The Basics of Powder Coating

 

As in painting with wet coatings, powder coating requires a number of steps be taken to ensure a quality coating is applied to the substrate being coated.

These steps include the following:

 
Pretreatment or Cleaning

Properly preparing parts for powder coating is essential for a quality finish. This includes cleaning, rinsing and drying. It is also the part of the process that causes the most problems and is typically given the least amount of attention.

Pretreatment can be broken down into two categories, Mechanical and Chemical preparation.

Mechanical preparation is referred to as any process where the surface is prepared mechanically such as sanding, grinding, media tumbling, sandblasting, shotblasting or glass bead blasting. This pretreatment process offers great bonding of the coating to the substrate due to the surface profile left by the process. Although there is no chemical protection offered if the coating is broken, the added surface area and bonding properties will often be adequate for most requirements. Some metals such as hot rolled steel have a layer of mil scale that is hard to remove chemically and mechanical preparation is a better system to use in such cases. Mechanical preparation does not remove oils well and is generally labor intensive and expensive.

Chemical preparation is any pretreatment process that cleans and lays down a surface film of iron phosphate, zinc phosphate or a chromate conversion coating. In the event that the coating is broken, these coatings are designed to slow down and isolate corrosion in the substrate. Chemical pretreatment is a wise choice if the volume of material processed is high. Chemical pretreatment can be accomplished in a number of ways, manual spray, dip or immersion tanks, or automated spray. With any chemical pretreatment it is wise to contact local authorities to see what regulations are regarding handling the chemical and dumping into city sewer systems. Not doing this could result in major problems down the line.

Solvent wiping is sometimes used to prepare substrates for coating, but has many drawbacks. It is not overly effective and contaminants tend to be spread over the substrate as opposed to being removed, health concerns involved with handling solvents are numerous, disposal of used solvents is expensive, production is slow and it is labor intensive and costly.

Manual spray typically uses a high pressure hot water washer with the capability of injecting a cleaner, a phosphate coating and a rinse or sealer. In smaller shops where production is not high and where parts sizes vary from small to very large, this system is workable. It is also initially the least expensive for the small shop.

 Dip Tanks typically have 3 to 6 stages with a cleaner tank or two depending on the substrate to be treated i.e.: aluminum or steel, a rinse, a phosphate tank, a rinse and a sealer. Dip tank systems are less expensive to install than an automated wash system and they have fewer wearing parts to replace. Disadvantages of dip tanks include but are not limited to:  first parts are cleaner than the last parts processed, oil floats on top of the solution and is carried out with parts as they are removed, production rate is not high as time in each stage varies with the chemical being used.

Automated spray systems use the same stages and chemicals as dip tanks but have the advantage of using impingement to assist the chemicals and allow a much higher production rate than dip tanks are capable of. 

All chemical pretreatment systems are dependant on time, temperature and chemical concentrations to provide quality preparation for powder coating.  Additional information regarding chemical pretreatment can be obtained from one or more of the chemical suppliers. 

Drying

Before powder coating, the substrate needs to be completely dry, both inside and out to ensure a quality finish. Sometimes this is accomplished in a drying tunnel with heat and  turbulator  fans and sometimes the cleaned product is put in the oven for a period of time to evaporate all moisture prior to coating.

 

Powder Application

Powder is applied to the substrate through the use of electrostatic powder spray guns. These can be either manual or automatic. Powder is placed in a container similar to a pressure pot in a wet system. The difference with powder is a fluidizing plate located at the bottom of the container that is porous. Air is forced through the plate and through the powder sitting on it. The fluidized powder is very much like a liquid coating at this point and is pumped out of the container and down the hose to the gun. When the trigger is pulled on the gun, an electrostatic field is generated in front of the gun and the powder is atomized and sent through the electrostatic field. This charges the powder ( usually negative ) and the part that is being coated is grounded ( which effectively give the part a positive charge ). As we all know opposites attract and the electrical charge holds the powder on the part until it can be cured by the addition of heat. 

