CHINA AUTOMOTIVE SUPPLIER QUALITY MANAGEMENT BRIEFING

(Continued from the last issue)

▌The impact of the machining process on having black spots on the surface of casting

The impact of oxidation blackening on the surface of castings produced by machining processes is mainly reflected in the cutting fluid used, the protection during storage and transportation of the machining process and the temperature and humidity of workplace.

In winter, when the die castings are transported from the die casting workshop to the machining workshop or stored, if there is air conditioner in the room and the temperature is significantly higher than outdoor, the water vapor in the air will condense the frost on the surface of the castings, which will cause rapid oxidation of the castings. Therefore, when the castings are just transported to the room, it is suggested that the package not be opened or parts not processed right away, so as to prevent the casting from frosting and oxidation blackening due to long time storage. In the high temperature season, the temperature of the products in the processing air conditioned room is relatively low, so they cannot be directly transferred to the warehouse with higher temperature. They must be transferred to a warehouse buffer zone for temperature transition, otherwise the products will be corroded by condensation water and produce black spots.

Aluminum alloy is amphoteric metal, it will change color when it meets either acid or alkaline material, so we should use neutral cutting fluid when choosing cutting fluid in machining.

▌The impact of cleaning and passivation treatment on having black spots on the surface of castings

After machining, the die castings should be cleaned and passivated as soon as possible to prevent the products from oxidation and blackening in the air.

The process includes the following: ultrasonic cleaning (oil removal) – secondary cleaning (removal of residual ultrasonic liquid on the surface) – surface conditioning (activation) – secondary cleaning (removal of surface conditioning liquid on the product surface) – surface passivation (phosphating) (uniform protective film is formed on the product surface) – three times cleaning (the first two times are for the removal of residual passivation liquid on the surface, and the last one is further cleaning to ensure that the product is clean, in order to facilitate product drying) – drying – offline.

In the whole process of cleaning and passivation, we should pay attention to the frequency of cleaning solution replacement; temperature and pH value of cleaning solution; pH value, temperature and passivation time of passivation tank; and drying temperature and time and other process parameters.

If it is not cleaned or cleaned thoroughly, the surface of die casting parts will be left with corrosive substances such as release agent, cutting fluid, passivation fluid and water stains, which will accelerate the speed of mildew forming and blackening of aluminum alloy die casting parts. Improper selection of cleaning agent or cleaning process will also cause corrosion and oxidation of die castings. Therefore, after machining, die castings should be cleaned, passivated (if applicable) and dried immediately.

▌The impact of air tightness test on having black spots on casting surface

Different customers have different requirements for various products. The product purchased by the author’s customer is thermostat housing, which is an aluminum die casting machine part. The customer requires air tightness test to prevent product leakage.

For air tightness test, the detection method generally used is water test, with water as the detection medium. The parts are placed in a special fixture and then immersed in water. It is then filled with a certain pressure of compressed and the pressure is held for a period of time. It is a good product if there is no bubble on the water surface. After the air tightness test, the product is taken out from the water and stored at the prepared working stations. At this time, we should pay attention to the position and direction of the product, and the direction that is easy to drain the accumulated water should be downward, otherwise the accumulated water cannot be drained out, which will cause water stain corrosion and blackening in the depression of the product. The natural drying environment temperature, humidity and drying time are also reasons for water stain corrosion and blackening, the author thinks that drying treatment should be carried out to eliminate the risk of water stain corrosion and blackening caused by excessive natural drying.

▌The impact of packaging on having black spots on the surface of castings

Packaging is generally decided by customers. Different customers require different packaging. The author observed that there are several packaging methods: first, placing the product in an outer box which has a large plastic bag inside. The products are placed in layers inside the large plastic bag, with each product touching with each other. Cardboard is used to separate each layer. After the placement, 1-2 small bags of desiccant will be placed on the top layer, then the mouth of the large plastic bag is twisted at the top of the box and the box cover is sealed. The advantage of this packaging is that the cost is the lowest and the most easy to operate; the disadvantage is that there will be collision and friction between each product in transit, resulting in oxidation and blackening. This packing is suitable for most customers with large order quantities and stable orders.

(To be continued)

By Felix SS YUAN

SHENTOU SUPPLY CHAIN MANAGEMENT CO. LTD. is a Shenzhen, China, based company serving international automotive clients in the implementation of their China strategies and programs. CHINA AUTOMOTIVE SUPPLIER QUALITY MANAGEMENT BRIEFING is a bi-monthly newsletter published by Shentou to address the specific and unique quality challenges and concerns international automotive companies face with suppliers in China. Comments are welcome at qms@shentou.com. Click here to subscribe.