Pre-Plate Finishing

Q. We chrome plate a small aluminum fitting. We first use a vibratory process on the part to produce a competitive, refined, pre-plate finish. This often creates small part dings and surface orange peel that only show up after plating. The vibratory polyester plastic media produces a matte finish that you can’t see through. What causes the surface damage, and how can we test for part damage on the hard-to-see-through matte finish prior to plating to avoid an expensive strip and re-plate process? —J.P.

A. The first thing that’s concerning is that you’re using a media harder that your part. This is one of the main causes of small dings and surface orange peel. Below is a list of possible reasons for part damage within a vibratory system:

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1. Media hardness (media harder than the part).
2. Media size (small media produces a tighter mass that protects part).
3. Part-on-part damage (too many parts within the mass).
4. Compound/water flow (soap and water cushions the mass).
5. Machine setting (running too aggressively).

Testing for part damage hidden by the vibratory polyester’s media matte surface prior to plating can be inspected or determined by:

1. Rubbing the part surface with a pencil eraser to remove the matte (created by the polyester media), exposing the underlying surface for visual inspection of part damage.
2. Adding a secondary vibratory burnishing (surface brightening) operation to expose or amplify part damage.

To determine if part-on-part damage is the cause of your dings, put only one part in the machine. If there is no damage, this could be the problem, however, this is not the cause of orange peel.

Increase your water/soap flow and decrease your machine energy as your next step, but eliminate one variable at a time. If increasing the soap and decreasing the machine’s energy fails to eliminate the surface damage, then media hardness and size is most likely your cause.

The polyester media is harder than aluminum. The media is approximately 45-50 Rockwell, while the aluminum is approximately 25-40 Rockwell. Harder media will ding and orange peel softer parts.

If the media is determined to be your problem, our recommendation is a softer, lighter, synthetic plastic media in a smaller, 3/8-inch cone. The synthetic media is less than 1 on the Brinell scale, which is much softer than aluminum. The synthetic is also the lightest of all preformed media at 45 lbs per cu ft.

The soft, light synthetic media is the first choice for vibratory pre-plate finishing and surface refinement on soft alloys such as aluminum, brass, copper, stainless, zinc, gold and silver. Surface refinement is often combined with high-energy centrifugal disc machines that flow the media by the part instead of imparting the hammering effect of vibratory machines.