THE BENEFITS OF POWDER COATING
The process of finishing metal surfaces with dry powder coatings is not new. It has been used in this country since the mid 1950s. It began with the coating of pipe, for corrosion protection, and electric motors, for insulation. With the growing need to reduce air pollution, including that associated with thin film liquid coatings, powdercoating has continued to develop.
From equipment to surface area covered per dollar of material applied, the costs of powdercoating are comparable to those of liquid coating.
However, there are distinct advantages to powdercoating when compared to solvent laden liquid coating:
- Powdercoating material is received by the applicator in ready-to-use state, thus eliminating any variable in mixing.
- Spray booths can be cleaned with a simple air jet as opposed to harmful solvents.
- Absolutely no solvents are used, eliminating the need for expensive air pollution control equipment. The absence of solvents virtually eliminates the fire hazard associated with paint shops.
- With the use of a powder recovery system, overspray can be collected for reuse. Liquid paints cannot be reclaimed.
- A 0.001 to 0.003 mil. thickness will be obtained in one powder application. Wet spray may require 2-3 coats to attain this thickness, greatly increasing the processing time and expense when higher film thickness is needed.
- Powdercoatings are cured at elevated temperatures (300F to 425F) as compared to wet coatings. This generally results in a tougher, more chip resistant coating. The high temperature and greater film thickness allow powdercoatings to achieve adequate bonding and coverage over less than ideal materials, resulting in less stringent precleaning and metal preparation requirements.
The powdercoating environment is much safer due to the elimination of solvents from the process. This increases employee and environmental safety and reduces insurance and permit costs compared to those of the wet spray industry.
Since powdercoated parts are fully cured when they leave the oven, unlike many wet coatings which have post cure aging requirements, the total processing time is shorter with powdercoating. This results in shorter turnaround time.
Parts are generally presented to the coating operation while they are at ambient temperature; the charged particles will be attracted to and accumulate on the article surface. The powder particles approach the surface, being attracted by their electrical charge.
Upon contacting the surface, that portion of the charge in contact with the surface leaks off to ground through the article and its support. Since the powder particles, generally speaking, are plastic and are poor electrical conductors, they will retain a portion of their charge and thus will continue to be attracted to the surface. The particles will be held in place.
Eventually, the accumulated charge on the surface will become sufficiently large and no more particles will be attracted. This represents a self-limiting effect. No amount of continued exposure to the spray will add to the thickness of the accumulated layer once this condition has been reached.
The total consumption of powder materials used will be apparent in the amount of reclaimed materials that are being collected. This will show as an increase in costs. Proper gun adjustments and material selection will help increase efficiencies.
Applying the powder to articles while they are at room temperature seems to be the process most widely adopted by industry at the present time. This is particularly true in those cases involving thin film requirements such as lawn furniture and appliance items.
Heavy film thicknesses for functional requirements such as motor winding insulation, and corrosion protection for pipe coatings may require that the parts be preheated prior to the application of the powder.
The charged powder particles will be attracted to the surface of the part whether the part is cold or heated. If the temperature of the part is above the melting point of the powder, that material attracted to the surface will melt and adhere. The molten material already on the surface will attract additional powder particles equally as well as the bare metal, so material will continue to accumulate on the part surface-so long as there is sufficient heat at the part surface to melt the newly arriving particles.
The material selected is important in this application. The powder selected must allow the high build film thickness to polymerize and cure without sagging or gassing. The powder and process technique must remain constant in order to maintain a given thickness. Proper attention to preheat and post heat dwell times is a must.
Often it is desirable to keep the powder material from reaching certain areas of the article being processed. Tight tolerances and electrical grounding contact areas will require masking. There is a number of ways in which to accomplish this; the following methods are some of the most popular.
Tapes--Heat resistant tape is a very common masking technique. Pre-cut to the desired size and shape, or purchased by the roll, tapes are a cost-effective way to mask most parts. Caution should be taken in the selection of the adhesive material that the tape is made of. During the baking process the tape's adhesive may also be baked on, leaving an undesirable gummy surface that would require a post cleaning operation.
Caps and Plugs--They are commercially available in a variety of shapes and sizes that will withstand post-bake temperatures up to 600 degrees.
Racking system--The design and configuration of the racking system can often serve as a masking device.
Air Mask--A low-pressure blast of air can remove powder that is electrostatically attracted to the ambient surface. This process must be accomplished prior to the baking cycle.
Vacuum masking--Can be accomplished in much the same manner as the air method described above. The advantages of this method are cleaner process and a sharper break line.
Foil wrap--Can hide the surface that does not require coating. This application is desirable on many large surfaces that would be difficult and timely to tape.
Other materials have been used such as silicone rubber, wood, cardboard, spray-on anti-stick compounds, brush-on compounds, high temperature grease, hot and cold dip plastisols, and a variety of others.
Any quality powdercoating job shop is geared up to finish the widest variety of part sizes and configurations. To be successful they must know every aspect of how to apply a quality finish in the most cost-effective manner. Utilizing their knowledge of powder application processes and procedures will enable them to maintain a high level of service.
It is essential that the job shop finisher know all the possible variables that could cause problems before they occur (both long and short-term problems). There is a line between what can and what cannot be accomplished with powder.
In most cases, a job shop is contacted when a new requirement is called for. For that reason, it is essential that the job shop finisher keep up-to-date on the latest developments and changes within this rapidly growing industry.
Finding and hiring a coating consultant is a good idea in many cases, although there are other ways to obtain the necessary information. Seeking information from sources such as job shop powdercoaters or powder suppliers can be useful. They can provide you with first hand information and advice based on years of experience.
Often, an application that may seem unique to you may be an everyday occurrence to another. The sharing of information has changed considerably in the last few years; more finishing professionals are benefiting from helping others find answers to their finishing questions.
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