Pretreatment system design for optimum end-use performance

systems and equipment

PRETREATMENT SYSTEM DESIGN FOR OPTIMUM END-USE PERFORMANCE CINCINNATI INDUSTRIAL MACHINERY, DIV. OF EAGLE PICHER, CINCINNATI A finishing system may have many possible arrangements, but only one is best suited to the user's plant conditions and needs. Only after working closely with the user and suppliers are you able to determine the system best suited to your requirements. Finishing systems are comprised of washers, dry-off ovens, incinerators, pretreatment, electtocoating, spray booths, flowcoaters, dip tanks, cure ovens, conveyors, waste treatment, and air makeup. A complete paint finishing system consists of an integrally designed combination of equipment (or single compact machine) that conveys parts through the cleaning, pretreatment, paint application, and baking steps to deliver a finished part - often without labor between loading and unloading. Systems can include makeup air equipment to replace exhausted air, loading and unloading devices, and other related equipment. A typical schematic is pictured in Fig.1.

DEFINING PARAMETERS Pretreatment can be accomplished in many different ways involving several technologies (see Table I). The optimum design for a family of parts evolves from understanding the following parameters and applying them with consistent design integrity: (1) quantity and configuration of parts; (2) material composition of parts; (3) desired material handling methods; (4) understanding soils and cleanliness desired; (5) facilities and utilities available; and (6) environmental considerations. Each of these parameters will influence the design of the system. Early involvement of competent representatives from chemical suppliers, equipment manufacturing companies, and paint companies will improve the design phase and enhance your objective. Once the parameters have been defined the design can begin.

Quantil)l and Configuration o/Parts The production rate obviously determines the level of automation, capacity of the equipment, energy consumption, chemical usage, etc. Selection for today's requirements may be inadequate for tomorrow's needs and sizing the system too large can waste money. Long-term planning will help determine production rate, future designs, and available financial resources. The proper analysis is essential. List all parts, their sizes, and annual production rate that you plan to process with the system. This will give you the yearly production requirements. Then, based on one, two, or three shifts, determine production time available. When comparing production requirements with production time available you can establish rate in feet per minute. The smallest, the largest, and the average part size must be defined in terms of dimensions. This allows equipment manufacturers to size the openings for washers (to minimize overspray), determine optimum drain length (minimize solution carryover), and size heaters, pumps, and fans. The overall weight of the part or batch of parts being conveyed must be known to calculate heat loss through the washer and ovens. Unusual shapes must be identified for early consideration. For example small, blind holes on surfaces could negate the power wash approach, whereas large, 205

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Table I. Comparison of Iron, Zinc, and Manganese Phosphating Phosphalin8 Material

Iron

Zinc

Manganese

206

Benefits

Drawbacks

Lowest chemical costs. lowest Not as corrosion resistant as equipment investment. good zinc phosphating. shorter paint adhesion. easiest to chemical life. conttol. gives higher gloss wi th less paint, can be applied in thinner coatings. minimum sludge formation. lowest heating requirements. adequate corrosion protection for most indoor and some outdoor applications. Highe:r chemical costs~lhan Excellent paint adhesion. excellent corrosion resistance iron. more sludge in all environments. better formation. higher maintenance costs than chemicalllfe than three-stage iron. moderate heating iron. more difficult to conttol than iron. requirements. Heaviest coatings. good tank Not used as a paint base. highe:r temperature llfe. excellent rust preventive. best for retention of oils and requirements. areatest drawing compounds. sludle formation.

Typical Applications

Metal cabinets. furlliture. electrical and electtollic enclosures. and toys and other novelties.

Water heaters. refrigerators. air conditioners. outdoor furniture. washing machine parts. Heavy gears. engine parts. parts to be deep drawn.

open or flat surfaces that can drain are ideal. Small parts that can withstand tumbling can be ideal for a drum washer; baskets of parts that cannot be tumbled can be immersed and agitated. The same situation is true with paint application. Smooth, flat parts can be easily automated, while complex shapes may require additional, manual reinforcement for complete coverage.

Material Composition o/Parts Knowing the base materials will allow compatibility with selected cleaners, subsequent waste treatment requirements, operating temperature, method of handling, and process specifications.

Desired Material Handling Methods Selecting the appropriate material handling method should be done based on the principles of increasing productivity. The method selected must incorporate reliability, economy, flexibility, and ease of installation. For example an overhead monorail-type conveyor system is relatively low in cost but not very flexible, while a power-and-free system is very flexible but higher in cost.

