Process Steps

The mechanical plating process requires a series of chemical additions. The quantity depends on the total area of part surface. The total area must be determined prior to each cycle. All of the plating steps are performed in the same barrel, normally without rinsing or stopping the rotation. The only exception is removal of heavy oil or scale; these contaminants should be removed before parts enter the plating barrel.

The first process steps include a series of chemical and metal additions designed to mildly acid clean and activate the substrate and then to apply a thin, uniform copper strike. The copper strike provides a clean, galvanically receptive part surface. The next step involves adding an "accelerator" or "promoter" agent, which creates a chemical environment that controls the rate of deposition and subsequent bonding of the plating metals. A defoaming agent is used during the process to control foaming and to prevent any associated loss of plating solution.

The plating metal is added as a dry, fine powder or a water slurry containing the powder. Platers add metal in a series of steps and in amounts proportional to the coating thickness desired. Commercial plating thicknesses of 5 to 12.5 pm (0.0002 to 0.0005 in.) usually require two to three additions of metal, while greater thicknesses or "galvanized" coatings can demand eight or more.

The following represents a typical sequence of operations for mechanical plating:

1. Alkaline or acid preclean (if necessary)

2. Prepare surface

3. Copper strike

4. Add accelerator/promoter

5. Add plating metal

6. Add plating metal

7. Add plating metal

The plating cycle is carried out at temperatures between 15 and 32 °C (60 and 90 °F). A pH between 1 and 2 is required at all times to ensure proper adhesion and a high plating efficiency. The low pH level acts to maintain an oxide-free condition at all times on both the part surface and the surface of the plating metal particles. These temperature and pH conditions facilitate mechanical bonding ("cold welding"). The process has an efficiency of about 92%; that is, approximately 92% of the plating metal added is actually plated on the parts.

The mechanical plating process usually takes about 45 min. At the conclusion of the cycle, the slurry of glass beads, coated parts, and water discharges into a vibrating surge hopper under the plating barrel. The capacity of this hopper should be large enough to accept the entire load, thereby freeing the barrel to begin plating the next load. The slurry dumps onto a vibrating screen or magnetic separator. Water sprays are used to wash and remove glass beads from the parts. A glass bead handling system consisting of a sump, a double-diaphragm pump, and an overhead conical storage reservoir with a pinch valve at the bottom is used to recycle the beads. More than one batch of glass beads can be used to speed production.

As described above, the mechanical plating process can be used to apply a variety of metals and codeposits with varying coating thicknesses, all with the same equipment. This capability offers advantages in equipment cost and space utilization over other forms of plating.

0 0

Post a comment