Unique NDC SR710 Sensor Meets the Challenge of Controlling the Coating Thickness on its Metallised Packaging Lines

The mechanical properties of aluminium make it ideal for demanding packaging applications, but in order to achieve reliable and consistent high quality heat seal bonding to the other lamination substrates, the aluminium must first be thinly coated with products such as acrylic lacquer, vinyl or other clear organic coatings.

NIR (near infrared) measurement technology is frequently used in the converting industry to measure coatings on a wide range of substrates. NIR has the advantage of being a selective measurement technique, able to measure water-based and organic constituents independently of the substrate and other constituents present. For the majority of applications in converting, it has completely replaced nucleonic measurement techniques which, being a subtractive method, require twice the hardware to measure before and after the coating station and then to subtract the one value from the other to gauge the coat weight. The NIR solutions also have the advantage of being non-contacting, and in applications such as coated papers for label stock, the most common measurement sensor format is “diffuse reflectance” also referred to as “backscatter”.

For converters wanting to measure these coatings as thin as 10g or less per square metre on a aluminium or a metallised polymer film, the phenomenon of optical interference precludes the use of diffuse reflectance instruments. Optical interference occurs with very thin transparent coatings when the coating thickness is of the same order of magnitude as the wavelength of light being used to measure it.

The SR710 had been developed by NDC to specifically measure thin organic coatings on metallic substrates. It differs from a diffuse reflectance NIR Sensor in two significant ways: firstly, the optical arrangement uses the metallic substrate itself to reflect a beam of light at a steep angle through the coating onto the detector, so the light is not diffusely reflected but reflected directly onto the detector. Referred to as specular reflection, this complex technique requires precision optics. Secondly, the Near Infrared region of the spectrum does not provide wavelengths which are absorbed strongly enough to enable accurate high-resolution measurement of these very thin coatings. The more strongly absorbed wavelengths lie in the Mid Infrared part of the spectrum, and these longer wavelengths that the SR710 uses also overcome the optical interference issue.

Attached to an NDC Accutrak™ electro-mechanical scanning frame, the SR710’s precision measurement data is processed with the positional information from the frame to create a coating profile display on the screen of the NDC 8000TDi or Pro.Net TDi control system, TDi standing for “Total Distributed Intelligence”. The measurement profile can also be generated in zone format and used in closed loop or semi-automatic control systems, to enable adjustment of the coating machine roller, line speed and other parameters.

Companies can safely rely on the sensor measurements to manage and control the coating process. Through comparison of the on-line measurement data from the SR710 with the values obtained using an off-line reference method, users have observed an instrument precision of ±0.1 gramme per square meter and the SR710 has completely replaced the former routine weigh-wash-weigh method of sample analysis.

Substantial payback is achieved on two fronts through the control of these expensive coating materials: firstly, production is controlled extremely close to specification – in real time – which results in material usage savings, improved product consistency, as well as shorter start-ups and avoidance of material wastage. Secondly, the resultant quality guarantees end-customer satisfaction and loyalty.

For more information, visit: www.ndcinfrared.com/converting

NDC Infrared Engineering: the Company

With manufacturing bases in Maldon, Essex, UK; Irwindale, California, USA; and Alleur, Belgium, NDC develops application-engineered measurement solutions for a wide range of web industries, including Paper, Film and Foil Converting; Film and Sheet Extrusion; Nonwovens and Textiles; and Rubber and Vinyl Calendering. The company boasts the industry’s widest range of measurement techologies ensuring no-compromise peformance for each application, including:

  • Infrared gauging for coating thickness, polymner thickness, moisture content measurement
  • Optical and Laser gauging for thickness measurement
  • Nucleonic gauging for basis weight measurement

Each measurement gauge is designed to be mounted on an NDC Scanning Frame and measurement data from the gauge combined with positional data from the frame to create high resolution scanning profile displays and outputs for process control.

Visit www.ndcinfrared.com for more details.