Dip Filling and Electroplating of Castings and Powder Metallurgy Parts

In recent years, in order to reduce costs, there have been an increase in the use of castings and powder metallurgy parts to replace the previously used turned parts without internal holes. In particular, powder metallurgy parts are used in small parts. As productivity increases, the surface treatment work of castings and powder metallurgy parts will inevitably increase accordingly. It is generally believed that the surface treatment of metal is not directly related to the internal structure of the metal, but it should not be ignored.

When parts such as castings and powder metallurgy parts are manufactured, the internal structure of the blank has many holes and the surface is covered. Although the unevenness on the surface is not serious, if you use a surface treatment method in an aqueous solution, sometimes the treatment liquid cannot be plated even after the treatment liquid is immersed inside the metal. Sometimes the coating that looks beautiful on the surface will expand from the inside, causing corrosion, Discoloration, especially the mechanical strength of powder metallurgy parts will be significantly reduced.

For powder metallurgy parts, it is usually treated with water steam at a temperature of about 600 to 650°C to generate an iron oxide layer to fill the surface holes. But theoretically speaking, it is difficult to fill the hole if it is too large. In addition, you can use shot peening as a remedial method to break the holes on the surface. This is also a method, but it is not ideal for dead corners and other places. In response to this situation, resin liquid and other methods can be used to block the holes, which is called the dip filling pre-plating treatment method. Originally, the dipping and filling treatment method can improve the pressure resistance and is a method often used in production. It is no exaggeration to say that the current automotive industry mainly uses aluminum alloy pressure casting parts, which cannot be realized without impregnation and filling. At the same time, it also hopes to reduce costs.

Originally, as the dipping and filling before electroplating treatment, the dipping liquid uses unsaturated polyester resin liquid or diallyl phthalate resin liquid, etc., and only one of them can be used. However, there are many kinds that can be used in this regard. The effect is also different. If this type of resin is to meet production requirements, it must meet the following conditions


1. The volume shrinkage rate when the impregnation liquid hardens is below 10% or less.
2. When dipping, the viscosity is between several centipoise and tens of centipoise or less.
3. At a relatively low temperature below 100C° (a hot water bath is enough), it will harden in a few minutes.
4. Maintain a long service life of more than several months.
5. Can be widely applied to various materials.
From the professional knowledge of polymer chemistry, generally speaking, the above 1 and 2, 3 and 4 are contradictory to each other. However, impregnation solutions that actually meet these conditions are under development. The vacuum impregnation filling method has been officially adopted after half a year of testing. In the past year and a half, more than 1 million powder metallurgy parts have been successfully produced.

This method uses corrosion-resistant resin with good adhesion to block the internal holes. This is a "temporary treatment". There is no need to check whether discoloration has occurred after plating. The mechanical strength of castings and powder metallurgy parts not only does not deteriorate for a long time, but also achieves unexpectedly good results of increasing mechanical strength.