Aluminum can lids are can lids with notches on the opening part and a pull ring for easy opening. According to the different opening methods, they are divided into hole-opening tear-off can lids (Ring Pull Type——RPT), hole-opening pressure-retaining can lids (Stay On Tab——SOT) and full-open can lids. Opening force is an important part of aluminum can lid quality control, to ensure that canned food is easy to open and not easily damaged at the notch during storage and transportation.
There are many reasons for the instability of aluminum can lid opening force
1. Aluminum factors affect opening force
The opening force of can lids produced by different alloy aluminum materials is different. Aluminum can lids are generally made of 5052, 5182 and other alloy aluminum materials with excellent can opening performance. The 5182 type containing 4%-5% magnesium and 0.35% manganese is a hard material with high compressive strength (630kpa) suitable for making beer or carbonated beverages. 5052 soft material is often used for can lids with low internal pressure resistance (310kpa) such as juice and tea. Experiments have shown that the opening force of can lids produced with 5182 material is about 1-2N greater than that of can lids produced with 5052 material.
The opening force of easy-open lids produced by different manufacturers of aluminum materials is different. In order to ensure the quality of easy-open lids, domestic aluminum easy-open lids are currently mainly imported.
The texture of aluminum affects the opening force. Aluminum is formed by secondary cold rolling. Its forming texture can be clearly seen under a microscope. Experiments have shown that the opening force of easy-open lids with the same texture as the pull ring is 0.3-0.5N greater than that of lids with transverse textures.
The storage time of aluminum affects the opening force. Magnesium-aluminum alloy softens during storage at room temperature.
Therefore, the storage period of aluminum and easy-open lids also has a certain influence on the opening force. In view of the influence of aluminum opening force, the inspection, batch differentiation and inventory management of incoming materials from suppliers should be strengthened, and appropriate modifications should be made to the processing technology and enterprise standards according to different profiles in accordance with the requirements of national standards.
2. The residual amount of the engraved line does not meet the specifications and affects the opening force
The specification of the residual amount of the engraved line is formulated according to the requirements of the national standard for the opening force, combined with factors such as materials and equipment. It is proportional to the opening force. The residual amount is small, because the material is thinner than the surrounding area, the strength is reduced, and it cracks and opens under the action of shear force. The residual amount is an important basis for adjusting the mold and controlling the opening force. Its precision requirement is very high, in microns. The changes in the machine oil temperature and mold temperature during production will change the change in the residual amount at the bottom dead point of the stamping, causing the opening force to exceed the standard. First, check whether the residual amount has a large positive and negative deviation or is not parallel to the left and right. If necessary, adjust the knife edge mold.
3. The abnormality of the engraved line mold affects the opening force
The wear of the engraved line knife edge causes a large opening force. The upper die blade of the engraved line is a 50° sharp angle, and the front end of the blade has a flat edge of 0.038+0.013-0.000mm. The section of the engraved line is a trapezoid. Under the action of external force, the trapezoidal cut corner is subjected to concentrated force and is easy to crack. After the blade is worn, the front blade becomes an R angle. The section of the engraved line after stamping is circular. Under the action of external force, the force is uniform, not easy to crack, and the breaking force is increased.
Abnormal influence of the engraved lower die. The RPT juice cap blade lower die is designed with two steps. The residual thickness at the opening is about 0.05-0.06mm smaller than the fully opened part to ensure that the engraved line is easy to open, and the fully opened part meets the requirements of the drop test. If the starting points of the two steps of the lower die are offset forward and backward, the R angle at the step joint is improperly polished, or the plane is not parallel to the left and right, and the surface finish is rough, it will cause abnormal opening. To solve the influence of the mold on the opening force, preventive inspection is mainly used. Generally, after the upper blade mold on the tangent line is punched 30-50 million times and the lower die is punched about 100 million times, it is necessary to use a microscope of more than 40 times to check its surface finish and blade wear, and combine the residual thickness and opening force data analysis to determine whether to repair or replace the mold.
4. The pull ring and the cover fail to fit tightly and affect the opening force
The riveting of the pull ring and the cover is a key point in the process of aluminum easy-pull covers. When the pull ring or the base cover is poorly formed, the pull ring cannot sink to the bottom of the base cover and rivet tightly with the cover, resulting in an unfixed opening fulcrum and affecting the opening force. There are two main situations:
The punching needle of the pull ring rivet is worn or the lubricating oil is insufficient during punching, causing the rivet hole burr to deform or reverse hole, and the pull ring bottom cannot sink to the bottom of the base cover during riveting.
The rivet forming mold at the third station of the cover mold is worn, resulting in a large R angle of the base cover blistering, and the pull ring cannot sink to the bottom of the base cover. To prevent the influence of riveting on the opening force, it is necessary to regularly grind the punching needle, regularly inspect the base cover and pull ring strips at the first three stations, and pay attention to the control of the lubricating oil amount of the pull ring.
5. Loose cover disc affects the opening force
The disc of the aluminum easy-pull lid is loose. When opening, the disc deforms and the stress is not concentrated. It is necessary to increase the opening force so that the notch can crack and open. The disc is loose, which mainly affects the opening force. There are three factors:
Improper adjustment of the disc reinforcement rib size. During the manufacturing process of the aluminum easy-pull lid, the disc deforms and relaxes after stretching and stamping. Therefore, a disc shrinkage reinforcement project is designed in the fourth station of the lid mold. If the project size is not adjusted properly, the expected disc shrinkage reinforcement effect cannot be achieved, which affects the opening force.
The abnormal riveting of the upper and lower dies at the sixth station of the lid mold affects the opening force. In order to ensure the precise riveting of the pull ring and the lid, a finger ring positioning punch for positioning the pull ring and a positioning cup (or positioning needle) for positioning the lid are designed in the sixth station. During stamping, the positioning punch and the positioning cup are lifted by the spring to contact the center and the shoulder of the back of the cover. If there are too many aluminum chips in the two molds or the spring is too hard, the finger ring positioning punch is easy to perform secondary stretching on the cover plate surface during high-speed stamping, causing the plate surface to loosen.
Improper adjustment of the engineering dimensions of the first three stations of the cover mold causes the plate surface to loosen. The bubble of the first station of the cover mold is too high, and the precision stamping of the second and third stations is too tight, causing serious deformation and relaxation, which affects the contraction and strengthening of the fourth station.
6. Non-standard testing instrument methods affect the opening force
Reasons for testing instruments. The RPT type easy-pull cover opening tester simulates the manual can opening method and is designed to tilt the cover surface 30 degrees positively. Domestic cap-making companies generally use this type of digital display automatic tensile tester, but some small companies still use hand-cranked flat pullers. Due to uneven manual force and abnormal opening angles, it will create a false impression of high opening force.
Abnormal testing methods. Aluminum easy-open lids need to be baked in an oven at 200°C for 10 minutes before testing their tensile strength. After natural cooling, the test can be carried out again. This is a simulation test based on the sterilization process of the can lids in the canning oven and the filling plant. This process is actually a heat treatment process for metal materials. If the temperature, time and cooling method are not well controlled, the opening force of the easy-open lid will also be affected to a certain extent.
Summary
The opening force of aluminum cans is affected by factors such as aluminum material, residual amount of score lines, mold, fit between pull ring and lid, looseness of lid plate, and testing instrument method. The opening force is an important part of its quality control, and it must be easy to open and not easy to break.
