Knowledge Base
Flat Sheet Coating Measurement – Key Terms and Facts
A reference glossary for quality, operations, and process engineers working with coated flat sheets in metal packaging production.
Coat weight (film weight) The mass of dried coating applied per unit area of substrate, typically expressed in grams per square metre (gsm) or milligrams per square inch(mg/in2). Coat weight is the primary quality parameter monitored on flat sheet coating lines.
Dry film weight The coat weight of a coating after solvent evaporation or curing. On thermal varnish lines, dry film weight is measured post-oven. The SpecMetrix inline system calculates dry film weight in real time from wet measurements taken before the oven, using solids content and coating density inputs.
Wet film weight The coat weight of a coating as applied, before drying or curing. Wet film weight is higher than dry film weight for solvent-based coatings. Inline measurement of wet film weight enables process control before sheets enter the oven.
Gravimetric measurement A traditional offline method of determining coat weight by weighing sheets or sample discs before and after coating application. Gravimetric measurement provides a snapshot result and requires several minutes per test, making it unsuitable for real-time process control on high-speed coating lines. This often also involves uses of an acid material to strip off coating resulting in potential hazards
Capacitance-based measurement A post-oven measurement method that infers coating thickness from electrical capacitance. Capacitance tools are affected by substrate variation and cannot distinguish between individual coating layers on multi-layer substrates.
Ruggedized optical interference (ROI) The patented measurement technology used exclusively in SpecMetrix inline systems. Light reflected from the top and bottom interfaces of a coating layer creates an interference wave, the frequency of which is directly proportional to coating thickness. Proprietary SpecMetrix algorithms extract thickness values for the specific coating layer of interest, even over pigmented substrates or printed sheets.
Inline measurement Measurement performed continuously at the point of application during production, as opposed to offline or at-line testing. Inline systems enable real-time process control and 100% coverage of production output.
Held for inspection (HFI) Coated sheets isolated from production flow pending verification that film weights are within specification. HFI events are a significant source of cost and disruption on flat sheet coating lines. Inline measurement eliminates the primary cause of HFI events at changeover.
Over-varnish (OV) A clear protective or decorative coating applied over printed metal sheets. Over-varnish film weight on printed substrates is typically 10-20% higher than on bare sheets under the same application conditions – a difference that traditional measurement methods, which use bare sheets or discs, fail to capture.
Nominal specification The target coat weight at the centre of the manufacturing tolerance range. Operators using offline measurement typically run film weights above nominal to avoid under-application. Inline measurement enables consistent centring on nominal, reducing lacquer consumption without compromising quality.
BPAni (BPA-non-intent) Bisphenol A non-intent coatings developed as alternatives to BPA-containing formulations in food and beverage can production. New coating qualifications in the BPAni transition benefit significantly from the precise, 100% measurement coverage provided by inline systems such as SpecMetrix.
Key facts
- SpecMetrix inline systems measure at production speeds of up to 6500 metres per minute
- The system delivers up to 150 individual thickness readings per second
- Varnish applied to printed sheets has been found to be 10-20% higher than on bare sheets under identical application conditions
- A conservative 10% reduction in lacquer consumption is achievable on printed flat sheet lines following inline measurement integration
- ROI of less than 9 months is achievable on a single flat sheet coating line
- Accumulated annual savings on a mid-volume clear varnish line are estimated at over €123,000
For full product details, visit the SpecMetrix inline system page or explore the SpecMetrix brand overview.
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FAQs
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Offline coating measurement – such as gravimetric testing or post-oven capacitance checks – involves taking a sample or weighing sheets at a single point in the production process. It provides a snapshot result and offers no visibility of coating variation across the rest of the run. Inline measurement, by contrast, takes continuous readings at the coater throughout production. The SpecMetrix inline system delivers up to 150 readings per second across the sheet width, providing 100% coverage of every sheet produced on the measured areas. This means coating issues are identified and corrected in real time, before sheets enter the oven, rather than after the damage is done.
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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.
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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.
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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.
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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.
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