The 4 E’s of Powder Coating

  • Energy — Less is required to operate a Powder Coating system
  • Ecology — Think Green and pollution free
  • Economy – Pollution-free Powder Coating systems are economical saving you $$$ in operating costs
  • Excellence – Powder coatings offer quality finishes that speak for themselves

This article is intended to be an informative guide that outlines and explains in detail the critical steps in designing and installing a successful Power Coating system. We cannot emphasis enough the word successful because when designed properly, a total powder coating production system will meet or exceed a client’s expectations, namely the 4 E’s … Energy, Ecology, Economy, and Excellence.

When researching systems that reduce atmospheric pollutants, employing systems such as Powder Coating, will perform an excellent job of eliminating pollution in order to meet EPA standards. Simultaneously, these applications are able not only to improve the quality of the coating process, but also provide an added benefit – an excellent return on investment.

Powder Coating is ranks Number 1 on the list since it is practically 100% efficient. All the powder material that is not applied to the part is collected, recovered, sieved, and re-used and because the material is collected for re-use, all the booth air can be re-circulated into the plant, thereby eliminating the need to exhaust the air since there are no solvents or harmful gases dissipated from the sprayed powder. The oven air exhausted from the plant is reduced making operating the system less costly.

Step one in designing a powder system is the foundation: identify the client’s requirements.

Following is a list of requirements and conditions for the proper design of a Powder system to help you understand the process. It is important to know:

  • What is the part (or parts) that require(s) coating and the expectations for the finished part?
  • Will the assembled part be used or installed indoors or outdoors?
  • What type of exposure will the part have to endure?

This information must be organized so that it is accurately conveyed to the equipment and material suppliers. This is known as a process specification. Process specification must define the ultimate goals of the equipment. It identifies how the part is presented to the coating process and in what quantities. It will also identify the life or performance expectancy of the product that will define the type of chemical pretreatment that the metal must be exposed to and the type of powder to be used. The process is comprised of the following components:

total-productino-coating-system

  • Chemical pretreatment, (Iron Phosphate, Zinc Phosphate, Sand Blasting, or the new Zirconium to replace the zinc phosphate process)
  • Blow-Off and Dry-off oven,
  • Electrostatic Application Equipment and Recovery System, (Cartridge Type, Cyclone Type for multicolor, quick color change etc.)
  • Cure oven, Convection, combination of IR and Convection or just IR, gas or electric)
  • Transport conveyor.

The total production coating system the close cooperation of the systems company working in tandem with the chemical company, the application equipment company, and the powder company … a unified team.

The single most important responsibility of the purchaser is to accurately define their production process and requirements. This information is vital to the equipment suppliers whose task it is to properly design the equipment, and to the material suppliers who must formulate the proper pretreatment and selection of the powder. Next, the purchaser must define the specific process goals and communicate them to the suppliers so that they can respond appropriately. It is sometimes more difficult to determine the process goals than it is to design the actual equipment or formulate the powder material. The purchaser must understand that the process definition requires clear and organized thought and accurate information that will include future projections (growth rate) to assure that the powder coating project is successful. A reference to the ASTM standards will be helpful.

Writing a Process Specification

A well-defined and written process specification will advise the equipment suppliers as to the type of equipment and process controls that you are interested in purchasing. It will inform them on how to integrate these components into a finishing process. Also this specification will communicate to the powder material suppliers the performance characteristics that must be formulated into the coating that will be applied to the product. This information needs to be well thought out, clearly defined and plainly written for all the parties to benefit. Verbal communication of this information leads to possible misinterpretation. Therefore, written documents are preferable to assure that the information is understood correctly.

Determine Process Equipment Requirements

The first goal that must be achieved with this specification is to determine the process equipment requirements. Descriptions of what you want to do, how you want it done and where you want to do it are imperative to clearly depict this process.