 
Powder Collection

Powder is an interesting and effective product for the finishing of manufactured goods. It does however, have properties that make it difficult to deal with if the over spray is not contained and collected in the spray area. By design, powder is both abrasive and mobile, cleanliness is paramount to a quality finished product. Powder collection is not an option; it is a must for powder applicators.


Without an effective collector, powder will migrate out of the spray area and into all areas of the building creating the following types of problems:

  • Color contamination of finished product from cross over of powder.
  • Health problems to employees through breathing in of dust contaminated air.
  • Increased maintenance costs to manufacturing equipment due to powder contamination of moving parts.
  • Increased costs associated with other equipment such as computers, heating and ventilation, etc.
  • Increased costs for all general maintenance – janitorial etc.
  • Powder booths exhausted to the out doors can result in contamination of buildings and vehicles, particularly on hot summer days where the powder can melt and cause damage similar to over spray from wet coatings.
  • The solution to the above is to effectively collect over spray powder and dispose of it or reuse it (in single color or reclaim applications).

 

The industry accepted airflow across the spray area to effectively contain and capture over spray powder is 100 - 120 FPM. Booth Brothers’ offering of powder collectors are sized to the spray area to ensure positive powder capture.

Most effective collectors on the market use cartridge style filters that collect the over spray powder. These cartridges are regularly cleaned with pulses of compressed air that blow the powder off and extend the life of the filter. Typically a cartridge filter will last around a year or 2,000 hours of operation, although some users get considerably longer life from a filter set. Cartridges that are not pulsed get blinded quickly and need to be replaced. At a cost of            $150.00 to $250.00 for each cartridge, the cost of operation and maintenance quickly mounts.

The size of the collector module is determined by the size of the spray enclosure and the size of the enclosure is determined by the size of the largest part or rack of parts that is to be sprayed in the booth. Usually you can add on 3’ around the sides of the part to arrive at an enclosure size. This gives ample room to move around the part while spraying. Beware of powder collectors that utilize bag and panel filters as they are ineffective and costly to maintain.

 

Curing

 The powder coated parts now need to be cured. In most shops this is accomplished by heating the parts in a convection style oven. Different powder coatings have different heat requirements in temperature and time at temperature to properly cure the coating, but most will cure in the 375 - 400 degree F range in 15 to 40 minutes. Specifications on powder coating regarding product, time and temperature to cure as well as performance can be obtained from your powder suppliers.

Ovens can be constructed with doors on one end only or doors both ends depending on allocated space and production speeds. The inside clear oven dimensions are based again on the largest part, rack or number of racks that you wish to put in the oven at one time with approximately 1'- 2' clear space around the parts to allow easy loading/unloading of the oven.

After the parts have cured and cooled they are finished and may be packaged and shipped or used. No additional cure time is required.

 

Conveyance


Another area that must be considered is moving the parts from one part of the system to the next. Typically in a manual system a wheeled cart is fabricated and hangers suspend the parts to facilitate spraying and baking of the parts. In more automated systems either a manual or powered conveyor is used to suspend the parts and move them through the various parts of the system. Over time the hangers will build up with baked on powder and cause the part being sprayed to be isolated from ground. Regular cleaning of hangers and hooks is critical if the powder system is to be efficient and safe. There are several cleaning processes available for hangers and hooks, including chemical stripping, blasting and high heat burn off.

 

Equipment Safety

Powder coating in general is much safer than comparable liquid systems. The application equipment is far less dangerous to operate and the powder material has fewer potential health risks. There is potential of electric shock if the equipment is not maintained and handled according to the manufacturer’s instructions. It is critical that all electrically conductive items in the spray area be grounded – this includes the spray enclosure, the rack and parts, the spray unit and the operator. Not being grounded allows a build up of electrical ions, which will discharge when a ground source comes close to the item. The most effective ground method is to provide a ground rod driven into the earth close to the application point. The resistance on the ground shall not be more than 1 megohm, as measured with an instrument that applies at least 500 volts to the circuit being evaluated.