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CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

CANADA'S LEADING INFORMATION SOURCE FOR THE METALWORKING INDUSTRY

Boosting TE

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Better transfer efficiency (TE) cuts liquid and powder coating waste and costs

by Pierre Bachand

Transfer efficiency (TE) is a critical component of a finishing operation, whether you’re applying powder coatings or liquid paints.

The goal of a cost-efficient finishing line is to have the highest possible TE.

In liquid paint applications high TE is not new; it’s been around for a long time – the paint brush. It offers close to 100 per cent TE, which is tough to beat. Yet it’s a slow process.

Transfer efficiency is generally accepted to be the percentage of paint actually on the part divided by the volume sprayed or atomized. Apply 100 ga , 30 ga on the part = 30/100 x 100 per cent  = 30 per cent.

There are other common definitions in the industry. For instance the Ohio EPA defines TE as “The percentage of total coating solids employed by a coating applicator which adheres to the object being coated.” The American Society for Testing and Materials (ASTM) describes it as “the ratio of the weight of paint solids deposited to the weight of the paint solids sprayed, expressed as a per cent.”

The widely industry-accepted ASTM method is the D5009

 

Example: 48 in. long panels six inches wide spaced six in. on a conveyer  with a couple of scavengers before and after to reduce the variations for first and last parts being coated. It’s quite evident that by using this test it is impossible to get over 50 per cent with non electrostatic applicators because half the space is empty.

History of TE

Allen DeVilbiss, nose and throat specialist, developed the first paint atomizer around 1888 to better apply medication to the throat of his patients. Some will remember the perfume atomizer with the round pompon.

The paint gun came shortly after. It became so successful that he quit his practice in 1890 to pursue the spray gun business.

In the 40’s what motivated Harold Ransburg in developing the first electrostatic liquid paint application equipment was the realization of the immense waste of paint when using conventional equipment and the associated costs. He worked at his father’s factory and one of his duties was to clean out the water wash paint spray booth tanks. He realized the low transfer efficiency of existing equipment from the amount of paint residue.

Here is a graphic taken from ITW Ransburg Literature to which I have added other automatic systems. The list goes from best to worst in TE.

No 2 Process manual spay gun from Ransburg: 100 per cent electrostatic applicator (no air or pneumatic pressure for the atomization) low volume/min, affected by Faraday Cage Effect. TE  greater than 95 per cent. This manual applicator is possibly the oldest original electrostatic unit still in production.

Automatic Disk System: rotary atomizer on a short or long hydraulically or electrically actuated vertical reciprocator  inside a Omega shaped paint spray booth offers up to 40k rpm TE.

Automatic Bell System: high speed rotary atomizer up to 55k rpm, fixed, oscillating or robot mounted. This system can apply very high solids paint (up to 98 per cent solid), can be set-up for very fast colour changes (seconds) and can be used with solvent or water Based materials

Improving TE for liquid finishing systems

The main elements for high efficiency are:

  1. The right spot size (particule size approx 20 microns)
  2. Typically, viscosity, flow abd atomization are the main factors. Low viscosity makes it easy for the atomizer to get the correct particule size or spot size. The less energy that is required the more efficient will be the application.
  3. The right voltage: typically the higher the better. Tests have found that there is little or no improvement above  100kV
  4. The right particule velocity. Just far enough to get to the target areas and still be able to benefit from the electrostatic charge for the overspray.
  5. The right grounding of the part, the most important element both for safety and efficiency.
  6. The optimum electrical resistivity of the particle. If too conductive it will lead to a short out and there will be no charge. Sometimes if too resistive, you get the same effect and you must add a polar solvent in order to charge the paint. For the No 2 Process, the resistivity range is from.1 to 1 MΩ
  7. Low atomization: less particle speed, a nice soft pattern that gently goes near the target where the electrostatic ¨weak¨force will bring it in and capture a maximum of overspray.

Improving TE for powder coating applications

Every manufacturer aims to boost first pass TE. In powder coatings processes, because of the reclaim and recycle systems used with large volume operations, first pass transfer efficiency is less important. Often, less than optimum settings will still improve results such as inside corner coverage.

With liquid paint, as soon as you have too much material on the surface, it sags and run. With powder, you get orange peel but it seldom runs like liquid paint.

In spray-to-waste powder operations where the non adherent powder is scrapped, optimization of the applicator is desirable. The operator wants a unit that charges all types of  paint  (epoxy, TGIC, urethane, TGIC-free, acrylic  and special finishes such as metallic (bonded and blended) easily.

How high is transfer efficiency with powder?

Very little hard numbers are available because there are too many variables. The absolute best powder, if applied to an improperly grounded part, will probably coat less that an average liquid coating material. Powder just wont  stick to the part. I have yet to see a list of TE on powder equipment for the powder.  I would venture that if one were to use a large round deflector with a good charging powder at low flow on a large flat target with a minimum of booth air, you could attain TE in excess of 85 per cent.

Here is a list of elements that will optimize TE (gun and system).

  1. The 3 ¨Cs¨of painting: Cleanliness. Cleanliness. Cleanliness.
  2. The right voltage for the right job.
  3. Optimise the amperage (if your units can): typically, large flat surfaces should have max amperage. With cavities, reduced amperage. Withre-coats, you need minimum amperage. (Combinations of amperage and voltage can be used to optimize results)
  4. Optimise environmental conditions: humidity and temperature.
  5. Optimise line speed. If you go too fast you get out of the optimum performance /powder charging range of the guns. It may be necessary to add a few units.
  6. Adjust the reciprocators’ (if in use) stroke, speed and gun to target distance. If you use fixed guns, adjust spray patterns and distance between applicators. In this case there is no powder blending so the powder flow must be more precise.
  7. Optimise line density by having the next part act as a powder scavenger.
  8. Optimise air flow velocity in the booth if necessary (while still meeting NFPA regulation).
  9. Choose the right deflector for the job.
  10. Fluidize the powder well. Poor fluidization can cause powder flow delivery problems; hence TE.
  11. Add automatic equipment/robotize if justifiable, such as powder triggering systems, in/out positioners, programmable control units with recipies that take away the constant variations of parameters.
  12. Adjust the supplementary air/power speed. Too little and the gun puffs too much and you increase blow-by.
  13. Put in a quality assurance system. Know what you are doing. What’s the point of having the best equipment in the world if your results are constantly off by one mil?
  14. Get input from your coatings and equipment suppliers of equipment.
  15. Get your people to realize the importance of doing the job right the first time, all the time.

Many of the added elements to the powder list can be used for optimization of the automatic liquid systems. One advantage of the liquid systems is that they can (usually) adjust/optimize the paint. That is, viscosity and conductivity in situ whereas with powder it has to be done in production. Whether you’re applying liquid paints or powder coatings, TE is a critical function and optimizing it will lead lower operating costs and improve finishing efficiency.

Pierre Bachand is president of finishing consulting and training firm ChromaTech Inc, based in St. Eustache, QC.

 

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