PETpla.net Insider 12 / 2015

INSPECTION 25 PET planet insider Vol. 16 No. 12/15 www.petpla.net Torus’ B300 Wall Thickness Gauge combines a confocal sensor, 4-axis motion control and sophisticated software to provide a measurement system not only suitable for produc- tion areas, but also ideal for R&D facilities. Dimensional scanning The B300 Wall Thickness Gauge also utilises the confocal sensor’s ability to measure distances. Enabling the profile of any bottle to be scanned ready for the operator to select the required measurement positions - including on the bottle base [Fig 3]. The high accuracy motion controller positions the sensor reliably any- where on the bottle, ensuring surface perpendicularity, achieving the highest accuracy base thickness measure- ments. Confocal base thickness The majority of material waste reduction in the bottle base is achieved in the preform design and tested with theoretical simulation models. Placing higher requirements on the blowing process, resulting in tougher specification limits. However, the only guaranteed method to deter- mine its effectiveness is to physically measure the blown bottle. Historically, base thickness measurements have been performed either by comparing section weights or using Hall Effect sensors; both methods having their limitations. Section weight A destructive measurement giving an indication of the amount of material used, but no information regarding material distribution. Outside toler- ance thicknesses go undetected, offering no empirical evidence that the bottle has been blown evenly through 360°. In addition, as bottle weights continue to reduce, the measure- ment percentage error is increasing to unacceptable levels. The B300 Wall Thickness Gauge can achieve accu- racy values of <10%. Hall effect A contact measurement where a magnetic ball-bearing is dropped inside the bottle and attracted to a magnetized probe manually posi- tioned on the outside. Introducing a magnetic field to a conductive sensor creates a voltage output proportional to the field strength: this is known as the Hall Effect or Hall voltage. The distance of the ball to the probe, set by the wall thickness, alters the mag- netic field and, as such, the voltage output. These changes are calibrated for thickness measurement. Whilst this is a proven and accepted method, it is heavily influenced by the opera- tor. Relying on manually positioning the sensor yields high reproducibility errors, with possible R&R percent- ages of >30%. The B300 Wall Thick- ness Gauge can achieve repeatability values of <10% [Fig 4]. Infra-red absorption Thickness measurement by elec- tromagnetic absorption is principally based on the Beer-Lambert law, sim- plified as; where A is absorption, ε is mate- rial molar absorptivity, c is compound concentration and l is the path length of the sample. PET has a complex electromagnetic transmission spec- trum [Fig 5], leading many absorption systems to use emission wavelengths specific to PET measurement. A wavelength with an optimum ratio is required to detect changes in material thickness. Unfortunately the effect of additives and barrier layers on transmissivity cannot always be anticipated. Further- more, the relationship between path length and absorption is logarithmic and typically requires a multipoint calibration to compensate. Absorp- tion technology uses an emitter and receiver on the same axis, often placed either side of the PET bottle. As a result the reported thickness is a combined average of both sides. The B300 Wall Thickness Gauge reports one-sided material distribution at specifiable heights and rotational posi- tions on the bottle shoulder, wall and base. Continued innovation Torus’ next innovations are re- imaginings of the industry recog- nised Top Load/Volume Gauge and a high-speed, high-volume Preform Inspection Gauge. It is important to recognise that the advances are not solely from the introduction of new technology, but also from the use of discernible metrology. Making the distinction between process monitor- ing and real measurement. Users are provided with reliable and traceable data subjected to stringent correla- tion, capability and R&R procedures. The importance of clearly defining and understanding terms such as Accuracy, Capability, Repeatability, Reproducibility, Precision, Resolution, Calibration and Traceability will be continued in Part 2: Metrology Princi- ples for the PET Packaging Industry. www.torus-group.com Fig. 3 Fig. 4 Fig. 5