222 Prakt. Met. Sonderband 30 (1999) 
pulled at a 60-degree angle to remove lose paint. 00 
The parts are then evaluated by comparing them to reference images contained in a VDA fre 
specification (2). The lower the damage the better the paint performance. 
[52 = 
o Single-Impact Test Bene 
This test is used to determine a painted surface’s resistance to impact from road debris. 
The test part is placed painted side down phasing a tube containing a wedge shaped steel anvil. A This € 
known weight is then placed on the back of the test part. The machine is set to an air pressure of 3 pes 
bar. The air pressure forces a small ball bearing up a steel tube striking the back of the wedge Tas} 
shaped steel anvil. The anvil is driven up striking the painted surface. The impact area is then flaw or 
covered with a specialized tape and pulled at a 60-degree angel. The impact is evaluated by The nl 
measuring the thickness of the impact area in millimeters. The lower the thickness the better the pares 
performance of the paint. 400, 60 
sen 
golyure! 
Cross-Sectional Evaluation of Painted Parts riz 
The above mechanical tests are used to improve and rate the performance of paint layers. 00V 
Procedures are also in place to improve the paint process itself. The goal is to reduce paint defects 
or irregularities to an absolute minimum. Defects are not only visually objectionable, but may also 
accelerate the degradation of the paint. 
A number of defects are possible in a high volume painting process. A few are listed below: 
e dirt inclusion 
® crater 
® paint over spray 
» metallic flake agglomeration 
To study and quantify paint defects, more than a visual surface evaluation is needed because of the 
difficulty assessing their cause. The painted surface is actually cross-sectioned to gain a better 
understanding how a defect may have occurred. This process is described in the next section. 
e Metallographic Specimen Preparation 
The painted surface or defect area to be analyzed has to be initially sectioned to a mountable size. 
The sectioning should not be done near the area of interest due to possible chipping and damaging 
of the paint layers during the rough cutting process. Then, the area of interest is sectioned with a 
precision diamond saw using a high-concentration diamond wafer blade. The cutting is always 
done in such a way that compression is applied toward the paint layers, otherwise the paint layers ars 
might be pulled away from the metal matrix. If a flaw has to be investigated, the sectioning should da 
be between 0.5 - 1 mm from the defect to be able to reach the center of the defect during the 
grinding / polishing process. 
The mounting should be performed using a slow curing epoxy resin to avoid gaps between the 
specimen and mounting media resulting in good edge retention during the polishing process. Best 
results are obtained using a vacuum impregnation method. The specimens are then mounted in 
plastic mounting cups using steel clips to support them during the curing process. The particular