Intervene early instead of reworking

A structural fingerprint of a coating by a portable interferometry measuring system allows detailed analysis of multi-stage coating processes - from substrate to clear coat. This allows early measures for process optimization and reduction of quality costs.

The later quality deficiencies or defects of a coating are detected in the process chain, the higher are the negative economic effects - this is not a new insight. However, establishing an inspection process in a paint store's production line that is capable of quickly and reliably providing criteria beyond the experienced expert's eye as to whether a surface actually meets specifications or whether expensive rejects are to be expected at the end of the process is not a trivial undertaking.

Particularly in the case of multi-layer coatings, several processes build on one another, and problems from previous processes generally propagate from the substrate through the cathodic dip coating to the clear coat. If defects are not detected until the final quality control, it is usually only possible to determine in which process step the original defect occurred by means of very complex analysis. Not least, the time delay that has accumulated by then makes it extremely difficult to investigate the cause.

In view of the fact that the coating process, taking into account energy, capital and environmental costs, accounts for around 30 percent of total vehicle production costs and that up to 70 percent of the energy required by an automotive assembly plant is generated in surface technology, the need for process optimization and minimization of scrap and rework cannot be overestimated. Despite great effort and expense in quality assurance, serious quality problems in the area of painting are sometimes not detected in time, even in the automotive industry: According to NHTSA.gov, more than 25 car models from different manufacturers have had to be recalled in significant numbers since 2000 due to poor paint quality. Such actions not only cause immense direct costs in the millions. Indirect damage is no less problematic, for example due to customer dissatisfaction resulting from such incidents and their impact on future brand loyalty.

Additional challenge multi-substrate

An additional challenge in the field of automotive coating is increasingly proving to be the trend toward multi-material bodywork, fired by the need for lightweight construction. Additively manufactured components as well as aluminum, plastics and composites have to produce an absolutely homogeneous paint finish side by side. Even a seemingly simple material such as steel can have very different surface properties, depending on whether it is galvanized with a ferroalloy, solution galvanized or double-layer galvanized. So even seemingly identical materials can interact differently with one and the same process [1].

In a word, achieving a high quality appearance with increasing cost pressure - including expensive raw materials and energy - on a modern automobile is becoming more demanding. Since the structure and optical effect of each applied coating layer is influenced by the previous layers, there is great potential for reducing quality costs if a decision could be made immediately after application and drying of a layer by means of a rapid inspection process as to whether the surface is in order and can be further processed or whether reworking is necessary or even discarding is to be expected at the end of the process chain. In particular, this makes it possible to draw conclusions at an early stage about the errors in process control underlying the surface defects.

Figures 3-5 show an example of the evaluation of a coating sequence by the SurfSpec with elevation display (Images: Soliton).

Mobile coherence scanning interferometry

At the suggestion of the automotive industry, a mobile measuring instrument based on coherence scanning interferometry, formerly known as "vertical scanning interferometry" [4]. has now been developed that makes it possible to characterize all phases of a coating process quickly and in production. It is called SurfSpec 4D and has been distributed since by the company Soliton for the German-speaking countries. It is a compact system that weighs only about three kilograms and can be positioned by hand. The SurfSpec's housing has a maximum edge length of 15 centimeters, vacuum feet couple it stably to surfaces in any orientation so that both horizontal and vertical structures can be measured. Vibrations or other environmental influences do not affect the measurement result. In contrast, commercially available measuring systems with comparable measurement properties have so far only been able to find application in the laboratory.

"I am not aware of any other measuring instrument on the market that is comparable to our measuring instrument in terms of resolution and function and flexibility," reports Michael Zerbin, sales engineer at Soliton. Each measurement and surface analysis takes less than 30 seconds, enabling real-time process control without cycle loss. The powerful device offers vertical resolution in the single-digit nanometer range, regardless of the field of view, and can measure both rough and smooth surfaces.

In terms of measurement, the device goes far beyond what previous research projects and studies have determined to be essential in many areas to provide information relevant to the human eye when analyzing surfaces [2]. To easily quantify structures up to 5 mm wide, the SurfSpec 4CD has a field of view of exactly 16 x 16 mm. The lateral resolution is 15 µm, and the vertical resolution is less than 15 nm. The RMS measurement range is from less than one nanometer to greater than two microns. Sheet metal and paint can be measured at almost all stages during production (Figure 1). This makes it possible to assign a structural fingerprint to each process step, from the raw substrate through the cathodic dip coating to the clear coat, and to check it again at the same positions after the following process step. The measurement can be performed in any orientation, whether upside down or on vertical or curved surfaces.

High-precision topography evaluation Filtering by frequencies

The data density is 1.4 million data points per measurement. No less important than the highly precise acquisition of 3D data of the topography of a surface is the evaluation of the data. The included software therefore offers automatic functions to identify elevated areas, for example, or to determine the number of pinholes, as well as their density, height and volume. The detection and geometric analysis of user-defined defects is also possible. Furthermore, data export for further analysis or for correlation with other instruments to common software systems such as Bandify 3D is possible.

By filtering the data according to specific frequencies, surface differences can be made visible. Research results presented by General Motors and Michigan Metrology at the "DCST Focus Conference" show how helpful the use of special filter settings can be. Figure 2 shows an example of how distinguishing features can be found between visually nearly identical aluminum samples. In this case, for example, the use of a frequently used "0.8 mm filter" did not yet lead to recognizable differences. However, filtering the data according to a defined frequency, which was recognized as decisive for optical effects, then detected so-called "roping" on two of four samples. If this "roping" occurs, it can be assumed later that the final color quality on the vehicle is insufficient.

The included software package has a variety of analysis tools as well as built-in functions in the equipment control software to detect such patterns. Since corresponding results can be evaluated for each material and process step, it is possible to prevent parts of unacceptable quality from being produced or further processed early in the manufacturing process. Figure 6 shows a painted surface using the height image, where higher and lower areas are indicated by false colors. Defects are thus very well represented. Last but not least, the comprehensive surface characterization by the interferometric measurement technology also allows the qualification of small features such as depressions. Therefore, a defect can be traced to determine whether it is covered or neutralized by subsequent coatings. This is an important aspect for avoiding unnecessary scrap, since it is often not known exactly how large a defect may be so that it no longer represents a quality defect after the final coating. For this reason, stricter criteria are often applied to be on the safe side, i.e. necessary. Surfspec can be used to precisely analyze such defects before a subsequent coating is applied and to check whether visible defects have occurred after the painting process has been completed. In the best case, the specifications can then be further defined and costs reduced. Thus, the SurfSpec measuring device offers a wide range of potentials for reducing quality costs through early detection and analysis of defects and thus increasing coating quality at reduced costs.

Bibliography

(1) Nelson K. Atafuan et al, "Evolution of the Automotive Body Coating Process-A Review," Coatings 2016, 6, 24.

(2) Don Cohen et al, "Substrate Surface Texture "Spectroscopy" and the Prediction of Final Paint Appearance," PNT1004 USCAR Painted Surface Prediction Project SO242895.

(3) N. Jouini et al, "Multi-scale analysis of high precision surfaces by Stylus Profiler, Scanning White-Light Interferometry and Atomic Force Microscopy," International Journal of Surface Science and Engineering 3(4):310 - 327, 2009.

(4) Caber, P. J. (1993). "Interferometric profiler for rough surfaces." Appl Opt. 32 (19): 3438-3441.