PHASE: VISUAL INSPECTION AND EXTERIOR STOCKINGS OF THE SEAM
A.- VISUAL INSPECTION
The first thing to do with a seam for analysis, is to visually inspect it from the outside. Many defects can be detected with the naked eye, or by direct touch, without the need to use any instrument and measuring material. The list of failures of this type are identified as “external defects of the seam”.
Some defects can be appreciated much more quickly by tactile recognition than by visual inspection, this first is done by running the finger inside and outside the seam, to check its roughness, presence of sharp edges, undulations, etc.
The main external defects that can be located by this system are:
Formation and unfinished crushing of the seam, caused by a 2nd incomplete operation, caused by a displacement of the container between the compression plate and the seaming mandrel. It is characterized because part of the seam has the profile of the 1st operation. See figure No. 17. It generates leaks. This defect is detected by a greater thickness of seam and a slight deformation of the bottom of the container.
Figure nº 17: Skating defect in a seam
This defect can be caused by:
– Insufficient pressure in the compression plate.
– The seaming rollers do not rotate freely.
– Worn-out mandrel
– Oil or grease on the mandrel or in the compression plate
– Inadequate positioning of the bottom in the mandrel by dimensions or shape, or also by too tight or loose.
– Bad timing of seaming operations
Presence of a sharp edge on the inside of the top of the seam. See figure no. 18. This very sharp edge is caused by the bottom material, being forced on the upper part of the lip of the mandrel during seaming. It normally occurs in the welding area of the body, but can also happen anywhere in the seam.
It is recognized by running with the finger the internal periphery of the seam. It is a dangerous defect, as it can cause a fracture of the metal below the cutting edge. If this happens there is a certain danger of leakage.
Figure nº 18: Sharp edge defect
It can be caused by:
– Wear on the seaming chuck.
– Profiles of too narrow routines
– Too tight of the seaming roll of second operation.
– Excessive pressure of the base plate.
– Mandrill too low in relation to routines.
– Compound of excessive seaming
This defect may appear more attenuated, without showing a sharp edge but simply a sharp edge. In this case the defect is minor and happens for the same reasons but at a lower intensity, it is designated as a living or straight edge.
Failure in the link between the body hook and the bottom hook. See figure No. 19. It is similar to the defect caused by a flattened flange, where part of it is bent against the body of the container, without wrapping with the bottom hook, but without protruding through the bottom of the seam. It supposes the presence of leaks.
Figure nº 19: False seaming
It can be caused by:
– Body eye damaged, hit or crushed.
– Eyelash of the body “achampiñonada”. That is, curved down, caused by poor adjustment of the eyelash, or by poor condition of the blinking tool.
– Damage to the curl of the bottom wing: bumps, marks …
– Incorrect positioning of the bottom on the seaming mandrel.
– Product or foreign matter deposited in the tab.
The false seams are a defect sometimes difficult to see, because they are covered by the bottom hook, they are always very dangerous. You have to detect them by observing the seam on the low side.
“Peak” in the assembly
Presence of a crest at the base of the seam, in the area of the lateral seam of the body. It consists of a projection of the bottom hook below the seam. See Figure No. 20. It involves the absence or decrease of the bottom hook in that area, and therefore the possibility of leakage. If it extends more than 1/3 of the length of the seam below it or by more than 10 mm along said seam, it is a major defect.
Figure nº 20: “Peak” in the seaming
It can be caused by:
– Excessive body hook
– Excessive amount of rubber
– Second operation performed with too much pressure
– Product or foreign matter inside the seam.
It is also called “fall in the union”.
Peaks in close
The peaks can occur not only in the area of the side seam, but at several points in the same seam, if these points are very pronounced they are called “V-peaks”.
Sometimes this peak can degenerate into a fracture at the base of the seam, very difficult to observe. It requires the use of a magnifying glass for your vision. Seams with a thickness less than that specified run the risk of presenting this defect.
The main cause of this defect is the excess pressure of the 2nd operation seaming roll and / or the base plate.
