Why and when should photo-thermal coating thickness measurements be used?

By means of a number of questions and answers, we will explain why and when the non-contact photo-thermal coating thickness measuring technique can be used the most effectively and which conditions are important. These questions and answers should help you to find approaches to how to make use of our coating thickness measuring technology.

· Why is it so important to determine the thickness of the coating? What are the consequences of coatings which are too thin or too thick?

· Which coating thickness measuring devices can be used for fulfilling the requirements described above?

· Which measuring method is used in our devices and what are its advantages compared to more traditional methods such as eddy current or magnetic?

· When is it more advantageous to determine the thickness of the wet coating and when is it better to determine the thickness of the dry coating?

· For corrosion protection, the correct coating thicknesses are very important, Car manufacturers make different demands in this area. How do we, as a manufacturer of devices, respond to the individual requirements and different standards?

· To what extent are car manufacturers already using online measuring devices during series production?

· Online measuring devices are probably only of interest for large series production. Are there requirements which might be of interest to medium-sized companies?

· What is the expected development of coating thickness measuring devices? Which improved or additional features will the devices of the future have?

Why is it so important to determine the thickness of the coating? What are the consequences of coatings which are too thin or too thick?

There are a number of reasons for determining the coating thickness. In principle there are four main requirements for a coating. The first is to provide surface properties such as colour, structure, tactile and mechanical properties, corrosion protection and information. The secondary requirements are of an economic nature. The first of those are low-cost production, which must in every case provide the surface properties desired by the customer, freedom from faults and uniform thickness of the coating applied. Furthermore, reduction in the energy and raw material consumption and the amount of waste is desirable. For economical production, the coating processes must be reproducible, testable and  as far as possible automatic. The tertiary requirements have to do with legal aspects. Particularly car suppliers with large production numbers must guarantee and be liable for the quality of their products for several years. Under unfavourable conditions, considerable costs can occur in the event of a warranty claim, which can completely consume the profit or the economic benefit. Very often, the value increase gained from the coating is relatively low compared to the value of the components, so that in the case of a faulty coating, components must be disposed of as waste which further reduces the economical benefit. For this reason, many customers require that evidence of the product properties in accordance with ISO 9000 is provided, and often this quality requirement is extended by their own quality audits. The ability to provide and document evidence of the quality can, if used properly, detect quality reductions early and remove the cause of the fault. This ensures that no quality flaw is detected by the customer who otherwise would then be fully within his rights to make a warranty claim. In the era of "just in time", time is very important. Every delay due to reworking or additional work costs money. The final requirements to be named are environmental requirements. Additional consumption of energy or raw materials and additional waste products affect the environment. Economical requirements alone require that the resources should be used as sparingly as possible during production.

The main purpose for the determination of the coating thickness  is to economically guarantee the properties given to the products by the coating.

Depending on the product, the companies using the coating thickness measurement have different aims. This is illustrated by an example.

A car manufacturer supplies large numbers of add-on components for engine production. The add-on component is screwed together with other components and then forms a module which is important for the functioning of the engine. The component is coated at the end of the production, which means that at the time of the coating process, the component is already relatively valuable. This coating is for corrosion protection on the one hand and on the other it must have certain mechanical properties. In this example, the coating must not be thinner than a certain value on the installation flange, in order to ensure a permanent screw connection. If the coating is too thick on the installation flange the coating structure might collapse under the high compression and the connection might become loose. The coating thickness should be measured and documented both on the installation flange and on the rest of the surface. Thickness in excess of the permissible thickness is to be indicated. It is obvious that, although the coating only contributes to a small degree to the value gain of this component, it is a decisive factor for its usability. If the component is viewed in isolation, one could to assume that excess thickness would not matter too much, and that sample testing followed by sorting would be sufficient for producing an adequate quality. However, the entire system must be considered. Although the component only corresponds to a small part of the entire engine, its quality will eventually determine the quality of the engine, because its function is relatively important for the functioning of the engine. Consider an unfavourable case, e.g. a batch of 5,000 components is delivered, 2 % of which have a faulty coating. The first components are installed and after 100 components the customer notices the fault. The consequence is that production is stopped and a warranty claim is made against the supplier. The supplier must collect the entire batch at his own expense and provide a replacement batch of the same size as rapidly as possible. Even if we disregard the fact that the components might require re-working, the same quantity of components must be available as a replacement batch. If this is not the case, and if a delay occurs, the customer can insist on a replacement batch from a different supplier so that his production is not endangered. In the worst case, the supplier might lose his customer. It rapidly becomes obvious that huge consequential costs can arise which do not compare at all with the cost of the coating and which might endanger the existence of a company. In the situation as described, a 100 % testing of the coating by online coating thickness measurement is important, so that the waste rate is lowered into the 0.1 % range.
Particularly medium-sized service coating companies bear an increased risk, because their value creation in comparison to the value of the component is relatively low. In addition, they struggle with small batch sizes and a large number of variants and types, which makes the start-up of the coating installation more difficult and also makes it difficult to achieve a uniform quality for all components.

