What is Flat Sheet Coating Measurement?

Traditional offline measurement methods – particularly gravimetric tests carried out on bare sheets or unprinted discs – do not reflect the actual coating behaviour on printed substrates. Research across multiple global metal packaging plants has shown that varnish applied to printed sheets is typically 10-20% higher than on bare sheets under the same application conditions. Because operators rely on offline snapshot data and cannot monitor the process continuously, they introduce a safety margin and run film weights above the nominal specification to reduce the risk of under-application. The result is systematic over-application across the entire production run. The SpecMetrix inline system measures coating directly on the printed sheet in real time, allowing operators to centre film weight confidently on nominal rather than above it – typically reducing lacquer consumption by around 10% per line.

The SpecMetrix inline system uses non-contact and non-destructive ruggedized optical interference (ROI) technology to measure coating thickness non-contact at production speeds of up to 500 metres per minute. Probes are mounted above the sheet at the coater – typically in a two or three-probe configuration measuring edge, centre, and edge. Light is directed at the coated surface and reflects from the top and bottom interfaces of the coating layer, creating an interference wave. The frequency of this wave is directly proportional to coating thickness. The system processes this signal continuously using proprietary algorithms, validates each reading, and displays real-time results and control charts to the line operator. There is no contact with the sheet, no slowing of production, and no requirement for sheets to be weighed or taken to a lab. You can see the system in action in the SpecMetrix inline system video. 

Savings vary by line conditions, but a detailed ROI analysis based on a representative flat sheet clear varnish line running 9 million sheets per year at 1,500 changeovers identifies six areas of saving. Faster changeovers – eliminating the five-minute gravimetric check at each start-up – contribute around €31,250 per year. Lacquer savings from centring film weight on nominal add approximately €31,500. Reduced spoilage from fewer held-for-inspection events accounts for around €11,250, with a further €6,750 from reduced running spoilage. A 2.5% improvement in line efficiency delivers approximately €32,000 in additional production value, and improved quality assurance reduces exposure to customer complaints by an estimated €10,000 per year. Combined, these savings total over €123,000 annually – delivering a payback period of less than nine months on a single coating line. 

Yes. This is one of the key differentiators of SpecMetrix technology. Many measurement tools based on optical principles cannot reliably distinguish the varnish layer from the underlying print or pigmented substrate. The SpecMetrix ruggedized optical interference technology uses proprietary algorithms to isolate and output the thickness of the specific coating layer of interest – even when applied over any colour, print, or pigmented base coat. Each measurement is individually validated, with results only output when the interference wave is reliable. This makes the system uniquely suited to over-varnish measurement on printed flat sheets – precisely the application where traditional methods have historically produced inaccurate results and led to over-application. 

For more information on the full range of SpecMetrix solutions, visit the SpecMetrix brand page.