This defect presents the flange bent against the body, without being wound with the bottom hook and appears clearly below the base of the seam. See figure nº 21. When it manifests in a severe way, it affects the contiguous and inferior part of the body. It generates a defective union. It is similar to false seaming but more visible.
Figure nº 21: Flattened tab
The most frequent causes that cause this defect are:
– Damaged tab during handling, transport, filling or in the seamer’s feeder.
– Very achampiñonada eyelash.
– Deformed flange due to deposit of product or foreign matter.
– Malignment of the container in the closed position.
A similar problem can occur when it is the curl of the bottom that has suffered damage, giving rise to the defect “crushed crush”. In this case part of the bottom hook is completely unhooked from the hook of the body, and further downward against the body, exposing the bottom edge.
Great deformation of the bottom in the area of the tray, resulting in the separation of the hooks from each other, and part of the curl of the bottom rotated, so that the flange is exposed.
Its main reasons:
– Mandrel too low compared to the base plate, or better compression plate too high.
– Excessive pressure in the compression plate
– Cuttings imprisoned in the head of the seamer.
Seaming area that is not tight, caused by a jump of the seaming routines. It always occurs in an area adjacent to the side seam, especially when the seam is engargolada, which is when it has greater thickness, and causes the routines to “stumble” with it, and rise slightly at the exit of the seam.
It is observed when disassembling the seam and examining the bottom hook with respect to the wrinkles on both sides of the seam. Its location will depend on the direction of rotation of the routines.
Its causes are:
– Excessive seaming speed.
– Sewing too thick.
– Breakage of the adjustment arm of the routines arm.
– Roller bearing of 2nd operation too weak or slow
It is a rubbing or abrasion of the wall of the bottom tray, which appears after seaming, and is caused by a sliding of the bottom with respect to the mandrel.
It can cause a detachment of the protective varnish, with the probability of a subsequent corrosion of the metal if it affects the tin.
Its possible causes can be:
– Insufficient compression plate pressure.
– Damaged seaming mandrel.
– Excessive vertical slack of the mandrel shaft
– Oil or grease in the mandrel
Precisely to avoid this defect, it is not advisable to use mandrels with the knurled lip.
Defect that is manifested because part of the seam is not properly tightened, presenting an irregularity inside, that is, on the wall of the bucket. It is caused by a hole or nick in the lip of the mandrel.
It originates because the lip of the mandrel has been chipped or “nicked” as a result of a jam or other reasons.
Expulsion of compound
Small particles of compound that emerge from the base of the seam. Its origin is in the expulsion of the same outside the seam, between the bottom hook and the body. It can be presented in a part of the seam or in its entirety. See figure nº 22
Figure nº 22: Expulsions of compound
– Compound of excess seam
– Compound of wet seam or without drying sufficiently.
– Seaming roll of 2nd operation too tight
Laminations at the base of the seam
Mark or stripe in the vicinity of the base of the seam, parallel to the summit thereof, which shows the presence of a lamination in this area. It can damage the varnish protection.
Its most likely cause is the use of a 2nd operation seaming roll too tight or very narrow profile. It can also originate a worn or large 1st operation seaming roll.
Irregularities in the seaming
Although it is very infrequent, irregular appearance seams can sometimes occur, with a lack of uniformity in thickness and length, slight laminations in some areas of the same or punctual peaks.
The main cause of these defects can be in an inclined mandrel or compression plate, either due to defective assembly or deformation. If the diameter of the mandrel is small it also produces irregularities in the seam.
B.- EXTERNAL MEASURES:
They will be made in several points of the periphery of the seam. Only the measurements of three points located in the vertices of a hypothetical equilateral triangle inscribed in the background will be recorded. The closest point of the side seam must be at least 10 mm away from it.
In non-cylindrical containers, measurements shall be made at three points, one at the point opposite the lateral seam and the other two at the ends of the straight part of the larger side seal. In non-cylindrical containers, the most conflicting points for the seam measures are the radii. An oval container is considered that the curved major sides are the straight ones. Values should not be averaged.
