Choosing the Media Mix
For mechanical plating, the usual "rule of thumb" is that for each cubic foot (by volume) of live load of parts, the plater uses one cubic foot of media. For plating cross recess screws, the ratio of media to parts is often reduced and the water level raised. For mechanical galvanizing (thicknesses over 0.001"), the general rule is to use 2 cubic feet of media to one cubic foot of parts. This provides additional cushioning to prevent chipping during the plating process. If the part type is difficult, the ratio of impact media to parts may be increased even more.
Most Common Media Mix:
- 4 volumes (50%) 4mm (4-6 mesh) or 5mm (3-4 mesh)
- 2 volumes (25%) 8-10 mesh or 10-12 mesh beads
- 1 volume (12½%) 16-25 mesh beads
- 1 volume (12½%) mush beads - usually 50 mesh beads
This mixture is sometimes calles a "4-ball" mix. A "3-ball" mix is similar to the above but with one intermediate size removed (i.e. 8 to 25 mesh beads).
Metal Plating Center Media Mix:
On some machines, the preferred four component media mix cannot be used. The most common example is the old 3M "Metal Plating Centers" which (usually) have 3/16" perforated holes in the separator unit which would trap the media with the parts. For these machines, we recommend: 6 parts 8-10 mesh beads; 2 parts 18-25 mesh beads; and 1 part 50-70 mesh beads.
Cross-Recess Screw Media Mix:
One Media size will lodge in the cross recess, this is media in the 10 to 25 mesh range. If any media size is capable of lodging it will lodge. Therefore, the plater must select a media mix that contains no sizes that will lodge.
2-Ball Media Mix:
Typical mixtures are 50% to 70% large beads (3 to 8mm) and 30% to 50% fine beads (50 to 100 mesh). The large beads are typically 3, 4, or 5mm beads but they can be even larger - such as 6mm, 7mm, or 8mm beads (available on special order from PS&T). The larger beads are typically made by a molding process, and are therefore both durable and expensive. A media mix like this will offer both the impact energy associated with the use of the large beads and the "throw" associated with the fine media.
"Mush" Media Mix:
All media is 50 mesh-100 mesh (i.e., 50-70 mesh [PS5070], 60-80 mesh [PS6080], or 70-100 mesh [PS7000]) with no larger media. This media mix has poor flow characteristics and typically plates at a lower efficiency than other media mixes. However, if the parts themselves act similarly to the media, this will work acceptably.
Fine Media - Pros and Cons:
Without fine media, the deposit is rough, the efficiency is low, and the throw into the recesses suffers. The mechanical plating process relies on the action of the fine beads to break up agglomerates of zinc that form in the (acidic) plating process. Without the fine beads, the agglomerates remain undispersed, resulting in a coarse deposit or an 'orange peel' effect.
During the plating process (including, in particular, the separation and media return) the fine media is typically lost from the system due to drag out. The finer the fine beads, the more these losses are encounted (i.e., 100 mesh is worse than 70 mesh and 70 mesh is worse than 50 mesh). These beads must therefore be periodically replaced.
Media should not contain an appreciable amount of broken media. Typically specifications are under 5%. Running heavy parts at a too high of speed will break down the media. The 'crush resistanace' of glass beads is about 31,000 to 36,000 psi. This is significantly in excess of the force needed to plastically deform (aka elastic deformation) the small (3-7 micron) zinc particales so as to 'cold-weld' the particales to the substrate. Thus, broken media is generally evidence of excessive mechanical energy being applied during the mechanical depsoition process.
Lodging of Media:
There is a simple test for lodging. Take the media that is being contemplated as the plating media mix and a few of the parts. Place them in a pint plastic bottle with water and shake vigorously by hand for two or three minutes. If the media can lodge in the parts, it will be evident.
It is impossible to completely separate media in such a way that 100% of the lodging size is eliminated: media in the sump, in cracks or crevices in the barrel, in the piping - all these contribute to the problem. For platers to whom this represents a significant risk, often the best solution is to set up an isolated system that only uses non-lodging media sizes.
Sampling of the media to determine the relative amounts of each of the various sizes may be performed. The actual separation of the various sizes is performed by vibrating a stack of U.S. Standard Sieves (available from many lab supply houses and from Gilson, who specializes in partical testing). The most common difficulty is obtaining a uniform sample of the media since the media tends to stratify with the larger beads rising to the surface. Dry media mixtures may be sampled with a tube or with a 'spinning riffler'. Damp or wet media may be sampled with a sampling probe such as those used to sample grain per ASTM C183. Either dry or damp media may be sampled by putting it in a pile and sectioning it (sometimes called "quartering it'). Slurries may be tested with sample cups designed with a long 'cutter' engineered to cut through the slurry and provide a uniform sample. Another sampling procedure is to take small samples continously from the batch of impact media as it is returned to the plating barrel; in this way, even if the media is stratified, a representative sample is obtained. Additional information and assistance on sampling and testing is available from PS&T.
Recessed Drive Screws Media Mix:
Plating of recessed drive screws is typically accomplished by using a ratio of about 3 cubic feet of parts to be plated to 2 cubic feet of media. This allows the media to flush in and out of the head of the fastener, allowing at least some throw into the recessed drive. Normally the only size of media used is "mush" or about 50 mesh beads. In essence, the head of the fastener is the substitute for the large beads the plater would otherwise use. For such fasteners, PS&T recommends the 433 High Throw Mechanical Plating Promoter.
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Glass Impact Media