Which coating thickness measuring devices can be used for fulfilling the requirements described above?

Decisive for the utilisation of coating thickness measuring devices is the production type of the coating. In the supply industry, highly automated coating installations are mainly used, while service coating companies mainly use manual or partially automated coating methods.
Highly automated coatings require measuring equipment which can measure a large number of moving components contact-free  and which can be integrated into the coating processes. Within a coating line, often only a few variants and types are coated using the same coating and subsequently measured in the measuring stations downstream of the actually coating.
Service coating companies require small, portable, easily operated devices which can be safely used for a range of coating processes. Most usually these coating thickness measuring devices are used for sample measurements during production with moving components.
In principle, coating thickness measuring devices can be categorised in two classes: contact and non-contact measuring devices. With the contact measuring devices, the easily handled eddy current measuring devices dominate with their simple and secure principle of functioning for standard applications, with ultrasound measuring devices for special applications. Because of the good price-performance ratio, eddy current measuring devices are mainly used by service coating companies. However, new materials such polymers or novel coatings are increasingly deployed, with which eddy current measuring devices cannot be used. This explains the desire to purchase small measuring devices with a wider range of features but a similarly good price-performance ratio as eddy current measuring devices.
The non-contact measuring devices are based on the photo-thermal or the ultrasound measuring principle and can be used on various materials such as metals, polymers and ceramics. Because of the current technical possibilities, non-contact measuring devices are currently designed for stationary applications in highly automated coating lines, and their use as mobile units is limited. Because of their higher price, these measuring devices usually cannot be reasonably used economically by service coating companies.

Which measuring method is used in our devices and which are its advantages compared to more traditional methods such as eddy current or magnetic?

The measuring devices in the "PaintChecker" series work using the photo-thermal measuring principle, a laser pulse is directed onto the coating which is then minimally warmed up. An optical receiver measures the time function of the temperature changes and an evaluation unit evaluates the coating thickness. Basically, thinner coatings can be heated more rapidly with the laser than thicker coatings. The time function is used as a measure of the coating thickness. The biggest advantages of this measuring procedure are that it is non-contact, measures at a high speed and is easily operated. The fact that it is non-contact allows dry and damp coatings to be measured on moving components, while the high measuring speed allows high local resolution of the coating thickness. The large measuring distance to the object to be measured  and the high distance tolerance facilitate application in automated environments. This means that the measuring head mounted on a robot can be moved around large objects to be measured, such as car bodies, without the risk of collision. Because of the small measuring spot and the high angle tolerance, even very small parts or very uneven parts can be measured, which eddy current measuring devices can only measure with difficulty.  Similarly, by means of mirror optics, cavities and other difficult to access places can be measured. It is also possible to measure warm components, because only the temperature change is used for the determination of the coating thickness.

On account of the principle of functioning, it is also possible to measure coatings on substrates other than metals. A limitation is, however, that it must be possible to photo-thermally excite the coating by means of a laser and that a temperature change must be noticeable. Nearly all the classical applications of eddy current devices with metallic substrates and dry or moist coatings are covered by our measuring devices. On non-metallic substrates, we always check the achievable temperature contrast on samples. The large number of coating/substrate combinations does not allow general statements to be made and a check needs to be made on a case by case basis. However, there are a number of empirical values which might be referred to.



When is it more advantageous to determine the thickness of the wet coating and when is it better to determine the thickness of the dry coating?

This in part depends on the aim, but also on the coating substrate combination and in some cases on the options for integration into existing installations.
If the aim of the coating thickness measurement is to rapidly correct coating faults. It is an advantage to measure moist coatings, because immediately after applying the coating, the reaction time is short. Usually, long drying units are installed downstream of the coating stations, which can hold many components and take a long time. In such circumstances, an entire batch could fit into the dryer and could only be tested after the leaving the dryer. In unfavourable cases, it is no longer possible to make changes if a quality loss is determined at this stage. In order to prevent such faults, installations are often started up in a very time-consuming way using individual test objects before the actual production is run. On polymer substrates, for example, in many cases only the moist coating can be measured, because only in this state can a sufficient temperature change be initiated on the object to be measured.
If the aim is to get a rapid high spatial resolution of the coating thickness, or if the components move very rapidly, it is best to measure the dry coating, because the measuring time is shortest on account of the thinness of the coating. For moist coatings, the measured coating thickness is higher, because a solvent such as water is still contained in the coating.
In some cases, the possibilities of integration in existing installations limit the application of the coating thickness technology, because, for example, only in certain places within the coating installation is sufficient space available.

