Coating Process in Food Three-Piece Can Manufacturing

Coating Process in Food Three-Piece Can Manufacturing

The main process of food three-piece can production includes shearing, welding, weld seam patching, drying, necking, flanging, beading, sealing, leak detection, coating, and drying, and packaging. Today, we will focus on the coating process:

Coating Process

Coating is the process of spraying a thin layer of paint on the inner wall of the empty can using a coating machine. The coating process belongs to the later stage of food and beverage can manufacturing and is of great importance. Although the inner surface of the food three-piece can has undergone printing and coating processes before can making, it is difficult to ensure that the coating is not damaged during the previous processing steps. Therefore, to improve the corrosion resistance of the tinplate inner surface coating and ensure the cleanliness of the inner wall surface of the can after manufacturing, a final repair spray coating is applied to the formed empty can. This is the significance of the coating process.

The coating material should meet the requirements of food safety and hygiene. Typically, epoxy resin or phenolic resin coatings are selected. To ensure compatibility with the inner coating material, matching coating manufacturers and models should be selected. Currently, the coating materials produced by ICI and PPG companies are mainly used in the industry, as well as spray products from other domestic and foreign coating manufacturers.

The quality requirements for coating include appropriate spraying volume and nozzle angle adjustment to ensure that the coating film is uniform and complete. The inner wall of the can should have no blisters or runs after spraying, and there should be no contamination on the outer surface. The residual material should be completely recovered. The coating volume is generally controlled between 80-180 mg/can, depending on the volume of the can. The spraying volume can be adjusted by changing the spraying delay time according to actual production needs. Spray guns come in single, double, and multiple gun types, and can be used in conjunction with nozzles of different shapes to better meet the coating needs of different can types. For example, for a single necking can and a single spray gun, the spraying angle and corresponding technical parameters are as follows:

The nozzle types, spraying positions, and angle parameters for different can types are as follows:

l Nozzle type 122000 <0.09 (gal)/min (GPM)/8in, distance A = 0.668in, distance B = 1.375in, spraying angle 25°, can type 65mmX122mnm;

l Nozzle type 122001 <0.09gal/min (GPM)/6in, distance A = 1.0in, distance B = 1.0in, spraying angle 15°, can type 452mmX133mm;

l Nozzle type 122001 <0.09gal/min (GPM)/6in, distance A = 1.15in, distance B = 1.375in, spraying angle 15°, can type 665mmX175mm;

The performance indicators of the integrity of the inner coating of the empty can should meet the requirements of the corresponding products. Inspection should be carried out after the inner wall coating of the empty can has been dried, using a conductivity tester. Please refer to the instrument manual for testing methods.

The structure and working principle of the coating machine are as follows:

The base of the coating machine is welded from steel plates or section steel. Above the base are installed explosion-proof main motors, cam indexing mechanisms, star wheel turntables, transmission gearboxes, can splitting screws, secondary motor-driven can rotation systems, various drive shafts and transmission components, spraying systems, and feed and recovery devices for sprayed mist particles outside the can mouth.

The cam indexing mechanism is designed based on the transmission principle of worm gears. When the input shaft rotates one circle, the worm wheel on the output shaft will rotate one tooth. According to this principle, the teeth on the worm wheel are changed into rollers, and the continuous curved surface on the worm can be divided by angles; 150 is defined as the length of the curved surface of the "worm wheel" rotating one tooth in a circle of 360°, and the other 210° is defined as the axial plane of the worm. In this axial plane, the "worm" does not rotate. Since the teeth on the worm wheel are changed into rollers, the rollers have a certain diameter size, so the axial plane of the "worm" is the axial plane of a groove. The function of the axial plane of this groove is to ensure that the "worm wheel" remains stationary within the other 210° of the worm. This principle design is used to ensure that the tank body can accurately enter the star wheel and ensure that there is enough residence time to complete the spraying process.

The pitch of the can separation worm increases by 11mm for each rotation of the can separation worm. Usually, a four-turn screw is used. It orderly separates the empty cans that are continuously and closely connected from the cage, and makes the distance between the cans open from 0 to 33mm, so that when the indexing mechanism drives the star wheel, the can body does not contact the edge of the rotating star wheel during the screw guide process. This is because the star wheel driven by the indexing mechanism is intermittent motion, and the screw is a continuous rotation motion. When the star wheel rotates 1/8 of a circle, the screw rotates 150°, and the distance the can body moves on the screw is 150/360X33mm, about 14mm. It can be seen that there is a 14mm adjustment margin between the can body and the star wheel, ensuring that the can body and the edge of the star wheel will not collide. However, when the can body is separated from the screw and falls smoothly into the groove of the star wheel, it is necessary to synchronize the moment when the can body is separated from the screw with the rotation pause gap of the star wheel. In addition, the can conveying speed of the screw is synchronized with the indexing speed of the star wheel, so there is a synchronous motion device designed in between.

The star wheel turntable is installed on the fixed flange, and together with the guide rail, it can guide the tank body and position it. If the height of the tank body is different, the spacing between the inner and outer star wheels can be adjusted, and tank bodies with different diameters should use star wheel turntables that match the tank diameter to meet production needs. The plastic belt driven by the secondary motor on the outer periphery of the star wheel of the single nozzle model (while the two synchronous active rubber wheels on the outer periphery correspond to the positions of the two spray guns on the double spray gun model) and the two auxiliary nylon wheels installed on both sides of the star wheel groove form a three-point support for the tank body, and ensure that the tank body reaches the same linear speed as the plastic belt when it stops between the star wheels. The tank body rotates at high speed driven by the friction of the plastic belt (the tank body rotation speed must reach more than 1500r/min, that is, the tank body rotates about 4.5 times in 0.18s); and the paint is sprayed into the high-speed rotating tank in a high-speed mist form under a pressure of 3920kPa (40kgf/cm?) to meet the need for uniform and comprehensive spraying of the inner wall of the tank body.

The spraying system is mainly composed of an electrical system and a CP pump. When the tank is about to move to the spraying position, the PLC controller has received the (sensor 2) signal from the fan-shaped piece installed on the input shaft of the cam indexing device after contacting the signal generator. When the tank enters the sensor 1 area, the nozzle starts spraying; when the fan-shaped piece is separated from the signal generator, the nozzle stops spraying and completes a spraying cycle.

Since the full spray paint is a volatile flammable liquid, the motor and electrical components must have explosion-proof functions; domestic and foreign manufacturers basically use the CP pump produced by Nordson Corporation of the United States as the full spray paint pump, because the CP pump produced by Nordson is characterized by high and stable pressure, durable pump body, and excellent sealing performance; in addition, all pneumatic components are used to realize the automatic operation of the cylinder according to the predetermined program. Since some mist residues will emit polluted air during the spraying process, a waste gas material recovery and suction device is installed.