Printed Sheet Registration in Thermoforming: Keeping Graphics on the Part

What registration means on a formed part

Registration is the position of the printed graphic relative to the part, not relative to the sheet. A printer can hold a label to within a fraction of a millimetre on a flat web, and the formed tray can still be rejected because that label sits 2 mm off the centre of the cavity or drifts across the pack.

There are two separate relationships. Print-to-form error is the position of the graphic relative to the formed feature, usually a pocket, a rim or a seal flange. Print-to-cut error is the position of the graphic relative to the trimmed edge. They can move independently, and a line that controls only one of them will still produce complaints.

On roll-fed thin gauge work the web is printed in repeat, so the pitch of the print has to match the pitch of the tool and the feed length has to match both. A 0.2 mm feed error per cycle does not sound serious, but over a few hundred cycles without correction the graphic walks steadily out of the window.

On sheet-fed work the problem is more often rotation. A square cut sheet placed against mechanical stops can still be a fraction of a degree out of square, and that angular error grows with distance from the stop. A logo at the far corner of a 600 mm panel shows it clearly while a logo near the stop looks fine.

The commercial consequence is that registration is a packaging line specification, not a printing nicety. A graphic that is out of place can obscure a seal area, sit under a lidding film, cover a barcode or simply fail the brand artwork check at goods-in. It is one of the few thermoforming defects that the end customer can see without knowing anything about plastics.

How the sheet moves before the tool closes

Printed sheet is heated to its forming window, and heating is dimensional change. PP has a high coefficient of thermal expansion and shrinks again as it cools against the tool; PET expands less but can crystallise and change dimensions if it is held too long at temperature. In both cases the printed repeat on a hot sheet is not the repeat the printer measured at 23 C.

The sheet is also constrained unevenly. The clamp frame grips the edges while the centre is free, so thermal growth is accommodated by sag in the middle. A graphic printed across the web can bow with that sag and then be frozen into a shape that is flatter than the artwork assumed.

Sag is not uniform along the oven either. The leading edge of an indexed sheet spends slightly longer under the heaters than the trailing edge, and the two sides can sit in different heater zones. That produces a gradient in both temperature and expansion, which shows up as a graphic that is correct at one end of the forming area and displaced at the other.

Then the draw itself stretches the print. Where the sheet pulls down over a deep cavity wall, the local stretch can be 30 to 100 percent or more. Any part of the graphic that crosses that region elongates with the polymer, and ink that cannot stretch with it either cracks or simply changes shape.

The practical conclusion is that a printed part is a cooperative design job. The printer needs to know which areas of the sheet will stretch, the toolmaker needs to know where the eye mark and the datums are, and the processor needs to know the thermal expansion of the specific resin and gauge. Three separate specifications that do not reference each other cannot produce a registered part.

Indexing the web: eye marks, encoders and servo feed

The usual control loop starts with a printed eye mark in the margin of the web. A photoelectric sensor sees the mark, the controller compares its arrival with the expected position, and the servo feed trims the next index length by the difference. That closes the loop on the print pitch, which is the only pitch that actually matters to the customer.

An encoder on the feed roll is still needed, because it gives the coarse position and the velocity profile between marks. Relying on the encoder alone assumes the print repeat and the roll circumference stay in a fixed relationship; slip, tension changes and thermal growth of the web all break that assumption.

Sensor choice is part of the specification. A high contrast black mark on a clear or white sheet is easy; a low contrast mark on a metallised or heavily flooded background is not, and a sensor that triggers on a colour boundary inside the artwork will produce a random index. Put the mark in a clean margin that is trimmed off, and agree its size and position with the printer.

The index has to be tied to the tool, not just to the feed. Form and trim stations have their own mechanical datum, and the controller needs a fixed offset between the eye mark and that datum. Record it in millimetres from the mark to the tool centreline, and re-establish it after any tool change or any work on the clamp frame.

Table 1 - Where a registration error comes from and where it appears.