MEASURE OF THE DEPTH OF CUBETA
The depth of the cuvette is measured from the top of the seam – top of the seam – to the base of the radius of the bottom cuvette wall. The depth of the original tray of the bottom is modified by the height of the lip of the seaming mandrel.
This depth varies according to the type of containers, and must not be less than the height of the seam. Its normal value after seaming must be 0.10 to 0.30 mm. deeper than the length of the seam, so that the mandrel allows the bottom and body a good coupling with the compression plate during the execution of the seam. In the case of the “mini seaming” this difference increases as we will see later.
In conventional container seams, the depth of the tub varies between 3 and 3.50 mm. although there are several exceptions, such as carbonated beverage containers, in which it has a greater value due to the convex configuration of the lid and the presence of the ring, and the same occurs in general with the easy-open covers.
The depth of the cuvette must be checked by means of an appropriate gauge, of the type shown in figure 23. To measure this depth, the gauge bar is placed on the top of the seam. The tip of the gauge is located at the lowest point adjacent to the wall of the tray, but away from the joint by at least 10 mm. It is also necessary to avoid that the horizontal bar rests on the joint or the tip touches some punched code on the bottom. In form containers, it is measured in four points, each located in the area of each radio
Figure nº 23: Measurement of the depth of the tray
Variations in the depth of the cuvette reveal the poor condition or adjustment of the tooling or the machine.
An increase in the depth of the tray, uniformly and to a much lesser extent than in the case of a “dingy bottom”, that is, without losing the double seam, is a defect caused by a mandrel adjustment in the bottom too hard , preventing the normal flow of material in the seam. It can also be caused by a lack of pressure in the base plate. Generates unbalanced hooks (short bottom hook).
If the depth increase of the bucket is not uniform, the causes may be the same or rather the lack of fullness the mandrel – mandrel warped -. The effects are the same.
MEASUREMENT OF THE SEAM LENGTH
It is about evaluating the height of the seam. To do it, a hook or micrometer is used, as can be seen in figure 24, that is, keeping the flat surface of the micrometer against the body of the container
Figure nº 24: Measurement of the length of the seam with hook or micrometer
The actual length of seam is also a good reference to intuit the quality of it, in relation to the theoretical type of seam that corresponds to it. This measure is related to the degree of tightening of the seam, the longer the seam, the greater the degree of tightness of the same, so it can be said that the length of seam depends on how the second operation seaming roll of the seamer works .
A first operation seaming roll of too wide or worn profile, loose or low in relation to the mandrel, will generate a large seaming length. If your profile is narrow, too tight or too high, it will cause a small seaming length.
A seaming roll of 2ª operation of big profile or worn, loose or low in relation to the mandrel, will even influence more on the length of the seaming, giving a big result. Otherwise it will give a small result.
The normal range of this measure is 2.6 to 3.2 mm, according to the formats and types of seam used, although there are also exceptions when it comes to a mini seam. In general, in the same package no difference in this length must be greater than 0.15 mm.
MEASURE OF THE FALL IN THE UNION
The fall in the joint is the increase in length that the seam undergoes at the point of intercession with the lateral seam. It is logical that at this point the seam increases its height, because in it, the body hook has a greater thickness, due to the overlap of two thicknesses of metal, necessary to perform the welding. This can translate into a decrease in the length of the bottom hook.
To measure this drop in the joint, use an ordinary micrometer or better a hookmeter, as indicated in figure # 25. It must be measured at the point of maximum fall.
The fall in the joint must never be greater than 20% of the average length of the seam. It comes expressed by:
Drop at the joint = Seaming length at the joint – Average length of the seam
Figure nº 25: Measurement of the fall in the union
Fall in the joint = A – average length of the seam.
In the area of the fall, the seam also has a greater thickness for the same reason mentioned above.
MEASUREMENT OF SEAM THICKNESS
It is another secondary indicator of the degree of tightness of the seam. It is a function of the metal thicknesses of the bottom and body and the pressure of the second operation seaming roll. It also depends on the weight of the compound applied. It should not vary more than 0.10 mm around the seam, although this reference is unreliable as there are many exceptions.
