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This article was first published in Glass International |
Introduction
How many of us have heard the story:
- A container load of whisky, shipped to Japan, was returned to a UK manufacturer after failing inspection at the port side. The ship's hold was opened and whisky fumes seeped into the air, this alone was enough for the importing authorities to refuse acceptance and to return the consignment unopened.
- The culprit? Line-over finish on a few of the containers.
- The cost? Enormous - lost customer, compensation, lost reputation, additional shipping, sorting, re-packaging and insurance costs.
- Whose fault was it? Certainly the container manufacturer for supplying poor quality containers but should the distiller have been inspecting containers at goods in on a sample basis?
Apocryphal? But it could be true! The example serves to illustrate that quality is directly linked with container manufacturer, the client and all points in-between. Break the chain at any point and the whole relationship fails. The container manufacturer must not supply faulty, or out of specification, goods and the supplier of the final product cannot afford to reject and waste a full and valuable container.
The supplier of final product, whether food, drink, cosmetics or other high value commodity, is interested in integrity, appearance and the lack of contamination. These three criteria can be applied to the container itself.
Glass container faults are of many different types but generally fall into three areas of concern: those that occur during manufacturing, handling or filling. Some examples include:
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Those faults that occur during handling can be gross and must be found at goods inward inspection. The manufacturing and filling faults are of much more challenging. However, before looking at the nature of some of these faults and discussing some of the more obvious pitfalls we should look at some of the more philosophical points behind the inspection process.
Generic approach to problem solving
The approach to solving the problem will be different depending on whether you are a manufacturer or user of containers.
Interestingly, the first problem to solve usually resolves around whose responsibility it is to solve the problem. Problem 'ownership' can involve a number of groups from management and design through to production. Ultimately it is a team problem but one person should be identified to champion the solution.
It is important to identify the correct problem to solve and then to agree what the inspection equipment is actually going to do. Is the measurement direct, thickness or height etc., or indirect such as a reflected light intensity that implies the presence of a crack? Will the inspection equipment simply register and count the number of defects observed or is some reject mechanism required, even statistical process control?
The next stage is sorting out how much of the problem can be solved by getting the correct mix between transducer and processing technology. One area where this is clearly illustrated is with the application of optical techniques to automated visual inspection. With careful optical design Diverse have reduced complex inspection tasks to relatively simple processes, such as reflected light intensity measurements to identify cracks, or, good optics enabling the replacement of photon counting devices with simple PIN photodiodes. Good systems design is always worth the investment.
Manufacturing faults
We have already indicated some of the more commonly encountered manufacturing faults. Some problems solved by Diverse have included high speed detection of check under ring (CUR) and check in thread (CIT) for United Glass containers [GP's box].
Faults and problems usually arise through mould wear, poor annealing, poor handling when hot, insufficient glass in the mould or poor withdrawal of the tooling. Contamination, process variation, melt quality and Murphy's law also have an impact.
Some defects are purely cosmetic and do not affect the function of the container whilst others, such as check in thread or stuck plug can have more unfortunate consequences. For example a container that exhibits check in thread may look perfectly good but after filling the automatic attachment of a threaded closure may break the neck from the container. This not only destroys the container but its contents have to be scrapped.
Where is it best to detect these faults? Hot end inspection is closest to the sharp end of manufacture for the container but causes problems for optical inspection techniques due to self radiation and thermal obscuration effects. Cold end inspection is simpler but carries the penalty of being remote from the hot end and therefore not suitable for most process control applications. However, detailed cold end inspection will prevent faults such as line over or checks from causing the return of a high value goods in shipment. Further, a statistical database can be created to track and correlate process variations against fault rates. Thereby feeding back into quality enhancement.
Filling faults
When a filling nozzle or some other mechanism is close to the container, indeed whenever the container is mechanically handled, there will be a risk of damage. Chips and flakes of glass are a risk, doubly so if the flake of glass enters the container.
Closure application is a second prime source of damage. Screw lids or tops, crown caps or similar closures all stress the glass container and any small cracks (perhaps as a result of poor annealing) can propagate and cause a catastrophic failure.
One very unpleasant fault that should be recognised is that of deliberate tamper with the container contents. Such occurrences as glass slithers, dirty hypodermic needles and others have been reported in the national press. The deliberate tamper is of course very difficult to detect as anyone with a knowledge of the process and its inspection will ultimately be able to find a way past the system. No system can be 100% foolproof. However a simple screening system will remove all of the obvious foreign inclusions.
Generic Solutions
How do we provide cost effective solutions to these manifold problems? Anyone attempting to provide a solution to a particular inspection problem should follow a rote similar to the one presented now.
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System Design |
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Illumination and Imaging |
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Image Capture |
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Image Processing |
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User Interface |
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Summary
Glass container inspection is hard and there are no easy answers. However, if you do not inspect you run the risk of contracting 'the whisky consignment syndrome'. This is applicable to both the container manufacturer and the supplier of the final 'contained' product.
The problems are definable and quantifiable. Sometimes formal problem solving techniques may have to be employed to determine exactly what the problem is, Taguchi methods may be useful here. Once the problem is identified and the function of the equipment defined then all that is required is commitment, some innovation, good engineering design and quality production!
The future in measurement of cold end quality will include more non contact automated visual inspection and more centralised inspection tasks. If one station can complete a number of different inspections then container line length and even line splitting may be significantly reduced. Processor speed is ever increasing and more capable, this together with the application of good optical engineering will ensure that apocryphal stories will disappear to the status of glassware inspection lore.
Tel. +44 (0) 1223 84 44 44