Tech tips for better TE

Maximizing the efficiency of your powder finishing system will result in a more cost effective operation. The first thing you need to do is understand transfer efficiency (TE) in powder finishing operations. TE is defined as a ratio:

POWDER ON THE PART X 100 =    % T.E

POWDER SPRAYED

This doesn’t mean necessarily using the best coating. Instead, it’s about using the powder coating that most effectively gets onto a part.

I had the opportunity to work for more than 15 years in the pretreatment area (chemical and mechanical) before spending the next 15 years selling, designing, installing, and sometimes repairing powder and liquid paint equipment. A good part of my time was dedicated to problem solving, which allowed me to learn different techniques for improving TE in powder finishing applications.

Tip  1: The three “C's” of powder painting

Cleanliness, cleanliness, cleanliness. This is the most basic, but also the most important factor to consider to improve the efficiency of your powder finishing system.

Tip 2: Improve line density

This is important for optimizing both manual and automatic systems. Optimizing line density ensures that powder is best directed at the part, that distances are  such  that coverage of the sides of parts is maximized and that overspray is reduced to a mininum.

Tip 3 : The mirror effect

This is a favourite tip of mine. You use a sheet of plastic to bounce back the powder to improve coating transfer to the back, the bottom or the sides of parts. It is not universally applicable but where it can be, you can achieve significant savings, particularly in spray-to-waste powder booths.

Tip 4 : Follow the needle

Quite a few painters use this technique to improve coating in recesses.

The painter uses the pointed tip as a lightning rod that attracts the electrostatic line forces and the paint inside the cavity. This seems to be most usefull for recoats. It is often used when hooks are coated with paint.

Tip 5: The ion collector

This is also marketed as the Super-Corona by one equipment manufacturer (ITW Gema, which was acquired by Graco recently). It improves penetration, reduces orange peel and optimizes paint thickness. Anything that reduces initiation of kV rejection improves TE. It also makes recoats easier.

Tip 6: The special kit for metallics

This is a simple solution to many application problems you may encounter working with metallics, even bonded metallics. Paint will often accumulate inside the flat jet or on the inside face of the round jet. Then it spits on the part.

To solve this problem, glue, two round deflectors, face to face before installing them on the electrode holder. The ion wind is maintained in front of the cloud and there is added turbulence that makes the coating or accumulation of paint on the unit virtually impossible.

Tip 7: Sandtext, veined and textured finishes

Many painters remove all types of deflectors when they want to optimize the special effect. A few months ago I was given samples of an aggressive  santext finish (called an anti-slip). One sample was done with a flat jet and the other sample was applied without a nozzle. The difference was impressive.

(One suggestion that has nothing to do with TE but improves the finish is to make sure the oven is hot when you introduce the parts. If the temperature rise is very slow there is a good chance that the powder will level off and kill the desired textured effects.)

Tip 8: The right ID hose for the right flow

The objective is to reduce powder paint velocity as it exits the front of the applicator. The right speed equals improved TE. Many operators  do not realize there are usually a couple of powder hoses with different IDs available from their supplier.

To improve very low flows, use a small ID hose. The smaller ID reduces or eliminates the need for supplementary air to keep the powder in suspension.

When shooting a large volume of powder, using a large ID reduces the linear speed of the powder as it exits the powder gun. As an added benefit, there is less impact fusion if you are using PVC.

Tip 9: Better recoats

To improve TE in both in batch and conveyor applications, there are a few rules of thumb:

• use the right settings

• do not stay in one spot: move, move, move, but not too fast

• clean hooks

• warm or hot parts coat better

For batch application there are a few tricks you can use. Wipe or wash the parts with a mixture of a fast solvent and a small quantity (less than 10 per cent) of a slow solvent (ex : butyl carbitol). Use a product like Recoat Magic, Ransprep or similar products mixed into water with the recommended solvents. I have mixed this type of product  with  Windex or Winshield Washer fluid. This type of product leaves a molecular film of hydrated salts on the surface of the painted part.

Tip 10: Write down the formula

When you are testing equipment to optimize TE, take note of the best settings then have everyone work with them. A few years ago, a multinational lighting company asked me to improve the touchup of some parts. The company was having varying results depending on the operator. The company filmed the operators and the settings on the control panels. Then it selected the best operator and his settings. This gave the company a good idea of the number of passes, how he adressed the part and the speed of his movement.

Tip 11: Slow down the powder when possible

As mentioned in past articles, it is important for the powder to move as slow as possible when the part geometry permits it. One element of today’s control panels is the concept of total air and percentage of powder.

Here’s a scenario: You have two guns painting the same parts with everything the same: hose length, line speed and paint. Gun number one is working at six cubic meter/hr  with 25 per cent powder flow. Gun Number two is working at three cubic meter/hr at  50 per cent powder flow.

The actual volume of powder per minute of each gun is the same. If the powder delivery is regular with no “sputtering”or coughing of the guns, this means that the velocity of gun number two is much lower than gun number one.

Why? Gun number one: 75 per cent of the total air is supplementary air = speed and suspension air (75 per cent of six cubic meters is 4.5). Gun number two: 50 per cent of  3.0 cubic meters is 1.5 or three times less.

It is always good to keep the hoses as short and straight as possible.

Tip 12: The right deflector for the right job

A lot of people think there are only two types of deflectors: the standard flat jet typically used and a round jet stored in the spare parts box. All the major powder coating equipment manufacturers offer a wide choice of nozzles for their applicator to maximize efficiency.

X nozzles are usually very good for difficult geometry parts. I once used one for deep lighting reflectors on a chain-on-edge system and achieved excellent results. Wide flat jets offer a wider pattern, which allows the gun to be closer to the target while keeping the same width as a standard flat jet nozzle. This improves target finish point while keeping the same stroke on a gun mover. One manufacturer uses a sleeve that can be slid over a round jet. This is useful for difficult to reach areas, particularly where one piece of metal lets the air pass. With a standard nozzle you remove the powder on the opposite side.

There are other nozzle shapes available, such as star, oblong and even blank ones that enable the operator to experiment.

Tip 13: Mix and match

In an automated line it is important to use simultaneously or alternatively fixed and moving guns to maximize TE.

Tip 14: Get close (enough) to the part

Use of extensions on the gun when the targets are so large or deep that the operator must shoot powder from over 12 inches away, are a good way to improve TE. Remember: A tired painter is not an efficient painter.

No 15: Invest in technology

In automatic powder paint systems, invest in in/out positionners, triggering packages, robots and other options that will improve transfer efficiency by optimizing application parameters.

Tip 16: Get assistance from your paint supplier

Coatings manufacturers can often improve charging qualities of powders when required. The improvement can be attained by changing the resin type, ratios of filler to resin, particle size or by adding charge enhancers. A note of warning though: this is not appropriate for finishers who don’t ground parts properly. I recommend you run a test before usage when finishing parts with complex geometries.

Information provided in part by Paul Boucher, ECE Canada and Brian Coutts, Erie Powder Coatings. CM

Pierre Bachand is president of Chromatech Inc., a finishing training and consulting firm based in Quebec. pbachand@chromatech.ca