For corrosion protection the correct coating thickness of very important, Car manufacturers make different demands in this area. How do we, as a manufacturer of devices, respond to the individual requirements and different standards?

Every car manufacturer has his own ideas about the requirements, and, of course, his own standards. At the centre, the function of the coating is to provide permanent corrosion protection, give colour, and to form a smooth, scratch-resistant and weather-resistant surface. These functions can technically be achieved with a range of coating structures. In these cases, specific requirements are also made on the coating thickness measuring technology. For example, several car manufacturers use the concept of online coating measurements using robots within the coating line solely for monitoring the coating thickness on all the car bodies. Others consider specialist offline measuring tables where, in addition to colour fidelity, shine and roughness, the coating thickness is also tested in samples. In the first case, the coating thickness measuring devices must cope under production conditions with the high mechanical loads typical for robots and provide high availability. This makes specific demands on the optics, the electronics and the mechanics. The measuring principle must cope with all kinds of tolerances, such as deviations in distance, angle errors, vibration, contamination and component wear, to name just a few.  Our devices were designed with the intention that the physical functioning principle of our devices should operate trouble free in such an environment and yet that they should still be easy to operate. The robot-aided coating thickness measurement poses without question the most demanding requirement, and other easier applications can profit from them.
In the second case, the offline measuring station, easy handling is most important. Here measuring systems must be manually and easily operable or it must be possible to adapt them rapidly, and without long setting up times, to a measuring robot. Service coating companies have similar requirements.

To what extent are car manufacturers already using online measuring devices during series production?

Online measuring devices are increasingly used by car manufacturers directly or indirectly during series production. The pioneer here is Audi in Ingolstadt, who, with their visionary character, have taken the lead and are an important figure in promoting progress in online coating thickness measurement technology. They were the first, in collaboration with the plant manufacturer Dürr, to integrate "PaintChecker" online coating thickness measuring devices by OptiSense into their coating installations. Currently, three coating installations are equipped with this measuring technology. Audi intends to achieve higher production reliability at constant quality and as a result save costs. Ford in Saarlouis has also decided to install online coating thickness measuring technology in one of its series production coating installations, pursuing similar aims to Audi.
Other car manufacturers, however, are still waiting and are currently not making any decision to use the technology in such a way. They carefully observe developments, keeping their own aims in mind, and check if the high expectations will be fulfilled during daily operation.
Increasingly, it is being seen that online coating thickness measuring technology is being used by suppliers. The main reason for this is the increased quality requirements of the car manufacturers, who require the quality to be guaranteed and verified. Especially suppliers for high-end or branded products, online coating measurement technology is mandatory, because here production if often working close to the edge of what is technically possible. Often, the online coating thickness measuring technology is not used by the car manufacturer himself, but is required by the supplier.

Online measuring devices are probably only of interest for large series production. Are there requirements which might be of interest to medium-sized companies?

Medium-sized companies wish to use online coating thickness measuring technology, but in most cases this is not economical on account of the wide application range desired and the high price of the devices. Our measuring devices currently only cover certain applications and are not universally applicable devices.
The best potential for the use of the online coating thickness measuring technology is found in large series manufacture with uniform automated coating processes. However, there are exceptions where the quality of the coating is the distinguishing feature and offers a competitive advantage in comparison to other companies. For example, with safety-relevant components or special optical effects, where the quality can only be guaranteed by means of online coating thickness measurements, its utilisation will also be economical in medium-sized companies.

What is the expected development of coating thickness measuring devices? Which improved or additional features will the devices of the future have?

Conventional eddy current measuring devices only offer a small margin for further development, because this principle of functioning has been largely optimised during the long time this product has been in use. Only details such as the size and the range of functions are still changeable.
The most frequently expressed wish is to be albe to carry out non-contact coating thickness measurements for non-metallic substrates, with similar functions, operation and purchase price as eddy current measuring devices. This means a sort of symbiosis of eddy current measuring technology and photo-thermal coating thickness measuring technology. These demands are mainly heard from service coating companies and medium sized companies.
The photo-thermal coating thickness measuring technology is profiting in a major way from the innovations in semiconductor technology. Increasingly, new, higher performance light sources and detectors become available at much lower costs, and these allow us to provide novel properties for our devices. Our devices have shrunk within 2 years from the size of a switch cabinet to that of a table top device. Modern light sources allow us to produce manually operable table top devices, which nonetheless fulfil the high requirements for laser safety. Our aim is to transfer the photo-thermal coating thickness measuring technology to hand-held devices. Important targets have already been reached, but further development work is required, particularly for miniaturising the optics and the energy management.