How Inline Flat Sheet Coating Measurement Works – and Why It Matters for Metal Packaging

The challenge with traditional coating measurement on flat sheet lines

For decades, metal packaging manufacturers have relied on gravimetric testing and capacitance-based methods to verify that coating film weights on flat sheets meet specification. Both approaches have a fundamental limitation in common: they are offline, snapshot-based, have measurement variability, and are reactive. 

With gravimetric testing, operators weigh sheets before and after coating – a process that consumes several minutes per changeover and offers no visibility of what is happening across the rest of the production run. Capacitance tools measure after the product comes out of the oven after 15 minutes, meaning an entire batch of sheets can be coated incorrectly before the problem is detected. 

The consequence is that most operators introduce a safety margin, running coating film weights above the nominal specification to reduce the risk of under-application. Research incorporating data from multiple global metal packaging plants has found that varnish applied to printed sheets is typically 10-20% higher than intended under the same application conditions. Because the most common measurement method involves weighing varnish on bare, unprinted sheets rather than printed ones, this discrepancy has gone largely undetected – and unaddressed – across the industry. 

The result is systematic over-application of lacquer, unnecessary material cost, and quality risk that offline measurement simply cannot catch in time.

How the SpecMetrix inline system works

The SpecMetrix In-Line coating measurement system is built on ruggedized optical interference (ROI) technology – a measurement principle that uses light to detect coating thickness directly at the coater while its wet, before sheets enter the oven. 

Here is how the process works: 

1. Light is directed at the coated surface The SpecMetrix probe illuminates the surface of the coated sheet with broadband light. Light reflects from both the top and bottom interfaces of the applied coating layer. These two reflected waves interfere with each other, producing an interference wave. 
2. Interference frequency is analysed The frequency of the interference wave is directly proportional to coating thickness. Proprietary SpecMetrix algorithms analyse the interference spectrum and extract the precise coating thickness values for the specific layer of interest – filtering out signals from the substrate or any underlying print layers. 
3. Every measurement is validated Each individual reading is validated before being accepted. This is particularly important when measuring over pigmented coatings or printed substrates, where signal quality can vary. Only reliable interference waves produce an output, ensuring measurement integrity across the full production run. 
4. Wet-to-dry conversion delivers real-time dry film weight When measuring wet coating inline before the oven, theSpecMetrix software automatically converts the measured wet thickness into a dry coat weight value. This conversion uses inputs for solids content and coating density, and results can be displayed in any desired coat weight unit. For UV varnish lines, where curing is instantaneous rather than oven-based, the system measures and displays the final dry coating thickness directly. 
5. Continuous data drives real-time process control The system delivers up to 150 readings per second across the sheet width. Mounted probes – typically in a two or three-probe configuration measuring left edge, centre, and right edge – provide a complete picture of coating uniformity. Control charts on the operator display plot a rolling average, making it immediately clear when film weights are drifting or a control limit has been breached. 

What inline measurement makes possible

The shift from offline snapshot testing to continuous inline measurement changes how coating lines can be managed and how costs can be controlled. 

Coating centred on nominal, not padded above it Because operators can see the actual coating weight in real time, there is no longer a need to run above the nominal specification as a precaution. Centring film weight on nominal rather than the upper tolerance limit typically delivers lacquer savings of between 6,000 and 9,000 litres per year on a line running 9 million sheets annually – a saving of around €31,500 at typical lacquer costs. 

Quality assured before the oven Detecting an out-of-specification coating weight at the coater, before sheets enter the oven, eliminates one of the most costly quality events in flat sheet production – an oven-load of sheets that must be held for inspection (HFI) or scrapped. With a line running 1,500 changeovers per year, eliminating HFI events typically saves in the region of €11,250 annually. 

Faster changeovers The gravimetric weighing routine required at the start of each coating run takes approximately five minutes per changeover. With the SpecMetrix system, the operator selects the coating from a drop-down menu and measurement begins from the first sheet. Across 1,500 annual changeovers on a line running at 2,500 sheets per hour, recovering those five minutes per run translates to approximately €31,250 in additional production value per year. 

Lean and Six Sigma process improvement Continuous, real-time data from a coating line makes it possible to apply Lean and Six Sigma methods to coating and decoration processes in a way that offline testing simply cannot support. A conservative 2.5% improvement in line efficiency – from 77.5% to 80% – generates around €32,000 of additional output value per year on a mid-volume line. 

Coating trial efficiency and new product qualification Development teams trialling new coatings or qualifying BPA-free and BPA-non-intent (BPAni) alternatives benefit significantly from having accurate, 100% coverage data for every trial variable. Qualification timescales are reduced, risk is lower, and the understanding of process windows is far more precise. 

The SpecMetrix inline system in practice

The system is a three-part installation: mounted probes at the coater, a control unit that powers the light source and processes the interference signals, and a display unit running SpecMetrix software. Remote slave screens can be added easily by plant IT teams, allowing visibility of coating data from multiple points on the line. 

The technology measures all wet or dry container coatings at production speeds, including pigmented base coats and varnish over any colour or print – capabilities that are not available from other optical measurement systems on the market. 

To see the system in operation and understand how it is installed on a coating line, watch the SpecMetrix In-Line system overview video. 

For more information on the full SpecMetrix portfolio, visit the SpecMetrix product range