- Email: [email protected]
Electrostatic spray processes
ELECTROSTATIC SPRAY PROCESSES by Joel Rupp, Eric Guffey, and Gary Jacobsen /JW Ransburg Nectrostatic Systems, Toledo, Ohio PRINCIPLES
OF ELECTROSTATICS
Electrostatic Theory Electrostatic finishing got its start in the early 1950s. Coatings engineers needed an application method that would significantly increase transfer efficiency and reduce finishing costs. They reasoned that particles and objects with like charges repel each other. and objects with unlike charges attract each other. The same would apply to charged spray coatings and a part to he painted. They discovered that by negatively charging the atomized paint particles and positively charging the workpiece to be coated (or making it a neutral ground), an electrostatic field would be created that would pull paint particles to the workpiece. (See Fig. 1.) With a typical electrostatic spray gun, a charging electrode is located at the tip of the atomizer. The electrode receives an electrical charge from a power supply. The paint is atomized as it exits past the electrode, and the paint particles become ionized (pick up additional electrons to become negatively charged). An electrostatic field is created between the electrode and the grounded workpiece. The negatively charged paint particles are attracted to the neutral ground. As the particles deposit on the workpiece, the charge dissipates and returns to the power supply through the ground, thus completing the electrical circuit. This process accounts for the high transfer efficiency. Most of the atomized coating will end up on the part. The degree to which electrostatic force influences the path of paint particles depends on how big they are, how fast they move. and other forces within the spray booth such as gravity and air currents. Large particles sprayed at high speeds have great momentum, reducing the influence of the electrostatic force. A particle’s directional force inertia can be greater than the electrostatic field. Increased particle momentum can be advantageous when painting a complicated surface. because the momentum can overcome the Faraday cage effect-the tendency for charged paint particles to deposit only around the entrance of a cavity. (See Fig.
2.) On the other hand. small paint particles sprayed at low velocities have low momentum. allowing the electrostatic force to take over and attract the paint onto the workpiece. This condition is acceptable for simple surfaces but is highly susceptible to Faraday cage problems. An electrostatic system should balance paint particle velocity and electrostatic voltage to optimize coating transfer efficiency. Electrostatic Advantages The main benefit offered by an electrostatic painting system is transfer efficiency. In certain applications clectroctatic bell\ can achieve a high transfer efficiency exceeding 90%. This high efficiency translates into significant cost savings due to reduced okct-spray. A phenomenon of electrostatic finishing known as ‘wrap” cause\ some paint particles that go past this workpiece to be attracted to the back of the piece, further increasing transfer efficiency. Increased transfer efticiency also reduces VOC emissions and lowers harardous waste disposal costs. Spray booth cleanup and maintenance are reduced. Coating Application Any material that can be atomized can accept an electrostatic charge. Low-. medium, and high-solids sol~entborne coatings. enamels. lacquers, and two-component coatings can be applied electrostatically. 198
SATA A U T O M A T I C A N D ROBOTIC GUNS
:' ,i:+~+u :+, IIIY++:Y:%-
CO
:i:/¸'/: .....
/ii
+ The finest atomization and performance • Special stainless steel head version available • Adaptable to nozzle extensions Wkether it's conventional, air assisted or HFLP, robotic or manual, SA TA has an automatic gun to suit any requirement.
Spray Equipment D i s t r i b u t e d b y D a n - A m Co ~, M e m b e r A S A Corporate Office: Phone: 800 533 80t6 West CoastBunch: One Sata Drive POB 46 SprlngVa~lev MN55975.0046 Order
Desk:
www.satausa.com
507 346 7776 Fax 8006337282 507.346.7481
1200 O~egon Avenue LongSeach, CAg0813
8OD-533-8016 • e-mail;
[email protected]
The Appficatlon Technology Of The Future
Circle 097 on reader information card
Coatings & Coating Processes for Metals byJ.H. Lindsay
Can-Am's 20 Years of Experience Guarantee's It
353 pages $150.00
The editor of this invaluable w o r k is a respected and well k n o w n authority. This new book is an encyclopedic listing of over 2,000 coatings and coating processes from more than 100 companies. The first section offers material and equipment by trade name. Section two lists the names, street and e-mail addresses, and phone a n d fax numbers of manufacturers. The third and fourth sections include a complete index of useful tables with representative standards and specifications. Send Orders to:
METAL FINISHING 660 White Plains Rd., Tarrytown, N Y 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-257 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and I20.00 for each book shipped express to all other countries.
Circle 020 on reader information card
Conveyor
High
_L
:e C;
IonizationArea
Grounded Work or Targets Material Source
®L Electrostatic Power Supply
/
I i I I I i
EaCh Ground Potential
Fig. 1. Electrostatic application circuit.l)w soh'ent-ba,~ed coatings. The various types of electrostatic systems can apply coatings regardless of their conductivity. Waterborne and metallic coatings can be highly conductive. Solvent-borne coatings tend to be nonconductive. Any metallic coatings can contain conductive metal particles. These metallic coatings must be kept in circulation to prevent a short circuit in the teed line. As high voltage is introduced into the system, the metal particles can line up to lkwm a conductive path. System modifications may be required because of coating conductivity to prevent the charge from shorting to gruund. (See Fig. 3.)
Operating Electrostatics Safely Electrostatic finishing is safe if the equipment is maintained properly and safety procedures are t~.~llowed. All items in the work area must be grounded, including the spray booth, conveyor, parts hangers, application equipment (unless using conductive/waterborne coatings), and the spray operator. As electrical charges come in contact with ungroundcd components, the charges can be absorbed and stored. This is known as a capacitive charge buildup. Eventually, enough charge is bui}t up so that when the ungrounded item comes within sparking distance of a ground, it
200
Fig. 2. Fanul~ O" cage effect. can discharge as a spark. Such a spark may have enough energy to ignite the flammable vapors and mists that are present in the spray area. An ungrounded worker will not know that the capacitive charge has been absorbed until it is too late. Workers should never wear rubber- or cork-soled shoes, which can turn then into ungrounded capacitors. Special shoe-grounding devices are available. If workers are using hand-held guns, they should grasp them with bare hands or with gloves with cut-outs for fingertips and palms that allow adequate skin contact. Proper grounding of all equipment that is not used for the high-voltage process is essential. Grounding straps should be attached to equipment and connected to a known ground. A quick inspection of all equipment, including conveyors and part hangers, can reveal improper grounding. Good housekeeping can pay dividends. Removing paint buildup from parts hangers can help ensure that workpieces are grounded. Ungrounded objects, such as tools and containers, should be removed from the finishing area.
PAINT PARTICLE CHARGING Electrostatic charging of paint particle got its start back in the early 1950s. Engineers were looking for methods to reduce the cost of finishing products. Harold Ransburg, the inventor of the electrostatic process, reasoned that since unlike electrical charges are attracted to each other, the same idea would apply to charged paint particles and a part to be painted. Everyone's heard the saying that "opposites attract, and likes repel." This is true with both a magnetic field and with the electrostatic process of charging paint particles. The electrostatic process is almost identical to the way a common magnet works. By creating an electrostatic field between a negatively charged paint particle and a positive grounded workpiece, the paint particles are attracted and deposit themselves onto the workpiece. The basic building block of electrical energy is the charged particle. All matter is made from electrically charged particles. These particles are either neutral, negative, or positive.
