PETpla.net Insider 03 / 2017

INSPECTION 22 PET planet Insider Vol. 18 No. 03/17 www.petpla.net Unique sensor technology improves PET blow moulding processes News in online wall thickness measurement Wall thickness is one of the most important and frequently measured properties in PET bottles. This is easy to understand because material distribution, and how well it is monitored and controlled, is correlated with a wide variety of bottles’ critical performance characteristics. Vertical resistance, pressure resistance, creep, CO 2 retention, performance in filling, transportation and storage are all affected by a bottle’s wall thickness. Thickness measurement also plays an important role in bottle lightweighting. A new and unique thickness measurement technology solution that addresses these issues efficiently is now available. the process properly. Due to process drifts between sampling intervals, too many sub-quality bottles could be produced before the problem was detected. Hence, in the early 2000s, through-bottle IR absorption and camera based measurement sys- tems began to appear on the market place. However, these methods have one important caveat; they combine thicknesses of a bottle’s two walls and cannot detect if one wall is too thin and the other is too thick. This means a through-bottle measurement system can report a bad bottle as a good one. Single-wall thickness measurement in real-time FocalSpec Ltd, headquartered in Oulu, Finland, is specialised in the development, manufacture and mar- keting of advanced optical sensors and systems for 3D and thickness measurement applications. Focal- Spec’s management has a long his- tory in the PET bottle testing field and was excited to re-enter the market with a completely new type of a prod- uct. The new Online Wall Thickness Measurement System was presented at the recent K 2016 in Düsseldorf, Germany. The FocalSpec system based on the company’s patented MCP 100 sensors, measures thickness of the nearest wall of passing bottles from one side of the flow. Having sensors on both sides of the conveyor en- ables the measurement of both walls individually. MCP 100 sensor The sensor that makes thickness measurement of individual bottle walls possible is based on safe IR light that was chosen because of its good penetration capabilities on pigmented materials. The sensor’s transmitter sends two IR beams into the bottle wall while the bottle is detected in the sensor’s capture area. A part of these beams is reflected from the bottles outside wall surface back to the sensor’s receiver. Some IR light penetrates the wall and reflects it to the sensor’s receiver from the inside wall surface. The sensor calculates the distance difference between these reflections which is adjusted for the material’s refractive index and is then sent out to a PC for processing and reporting. A little bit about history Since the invention of the PET bottle, manufacturers have been using various methods to measure wall thickness and applying the results in their processes. Micrometers were a logical first choice, and were soon followed by other manual methods such as section weighing, ultrasonic, capacitive and magnetic devices. The first semiautomatic laboratory instru- ment based on infrared absorption was introduced in the 1980s. As blow moulding machines became larger and faster, sampling- based off-line thickness measure- ment, combined with statistical process control methods, was often deemed inadequate for controlling FocalSpec wall thickness system INSPECTION S P E C I A L