Understanding Soils and Cleanliness Contaminants determine chemical selection and spray or immersion selection, and ancillary equipment for handling the effluent (chips, oil, or heavy metals) will impact the waste treatment stream. If the cleaning process is not defined it is important to get contaminated parts to the chemical and equipment suppliers and have the parts tested in a laboratory. While in the laboratory your suppliers can simulate conditions and test the different variables. Minimizing or avoiding contamination is the key to keeping your factory and parts clean. Know where your soils are coming from and why. This allows you to take steps to contain soils in the area where they occur. Effective and inexpensive means to check for clean parts are the "water-breakfree surface test" and "white towel test." If organic soil has been effectively removed a uniform sheeting of the rinsewater will occur as parts exit the last pretreatment stage. If the surface has beaded water standing you have not adequately removed organic soils. Inorganic soils can be checked by using a white towel after they have passed through the dry-off oven. If the towel is dirty you have not adequately removed the inorganic soils. More sophisticated is the millipore test. This test requires a vacuum pump, flask, funnel, filter papers, isopropyl alcohol, oven, and scale. This test can detect micron particle size and weight in milligrams.

Facilities and Utilities Available A floor layout with height clearance is important for design considerations. Often, space constraints dictate process times. Although it may not be the optimum process it may achieve acceptable results. Availability of electrical service, heating preference, and plant conditions (such as availability of truck dock, building door size,floor and roof construction, distance from unloading site to erection site, and whether there is a clear path) must also be considered.

Environmental Considerations Local, regional, and federal regulations are continually being added and changed. The recent Clean Air Act Amendments of 1990 established procedures for a national permit system for air pollutante missions, as well as establishing a basis for more strin-

207

gent controls on emissions from all manufacturing operations. Waste stream compliance is forcing many manufacturing companies to invest in waste treatment plants. Understanding your local laws and knowing your process will determine your direction. If you are not familiar with either, outside help must be sought.

COMPONENT DESIGN

Material Handling Selection of the material handling method varies directly with production volume and desired cleanliness. In general the monorail is the most economical, flexible, and reliable method of handling product,but it is not the ideal solution for all cases. If you handle product in batches/baskets a belt internal to the machine can provide an excellent and consistent means of transporting product through the washer. If your product can tolerate "bumping," such as nuts or bolts, a drum machine is a cost-effective means of conveying high volumes with excellent results. There are also situations where combinations of belt and monorail are appropriate. Rather than having two machines you can easily assimilate dual lines into one washer. Conveyor systems are discussed in further detail in a separate section of this Guidebook. Syste~ size is a function of conveyor speed, which is actually based on part density rather than raw production rate. As an example, to determine the line speed for a given production shift divide the number of parts desired by the number of parts per rack times the rack spacing divided by the number of minutes available per shift.

Washers/Pretreatment The choice of a spray wand, three-stage, or five-stage machine is based on a number of variables: incoming soil loads, space available, results required, energy consumption, rotal initial cost, estimated total operating cost, production volume, size of part, etc. The initial cost of a five-stage machine (clean, rinse, phosphate, rinse, rinse) is somewhat higher than a three-stage type but operating economies and higher quality quickly offset the investment. The five-stage machine ensures longer chemical life. ----f ~ The high-pressure heated spray wand is an excellent choice for low-volume, 2'.0· hard-to-handle parts. This approach is a cost-effective technology with a great deal offlexibility and versatility. From experience and the recommendation of various chemical companies, to remove shop dirt and light machine oil a three-stage machine is adequate. The typical process would be a one-minute

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wash stage (heated), 30-second rinse (am-

bient), and a 30-second rinse/inhibit (ambient or heated). If you have a more critical cleaning specification or phosphating requirement a five-stage washer with oneminute wash (heated),30-second rinse

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(ambient), one-minute phosphate (heated), 30-second rinse (ambient), and 30-second rinse/ inhibit (ambient) will be required

Spray versus Immersion In general, spray processing provides the most effective cleaning and rinsing capabilities owing to increased mechanical action, liquid impingement, and natural draining; however, for impingement of recessed or hidden surfaces of complex parts and assemblies, immersion processing is more appropriate. The addition of ultrasonics or agitation can enhance rhe impingement capability, but it is costly. When processing complex parts or heavily loaded baskets of parts a combination of immersion and spray stages is often required. Testing dirty parts in a laboratOlY will provide proper selection.

