1 Overview
The metal packaging container has many advantages, for example, it has excellent barrier properties, mechanical properties, good convenience, good surface decoration, and convenient waste disposal. However, the chemical stability and economic efficiency of metal packaging containers are poor.
Leaking cans is a common phenomenon in the beverage industry. Poor seal sealing is one of the reasons for this phenomenon. Therefore, it is important to grasp the principle of sealing of metal packaging containers and strictly produce them in accordance with the process specification, so as to increase the operational level of operators and reduce the occurrence of this phenomenon.
Sealing of metal packaging containers is generally achieved by crimping. Crimping can be further divided into double crimping, triple crimping and micro crimping.
2 Double crimping technology
2.1 double curl form
Double curling is divided into two types: flat roll and round roll, and the application of flat roll structure is more. In addition to the double hemming and double hemming, there is a special double hemming form, which is the welded double lap joint shown in Figure 2 [3].
2.2 The principle of double hemming
The double curl is completed by the head roll and the two rolls, respectively. The head roller is moved radially relative to the can body. The friction force between the roller groove and the round edge of the can lid causes the round edge to curl normal to the groove curve, completing the initial shaping of the curl, as shown in FIG. 3 . After the first roll operation is completed, the roll seal position is exited. The two rollers will begin to be in contact with the pre-formed hemming, and will begin to move radially to the can, pressing the hemming to complete the enclosing operation, as shown in Figure 4.
There are two cases according to the different sealing mechanism of the sealing mechanism [2]: First, the tank rotates with the supporting tray and the indenter, and the crimping roller rotates and moves radially to the tank axis to complete the double curl operation; The second is that the tank body is fixed and the roll seal roller rotates and rotates around the tank body. At the same time, it moves radially to the tank body shaft to complete the curling operation.
When the special-shaped cans are sealed, the above-mentioned second method is adopted, the cans are not moved, and the rollers are rotated around the cans to complete the curling operation.
3 Triple hemming technology
3.1 Triple hemming structure
Tri-folding, also known as seven-layer crimping, is the primary form of closure that connects the closed drum body to the bottom (top) of the barrel. Compared with the double crimping, there are two more curls, which increase the overlap of the barrel body and the top (bottom) of the barrel, thereby improving the sealing performance of the steel barrel and giving the steel barrel a strong impact resistance.
In addition, compared with the double crimping, the triple crimping has higher technological requirements for the crimping process, and generally requires three rollers for forming, so the equipment and technical requirements are relatively high.
3.2 Triple hemming processing
Except for a few barrel mills equipped with three pressure rollers on the sealing machine, most of the manufacturers are working on two devices, the pre-roller and the sealing machine, that is, installing the first roller on the pre-roller, that is, the pre-rolling pressure. The roller completes the pre-rolling deformation of the bottom (top) semi-finished product, and the sealing machine is equipped with the second and third rollers to complete the seven-layer hemming process.
4 Micro-crimping technology
Micro-crimping is a new molding process technology that optimizes metal cans' weight and processing costs. Compared with traditional crimping, the micro-crimping structure has only about half the size of the traditional standard crimping, as shown in Table 1.
Miniature rolled metal cans are reduced in size. Can they meet the protection of goods and the integrity of packaging containers during the distribution process? To this end, the Center for Packaging Technology of the Brazil Academy of Science and Technology made a series of comparative tests. The test results show that the properties of the miniature crimped metal packaging containers all meet the requirements in circulation.
The micro-crimping technology shows outstanding advantages in all kinds of metal containers. Not only reduce the size of the structure, but also reduce the amount of sealant. And productivity has increased significantly, which in turn has reduced packaging and transportation costs.
5 Curling quality defect detection and solution
The quality of crimping directly affects the protective function of the metal packaging container on the interior products. The curling defect [1] can lead to a tight seal, which can cause the loss or deterioration of the contained product, and at the same time it also makes it possible to potentially breed bacteria that may cause gourd poisoning (sausage poisoning). Each metal can must be checked for sealing quality before it is used. Once the crimp quality defect is found, the sealing process must be re-crimped in accordance with the relevant procedures and requirements.
5.1 Influence of Roll Seal Groove Curves
Rolling rollers, upper pressure heads, and tray plates are often referred to as the three elements of the sealing device. As a basic component, it is also a basic factor that affects the curling.
Experience has shown that the quality of seals depends critically on the profile of the curved wheel groove. In double hemming, the main function of the hemming is the curve of the groove rolling of the head roller. The curve of the main part is the Archimedes spiral, the curvature of which is determined by the winding speed, and the width and depth are Tank type is related to material properties and thickness, and the groove curve widens and deepens as the diameter of the finished product tank increases, and widens and deepens as the material is thickened. As shown in Figure 5. In order to facilitate production, the design is replaced by 2 to 4 arcs. The extent of the arc depends on the width and width of the groove curve, while the width of the curve replaces the arc section.
