I P R Ltd.
The PET Specialists
As with so many aspects of the beverage business the choice of an appropriate cap is based as much on the demands of the market as on technical and financial considerations. However, it is important that the correct choice is made. The cap is the only “moving part” in the package and plays an important role in generating positive customer acceptance of the product. Consumer demands include ease of opening (and ease of resealing where appropriate), protection against spillage or leakage, and tamper evidence.
Should a one-piece cap be used, or is a liner required to improve sealing? Is the product sensitive to oxygen, and is an oxygen scavenger insert desirable? Is a tamper-evident cap needed or does the market call for a separate tamper-evident seal. The type of closure to be used affects not only the specification of the preform, or bottle, to be used, but also the type of closure equipment.
Plastic screw caps of different types and diameters can all be fitted using the same basic capping equipment simply by changing capping “heads” to suit the cap diameter., although there may also be minor changes to be made to the cap orienting equipment, feed chute and cap placer.
Aluminium caps are applied in a different way, but can still be fitted using the same basic capper. A different type of capping head is required here. Aluminium caps, when supplied to the bottler, normally have no thread form. They are placed over the bottle neck as a smooth sleeve and are then formed by special rollers to conform to the shape of the bottle's screw thread.
PET bottles designed for Carbonated Soft Drinks (CSD) generally have vertical vent slots in the screw thread to allow excess pressure to escape in a controlled manner when the bottle is opened. These vertical slots make it slightly more difficult to roll the thread on an aluminium cap and is one of the reasons why they are not so frequently seen on PET bottles, especially on carbonated drinks. Nevertheless, as aluminium companies recognise the strength of the trend to PET containers, we are now seeing the introduction of aluminium caps suitable for PET bottles, and which can even include a plastic tamper-evident ring. A major advantage of aluminium is the fact that it provides an extremely high barrier against permeation.
When bottling carbonated drinks the question of internal pressure on the cap must be considered. Beer is normally given much less carbonation than soft drinks and so a 43 mm screw cap can withstand the internal pressure. Carbonated soft drinks, on the other hand, have much higher carbonation levels and consideration must be given to the danger of pressure build-up blowing off a large diameter cap.
Plastic and aluminium screw caps are available with integral, tear-off tamper evident rings, On an all-aluminium cap the ring is rolled under the bottle neck at the same time as the thread is rolled. Plastic caps are moulded such that the tamper-evident ring will snap into place over the bottle neck when the cap is applied but will resist attempts to remove it.
A two-piece cap will have a separate insert or liner to improve the seal on the bottle neck. Aluminium caps always require a liner, but one-piece plastic caps are common. In the case of oxygen sensitive drinks the liner may also be made from a gas barrier material or an oxygen scavenger material (or both) to remove oxygen from the bottle headspace and improve the barrier performance of the cap.
To maintain the traditional look of the glass beer bottle with PET it is perfectly possible to use crown corks, which would also include liners with the oxygen protection properties described above. For ease of opening there is also a crown type of closure with a ring-pull opening, similar to the ring-pull on a beer can. This type of closure can be applied equally well to PET bottles with a crown-cork neck finish. Capping equipment is available which will handle these ring-pull crowns as well as conventional crown closures.