Weld Seam Liquid Coating: Equipment, Processes, and Troubleshooting

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(1) Roller Coating Equipment: Inner coating device for weld seams, with a coating box mounted on a coating arm connected to the welding arm. When the tank body is transported to the coating wheel after welding, the coating of the weld seam is performed. This method involves applying liquid coating to the weld seam on the tank body using a roller, driven by a motor. The coating amount on the roller is controlled by a scraper. The coating wheel surface is processed into different curves to control the thickness of the liquid coating. The coating wheel system is placed in a coating box, and the coating material is supplied to the coating box from a coating tank through pneumatic pressure. The coating box also has a coating liquid level detection device to control the amount of coating material supplied from the coating tank to the coating box. The coating wheel’s coating surface is designed with different curves, including convex curves, straight lines, and concave curves, to control the thickness of the coating. The coating box is mounted on a support, and the coating liquid control is simple, maintaining a constant coating liquid level using the principle of pressure balance inside and outside the coating bottle.

Most external weld seam coatings use roller coating devices, which work in conjunction with the oxygen-free protection process to enhance the aesthetic appearance and rust prevention of the tank body weld seam.

For external weld seam coating devices, refer to Figure 3-71, including the coating box, tank conveyor belt, and control system. Liquid coating accessories are shown in Figure 3-72.

(2) Spray Equipment: No-air spray system, commonly used for internal weld seam liquid coating equipment by most manufacturers. Generally, it is an improved system based on the one invented by the American company NORDSON. The main components include an air pressure coating pump (air pressure-to-hydraulic ratio of 1:16), filter, coating heater, coating hydraulic pressure regulating valve, spray gun, nozzle, and a circulation valve group forming a closed-loop circuit. An electrical control system controls the coating heater temperature and the spray switch time of the nozzle. This coating process involves atomizing the liquid coating through a special nozzle, spraying it onto the weld seam. Good coating results are achieved by adjusting nozzle hole size, coating viscosity, coating temperature, spray pressure, and diluents. There are air and non-air atomization nozzles. The principles of non-air spray are shown in Figure 3-73.

Coating viscosity should be determined based on data provided by the coating manufacturer or experimental results. DIN4 viscosity cup (volume 100 mL, hole diameter 4 mm, ambient temperature 20°C) is commonly used for viscosity measurement. The coating pump sucks the coating into the pipeline (see Figure 3-76), and the pressure can be increased to 2-4 MPa. The main components of the spray system are as follows:

1. Liquid Coating Boost Pump: Driven by compressed air, the piston cylinder moves up and down, pressurizing the liquid coating to the required pressure (usually 2-4 MPa) in a ratio of 16:1. The pressure is determined based on the coating width, selected nozzle, coating viscosity, and coating temperature.

2. Coating Heater (see Figure 3-77): Reduces the viscosity of the liquid coating to increase the solid content after spraying, enhances the evaporation rate of the solvent, reduces the surface tension of the coating, and improves the adhesion of the coating layer. The liquid coating pipeline system usually has a circulation valve.

Notably, in non-air spray nozzles (see Figure 3-81), when working, they are mounted on the spray gun. The head is made of hard alloy material, and the special-shaped small holes are processed by a special method, allowing the liquid coating to form a fan-shaped atomization under high pressure. These nozzles come in various styles and different aperture sizes to adapt to different coatings. There are three types of hole shapes:

1. Standard Type (S-standard): The nozzle has “cat-eye-shaped” small holes, producing a fan-shaped atomization, suitable for certain liquid coatings.
2. Cross-Cut Type (X-cross-cuts): The nozzle has a square hole, providing clear fan-shaped atomization and higher adaptability to various coatings, resulting in a more even distribution of the coating. The cross-cut type may perform better than the standard type for some coatings but similar for others.
3. Flow Coating Type (F-FLOW-COATING): This nozzle offers the best coating effect, with minimal overspray and excellent coverage. It is suitable for both traditional and some special liquid coatings. After prolonged use, nozzles may wear differently, affecting their performance.

When selecting the nozzle model, it is essential to conduct tests based on different liquid coatings or follow the recommendations from the liquid coating supplier.

Additionally, when changing coatings, it is crucial to clean the pipeline, use appropriate diluents, and avoid mixing different liquid coatings, which may result in adverse effects on the adjustment, debugging, and final coating results.

(3) Baking and Curing Process of Liquid Coatings: After applying liquid coatings to the weld seam area, a curing process is required to polymerize the coatings. The protective effect of the coatings applied to the weld seam also depends on the baking and curing temperature and time, especially for applications like high-temperature sterilization at 121-129°C for 90 minutes in the food industry.

