Induction Cap Sealing
This article was originally written in the period 1995-2000
The latest induction cap sealers have added considerably to the flexibility offered by this process for sealing packaging containers.
Although there is a wide range of closures available, the traditional bore seal has, to date, been the most widely used method of sealing containers in a variety of packaging applications. Bore seals are inexpensive to manufacture and are relatively easy to fit.
But traditional bore seals do not form hermetic, flexible or even reliable seals – they leak!
There are three main types of bore seals, which are either inserted into the neck of the container, slide over the neck or form a combination of the two. The efficiency of all three types of seal can be affected by temperature, natural relaxation and mechanical properties, all of which will cause the seal to leak, especially while in transit.
Plastic and glass containers can undergo dramatic changes in temperature during storage and sale – typically this can occur as the container is moved from refrigeration at the point of purchase to higher external temperatures. As a result, the container and the seal, which are generally manufactured from dissimilar materials, will expand or contract at different rates, causing a leakage of the product and allowing air and bacteria to enter the container.
Most plastic materials go through a process of shrinkage, where the plastic components, after manufacture, contract or relax slightly. If the plastic container is manufactured, filled and then capped in a short period of time, there is a risk that the plastic will not relax fully prior to capping. If relaxation continues after capping, or if the container and seal relax at different rates, there is again the potential for an air gap to open between them.
Thin-walled containers and low cost polyethylene closures are commonly used in the dairy sector to reduce costs. These however increase the possibility of leakage, as containers and closures are relatively flexible, so the two flex at different rates as the pack is handled and transported.
Using the latest developments in induction cap sealing technology, it is now possible to create alternative, airtight, hermetic seals on the necks of containers. The process can produce significant cost savings, as fewer damaged packs result.
The seal is strong, flexible, yet easy to remove, and provides protection against cross contamination and an effective tamper-evident layer.
A cap seal is inherently flexible, so move with the neck of the container as it distorts. A cap seal will withstand considerable pressure both internally and externally – a typical on-site test is to stand unsupported on a sealed container.
Each seal is chosen for its particular application, which can vary from permanent bond seal for dangerous agrochemical goods, to easy peel seals for recyclability on milk containers.
Until recently, induction cap sealing systems have been limited in application, because older systems running at higher frequencies can be difficult to control, giving a very narrow window of consistent operation. In practice, this has meant that system set-up has generally had to be extremely accurate. Once set, there has been limited opportunity to cope with subsequent change elsewhere on the production line. For example, badly adjusted capping units, fluctuations in line speed and poorly manufactured containers and closures can all affect seal integrity.
New induction systems and control use a different tunnel heating coil, which runs at frequencies between 20 and 80kHz and concentrates the electromagnetic energy into a controlled zone directly under the sealing head. Combined with PLC control and linked to automatic closed loop circuits for frequency and flux density within the sealing head, this enables the energy applied to the foil seals to be adjusted automatically within a wide band. Consistent seals can be produced on almost any type and size of container, closure or induction foil, regardless of line speed or the spacing between containers.
Sensors are fitted to monitor the position of containers, height of closures and the presence of foils. These are linked to line management equipment and to the induction sealing itself, which simultaneously monitors the output frequency applied by the heating unit, the proximity of closures to the sealing head, generator output loading and the dwell time.
In recent years, the design of the seals themselves has also been improved, with peelable tabs now being available. These can be removed without leaving any foil residue, thus overcoming objections that induction cap sealed containers cannot be recycled.
Induction cap sealing
- Induction cap sealing is a cost-effective method of sealing containers.
- A thin flexible foil is placed across the opening by localised heating, achieved by coating the foil with a layer of polymer and then passing the container and closure under an electromagnetic sealing head.
- The heat induced in the foil melts the polymer, which subsequently cools to form a strong, flexible hermetic bond with the neck.
- The entire process is completed in seconds and can be carried out on-line.
- Enercon
- Tel: 01296 432096
- Fax: 01296 432096