The caps cover a decisive role in preserving quality and are the main interface between product and consumers.

The pre-threaded plastic cap is the most commonly used method for hermetically sealing PET bottles upon completion of the filling cycle. The pre-threaded caps in plastic are obtained by means of injection moulding of PVC, HDPE and other plastic materials. They are normally supplied with a tamper evident seal that stays on the cap once the closure is opened for the first time.

The market now offers a number of caps with different diameters; the diameters most frequently used are 28mm for sparkling beverages and 38mm for non-sparkling beverages. Depending on the number of threads, the cap is either single-threaded, double-threaded or triple-threaded.

There are also sport caps that permit consumers to drink the beverage directly through the cap. In some cases, an aluminium foil sealed on the bottle neck guarantees the bottle seal.

The perfect sealing between cap and bottle neck is a fundamental target for the success of the entire aseptic filling process, whereas the inviolability represents a requirement, above all in the low-acid aseptic field. For this reason the analysis of cap performances represents an important step in the design stages of the container for aseptic applications.

In the last few years the caps have had to respond to increasingly higher performances under diverse aspects:

  • Sealing Guarantee
    1. to maintain the sealing during the entire shelf-life of the product;
    2. to maintain the sealing even if the product is exposed to temperatures above or below those of storage;
    3. to maintain the sealing also in the event of mechanical stress due to transport;
    4. in the event of product contamination and consequent fermentation, it is possible that the pressure inside the single bottle will increase significantly. This will create a risk for the consumer, as when the bottle is opened, the over pressure inside will cause the cap to thrust open and therefore may end up harming the consumer. This is why the cap must be manufactured so that a gradual depressurization occurs when it is still applied to the bottle.
       
  • Tamper evident band used to ensure product integrity
    Every cap has a seal ring connected to the rest of it by means of bridges. The opening of the cap normally leads first to the breakage of the bridges and afterwards, to loss of sealing. In this way it is possible to visually verify that the bottle is no longer in aseptic conditions.
     
  • Reduction in weight and dimensions
    The dimensions of the caps have gradually been reduced with the introduction of double and triple thread caps, as an alternative to the single-threaded cap. The risk is that a minimum rotation could lead to loss of sealing. In order to minimize this risk, it is fundamental to ensure that the capper performs a perfect closure torque application. This is why many cappers are now equipped with brushless gear drives that apply a certain closure torque regardless of the line speed.
     
  • Easy-to-open closures for the consumers
    Possibility of attaining the sealing performance without having to apply such a strong closure torque that makes it diffcult for a certain category of consumers to open (children, elderly, ecc...).
     
  • Active functions
    The cap can also be considered as active packaging: there are oxygen scavenging caps that absorb the oxygen inside the head space of the bottle and thereby prevent the product from oxidising.