Classification and characteristics of aluminum foil

Classification and characteristics of aluminum foil

Due to its excellent properties, aluminum foil is widely used in food, beverages, cigarettes, medicines, photographic plates, household daily necessities, etc., and is usually used as its packaging material; electrolytic capacitor material; insulation material for buildings, vehicles, ships, houses, etc.; it can also be used as decorative gold and silver wires, wallpapers, and decorative trademarks for various stationery prints and light industrial products. Among the above-mentioned uses, the most effective way to play the performance points of aluminum foil is as a packaging material. Aluminum foil is a soft metal film. It not only has the advantages of moisture-proof, airtight, light-shielding, abrasion-resistant, fragrance-preserving, non-toxic and tasteless, but also because of its elegant silver-white luster, it is easy to process beautiful patterns and patterns of various colors, so it is more likely to be favored by people’s aluminum foil tape.

Especially after aluminum foil is compounded with plastic and paper, the shielding property of aluminum foil is integrated with the strength of paper and the heat sealing property of plastic, which further improves the shielding performance of water vapor, air, ultraviolet rays and bacteria required as a packaging material, and greatly broadens the application market of aluminum foil. Since the packaged items are fully isolated from the light, moisture, and air outside, the packaging is well protected. Especially for the packaging of steamed food, the use of this composite aluminum foil material can ensure that the food will not deteriorate for at least one year. Moreover, it is very convenient to heat and open the package, which is very popular among consumers.

With the improvement of people’s living standards and the development of tourism, the demand for beverages such as beer, soda and canned food is increasing, which requires modern packaging and decoration to facilitate competition in the international market. In recent years, in order to meet market requirements, people have developed packaging materials such as plastic films and spray-coated foils with good shielding properties, but their comprehensive performance is not as good as that of over-coating and lamination processing. Therefore, it can be said that aluminum foil is a relatively perfect packaging material with multiple excellent properties, which fully demonstrates its broad application prospects in many fields.

In order to improve the rolling efficiency and the quality of aluminum foil products, modern aluminum foil rolling mills are developing in four directions: large coils, wide widths, high speeds and automation. The roller width of contemporary aluminum foil rolling mills has reached more than 2200mm, the rolling speed has reached more than 2000m/min, and the coil weight has reached more than 20t. The corresponding rolling mill automation level has also been greatly improved, and thickness control systems (AGC) are generally installed, and most are equipped with flatness gauges (AFC). The aluminum foil industry is facing a period of rapid development.

Classification of aluminum foil

According to thickness

Aluminum foil can be divided into thick foil, single zero foil and double zero foil according to thickness.

• ① Thick foil: foil with a thickness of 0.1~0.2mm.
• ② Single zero foil: foil with a thickness of 0.01mm and less than 0.1mm/.
• ③ Double zero foil: The so-called double zero foil is a foil with two zeros after the decimal point when its thickness is measured in mm, usually aluminum foil with a thickness of less than 0.01, that is, aluminum foil of 0.005~0.009mm.

By shape

Aluminum foil can be divided into roll aluminum foil and sheet aluminum foil according to its shape. Most of the aluminum foil deep processing raw materials are supplied in rolls, and only a few handicraft packaging occasions use sheet aluminum foil.

Aluminum foil can be divided into hard foil, semi-hard foil and soft foil according to its state.

• ①Hard foil: Aluminum foil that has not been softened (annealed) after rolling has residual oil on the surface without degreasing. Therefore, hard foil must be degreased before printing, laminating, and coating. If it is used for forming processing, it can be used directly.
• ②Semi-hard foil: Aluminum foil with hardness (or strength) between hard foil and soft foil is usually used for forming processing.
• ③Soft foil: Aluminum foil that has been fully annealed after rolling and becomes soft, with soft material and no residual oil on the surface. At present, most application fields, such as packaging, composite, electrical materials, etc., use soft foil.

According to the surface state

Aluminum foil can be divided into one-sided smooth aluminum foil and two-sided smooth aluminum foil according to the surface state.

• ① Single-sided smooth aluminum foil: double-rolled aluminum foil, one side is bright and the other side is dark after rolling. Such aluminum foil is called one-sided smooth aluminum foil. The thickness of one-sided smooth aluminum foil usually does not exceed 0.025mm.
• ② Double-sided smooth aluminum foil: single-rolled aluminum foil, both sides of the foil are in contact with the roller, and the two sides of the aluminum foil are divided into mirror-finished two-sided smooth aluminum foil and ordinary two-sided smooth aluminum foil due to the different surface roughness of the roller. The thickness of two-sided smooth aluminum foil is generally not less than 0.01mm.

According to the processing technology

Aluminum foil can be divided into plain foil, embossed foil, composite foil, coated foil, colored foil and printed foil according to the processing technology.

• ① Plain foil: Aluminum foil that is not processed after rolling, also known as plain foil.
• ② Embossed aluminum foil: Aluminum foil with various patterns on the surface.
• ③ Composite foil: A composite aluminum foil formed by laminating aluminum foil with paper, plastic film, and cardboard.
• ④ Coated foil: Aluminum foil with various resins or paints on the surface.
• ⑤ Colored aluminum foil: Aluminum foil with a single color on the surface.
• ⑥ Printed aluminum foil: Aluminum foil with various patterns, designs, texts or pictures printed on the surface, which can be one color, up to 12 colors.

