Process
The process begins with a descaling and removing soil from the workpiece. This can be done in the tumbler or in a separate cleaning system. After cleaning, the parts are prepared by combining them with water, media, and a surface conditioner. The surface conditioner lightly coats the workpiece in copper, while the media removes any residual mill scale or oxides. Finally, accelerators, promoters and metal powder are added to the mix. The accelerators and promoters provide the proper chemical environment for the plating to occur, such as the maintenance of a pH level of 1 to 2 to prevent oxidation and promote adhesion. The media that is already in the mixture cold welds the metal powder to the workpiece through impacts that are induced by the tumbling action of the tumbler. At this point the surface finish is typically matte to a semi-bright finish, however the finish can be improved with a water polish. The time required for the above process is approximately 50 minutes.
For some thinly coated workpieces a chromate passivation is necessary. Finally, the workpiece, whether passivated or not, is dried.
The media material is usually soda lime glass or a ceramic. It is usually spherical in form, but angular shapes are also used. For plating, media usage is usually 1 part media for every workpiece, but for galvanization the ratio is 2:1. However, various sized media are used in each batch with a typical batch consisting of 50% 4–5 in (100–130 mm) sized beads, 25% 2–2.5 in (51–64 mm) sized beads, and 25% 1–1.25 in (25–32 mm) sized beads. The smaller media are omitted when the workpiece has a cavity that the media can get caught in, such as a fastener's recessed head. Note that the media is reused many times.
This process works better if the workpieces' surface finish is slightly rough.
Read more about this topic: Mechanical Plating
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