A sheet is fully lacquered when every area that should carry coating does, on every side that should be coated. On a production line, the only reliable way to confirm it is to inspect every sheet at line speed, because a sheet with missing lacquer looks exactly like a good one. The IS651 Missing Lacquer Detector does this at up to 1,200 sheets per minute on white, colored and clear lacquers, and with an opposing pair of sensors it checks both sides of sheets that are coated on both sides.
Key takeaways on fully lacquered sheets
- Missing lacquer is hard to see. Clear varnish on bright tinplate is virtually impossible to confirm by eye.
- The damage depends on where the sheet goes next: corrosion after filling, blunted DRD press tooling, crown caps that do not seal, or inverted and inside-out cans.
- The cheapest place to catch it is the coater outfeed or the wicket oven outfeed. A second check at the press entrance or the slitter protects the tooling and the can line.
- For sheets coated on both sides, an opposing pair of IS651 sensors checks each side separately.
- The IS651 detects missing lacquer. It does not measure how much lacquer is on the sheet; that is the job of a coating thickness gauge.
Why is it hard to confirm that a sheet is fully lacquered?
A sheet that leaves the coater uncoated or only partly coated does not look wrong. It looks like every other sheet on the stack. Clear varnish on bright tinplate is the hardest case: there is no color and very little contrast, so it defeats both the naked eye and sensors that work on color or contrast.
Spot checks only cover the sheets you sample. Because one coated sheet becomes many can bodies, a single sheet that escapes uninspected produces a batch of defects, spread through a run that has already moved downstream.
The IS651 uses a specialized light source that responds to the presence of the lacquer film rather than its appearance. That is what makes clear varnish detectable at line speed, down to a minimum lacquer weight of 2 g/m², depending on the lacquer.
Where does missing lacquer cause damage?
The same defect causes a different failure depending on where the sheet is used. These are the four places canmakers most often need to confirm that a sheet is fully lacquered.
| Where | What goes wrong | Where to inspect |
|---|---|---|
| Coating line | Lacquer starvation and overlapped sheets leave bare areas | Coater outfeed or wicket oven outfeed |
| DRD and deep-draw presses | An uncoated blank can blunt the press tooling | Press entrance |
| Crown cap presses | The sealing gasket does not adhere, so the cap may not seal | Before the sheets reach the press |
| Can lines and slitters | Blanks lacquered on the wrong side become inverted or inside-out cans | Slitter first operation table |
1. On the coating line
Two common faults leave sheets without lacquer on the coating line. Lacquer starvation happens when the day tank is not topped up or refilled in time: the coating fails at the edges of the sheet first. Partially overlapped sheets leave a bare band across the full width of the lower sheet, where the upper sheet shielded it from the coating roll.
Inspecting at the coater outfeed or the wicket oven outfeed catches both before the sheets are stacked, which is where a defect costs the least to contain. For the pattern each fault leaves and how many sensors each one needs, see Missing Lacquer on Coated Sheets: 3 Causes and How to Detect Them.
2. On DRD and deep-draw presses
DRD blanks are designed to be pre-coated, and the coating is part of how the blank behaves in forming. A bare blank entering the die is outside the condition the tooling was set up for, and it can blunt the tooling. An IS651 at the press entrance confirms the base plate is fully lacquered or coated before it reaches the die.
3. On crown cap presses
Crown sheets carry an internal varnish so that the sealing gasket adheres to the shell. A shell that missed the varnish will not seal reliably, and the leak shows up only after the bottle is filled and capped. Checking that the base plate is lacquered before the sheets reach the press keeps those shells out of production.
4. On can lines and slitters
A blank that is lacquered on the wrong side produces an inverted or inside-out can: the coating ends up on the outside and the product sits against bare metal. The IS651 detects blanks lacquered on the wrong side at the first operation table of the slitter. On the can line, the IS631 Inverted Blank Detector detects inverted can bodies or blanks with lacquer on the wrong side.
How do you check both sides of a sheet?
Each IS651 looks at one side of the sheet. For sheets coated on one side, a single-sided fitting is enough. For sheets coated on both sides, a second sensor is mounted opposite the first, so each side is checked on its own. If either side arrives without lacquer, the sensor on that side sends the reject signal.
The IS651 detects the absence of lacquer, not an excess. It will not tell you that one side received two coats. To measure how much lacquer is on a sheet, use a coating thickness gauge such as the IS860 CoatPro™.
How many sensors does a fully lacquered check need?
Each IS651 inspects a strip about 15 mm wide, running in the length direction of the sheet, so sensors go where defects actually occur rather than edge to edge. For most canmakers the starting point is one sensor, fitted about 100 mm in from the edge of the narrowest sheet they run. It detects effectively all partially overlapped sheets, lacquer starvation on the edge it guards, and some contamination defects.
Run it, count what it catches, and add sensors only if your own quality data says you need them. On sheets coated on both sides, each position takes an opposing pair.
What does the IS651 send to the line?
- Reject signal. A 45 millisecond pulse when missing lacquer is detected, which triggers a user-defined response to isolate the sheet.
- Heartbeat signal. A signal for every sheet inspected, which tells the control system that the sensor is live and coated sheets are passing. It also works as a sheet counter for process monitoring and SPC.
- No operator settings. The IS651 calibrates itself, detects a lacquer changeover and learns the new coating on its own. There are no dials, counters or control box, and no separate controller is required.
It responds in 3 milliseconds and inspects at up to 1,200 sheets per minute.
Recommended solutions
- Missing Lacquer Detector IS651 – detects uncoated and partially lacquered sheets at line speed, on one side or both, including clear varnish.
- Inverted Blank Detector IS631 – detects inverted can bodies or blanks with lacquer on the wrong side.
- Sheet Skew Measurement System IS416 – protects against poorly placed coating by detecting skewed (rotated) sheets at the coater.
- Double Sheet Detector IS231 – protects production against double sheets without requiring any operator adjustment.
Which sheets leave your coater without lacquer?
Send us your sheet sizes, lacquers and line layout, and we will suggest where the IS651 should go.
FAQs about fully lacquered sheets
How can I tell if a sheet is fully lacquered?
Not reliably by eye, especially with clear varnish on bright tinplate. An inline sensor that inspects every sheet at line speed, such as the Innosen IS651 Missing Lacquer Detector, sends a reject signal when a sheet is uncoated or only partially lacquered.
Can the IS651 check both sides of a sheet?
Yes. A single-sided fitting covers sheets coated on one side. For sheets coated on both sides, a second IS651 is mounted opposite the first, so each side is checked separately.
Does the IS651 detect double lacquer or measure lacquer weight?
No. The IS651 detects missing lacquer, down to a minimum lacquer weight of 2 g/m² depending on the lacquer. To measure how much lacquer is on a sheet, use a coating thickness gauge such as the IS860 CoatPro™.
Why does missing lacquer damage DRD press tooling?
DRD blanks are designed to be pre-coated, and the coating is part of how the blank behaves in forming. A bare blank entering the die is outside the condition the tooling was set up for and can blunt it.
Why does a crown cap need a lacquered base plate?
The internal varnish on crown sheets allows the sealing gasket to adhere to the shell. A shell that missed the varnish will not seal reliably.
What causes inverted or inside-out cans?
Blanks that were lacquered on the wrong side. The IS651 detects them at the slitter, and the IS631 Inverted Blank Detector detects inverted can bodies or blanks with lacquer on the wrong side.
Where should a missing lacquer detector be installed?
On coating lines, at the coater outfeed or the wicket oven outfeed. On DRD and deep-draw presses, at the press entrance. On slitters, at the first operation table.
