Switching from post-plate to pre-plate

A. Hi, Cliff. Normally, the cost advantages of preplating accrue from the fact that the raw material is plated as a continuous strip instead of individual parts which must be jigged. That greatly reduces material handling cost, especially labor cost. The downside is that when you shear pieces out of this, or put holes in it, there will be no plating on the edges.

Whether an unplated steel edge will be a satisfactory finish depends on what metal you are electrodepositing, the environment, and the application. Zinc, for example, is anodic to steel and can galvanically protect those bare cut edges to a good degree; whereas nickel plating is cathodic to steel and the edges will rust aggressively as the exposed steel tries to galvanically protect the plating. Unless you can roll it under to keep it away from the elements, or the environment is benign (indoor, air conditioned) nickel preplate is unlikely to be acceptable.

So, if the trays are zinc plated and will be used indoors, chances for success are very good. Best of luck.

A. Hi Cliff,

It would be extremely helpful to us if you informed us of the materials of construction and the plated materials. In addition to sheared edges, if the material is to be worked in a forming operation the plating may crack and delaminate as well, unless the deposition process has been specifically tuned for a workable deposit. Electronics mfgrs. runs into this as pre-plated IC leads must survive trim & form operations.

Regards,

A. I have spoken with mechanical engineers and designing for many electronics OEM's, and there is definitely a trend toward using preplated material. Personal computers and other chassis are now frequently made from preplate; the exposed steel on the sheared edges is not a problem to them. I am talking about companies like HP etc.

Regards,

A. Thanks Pat. The folks above are using preplate, so it can certainly be feasible.

But it still depends on the environment and the plating material. Zinc is sacrificial, and protects steel despite the exposed edges; plus, most personal computers are exposed to very benign environments.

Using a non-sacrificial plating like tin or nickel with exposed edges in a less benign atmosphere is a problem though. As an extreme counter example, I've seen electronic controllers on the plant floor in a plating shop where integrated circuits actually broke off the board (all 16 legs) in under 3 years (that couldn't have been a great environment for people either). The acceptability of preplate and its bare edges is a matter of specific cases.

A. I represent the United States largest producer of preplated strip products. We supply a wide range of pre-plated nickel and zinc products into the automotive, electrical, computer, household, and office equipment markets. Not every application can be switched to preplated, but a great deal of them CAN and DO.

A. Hi Dan. If powder coating is doable, you might add it to international units. Post plating probably will not match the corrosion resistance of pre-plate plus powder.

Regards,

A. I've had numerous experiences with conversion from post-plating to pre-plate . . . as an engineering manager at HP and Eaton . . . and as VP/Dir of Engineering at 3 metal fabricating companies . . . now managing projects at a manufacturer of medical products.

Post plating is a big headache to the fabricator. Most have to send parts outside for plating. They have to be counted, packaged, shipped, tracked, expedited, recounted on return, detrashed, and often put back into the production process. Parts get lost and damaged. It is expensive, problem prone, and adds one week to inventories and lead times. Your fabricator does NOT like doing this for you.

In the 1990's, ASTM A591/A591M (now ASTM A879 ) electrogalvanized steel became popular for electronics equipment. It has a very thin zinc coating. The appearance was much better than the "garbage can" look of hot dip galvanized steel.

In recent years, the appearance of ASTM A653 has improved dramatically. It now commonly has a smooth, spangle-free appearance. Its zinc coating is several times the thickness of electrogalvanized, which translates into improved corrosion protection.

Punched and sheared edges will be exposed steel. A small amount of zinc tends to be smeared across the edge. Galvanic action causes zinc to partially protect bare steel in near proximity. Edges may show discoloration (rust) over time, but is usually minimal.

Post plating offers the benefit of depositing zinc on the edges of the part. However, it can bring a host of other problems. Many process variables in the plating process offer opportunities for defects. Plating can look bad or simply peel off. If you can find a good plater with reliable delivery, pay him well.

As a fabricator, I was presented with many designs from high tech customers. Over time I learned that whenever the customer specified Zinc & yellow chromate, it meant the customer had abandoned continuing engineering of the product or was technologically uncompetitive. Zinc & yellow chromate came to mean "old technology" and "dinosaur".

Certainly, your design will be the major factor in deciding how to coat your product. Be very careful that you don't choose post-plating because "that's the way we've always done it." . . . or "our customers expect it." That's not true anymore.

Thanks for the great update, Mark, you make many excellent and informative points that are very easy to agree with!

The only thing I take issue with is a prejudice against zinc plating plus chromating as a "dinosaur". Cyanide zinc plating is unheard of today, as is ammoniated acid zinc, and there is strong continuing movement to alkaline zinc plating. That means that very few plating shops are using the plating process they were using just a few years ago. And with the impact of RoHS, almost no shop in the world is using the hexavalent chromating technology they were using less than five years ago; they are using thick film trivalent chromates or silane and zirconium topcoats completely unheard of a few years ago -- spanking new plating technology, followed by spanking new conversion coating technology, followed by spanking new top coating technology is hardly "dinosaur" stuff :-)

Rather, zinc plating simply remains the least expensive plating process, yet it offers cathodic protection -- which so few other processes do. So, if the driving force behind the widespread switch to preplating is, as you say, cost reduction, it hardly seems fair to criticize post platers for not moving, for no reason whatsoever, towards a more expensive finish when the finish they are offering is already superior but more expensive than the preplate they are competing against :-)

But, yes, a plating shop which decides to do alloy plating like zinc-cobalt, zinc-iron, zinc-nickel, or tin-zinc rather than plain zinc plating has at least taken a first small step away from the "commodity" market, where they are constantly beat to death by penny-pinching multi-national OEMs; plating shops are almost certainly better off recusing themselves from that market :-)

Thanks and Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 17, 2010

A. Hi Corey. I'm sure the suppliers of pre-plate have had that question before and can address it for the specific material you have in mind. My bet is that there is no difference.

Regards,

A. Hi Kishore,

Two suggestions, firstly cut the parts before electrogalvanising, that way there will be no bare edges after treatment.
Secondly look at the possibility of brush plating (often also called Selectron or Dalic plating, although these are brand names rather than the process) the edges.

A. Hi Kishore. In addition to Terry's good advice, you might simply make sure that the parts are promptly and thoroughly dried. Additionally, sometimes the most economical solution is to use preplate, but to roll/turn the edges so the bare edge is not visible and exposed. Good luck.

A. Hi Woody. If the vendor doesn't want to laser cut zinc coated steel, that's up to him -- it won't do much good to interject third parties. And as for the differences between the two vendors, that's another thing I can't help you with :-(

I can say, though, that there is no difficulty in zinc electroplating the finished components if you want to go that way. But I can also say that your existing preplate, with a proper pretreatment and proper powder coating should be very highly rust resistant, even with bare edges because powder coating on bare steel can be excellent, and with preplate you're a big step beyond that. Good luck.

Regards,