Effect of Powder Coatings on the Appearance Quality of Castings

Effect of Powder Coatings on the Appearance Quality of Castings

ABSTRACT: In this paper, the adhesion problem of white powder material on the appearance of an aluminum alloy die casting in our company is discussed. Through the comparative analysis of white powder adhesive and other normal positions, the origin and formation mechanism of white powder adhesives are found, and the pertinent adjustment and improvement are carried out to improve the appearance quality of the casting.
1. Background
  As shown in the following figure, the part of an aluminium alloy die castings produced by our company is made of ADC12. The product requires uniform appearance, no shot blasting and surface oxidation treatment. Fig. 1 is a normal appearance castings, and Fig. 2 is a white powder castings with bad appearance. The proportion of bad occurrence is 10%. Impacts the appearance quality seriously.
Good die casting appearance
Fig.1 Good die casting parts appearance
No good die casting parts appearance
Fig.2 No good die casting parts appearance
2.Reasons Analysis
  As shown in Figure 2 above, the appearance of the white powder of the defective castings is not uniform in color, irregular in shape, and the location of adhesion is not fixed. This phenomenon was found in the investigation of different batches of products in stock. According to these appearances and without surface treatment such as passivation and anodic oxidation, it is preliminarily judged that this phenomenon is a mildewed white spot produced by environmental impact during casting circulation or storage. Usually, mildewed white spot of this kind of aluminum casting can be erased slightly, so part of the products with this phenomenon are collected. Don't clean it slightly and use ultrasonic cleaner and cleaning liquid to clean it (condition: cleaning temperature 50 C, cleaning time 5 minutes). The test found that the white spot can not be effectively removed by the above two ways. It is concluded that the white powder is not mildewed white spot, but mildewed white spot. In order to clarify the origin of the white foreign matter thoroughly, SEM analysis was made for the castings with normal appearance and white spot respectively.
1) SEM analysis of the local position on the surface of normal castings is carried out. The results are as follows (see Fig. 3 (a) and Table 1 (a).
2) Scrape off the white adhesives with a knife for SEM analysis of castings with white powder adhesives. The results are as follows (see Fig. 3 (b) and Table 1 (b).
SEM analysis of good and NG die casting surface
Fig.3 SEM analysis of good and NG die casting surface
Table 1 Analysis results
Location Element Weight%
C O Al Si Fe Mg
a 20.72 16.15 46.36 6.9 4.49 0.15
b 27.85 42.54 6.21 11.13 0.28 8.12
  The results of SEM analysis and comparison between normal and white adhesives show that the content of C, Al, O, Si and Mg in white adhesives is 7.13%, 40.15%, 26.39%, 4.23% and 7.97% higher than that in normal castings. The content of C, O, Si and Mg in white adhesives is obviously higher than that in normal castings. Element content, from the paint manufacturer know that these elements are widely contained in GRAPHACE series of a certain model release agent, of which O, Si, Mg are the main components of the powder, and the powder has strong high temperature resistance, appearance characteristics of white powder, consistent with the actual situation of casting appearance. It can be concluded that the powder substance in GRAPHACE coating (after dilution, the content is very low) accumulated and remained on the surface of the casting during the die casting process, forming the white matter adhesion on the surface of the casting.
3. Improvement measures
  Because of the particularity of the process of the product, it is impossible to solve this problem by changing the coatings for the time being. Therefore, the following countermeasures are adopted to improve the quality of the product.
1) Adjustment and control from coating accumulation itself: By observing the actual production situation on the spot, it is found that after 4-5 hours of continuous production, the phenomenon of coating accumulation appears on the local position of the die cavity surface. As shown in the box position of Figure 3(a) below, in order to solve this problem, the copper wire brush is added every 4 hours to clean the die. As shown in Figure 3 (b) below, the local cavity of the cleaned die is obtained.
Before and after mold cavity cleaning
Fig.4 Before and after mold cavity cleaning
2) Improvement in the cause of coating accumulation:
Under the condition of guaranteeing the demoulding effect, the dilution ratio is adjusted from 1:80 to 1:100 by adjusting the coating concentration, which reduces the probability and speed of coating accumulation in the cavity.
4. Improving results
  Through the implementation of the above measures, in the batch production situation, the problem of white powder adhesion has been well controlled, and the proportion of undesirable products has been reduced from 10% to less than 0.5%, which greatly improves the appearance quality of castings. At the same time, by reducing the concentration of the coating, the starting cumulative time of the cavity coating is extended from 4-5 hours of continuous production to 10-12 hours of continuous production, which reduces the cumulative speed of the coating and reduces the cleaning frequency.
5. Concluding remarks
1) The improper use of powder coatings will accumulate in the die cavity, which will have a negative impact on the appearance quality of castings.
2) The concentration change of powder coatings will affect the accumulation rate of coatings, and the proportion concentration should be lowered as far as possible without affecting demoulding. For the adhesion of foreign bodies on the surface of castings, it is a kind of problem to analyze the adhesion on the surface of castings by SEM, to clarify its composition, to trace its origin and to improve it pertinently. Effective solutions.
 


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