This measurement, like the previous ones, is carried out in three positions in the form of an equilateral triangle. To perform this measurement, a special gauge or a hook can be used. The use of the gauge gives us an accuracy much higher than the micrometer and is less susceptible to errors of measurement by the operator. See figure nº 26.
Figure nº 26: Measurement of the seaming thickness by means of gauge
When the hook meter is used, it should be slightly balanced with the index once it is placed over the seam, until the angle it forms with the horizontal is the same as the wall of the bucket forms with the vertical, as shown in figure no. 27, having to take special care not to press excessively the seam, as it may lead to a false measurement.
Figure nº 27: Measurement of the seaming thickness by means of a hook
There is another very simple means to measure approximately the thickness of the seam, is to use a “rule of seam control” (see figure No. 16), it presents a series of notches calibrated with different measures. The one that best fits the thickness of the seam will give us a fairly approximate assessment of its value in mm.
2nd PHASE: DISMANTLING THE SEAM
In order to determine a series of key factors for the integrity of the seam, such as degree of tightening, hooks of body and bottom etc, it is necessary to disassemble it. There are two ways to prepare the seam for your study:
1.- Total dismantling or “scrapping” of the same, for direct measurement.
2.- Sectioned for evaluation by projector
Be part of a series of common operations for both roads, although after each one requires a final preparation.
The steps to take are the following:
Common operations :
1º.- Cutting and extraction of the central panel of the bottom.
2º.- Cut of two opposite sections of the seaming
3º.- For study by disassembly
a.- Elimination by tearing of the rest of the panel
b.- Disassembly of the bottom hook
c.- Study of the disassembled elements of the seam
4º.- For study by projector
a.- Fold back the sections produced and separate them from the container.
b.- Uniformize the edges of the sections with fine sandpaper.
c.- Place the sections in the projector for evaluation
d.- Detach the cut sections of the bottom hook, for the independent study of both hooks.
Enter in detail in each of the steps marked
1º.- CUTTING THE CENTRAL PANEL OF THE ENDS
It is the operation prior to the scrapping or sectioning of the seam. It is a necessary task to be able to access it easily. For this, it is necessary to use a bacteriological opener or, in its absence, a curved tip scissors for sheet metal cutting.
Figure 28 illustrates the operation of the bacteriological opener. It is necessary to nail it in the center of the bottom, and adjust the form opener, to cut a disc with such a radius, that it leaves a strip of material without splitting between 6 to 12 mm, from the wall of the bucket to the cutting edge .
Figure nº 28: Cutting the central panel of a background
Once the central disk is cut, the same is extracted. It is advisable to use safety gloves for this operation, and thus prevent risks of injuries with the sharp edges of the metal.
2º.- CUTTING A SEAM SECTION
This operation is imperative only if the seam is to be inspected by means of its projection, but it is advisable to do it in either case. To cut the cut, starting from the container with its central panel removed, you can use a special saw of fine cut, making two cuts in the seam. To obtain a clean cut, without burrs or deformations, it is necessary to use specific saws for this purpose in the market, which have two circular blades of about 100 mm in diameter, with a thickness of 0.35 mm, with a pitch of 24 teeth / inch and a spin speed of 520 rpm See figure # 29
Figure nº 29: Saw for cutting seams
The cuts must be made aligned with the diameter of the container and parallel to its axis, they should be located at 90º to the right of the side seam. Then another section must be cut on the opposite side of the container. See figure nº 30
Figure nº 30: Cutting a section of the seam
3º.- PREPARATION OF THE SEAM FOR DISMANTLING
Its purpose is to detach the bottom hook of the container, to then be able to analyze it separately.
Photo nº 31: Sectioned container
Starting from the sectioned container (Photo nº 31) it is necessary to tear with the help of pliers or special tongs for this purpose, the rest of the background that has remained after cutting the central panel, between the cut and the top of the seam (Figure nº 32).