Sample Three-Stage Washer The first step in the design is to layout the system (using a conveyor speed of 4 fpm and a largest part size of2'1z fr x 18 in. x 18 in.). The entrance profile s hould include a 3-in. clearance around the part. This will provide flexibility to process larger parts if required (see Fig. 2). A housing space of 1 ft on each side of tunnel openings allows for spray risers and piping. The cleanout section can be 2 to 21f2; Viz ft from the fl oor to the bottom of the tunnel opening will provide enough space to have th e tank capacity to keep the pump-to-tank ratio around 3: 1. The overall height of the machine is 71f2ft, but we need to allow another 3 ft minimum for ventilating ductwork. The machine length for a three-stage washer, allowing three minutes of drying, is shown in Fig. 3. You know that one minute in the wash stage is req uired and that the conveyor is traveling at 4 fpm , so yo u can size the wash stage at 4 ft. Similarly,

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Considering both energy and poUution problems. ,DeW processes allow lower processing IempentUres in some cases.

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C<>osidering - . enetg)' and pollution problems, new processes allow lower processing temperatures in some cases.

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rinse stages are 30 seconds; thus, they are each sized at 2 ft in length. Good design practice dictates that the approaches and exits should be two times the runnel width; therefore, they should be 4 ft long. The drains should be three times the tunnel width, minimizing spraying and carryover contamination; therefore, they are 6 ft in length. Special consideration for drain lengths may be necessary for long parts or slow line speeds. The hot air blow-off will require three minutes of drying time, so size that at 12 ft. You also must have 1 ft between washer and hot air blow-off fan for ventilating.

ENVIRONMENTAL CONCERNS Most washer systems require waste treatment of the effluent from the system prior to disposal to the sewer. In some cases, where the potential discharge of cleaners and phosphates to the sewer are significant, a self-contained treatment system is required. Most municipal waste treatment authorities have a list of chemicals they can accept and in what concentrations. Check with the local authorities for additional information, or consultants are available to assist in your decision-making process.

Fig. 6. Three-stage drum-type washer with blow-off (see Table IV for details). 212

TYPICAL DRUM FOR SPRAY

SPECIAL SCOOP-TYPE DRUM WHEN SCOOPS ARE USED IT ~AY ELIMINATE THE NEED FOR PUMPS

Fig. 7. Drum washer construction details.

FIVE-STAGE WASHERS A five-stage monorail-type zinc phosphating machine is shown in Fig. 4. Table II provides details for each stage.Table III provides similar information for the five-stage iron phosphating system pictured in Fig. S.

DRUM-TYPE WASHERS Figure 6 depicts a typical three-stage drum-type washer. Concentration details are shown in Fig.7. Table III gives further details on drum-type systems.

OVENS In former years there was a tendency to over simplify oven problems. There was a widely held idea that an oven was "just a heated box" through which parts were passed to dry or bake a finish or to evaporate and remove moisture or otherwise

Table IV, Typical Dnun·Type COUStruCtiOD for Three·Stale Washer with Blow·Off Stage No.1 Wash

Stage No.2 Rinse

Stage No.3 Rinse

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1/4-in. mild steel If'3:in. mild steel

Nozzles

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Mild or stainless steel Steel Iron Mild steel

Mild or stainless steel Steel Iron Mild steel

Piping Pump Immersed heating equipment Air heating equipment

Blow.off/Dryer 220'F 180

1/8·in. mild steel

Mild steel

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process by heat application. In more recent years the scientific aspects of heat engineering have come to be appreciated. True economy in operation matched by superior results can only be obtained by expert design based on sound engineering followed by quality manufacturing and conscientious installation. The ovens may be on the plant floor or elevated overhead, either inside the plant or outside, on the ground or on the roof. With the ovens elevated the factory floor may be used for manufacturing or storage. The oven panels should be tongueand-groove, all-welded construction, fabricated of heavy-gauge sheet steel, with a minimum of through metal, which reduces the transmission of heat by conductance. To avoid "insulation sag," which would leave an open space at the top of the wall panels, oven panels should be insulated with mineral wool batts If, in. greater in thickness than the oven panels. Access doors should be fully insulated or of nonsagging construction and equipped with explosion latches. These doors should allow opening from inside or outside the oven. The relatively few ovens shown here are merely examples of the many types being built, each representing dozens and even hundreds of similar installations. Many special types are not shown because oflimited use. For standard ovens, custom designs, and even economical prefabricated components it pays to consult true experts in heat engineering.

