If you are a production or quality engineer in a blow-moulding factory making PET soft drink bottles, why should you be concerned about the plastic wall thickness and how do you measure it? Is it the same for other blow-moulded and thermoformed packaging containers? Alison Fox, Dyne Testing sales manager at Intertronics, presents some answers.
The structural integrity and safety of the bottle is paramount. Insufficient wall thickness can lead to bottle deformation, cracking, or bursting under pressure. On the other hand overly thick walls waste materials and increase your costs without proportionate performance benefits. PET resin represents a substantial cost of the bottle. Understanding the wall thickness helps control resin use, which helps with your profitability; in an intensely competitive industry, profit improvement is always welcome. If you can safely reduce resin use (and still comply with regulatory standards and customer specifications), then this can help with sustainability goals, and lessen environmental impact through minimising waste and recycling efficiency.
So how do you measure wall thickness? You could take a scalpel, cut out a section (watch your fingers!), and measure the thickness with calipers. You might need to repeat this on the neck, shoulder, label area, body, base, and critical stress points, as these are areas prone to variation.
These days, a non-destructive method is usually preferred. It will be faster, simpler, accurate and repeatable, and it should be fully digital, so that your regular sampling from production runs is collected and tracked over time. You can use SPC software to detect trends and variations early, enabling proactive process adjustments.
Ultrasonic thickness gauges are more often used for large, rigid parts which are eventually used across manufacturing in industries such as plumbing and piping, automotive, and heavy industrial applications.
For your PET blow-moulded bottles, thickness measuring instruments based on the Hall effect are in common use. They are suited to thin-walled, complex and small components, suitable for blow-moulded and thermoformed packaging containers or other parts that require tight tolerances or have irregular shapes. These instruments work by putting a small magnetic steel ball inside the bottle, and the sensor on the outside. The ball is attracted by a powerful magnet positioned directly behind the sensor tip. The sensor measures the magnetic field strength through the PET wall, and the intensity of the detected magnetic field correlates precisely to the distance between the ball and the sensor, essentially the bottle’s wall thickness.
Like those games you have as a child, where a magnetic wand is used to guide metal balls through a maze under a clear cover, the ball and sensor can be moved along the entire surface of the part, even into the small, complex, or curved areas. This straightforward thickness measurement can be used on non-magnetic materials such as the plastics used in packaging, but also glass, synthetics, non-ferrous metals, and composites.
The latest instruments using this technology, like the ElektroPhysik MiniTest FH, should offer you measurement accuracy better than ± (1 µm + 0.5 per cent of reading) on up to a 24 mm wall thickness. It should have an intuitive, menu driven interface to allow you to capture the data, store as much data as you need and interfaces (USB, RS232C or Blue-Tooth) for transferring this data to a computer or directly into your QA software. It should be quick and easy to calibrate and offer a range of sensors to deal with your varied product sizes and geometries. And it should have an industrial grade build quality, robust enough for your factory.
You might still want to cut apart some bottles and wield your calipers from time to time. Periodic destructive testing complements routine non-destructive methods, ensuring high accuracy, validating equipment, diagnosing complex issues, and ensuring rigorous process control and compliance. But the use of thickness measuring instruments based on the Hall effect might be your technique of choice for routinely managing wall thickness. With robust measurement and control processes, you can ensure high-quality parts, minimise costs, enhance product safety and support sustainability objectives.
For the latest packaging news please visit: https://convertermag.com/category/latest-news/