  1. Production Rate — Production rate is based upon the immediate or present requirement and projected growth of the company over the next five years. First, define all the parts that must be coated. Identify the largest piece. With that information in hand, make an accurate listing of the total number of parts that will be required to meet the production output within a specific period of time (hour, day of the week or month). List the total weight of the parts produced (pounds per hour or per day). By knowing the size, list the area produced in the same period of time (square feet per hour or per day). What is the size of the largest piece and how many units of the largest piece are required in a day? Of all the parts produced, what is the material of the substrate (steel, aluminum, brass, etc.)? Is it a sheet metal, a machined part or a casting? This is the information required to design the type and size of the conveyor (pounds per hook required) and the speed of the conveyor (feet per minute “FPM”). Once this is established, the oven size and washer are calculated. Production rate will also allow the equipment manufacturer to determine the number of guns required to coat the parts at the production rate required. The supplier must plan for a certain amount of rejection rate, etc., to establish the correct process.
  2. The Composite Part Profile of the Parts (Height, Length, Width) — This composite profile consists of the tallest part’s height, the widest part’s width and the longest part’s length. These dimensions may not occur on the same part but may be a composite of three different parts. In addition, if some existing equipment is to be used, then describe the existing part openings, top of part to top of conveyor rail dimensions and specify the existing conveyor type. This will provide the equipment supplier with the information that is required to integrate the new equipment into an existing system.
  3. The Design Conveyor Line Speed — The design conveyor line speed that had been calculated in the production analysis should be specified in feet per minute (FPM) and reflect the actual line speed that is planned for this system. All the components are designed based on this (design speed) FPM. For example, the conveyor speed may have a range varying from 8-14 FPM but the design speed specified is 10 FPM. The design speed (10 FPM) will be the critical speed that will affect the design of all of the components of the system. Also, list the hanging centers for the parts that were chosen for this design line speed when it was calculated in the production analysis. Also it is imperative to mention if you plan to you a load bar, a rack etc. depending on the variety of parts being coated. This information will be used to gauge the size and length of the washer, ovens and spray booth so they are compatible with the process times required to attain the coating performance and finish specifications. The load bar will have an important impact on the turn radii that will affect the overall dimension of the system.
  4. The Plant Working Schedule (Hours Per Shift, Shifts Per Day, Days Per Year) — This working schedule is used to determine the design line speed. This will provide additional information to the equipment supplier to reinforce the fact that all the plant time that is available to process the production part quantities has been considered. Sometimes, if this information is omitted, the supplier can incorrectly assume that the production analysis is based upon a single shift when actually the line speed was calculated to reflect this production rate over three shifts.
  5. Describe the Type of System Desired (Batch, Conveyorized, Manual, Automatic, Highly-Automatic, etc.) — This will eliminate any confusion that may occur through inaccurate interpretation of the process specification. When in doubt, always state what may be obvious so as to eliminate the possibility of any confusion.
  6. List All Available Plant Utilities (Gas, Electric, Water, Steam, Wastewater or Environmental Limitation, etc.) — Be as specific as possible with this information. Include all voltages, maximum current or power limitations (amps or KVA), gas pressures, gas main size, steam pressures, water volume limitations, and all information on wastewater and other environmental limitations. The locations of these utilities should be specified on a plant layout drawing, if available. This data will let the equipment manufacturer size the motors, burners, gas regulators, gas valve train, washer fill valves, washer tank heat exchangers, drain pipes, etc. The location will allow them to estimate what it will take to provide the utilities to the actual locations where they are needed. Often they will be able to locate the equipment components close to the utilities that are required to operate them. This can be seen in installations where the pretreatment system is located close to the plant wastewater and sewer locations to minimize the pipe connections. Furthermore, this can possibly eliminate pump stations required when the distances are longer than desired.
  7. Define How Many and Which Colors will be Sprayed in This System — A breakdown of production rate by color is also necessary to completely describe the process (i.e., 50% black, 25% blue, 10% white and 15% six other colors). The equipment manufacturer will use this information to determine which colors are cost justified to reclaim for reuse and which colors should be collected for disposal. Annual powder usage (in pounds), if available, is also helpful in determining reclaim requirements.
  8. Plant Floor Space Limitations – Where will the System be Installed? — If construction of a new building is planned then state that fact. Be sure to include clear span building height, total roof height, truss or beam size and their locations, column size and locations, existing equipment locations, and any other encumbrances (air conditioning ducts, fire sprinklers, pipes, etc.) within this part of the building. Storage areas, personnel aisles, doors, windows, floor pits, etc., are also to be included in this information. Reference and include a plant drawing layout, if available. Show desired locations for load and unload areas required to incorporate this finishing system into the plant manufacturing process flow. These facts are necessary for this finishing system to be integrated successfully within the plant manufacturing process and fit within the plant space limitations. Communicate this information to facilitate equipment installation.
  9. Statement of Product Quality — A statement of anticipated product quality that this system is expected to produce should be noted with this information. For instance, describe the grade or type of finish that is desired (i.e., appliance, automotive, general metal, etc.) along with the maximum allowable reject rate. Include any quality control check sheets and specifications for paint finish that may already be in use at this facility. This information will help the supplier direct the equipment design (such as sieve screen mesh size or oil skimmer for the wash stage) towards what is required to meet these quality expectations.
  10. Powder Coating Performance Requirements — State the powder coating performance requirements that are to be achieved with this finishing process (corrosion resistance, chemical resistance, post-forming or post-machining, UV resistance, film thickness and dielectric strength).

