Intelligent package sealing and inspection

Report by DAVID KERR

The integrity of seals on plastic food packages is of critical importance. Seal leakage is a major problem, giving rise to poor perceived quality or to environments that favour growth of microorganisms.

Whilst certain limited forms of inspection may be carried out during production, it is often at a later date that seal failure becomes apparent. Failures sometimes result in a need to quarantine packages, prior to inspection and release to the customer, with concomitant costs. Conventional thermal-pressure sealing techniques have low flexibility and a high capital cost in terms of machine tooling. Changing and cleaning sealing heads between relatively short production runs involves prolonged downtimes with consequent high operating costs.

The aim of this research programme was to develop an innovative food package sealing and inspection system for use with trayed, ambient or chilled oven-ready meal products.

Laser sealing system

A laser sealing machine has been developed that will seal lidding film to filled food trays without the need for tooling. The technique is non-contact and flexible, permitting multiple package shapes to be processed on a single production line, or fast product changeover without the need to reset the sealing machinery. The lidding film is also cut by the laser beam once the seal has been made. Cuts may be made either inboard or outboard of the tray edge.

A prototype of the new sealing system is shown in Fig I. The film is transported within the machine using a conventional roller-fed system driven by an electric motor via a slipping clutch mechanism to maintain constant lateral tension. Trays are transported to and from the sealing station by a chain-driven conveyor unit. A 50W, RF excited CO2 laser is used for sealing the food trays and cutting the film afterwards. The laser beam is steered using two galvanometer mirrors rotating around mutually orthogonal axes. The mirrors are controlled from a central processor unit, which also controls the film advance and conveyor mechanisms. Sealing and cutting paths can be defined off-line and stored for future use in the form of files compatible with many CAD packages. The laser leaves very cleanly cut edges in the remaining film web, making it less prone to breakage and reducing film mis-alignment or tension variations.

Computer vision techniques are used to inspect every tray for seal quality. High spatial resolution images of the sealed areas of each tray are acquired automatically as the product leaves the machine. The seal inspection vision system was initially developed as a high resolution imaging system, based on the analysis of textural properties of the sealed surface.

A second computer vision technique, based on a special type of embedded signature mechanism (UK patent pending, Application no. 0427321.5) has also been developed, which has proved to be even more reliable and sensitive to fine seal variations, contamination, etc. It is capable of detecting localised variations in seal quality (see Fig. II) as well as more serious failures caused by poor film handling, tray contamination, tray distortion, etc.

Benefits of the system

The new method will bring numerous important benefits to the food industry, including:

  • Non-contact, tooling-free process.
  • Improved seal quality and reduced product wastage.
  • Enhanced customer confidence and perceived product quality.
  • Potentially reduced film wastage and packaging costs.
  • Reduced tooling, consumable and maintenance costs.
  • Minimal machine downtime.

We have benchmarked the laser-produced seals against conventional thermal-pressure seals using the same film and tray materials (both polypropylene and PET). Our results show that the laser seals have superior characteristics in that they can be made stronger (resistance to bursting) yet are easier for the consumer to peel. The process is also applicable to multi-layer, oxygen barrier films used in ambient storage food packaging.

The project consortium is planning to commercialise the machine and the new sealing and inspection concepts. The first machines will be intended for oven-ready convenience meals using CPET trays and conventional lidding films. The computer vision inspection system may also be used on conventionally sealed trays and this technology will be exploited in the near future.

Project partners

Industrial partners include The Recipe Dish Company, RPC Containers Ltd, Packaging Automation Ltd, GSI Lumonics Ltd, and FFP Packaging Solutions Ltd.

FURTHER INFORMATION - AFM 114

Dr D. Kerr

Wolfson School of Mechanical & Manufacturing Engineering

Loughborough University

Ashby Road

Loughborough

Leicestershire, LE11 3TU

Tel: 01509 227559

E-mail:

Fig. I: The prototype laser sealing machine showing the chain conveyor, film handing, laser scanning and tray inspection systems

Fig. II : Output from laser seal inspection using embedded signature technique

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