Screw machined parts with blind holes, present many challenges. Full electroplating coverage is just one of the challenges, but you can be sure your customer has a high expectation that full plating coverage is easily attained. Risks should be identified and communicated early on, preferably during the quoting or contract review stages. Luckily some design engineers plan for this by adding a weep hole which allows for solution transfer but that’s not always possible. Leaving the electroplater and obstacle to overcome.
The Importance of Cleaning & Activation
As with any plating process, good cleaning and activation are essential. Picture trying to fill a soda bottle by submerging it into a pail of water. The narrow neck of the soda bottle will only allow a certain amount of solution to transfer because of the air trapped inside. If the diameter of the hole in the neck of the bottle was as wide or wider than the body of the bottle there would be little to no problem with getting the water in. The same theory applies to a screw machined part. Adding a weep hole to the bottom allows for more solution transfer. The bigger the weep hole the better the solution flows. The position of the hole can also be critical.
In the case of screw machined parts, chemicals that flow into the blind holes must still be exchanged with fresh solution. This is necessary so that the cleaning or activation is effective. Flushing out those chemicals that you worked so hard to get in is equally important. No chemical residue can be left behind, especially if the chemical isn’t compatible with the electroplating solution.
As a rule, blind holes with a depth to diameter ratio of one-to-one will rinse properly and have good plating coverage. When the depth of the hole is greater than the diameter, the risks of residue build-up and poor coverage increase.
Choosing the Right Process and Equipment
The right choice of electroplating equipment is essential to your success. Tumbling action of plating barrels can be increased but only if the parts are robust enough to handle the movement. Soft brass parts with an outer diameter thread would get damaged this way,
Vibratory plating is gentler on parts, especially long thin annealed brass parts. The vibration liberated air bubbles which get replaced with chemicals being used.
Spouted Bed Electrode (SBE) plating uses a high velocity solution to pump the chemicals into the holes. This process is great for small quantities of miniature parts.
Success is dependent on the interaction of these variables, but it is possible. Plating adhesion shouldn’t be compromised if the chemicals exchange properly. Electroplating thickness in the recessed areas will be less than the outer diameter simply because of the natural flow of electrical current. The inner diameter is a lower current density area than the outer diameter.