Determine System Location

Selecting the best location for the new powder system within the plant requires considerable forethought. Be sure to allow adequate access for incoming and outgoing parts that will be coated by this process. Batch part storage is also important. Access to all the equipment for maintenance, operation and chemical and powder replacement is a must so don’t skimp on room.

The system location should not interfere with other plant operations and should not be located next to other plant processes that may cause problems with the powder coating system. Locating adjacent to sanding and grinding operations guarantees a high incidence of contamination problems. High traffic areas through the powder system location should also be avoided. Isolating the powder application areas from the plant environment using an enclosed room may be required.

Location and access to the plant utilities or wastewater treatment, drains, etc., must be considered when determining the system placement. Often the cost of bringing these utilities to the equipment can be minimized if the location is well thought out beforehand.

As stated previously, load and unload areas should be selected to provide for the best incorporation of this finishing process into the overall manufacturing flow. Additionally, selection of conveyor-type (power-and-free, or power only) and conveyor routing can be a good way of eliminating manual material handling now required in the existing manufacturing process.

The best way to help equipment suppliers determine ideal system locations is to provide a plant layout. CAD drawing programs or paper cutouts of the equipment are extremely useful tools when examining different equipment locations. In this way the scale sized equipment components can be examined in different areas prior to selecting the final system location.

Prior to the procurement of any equipment the purchaser should approve a final system layout, showing all equipment components and locations. There should be adequate detail on this drawing to carefully determine how the system would be installed, operated and maintained and how the materials (powder, chemicals and parts) will be delivered to this process. Powder storage, hanger maintenance and rejected part rework stations should be located close to the system without interfering with the powder coating process.

Selecting the Right Powder: Equipment and system location considerations are only part of defining a powder coating process. The right powder material must also be selected using techniques that are specific for this task. As always, determining the process goals and/or requirements is the best place to start in this effort.

Define Coating Performance Requirements

Powder manufacturers are well equipped to assist coaters and equipment suppliers in selecting the proper coating. However, it is important for the end user to be familiar with all of the options available and especially have a well-focused understanding of what is expected from the finished part. It is therefore wise to define the coating by its performance rather than its chemistry. This knowledge will facilitate communication as well as ensure that the job will be done right the first time.

A performance-driven approach to coating selection starts with a few simple questions:

  • What do we expect the coating to add to the finished part?
  • To what type of environment will the finished part be subjected?

Each department within a company (such as sales, marketing, engineering, manufacturing, customer service) may have different answers to these questions, so it is important to include all departments in the team to cover the product’s full range of needs.

A second step in the performance-driven approach to powder selection involves the careful consideration of various coating characteristics and some ranking of their importance with respect to end use, application and other processing requirements. Some or all of these characteristics may have been uncovered in answering the above two questions earlier; however, the ranking process now adds relative importance to each characteristic.

