Q. We make steel stampings from 10 gage CRS. They weigh an average of 120 grams (1/4 lb) each. The parts are deburred in a through-feed vibratory finisher at a rate of 1,200 parts/hr. After deburring, they free fall into large, unlined, cardboard boxes. We have shipped these boxed parts for years to domestic customers with no corrosion problems. Now, some of our customers have moved their assembly plants to Mexico and other distant sites and the parts are showing signs of rust by the time they are unpacked—maybe as much as a month after we ship. One of our vendors suggested dipping the parts in a solvent based, water displacing inhibitor. We sent samples of dipped parts to three customers and two of them rejected the process because it adds a cleaning step at their end. We are now investigating a dryer to follow the vibratory operation so the parts will be packed dry. Can you suggest the best method for drying following deburring? J.G.
A. There are a variety of drying methods that will work for you, and I’ll give you some options to consider. First, however, let’s consider what you are really trying to accomplish. It seems to me that drying the parts may be putting an expensive band-aid on the problem that is to get the parts to the customer without corrosion. If the parts arrive corrosion free, do you really care whether they were wet or dry when packaged? I think not, especially when you consider the very high cost of drying. Much of my time and effort is focused on recommending corrosion control during manufacturing and shipping. The interest in this subject runs high when it is included in seminars and workshops. Often the best answer is in chemistry rather than drying.
The first suggestion I have is to conduct Cast Iron Chip tests with the fluids you are using in the vibratory machine. This test is easy to conduct in your own facility, it is available in various labs around the country, and it gives results within 24 hrs. Humidity cabinet testing is also valuable but costs more and takes days, or weeks, for results. Contact me through PRODUCTS FINISHING for details on doing your own Cast Iron Chip tests. I am suggesting this because the cheapest way to protect your parts will be to use compounds that give adequate protection. Using corrosion testing, determine the failure concentration for your compound. Use at least twice that concentration in your finishing machine. For most ferrous parts, this is adequate, and you can easily do a production test to prove it.
Next, prepare a garden type sprayer with the proper concentration of your compound and use this to spray any cardboard that will contact the parts during shipment. The reason is that normal cardboard is very acidic. When wet parts come into contact with cardboard, they activate that acid. Just to see how dramatic this is, put a few shreds of your box and/or divider material into a glass of water and let it soak overnight. In the morning you will find that the water is between 3.5 and 4.5 pH. Acid corrodes ferrous parts. Period. By pre-treating the cardboard, you neutralize the acid with the normal alkalinity of the finishing compound.
Even with this method, it is good to let the parts air dry in the box as long as possible before closing the box. Containers make good humidity cabinets. As they ship through warm and cold climates, humidity in the box will go up and down, often challenging the best inhibitors and especially the water based ones.
One last thing to consider before investing in a dryer is dipping or spraying the parts, cold, with a water-based inhibitor that won’t interfere with your customer’s next operation. There are many inhibitors on the market that will work this way, but you still will want to spray the boxes as extra security.
Now, let’s look into drying methods. Starting with the least expensive, let the parts air dry, either in the packing boxes, or better yet, in wire totes. This can take at least a day, but can be improved with fans blowing over the parts. The floor space and handling is the only cost and probably much less than other methods. There are lots of equipment methods to consider. I’m not prepared to give a comparison of energy costs for these different approaches, so be sure to include energy costs in your evaluation. They can be significant. Perhaps the least expensive method will be to convey your parts under warm air heated by a turbine type, regenerative blower. The high volume, low pressure air is about 105–120°F, warming the surface, but not really heating the part while blowing off the water. Most other methods, except cold spinning, usually heat the entire part—a very expensive process.
My next priority choice for your type of parts is a heated corncob dryer. These are available as through-feed tumblers, or in specially designed vibratory finishers. The tumbler styles are generally gas heated for better economy, while the vibratory models are most often electrically heated. Corn cob has the added advantage of cleaning and polishing the parts, so they will look very nice and may give you a competitive advantage. The downside of cob drying is that the grains may get lodged in small passageways or tapped holes. A suggestion: If you use cob, it is also a mild acid. Treat new cob with your compound to neutralize the acidity.
A heated dip tank, or spray system, with a water based inhibitor is another possibility. If the temperature is above 160°F the parts will dry very quickly when spread on a conveyor. Air blow-off will speed the evaporation process.
Convective and infrared conveyor dryers are also very popular. These are tunnel ovens that may be supplemented with blowers, hot air fans, air-knife blow-offs, or other options. Any way this is done, it is expensive. When you are evaluating energy usage, don’t forget that any compressed air you use is also expensive.
The above equipment suggestions can all be in-line, and automatic as parts emerge from the screen separator of the finishing machine. Some, such as hot spraying or turbine regenerative air blow off can be applied right on the separator, if it is long enough.
A last drying method is centrifugal drying. These can be very low cost if used cold, but they are batch operations, not easily automated. The labor cost can exceed the energy costs of the systems already discussed. You can get these dryers with optional hot air blowers to reduce cycle times and increase throughput, but you still have a batch operation.
Analogous to this discussion is a dishwashing machine. Did you know that the energy cost is greater than the cost of the detergent?