Improving Your Liquid Finishing System

Q: My finishing system is not working as well as it was. Is there anything I should be looking at to try to bring it up to speed?

A: Over the years, I have been asked this question countless times. Everyone always thinks the equipment is at fault. This is a short review of some of the items I have seen that cause problems with the metal and wood liquid application systems.

The Real Transfer Efficiency (TE) of a System: I always use application transfer efficiency (ATE) instead of TE. An example of TE would be a glass of water on my desk. The TE of the water to the desk is 100% (nothing is lost), but the film build is not uniform. But, if you use ATE, this is a combination of what the film build requirement is and the TE of the coating. So if we put the same glass of water uniformly over the entire surface of the desk, the ATE would be 100%. In the same vein, if the requirement is 1 mil and we are applying 2 mils at 100% TE, we are overcoating by 1 mil (or waste of 1 mil). So the ATE is only 50% or a lot of wasted paint and paint dollars.

Changes in Coating Resistivity: Once, we had two large electrostatic application systems that ran at about 30 FPM with great ATE results. Over time and several batches of paint, the film builds were varying greatly and the paint usage was very high. Also, the film builds were varying greatly with the line-of-site areas having much higher film builds. We checked the paint retains at the coating manufacture and found that the resistivity of the coating was getting hotter with each batch. The coating formula had not changed, but the resin supplier had changed its formulation and it was getting more conductive (hotter), which resulted in a change in coating resistivity. We implemented a quality check at the paint supplier for their raw materials and the problem was completely solved.

The problem was not the end user or equipment supplier, but the resin supplier to the coating supplier. I like to say that a strong finishing system is a finishing triangle. Everyone must work together to assure repeatable system results. This is one of the reasons I recommend that the end user always have a resistivity meter to eliminate one possible coating variable. The finishing triangle is a three-part team that includes the end user, paint supplier and finishing equipment supplier. All three must work together to form a strong triangle. If any one of the three fail, the triangle falls apart.

Problems with the Process: Recently, I was called in on a finishing problem. Sometimes the system ran great, but the results were varying all the time. This company ran all coatings from 55-gallon drums. They would come in and start all the systems at the same time, which included turning on the conveyor, loading the paint in the system, start finishing and turning on the paint agitator. But the results were varying all the time — from clogged tips to sags in the coating and transparency in the film — all out of the same batches of coating.

As we know, the pigment in coatings are normally heavier than the resins, so the pigments sink to the bottom of the drum when not agitated. When they turned the system on at the same time, for the first several minutes, a higher percentage of pigments was being removed from the coating; resulting in the tip clogs and heavy film builds. Later, when the pigments were fully mixed in the coating, the percentage of solids in the coating was less than a new drum of paint. At times, the coating at the correct film builds would sag. This occurred because a large percentage of the pigments were removed during early morning startups.

The problem was not due to a coating or equipment issue, but due to the process. The solution was to turn the agitator on 15 minutes before it was used and that completely solved the problem. While this is a simple process change, many end users have this common minor process problem that can generate major finishing problems.

Constant Viscosity. To have repeatability, we need constant viscosity control. The problem with coatings is they are temperature sensitive. The system will have varying flow rates as the coating viscosity changes. An example is a normal day of operation. In the morning, everything is cool (or even cold) and the viscosity is higher. So the pressure controls must be increased to compensate for the viscosity increase. But, as the coating gets warmer, the viscosity comes down and the fluid pressures need to be reduced to maintain the same film builds. If not, you can get sags and, again, while the TE might be high, the ATE is being reduced, resulting in coating waste.

The solution is to heat the coating to a point where the viscosity is always the same. This is where the coating supplier needs to provide a thermal viscosity flatline temperature. This is a temperature where the viscosity is always constant. Some people believe heating the coating helps dry the coating. This is partly true, but only because, at a constant viscosity, the atomization will remain constant; therefore, the atomized particle size is constant, which will result in uniform film builds. It should be noted that when heated paint is about six inches from the atomization tool, it will return to the ambient temperature. So, what we are really doing is making the finishing system constant, which is much easier to operate and control. And, by making the film builds uniform, the parts will dry faster.

Conveyor Grounding: Every conveyorized finishing system needs to have a grounding for both system safety and higher ATE percentages. I have seen systems where the grounding on the conveyor is basically zero and the ATE is very poor, at best. The areas facing the application tools had heavy film builds, while the side areas are much lighter films. To get acceptable products, the varying film builds resulted in a very poor ATE percentage. The operators tried to change the system setup parameters to compensate for the changing system performance. Regardless of what they tried, the results got worse and worse. The conveyor grounding must be checked on a regular basis and cleaned if the grounding is being reduced. Good grounding is a result of good preventive maintenance programs by the end user. The conveyor was cleaned, which returned the conveyor to a grounded system and the ATE came back into balance.

These are just several areas that I have encountered while visiting end users. It’s normally the little problem areas that cause the major problems. Just remember, if the film build requirement is 1 mil, an actual film build of 1.2 mils is a 20% paint waste. Control over the small items saves major problems.

Steve Romer is a senor systems application engineer at Sames Kremlin. Visit sames-kremlin.com