When wrapping profiles and panels in PVC with a slot nozzle glue head, the quality of the final result depends on a chain of factors that need to be checked carefully, one by one. When the decorative foil detaches from the substrate (the phenomenon known as delamination), the cause is rarely a single one: it’s almost always the sum of small imprecisions that, taken individually, might seem negligible.
In this article we run through the essential checks to carry out before and during bonding, for anyone working in PVC profile lamination.
The starting point: compatibility between adhesive, decorative foil and substrate
Before even starting up the machine, you need to check that the decorative foil and the substrate are compatible with each other. The key parameter is wettability, that is, the ability of a liquid — in this case the adhesive — to spread evenly across a surface.
The values to consider are the surface energy of the profile (or the decorative foil) and the surface tension of the adhesive. The surface energy must be higher than the surface tension.
Measuring wettability: Dyne Test Inks
To check wettability there are specific tools, by now a standard in quality-control laboratories: the Dyne Test Inks, test inks that make it possible to measure the surface tension of plastics and films, expressed in dyne/cm, and to assess the effectiveness of the treatments applied.
The test is carried out as follows:
- apply a thin layer of ink across the full width of the profile, at various points;
- if after a few seconds the mark stays put and even, the wettability value is correct;
- if instead the ink retracts into droplets in less than two seconds, it means the surface tension of the material is lower than the value of the ink used, and a different ink should be tried.
With PVC profiles, the result of the test is often visible to the naked eye, because the ink mark can’t spread out: a clear sign that the profile needs pre-treating before you proceed.
Pre-treatment with primer
Primer is a liquid product that modifies the surface of the profile, preparing it for the subsequent wrapping with no risk of detachment.
There are several families of primer, each designed for specific combinations of substrate and decorative foil, and it’s important to choose the right one according to the material to be treated.
One last check, often underestimated: the ink test should be carried out on the decorative side too, not just on the profile. Only by verifying the wettability of both surfaces can you rule out any adhesion problem in advance.
PVC profiles: the seven process points to check
Once the profile has been pre-treated, attention shifts to the actual process parameters. There are seven main checks to carry out to ensure a stable, long-lasting bond:
- Profile cleanliness: there must be no trace of dust, extrusion residue or moisture on the surface to be laminated.
- Amount of primer dispensed, calibrated according to the type of primer used, and correct drying, determined by the orientation, distance and number of hot-air blowers installed. The temperature of the freshly dried profile should be measured with a pyrometer at several points along the pre-bonding path.
- Profile temperature, to be kept between 45 and 55 degrees just before bonding, always verified with a pyrometer and/or with the help of an IR thermometer.
- Foil tensioning, which must remain constant throughout the wrapping stage.
- Position of the pressing wheels, which must stay firmly against the profile, with the lateral pressing wheels positioned correctly to ensure even pressing on the edges too.
- Amount of glue applied: it’s good practice to check periodically that the value set on the operator panel really matches the actual one, by comparing the coat weight of a coated foil sample against an uncoated one.
- Open time, that is, the interval between applying the adhesive and pressing: too long or too short a time compromises final adhesion.
Continuous process control, not a one-off intervention
Foil delamination is almost never the result of a single error, but of a deviation — even a minimal one — in one or more of these parameters.
That’s why the most effective way to prevent it is to treat these checks as an integral part of the production process, to be verified regularly and not only when problems have already appeared: from compatibility between decorative foil and substrate through to the positioning of the pressing wheels, every stage has a bearing on the final result.
If you need support in understanding how to check even just one of these parameters, our Academy can help with a dedicated course tailored to your needs