One of the most important considerations in defining the end use performance is weatherability. Outdoor exposure results in absorption of energy in the ultraviolet region of the electromagnetic spectrum. This energy can attack both the organic binder that forms the film and the pigmentation resulting in gloss loss and color change. Due to a tendency to chalk, epoxies and epoxy-containing hybrids are generally not recommended for outdoor use when aesthetics is a primary concern. Polyesters and acrylics, on the other hand, provide excellent UV light stability and typically find use in architectural, automotive, lawn and garden as well as outdoor furniture markets.

Functional Characteristics

Powder coatings are used to provide protection to the substrate and as such several other functional characteristics must be considered. These include the following:

Table 1: Powder Properties and Process Control

Powder Characteristics Example Test Methods
Powder Characteristics Example Test Methods
Weatherability Outdoor Exposure Clarity Visual to Standard
Corrosion Resistance Salt Spray Flamboyant/Tint Visual to Standard
Chemical Resistance Immersion Color Visual to Standard
Insulating Ability Dielectric Strength Gloss Gloss Meter
Heat Resistance Hold at Temperature Opacity Contrast Ratio
Abrasion Resistance Taber Abrasion Cure Solvent Rub
Impact Resistance Gardner Impact Overbake Resistance Visual to Standard
Mar Resistance Pencil Hardness Intercoat Adhesion Crosshatch Adhesion
Smoothness Visual to Standard Particle Size Sieve Analysis
Texture Visual to Standard Adhesion Crosshatch Test
Wrinkle Visual to Standard Edge Coverage Microscopic Inspection
Metallic Visual to Standard Formability Zero T Bend
Hammertone/Vein Visual to Standard Printability Over Print
  • Corrosion Resistance — Powder coatings act as a barrier to corrosive chemicals and moisture that are essential components of the corrosion process.
  • Chemical Resistance — Chemicals are not limited to chemical manufacturing plants. They can be cleaners used around the home and office, lubricating oils, gasoline and antifreeze used in the garage as well as many other compounds that may come in contact with a coating during a manufacturing process or in subsequent end use. All should be identified during the coating selection process to ensure that the substrate is adequately protected.
  • Electrical Insulation Resistance — Most powder coatings are excellent electrical insulators. Some, however, are specifically designed and tested for use on electrical components. It is important to define the dielectric strength required as well as any other related electrical property. Frequently, these are spelled out as “standards” by trade groups or organizations, such as Underwriters Laboratories.
  • Heat Resistance — Coatings are frequently subjected to elevated temperatures for constant or intermittent periods while in use. Higher temperatures generally cause some degradation that may reduce the useful life of the coating. Despite this fact, successful powders have been developed for barbecue grills and cookware, under-the-hood automotive and many other high temperature environments.
  • Abrasion Resistance — Powder coatings generally provide outstanding abrasion resistance. Store and library shelving and home and office furniture are just a few of the markets where powder’s superior abrasion resistance has been recognized. As with many properties, standard tests, such as Taber Abrasion, or in-house developed tests can be used to define the degree of performance required. The coating supplier, however, is more likely to have actual data or a close approximation for industry-wide test methods, such as those provided by the American Society for Testing and Materials (ASTM).
  • Impact Resistance — Like abrasion resistance and hardness, impact resistance is a measure of the coating’s toughness. Powder coatings are formulated to withstand blows from hammers and wrenches on an oil rig, stone damage to lawn mowers and automotive components as well as the everyday wear and tear of children’s toys, furniture and playground equipment.
  • Mar Resistance — Mar resistance is the ability of a coating to resist surface marking. Test methods used to determine a coating’s resistance to this surface marking includes several on-the-spot varieties such as fingernail mar, nickel scratch and Pencil Hardness tests. However, both the test method and interpretation of results tend to be subjective. Some very high-gloss and some low-gloss coatings can mar due to reorientation of components on the surface. Therefore, it is very important to accurately define the performance needs and to communicate these to the coating supplier via a reproducible test method. Other use-specific functional characteristics may be appropriate to further describe performance needs. These include edge coverage, thermal shock resistance, flammability and any other property essential to the end use.