For this you need a little practice, although it is an easy task. Using the tongs, pull in the opposite direction to the radius of the panel and around the entire length of the seam. A strip of metal that includes the rest of the panel and the wall of the tray will be torn. In this way we will have the bottom hook independent of the rest of the container.
Figure nº 32: Preparation to extract the bottom hook
To remove the hook from the bottom you have to hit, gently and carefully, down the cutting edge of it with the help of the flat part of the tongs. See sequence of extraction in photos 33. This way we will be able to disengage little by little the hook of the bottom of its seaming.
Figure nº 33: Way to extract the body hook
In this way, we will detach the hook from the body with great caution, leaving the assembly as shown in drawing no. 34. We have finished the preparation of the can for the examination of its seam.
Figure nº 34: Package prepared for the examination of the seaming by scrapping
4º.-PREPARATION OF THE SEAM FOR PROJECTION
Starting from a container previously prepared as indicated in figure 230, that is to say, once two sections or sections of seam have been cut, the section produced must be folded back, as shown in figure 35, and separated from the rest of the container by cutting it with pliers, taking care not to affect the seam.
Figure nº 35: Separation of the cut section of a seam
The surface of the section cut, will present a rough and burned by the action of the saw, it is necessary to soften and standardize the section with the help of a fine sandpaper. Now we can take this fragment on the projector and visualize it for evaluation.
With great care, you can detach the hook from the bottom of the seam fragment, and thus be able to check the trace produced by the routines when acting on the mandril, routing jump, drop in the joint and other defects.
3rd PHASE: INTERNAL MEASUREMENTS
With the seam prepared we enter the measurement phase of the same parameters
5º.- METAL THICKNESS OF THE BODY METAL AND ENDS
The actual thicknesses of the metal of the body and background must be known, since they are necessary data for later calculations. To perform this measurement, a gauge of the type shown in Figure No. 36 will be used.
Figure nº 36: Micrometer for measuring metal thickness
A micrometer with digital or dial reading can also be used. Whatever the instrument used, it must be checked at regular intervals, to be sure of its accuracy.
To make these measurements on a sample container, triangles should be cut in the manner indicated in figure 37, on which we will determine the metal thickness.
Figure nº 37: Points of thickness measurement on a container
The point P, in which we will perform the measurement, must be located at least 10 mm from the edge of the container.
If the triangular samples are varnished and / or lithographed, if you want to have a reading without errors, you must remove these coatings, applying a cotton impregnated with solvent.
6.- TIGHTENING OF THE SEAM. DETERMINATION OF FREE SPACE
It is a critical parameter and about it we refer to what we will discuss in detail in the section “ACCEPTABILITY OF SEAM”
7.- MEASUREMENT OF HOOKS
The bottom and body hooks should be measured in two opposite points in two pieces, and in the 10 and 14 o’clock position, with 12 being the side seam, in three pieces. The lengths of the hooks of body and bottom, can be measured by means of a hookmeter if the seam has been disassembled, but if we want to obtain a much more precise measurement, a seaming projector should be used if the seam has been previously sectioned.
If we use the measurement by ganchimetro with disassembly in the seaming, we can take stockings in several points of both hooks and then remove the stocking, with what we will obtain a more real value of its measure. If a projector is used, the measurement will be more accurate but only at two points of the seam, which makes its general vision more incomplete.
The projectors used to control seams are of the type shown in Figure No. 38 or the like.
Figure nº 38: Seaming projector
In this equipment you can take direct values of the lengths of hooks, through an appropriate placement of the seam, by means of the reading that the mobile arms give us in the graduated rule. See figure nº 39.
Figure nº 39: Measurement of a seaming on the projector
The hook must be placed on the device so that a clear image is obtained on the screen.
