Coating structure
For the
coating structure, several aspects have to be considered. The
colour and the degree of pigmentation have an influence on how
intensively the
coating can be heated by the laser. The measuring method only works if
the
laser pulse can create a change in the temperature within the coating.
For most
applications, very opaque paints are used, which can be measured very
well. In
principle, dark coatings can be heated with less laser energy than
light-coloured coatings. The lighter-coloured coatings reflect a large
part of
the laser energy at the surface of the coating and only a small part
contributes to the heating. The PaintChecker compensates for this
effect by
increasing the laser output. The pigmentation determines the colour
saturation,
and in this way also the energy absorption. With transparent or clear
coatings,
the coating itself is hardly heated, it is mainly the substrate that is
heated.
In many cases, it is still possible to measure the coating thickness if
the
substrate is dark. In this case, the laser pulse can heat the coating
indirectly via the substrate.Because the supplied laser energy is dispersed via the substrate, the heat transfer properties between coating and substrate are very important. If the coating does not adhere sufficiently to the substrate, the heat is not dispersed and the measurement of the coating thickness is more difficult. A similar effect can occur with multi-layer coatings, when the same coating material is layered on top of dry coating layers for the desired coating thickness to be reached. Every separating layer reduces the thermal conductivity. If, however, the same thickness had been produced in a single application, the measured values would have been different. For photo-thermal coating thickness measurements, such effects can be taken into account during calibration, but care must be taken that the coating application process is the same for every measurement.
The substrate roughness and the coating waviness also influence the measurement of the coating thickness. Excessive roughness or waviness seemingly reduces the measuring precision on account of inadequate local referencing, because an average value is determined for the measured spot. This imprecision does not originate in the measuring technique but only in the local referencing, and it can only be improved by more precise positioning technology.
Using the photo-thermal measuring technique, only the complete coating structure can be measured. If individual layers are to be analysed, a differential measurement has to be carried out after every single coating application. This also applies when measuring a moist coating on top of a dry multi-layer coating. To do this, the coating thickness of the dry multi-layer coating must be known, so that it can be subtracted from the overall coating thickness. The coating thickness of the dry coating structure does not need to be determined precisely at the same location as the measurement is carried out. A statistical average value is often adequate, because the thinner, dry multi-layer structure contributes much less to the overall coating thickness compared to the thick, moist layer. Measuring imprecision and faults in the dry coating structure are for this reason much smaller.
In some exceptional cases, it is possible to analyse a double-coating structure, such as a clear varnish on top of a thick dry coating, with a single measurement, if the thickness of the two coating layers is very different.
Coating state
The coating can be in one of several states during the measurement. For the most common measuring method, the coating is dry, i.e. the solvent content in the coating has evaporated and a solid surface has formed which is firmly bonded to the substrate. In this state, the coating does not change to any relevant degree during the measurement.
The coating will show a different behaviour if it is to be measured while it is still moist. Immediately after application, large quantities of solvent are found in the coating, part of which will already have evaporated during application. The remaining proportion of solvent makes the coating appear to be thicker if measured moist compared to after drying. Because the evaporation progresses continuously, it is important to adhere to a defined time lapsed after application for the measurement. Our experience shows that no measurement should be carried out during the first 60 seconds after application, because during this time the coating structure is very unstable and the surface not completely cross-linked. If the measuring time is not adhered to precisely, further evaporation will lead to a coating thickness error of approx. 2 % for water-based coatings and 4 % for solvent-based coatings per minute. For the coating line, it must be ensured that if the feed is stopped, the measurement is skipped for several parts.
If unbaked powder coatings are to be measured, they should be measured immediately after application. Although no solvent will evaporate from the coating, the powder will absorb moisture and the powder layer will settle with time. The measurement should be carried out no later than 20 minutes after application.
In principle, all moist coatings are affected by gravity. Particularly when vertical surfaces are coated, the coating will flow downwards depending on viscosity and time and result in a sloping coating thickness. For this reason, the time between measurement and drying is also important, because the thickness of the baked coating can be different from that at the time of the measurement.
Because measurements of moist and unbaked coatings are always a measurement of the current coating thickness, a calibration must be carried out using moist and dry layers so that the proportion of solvent can be determined.
Movement
The
components can be moved during the measurement, because the laser
spot is larger than the measuring spot and aligned to compensate for
the
movement. The movement compensation depends on the size of the
rectangular
laser spot. In the direction of movement, the laser spot is larger than
perpendicular
to the direction of movement. For this reason, a higher speed is
allowed in
parallel to the direction of movement than at right-angles to it. In the practical application, it is also
possible for the measuring head to be moved while the component is at
rest. It
is also possible for both the measuring head and the component to move
at the
same time, e.g. in robot applications. In this case, it must be
observed that
several movement directions and speeds superimpose and that speeds can
be added
or subtracted. Because the laser spot is not symmetrical with
respect to
the measuring spot, there is a preferred direction for measurements
taken during
motion. It must be ensured that the relative movement between the
measuring
head and the component does not exceed the permissible speed and that
the
preferred direction is used. The best measuring results are obtained
when a
constant speed is used. Movement errors are most easily recognised if
the
measured values jump around when an homogeneous surface is measured.Measuring distance
The large measuring distance has been realised by means of the receiver optics. The heat radiation from the coating is focussed by a large mirror onto a heat detector. Because the photo-thermal method analyses only the changes with time and the receiver optics have an adequate depth of field, the measurement will, within certain limits not depend on the strength of the measuring signal and compensation can also be made for tolerances in the measuring distance. If the permissible tolerances are exceeded, worse measuring results must be expected, which become apparent by an increase in standard deviation.