201
Conveyor High Voltage Cable
_L
Ionization Area
Atomizer
. . . . . . . . . . . . Grounded Work or Targets Material Source
ion Device
Earth Ground Potential Electrostatic Power Supply
±
I
I I I
Fig. 3. lsolaled electrostatic application cin'uit,fbr waterborne and metallic coatings. Back in the early days of particle charging, a process referred to as the Number One Process was developed by Harold Ransburg to charge paint particles. Paint particles were sprayed into an electrostatic field by conventional air spray guns. Two wire grids were aligned parallel to each other at a certain distance, then the parts were conveyed through these grids. At one end of the grids, atomized paint particles were sprayed into the electrostatic field. The paint particles would become negatively charged and would be attracted to the positively grounded parts. These wire grids are now the wire electrode in an electrostatic spray gun. The three most common ways of charging paint particles are the electrostatic spray gun, a rotary bell, or a rotary disk. All three of these methods work by the same common principle of the electrostatic field between the atomizer and the workpieee then introduce atomized paint particles into the field and they will be attracted to and deposit themselves on the positive grounded workpiece. With an airspray or an HVLP electrostatic spray gun, a high voltage DC charge is supplied to the applicator's nozzle electrode, creating an electrostatic field between the gun and the grounded target object. (See Fig. 4.l The coating materials are charged at the point of atomization. The charged paint particles are attracted to and deposited on the grounded target
202
Fig. 4. 7~vpical electrostatic hand gun application. object. This electrostatic charge allows a more efficient, uniform application of the coating material to the front, edges, sides, and back of the product. The electrostatic forces allow for a high percentage of the charged paint particles to be deposited on the workpiece. The electrostatic process can also be used to charge paint particles using airless and air-assisted airless electrostatic spray guns. The only difference is the coating material is atomized by different methods. An air spray or HVLP electrostatic gun utilizes much lower air pressure to atomize the coating material, the airless and air-assisted airless methods use a much higher pressure. Coating material is delivered at high pressure to the atomizer. There, the material is atomized by passing through a very small orifice under high pressure. The resulting spray mist particles then become electrostatically charged and are attracted to the workpiece in the same manner as electrostatic air spray or electrostatic HVLE Today, rotary bells are generally about 1 to 3 in. in diameter and rotary disks are about 6 to 12 in. These atomizers operate on the same principle except they are positioned differently to the workpiece. Bells are positioned with their axis horizontal to the part, and disks are positioned vertically. A rotating disk or bell distributes a thin, even coating to the edge of the atomizer. There the coating is atomized either by the electrostatic force or centrifugal force. A low speed rotary atomizer utilizes almost all electrostatic forces, a high speed rotary atomizer relies on the centrifugal force of the atomizer to atomize the coating material. A DC high voltage charge is then supplied to the rotating atomizer, creating an electrostatic field between it and the grounded target object. The negatively charged paint particles are attracted to and deposited on the positive grounded workpiece, The forces between the charged particles and the grounded target are sufficient to turn normal overspray around and deposit it on the back surface of the target; therefore, a very high percentage of the paint particles are deposited on the part. Paint resistivity, often referred to as conductivity, is critical when spraying materials electrostatically, Waterborne materials are very conductive; therefore, measures such as voltage blocking devices, external charging probes, or completely isolating the fluid supply
203
and fluid lines must be taken or the paint particles will not be able to Fnaintain the electrostatic charge. Due to the low resistance of waterborne materials, all of the electrostatic voltage will drain off to ground and short out the system. If one of the three previous methods mentioned are not used, the paint particles cannot be charged electroslalically. Solvent-borne materials paint resistivity will vary fl-om one material to another. When spraying solvent-borne coatings with electrostatics, it is critical to measure and monitor the resistivity of the paint being sprayed. Materials that are too conductive, (very low resistance, often referred as "hot") will also drain some or all of the electrostatic voltage off to ground. This will greatly reduce the electrostatic efl'ects on the paint particle. On the other hand, when using materials with a very high resistance, often referred to as "'dead," the paint particles will not readily accept the electrostatic charge and the transfer efficiency will be very poor. Coating suppliers can easily ti)rmulate their solvent-borne materials to be within a specific resistivity range. The optimum resistivity may difter depending on the tool used for application. For example, with an electrostatic disk or bell, the optimum resistivity range is between 0.05 and 1 megohms on a (Ransburg) paint resistivity meter. An electrostatic spray gun however, can effectively spray coating materials between 0. I to ~ megohms of resistance. Another example is the No. 2 Process on-site electrostatic spray gun. This gun requires a more precise paint resistivity because it relies solely on the electrostatic charge to atomize the coating materials. The paint used with this gun must read between 0.1 to 1 megohms on the (Ransburg) paint test meter to work properly. Another key element in the electrostatic process or charging of paint particles is particle size. Large particles sprayed at high speed have greater momentum and reduce the influence of the electrostatic force. Increased particle size and momentum can be an advantage when coating complicated surfaces because the momentum can overcome the Faraday cage areas (where paint particles are attracted to the edges of a workpiece while avoiding inside corners and recessed areas). On the other hand, small paint particles sprayed at low velocities have low momentum, thus allowing the electrostatic force to take over and attract the coating material to the target object. This conditiun is acceptable for simple surfaces but is highly susceptible to Faraday cage problems.
ELECTROSTATIC PROCESSES/EQUIPMENT The electrostatic application of atomized materials was developed to enhance finish quality and improve transfer efficiency. (See Fig. 5.) Presently, there are seven types of electrostatic processes for spray application: • • • • • • •
Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic
air spray atomization high volume, low-pressure (HVLP) atomization airless atomization air-assisted airless atomization electrical atomization rotary-type bell atomization rotary-type disk atomization
Regardless of the electrostatic finishing systems, each has its advantages and limitations. What may be suitable for one situation may not be suitable in another. (See Table 1.)
Electrostatic Air Spray Atomization Electrostatic air spray uses an air cap with srnall precision openings that allows compressed air to be directed into the paint for optimum atomization. Electrostatic air spray is the most widely used type of atomization in the industry today due to its control and versatility. Electrostatic air spray provides very high transfer efficiency by utilizing the
204
Table 1. Advantages and Characteristics of Electrostatic Spray Processes Process
Advantages & Characteristics
Electrostatic Air Spray
Complete pattern control Finest atomization Higher TE than non-electrostatic Can be used with all materials (2k, waterborne, high solids) Fast application Excellent finish quality (Class A) Complete pattern control Good atomization Higher TE than air spray Fast application Used with all materials 12k, waterborne, high solids) Good atomization Very high TE (due to electrostatic wrap) Very fast application Larger pattern/high delivery Good atomization Very high TE Very fast application Excellent finishing quality Highest TE of any electrostatic gun (no overspray) Excellent finish quality Excellent alonlization Very high TE Used with all materials (2k, waterborne, high solids) Automatic application Excellent finishing quality Excclle0t alomizalion Very high TE Used with all material (2k, water, high solids) Very fast application Automatic application
Electrostatic HVLP Spray
Electrostatic Airless Spray
Electrostatic Air-Assisted Airless Spray Electrostatic Electrical Atomization Electrostatic Rotary Bell Atomization
Electrostatic Rotary Disk Atomization
TE - transferefficiency. electrostatic charge to wrap paint around edges and capture overspray that would have been unusable waste. Standard electrostatic air spray provides transfer efficiencies in the 40 to 90% range depending on the type of material and application.