Direct versus Indirect Heating Systems In a direct fuel-fired system the products of combustion are allowed to come into contact with the work; the combustion equipment can be located inside or outside the oven. Although gas is generally used with a direct fuel-fired system, modern processing of fuel oils, along with improved handling and firing equipment, has increased their use in this area. An indirect-fired system does not allow products of combustion to come into contact with the work. Electric and steam heating are common choices and fuel-fired equipment may be used in conjunction with a heat exchanger to separate the products of combustion from the oven atmosphere.

Fuels The brief outline that follows describes the most popular fuels being used today. You may want to investigate the possibility of using one as an alternative to back up your present system or as a supplement. Gas: This is generally considered a clean, convenient, and easy-to-use fuel. Work loads are commonly heated by the direct-fired method with no adverse effect from the products of combustion; however, because the availability and overall costs have changed it may be to your advantage to check the alternative fuels in your area. Oil: Many of the problems that once plagued the use of oil with direct-fired oven equipment have been eliminated, and indirect-fired systems are easily designed and installed when needed.In either case plan for a safe, convenient storage space. Steam: A very clean source that is simple, easy to control, and reliable for low-tomedium temperature operations, steam can be produced in a number of ways that are most economical. Electricity: This is a clean, simple, and efficient source of heat. High temperatures are easily obtained and heat recovery systems are available to economically reduce operating cost. Combination: Combination fuel systems are very popular with people who have been plagued by fuel price increases and shortages because they allow the ability to switch from one fuel to another without stopping production or adversely affecting the product. 214

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Recovery: Recovery systems have recently become very important to all users of energy. As fuel costs increase heat recovery systems become more practical. Radiant: This is particularly applicable to flat parts and batches processed on the basis of part shape and size. It is a clean, high-energy source of heat where high surface temperatures are easily obtained in short periods of time. Although expensive to control it can provide shorter cure times and minimal floor space.

Features Knowledgeable plant operators want panels and the general construction to provide maximum strength and rigidity, minimum heat transfer or loss, and an attractive appearance. The panels should be filled with insulation of full thickness, formed in batts to resist sagging and settling that can leave uninsulated areas. Tongue-and-groove construction mates into a strong, neat joint. Ovens should be able to be readily disassembled and moved if necessary. A typical oven system is shown in Fig. 8. Air seals: Ovens can be provided with a variety of seals to prevent escape of heated air and fumes. The bottom entry oven, which must be elevated, has a natural type of seal because the heated air rises into the upper, sealed portion of the oven. Exhaust-type seals, where conveyor openings into the oven are enclosed by a hood with an exhaust fan, are also available. In theory the most practical type of seal is the recirculating seal, which has been far from perfect in the past. Companies have invested considerable time and money in this seal and they are now producing recirculating seals that allow adjustment to compensate for various oven temperature losses. By providing a recirculating fan with adjustable vents at least 80% of the normally escaping heat can be retained. Ovens can be designed as a high-velocity type or low-velocity type depending on the application. In most cases ovens are designed with both types in special zones to control paint popping, blistering, or in the case of a powder oven, dry powder blow-off Low-velocity or "quiet" zones allow the coating to cure slowly, allowing solvents to evaporate before the surface film is set, which minimizes pops and blisters. Radiant and infrared ovens can fit into this category. High-velocity zones allow direct impingement of the air to the work,which allows for quicker curing of the coating or evaporation of water. This air movement helps to obtain even temperatures throughout the oven as welL The air movement is induced by high-velocity nozzles designed to improve direct impingement of heated air on the work surface. Attempting to dry or cure a painted part too quickly is a notable cause of paint "skinning," resulting in bubbles, blemishes, and powder blow-off. Rolling air or high turnover rates: This induces an air movement that helps to obtain re215

markably even temperatures. The air movement is induced by high-velocity nozzles that also prevent direct impingement of heated air on the work surface, anotable cause of "skin-drying," resultant bubbles and blemishes, and powder blow-off Burner size: When selecting burners for an oven concentrate on the total needs of the job at hand; then allow for a reserve capacity to handle higher work loads without expensive alterations. This provides the greatest overall economy and equipment flexibility because the burners are efficient throughout their operating ranges. Combustion chambers: Ovens are usually designed with the combustion chamber as an integral part of the unit, streamlining the appearance and eliminating the need for exterior ducrwork. This compact arrangement also reduces power requirements connected with excessive ductwork, saving floor space as well. Disposable filters: Intake air filters can minimize the intake of dust, which can cause finish flaws. The filters should be arranged for quick and easy changing without use of tools. The best disposable filters to use are frame less, eliminating the usual cardboard frame as a possible fire hazard. Simple erection: Ovens should be constructed of prefabricated panels for easy erection. All componenrs-panels, structurals, ducrwork, wiring, and piping- should be keyed to assembly drawings and manufactured to fit easily, quickly, and precisely in the field. The purchaser can in many cases erect the oven with existing plant person-

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nel under supervision for significant savings. For more complex ovens suppliers provide a complete service ranging from basic erection to complete turn-key jobs.