The curing device consists of an oven and a conveyor belt. The conveyor belt transports the coated tank body through the oven at a specific speed and holds it for a certain period. The hot air in the oven cures the coatings, forming a chemically stable coating layer. The ends of the oven can extract volatile solvents, as shown in Figure 3-82. The baking and curing process of liquid coating generally consists of two stages:

1. Solvent Evaporation Stage: This is a physical reaction. During the initial stage of baking in the oven, the solvent in the wet coating layer needs to evaporate fully before the solid components of the coating cure. Otherwise, if the coating surface begins to cure while the solvent has not completely evaporated, continued solvent evaporation can create bubbles on the surface of the cured coating, damaging the protective layer.

2. Coating Curing Stage: This is a chemical reaction. After the solvent in the coating layer applied to the tank body weld seam has completely evaporated, the solid content undergoes a polymerization reaction at the high temperature of the oven for 10-20 seconds (different coatings have different properties). This process transforms low-molecular-weight compounds into high-molecular-weight compounds, giving the coating layer the characteristics of high-molecular-weight compounds and achieving rust resistance and corrosion resistance. The distribution of coating thickness after baking and curing is shown in Figure 3-83.

Experience shows that approximately 1 second of baking time is required for every 1 μm of cured liquid coating layer. By using an improved solvent formula that ensures the required solder melting temperature for the coating and without bubbles, it is possible to reduce baking time by 30%.

The curing curve of liquid coating is shown in Figures 3-84 and 3-85, with the initial stage being the solvent evaporation period at lower temperatures and the later stage being the curing period at higher temperatures.

During the baking process, temperature and time are crucial parameters. Higher baking temperatures may lead to the following results:

– Good curing of the coating

layer.
– The coating layer may become too thin.
– The external ink layer may change color, and the tank may be scratched during transportation.
– Shorter baking time for curing.

The impact of baking and curing time is as follows:

– Less prone to bubbling during baking and curing, resulting in a well-cured coating layer.
– Favorable for subsequent processes, such as high-temperature sterilization.
– Adequate coating film thickness. Lower baking temperatures are beneficial for the equipment’s lifespan.

The distribution of the coating belt during baking, the actual state of the liquid coating layer, and the ideal state may differ significantly, especially in thinner areas of the weld seam (see Figure 3-86).

(4) Common Issues in the Baking and Curing Process of Liquid Coating Belts:

1. Coating Belt Deviation: Poor adjustment of the tank conveyor belt or poor connection between two conveyor belts can cause the tank body to rotate on the conveyor belt, misaligning the nozzle or internal roller coating wheel with the weld seam, resulting in coating belt deviation (see Figure 3-87). This affects the protective effect of the coating layer. The solution is to patiently adjust the conveyor belt to ensure smooth movement of the tank body on the conveyor belt without rotation.

2. Anomalies in Four Scenarios:
a. Coating Layer Too Thin After Curing: Possible causes include low solid content in the liquid coating, requiring an increase in solid content or coating viscosity. For roller coating, choose a roller with a thick coating curve. For spraying, increase the coating pump pressure and coating heater temperature, or switch to a larger nozzle. Alternatively, slow down the conveyor belt speed.
b. Coating Layer Porous After Curing (Microbubbles): This may result from too fast curing, causing the coating to boil and form bubbles and blister-like micropores. The solution is to slow down the curing speed by reducing the conveyor belt speed or lowering the temperature of the heating oven in the early stage.
c. Coating Layer with Bubbles: Bubbles may form in the coating layer during solvent evaporation or spraying. If the curing speed is too fast and the surface coating begins to cure while the solvent continues to evaporate, or if bubbles from the coating are not eliminated during spraying, bubbles and small pores may be left on the surface of the cured coating. Using the wrong solvent, such as toluene or banana water commonly used for regular paints, can also lead to bubbles that are difficult to eliminate during the curing of the liquid coating belt, as shown in Figure 3-89. Sometimes, bubbles form on the weld seam, as shown in Figure 3-90. The cause may be excessive heat on the weld seam and slightly higher viscosity of the coating. The solution is to add a cooling device after welding or reduce the viscosity slightly.
d. Poor Coverage of the Mouth Iron Edge Weld Seam Area: The quality of welding has a significant impact on the protective coating layer. Reasons and solutions for this issue include: i. Weld seam splatter: Slightly increase the overlap amount of the weld seam during welding or reduce the welding current to smooth the weld seam, facilitating coating repair. ii. Insufficient coating viscosity: Increase coating viscosity and correspondingly increase coating temperature.

3. Overspray: Some coating material outside the welding seam repair belt causes an impact on aesthetics (see Figures 3-91 and 3-92). Sometimes, these coatings are difficult to cure. Causes and solutions include: i. Coating viscosity is too low, requiring an increase in viscosity. ii. Incorrect nozzle type, meaning the selected nozzle has a width that is too narrow, causing the nozzle-to-weld seam distance to be too far, resulting in atomized coating drifting to other parts of the tank body. The solution is to choose the correct nozzle type.

