Coating Standards for Tinplate Cans: Ensuring Food Safety and Quality

Tinplate cans remain a cornerstone in food packaging due to their ability to preserve freshness, flavor, and safety during storage, transportation, and sale. A critical factor in this reliability is the internal coating of the tinplate, which prevents chemical reactions between the food and the metal surface, minimizing contamination and maintaining quality.

How Internal Coatings Protect Food in Tinplate Cans

Internal coatings act as a protective barrier between the can’s tinplate surface and its contents. The coating prevents corrosion and the release of trace metals, ensuring the food retains its original taste and nutritional value over time.

Coating Application Process

Before tinplate sheets are cut into can bodies, a roller coating machine applies the coating evenly across the smooth surface. This process can be independent or integrated into a printing-coating production line.

The coating process involves:

1. Feeding the coating from a supply tray to a stainless steel roller partially immersed in the coating.

2. Transferring the coating to a rubber roller, which works with an embossing roller to apply the coating to the tinplate.

3. Controlling the thickness of the coating through roller gaps and feeding mechanisms.

4. Collecting excess coating for reuse, ensuring minimal waste.

Considerations for Welded Seams

Areas where the can body will be seam-welded must remain free of coating, as applying coating there may lead to electrical resistance welding failure. Operators adjust the rubber roller with grooves to leave a precise uncoated strip for welding.

Types of Coatings and Their Applications

Choosing the right coating is critical and depends on the food type and processing conditions. Below is a summary:

Coating Thickness Requirements

• Typical food cans: 3.5–6.0 g/m², allowing controlled migration of tin ions for flavor and preservation.

• Acidic vegetables (e.g., asparagus, bamboo shoots, cauliflower) may require “composite coating layers” applied in a three-coat, two-bake method, achieving a total thickness of 10–13 g/m² for maximum density and durability.

Curing and Quality Control

Proper curing is essential to achieve uniform coating quality:

1. Preheating Section – heats tinplate to the baking temperature.

2. Constant Temperature Section – ensures full polymerization.

3. Cooling Section – gradually reduces temperature to prevent stress.

Temperature monitoring at five fixed points on tinplate surfaces ensures uniform heat distribution within ±10℃.

Preventing Mechanical Scratches in the Oven

Scratches can expose the metal, causing corrosion, gas generation, and can deformation. Common causes include:

• Tinplate rubbing against hot oven racks.

• Chain or rack vibrations during operation.

• Oxidation or dirt on racks adhering to wet coatings.

Preventive Measures: Regular rack cleaning, heated NaOH washing cycles, proper chain maintenance, and controlled airflow reduce scratches and ensure coating integrity.

Manufacturer’s Perspective: Industry Insights

As a tinplate packaging manufacturer, we view internal coatings as the core technology that defines can quality. While tinplate itself offers strength and durability, the coating ensures compatibility with diverse foods, from acidic vegetables to protein-rich meats. Selecting the appropriate coating type, thickness, and curing conditions is not just a technical choice—it directly impacts shelf life, consumer safety, and brand reputation.

Tip for Food Brands: Coordinating coating selection with the type of canned food allows manufacturers to optimize both appearance and taste, reducing consumer complaints related to discoloration or perceived spoilage.