Sheet temperature is the variable that most often gets missed. A change of 10 C in the forming window alters the expansion of a PP web enough to move a graphic by a visible fraction of a millimetre over a 500 mm index, so the heater settings belong on the same setup sheet as the eye-mark offset.

Pre-distorting artwork for the draw

Artwork that has to sit on a formed wall is deliberately printed in a distorted form so that it looks correct after stretching. This is the same idea as a map projection: the flat image is the inverse of the deformation the sheet will undergo. The printer cannot do it from a drawing of the finished part alone.

Start with a formed sample and a grid. Print a square grid onto a plain sheet at the production gauge, form it under production conditions, and photograph the deformed grid on the part. Measure the local stretch in both directions at each region of interest. That map is the input to the artwork correction.

Keep critical graphics out of high-stretch zones wherever the design allows. A logo on a flat flange or on the bottom of a shallow pocket needs little or no correction; the same logo wrapped over a deep corner needs a large correction and will still vary with sheet temperature and plug timing. Design is cheaper than compensation.

Allow for direction. The stretch ratio along the direction of draw is rarely the same as across it, so a circle printed on the flat sheet becomes an ellipse on the formed wall. If the brand artwork contains circles, text or a fixed aspect ratio, the correction has to be two-dimensional, not a single scale factor.

Re-validate the correction whenever the forming conditions change. A different plug material, a different sheet gauge, a hotter oven or a change in pre-stretch all alter the deformation map. The artwork is tied to a process window, and a new process window needs a new grid trial.

Measuring print-to-form and print-to-cut error

Measure both relationships separately, on the same part, with a defined datum. For print-to-form, reference a formed feature such as the pocket centreline or the inside edge of the seal flange and measure to a printed feature. For print-to-cut, reference the trimmed edge to the same printed feature. Two numbers, two acceptance limits.

Use an optical method. A flatbed scan of the formed part against a fixture, a vision camera with a telecentric lens, or a simple overhead camera with a calibrated scale all avoid pressing on a thin tray. A calliper on a soft flange will distort the part and add its own error to the reading.

Define the sample in terms of cavity position. On a multi-cavity tool, take parts from the four corners and the centre, because a temperature gradient across the sheet will show up as a position-dependent error that an average hides. Record the cavity number with every measurement.

Log the direction of the error, not only its size. A consistent offset is a setup problem and can be corrected in the index; a growing drift along the run is a thermal or tension problem; a random scatter is usually sensor triggering or sheet slip. The sign of the number tells you which lever to use.

A practical acceptance band for printed thin gauge packaging is in the region of plus or minus 0.5 to 1.0 mm for print-to-form and tighter for print-to-cut, but the right value is the one the filling and lidding equipment can tolerate. Ask the packaging line, not the print supplier.

Table 2 - What belongs in a printed sheet registration instruction.

Keep a golden formed part from the approved setup at the line. A quick visual comparison at start-up and after every roll change catches a drifting index long before a formal measurement does.

What to put in the specification

Write one document that the printer, the toolmaker and the forming line all sign. It should state the print repeat, the eye-mark size, contrast and margin position, the mark-to-tool offset, the sheet resin and gauge, the forming temperature window, and the two registration tolerances with their measurement method.

Specify the artwork correction as a controlled file with a revision, not as an instruction to adjust the print. When a new grid trial produces a new distortion map, the artwork revision changes and the old one is withdrawn. That prevents two suppliers running different corrections on the same part number.

Agree what happens at a roll change. New print rolls have slightly different repeat lengths, ink density and eye-mark contrast, so the first formed parts from a new roll need a check and possibly a small index trim. Put that in the start-up checklist rather than discovering it in the packed stock.

Include the trim tool in the datum chain. A form-cut-stack line registers the cut to the formed part; an offline trim press registers it to a fixture. If those two datums are not derived from the same eye mark and the same tool centreline, print-to-cut error will never be stable.

Finally, treat a registration complaint as a measurement question. Ten minutes with a formed grid sample and four measured parts will tell you whether to change the index, the heater balance, the clamp frame or the artwork. Guessing usually ends with a new print cylinder that was never needed.