Electrostatic HVLP Spray Atomization Electrostatic HVLP spray utilizes the same atonfization characteristics as electrostatic air spray technology with slight modifications. When using air HVLR the pressure of the compressed air at the aircap must be reduced to a range of 0.1 to 10 psi. Transfer efficiency is greater when using HVLP spray to lower the particle velocity and atomize the material thus causing less waste and blow-by of material. Some electrostatic equipment can be easily converted or transformed between air spray and HVLP spray technology by simply changing four parts. HVLP spray technology helps meet stringent EPA codes requiring reduced VOCs and waste. Electrostatic HVLP spray provides transfer efficiencies in the 60 to 90% range depending on the type of material and application.
Electrostatic Airless Spray Atomization Electrostatic airless spray technology utilizes the principle o f fluid at high pressures (500-5,000 psi) atomizing through a very small fluid nozzle orifice. Size and shape of the 205
I
I
I
I
I
I
I
I
Electrical Atomization (95 - 98%) [ ~
Elect. Disk Atomization (80 - 95%) Electrostatic Bell Atomization (70 - 95%) Electrostatic HVLP (60 - 90%) Elect. Air-Assisted Airless (50 - 85%) ~ Elect. Air Spray (40 - 8
0
%
~
N )
'
,
,
~
~
Electrostatic Airless (40 -70%) ~ , , ~ ' ~ N ~ ~ (30 - 60%)[
]Conventional HVLP
(30 - 60%)[
]Conventional Air-Aissisted Airless
(20 - 50%) L_
J Conventional Airless
(15 - 40%) I
L
J Conventional Air Spray
I
I
I
I
I
I
i
i
Fig. 5. T3pical tran.sfer efficiencies for t'arious electrostatic and conventional spray processes
As~w~l~y
Fig. 6. Typical bell-oT~e installation.
206
spray pattern along with fluid quality is controlled by the nozzle orifice. Airless spray technology evolved after air spray to aid in faster application rates using higher delivery and heavier viscosities on larger parts.
Electrostatic Air-Assisted Airless Atomization Electrostatic air-assisted airless spray technology uses the airless spray principle to atomize the fluid at reduced fluid pressure with assisted atomizing air to aid in reducing pattern tailing and affect pattern shape. Air-assisted airless spray technology offers some of the desirable characteristics of both airless spray and air spray. The desirable characteristics being medium to high delivery rates, ability to spray heavy viscosities at low velocities, and high transfer efficiency.
Electrostatic Electrical Atomization Electrostatic electrical atomization is accomplished by using a rotary bell on the end of a gun to evenly dispense paint to the edge of the bell. Once the coating material reaches the edge of the bell it is introduced to an electrical charge. The electrical charge at the sharp edge (approximately 100 kV) causes paint of a medium electrical resistance range (0. l to 1 megohms) to disperse onto the product. The pure electrical application is a slightly slower process than an air spray or air-assisted airless technology and requires a rotational type spray paint technique, due to the bells spray pattern, but is the most transfer efficient spraygun process in the industry today. The ultrasoft forward velocity of the spray pattern achieves transfer efficiencies of nearly 100% on most products. This high transfer efficiency spawned the industry of painting and refurbishing machinery and furniture in place.
Electrostatic Rotary-Bell-Type Atomization An electrostatic bell atomizer is a high-speed rotary bell that uses centrifugal force as well as electrical atomization to atomize material and efficiently transfer material from the bell edge to the target being painted. (See Fig. 6.) The bell is used on a turbine motor where the pattern is carefully directed by the use of compressed air, introduced to the pattern at the edge of the bell cup. The compressed air gives the material forward velocity to aid in penetrating recessed areas. The bells are usually mounted stationary or reciprocated to coat products on straight line conveyors. The bells may also be positioned on both sides of the conveyor. Rotary-bell-type atomization provides transfer efficiencies in the 70 to 95% range.
Electrostatic Rotary-Disk-Type Atomization An electrostatic rotary-disk atomizer is a high-speed flat rotary atomizer that uses centrifugal force along with electrical atomization to atomize coating material and efficiently transfer the material from the disk edge to the target being painted. The disk is used in an enclosed omega shape loop (see Fig. 7) to coat the product. Disks may be mounted stationary and tilted (up to 45 °) to coat small parts of 12 in. or less, or mounted on reciprocating arms to coat parts up to 40 ft. in height but generally no wider than 4 It. in width. The disk produces transfer efficiencies in the 80 to 95% range.
WATERBORNE ELECTROSTATICS Over the last several years, government regulations on VOC emissions coming from paint application facilities, have fueled the need for coating manufacturers to reduce the amount of VOC from their coating materials. Waterborne coatings have been around for many years, but due to tougher government regulations they are rapidly gaining more and more
207
~mze
t,enk P~
C ~ Vd~l A~bly
Fig. Z T37~ical disk-ope installation. momentum in today's finishing industry. Many of current users of solventbome coatings will be forced to make the switch to a more compliant coating in the future. And many of these manufacturers, in an effort to utilize as much of their existing finishing equipment possible. will make the move to waterborne coatings. Although the application of these waterborne coatings is basically the same as with solventborne coatings, many factors must be taken into consideration. Are my system's components compatible with waterborne materials'? Many alloys and metals will rust and corrode over time when coming in contact with waterborne materials: therefore, you must ensure that all components such as pumps, valves, piping and the atomizer itself are constructed of materials compatible with waterborne coatings such as 316 stainless steel or Teflon. A decision must be made as to how the system will be isolated fl'om high voltage grounding out back through the to waterborne fluid supply. Water is a good conductor of electricity, and all components that come in contact with the waterborne material will be at high w)ltage. This includes all atomizers, fluid supply hoses, pumps, regulators, valves, and the fluid supply itself. In today's finishing environment waterborne materials must be safely isolated. This is accomplished by: (I) complete system isolation; (2) voltage blocking device; or {3) indirect charging of the coating material.
Complete System Isolation Complete system isolation is the most commonly used method of isolating high voltage from the waterborne fluid supply. This low-tech approach has been around for decades. (See Fig. 3.)
