Defects in aluminum foil

- Dec 29, 2018-

The main defects of aluminum foil are: 

(1) pinhole. Pinhole is the main defect of aluminum foil. In the raw materials, rolls, rolled oil, and even the dust size in the air to reach the 6μm or so into the roller seam will cause pinhole, so 6μm aluminum foil without pinhole is impossible, can only use how much and size to evaluate it. Because of the improvement of aluminum foil rolling conditions, especially the effective filtration of dust-proof and rolling oil and the setting of convenient roller replacement system, the number of aluminum foil pinhole is more and more dependent on the metallurgical quality and processing defects of raw materials, because the pinhole is often the loss of raw material defects, it is difficult to find the corresponding relationship with the original defects. It is generally believed that the pinhole is mainly related to gas content, inclusions, compounds and component segregation. The use of effective aluminum purification, filtration, grain refinement are helpful to reduce pinhole. Of course, the use of alloying and other means to improve the hardening characteristics of the material also helps to reduce the pinhole. High-quality hot rolled rolling 6μm aluminum foil pinholes can be below 100/㎡. When the purification is better, the 6μm aluminum foil pinhole is below 200/㎡. In the aluminum foil rolling process, other causes of pinhole factors are also many, even catastrophic, thousands of pinholes per square meter is not unusual.

The effective filtration of rolling oil, short-term replacement of rollers and dust control measures are necessary to reduce aluminum foil pinhole, and the use of large rolling force, small tension rolling will also be helpful to reduce pinhole. 

(2) Roller printing, roller eyes, luster uneven. It is mainly caused by the roll aluminum foil defects, divided into points, lines, faces three kinds. The most striking feature is the emergence of three cycle periods. The main causes of this defect are: incorrect grinding of rollers; damage rolls of foreign objects: incoming defects, roller fatigue; roller impact, skidding, etc. All the factors that can cause the surface damage of the roll can create harm to the aluminum foil rolling. Because the aluminum foil rolling roller surface finish is very high, the slight gloss unevenness will also affect its surface state.

Regular cleaning of the mill, to maintain the cleaning of the mill, to ensure the normal operation of the roller, regular roller change, reasonable grinding, are to ensure that the aluminum foil after rolling surface uniform and consistent basic conditions. 

(3) Wrinkle. Because of the serious bad shape of the plate, in the aluminum foil coiling or expansion will form a wrinkle, its essence for tension is not enough to make the foil surface leveling. For tension dimension 20MPa device, the plate shape of the foil surface must not be greater than 30I, when greater than 30I, it is bound to wrinkle.

Because the aluminum foil tends to bear more tension than the subsequent processing, some in the rolling only show bad plate shape, including the roll grinding is not correct, the roll type is not right, the incoming plate shape is not good and the adjustment plate shape is not correct. 

(4) highlights, bright marks, bright spots. Duplex surface due to the improper use of double oil caused by the bright spots, bright marks, bright spots, mainly because of the insufficient strength of the double oil film, or the uneven roll surface caused by uneven rolling deformation, the appearance of hemp or foreign body pressure into the shape. It is an effective measure to solve this kind of defect by selecting reasonable double oil and keeping incoming cleaning and roll surface uniformity.

Of course, it is also necessary to change the amount of pressure and select excellent aluminum plate. (5) Thickness difference. Thick difference is difficult to control is a feature of aluminum foil rolling, 3% of the thickness difference in the plate production may not be difficult, but in the production of aluminum foil is very difficult. The reason is that the thickness is thin, other trace conditions can cause effects, such as temperature, oil film, oil and gas concentration and so on. Aluminum foil rolling a roll up to hundreds of thousands of meters, rolling time up to 10h, with the extension of time, thick difference is easy to form, and the thickness of the means to adjust only the tension speed.

These factors have caused the thick control of aluminum foil rolling difficulties, so, the real control of the thickness difference within 3%, the need for many conditions to ensure that the difficulty is quite large 

(6) Oil pollution. Oil pollution refers to the rolling aluminum foil surface with excess oil, that is, in addition to rolling oil film outside the oil. These oils are often thrown, splashed and dripped on the foil surface by the roller neck or on the outlet of the mill, and are dirtier and more complex in composition. Aluminum foil surface oil is more harmful than other rolling strip oil pollution, one is because most of the aluminum foil finished products as decorative or packaging materials, must have a clean surface, the second is its thin thickness, easy to form bubbles in the rear annealing, and because the amount of oil more in the place to form too much residue and affect the use.

Oil pollution defects are more or less a very important index to evaluate the quality of aluminum foil. (7) Water spots. Water spot refers to the before rolling there is water droplets on the foil surface, rolling after the formation of white spots, slightly will affect the surface condition of the foil surface, serious will cause a broken belt.

Water spot is due to the oil beads or the mill has water droplets dropped on the foil surface formed, control the oil moisture and water source is the only measure to avoid water spots. 

(8) Vibration marks. Vibration marks refer to the periodic transverse waves on the surface of aluminum foil. There are two reasons for vibration marks: one is due to the formation of roll grinding, the cycle is around 10~20mm, the other is rolling due to the oil film discontinuous formation of vibration, often produced in a speed interval, the period of 5~10mm.

The root cause of vibration marks is the insufficient strength of the oil film, which can usually be eliminated by improving the lubrication state. 

(9) Tension line. When the thickness reaches less than 0.015mm, parallel stripes are formed vertically in the aluminum foil, commonly known as tension lines. The tension line spacing is around 5~20 mm, the smaller the tension, the wider the tension line, the more obvious the stripes. When the tension reaches a certain value, the tension line is slight or even disappears. The more small the thickness is, the more likely it is to produce tension lines, and the more likely it is to produce tension lines in double-hop rolling than for single sheets.

Increasing tension and roll roughness is an effective measure to reduce and eliminate tension line, and large tension must be based on good plate shape. 

(10) Open the seam. The open seam is a special defect of foil rolling, which cracks straight along the longitudinal direction when rolling, often accompanied by wire wire. The root cause of the opening seam is the inlet side discount, often occurs in the middle, mainly due to the incoming middle pine or poor roll.

Severe seams cannot be rolled, while minor seams crack at a later cut, which often results in a large amount of scrap.

(11) Airway. In the rolling intermittent occurrence of strip crushing, the edge of the droplet curve, there is a certain width, the mild airway is not crushed, white stripe and dense pinhole. The presence of dense pinholes at the front and rear ends of crushed aluminum foil is the main marker for judging airway and other defects.

Airway comes from raw materials, it is very important to choose materials with low gas content as aluminum blank. 

(12) coiling defects. Coiling defects mainly refer to loose rolls or nesson external tightening. Because the tension of aluminum foil is limited, it is difficult to roll the hard coil. It is ideal to obtain a coil with tight external pine, and enough tension is the condition for forming a certain tension gradient.

Therefore, the coiling quality ultimately depends on the shape of the plate, the nesson outside the tight coil will form a transverse edge, and the loose coil will form an ellipse, which will affect the future processing. Although there are many kinds of aluminum foil rolling defects, but in the end, the main performance is: pinhole, roller eye, open seam, airway characterized by holes, oil pollution, uneven luster, vibration marks, tension lines, water spots, bright spots characterized by surface conditions, with the influence of post-process processing of the plate shape, wrinkle, discount, bad coiling, with the size of the characteristics of thick and so on.

In essence, aluminum foil unique defects only one type of pinhole, the other types of defects are also available, but the severity of the performance is different or the requirements are different.