Dry-OffOvens The dry-off oven is usually a continuation of the washer. Drying metal pans with no "puddling" of water requires three to five minutes, depending on temperature. At 4 fpm you will need 12 [0 20 ft of dry-off oven, excluding air seals. The next unit requiring the largest amount of floor space is the cure oven. Assume for design purposes that the paint you are using requires a 20-minute cure at 350 0 F. Again, conveyor speed is 4 fpm so you have [0 be in the oven at temperature for 80 ft of conveyor travel. The oven can be fabricated in practically any configuration, depending on the floor space available. It could be a single-pass oven (80 ft long) or a two-pass oven 216

(40 ft long) or a four-pass oven (20 ft 10ng).This you can determine from the floor space available. Iffloor space is a premium and the building height will allow it the oven can be hung from cross-beams supported from the floor or mounted on the roof Available space usually dictates the position of the oven. The tunnel opening of the oven is normally the same dimensions as the washer except that usually 18 in. to 2 ft from the bottom of the tunnel opening to the floor is ample for ductwork. To sketch the oven layout you know that the maximum part width is 1liz ft and that they are spaced on 24-in. centers. This indicates that a 3-ft-diameter wheel turn will permit clearance of parts on a turn, so the proposed oven is outlined as shown in Fig. 9.

PAINTING SYSTEMS The major types of paint systems fall into five categories: conventional solvent systems, water-reducible systems, high-solids systems, powder systems, and electrodeposition systems. Defining your criteria relative to operational characteristics, coating properties, initial capital expenditure, and operational costs will determine the paint system. A powder system and electrodeposition system have been included to demonstrate layout. The following are component design considerations for an electrodeposition system.

ELECTROCOAT (ELECTRODEPOSITION) PAINTING

Coating Thickness Control The main factors controlling film thickness are the applied voltage and the film resistance. Increasing the coating voltage or lowering the specific film resistance causes an increase in film thickness. You simply dial the desired coating thickness. The electrocoating process will continue until the organic film deposited provides an electrical insulating resistance, which prevents further current flow. When the coated parts are removed from the bath they are rinsed in permeate and deionized water to remove nondeposited paint particles.

Tank Design Electrocoat tanks are designed for an immersion time of 1% to 2 minutes. It is possible to deposit approximately 1.0 mil organic coating in the first 15 seconds; however, for heavier film deposits a longer time is required. Tank equipment includes dual pumps with each pump able to maintain the bath and prevent the setting of paint solids. Plate-and-frame heat exchangers are used with chiller units to maintain proper tank temperature.

Tank Design is Vital In the design of the electro coat tanks some of the most important items are circulation rate, circulation flow, and density of the paint. With the paint solids normally at 8 to 10% density a flow rate and pattern is determined to prevent settling. The flow rate in the average tank is accomplished by the use of headers with eductors. The flow pattern in the bottom of the tank is opposite that of the conveyor movement and with the conveyor at the top of the tank. The exit end of the tank is equipped with an overflow weir tank designed to prevent foaming without dropping or aerating the paint. The recirculating pump suctions are also connected to this tank. 217

Filter Systems Conventional filter systems are provided with approximately 50 micron filter media to remove foreign debris that may enter the bath. An ultrafiltration system will be used to remove soluble salt and water carried into the bath from the cleaning process by the parts being coated. Ultrafiltration may also be used to recover paint solids from the postrinse so they may be returned to the bath. A virtually closed system exists when ultrafiltration is used to provide rinse water in the place of deionized water. This arrangement will aid considerably in the prevention of water pollution.

SUMMARY Getting value from your finishing system involves a comprehensive review of your requirements and, as necessary, applying some or all of the many technologies available into your system design. Normally, one person or one company does not possess all resources to do this task. A good approach is to consult reputable suppliers and follow their recommendations. The best solution to your system design is one in which every selected supplier works as part of your team toward the common goal of a successful system installation, start-up, and operation.

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