Background Information

Workpiece name: Grille body

Post-processing: Electroplating + painting

Project name: NX4C

Material grade: ABS710 (natural color)

Problem Description:

There is a black spot problem in the grille body produced by the material, and the coating of the black spot parts is defective after electroplating, which needs to be solved by process technology at the customer's site, otherwise the product can only be scrapped.

Defect analysis:

1. The position of the black spot of the product is not fixed, and it randomly appears on the large surface of the product without regularity;

2. The black dots have a sheet-like structure and are not in the shape of dots, there is no smooth interface at the edge of the black dot, and the edge structure is more prominent;

3. The position of the black spot of the product is visually confirmed as dark brown, no silver white or pure black;

4. The defective black spots all appear on the surface of the product, and there is no defective black spot in the inner layer or core layer of the product;

5. The black spot position of the product is viewed under the microscope as debris, non-integral or continuous;

6. The black spots are not caused by oil stains or sludge on the parting surface of the mould split;

7. There is no black spot problem in the production products after starting-up the machine. After 48/72 hours of continuous production, black spots appear, and the probability of black spots appearing is high with the long production time;

8. Through the analysis of elements, the analysis result is: the main component is iron.

Cause analysis:

According to the above typical characteristics of black spots, it can be directly judged that the source of black spots is caused by the presence of rust spots on the parting surface of the mould split or the perforation; The rust spots fall off to the surface of the mold after the mold is opened, or are electrostatically adsorbed to the surface of the mold after the mold is opened, and the melt is formed on the surface of the product when the next mold is injected.

For example, the carbonized decomposition material in the barrel or the residual material in the hot runner or the impurities in the material will not appear on the surface of the product 100% of each mold, but will definitely appear in the inner layer of the product or in the shape of an ellipse or a dot, which is basically an overall continuous state under the microscope; For example, the mold ejector pin or the ejector block is silver-white instead of black-brown. 

Troubleshooting:

In order to verify the rust of the mold for a long time, the parting surface and the perforation of the mold are cleaned and maintained on the machine (the fixed mold is polished with whetstone, and the movable mold is cleaned with a dry ice machine), and the continuous production is tracked. After confirming the parting surface and the perforated surface of the mold in continuous production for 72 hours, there are obvious brown rust spots in the exhaust and areas other than the exhaust, which can be wiped off by hand, and the black spot problem also appears on the surface of the production product.

Corrective measures:

1. Add the exhaust gas port at the parting surface of the mold and the position of the perforation;

2. Increase the frequency of mold cleaning and maintenance, and clean up the mold surface in time after corrosion and rust spots appear to ensure the cleanliness of the mold;

Conclusions:

1. Through multi-directional analysis of failure mode and failure phenomenon, it is directly judged that the black spot is caused by mold parting surface, perforation and corrosion, not our company's material problem;

2. This case analysis reflects the technical expertise of our company.