Appearance Characteristics

Appearance characteristics can be equally important to the coater and the OEM, providing brand name type recognition, styling options and quality that can be immediately seen. These properties are often best defined with a physical standard although instrument-based test methods have become very reliable for some characteristics.

Powder coatings are available to cover a wide variety of appearance needs, including the following:

  • Smooth coatings that rival the best liquid finishes are available in a variety of gloss ranges. Smooth, high-gloss coatings can offer high distinctness of image (DOI) that translates to an illusion of depth. Smooth, matte finishes approach anodized aluminum or black oxide processes in appearance.
  • Textured coatings are often employed to hide substrate irregularities that may “telegraph” to the surface of a smooth finish. They can also effectively hide fingerprints and a distinctive feel to a product as well as provide non-slip characteristics.
  • Wrinkle finishes are a special class of textures that offer styling variation and a consistent appearance. As with color, there is no substitute for a physical standard when defining the desired type of texture.
  • Metallic coatings that add sparkle highlights to a base color are very popular styling tools. Solid metallics that reproduce the appearance of the base metal are used as alternatives to plating and add richness.
  • Hammertones and Veins give antique or distressed looks and are widely used on metal furniture.
  • Clear coatings provide protection for plumbing fixtures, aluminum wheels and other parts. They also serve as top coats over metallic base coats to bring out the luster and to provide protection for these pigment-rich surfaces. Reduced gloss versions are available to give a satin look.
  • Flamboyants and Tints are non-hiding coatings that add highlight color to a substrate or base coat. Brass tints, for example, can create a brass-like appearance over polished metal substrates.
  • Color variety is almost limitless and new special effects are continuously developed to meet the demands of stylists and designers. Many coating suppliers offer a pallet of off-the-shelf colors as well as custom color match services. It is impossible, however, to match an “idea” due to differences in color perception from person to person. Even computer generated reflectance or chromaticity data cannot be used to provide an exact match. There is no suitable substitute for a good physical standard when it comes to defining color requirements.
  • Gloss ranges from flat to high gloss are available in most powder chemistries. Even though reliable gloss measuring instruments are available and a number more easily describes gloss, it may be important to consider a physical standard in the proper range due to the impact of gloss on perceived color.   For example, a bright high-gloss red may appear to be off-color when reformulated at a 60° gloss of 10 even though the same pigmentation is used. Gloss and color are inseparable.
  • Opacity governs the hiding power of a coating and is directly related to pigmentation. In detailing the process requirements it is important to consider the minimum film thickness that will be required to completely hide the substrate. A test in which the coating is applied over panels that a half-black and half-white can be used to develop a numerical value is known as contrast ratio.

Application System Characteristics

Along with weatherability, functional and appearance requirements, the performance characteristics that are affected by the application system still remain to be defined. Your coating supplier needs to know the following:

  1. Cure Cycle – To provide effective and reproducible performance in all properties, the proper degree of cure is essential. Cure cycle is best described by the amount of time the substrate is at a given cure temperature (metal temperature). Air temperatures, oven set points, total time in the oven, or line speed do not effectively convey this information. The efficiency of the oven and part mass make it necessary to conduct a complete oven survey to identify temperature profiles which can differ from part to part and top to bottom of the oven. Using this information, the coating manufacturer should be able to supply a coating that meets the process requirements or offer advice on any inadequacy of an existing oven. For new installations, the coating manufacturer can supply a cure schedule for a pre-selected coating and this can be used to define the oven requirements of the new system. The source of heat also is an important factor that must be communicated to the material supplier. Byproducts of combustion from natural or synthetic gas or propane, and/or the rapid heat-up rate from infrared ovens are just a few examples of conditions that can impact the performance of the coating and, therefore, must be discussed with your coatings supplier promptly.
  2. Over-Bake Resistance – Excessively high peak metal temperature or extended time at temperature can cause coating surface degradation that may initially be noted as gloss reduction or color change. High temperatures will be apparent in the oven survey while extended times usually result from a conveyor line shutdown for maintenance, breaks, etc. Some degree of over-bake resistance is generally built into custom formulated products; the supplier is advised of this condition up front. The best solution, however, is to prevent over-bake conditions by designing safety procedures in case of line shut down or other unforeseen factors.
  3. Inter-Coat Adhesion – While the powder coating process is generally very forgiving, some rejects do occur due to lack of adequate process controls. These parts are salvaged frequently by re-coating. In other cases, clear top-coat, or color coat over a primer, requires good inter-coat adhesion. Some powder chemistries and some formulation additives do not lend themselves to re-coating. Therefore, it is essential that these possibilities and alternative plans are discussed with the supplier during the coating selection process. Also liquid touch up or liquid top-coat on a powder prime coat should be approved by your coating supplier. Powder top-coat on a liquid primer is even more problematic.
  4. Powder Particle – Particle size plays an important role in the application process. Size distribution affects many other properties critical to consistent performance of the coating system, including:
    • Fluidization
    • Powder transports in hoses
    • Uniform cloud density
    • Powder movement in air
    • Electrostatic charging
    • Deposition and build rate
    • Transfer efficiency
    • Faraday cage penetration
    • Wrap-around
    • Film smoothness

