How is the sealing achieved in composite aluminum caps?

The sealing performance of composite aluminum caps is achieved through the synergistic structure of “rigid aluminum cap fixation + elastic plastic inner plug fit.” Its core principle leverages the complementary properties of the two materials to block leakage pathways by providing both mechanical fixation and gap filling.
I. Core Sealing Structure and Function
Plastic Inner Plug (Sealing Core Component):
Typically made of PE (polyethylene) or PP (polypropylene), it possesses inherent elasticity and flexibility. Some variants incorporate embedded silicone/butyl rubber sealing rings.
The inner plug is designed to match the bottle opening's shape (e.g., conical, cylindrical). During assembly, it deforms under compression, tightly filling the minute gaps between the bottle opening's inner wall and the plug to form the first sealing barrier.
Aluminum Cap Outer Layer (Fixation and Auxiliary Sealing):
Made of rigid aluminum alloy, it is formed through stamping into a structure compatible with the bottle neck (e.g., flanged, threaded). After assembly, it snaps tightly or screws onto the outer side of the bottle neck.
The rigidity of the aluminum cap restricts the displacement of the plastic inner stopper, preventing loosening due to vibration or temperature changes during transportation or storage. Simultaneously, it blocks external air and moisture from infiltrating through the joint between the cap and bottle neck.
II. Key Sealing Principles
Elastic Compression Seal: The plastic inner stopper's diameter or thickness slightly exceeds the bottle neck's inner diameter. During assembly, it is forcibly compressed into the neck, generating sustained elastic tension. This ensures constant contact with the inner wall, filling minor gaps caused by manufacturing tolerances to achieve a “surface seal.”
Mechanical Compression Seal: The aluminum cap applies sustained axial pressure to the plastic inner stopper through methods like flange engagement, threaded tightening, or snap-fit locking. This enhances the stopper's contact with the bottle neck, further increasing sealing pressure and preventing gas or liquid permeation.
Auxiliary Sealing Design: Some plastic inner stoppers feature annular ridges or grooves at the bottle mouth contact area. Upon assembly, these ridges are compressed to form multiple “line seals,” effectively adding layers of protection and enhancing sealing reliability. Some aluminum caps incorporate internal sealing coatings to further block leakage pathways.
III. Ensuring Sealing Stability
Plastic inner stoppers utilize food-grade/pharmaceutical-grade elastic materials with strong chemical stability. They resist swelling or aging when exposed to liquors, pharmaceuticals, or other media, maintaining long-term elastic conforming performance.
The rigid structure of aluminum caps resists deformation caused by external pressure or temperature fluctuations, preventing seal loosening and ensuring consistent sealing performance during extended storage or transportation.