4. Splattering: The repair belt is too fine, accompanied by splattering at both ends (see Figure 3-93). Causes and solutions include: i. Low spray pressure or pipeline blockage, requiring an increase in pressure. Check the pipeline. ii. Coating viscosity is too high, requiring a reduction in viscosity and an increase in the coating heater temperature. iii. The nozzle is too close to the tank body, requiring adjustment.

Essential Welding Machine Operations and Quality Inspections

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In the actual operation of welding machines, although different welding machines may have slight variations, the basic principles are the same. Therefore, it is crucial to adjust each part of the welding machine according to the welding machine manual, with special attention to the following key points:

(1) Sheet Metal Specifications and Dimensions, Right Angles, and Burr Compliance, Correct Adjustment of the Magazine:
Ensure that the sheet metal meets the specified size, right angles, and burr requirements, and adjust the magazine correctly.
(2) Correct Adjustment of Sucker, Pusher, Double Sheet Detector, Flexer, Pre-roll forming, and Roll-forming:
Ensure the proper adjustment of sucker, pusher, double sheet detector, flexer, pre-roll-forming, and roll-forming.
(3) Working Height Adjustment of Z-Bar and Lower Welding Wheel Welding Position:
Adjust the working height of the Z-bar and lower welding wheel welding position according to the correction amount of the lower welding wheel.
(4) Proper Adjustment of Roller Guider and Welding Calibration Crown:
Ensure the correct tension, up-down, and front-back adjustment of the roller guider and welding calibration crown. Ensure that the can body’s opening shape meets the requirements, and maintain a deviation of less than 0.10 mm in the overlapping amount between the front and rear ends.
(5) Correct Adjustment of Synchronization and over push Amount:
Ensure the correct adjustment of synchronization and over push amount.
(6) Copper Wire Size and Forming Conformance:
Ensure that the dimensions of the copper wire meet the requirements, and the entire process operates smoothly. The tin traces on the upper and lower welding wheels should be centered, with a gap of approximately 1.5 mm. After passing through the welding station, the copper wire elongation rate should be less than 2%-3%, and the temperature should feel normal.
(7) Adjustment of Welding Pressure Based on Plate Thickness and Quality:
Correctly adjust the welding pressure based on plate thickness and quality. Adjust the welding current to the upper limit until the weld seam splatters irregularly, Then adjust it to the lower limit until the weld seam opens or breaks mechanically. Finally, adjust the welding current to its optimal value, usually slightly above 1/3 between the upper and lower limits.

Welding Quality Inspection:
(1) Visual Inspection of Weld Seam Appearance:
During the production process, visually inspect the quality of weld seams inside and outside the can. Visual inspection can identify obvious defects such as weak weld points, virtual welding, burn-through, overheating, uneven weld points, missed welds, splatter points, lack or excess of overlapping amount at the can body’s start or end, etc.
(2) Weld Seam Strength Inspection (Spherical Tester):
Place the sample can on the spherical tester, align the ball with the weld seam to form a straight line, and move the ball along the weld seam of the can body. Expand the weld seam outward by 1-2 mm (adjustable). Remove the can body from the spherical tester and check for any cracks or fractures in the weld seam.
(3) Tear Test:
Take two sample cans, cut 5 mm at one end of a can using scissors or pliers, bend the cut surface upwards, insert the can body onto the test mandrel, clamp the cut surface with pliers, and continuously and uniformly tear the weld seam upwards (at a 45° angle to the seam). Repeat the process on the other end of the second can. Visually inspect the torn weld seam: if the entire weld seam comes off clearly, feels smooth when shaken between the index and middle fingers, the weld seam is of good quality. If the weld seam breaks at a certain point during separation, the welding current is too weak. If the torn edge is irregular and feels rough, it indicates splatter points and excessive welding current. A good-quality weld seam should be flexible and not brittle.
(4) Cone Test:
The main purpose is to test the ductility of the weld points at the front and rear. Place the test can on the cone tester, press the conical body proportionally into the welded can body at a constant speed until it expands into a cone shape. Remove the can body and observe the expansion of the can: if the expansion value is above 20%, the flanging performance is good; 10%-20% expansion is acceptable; if the expansion value is ≤10%, it is considered unacceptable.
(5) Weld Seam Flip Curl Test:
The main purpose is to test the ductility of the weld points at the front and rear. Extract one can body from the welding machine, select the corresponding mold, fix it on the bottom plate of the flip curl tester, and place the can body on the mold. Operate the piston valve, press the plate to contact the can body for flip curling, curling about 10 mm, remove the can body, and observe and identify whether there are cracks in the flipped part. Cracks indicate failure.
(6) Inspection of Welding Before Overlapping, Welding Width, and Thickness:
Generally, use a magnifying glass with a scale or a projector to measure the overlapping width before welding and the width of the weld seam after welding. The weld seam thickness is measured using a pointed micrometer.
(7) Inspection of Inner Diameter Using Internal Diameter Calipers:
Check the inner diameter using internal diameter calipers, ensuring that the difference between the front and rear can diameters is less than 0.10 mm.