    It is important to note that other conditions also influence these properties and particle size is not the sole determining factor. Additionally noteworthy is the fact that the coating process, recovery system, frictional particle breakdown, and in-line-sieving systems make particle size a dynamic issue that must be managed during the operation of the system. The size distribution produced by the powder manufacturer must take the coating system design and anticipated changes into account to provide a suitable product.

  5. Substrate – The composition and condition of the substrate must be communicated to the coating supplier to prevent potential problems. Porous castings tend to out-gas during cure resulting in bubbles and blisters. Some coatings are available that help to overcome these problems.
  6. Film Thickness – As mentioned in the discussion of opacity, film thickness will dictate the pigmentation characteristics for hiding. Low film thickness may also require special resins or additives for smoothness. To prevent drips and sags from heavy films, different resins may be required.

Subsequent Manufacturing Process Characteristics

Finally, any process requirements driven by subsequent manufacturing processing must be identified and communicated to the powder material supplier. These may include the following:

  • Formability – Bending, crimping, punching, drilling, etc., all require a high degree of film flexibility.       Some powder chemistries are more readily post-formable than others and therefore this manufacturing process should be discussed with the coatings supplier.
  • Printability – As in the case of re-coatability, some coatings do not easily accept over-printing, for example, silk-screening. This is frequently due to the excellent solvent resistances provided by many powder coatings, coupled with the fact that printing inks are moving toward less aggressive solvents. Decal adhesion can have similar problems and solutions. The proper combination of powder coating and ink or adhesive must be selected.

In some instances, it can be impossible to have optimum performance for every property identified therefore compromise becomes necessary. Realistically, the formulation characteristics needed to achieve the highest level of performance with one property may be completely in opposition to the direction needed to achieve the same level of performance with another property. Thus a simple ranking of properties, in order of importance, may help form the basis of this compromise without sacrificing overall product performance.

Powder coatings used on reinforcement bar (rebar) for concrete highways exemplify the above condition. These coatings must be extremely tough, corrosion and chemical resistant, and capable of achieving cure very rapidly. During road construction, they are bent at an angle less the 90° so the coating must exhibit good flexibility and adhesion with minimal substrate penetration. To achieve the rapid cure with high corrosion and chemical resistance, specific resins of the cure systems are required. These generally result in cured films with very high cross-link density. This degree of structure in the polymer usually translates to brittleness and tends to bind the sites on the molecule that provide adhesion to metal. On the other hand, flexibility and adhesion can be provided by different chemistries that do not offer adequate cure, corrosion resistance and chemical resistance. A compromise was necessary to achieve the most important performance characteristics required by this demanding application.

With an understanding that first, you must be capable of coating the rebar, and second, its basic function is to protect the metal from corrosion, it follows that flexibility must be optimized in a highly cross-linked system. To do this, the exact degree of flexibility must be defined. This will minimize any sacrifice of corrosion resistance. The fact that the relationship between the two properties is non-linear forms the basis for a potential solution. That is, a 50% increase in flexibility does not require a 50% reduction in corrosion resistance.