Soft aluminum foil can also be further pressed into 40 foil for high-end decoration.

Characteristics of aluminum foil

Aluminum foil has a clean, hygienic and shiny appearance. It can be integrated with many other packaging materials. The printing effect of aluminum foil surface is better than other materials. In addition, aluminum foil has the following characteristics:

• (1) The surface of aluminum foil is extremely clean and hygienic. No bacteria or microorganisms can grow on its surface.
• (2) Aluminum foil is a non-toxic packaging material. It can be in direct contact with food without any harm to human health.
• (3) Aluminum foil is a tasteless and odorless packaging material. It will not cause any peculiar smell to the packaged food.
• (4) If the aluminum foil itself is non-volatile, it will never dry or shrink itself or the packaged food.
• (5) Whether at high temperature or low temperature, aluminum foil will not have grease penetration.
• (6) Aluminum foil is an opaque packaging material. Therefore, it is a good packaging material for products exposed to sunlight, such as margarine.
• (7) Aluminum foil has good plasticity, so it can be used to package products of various shapes. It can also be made into containers of various shapes.
• (8) Aluminum foil has high hardness and tensile strength, but low tear strength, so it is very easy to tear.
• (9) Aluminum foil itself cannot be sealed by heat, and a heat-resistant material such as PE must be applied to its surface to heat seal it.
• (10) Aluminum foil may have adverse reactions when it comes into contact with other heavy metals or heavy metals.

Characteristics of aluminum foil rolling

In the production of double foil, the rolling of aluminum foil is divided into three processes: rough rolling, medium rolling and finishing rolling. From the perspective of technology, it can be roughly divided according to the thickness of the rolling outlet. The general classification method is that the outlet thickness is greater than or equal to 0.05mm for rough rolling, the outlet thickness is between 0.013 and 0.05 for medium rolling, and the outlet thickness is less than 0.013mm for single finished products and double rolled finished products for finishing rolling. Rough rolling is similar to the rolling characteristics of aluminum strips. The thickness control mainly depends on the rolling force and post-tension. The rough rolling processing rate thickness is very small. Its rolling characteristics are completely different from the rolling of aluminum strips. It has the particularity of aluminum foil rolling. Its characteristics are mainly in the following aspects:

(1) Aluminum strip rolling. To make the aluminum strip thinner, it mainly depends on the rolling force. Therefore, the plate thickness automatic control method is a control method with constant roll gap as the main body of AGC. Even if the rolling force changes, the roll gap can be adjusted at any time to keep the roll gap at a certain value to obtain a plate with consistent thickness. When aluminum foil is rolled to the intermediate finishing rolling, due to the extremely thin thickness of the aluminum foil, it is easier to increase the rolling force during rolling to make the roller produce elastic deformation than the rolled material produce plastic deformation. The elastic flattening of the roller cannot be ignored. The elastic flattening of the roller determines that the rolling force in the rolling of aluminum foil can no longer play the same role as that of rolling plates. Aluminum foil rolling is generally a roll gapless rolling under constant pressure conditions. The adjustment of the thickness of the aluminum foil mainly depends on the adjustment of the tension and rolling speed.

(2) Overlap rolling. For extremely thin aluminum foil with a thickness of less than 0.012mm (the thickness is related to the diameter of the working roll), it is very difficult to use a single sheet rolling method due to the elastic flattening of the roller. Therefore, the double-combined rolling method is adopted, that is, adding lubricating oil between two sheets of aluminum foil and then rolling them together (also called overlap rolling). The overlapping rolling process can not only produce extremely thin aluminum foil that cannot be produced by single sheet rolling, but also reduce the number of strip breaks and improve labor productivity. This process can mass-produce single-sided smooth aluminum foil of 0.006mm to 0.03mm.

(3) Speed effect. During the rolling process of aluminum foil, the phenomenon that the thickness of the foil becomes thinner as the rolling speed increases is called the speed effect. The explanation of the speed effect mechanism still needs in-depth research. The reasons for the speed effect are generally believed to be the following three aspects:

• 1) The friction state between the working roll and the rolled material changes. As the rolling speed increases, the amount of lubricating oil brought in increases, thereby changing the lubrication state between the roll and the rolled material. The friction coefficient decreases, the oil film becomes thicker, and the thickness of the aluminum foil decreases accordingly.
• 2) Changes in the rolling mill itself. For a rolling mill using cylindrical bearings, as the rolling speed increases, the roll neck will float in the bearing, so that the two interacting rolls will move towards each other.
• 3) Process softening when the material is deformed by rolling. The rolling speed of the high-speed aluminum foil rolling mill is very high. With the increase of rolling speed, the temperature of the rolling deformation zone increases. It is calculated that the metal temperature in the deformation zone can rise to 200°C, which is equivalent to an intermediate recovery annealing, thus causing the processing softening of the rolled material.