Tilting
Because heat radiation dissipates in all directions into the room, a measurement can also be made at an angle. This way it is possible to access complicated measuring points on edges or within hollows. The heat radiation also obeys the laws of optics and can be deflected by mirrors just like ordinary light. With the use of mirror systems, the amount of movement required can be significantly reduced for complicatedly shaped components, whereby the measuring head is mounted in a fixed position and only the mirrors are moved. For such cases, we can determine the best value solution for your measuring task and realise it.
Temperature
The
components can be at different temperatures during the coating
process. When measuring while moist, the components are usually at the
same
temperature as their environment. After drying, it is quite possible
that the
temperature will reach between 60...120°C. The PaintChecker only
registers
temperature variations, which means that the absolute temperature is
almost
irrelevant. It is possible to measure components up to approx.
80°C. The high
temperature, and slow temperature changes less than 10°C, are
compensated for
by the PaintChecker by compensating for the background temperature. The
calibration must, however, be carried out at the actual temperature and
should
not be used for measuring cold components. For measuring cold
components, a
separate calibration when cold is required. Non-permissible temperature
changes
can lead to changes in the coating thickness and are difficult to spot.
Non-permissible temperature changes can occur, in particular, when
controlled
drying ovens are used. Please contact us if you plan to measure at an
elevated
temperature. We can offer you solutions for your problem and assist you
with
suitable temperature measuring equipment.Contamination
If the PaintChecker is used in the immediate vicinity of the coating equipment, overspray can contaminate the measuring head. With powder coatings, the powder can simply be blown away or wiped off. In the case of solvent-based coatings, it can be cleaned using a suitable solvent and following the cleaning instructions. Contamination by overspray leads to a decreased measuring signal and is tolerated by the PaintChecker within certain limits. If the contamination is excessive, a fault message is sent to the evaluation unit. Increasing contamination becomes apparent by an increase in the standard deviation.
Environment
The PaintChecker is also affected by influences from the environment. These include EMC interference from the coating line and control technology which is transmitted to the cable system and leads to faults. Our measuring technology is designed in such a way that it can also be used in such environments. For optimum measuring results, these interferences should, however, be localised and limited. The conveyor systems in particular create a high interference level because of the pulse controls. For this reason, all the cables for the PaintChecker should be screened and should be laid in their own cable ducts separate from the power cables. A careful earthing plan for the control technology prevents mass loops and cross currents from linked control systems and helps to prevent faults.
When utilising a movable measuring head, vibrations frequently occur which must be reduced to a reasonable level. If the positioning system is not sufficiently rigid, vibrations will lead to small deviations which, on account of the large working distance, can lead to large deviations at the measured object. If the coating thickness deviation also is irregular, the measuring spot will wander in an irregular manner, all the time measuring varying coating thicknesses. The result is a very large standard deviation, because the measuring values will fluctuate strongly.
Furthermore, chemically active gases or liquids can affect the measuring head. The measuring head is only protected to some degree against direct penetration. In particular, strongly oxidizing substances are to be avoided, because they may blind the optical system.
The PaintChecker measuring head is not in a gas-tight enclosure and can therefore not be used in explosion hazard areas without modification. In most cases, however, the explosion hazard can be eliminated by ventilating the measuring chamber.
Calibration
The
calibration critically determines the quality of the absolute
thickness measurement. Two topics will be discussed below, the making
of
samples and the referencing. By means of the calibration, a direct
correlation
between the measuring signal and the actual coating thickness is
determined.
For this, the coating thickness is usually measured using a reference
measuring
device, or known reference samples are used. The calibration is made by
applying a coating which is then measured both with the PaintChecker
and with
the reference device. The calibration software supports the user with
this and
calculates the calibration function. The best calibration precision is
achieved, if a tapering or step-shaped coating structure is created and
then a
multi-point calibration is carried out.Sample creation
The calibration samples must be made very carefully and they should include the coating thickness range which is to be measured later. The sample sheets should have a smooth substrate and, in the case of multi-layer systems, a uniform basic structure. The uniformity of the basic structure can be checked using the reference tool. A small standard deviation indicates a uniform coating. The actual coating layer thickness should ideally be made using the same coating process and under identical production conditions, in order to achieve the best possible calibration sample. This ensures that the same conditions are present as during use in the production process. In exceptional cases, other methods for making the samples are possible. We will be pleased to assist you with the production of your calibration samples.
Referencing
Using an alternative measuring method, usually the eddy current method, the referencing determines a direct correlation with the actual coating thickness. To do this, conventional measuring tools are used, which also have limited measuring precision. The precision of the reference measuring tool has an important influence on the result of the calibration. No better measuring precision can be achieved than that of the reference measuring tool. For this reason, the reference measuring tool should have an adequate measuring precision and itself be calibrated before every measurement. The standard deviation is a good measure for assessing the sample quality . Non-metal substrates are often used for which eddy current measuring tools are not suitable. In such cases, specially prepared samples must be used onto which metal platelets are applied. Alternatively, a different measuring method can be used. In critical cases, destructive measuring methods, such as measuring a ground cross-section can be deployed, however these should only be performed in specialist laboratories.
For companies which do not wish to utilise additional reference technology, we can offer the production of standard calibration samples as a service. Using the standard calibration samples, the calibration can be carried out without a reference measuring tool. Then no coating substrate combination can be measured other than that of the standard calibration sample.
The measuring system capability can be verified by means of the special PTB certified reference sample.