208
In an isolated system, any components that come in contact with the waterborne material must be kept isolated from any possible grounds. The fluid supply must be enclosed in a caged area with the supply bucket, drum, or tote on an isolation stand. The gates to these cages must be equipped with safety interlocks. When an operator opens the gate to enter the cage, a pneumatically operated ground rod must short the systems" high voltage to ground. This ensures that the operator will not come in contact with a charged waterborne fluid supply. In addition, one of the isolation stand's legs should have a 1,050 megohm bleed resistor installed inside it and attached to earth ground so that when the high voltage is turned off the voltage can bleed off to ground in a timely manner. Despite the fact that these properly confirmed waterborne systems may have safety interlocks and bleed resistors, n e v e r a s s u m e that all of the high voltage has been discharged to ground. Before approaching any of the wetted systems components, always take a secondary ground wire and touch it to all system components to make sure that the system is fully discharged. Failure to do so could result in a painful shock to the operator, Failure to keep the entire system properly isolated from ground can result in a shorting condition. This can potentially short some or all of the high voltage to ground. This can greatly reduce the electrostatic affect which can lead to poor transfer efficiency. Example: A fluid supply hose, of a fluid supply container too close to ground, can short the system out completely or create a high load (high microamp reading) on the power supply which in turn lowers the actual voltage at your applicator. This can significantly reduce transfer efficiency. In addition to keeping all the equipment isolated, the cages (fluid supply) must be kept relatively close to the application equipment. This can result in a significant amount of lost floor space. In many occasions, the amount of floor space it takes to enclose the fluid supply may not be available. In many installations, floor space is extremely valuable and cannot be afforded when lost.
External Charging (Indirect Charging of Material) External charging of waterborne coatings allows the fluid supply to remain grounded. The fluid supply area can remain the same as it was configured for a solvent based coating. Since the paint particles are charged externally, or as some say "indirect," the high voltage does not follow the conductive path through the fluid lines back to ground. The indirect charge of the material is accomplished by placing a probe, which is at high voltage, a few inches away from the gun electrode. This probe creates the electrostatic field to charge the paint particles without coming in direct contact with the waterborne material. Thus, the high voltage does not follow the conductive path back through the fluid lines. With automatic applicators such a rotary atomizers, a ring of probes (6-8) is placed around the applicator a few inches back and away from the rotary bell. This configuration is often referred to as a "Copes" ring. Many U.S. automotive assembly plants have switched to waterborne basecoats and the Copes bells have become widely accepted in the automotive market. Utilizing Copes technology, color changes in the six to ten second range can still be achieved. Unfortunately, of the three common methods of spraying waterbornes electrostatically, the external or indirect charging method is the least efficient. Voltage blocks and isolated systems have been proven to provide higher transfer efficiencies+
Voltage Blocks In recent years, the application of waterborne coatings has become simpler and safer with the development of voltage blocking devices. Voltage blocking devices isolate the spray applicators from the grounded fluid supply. This prevents the high voltage from following the conductive path through the fluid lines back to the ground fluid supply and grounding (shorting) out the system high voltage. These devices can be used to feed both manual and automatic spray applicators. In a
209
handgun situation, only one applicator can be fed from a single voltage blocking device. Where as with an automatic applicator the voltage blocking device can feed multiple applications. This is due to the fact that any and all applicators will be charged back through their fluid lines when connected to one blocking device. Voltage blocking devices eliminate the need for safety cages and interlocks and protect the operator from coming in contact with a charged fluid supply. This eliminates the need for isolation stands and the isolation of the fluid supply from ground. It is now a grounded fluid supply. This can lead to a significant amount of savings in floor space.
Summary Of the three methods discussed for spraying waterbornes electrostatically all have their advantages and disadvantages. The end user must decide as to which method is best suited for their application. Voltage blocks are the simplest and can be used with any type of fluid supply, but up front cash can sometimes be a factor in the mind of the decision maker. Isolated systems can be cheaper on most occasions, but can also take up a lot of valuable floor space. Isolated systems are also the least safe and may be impractical when your fluid supply is a remote paint kitchen. Although indirect charging may be the least efficient of the three methods discussed, it may be the most practical in some applications. For example, in automotive assembly plants where a large paint kitchen is involved or extremely fast color changes are necessary. Voltage blocks and isolated systems have been proven to provide higher transfer efficiencies.
ELECTROSTATIC PROCESS FOR PLASTICS & OTHER NONCONDUCTIVE SUBSTRATES The ideal application for the use of electrostatics is metal because the only thing that needs to be done to spray electrostatically is to connect a ground wire to the product: however, when you try to electrostatically spray a nonconductive substrate, such as plastics, it must be made conductive. There are several ways of making the part being coated or the application conductive. The most common of these being: 1. Build a bracket of grounded metal and place the nonconductive part between the applicator and the conductive fixture. (The charged particles will see the ground and be drawn to the part being coated. Examples for utilizing this method of technology would be the coating of fabrics, paper or other thin structures.) 2. Certain materials become conductive with heat. Materials such as glass, rubber products, and some plastics may be heated until they are conductive and electrostaticaIly sprayed while warm. 3. All nonconductors, such as wood, rubber, plastic and glass, may also be treated with chemical sensitizers. These are generally hydroscopic chemicals that attract moisture onto the surface of the product to create conductivity. Controlled concentrates of the sensitizer may be applied by dipping, wiping, spraying or a mist chamber. After treatment, the part becomes conductive when exposed to adequate humidity such as a humidity chamber or high ambient humidity (70% relative humidity). Sensitizers are nonfilm-forming liquids. 4. Another method of making the part conductive is by using a conductive primer. The conductive primer can be applied to the substrate by conventional means, thus allowing the top coat to be applied electrostatically. Conductive primers may be sprayed, dip coated, flow coated, or molded in. The reason for making nonconductive parts more acceptable to an electrostatic charge is to utilize the most efficient process with the highest quality finish at the most minimal cost. By utilizing the electrostatic process, you will achieve each of these benefits.
210
Table II. Paybaek in Dollars per Year (225 Working Days) Paint Usage (gal/day) 5 10 15 20 25 30 35 40 45 50 100 150 200 250 300 350 400 450 500 550 600 651) 700 750 800 850 900 950 1,000
Trans2fer Efficiency Improvement 4.00%
8.00%
12.00%
16.00%
20.00~
675 1,350 2,025 2,700 3,375 4,050 4,725 5,400 6,075 6,750 13,500 20,250 27,000 33,750 40,500 47.250 54,000 60,750 67,500 74,250 81,000 87,750 94,501) 101,250 108,000 I 14.750 121,500 128,250 135,000
1,350 2,700 4,050 5.400 6,750 8,100 9,450 10,800 12,150 13,500 27,000 40,500 54,000 67,500 81.000 94,500 108,000 121,500 135,000 148,500 162.000 175.500 189,1)00 202.500 216,000 229,500 243,000 256,500 270,000
2,025 4,050 6.075 8,100 I 0,125 12,150 14,175 16.200 18,225 20,250 40,500 60,750 81,000 ] 01 +250 121,500 141,750 162,000 182,250 202,500 222,750 243.000 263,500 283,500 303,750 324,000 344,250 364,500 384,750 405,000
2,700 5,400 8, I 00 10,800 13.500 16,200 18,900 21,600 24,300 27,000 54.000 81.000 108,000 135.000 162,000 189,000 216.000 243,000 270.000 297,000 324,000 351,000 378,000 405.000 432,000 459,000 486.000 513,000 540,000
3,375 6,750 10,125 13,500 16.875 20,250 23,625 27,000 30.375 33,750 67,500 101,250 135,000 168,750 202,500 236,250 270.000 303,750 337,500 371,250 405,000 438,750 472,500 506,250 540,000 573,750 607,500 641,250 675,000
Cost of paint: $15/gaI.
