QMT Features: February 2012
Measurement of coating thickness of duplex coatings for car bodies.
A combination of a sophisticated phase-sensitive eddy current sensor and a magnetic-inductive sensor is capable of automatically carrying out many typical measuring jobs including measuring duplex coating thicknesses


Quality control of painting processes in automobile manufacture is becoming increasingly important. The requirements on efficient corrosion protection and paint quality are growing with the simultaneous target of reducing costs, for instance. This has led to a reduction in the Zn coating thickness on galvanised steel car body parts, for instance, and a tightening of the tolerances on thickness for the individual paint coating.

This process becomes even more complex with the different methods of galvanising the steel parts used, the additional further corrosion protection measures and the increase in the use of substrate materials other than steel, e.g. aluminium. The subsequent painting process for all these different parts and methods must be monitored efficiently and compliance with the tolerances must be ensured.
The traditional approach to measuring the individual paint layers on galvanised steel parts using a magnetic inductive sensor, with the zinc thickness being deducted as a value assumed to be constant, cannot meet these current requirements. Even within the same car model, the zinc coating thickness can vary greatly because of the different suppliers of the body parts and be further changed by subsequent forming operations. The separate measurement of thin electrophoretic dip applied coatings (see Case 1 component (2) of the paint system, e.g. 20 µm) in particular, therefore, demands precise measurement of the zinc coating (approximately 5-10 µm) to allow verification of compliance with the thickness tolerances demanded. At the same time, however, it should be possible to measure paint coatings on aluminium, if possible without needing an additional sensor. The Fischer ESG20 probe was developed specifically for these requirements in automotive production. It is a combination of a sophisticated phase-sensitive eddy current sensor and a magnetic-inductive sensor that is capable of automatically carrying out many typical measuring jobs (duplex-mode application, Figure 1):

1. Paint system/zinc/steel) simultaneous, very precise measurement of the zinc coating and the coating thickness of the paint system; Paint and zinc coating thicknesses are shown in the display.
2. (Paint system/aluminium) measurement of the paint coating thickness on aluminium parts.
The substrate material is detected automatically by the sensor, i.e. the operator need not worry whether he is measuring the paint thickness on galvanised steel or on aluminium parts. The zinc coating is measured with high precision in Case 1 by the phase-sensitive eddy current method through the paint coating (lift-off compensation).
Practical software algorithms ensure that this eddy current probe can be calibrated without additional zinc reference standards. If, on the other hand, the measurement is being taken on aluminium parts in Case 2, special conductivity compensation ensures that different aluminium alloys (variation in conductivity) have no influence on the measurement of the paint thickness.  The correct coating thickness is always measured very precisely and reproducibly without additional calibration.
Other manufacturers have employed dual probes for further measuring jobs in the past. To avoid additional measuring systems being required for these applications, the ESG20 probe can also be operated in a dual mode. Thus, the sensor can also carry out the classical measuring jobs of «paint/steel» or «paint/aluminium» with automatic detection of the substrate material.
The ESG20 may therefore be used as a universal sensor, tailor-made for process control in automotive production.

Article By Dr. Hans-Peter Vollmar

Photos courtesy Volkswagen AG Wolfsburg

www.helmut-fischer.com
  
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