Managing a changeover from cyanide to a noncyanide alkaline zinc electroplating over a weekend and without the usual headaches...
|Customer gripes.||Quality losses.|
These were some of the concerns facing Independent Plating when high reject rates led us to consider converting one of our workhorse plating systems, an automated 15,000-gallon, eight to 10,000 amp cyanide-zinc rack electroplating line. The reject rate on the line was running unacceptably high, and no amount of effort had been able to improve the yield.
The first consideration was whether to make the conversion at all. Years ago, when cyanide baths no longer performed satisfactorily, they could be dumped and replaced at relatively little cost. Not so today. It made sense to look at an alkaline, noncyanide system that could give us environmental advantages as a secondary benefit.
Maintaining quality primarily involved product appearance (notably color), adhesion and corrosion protection. Chemicals used in the converted line would affect these factors. Following a lengthy search for a chemical supplier, we selected MacDermid because of its technical expertise and commitment to walk us through the conversion. MacDermid's satellite laboratory in the Massachusetts area was to prove a big help in solving problems that lay ahead.
Initial Makeup. A key problem at the outset was the initial makeup of the zincplating bath. We had two options: 1) Put zinc anodes in the tank and dissolve them with caustic soda. This would require several days; 2) Purchase a purified zinc concentrate and add water and caustic to it. This would be relatively speedy, and use of purified zinc would assure a startup bath with less contamination than a bath made up by dissolving zinc anodes. We decided to use the zinc concentrate for that reason and it would save time on the conversion.
Wall of Steel. Another quality concern in planning the new line was the potential of zinc plateout. We use densely hung racks, literally running a "wall of steel" through the line. This requires us to keep the caustic and zinc concentrations high in the plating tank.
In some instances, more likely in an alkaline than a cyanide tank, trying to maintain the required zinc caustic concentrations causes zinc to go into solution as undissolved particles. These particles or fines can settle on flat, horizontal part surfaces as they are plated, causing surface roughness. (Zinc fines are not as bothersome on a barrel line because parts are tumbled.)
In addition, use of zinc anodes can cause the zinc concentration to rise during downtime, such as week-ends, because the zinc continues to dissolve.
Because high concentrations of caustic and zinc are inherently difficult to maintain effectively with active (zinc) anodes, we chose steel anodes in the zinc plating tank rather than zinc.
Customer Concerns. End users were a concern. How would customers (and customers' customers) react to the conversion? The reject problem had never been a concern to them, since rejects were caught at the plant. But we knew that adhesion and appearance were two things customers worried about with a noncyanide bath.
Poor adhesion can cause premature and delayed blistering. Because many of our products are post-formed, we knew adhesion would be a customer concern.
Appearance is critical. We apply decorative zinc primarily, which means we are looking for a bright finish, a sort of low-cost duplicate of nickel chrome plating. For example, you naturally want all four legs of a chair or table to be consistent in appearance.
To offset such customer doubts, the supplier plated some of our parts offsite using Isobrite® 424 noncyanide alkaline zinc brightener. We sent the samples to our customers. Although the appearance was somewhat altered, it was actually closer to nickel chrome, customers liked what they saw.
Redesigning the Line. This campaign to allay fears (ours and our customers') progressed simultaneously with conversion plans. We redesigned the line as follows:
Consequently, additional pre-cleaning would be required for the new line. A caustic soda predip (1216 oz/gal concentration) was positioned immediately ahead of the non-cyanide zinc plating tank. Agitation was provided. In addition, the rinse preceding the caustic soda predip was altered from immersion to spray to decrease water carryover. The water for both tanks was changed from a well to a city source to ensure a consistent, contaminationfree supply.
A nitric acid post dip rinses off any films remaining from the zinc plating solution, conditions the zinc surface to accept an Iridite® chromate conversion coating and extends the life of the chromate bath. In addition, additional agitation was installed on the chromate tank to improve performance.
The lacquer coating was retained. Most products on the line, except those to be painted, are coated with a waterborne lacquer. Because the lacquer improves corrosion resistance, we retained this bath.
Implementation. Before shutting down the cyanide line, a crew ran the plating racks and new steel anodes through the system's first six tanks: cleaners, immersion rinses and a three-stage 50 pct HCl solution. This was done to ensure that the parts were thoroughly cleaned.
Then, on a Thursday at 9:00 p.m. (after the second shift), a crew pumped out the 15,000-gal cyanide bath, steam cleaning equipment and piping, then changed and added filters and made repairs.
The maintenance staff changed piping, went over the electrical systems, added a filter and a solution agitation pump, changed an immersion rinse to spray, and a myriad of other preparations. There were many people climbing about the machine, but tight prescheduling kept things coordinated. Most failures in a conversion occur because suppliers and their customers do not sit down and work up a plan. It is important, particularly when it involves a different technology.
Cost Savings. The smoothness of the Thursday to Tuesday changeover was surprising, giving us no downtime. When we started running the line, the first six racks were acceptable, but not as good as everything that came off afterward. From day one, everything except the first six racks was 100 pct. The alkaline, noncyanide bath has not slowed throughput and has certainly improved output.
A cost analysis indicated several key savings the conversion provided:
And, of course, the virtual elimination of rejects translates into greater yield for significant savings in costs.
Independent Plating Co., is one of three firms on New Street in Worcester, Massachusetts, I manage. The other two companies are Ivy League School Furniture and Worcester Chrome Furniture. The three firms are divisions of Worcester Manufacturing, which also owns Babyhood Plating and Level Plating. The three New Street companies produce five million feet of tubing and 1.5 million pounds of wire and fabricated nonwire products annually. Customers include manufacturers of bleachers, satellite dishes, school furniture and supermarket equipment.