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Main Factors Affecting Weld Quality
After welding, the original protective tin layer on the weld seam is completely removed, leaving only the base iron.
Therefore, it must be covered with a high-molecular organic coating to prevent corrosion from contact between the iron and the contents and to avoid discoloration caused by corrosion.
1. Types of Coatings
Repair coatings can be divided into liquid coatings and powder coatings. Each type has unique properties due to differences in composition, application, and curing processes.
1. Liquid Coatings
These include epoxy phenolic, acrylic, polyester, organosol, and pigmented coatings, suitable for weld seam repair in most food and beverage cans.
▶ Epoxy Phenolic Coatings: Have few micropores, excellent chemical and sterilization resistance, but require high baking heat. Insufficient baking leads to incomplete curing, causing the coating to whiten after sterilization, affecting performance and food safety. Excessive baking reduces flexibility and adhesion, making the coating brittle and prone to cracking.
▶ Acrylic and Polyester Coatings: Offer excellent adhesion, flexibility, chemical resistance, and sterilization resistance. However, acrylic coatings may absorb food colors and have limited resistance to sulfide corrosion.
▶ Organosol Coatings: Characterized by high solid content, forming thick coatings on weld seams without bubbles, with excellent flexibility and processability. They require less baking heat than other coatings but have poor penetration resistance and are prone to sulfide corrosion, making them unsuitable for sulfur-containing foods.
▶ Pigmented Coatings: Typically made by adding titanium dioxide or aluminum powder to organosol, epoxy, or polyester coatings to mask corrosion spots under the film, suitable for weld seam repair in cans like luncheon meat.
2. Powder Coatings
Powder coatings form thick, complete films, providing the best protection for weld seams. They have no solvent emissions during processing, reducing environmental pollution, and are widely used in food and beverage cans with high corrosion resistance requirements.Powder coatings are divided into thermoplastic and thermosetting types.
▶ Thermoplastic Coatings: Mainly composed of polyester powder, titanium dioxide, barium sulfate, etc. Film formation is a simple melting process, so during the baking after full-can spraying, when the temperature reaches the melting point of the powder coating, the repair coating will remelt and form. These coatings are highly flexible and withstand various mechanical processes but have poorer chemical resistance than thermosetting coatings, easily absorbing food colors. Their adhesion to the base coating is lower than to the weld seam, resulting in a bridge-like arch shape.
▶ Thermosetting Coatings: Primarily composed of epoxy/polyester, they cure into high-molecular compounds through polymerization after heating, forming thinner films than thermoplastic coatings with excellent chemical resistance but inferior processability.
Repair coatings can be divided into liquid coatings and powder coatings. Each type has unique properties due to differences in composition, application, and curing processes.
1. Liquid Coatings
These include epoxy phenolic, acrylic, polyester, organosol, and pigmented coatings, suitable for weld seam repair in most food and beverage cans.
▶ Epoxy Phenolic Coatings: Have few micropores, excellent chemical and sterilization resistance, but require high baking heat. Insufficient baking leads to incomplete curing, causing the coating to whiten after sterilization, affecting performance and food safety. Excessive baking reduces flexibility and adhesion, making the coating brittle and prone to cracking.
▶ Acrylic and Polyester Coatings: Offer excellent adhesion, flexibility, chemical resistance, and sterilization resistance. However, acrylic coatings may absorb food colors and have limited resistance to sulfide corrosion.
▶ Organosol Coatings: Characterized by high solid content, forming thick coatings on weld seams without bubbles, with excellent flexibility and processability. They require less baking heat than other coatings but have poor penetration resistance and are prone to sulfide corrosion, making them unsuitable for sulfur-containing foods.
▶ Pigmented Coatings: Typically made by adding titanium dioxide or aluminum powder to organosol, epoxy, or polyester coatings to mask corrosion spots under the film, suitable for weld seam repair in cans like luncheon meat.
2. Powder Coatings
Powder coatings form thick, complete films, providing the best protection for weld seams. They have no solvent emissions during processing, reducing environmental pollution, and are widely used in food and beverage cans with high corrosion resistance requirements.Powder coatings are divided into thermoplastic and thermosetting types.