COST SAVINGS Transfer Efficiency/Paint Savings The cost savings associated with the use o f electrostatic e q u i p m e n t can be realized in m a n y different areas. The most obvious savings is in paint usage. With the increase o f high-solids, plural-component, and base/clear finishes, it is not unrealistic to pay $100 per gallon for these coatings. C o n s i d e r i n g this cost. it is crucial that the coating is applied to the product as efficiently as possible, With a conventional air spray gun, r o u g h l y 15 to 4 0 % of the paint sprayed f r o m the g u n is applied on the part. This is k n o w n as transfer efficiency. The r e m a i n i n g 60 to 85% is lost in the filters or left as overspray on the floor a n d walls. Conventional H V L P g u n s are m o r e efficient than conventional air spray guns. H V L P g u n s will typically yield transfer efficiencies of 30-60%. Electrostatic g u n s can obtain even greater transfer efficiency. An electrostatic air spray g u n is normally in the 4 0 to 8 0 % transfer efficiency range. This m e a n s you c a n coat twice as m a n y parts with an electrostatic air spray gun, c o m p a r e d to a nonelectrostatic air spray g u n given the s a m e quantity of paint. As with noneleetrostatic guns, H V L P t e c h n o l o g y s h o w s significant i m p r o v e m e n t in transfer efficiency. The same holds true with electrostatic H V L P t e c h n o l o g y as well. In some cases, electrostatic H V L P has obtained efficiencies as high as 90%.
211
Table III. Maintenance Costs Estimated in Dollars A[,~l);'oxmtate Paint Usage Gall &O 5 10 15 2(I 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10('1
TE
Gall day
Waterwetsh Chemi~ol U~age TE (65~ )
TE (75%)
1,250 104 64 2,50('1 208 129 3,75(I ~ 12 193 5,(XX) 416 257 6.250 521 322 7.5('10 625 386 8,75('1 729 450 10.000 833 515 I 1.250 937 579 12,500 1,041 644 13,75('1 1.145 708 15,000 1,249 772 16,250 1,353 837 17,500 1,457 901 18.750 1.562 965 20,000 1.666 1,030 2 t,250 1,770 1.094 22.5(X) 1,874 1.158 23,750 1,978 1.223 25.000 2.082 1.287 Note: Note: 251) days/yr I ~)~'/ib of overspray Estimated $ 1.4 I/Ib of chemical
Wowru'a.dl Slud,,¢e Retnot'ol TE (65q) 295 591 886 1.181 1.477 1.772 2,067 2,362 2,658 2,953 3.248 3,544 3,839 4,134 4,430 4,725 5,020 5,315 5,61 I 5,906 Note: 400 Ib/drum $ I00.(X)/ disposal drum
TE (75%) | 83 366 548 731 914 1,097 1,280 1.463 1.645 1,828 2,01 1 2.194 2.377 2,559 2.742 2,925 3,108 3.291 3,473 3.656
Drr Filter Divposal Co~t TE (65%)
TE (75q)
1.023 632 2,045 1.265 3,068 1,897 4,091 2.53(I 5.113 3.162 6.136 3,795 7.159 4.427 8,181 5.060 9,204 5,692 10,227 6,325 I 1,249 6,957 12,272 7.590 13.294 8,222 14.317 8,855 15.34('1 9.487 [6,362 [().120 17,385 10.752 18,408 I 1.385 19.43('1 12,017 20,453 12.650 Note: 60 tilter~,/drum $200.(X)/ drum o f tilters
D 0 1.71wr Usage IF. (65q)
TE (75q)
307 190 6,14 380 920 570 1,227 760 1.534 950 1.841 1.141) 2.148 1,330 2.454 1.519 2,76 l 1.709 3,008 1,899 3,375 2,089 3,682 2.279 3,988 2,469 4,295 2.(~59 4.602 2.849 4.909 3.039 5,216 3,229 5,522 3,419 5,829 3,609 6.136 3.799 Note: S 1.00/fihcr
transfizr efficiency.
Typically, cost justification is obtained fi-om paint cost savings alone. Its typically enough to cost justify the purchase of the electrostatic applicator. Table ll displays the dollar figure in paint savings that can be achieved by slightly increasing transfer efficiency.
VOC Reduction Another savings area is emission reduction. With tederal and local regulations becoming tougher by the day, VOC (volatile organic compound) emissions has become a major issue. We arc constantly trying to reduce the amount of VOCs enfitted into the atmosphere. By increasing transfer efficiency you lower VOC emission. (See Fig. 8.) This is a result of more paint being applied on the part and less paint being deposited into the booth filters or atmosphere, Many states, such as California, now mandate that you use either H V L P or electrostatic technology to qualify for a permit to install a new spray booth. A manufacturing facility is permitted to emit a specified amount of VOCs (in tons) per year. It" the tonnage limit is exceeded, strict fines are enforced. These fines can easily exceed thousands of dollars. As a result of these laws many companies have invested in electrostatic finishing equipment to comply with V O C regulation.
D e c r e a s e d M a i n t e n a n c e Costs As stated earlier, when transfer efficiency increases, the amount of overspray deposited into the spray booth decreases. This means spray booths that previously required weekly cleanings and filter changes may now require biweekly maintenance. Water-wash booths that previously consumed 55 gallons of chemicals in one month may now use only 30 gallons. Not 212
10
20
[::!:::ilili i :i:ii::!:i:3:.] 50%
30
40
SO
60
70
80
90
100
67%
75%
80%
83%
86%
88%
89%
90%
33%
50%
60%
67%
71%
75%
78%
80%
25%
40%
50%
57%
63%
76%
70%
20%
33%
43%
50%
56%
60%
44%
50%
25%
33%
40%
13%
22%
30%
IllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllU[llll 14% i:~i~.~iiiiiii~i!i~i~iiiii~iii~i~.~.~i~i~i~.~ii~
~ ~
i.~.~.~`~.~.£~.~:.~.~.~.~.~.~;.~.~.~.~5~.~.~.~.~.%.~.~.~.~.~.~.~:~`..~5.~.~%.~.~£~.~.~.~:.~.~.~A~.~.~$.~.~:~.~.~.;~;.~.~.~.~.'~.~.~.~.~?5.~.~`3.~.~.~.~.~`~.~.~I.~.~.`.~.~`..
Fig 8. Emission reduction fi'om installation ~'new equipment. Etample: If era'rent TE is 40~ and new equipment is rated at 90%. the emission redaction would be 56%. only is the cost to purchase these materials reduced, but the cost to dispose of them is decreased as well. Dirty booth filters and contaminated booth water must typically be disposed of as hazardous waste. In recent years, the cost associated with the disposal of hazardous waste has sky rocketed. Not only is there a direct cost reduction, such as filters, chemicals and disposal (see Table Ill), there is also an indirect savings in labor cost, due to the tact that booth maintenance can easily consume on an average up to 8 man-hours per week.
