In modern bottle cap production, aluminum coil sheets must not only have excellent mechanical properties but also meet demanding requirements for deep drawing, printing/painting, and food safety. Chromate (chemical conversion coating) treatment and DOS (food-grade lubricant coating) treatment are the essential surface processes that enable aluminum sheet to meet these high standards.

Through precise surface pretreatment and lubrication technology, these treatments improve the aluminum’s corrosion resistance and formability, while also delivering efficient, stable, and sustainable solutions for high-speed production lines—making them the ideal choice for premium food, beverage, and pharmaceutical closures.

What Is Chromate Treatment?

Here, “chromate” generally refers to chromate conversion coating, not metallic chrome plating.

Purpose:

• Enhanced corrosion resistance:
  While raw aluminum naturally forms an oxide layer, chromate conversion creates a denser, more stable protective film that effectively prevents corrosion. This is crucial in food and beverage packaging because it prevents chemical reactions between contents and the aluminum.

• Improved coating adhesion:
  The chromate layer provides an ideal base that significantly increases the adhesion of subsequent coatings (like printing inks, lacquers, or internal linings). Without proper pretreatment, coatings can peel or flake.

Application:
Chromate pretreatment is typically performed before printing and forming of the aluminum coil, laying the foundation for color printing and interior protective coatings on closures.

What Is DOS Treatment (Dioctyl Sebacate)?

DOS treatment generally refers to applying a thin layer of Dioctyl Sebacate (DOS), a common food-grade lubricant, on the aluminum sheet surface.

Purpose:

(1) Improved forming performance:
Bottle cap production involves high-speed stamping, deep drawing, and forming. DOS acts as a lubricant to significantly reduce friction in the die, which helps:

Reduce mold wear

Prevent scratches, cracks, or damage during forming

Improve forming precision and production efficiency

Lower “earing” rate (earing refers to wavy protrusions along the drawn edge caused by material anisotropy; reducing earing cuts waste)

(2) Auxiliary corrosion protection:
DOS coatings also provide short-term corrosion protection, especially during storage and transport of aluminum coil.

Application:
DOS treatment is typically applied in the final processing stage of the aluminum coil, just before slitting and coiling, to ensure optimal performance in subsequent stamping and forming operations.

Typical Specifications for Aluminum Closure Sheet

Main Advantages of Chromate and DOS-Treated Aluminum Sheets

• Excellent Forming Lubricity:
  DOS lubricant layer significantly lowers surface friction, optimizing deep drawing, stamping, and rolling processes. It reduces mold sticking and scratching, ensures smooth, flawless cap appearance, and improves production efficiency and yield. Widely used for tamper-evident caps and small-bottle sealing.

• Improved Production Efficiency:
  Enhanced surface lubricity and adhesion enable smoother operation on high-speed stamping lines, reducing downtime for cleaning, mold maintenance, and scrap rate—lowering total production costs.

• Food Contact Safety:
  Complies with international food-contact regulations (FDA, EU 1935/2004, etc.), making it suitable for food, beverage, and pharmaceutical packaging that demands the highest hygiene and safety standards.

• Superior Coating Adhesion:
  Chromate pretreatment ensures strong bonding with printing inks, lacquers, and interior coatings, preventing peeling, cracking, or distortion—enabling complex multicolor printing and full branding customization.

• Sustainability and Environmental Compliance:
  Supports chrome-free and eco-friendly pretreatment options to reduce harmful emissions, meeting the packaging industry’s global sustainability goals.