5.2 crimp sealing technical requirements
The corresponding explanation of the terminology used in the crimping sealing technical requirements is shown in Figure 6 [5]. In order to guarantee the qualified production of metal cans, the crimping must meet the following requirements during the production process.
1) Tightness ≥ 50%. Tightness refers to the tightness of the crimp seal, which is generally measured by the degree of wrinkle of the cover hook. The degree of wrinkle refers to the degree of wrinkling of the inner periphery of the lid hook after the curl is disintegrated, that is, the percentage (%) of the buckling portion to the length of the entire lid hook. Tightness = 100% - wrinkle degree. Qualified tanks require tightness greater than 50%, ie tanks with a wrinkle degree greater than 50% are unqualified.
2) The joint cover hook integrity rate is ≥50%. Cover hook integrity refers to the extent to which the cover hook hangs. The crimping cover hooks at the seams hang downward, causing the cover hooks to have insufficient width, thereby affecting the tightness of the bead.
3) Splice rate (K) ≥ 50%. The overlap ratio indicates the degree of overlapping of the inner cover hook and the body hook, and its size is equal to the ratio of the spliced ​​length, the spliced ​​length plus the gap length at both ends, and the higher the splicing ratio, the better the hem tightness. The clearance between the cover hook space and the hook space should be as small as possible to improve the overlap ratio of the curling edge.
5.3 Crimping defects and solutions
1) Vertical lip. A curled lip is a smooth overhang that appears below the normal curl. As shown in Figure 7-(a). This defect usually occurs at the edge of the three-piece can, because the thickness is greater here than elsewhere.
Factors that may cause this kind of defect are: excessive pressure on the rotary table of the tray support, loose pressure during the operation of the first pass, residual content on the crimped edge, defects in the metal can (such as bends or dents, etc.) ), the first round of roller wear and so on.
For different factors, there are different solutions: reduce the pressure on the rotating stand of the tray; tighten the operating pressure of the sealing on the head channel; carefully remove the lid of the can and the edge of the can before sealing the lid to ensure that there is no content or other impurities Residues; detect damage before use in metal cans; replace new head roller seals.
2) Sharp edges. Crimped sharp edges are sharp edges on the inside of the seal. As shown in Figure 7-(b). Possible causes of sharp edges: The operating pressure of the head and/or two roller seals is too tight; there is residue in the curling; the pressure on the rotary table of the tray is too high; the bead roller and/or chuck are worn out. .
Remedy: Make the head and/or roller operation pressure loose; Carefully remove the lid of the can and the edge of the can before the lid is sealed to ensure that no contents or other impurities remain; Reduce the rotation of the tray Pressure on the bench; replace the roll seal and/or chuck.
3) Broken. Crimping refers to the phenomenon that the curled edge is too tight and the curled edge of the outer layer is broken. As shown in Figure 7-(c). Generally, it is caused by the over-tightening of the head roll and the second roll. The solution is to loosen the pressure of the two roller seals appropriately.
4) Fake volumes. Fake rolls are a serious flaw in the crimping process and can lead to leaks in the contents of the metal container. A detailed inspection of the metal cans can be performed from the appearance. If the can lid and the can body are not hooked together but are only flattened together, then this is a fake roll. As shown in Figure 7-(d).
Factors that may cause false roll: The lid and can body are not evenly bent or damaged before crimping; there is residue in the curl or the product in the metal container is filled (filled) too full; Loose; two rollers are over-tight.
Refer to the solution: check the lid and can body before wrapping to see if there is any damage; carefully remove the can lid and can body edge before sealing the can lid to ensure that no contents or other impurities remain in it, and check whether the contents are filled ( Filling) is too full; increase the pressure of the head roller sealing roller; reduce the pressure of the second roller sealing roller.
5) Incomplete crimping. Incomplete crimping refers to a curling defect that occurs because the crimping process is not completed or the pressure in some areas of the crimping process is too loose. As shown in Figure 7-(e).
The possible causes are: too tight or too loose pressure on the rotary table of the tray holder; wear of the sealing cartridge; the roller cannot rotate freely; there is grease on the sealing chuck and/or rotating table.
Corresponding solutions: reduce the pressure on the rotating tray of the tray; replace the sealing chuck; clean, oil, and overhaul the roll to ensure that it operates without obstruction; clean the sealing chuck and/or tray Tank plate rotating table.
In addition to the above-mentioned defects, in addition to avoiding the above-mentioned defects, the metal sealing cans have to be tested for flaws such as fast openings, large collapses, curled teeth, tongues, skipping, and crimping [6]. According to the standard in the pressurization test or decompression test whether there is a leak. Other industrial metal cans require uniform crimping, no wrinkles, burrs, and iron tongues, and airtight tests, hydraulic tests, and drop tests should comply with the relevant standards.
6 Conclusion
Sealing of metal packaging containers is generally based on crimping, improved as needed, and accompanied by relevant metal seals. For the sealing defects that may appear in the crimping seal, the causes of these defects are analyzed, and the corresponding solutions are given.
references
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