Workshop practice
When the product has no registration mark
A reel-fed printing machine needs to know where each piece starts. Usually, a printed mark tells it. When that mark disappears, the reference must be found in the product itself, and that changes more than you might expect.
Every machine that processes a reel — printing, cutting, encoding or labelling — needs to know where each piece is at every moment. Where it starts, where to stop, where to print.
The usual approach is a registration mark printed on the material: a small contrasting rectangle detected by a photoelectric sensor as it passes. It is simple, reliable and effective.
Until a product arrives without one.
The case
A machine that encodes and personalises RFID tickets on a reel. Previous versions used printed references to determine where printing began and how far to advance.
Then tickets started arriving without that mark. The only available reference was in the shape of the ticket's edge. It was very small: too small for the detection system previously used.
The symptom on those jobs was missing prints: tickets passing through without personalisation. The machine did not detect the ticket's starting point in time and did not print.
Why replacing the sensor is not enough
The obvious response is a better sensor. That is needed, but it is not enough.
Detecting a printed mark and detecting an edge detail are different problems:
- A printed mark is designed to be detected. Its size, contrast and position suit the sensor.
- An edge detail is where it is, at whatever size it happens to be, and was not designed with a detector in mind.
So the problem is not just "seeing something smaller". It is seeing it in the same place every time, on a product that moves, vibrates and does not always pass at the same height.
The solution: three things at once
1. A sensor suited to the detail
Fork-type optical sensors were incorporated to detect the reference at the ticket edge.
A fork sensor has an emitter and receiver facing each other in the same housing, with a slot between them for the material to pass through. As both are fixed opposite each other, alignment is maintained, and the beam crosses the material path rather than reflecting off it. For detecting an edge or a small cut-out, this is a very reliable solution.
2. Mounts that place it exactly where it is needed
A good sensor in the wrong position is no use. New mounts for the sensor arms and photoelectric sensors were developed:
- The arms position the sensors across the ticket, wherever the reference lies on each format.
- Longitudinal movement combines coarse positioning along a guide with fine adjustment.
- Position references help restore known settings when returning to a job.
3. Keep the product steady as it passes
This is the most underestimated part.
A large reference tolerates some material movement. A small reference does not. If the web flutters, rises or drops as it passes the sensor, that tiny edge feature can move outside the beam or pass a moment later. The result is a missed or delayed detection, which is exactly what causes a missing print.
That is why the assembly incorporates web support and stabilisation in the detection area. A small reference requires the product to maintain its position as it passes.
Keeping what already worked
An important design detail: the machine did not stop working with printed marks. It retains the ability to use marks on either side and adds detection for the unmarked format.
The mode is selected on the control screen. The operator selects the product type, and the machine uses the appropriate reference.
It is a rule worth following whenever a machine's capabilities are extended: adding a new mode should not mean giving up the old ones. Existing orders keep coming in.
A second level of control
The change resolved the missing prints on those jobs. But the machine also has downstream vision inspection: if it detects an incorrect ticket, the process stops for the operator to intervene.
This is a good example of the two working together. Reference detection prevents the error. Inspection checks that it has not occurred. Neither replaces the other.
The summary
If your product changes and the registration mark disappears:
- Look for a reference in the product itself: an edge, a cut-out, a change in shape.
- Choose the sensor for the detail, not based on what you already had. For small edges and cut-outs, a fork sensor is often the most reliable option.
- Allow precise sensor positioning, across and along the web, with a return to the same position.
- Stabilise the material in the detection area. The smaller the reference, the more it matters.
- Keep the previous modes. Add the new one and let the operator choose.
- Check afterwards as well. Preventing the error does not remove the need to verify that it has not occurred.
At 7Axis Industrial, we adapt existing machines to new products and formats without losing what they could already do.
See the full CCPB7 case study → Machinery modifications and upgrades →