▶ Thermoplastic Coatings: Mainly composed of polyester powder, titanium dioxide, barium sulfate, etc. Film formation is a simple melting process, so during the baking after full-can spraying, when the temperature reaches the melting point of the powder coating, the repair coating will remelt and form. These coatings are highly flexible and withstand various mechanical processes but have poorer chemical resistance than thermosetting coatings, easily absorbing food colors. Their adhesion to the base coating is lower than to the weld seam, resulting in a bridge-like arch shape.
▶ Thermosetting Coatings: Primarily composed of epoxy/polyester, they cure into high-molecular compounds through polymerization after heating, forming thinner films than thermoplastic coatings with excellent chemical resistance but inferior processability.
2. Coating Thickness
3. Integrity of Coating
1. Weld Quality
The integrity of liquid repair coatings largely depends on the geometric shape of the weld seam. If the weld seam has spatter points, severe extrusion, or a rough surface, liquid coatings cannot completely cover it. Additionally, the thickness of the weld seam affects the coating effect; generally, the weld seam thickness should be less than 1.5 times the plate thickness. For secondary cold-rolled iron or high-hardness iron, the weld seam thickness is 1.5 to 1.8 times the plate thickness.
Weld seams made without nitrogen protection may have poor adhesion of the repair coating due to excessive oxide layers, leading to coating cracks during subsequent processes like flanging, necking, and beading, affecting the integrity of the repair coating.
Powder coatings, due to their sufficient thickness, can perfectly address metal exposure issues caused by weld defects, providing excellent protection for the weld seam.
2. Bubbles
Unreasonable solvent formulations in liquid repair coatings can affect the integrity of the coating. When liquid coatings contain more low-boiling-point solvents, or if the temperature rises too quickly during baking, or if the weld seam temperature is too high, a large amount of solvent evaporates during baking, leaving strings of bubbles or micropores in the coating, reducing coverage and the protective effect on the weld seam.
4. Baking and Curing
1. Curing Process of Repair Coatings
The baking and curing of liquid coatings can be roughly divided into the following stages: the coating first levels and wets the weld seam and blank areas (about 1–2 seconds), followed by solvent evaporation to form a gel (should be completed within 3–5 seconds; otherwise, the coating will flow away from the weld seam), and finally polymerization. The coating must receive sufficient total heat, which significantly affects the thickness and performance of the repair coating. As mentioned earlier, rapid temperature rise during baking can easily produce bubbles, while slow temperature rise may result in insufficient curing due to short peak temperature maintenance.
Different coatings have varying peak times during baking; epoxy phenolic coatings require longer times than organosol coatings, meaning they need more heat for baking.
For powder coatings, thermoplastic coatings simply melt to form a film during baking without polymerization, while thermosetting coatings undergo addition polymerization after pre-polymerization and melting to crosslink into high-molecular compounds. Therefore, the baking heat is closely related to the performance of the repair coating.
2. Impact of Curing Degree on Coating Performance
Repair coatings can only exhibit their characteristics when fully baked and cured. Insufficient baking leads to many micropores and poor processability; for example, insufficiently baked thermoplastic powder coatings may wrinkle during flanging. Excessive baking affects adhesion; for instance, overbaked epoxy phenolic coatings become brittle and prone to cracking during flanging, necking, and beading. Additionally, sufficient cooling after baking is crucial for the performance of the repair coating. For example, if thermoplastic powder coatings are not rapidly cooled to room temperature after baking, the coating may crack during flanging. Adding a cooling device after the oven can prevent cracking issues in the repair coating during flanging.
In summary, to ensure the quality of the repair coating—i.e., low porosity and good processability—it is essential to control the thickness and curing degree of the coating.
Changtai Intelligent provides three-piece can body rounding machines and weld seam repair coating machines. Changtai Intelligent Equipment is an automatic can equipment manufacturer and exporter, offering all solutions for tin can making. To get prices for three-piece can making machines, choose quality can making machines at Changtai Intelligent.
Chengdu Changtai Intelligent Equipment Co., Ltd.- A automatic can equipment Manufacturer and Exporter, provides all the solutions for Tin can making. To know the latest news of metal packing industry, Find new tin can making production line, and get the prices about Machine For Can Making,Choose Quality Can Making Machine At Changtai.
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Post time: Jul-16-2025