Methods of manufacturing metal cans
Production of cans made of coated tin sheet, contrary to the public opinion that it is easy, requires knowing many new skills and modern technologies. The accuracy and dimensions of parts in production equipment and machines are subject to micron tolerances, which places this industry in the group of precision industries. The most important types of cans are food or non-food metal cans in three pieces or two pieces.
- three-piece cans (3-piece can)
Double can (2-piece can)
1. Draw & Wall Ironed cans (DI: Drawn & Wall Ironed cans: long double cans that are formed by the operation and ironing of the sheet in a stretch form, and their bottom and body are of the same material.
2. Two-piece stretch cans: Two-piece cans are produced with a stretch operation with a lower stretch ratio and material flow into the mold, and the body and bottom of the can are made of the same material.
Three -piece cans
1. Varnishing of coated tin sheet in the varnishing line
2. Cutting the can body parts (Body Blank) in the Doublex Slitter machine
3. Can body cylinder production in the body making machine
4. Covering the weld of the body from the inside with powder or varnish in an electrostatic way to protect this part from corrosion
5. Baking and sticking the powder (or varnish outside the seam) to the body cylinder in a special oven (Seam Coating Curing Oven)
6. Carrying out operations: dividing the body (Parting), beading, changing the shape of the cross section of the can, turning the edge of the body (Flanging), sewing the floor to the body (Seaming) which is done in one or more steps and in one or more machines with Pay attention to the need and type of equipment used at the end of the production operation.
The process of producing two -piece cans
1. Varnishing of coated tin sheet in the varnishing line
2. Cutting body sheets in zigzag shape or simple or without cutting in sheet feeding method
3. Feeding the coated tin sheet into the press and producing two-piece stretchable cans with turned edges in a special press where drawing & redraw, trimming and turning the edges are done at the same time. Most canning lines in Iran have been equipped with new and modern machines during the last decade. The tendency to use thinner coated tin sheets in order to save on the consumption of raw materials is one of the main characteristics of these lines.
Reducing the thickness of sheets, along with increasing the speed of production, requires compliance with the various qualitative ranges required in the production of raw materials, especially coated tin sheets. It should be mentioned that the design printed on the two-piece stretchable cans is designed with a special software. In these types of cans, the design printed on the sheet finds its original shape after being stretched. The Spanish company Logitech is one of the centers that provides this service, and many Iranian companies order their designs there.
Some of these methods are as follows:
The amount of oil and its uniform distribution and life are very important in the quality of varnishing and sheet coating operations. It is better to control and examine this oil with conventional methods before entering the coating process. Some of these methods are as follows:
1. Water-break test: It is the easiest method to ensure the presence of oil on the sheet.
2. Ellipsometer test: It is the most common method to determine oil thickness, which is done with the help of a device with the same name based on optical principles.
3. Test by Soxhlet Method: In the past, the main method of determining the thickness of oil on a coated tin sheet was considered, and now it has been replaced by easier and faster methods. This method is described in the ASTM A623 standard and its basis is based on removing oil from a certain surface of the sheet with the help of a boiling solvent such as chloroform and measuring the remaining oil after the evaporation of this solvent.
9. Tin-coated sheet packaging: Improper packaging and the ability to penetrate the humidity of the environment until the time of use, along with the strong permeability caused by the surface tension property between the sheets, causes irreparable damage to this industrial production.
Combined classification
Can manufacturers consider a combination classification for coated tin sheets. This classification includes seven combinations, from the softest sheet that can be produced in the factory (T-1) to the hardest sheet that can be used by can manufacturers, it can be said that:
T-1 and T-2 sheets are used for stretched containers such as the body of fish cans and lids. T-4, T-3 sheets are special for the body of processed food cans.
T-5 sheets are special for cases that require more resistance in terms of bending or high resistance to pressure.
T-6 sheets are not used for food cans and are only used for two ends of beer cans.
T-U sheets These sheets are continuously resistant to heating and cooling. With the electrolyte method, we can put different amounts of tin on the surface of the steel sheet, that's why we have the following divisions in terms of the weight of tin per square meter:
E1=2.8 grams of tin per square meter of steel sheet
E2=5.6 grams of tin per square meter of steel sheet
E3=4.8 grams of tin per square meter of steel sheet
E4=11.2 grams of tin per square meter of steel sheet
In this case, the sheets are divided based on the weight of tin used per square meter of steel sheet.
DWI process
In America, in the late 1950s, the first series of DWI aluminum cans was prepared, and in 1971, the first series of DWI tin-coated steel cans was released to the market by Seal Crown Crack Company.
In the DWI process, steel or aluminum is first rolled into a coil with a specific thickness. (for steel 0.3 mm and aluminum 0.43 mm) from the said coils, plates or circles are made using a stapler machine, which are made into a glass using a press machine. The name Wall Airend is successively hammered by passing through an iron ring, and during this process, the thickness of the glass reaches one third of the original thickness and its height reaches three times the original height with a fixed diameter, and then in order to obtain the desired height and shaping Scissors the extra height of the can and finally the extra oil in the steps
The added shaping is removed from the can by washing and by passing the clean can through the printing rollers, the outside of the can is decorated in a suitable way.
DWI cans of soft drinks are given throat and rim after the above steps, the internal varnishing system of the can is done by spraying phenolic or vinyl lacquer along with the solvent on the inner surface and then placing the can in the oven to evaporate the solvent. It is obvious that the type of varnish system The amount and number of varnish layers depends on the material of the can (steel or aluminum) and the type of product that is going to be packed in it.
TFS sheets are not suitable for the DWI process because the chrome layer cannot withstand the hammering of iron rings.
DRD process
In practice, DRD cans were prepared with the same initial steps as described for DWI cans. But in these cans, the height and final diameter of the containers are further processed during the process or stretching by impact on the glass, and in this way containers with a smaller diameter are prepared. DRD containers are commonly made from TP, TFS sheets. The varnish used inside the containers depends on the type of can and the type of food used in the packaging, and epoxy phenolic or vinyl varnishes are usually used for this purpose. Two-cloth cans have many advantages, especially in terms of economy and appearance compared to three-cloth solder or resistance welding cans. In general, the containers produced by the DWI method have a relatively thin wall, which is suitable for the packaging of carbonated drinks. be Because they have the ability to withstand internal pressure, but in DRD cans, the body has a thicker body, so they are used for packaging foods that require heat after sealing. Choosing TP or TFS or aluminum sheets for the production of two-piece cans is an economic issue that depends on the price of these sheets.
