Beverage & Packaging Sealings - 18’th Edition: Plastic Closures introduction - focusing on 1-Piece Cap.

Metal Packaging in general and metal caps in particular, have been my first love. Hence I still wish to continue with metal caps, going deeper into their application and functionality and performance aspects….. but then decided to start this edition with Plastic Caps, in order to break the monotony and to inject interest and some freshness. Will come back to Metal Caps later. 

Unlike metal caps, Plastic caps are pre-threaded, which means that threads are already produced into a plastic cap during molding stage. Whereas Metal caps (like Crown Caps or ROPP Aluminium Caps) doesn’t have any threads when produced. In fact ROPP aluminium caps are just the shells which are placed on a bottle neck, and then threads are replicated during capping operation.

Hence, let me start with basic classification & nomenclature of plastic caps, which will make the contents of my next editions easier to grasp. Names of various functional parts of cap are identified/ highlighted in a sketch pasted below. Physical significance and function of each closure part will be explained in next edition.

Main Functional Parts of the Cap are:

1.     Sealing Zone : is combination of sealings (inner sealing, outer sealing & top sealing) and is responsible to guarantee the safe containment and self-life of beverage packs.

2.     Threads: moulded inside of the closure wall are responsible to retain the closure in it’s position during the entire life of package. Strength of these threads and a good interference between closure threads and bottle neck threads has an utmost importance to grant a good sealing performance.

3.     Temper-Evidence Band: This is a ring shaped part of Cap which remains attached to closure body through fine bridges. Once closure is unscrewed (removed) from the neck, these fine bridges breaks and this ring remains on the neck finish, while closure comes out of the neck on unscrewing.

TYPE OF CAPS: Here we are limiting the scope of this write-up to flat-top beverage caps only (excluding sports cap/ multi component caps /functional caps…). There are two types of the caps:

1.       Single Piece or 1-Piece caps: As the name suggests, these caps are made of a single material only. So all the parts of caps including sealing zone is made of a single material. The advantage of this cap is simple manufacturing process wherein the cap is moulded in single operation on one machine.

2.       Double Piece or 2-piece caps: As the name suggests, this type of caps are produced with 2 materials. Main closure shell is moulded with one material on a moulding machine. Then a soft liner is moulded into the cap on a second moulding machine as a secondary operation. The cost of this cap(compared to 1-piece cap) is comparatively more due to extra and expensive lining material. Investment in secondary lining operation also contribute to higher costing.

SACMI supplies complete turn-key lines to produce both 1-piece as well as 2-piece caps.

Today we are going to cover 1-piece caps in this edition. Here below is a sketch/picture showing the cross-section of a plastic closure applied onto a PET Bottle neck.  

In above sketch, the geometry of sealing plugs/ sealing zone is for representative purpose only. Actual geometry may differ, depending upon closure designs and closure applications. 

Single piece caps can further be divided into following groups, based upon sealing geometries involved:

         I.            Caps with Outside Seal : Here the Sealing is provided by a ring-shaped sealing in the cap that sits on the outside edge of the bottle neck. This kind of caps are hardly used these days because outside edges of the preforms/bottle necks are exposed/prone to scratches/damages during transportation and handling…. Hence sealing becomes difficult….. Instead the inside bore of the preform/bottle remains free from handling abuses/scratches, hence inside seal is preferred these days….

       II.            Caps with Inside Seal (also called Bore-Seal): Here the sealing is provided by a circular ring/ plug, that enters into the bottle neck bore. The cross-section of this sealing-plug wall, could either be straight or of an olive shape. Diameter of this inner-sealing plug is little more than the bottle bore diameter in order to have an interference which is necessary for an effective sealing. This seal is preferred over outside seal for the reasons explained in preceding point above.

     III.            Caps with Dual Seal (both outside as well as inside seal). This is a combination of above 2 Seals.

    IV.            Caps with Triple Seal (inside, outside & top seal). Above and over inner and outer plug-seals; there is a small ring in-between outside and inside sealing plugs; and this third ring sits on the top surface of bottle neck. In case of Carbonated Soft Drinks, this top-seal is not effective al all, but acts as a stopper during closure application. Due to internal pressure in the CSD bottles, the closure top surface becomes spherical (doming effect) and that results into following:

·       Top seal is lifted up, hence losing the sealing.

·       Inner plug have a tendency to go away from bottle wall, hence inner sealing weakens (if not lost completely) under high temperatures and carbonation pressure.

·       Outside seal becomes more strong and get reinforced by the internal pressure/ doming effect. Hence in CSD application outside seal is the PRIME & most important seal.

To be continued in next edition......