With some order of importance and well-defined critical needs, it is possible to fully satisfy the overall performance objective as in the above-mentioned example. Remember…a performance-driven approach to coating selection begins with these two questions:

  1. What do we expect the coating to add to the finished part?
  2. To what type of environment will the finished part be subjected?

The tendency to concentrate too heavily on single properties and lose sight of these two questions usually results in an over-engineered or over-specified product. This does not provide the degree of insurance that may be intended. Instead, over-engineering tends to add cost and obscure the real objective.

With the performance criteria selected, it now becomes a matter of defining test methods and establishing target values. With these tools, the powder supplier can provide samples for laboratory test or trial runs at an equipment vendor. The most meaningful results, however, will be achieved with parts cleaned, coated and cured on the actual line that will be used.

Results from this series of events can be organized into a material specification developed from proven performance.

Clearly a great deal of planning and communication must be invested in selecting the right powder coating for the job at hand. It is a process that can take minutes to months depending upon the application, level of understanding of all parties involved and status of the coating line (i.e., an existing line versus an installation in process versus the planning stage). Like all good investments, the time spent in making a careful selection will usually result in dividends down the road. In this case, that could translate to a smooth-running, cost-effective coating operation as well as satisfied customers.

Define Quality Requirements

With a material specification and physical coating standard in place, it becomes possible to discuss quality. Quality is a net result of the entire coating system running effectively. Some film properties, such as dielectric strength, are heavily dependent upon the coating material or formulation. However, all film properties are dependent upon proper application. The following table attempts to quantify application dependence on a scale of 1-to-10, and it also list examples of key process control points that affect each property. A handful of characteristics may be used to define the quality of the coated part. These have implications far beyond the property itself and therefore become good tools with which to evaluate film quality. They include coverage, film thickness, appearance defects, cure and adhesion.

Table 2: Thermosetting Powder Properties and Applications

Generic Powder Typical Properties Typical Applications
Epoxy
  • Excellent chemical and corrosion resistance
  • Excellent mechanical properties
  • Poor exterior color/gloss retention
  • Available in all colors, clear and textures
  • Metal furniture
  • Underbody automotive parts
  • Major appliances
  • Microwave ovens
  • Shelving
  • Power tools
Epoxy
Polyester
Hybrids
  • Very good chemical and corrosion resistance
  • Very good mechanical properties
  • Fair exterior color/gloss retention
  • Available in all colors, clear and textures
  • Water heaters
  • Radiators
  • Transformer covers
  • Office furniture
  • Fire extinguishers
  • Toys
Polyurethane
  • Good chemical and corrosion resistance
  • Good mechanical properties
  • Very good exterior color/gloss retention
  • Available in all colors, clear and textures
  •  Automotive wheels and trim
  • Playground equipment
  • Garden tractors
  • Fence fittings
TGIC
Polyester
  • Very good chemical and corrosion resistance
  • Very good mechanical properties
  • Very good exterior color/gloss retention
  • Available in all colors, clear and textures
  • Architectural aluminum
  • Outdoor furniture
  • Farm equipment
  • Fence poles
  • Air conditioning units
Acrylic
  • Very good chemical and corrosion resistance
  • Good mechanical properties
  • Very good exterior color/gloss retention
  • Available in all colors, clear and textures
  • Washing machines
  • Oven parts
  • Refrigerators
  • Microwave ovens
  • Aluminum extrusions

Other characteristics important to the end use or film quality required may also be selected. In cases where weatherability is a critical property, test coupons may be cut from raw stamped metal stock and hung on line for cleaning, coating and curing. These can act as control pieces for further evaluation including accelerated indoor tests or outdoor weathering tests.

Log sheets that contain test results, operating parameters and other pertinent information can help assure that the process is under control. Changes in results then become important indicators that something is wrong. When this occurs, the log sheets will help to connect cause and effect, thus becoming an important tool in problem solving and prevention. During system start-up, logs can help to define process capability with ranges used to establish product or process specifications as well as providing feedback into powder, pretreatment, chemical or other supplier specifications.