Penetration of the body hook
It is a critical parameter and about it we refer to what we will discuss in the section “ACCEPTABILITY OF SEAM”
It is a critical parameter and about it we refer to what we will discuss in the section “ACCEPTABILITY OF SEAM”
9.- VERIFICATION OF INTERNAL SEAM DEFECTS
A good indication of the seaming pressure is the impression produced on the internal part of the body, by the pressure exerted by the seaming routines on the mandrel. This indication is not a guarantee by itself of adequate seaming pressure.
One way to improve the grip of the mandrel, is to knurl the surface of the lip of the same, but this practice is not recommended, as it damages the protective varnish of the bottom in this area. It is always better to use a smooth mandrel and ensure that its adjustment is adequate to ensure a good fixation.
Let’s enter in the description of the main visual defects that can occur in a disassembled seam:
1º.- Fall in the union
It is the presence of a ripple, more or less important, on the inner face of the bottom hook, in the area of the side seam. The reason that a ripple occurs at this point is due to the higher pressure generated there, due to the greater number of layers of tinplate existing in it. See figure nº 40
Figure nº 40: Fall in the union
In the case of containers with lateral seam engargolada – or encased -, with or without side cement in it, this ripple will be quite greater caused by the metal layers that form the engargolado in this area.
It is therefore necessary to examine the interior of the hook, and assign it a value as “drop in the joint” according to a reference scale or by measurement.
The “internal drop of the joint” must not exceed 50% of the length of the bottom hook. A 50% drop produces an overlap in the area of the union of only 20% of the normal value
It is also convenient to carefully examine the inside of the bottom hook on both sides of the assembly, to detect irregularities, seaming roll jump, etc.
2nd.- Mandrel’s lipprint
It is the impression produced in the internal part of the body as a reaction to double seam. See figure 41. It is the result of the pressure exerted by the seaming roll of the second operation during the seam.
His visual inspection allows, together with other factors, to appreciate if the tightness of the seam is correct. Its total absence indicates lack of tightness, although other measurements are correct. It should have a light, uniform, clearly visible matte appearance, free of irregularities and always around the internal part of the body in the seam.
An excessive footprint, a marked step, is an indication that the routines or mandrel used may be erroneous, or that the seaming operation has been carried out with too much pressure. A too large mandrel will logically produce an excessive footprint.
Figure n ° 41: Lip imprint of the mandrel on the body hook
It is a good indication of the seaming pressure but it does not guarantee that it was correct, so it must be used in conjunction with other inspections.
3º.- Visual inspection of wrinkles, waves and other irregularities in the hooks of the body and / or bottom.
. All these irregularities are produced by lack of adjustment in the tooling, or by not being the same ideal, and can be of many different types. Among others are:
– Eyelash of the body with wrinkles
– Sliding container
– Diameter of the small mandrel
– Inclined mandrel
– Seaming roll 2nd operation loose
– Profile of the second operation seaming roll worn
– Clearance of the seaming roll of 2nd operation on its axis
4th PHASE: LEAKS
Regardless of the analysis of seam due to its destruction, it is necessary to verify in other complementary samples, the absence of leaks and therefore the adequate sealing of the containers. The amount to be tested will be taken according to a predetermined sampling type, with the help of the corresponding tables, depending on the size of the batch and the chosen quality level.
The procedures to be applied vary according to whether it is empty or full.
a.- Empty boat
Air under pressure is used in two possible variants.
1ª.- By means of a head, pressurized air is supplied inside the container, applying at the same time with the help of a brush, soapy water in the seams. If there are air leaks through them, bubbles will form at the exact point of the leak.
2nd.- Submerge the container completely in a tank with water, and apply air inside by means of a suitable head. If the seam is defective or has pores, a bubbling occurs due to the exit of air through the pores.
The air pressure used in both cases is of the order 1.5 Kg / cm2 for electrically welded side seam packs and size equal to or less than 1 kg. For larger sizes or cemented seam is lower.
These procedures are used to sample lots, but in manufacturing packaging lines other techniques are used to verify the tightness in 100% of the production.
b.- Boat full or closed at both ends
There are complex procedures at the laboratory level, such as the Lefebvre method, which requires a certain specialization. There are also simpler procedures like the one we indicate in our work on this Web: