Improving First-Pass Transfer Efficiency

Article From: Products Finishing, from The Sherwin-Williams Company

Posted on: 12/1/2005

Want to boost powder coating productivity? Look to your process, not your powder.

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Hangers should position parts away from the booth ceiling and floor

Hangers should position parts away from the booth ceiling and floor, and parts should be oriented to allow coverage of all key surfaces.

Savings

Savings resulting from improved application can be 5 to 50% or more of annual coating expenditure.

Manufacturers are always looking for ways to reduce costs and improve their bottom line, and some applicators of powder coatings look to a reduction in coating cost to achieve savings. A saving of 5 to 10% of the price per pound of the coating can temporarily satisfy these consumers, but coating manufacturers do not always have the ability to lower their process costs or selling prices to accommodate customer demands.

More often than not, the actual coating application is overlooked as a means of saving money. This is especially true when reclaim systems are employed to collect, for reuse, the powder coating overspray which might otherwise be lost in spray-to-waste application systems.

Manufacturers may not be aware of all of the costs associated with coating application. Uneven or excess coverage, for example, can increase costs by 25% or more. Savings resulting from improved applications can be substantial—5 to 50% or more of annual coating expenditure. For this reason, applicators should analyze their entire process to determine methods for improvement and subsequent cost reductions.

Transfer Efficiency

In powder coating, transfer efficiency is the ratio of the quantity of powder deposited on the part to the quantity of powder directed at the part. Transfer efficiency is given as a percentage, with 100% being most desirable.

It is always desirable to improve first-pass transfer efficiency—maximize the efficiency of the initial coating application—to minimize costs. Collection of any overspray, which can be conditioned for reuse, will affect the second or subsequent multi-pass transfer efficiencies. Although overall transfer efficiency may be high with the use of a reclaim system, more cost benefits can be realized by achieving a high first-pass transfer efficiency.

The most efficient application occurs when 100% of the applied coating is deposited uniformly on the part, leaving no excess coating for collection, disposal or reuse. It can be said, then, that the best reclaim is no reclaim! Although this is desirable, it is not achievable on most production lines.

Use of a reclaim system can push overall efficiency to 90% or greater. But, while the reclaim apparatus reduces some loss factors, it creates others. It is advisable, for example, to recondition the collected powder overspray before it is reused. Ideally, reconditioning should remove the dust, dirt and powder fines as well as very large powder agglomerates from the collected overspray.

Maximizing first-pass transfer efficiency will reduce the amount of powder oversprayed and subsequently the amount of reclaim generated. A high first-pass transfer efficiency will generate smaller quantities of reclaim, which should enable reintroduction of reclaim into the virgin powder without grossly affecting particle size distribution of the blend. It is desirable to condition the reclaimed powder prior to the reintroduction into the virgin material for optimal results.

The amount of reclaim that can be added back to the virgin material varies with each application. A good rule of thumb is to maintain reclaim at less than 25% of virgin powder by volume. Better is 15% by volume, which keeps particle size distribution—and the application properties of the powder blend—more consistent.

An increase in transfer efficiency is only one improvement that can lower costs, improve productivity and increase quality. A review of the entire application process and system may yield further improvements. Some of the areas to consider follow. These are not listed in order of importance, nor is this an exhaustive list.

Application Area

The application room should be free of dust, dirt and clutter, and should be well illuminated. The walls and ceiling should be painted with a high-quality white gloss coating, while the floor should be painted with a light color (gray, beige, white, etc.) gloss or semi-gloss coating to provide easy cleanup and minimal dust and dirt generation.

Ideally, temperature in the application room should be held constant from 65 to 75°F, and humidity from 40 to 60%. Higher temperatures and humidity levels will usually cause a decrease in the retention of the electrostatic charge. Although lower temperature and humidity can increase charge retention and improve powder handling characteristics, a very low level can adversely affect charge retention and transfer efficiency.

Powder storage areas should also be maintained at 75°F or less and below 60% relative humidity. Powder stored at a lower temperature or humidity level than the application area should be allowed to equilibrate before use.

Other dos and don’ts for the application room:

  • Maintain slight positive pressure to prevent dust and dirt from entering the room. Average velocity of the air exiting
    room openings should be 50 fpm or more.
  • Don’t store infrequently used items in the application room.
  • Clean the area routinely to minimize dust and dirt. A vacuum (portable or central) with a HEPA filter designed for powder coating applications should be used to cleanup any loose powder in the room. Minimize use of compressed air blowoff guns for cleanup.

Preventive Maintenance

If you haven’t implemented a preventive maintenance program for all your application equipment, you should. Cleanup of the room and apparatus should also be on a scheduled basis for optimum results.

Table I: Maximizing Transfer Efficiency
Transfer Efficiency Variable
Impact on Efficiency
Ease of Change
Charging Efficiency

 

 

Gun Voltage
10
2
Gun-to-part distance
10
4
Powder Particle Size
6
6
Powder Dielectric Constant
4
10
Powder Output
8
2
Deposition Efficiency

 

 

Recovery System/Air Velocity
10
2
Transport Air Pressure
10
2
Powder Cloud
8
4
Gun Movement, Directability, Number, Spacing
10
4
Ware Size, Shape, Complexity
8
10
Part Indexing & Gun Triggering
10
10
Ware Batching
6
8
Grounding, Hook-Hanger-Loadbar Cleaning
10
6
Line Speed
4
8
Amount of Hand Augmentation
8
2
Film Thickness
8
6
Virgin-to-Reclaim Ratio
10
4
Powder Resistance
2
10
Powder Specific Gravity
6
8

Maintain an average face velocity of 100-120 fpm. Air should flow uniformly into each booth opening. It may be necessary to increase average face velocity to 120-150 fpm to retain powder dust in booths with tall part openings (6 feet or more).

Keep water, oil and dirt from the compressed air system. The moisture level in the compressed air should be maintained below a +38°F dewpoint. The compressed air should contain less than 0.1 ppm oil and have no particulate contaminants greater than 0.3 microns. These are upper limits—lower levels would be better, and requirements may vary for a specific powder.

Data on operating parameters should be gathered on a daily basis at least once per shift, or more often when production levels are high. This information will be invaluable for spotting trends on the line and for determining causes of coating problems.

Also, frequently measure and record film thickness on various parts during a shift, and inform operators so they can maintain the desired film thickness. This is especially important if the shape, number or size of the parts or the powder color, type or brand changes frequently.

Maintain adequate lighting in the powder booth. A light intensity of 100 to 150 foot-candles at the point of application is desirable for good visibility. Light should come both from above and the side of the part for good illumination, and care should be taken to minimize shadows.

Application Issues

Powder spray gun and pump pressures should be adjusted to the minimum settings required for the proper film deposition, application and coverage; high pressures waste powder in a spray-to-waste application and generate more overspray—and subsequently more fines—in a reclaim system. High pressures also increase gun and pump wear and can cause impact fusion on gun tips and powder pumps.

The powder cloud during application should be adjusted to a minimum level. Cloud density should not impair visibility from one end of the booth to the other, even at line speeds of 20 fpm.

Increase line load density—the number of parts per linear foot of conveyor—whenever possible. This minimizes powder wasted or collected for reuse while increasing productivity.

Establish a gun-to-part distance which allows for optimum coverage and application efficiency. Typically, a distance of 8-12 inches can provide the desired results depending on nozzle type, part configuration and number of guns used. Adjust powder pump delivery rates and application pressures to provide needed delivery with minimal powder overspray.

Use a fan pattern that closely matches the size of the parts being coated. Excess fan patterns waste powder. This is especially true with manually applied powder and in a manual or an automatic spray to waste application.

Parts with a more complex configuration will require a nozzle with better ability to penetrate the recessed areas. Select a nozzle which will provide good penetration into the recessed areas while providing the needed film build with uniform coverage. It may be best to utilize line trials to determine the best nozzle design for the specific line conditions and part configurations. Match the spray pattern to the shape, size and geometry of the part.

Where possible, use a low-powder velocity to penetrate Faraday Cage areas (recesses, enclosed areas, etc.). The best pattern control device for this application is a small conical deflector tip with a low-powder-velocity spray pattern. Air-assisted spray pattern control devices offer no advantages in such applications.

Gun trigger controllers can reduce coating consumption in automatic systems. A controller with horizontal zones can trigger the guns off between parts. When vertical zones are added, guns can be triggered off for short parts. These capabilities provide the greatest flexibility in hang patterns and subsequent powder savings.

It’s also beneficial to use gun positioners, especially when coating parts of various depths. The positioner can move the reciprocator in closer for narrow parts and out for wider parts to maintain gun-to-part distance. A short gun-to-part distance may improve coverage in Faraday Cage areas.

In manual applications, the operator should employ a steady, even spray stroke, keeping the gun perpendicular and the spray stroke parallel to the part surface. Strokes should follow part contours.

Evaluate coverage (spray pattern and deposited powder) of spray guns on each side of an automated booth. If necessary, increase the number of guns on each side. Added overlap of the spray patterns resulting from the additional guns will provide more uniform coverage while reducing the quantity of powder sprayed per gun. Gun reciprocation can also help to provide more uniform powder deposition. It is important to synchronize conveyor speed with the reciprocator cycle rate such that the spray starts and finishes at the same part of the reciprocator stroke.

Control of spray gun pattern size is best accomplished by selecting the proper nozzle and varying the distance between the gun and the part. Some applicators try to coat too much surface area with one spray gun. There are practical limits on gun-to-part distance, and powder delivery rate and velocity should not be increased to compensate for excess distance. Higher delivery rate and added velocity reduce dwell time in the electrostatic field; this, along with increased gun-to-part distance, dramatically reduces first-pass transfer efficiency.

Optimize the electrostatic voltage potential applied to the spray guns to provide the maximum deposition and uniformity of coverage for the parts. Consideration must be given to part configuration, powder composition, gun-to-part distance and powder delivery rate and pressure. Take care to minimize powder buildup on part edges while improving penetration into recessed areas and coverage uniformity.

Improve the electrical continuity between the part and the ground. Ground continuity greatly affects transfer efficiency. A high resistance decreases transfer efficiency, while a low resistance increases transfer efficiency. A maximum resistance of one megohm is the limiting factor for the safe application of powder coatings; higher resistance creates a potential sparking condition. A resistance of 0.5 megohm can be an attainable control point for applicators desiring good transfer efficiencies and safe application conditions.

Hangers or fixtures should be cleaned regularly to provide good electrical continuity and reduced particulate in the applied films. Square stock can enhance electrical contact. The corners of the square stock act as knife edges, which can cut through much of the accumulated buildup on the components, conveyor and parts. Hangers fabricated from square stock can also reduce the resistance to ground, while extending the period between cleaning.

Hangers and fixtures should be constructed from materials which will extend their useful life. Materials selection should take into consideration the cleaning method to be employed. For example, plain steel can be used for hangers cleaned using mechanical means, such as wire brushing or chipping. Stainless steel should be considered for fixtures cleaned using chemical strippers. Inconel is the material of choice for hangers cleaned in a burnoff oven.

Minimizing hanger size reduces the amount of powder they attract. Hangers should position parts away from the booth ceiling and floor to better utilize the applied powder, and parts should be oriented in a manner to allow coverage of all key surfaces. The parts should be racked at a high density to reduce powder blow-by.

The table in this article lists some application variables, their impact on transfer efficiency, and the difficulty associated with changing each. The impact scale runs from 1 (low impact) to 10 (high impact). The ease of change scale runs from 1 (easiest) to 10 (most difficult).

 


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