PETpla.net Insider 12 / 2015

INSPECTION 24 PET planet insider Vol. 16 No. 12/15 www.petpla.net Part 1: Innovating PET measurement and introducing new technologies Base and wall thickness measurement by Chris Freshman MEng(Hons) CEng MIMechE, Torus Measurement Systems, Telford, UK This article describes the need for sophisticated thickness gauging to meet the PET industries grow- ing requirements to reduce bottle weight. Detailing the principles behind confocal technology and its imple- mentation to acquire high accuracy base and wall thickness measure- ments. Confocal sensors have been compared with section weights, Hall Effect sensors and infra-red absorp- tion, exploring various advantages and limitations. The continued weight reduction of PET bottles minimises wasted raw material, but places greater empha- sis on developing better production processes and creating more efficient preform and bottle designs. Recent years have seen significant advances in bottle forming technologies, produc- ing thinner walls and lower amounts of excess material in the bottle base, whilst preserving structural rigidity and functionality. These increasing demands on Research & Development (R&D) and Production require accurate and repeatable measurement systems, to not only report wall thickness, but also provide reliable measurement data for the bottle base - peak and valley. Furthermore, it is ever more important to verify material distribution. Poor distribution in the blown bottle can be attributed to a number of reasons, for example, preform quality and blow moulder temperature variation. Thick- ness results at detailed heights and rotational positions are vital for mean- ingful material distribution analysis. Chromatic confocal sensors To meet these demands Torus Measurement Systems have devel- oped the B300 Wall Thickness Gauge. Doing away with non-linear, infra-red, PET specific, transmission monitoring. Instead utilising Chromatic Confocal Technology for high accuracy, high lin- earity and flexible one-sided thickness measurement. The sensor focuses white light through a series of high precision lenses creating controlled chromatic aberration; splitting the lights wave- lengths, and thus its colours, across the working distance of the sensor [Fig1]. A factory calibration assigns every wavelength of colour to a dis- crete distance from the sensor. The light reflected from the target surface is directed onto a spectrometer and the wavelength in focus produces a high intensity peak on the spectral graph. Linear thickness measurement When measuring transparent material, such as the PET bottle wall, a proportion of light is reflected off the outer surface of the bottle. The rest passes through the material and reflects off the inside surface, forming two high intensity peaks on the spec- tral graph [Fig 2]. When combined with the materials Refractive Index, defined as: where c represents the speed of light in a vacuum and v is the phase velocity of light in the material, it cre- ates an accurate, one-sided, non-con- tact, thickness measurement for the bottle wall and base. Advantages - Including, but not limited to:  Linearity of <30μm over its full 3mm measurement range (cus- tomisable). Removing the need for offline verification of individual positions to compensate for thick- ness, geometry and colour.  A measurement spot size of 7.2μm increases positional accu- racy and removes large area averaging.  Single sided wall thickness meas- urements give an accurate repre- sentation of material distribution.  Ability to measure any transpar- ent material with a known refrac- tive index or a single reference sample - not just PET.  Absolute measurement – non-comparative.  Multi-layer thickness measure- ment is possible, given a refrac- tive index change between layers. Measuring principle Lenses Beam Target Fig. 1 Fig. 2