Article posted in The Official Publication of the Powder Coating Institute, Powder Coated Tough
Written by Larry Ensley, technical service manager at Hubbard-Hall.
It is generally accepted that paint and powder coating processes suffer from the overspray that builds up on hooks, racks, and hangers. Consequences include a drop in electrical conductivity, which reduces coating adhesion, and results in a poor visual appearance.
The solution is to remove the build-up as often as possible, but that’s something many coaters find difficult and expensive. Chipping, burning, and blasting the excess buildup of paint and powder are slow, labor-intensive processes that end up damaging the hooks and racks. Furthermore, swapping out hangers and racks so they can be cleaned in an off-line process creates downtime, which is another expense most in the industry would like to reduce or eliminate.
One of the major efficiencies with implanting an in-line paint stripping process is that it drives towards LEAN manufacturing. A prevalent movement in today’s manufacturing is LEAN – a methodology that focuses on minimizing waste within production systems while maximizing productivity.
Fortunately, there is a better way. This article explains how coating process operators can save money and improve quality by incorporating an in-line rack and hook stripping process.
The “How” of In-line Paint Stripping
Parts to be powder coated are hung on metal hooks or racks. This provides an electrical path that creates an electrostatic field. Charged powder particles are drawn onto the parts, and onto the hooks, hangers, and racks. As this coating builds up over multiple cycles, it reduces the electrostatic effect, coating adhesion suffers, and so too does the appearance.
Coating equipment operators have tried a variety of ways to remove this build up. These include:
• Physically chipping it away by hand with hammers and pliers.
• Sand blasting.
• Burn-off ovens.
• High temperature molten salt baths or fluidized sand beds.
• Off-line chemical stripping processes.
All the methods above incur downtime and add labor cost during the cleaning methods. They are labor-intensive, risk damaging the hangers and racks, and most importantly, require coaters to maintain surplus inventory of the racks and hangers in order to continue the paint and powder coating processes. The surplus inventory of racks and hangers also adds cost and consumes valuable storage space while they are not in use.
An alternative to these methods is an in-line stripping process, which adds an immersion tank to the coating process after the part unload point. Empty hangers travel through the stripping solution to remove a single coat of paint or powder build-up. The immersion tank contains heaters, circulation pumps, and eductor nozzles to reduce the time required to strip the racks. The racks are rinsed and cycled back to the rack loading station, where the finishing process is repeated.
Another important consideration when designing the in-line paint stripping process is to ensure that the proper paint stripping solution is installed in the strip tank. The paint stripping solution must be compatible with the construction material of the hooks or racks (steel, aluminum, plastic, etc.) which can be attacked by certain chemistries.
What happens to coating that’s been removed? The in-line paint stripping system includes in-tank agitators and a circulation system with pumps and filters. Agitation keeps the coating residue in solution until it’s drawn into the pumps. The pumps push it through filter bags that separate out much of the stripped coating. The clean stripper solution is then returned to the tank where it continues to do its job. This filtration process means there is minimal consumption of stripper and less tank maintenance, which keeps operating costs low. This is very important to the life of the bath. The time required to do this will be dependent upon routine maintenance performed on the stripper solution, as a bath that is well maintained can last indefinitely.
An in-line paint stripping system is close to being maintenance-free. However, despite agitation and filtration, some residue will accumulate at the bottom of the dip tank. This needs to be decanted periodically and disposed of per local regulations. The resulting downtime is just a few hours per year, and is minimal when compared to the maintenance required for other paint stripping processes. Likewise, filter bags will need periodic replacement, but again this maintenance is minimal.
Paint and powder coat operators who install in-line stripping realize benefits in four areas:
• Coating quality.
• Ergonomics and safety.
It’s hard to measure how well a coating adheres to the substrate. Sometimes the first indication of a coating problem is when it starts to flake off. While poor adhesion has many causes, inadequate electrical conductivity is a common reason. By stripping excess coating from racks and hangers on each cycle, not only is good conductivity assured, but it’s also consistent every time. Poor electrical conductivity also impacts appearance which can lead to an uneven coating and defects.
Ergonomics and Safety
Removing excess coating by hand comes with serious health and safety issues. Furthermore, a manual paint removal process is time consuming and labor intensive. Physically inspecting each contact point on a rack or hanger is inefficient, and there is wasted motion when racks and hooks are removed from a conveyor and replaced with a clean one. An in-line paint stripping process eliminates these costly inefficiencies.
If the coating line isn’t running, it’s not making money, and with many coaters operating on thin margins, any loss of capacity eats into profitability. If spare racks and hangers are available, swapping them out costs output. And if spares aren’t available, shutting the line down for cleaning can deliver a financial hit.
Nearly all of the paint stripping options, other than an in-line process, add unnecessary costs and labor. In addition, in-line stripping virtually eliminates the damage and wear caused by other removal methods. Operations that have gone to in-line stripping report labor savings of up to 67 percent as well as consistent improvements in quality yields. Costs associated with rack and hanger replacement are also avoided by going to in-line stripping. On a high-volume line this can easily add up to thousands of dollars saved per year. Yet more savings are realized in terms of consumables used for off-line stripping.
What if you are a small facility?
Unfortunately, not every organization has the footprint or capital to invest in an in-line paint stripping operation. In those instances, an off-line chemical immersion process sized to their specific needs is recommended.
Many coating plant operators look at stripping as a necessary evil. Some don’t even calculate the costs of the process. Those who do see in-line stripping as an opportunity to reduce costs while increasing capacity and improving product quality. The benefits that an organization can experience when implementing an in-line paint stripping process include reduction in rack repair costs, reduction in real time accidents, and reduction in strip time from hours to minutes.
Larry Ensley is technical service manager at Hubbard-Hall.
February 10, 2020