One of the common complaints voiced to the finisher by fastener manufacturers is the presence of mixed parts. While annoying, this condition causes many more problems and is a greater headache than most coaters realize. With today's high-speed assembly systems, bowl feeders are often employed to bring parts from holding areas and reservoirs to the assembly point. Clogging the systems with alien fasteners can close down the feed system for minutes and in a worse case, jam it so tightly that the feed track or bowl has to be taken off line to be freed (it was estimated several years ago that downtime in an assembly plant could cost about $50,000/minute).
Robotic assembly, a growing assembly tool, operates exactly as programmed. Whereas a human operator would pick up a foreign part and toss it aside, the robot will follow its programming and attempt to install the part. Fit or no, it will go in or the robot will stop. This usually necessitates stopping the process and adjusting and resetting the robot, at expensive downtime rates. Finally, in the statistically run world, a minute imperfection in a lot of parts is considered grounds for rejection of the whole order.
Where do these interlopers come from? A study made several years ago, and still true today, found that many parts were commingled at the plating line. It was found that many parts stuck into corners and catch points of plating barrels and were not fully emptied when the barrel was discharged. When the next lot of parts was run, the caught parts broke free and were carried along. More rare, but still happening, are mixes caused by handling. Especially critical was the condition where the plater had multiple tubs of the same lot. Operators were found to have emptied an odd tub into the hopper. Instances of single and extremely low occurrences of mixed parts were attributed to open-topped tubs being the receptacle for loose, picked-up-off-the-floor items.
The study found that a significant number of mixed parts came from the heat treaters (the numbers were 56% from heat-treated parts as received, 40% from internal plating operation mixes and from four to as high as 9% from manufacturers output).
What to do to avoid these complaints? A very good step is to look over the incoming stock. Although hand sorting surely is not practical, a quick overview of the quality of the incoming parts could save a few black marks on your record. Certain users are now requiring that all parts destined for robotic installation be in 100% conformance. This condition requires a sort stage after the coating, which is sometimes an extra, sub-contracted operation. Numerous firms have sprung up that specialize in vision and other types of sorting. Unless the lot is small, in-house sorting usually does not pay. Internal, manual sorting only offers a 90% plus efficiency and would not conform to the requirement that the carton contain 100% of the specified fastener.
Another safeguard for your plating reputation is to have the source notified of mixed parts when you spot them and get written documentation of who was called, when and what the source asked you to do about them. This saves a lot of blame and finger pointing later when the final user kicks the job back. If possible, try to save some of the mixed parts for late determinations. Parts with different head markings (different sources) obviously came from heat treater mix; parts with the identification may or may not be a manufacturer or heat treater problem.