The main defects of aluminum foil are:
(1) Pinholes. Pinholes are the main defect of aluminum foil. The dust size in the raw materials, rolling rolls, rolling oil, and even air reaches 6 μ Entering the roller gap around m will cause pinholes, so 6 μ It is impossible for aluminum foil to have no pinholes, and it can only be evaluated based on its quantity and size. Due to the improvement of aluminum foil rolling conditions, especially the effective filtration of dust and rolling oil, and the convenient setting of roll changing systems, the number of aluminum foil pinholes is increasingly dependent on the metallurgical quality and processing defects of the raw materials. As pinholes are often the shedding of raw material defects, it is difficult to find a corresponding relationship with the original defects. It is generally believed that pinholes are mainly related to gas content, inclusions, compounds, and component segregation. Effective aluminum liquid purification, filtration, and grain refinement can all help reduce pinholes. Of course, using alloying and other methods to improve the hardening characteristics of materials can also help reduce pinholes. 6 high-quality hot-rolled materials rolled μ The number of pinholes in m aluminum foil can be below 100 per square meter. When the purification of cast and rolled materials is good, 6 μ The number of aluminum foil needle holes is below 200 per square meter. During the aluminum foil rolling process, there are many other factors that can cause pinholes, even catastrophic, with thousands of pinholes per square meter not uncommon. Effective filtration of rolling oil, short-term replacement of rolling rolls, and dust prevention measures are all necessary conditions for reducing pinholes in aluminum foil. Using high rolling force and low tension rolling can also help reduce pinholes.
(2) Uneven roller printing, roller eyes, and gloss. It is mainly caused by aluminum foil defects caused by rolling rolls, divided into three types: point, line, and surface. The most prominent feature appears in three cycles. The main reason for this defect is: incorrect grinding of the rollers; Foreign object damage to the rolling mill: incoming material defects imprint damage to the rolling mill; Roll fatigue; Roll to roll collision, slipping, etc. All factors that can cause surface damage to rolling rolls can pose a threat to aluminum foil rolling. Due to the high smoothness of the aluminum foil rolling roller surface, even slight uneven gloss can affect its surface condition. Regular cleaning of the rolling mill, maintaining its cleanliness, ensuring the normal operation of the roller cleaner, regular roll replacement, and reasonable grinding are the basic conditions to ensure the uniform and consistent surface of the aluminum foil after rolling.
(3) Wrinkles. Due to severe poor flatness, wrinkles may form during aluminum foil winding or unfolding, which is essentially due to insufficient tension to flatten the foil surface. For a device with a tension of 20MPa, the shape of the foil surface should not exceed 30I. When it exceeds 30I, it will inevitably wrinkle. Due to the fact that aluminum foil often bears greater tension during rolling than subsequent processing, some of them only exhibit poor plate shape during rolling, including incorrect roller grinding, incorrect roller shape, poor incoming plate shape, and incorrect adjustment of plate shape.
(4) Highlights, bright marks, bright spots. The bright spots, marks, and spots on the double joint surface caused by improper use of double joint oil are mainly due to insufficient strength of the double joint oil film or uneven rolling surface, resulting in uneven deformation during rolling, with a rough or foreign object pressed in appearance. The effective measures to solve such defects are to choose a reasonable double oil, maintain the cleanliness of the incoming material, and ensure the uniformity of the roller surface. Of course, it is also necessary to change the amount of compression and choose excellent aluminum plates.
(5) Thickness difference. Difficulty in controlling thickness difference is a characteristic of aluminum foil rolling. A thickness difference of 3% may not be difficult in sheet production, but it is very difficult in aluminum foil production. The reason is that the thickness is thin, and other trace conditions can have an impact, such as temperature, oil film, oil and gas concentration, etc. Rolling aluminum foil can reach several hundred thousand meters per roll, with a rolling time of about 10 hours. As time goes on, thickness differences are easily formed, and the only means of adjusting thickness is through tension speed. These factors have all caused difficulties in thickness control during aluminum foil rolling, so truly controlling the thickness difference within 3% requires many conditions to ensure, which is quite difficult.