Better Coverage/Improved Quality Electrostatic finishing has many other benefits in addition to cost savings. Application time may be reduced with the aid of electrostatic wrap. Electrically charged paint particles can change direction and be deposited on the top. bottom, and sides of the part when sprayed from one direction. Depending on part size and configuration this wrap-around may sufficiently coat all of the product at once, eliminating the need for additional passes. When using a nonelectrostatic gun you must point the gun at every area that requires paint, if you miss a particular angle it will not be painted. With electrostatic applicators, the wrap-around may coat these areas producing a more uniform finished product. As a result of a more uniform finish, many manufacturing facilities have experienced a lower reject rate in their production. In the case of on-site furniture refinishers or appliance refurbishers, it would be virtually impossible to paint without electrostatic finishing equipment.
213
OF ELECTROSTATICS
Electrostatic Theory Electrostatic finishing got its start in the early 1950s. Coatings engineers needed an application method that would significantly increase transfer efficiency and reduce finishing costs. They reasoned that particles and objects with like charges repel each other. and objects with unlike charges attract each other. The same would apply to charged spray coatings and a part to he painted. They discovered that by negatively charging the atomized paint particles and positively charging the workpiece to be coated (or making it a neutral ground), an electrostatic field would be created that would pull paint particles to the workpiece. (See Fig. 1.) With a typical electrostatic spray gun, a charging electrode is located at the tip of the atomizer. The electrode receives an electrical charge from a power supply. The paint is atomized as it exits past the electrode, and the paint particles become ionized (pick up additional electrons to become negatively charged). An electrostatic field is created between the electrode and the grounded workpiece. The negatively charged paint particles are attracted to the neutral ground. As the particles deposit on the workpiece, the charge dissipates and returns to the power supply through the ground, thus completing the electrical circuit. This process accounts for the high transfer efficiency. Most of the atomized coating will end up on the part. The degree to which electrostatic force influences the path of paint particles depends on how big they are, how fast they move. and other forces within the spray booth such as gravity and air currents. Large particles sprayed at high speeds have great momentum, reducing the influence of the electrostatic force. A particle’s directional force inertia can be greater than the electrostatic field. Increased particle momentum can be advantageous when painting a complicated surface. because the momentum can overcome the Faraday cage effect-the tendency for charged paint particles to deposit only around the entrance of a cavity. (See Fig.
2.) On the other hand. small paint particles sprayed at low velocities have low momentum. allowing the electrostatic force to take over and attract the paint onto the workpiece. This condition is acceptable for simple surfaces but is highly susceptible to Faraday cage problems. An electrostatic system should balance paint particle velocity and electrostatic voltage to optimize coating transfer efficiency. Electrostatic Advantages The main benefit offered by an electrostatic painting system is transfer efficiency. In certain applications clectroctatic bell\ can achieve a high transfer efficiency exceeding 90%. This high efficiency translates into significant cost savings due to reduced okct-spray. A phenomenon of electrostatic finishing known as ‘wrap” cause\ some paint particles that go past this workpiece to be attracted to the back of the piece, further increasing transfer efficiency. Increased transfer efticiency also reduces VOC emissions and lowers harardous waste disposal costs. Spray booth cleanup and maintenance are reduced. Coating Application Any material that can be atomized can accept an electrostatic charge. Low-. medium, and high-solids sol~entborne coatings. enamels. lacquers, and two-component coatings can be applied electrostatically. 198
SATA A U T O M A T I C A N D ROBOTIC GUNS
:' ,i:+~+u :+, IIIY++:Y:%-
CO
:i:/¸'/: .....
/ii
+ The finest atomization and performance • Special stainless steel head version available • Adaptable to nozzle extensions Wkether it's conventional, air assisted or HFLP, robotic or manual, SA TA has an automatic gun to suit any requirement.
Spray Equipment D i s t r i b u t e d b y D a n - A m Co ~, M e m b e r A S A Corporate Office: Phone: 800 533 80t6 West CoastBunch: One Sata Drive POB 46 SprlngVa~lev MN55975.0046 Order
Desk:
www.satausa.com
507 346 7776 Fax 8006337282 507.346.7481
1200 O~egon Avenue LongSeach, CAg0813
8OD-533-8016 • e-mail;
[email protected]
The Appficatlon Technology Of The Future
Circle 097 on reader information card
Coatings & Coating Processes for Metals byJ.H. Lindsay
Can-Am's 20 Years of Experience Guarantee's It
353 pages $150.00
The editor of this invaluable w o r k is a respected and well k n o w n authority. This new book is an encyclopedic listing of over 2,000 coatings and coating processes from more than 100 companies. The first section offers material and equipment by trade name. Section two lists the names, street and e-mail addresses, and phone a n d fax numbers of manufacturers. The third and fourth sections include a complete index of useful tables with representative standards and specifications. Send Orders to:
METAL FINISHING 660 White Plains Rd., Tarrytown, N Y 10591-5153 For faster service, call (914) 333-2578 or FAX your order to (914) 333-257 All book orders must be prepaid. Please include $5.00 shipping and handling for delivery of each book via UPS in the U.S., $10.00 for each book shipped express to Canada; and I20.00 for each book shipped express to all other countries.
Circle 020 on reader information card
Conveyor
High
_L
:e C;
IonizationArea
Grounded Work or Targets Material Source
®L Electrostatic Power Supply
/
I i I I I i
EaCh Ground Potential
Fig. 1. Electrostatic application circuit.l)w soh'ent-ba,~ed coatings. The various types of electrostatic systems can apply coatings regardless of their conductivity. Waterborne and metallic coatings can be highly conductive. Solvent-borne coatings tend to be nonconductive. Any metallic coatings can contain conductive metal particles. These metallic coatings must be kept in circulation to prevent a short circuit in the teed line. As high voltage is introduced into the system, the metal particles can line up to lkwm a conductive path. System modifications may be required because of coating conductivity to prevent the charge from shorting to gruund. (See Fig. 3.)
Operating Electrostatics Safely Electrostatic finishing is safe if the equipment is maintained properly and safety procedures are t~.~llowed. All items in the work area must be grounded, including the spray booth, conveyor, parts hangers, application equipment (unless using conductive/waterborne coatings), and the spray operator. As electrical charges come in contact with ungroundcd components, the charges can be absorbed and stored. This is known as a capacitive charge buildup. Eventually, enough charge is bui}t up so that when the ungrounded item comes within sparking distance of a ground, it
200
Fig. 2. Fanul~ O" cage effect. can discharge as a spark. Such a spark may have enough energy to ignite the flammable vapors and mists that are present in the spray area. An ungrounded worker will not know that the capacitive charge has been absorbed until it is too late. Workers should never wear rubber- or cork-soled shoes, which can turn then into ungrounded capacitors. Special shoe-grounding devices are available. If workers are using hand-held guns, they should grasp them with bare hands or with gloves with cut-outs for fingertips and palms that allow adequate skin contact. Proper grounding of all equipment that is not used for the high-voltage process is essential. Grounding straps should be attached to equipment and connected to a known ground. A quick inspection of all equipment, including conveyors and part hangers, can reveal improper grounding. Good housekeeping can pay dividends. Removing paint buildup from parts hangers can help ensure that workpieces are grounded. Ungrounded objects, such as tools and containers, should be removed from the finishing area.