(6) Oil stains. Oil stains refer to the excess oil on the surface of aluminum foil after rolling, which is not the rolling oil film. These oils are often thrown, splashed, or dropped on the foil surface from the neck of the roller or from the outlet of the rolling mill, and are dirty and have complex compositions. Oil stains on the surface of aluminum foil are more harmful than other rolled materials. Firstly, as most aluminum foil products are used as decorative or packaging materials, they must have a clean surface; The second reason is that its thickness is thin, which can easily form bubbles during subsequent annealing, and due to the high amount of oil, too much residue is formed in this area, which affects its use. The number of oil stains is an important indicator for evaluating the quality of aluminum foil.
(7) Water spots. Water spots refer to white spots formed by water droplets on the foil surface before rolling, which can affect the surface condition of the foil surface in mild cases and cause strip breakage in severe cases. Water spots are formed due to water droplets in the oil or falling on the foil surface in the rolling mill. Controlling the moisture and water source in the oil is the only measure to avoid water spots.
(8) Vibration marks. Vibration marks refer to periodic transverse waves on the surface of aluminum foil. There are two reasons for the occurrence of vibration marks: one is due to the formation of roller grinding, with a cycle of about 10-20mm; Another type is vibration caused by discontinuous oil film during rolling, which often occurs in a speed range with a period of 5-10mm. The root cause of vibration marks is insufficient oil film strength, which can usually be eliminated by improving the lubrication state.
(9) Tension line. When the thickness reaches below 0.015mm, parallel stripes are formed in the longitudinal direction of the aluminum foil, commonly known as tension lines. The spacing between tension lines is about 5-20 mm. The smaller the tension, the wider the tension lines, and the more obvious the stripes. When the tension reaches a certain value, the tension line is very slight or even disappears. The smaller the thickness, the greater the possibility of producing tension lines, and the greater the possibility of producing tension lines in double rolling compared to single rolling. Increasing tension and roller roughness are effective measures to reduce and eliminate tension lines, and large tension must be based on good plate shape.
(10) Sewing. Slitting is a unique defect in foil rolling, which cracks straight along the longitudinal direction during rolling, often accompanied by metal wire threads. The fundamental reason for seam opening is the entrance side folding, which often occurs in the middle, mainly due to loose middle or poor rolling of incoming materials. Severe slits cannot be rolled, while slight slits may crack during subsequent slitting, often resulting in a large amount of scrap.
(11) Airways. During the rolling process, there is a strip like crushing with a droplet like curve at the edge and a certain width. The mild airway is not crushed and appears as a white strip with dense pinholes. The presence of dense pinholes at the front and rear ends of the crushed aluminum foil is the main indicator for identifying air passages and other defects. The airway comes from raw materials, so it is very important to choose materials with low gas content as aluminum blanks.
(12) Rolling defects. The defects in coiling mainly refer to looseness or looseness inside and tightness outside. Due to the limited tension that aluminum foil can withstand, it is difficult to roll hard coils. The most ideal approach is to obtain a tight and loose roll inside and outside, while sufficient tension is a condition for forming a certain tension gradient. So, the quality of coiling ultimately depends on the shape of the plate. The inner loose and outer tight coils will form transverse edges, while the loose coils will form ellipses, which will affect future processing.
Although there are many types of defects in aluminum foil rolling, the main manifestations are pinholes, roller holes, slits, and air passages characterized by pores; Oil stains, uneven gloss, vibration marks, tension lines, water spots, and bright spots characterized by surface conditions; Poor flatness, wrinkling, folding, and coiling that affect subsequent processing; Thickness differences characterized by size. In essence, the only type of defect unique to aluminum foil is pinholes, and other types of defect boards also have the same, but the severity or requirements of the performance are different.
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