PAINT PARTICLE CHARGING Electrostatic charging of paint particle got its start back in the early 1950s. Engineers were looking for methods to reduce the cost of finishing products. Harold Ransburg, the inventor of the electrostatic process, reasoned that since unlike electrical charges are attracted to each other, the same idea would apply to charged paint particles and a part to be painted. Everyone's heard the saying that "opposites attract, and likes repel." This is true with both a magnetic field and with the electrostatic process of charging paint particles. The electrostatic process is almost identical to the way a common magnet works. By creating an electrostatic field between a negatively charged paint particle and a positive grounded workpiece, the paint particles are attracted and deposit themselves onto the workpiece. The basic building block of electrical energy is the charged particle. All matter is made from electrically charged particles. These particles are either neutral, negative, or positive.
201
Conveyor High Voltage Cable
_L
Ionization Area
Atomizer
. . . . . . . . . . . . Grounded Work or Targets Material Source
ion Device
Earth Ground Potential Electrostatic Power Supply
±
I
I I I
Fig. 3. lsolaled electrostatic application cin'uit,fbr waterborne and metallic coatings. Back in the early days of particle charging, a process referred to as the Number One Process was developed by Harold Ransburg to charge paint particles. Paint particles were sprayed into an electrostatic field by conventional air spray guns. Two wire grids were aligned parallel to each other at a certain distance, then the parts were conveyed through these grids. At one end of the grids, atomized paint particles were sprayed into the electrostatic field. The paint particles would become negatively charged and would be attracted to the positively grounded parts. These wire grids are now the wire electrode in an electrostatic spray gun. The three most common ways of charging paint particles are the electrostatic spray gun, a rotary bell, or a rotary disk. All three of these methods work by the same common principle of the electrostatic field between the atomizer and the workpieee then introduce atomized paint particles into the field and they will be attracted to and deposit themselves on the positive grounded workpiece. With an airspray or an HVLP electrostatic spray gun, a high voltage DC charge is supplied to the applicator's nozzle electrode, creating an electrostatic field between the gun and the grounded target object. (See Fig. 4.l The coating materials are charged at the point of atomization. The charged paint particles are attracted to and deposited on the grounded target
202
Fig. 4. 7~vpical electrostatic hand gun application. object. This electrostatic charge allows a more efficient, uniform application of the coating material to the front, edges, sides, and back of the product. The electrostatic forces allow for a high percentage of the charged paint particles to be deposited on the workpiece. The electrostatic process can also be used to charge paint particles using airless and air-assisted airless electrostatic spray guns. The only difference is the coating material is atomized by different methods. An air spray or HVLP electrostatic gun utilizes much lower air pressure to atomize the coating material, the airless and air-assisted airless methods use a much higher pressure. Coating material is delivered at high pressure to the atomizer. There, the material is atomized by passing through a very small orifice under high pressure. The resulting spray mist particles then become electrostatically charged and are attracted to the workpiece in the same manner as electrostatic air spray or electrostatic HVLE Today, rotary bells are generally about 1 to 3 in. in diameter and rotary disks are about 6 to 12 in. These atomizers operate on the same principle except they are positioned differently to the workpiece. Bells are positioned with their axis horizontal to the part, and disks are positioned vertically. A rotating disk or bell distributes a thin, even coating to the edge of the atomizer. There the coating is atomized either by the electrostatic force or centrifugal force. A low speed rotary atomizer utilizes almost all electrostatic forces, a high speed rotary atomizer relies on the centrifugal force of the atomizer to atomize the coating material. A DC high voltage charge is then supplied to the rotating atomizer, creating an electrostatic field between it and the grounded target object. The negatively charged paint particles are attracted to and deposited on the positive grounded workpiece, The forces between the charged particles and the grounded target are sufficient to turn normal overspray around and deposit it on the back surface of the target; therefore, a very high percentage of the paint particles are deposited on the part. Paint resistivity, often referred to as conductivity, is critical when spraying materials electrostatically, Waterborne materials are very conductive; therefore, measures such as voltage blocking devices, external charging probes, or completely isolating the fluid supply
203
and fluid lines must be taken or the paint particles will not be able to Fnaintain the electrostatic charge. Due to the low resistance of waterborne materials, all of the electrostatic voltage will drain off to ground and short out the system. If one of the three previous methods mentioned are not used, the paint particles cannot be charged electroslalically. Solvent-borne materials paint resistivity will vary fl-om one material to another. When spraying solvent-borne coatings with electrostatics, it is critical to measure and monitor the resistivity of the paint being sprayed. Materials that are too conductive, (very low resistance, often referred as "hot") will also drain some or all of the electrostatic voltage off to ground. This will greatly reduce the electrostatic efl'ects on the paint particle. On the other hand, when using materials with a very high resistance, often referred to as "'dead," the paint particles will not readily accept the electrostatic charge and the transfer efficiency will be very poor. Coating suppliers can easily ti)rmulate their solvent-borne materials to be within a specific resistivity range. The optimum resistivity may difter depending on the tool used for application. For example, with an electrostatic disk or bell, the optimum resistivity range is between 0.05 and 1 megohms on a (Ransburg) paint resistivity meter. An electrostatic spray gun however, can effectively spray coating materials between 0. I to ~ megohms of resistance. Another example is the No. 2 Process on-site electrostatic spray gun. This gun requires a more precise paint resistivity because it relies solely on the electrostatic charge to atomize the coating materials. The paint used with this gun must read between 0.1 to 1 megohms on the (Ransburg) paint test meter to work properly. Another key element in the electrostatic process or charging of paint particles is particle size. Large particles sprayed at high speed have greater momentum and reduce the influence of the electrostatic force. Increased particle size and momentum can be an advantage when coating complicated surfaces because the momentum can overcome the Faraday cage areas (where paint particles are attracted to the edges of a workpiece while avoiding inside corners and recessed areas). On the other hand, small paint particles sprayed at low velocities have low momentum, thus allowing the electrostatic force to take over and attract the coating material to the target object. This conditiun is acceptable for simple surfaces but is highly susceptible to Faraday cage problems.
ELECTROSTATIC PROCESSES/EQUIPMENT The electrostatic application of atomized materials was developed to enhance finish quality and improve transfer efficiency. (See Fig. 5.) Presently, there are seven types of electrostatic processes for spray application: • • • • • • •
Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic Electrostatic
air spray atomization high volume, low-pressure (HVLP) atomization airless atomization air-assisted airless atomization electrical atomization rotary-type bell atomization rotary-type disk atomization
Regardless of the electrostatic finishing systems, each has its advantages and limitations. What may be suitable for one situation may not be suitable in another. (See Table 1.)
Electrostatic Air Spray Atomization Electrostatic air spray uses an air cap with srnall precision openings that allows compressed air to be directed into the paint for optimum atomization. Electrostatic air spray is the most widely used type of atomization in the industry today due to its control and versatility. Electrostatic air spray provides very high transfer efficiency by utilizing the
204
Table 1. Advantages and Characteristics of Electrostatic Spray Processes Process
Advantages & Characteristics
Electrostatic Air Spray
Complete pattern control Finest atomization Higher TE than non-electrostatic Can be used with all materials (2k, waterborne, high solids) Fast application Excellent finish quality (Class A) Complete pattern control Good atomization Higher TE than air spray Fast application Used with all materials 12k, waterborne, high solids) Good atomization Very high TE (due to electrostatic wrap) Very fast application Larger pattern/high delivery Good atomization Very high TE Very fast application Excellent finishing quality Highest TE of any electrostatic gun (no overspray) Excellent finish quality Excellent alonlization Very high TE Used with all materials (2k, waterborne, high solids) Automatic application Excellent finishing quality Excclle0t alomizalion Very high TE Used with all material (2k, water, high solids) Very fast application Automatic application
Electrostatic HVLP Spray
Electrostatic Airless Spray
Electrostatic Air-Assisted Airless Spray Electrostatic Electrical Atomization Electrostatic Rotary Bell Atomization
Electrostatic Rotary Disk Atomization
TE - transferefficiency. electrostatic charge to wrap paint around edges and capture overspray that would have been unusable waste. Standard electrostatic air spray provides transfer efficiencies in the 40 to 90% range depending on the type of material and application.
Electrostatic HVLP Spray Atomization Electrostatic HVLP spray utilizes the same atonfization characteristics as electrostatic air spray technology with slight modifications. When using air HVLR the pressure of the compressed air at the aircap must be reduced to a range of 0.1 to 10 psi. Transfer efficiency is greater when using HVLP spray to lower the particle velocity and atomize the material thus causing less waste and blow-by of material. Some electrostatic equipment can be easily converted or transformed between air spray and HVLP spray technology by simply changing four parts. HVLP spray technology helps meet stringent EPA codes requiring reduced VOCs and waste. Electrostatic HVLP spray provides transfer efficiencies in the 60 to 90% range depending on the type of material and application.
Electrostatic Airless Spray Atomization Electrostatic airless spray technology utilizes the principle o f fluid at high pressures (500-5,000 psi) atomizing through a very small fluid nozzle orifice. Size and shape of the 205
I
I
I
I
I
I
I
I
Electrical Atomization (95 - 98%) [ ~
Elect. Disk Atomization (80 - 95%) Electrostatic Bell Atomization (70 - 95%) Electrostatic HVLP (60 - 90%) Elect. Air-Assisted Airless (50 - 85%) ~ Elect. Air Spray (40 - 8
0
%
~
N )
'
,
,
~
~
Electrostatic Airless (40 -70%) ~ , , ~ ' ~ N ~ ~ (30 - 60%)[
]Conventional HVLP
(30 - 60%)[
]Conventional Air-Aissisted Airless
(20 - 50%) L_
J Conventional Airless
(15 - 40%) I
L
J Conventional Air Spray
I
I
I
I
I
I
i
i
Fig. 5. T3pical tran.sfer efficiencies for t'arious electrostatic and conventional spray processes
As~w~l~y
Fig. 6. Typical bell-oT~e installation.
206
spray pattern along with fluid quality is controlled by the nozzle orifice. Airless spray technology evolved after air spray to aid in faster application rates using higher delivery and heavier viscosities on larger parts.
Electrostatic Air-Assisted Airless Atomization Electrostatic air-assisted airless spray technology uses the airless spray principle to atomize the fluid at reduced fluid pressure with assisted atomizing air to aid in reducing pattern tailing and affect pattern shape. Air-assisted airless spray technology offers some of the desirable characteristics of both airless spray and air spray. The desirable characteristics being medium to high delivery rates, ability to spray heavy viscosities at low velocities, and high transfer efficiency.
Electrostatic Electrical Atomization Electrostatic electrical atomization is accomplished by using a rotary bell on the end of a gun to evenly dispense paint to the edge of the bell. Once the coating material reaches the edge of the bell it is introduced to an electrical charge. The electrical charge at the sharp edge (approximately 100 kV) causes paint of a medium electrical resistance range (0. l to 1 megohms) to disperse onto the product. The pure electrical application is a slightly slower process than an air spray or air-assisted airless technology and requires a rotational type spray paint technique, due to the bells spray pattern, but is the most transfer efficient spraygun process in the industry today. The ultrasoft forward velocity of the spray pattern achieves transfer efficiencies of nearly 100% on most products. This high transfer efficiency spawned the industry of painting and refurbishing machinery and furniture in place.
Electrostatic Rotary-Bell-Type Atomization An electrostatic bell atomizer is a high-speed rotary bell that uses centrifugal force as well as electrical atomization to atomize material and efficiently transfer material from the bell edge to the target being painted. (See Fig. 6.) The bell is used on a turbine motor where the pattern is carefully directed by the use of compressed air, introduced to the pattern at the edge of the bell cup. The compressed air gives the material forward velocity to aid in penetrating recessed areas. The bells are usually mounted stationary or reciprocated to coat products on straight line conveyors. The bells may also be positioned on both sides of the conveyor. Rotary-bell-type atomization provides transfer efficiencies in the 70 to 95% range.
Electrostatic Rotary-Disk-Type Atomization An electrostatic rotary-disk atomizer is a high-speed flat rotary atomizer that uses centrifugal force along with electrical atomization to atomize coating material and efficiently transfer the material from the disk edge to the target being painted. The disk is used in an enclosed omega shape loop (see Fig. 7) to coat the product. Disks may be mounted stationary and tilted (up to 45 °) to coat small parts of 12 in. or less, or mounted on reciprocating arms to coat parts up to 40 ft. in height but generally no wider than 4 It. in width. The disk produces transfer efficiencies in the 80 to 95% range.
WATERBORNE ELECTROSTATICS Over the last several years, government regulations on VOC emissions coming from paint application facilities, have fueled the need for coating manufacturers to reduce the amount of VOC from their coating materials. Waterborne coatings have been around for many years, but due to tougher government regulations they are rapidly gaining more and more
207
~mze
t,enk P~
C ~ Vd~l A~bly
Fig. Z T37~ical disk-ope installation. momentum in today's finishing industry. Many of current users of solventbome coatings will be forced to make the switch to a more compliant coating in the future. And many of these manufacturers, in an effort to utilize as much of their existing finishing equipment possible. will make the move to waterborne coatings. Although the application of these waterborne coatings is basically the same as with solventborne coatings, many factors must be taken into consideration. Are my system's components compatible with waterborne materials'? Many alloys and metals will rust and corrode over time when coming in contact with waterborne materials: therefore, you must ensure that all components such as pumps, valves, piping and the atomizer itself are constructed of materials compatible with waterborne coatings such as 316 stainless steel or Teflon. A decision must be made as to how the system will be isolated fl'om high voltage grounding out back through the to waterborne fluid supply. Water is a good conductor of electricity, and all components that come in contact with the waterborne material will be at high w)ltage. This includes all atomizers, fluid supply hoses, pumps, regulators, valves, and the fluid supply itself. In today's finishing environment waterborne materials must be safely isolated. This is accomplished by: (I) complete system isolation; (2) voltage blocking device; or {3) indirect charging of the coating material.
Complete System Isolation Complete system isolation is the most commonly used method of isolating high voltage from the waterborne fluid supply. This low-tech approach has been around for decades. (See Fig. 3.)
208
In an isolated system, any components that come in contact with the waterborne material must be kept isolated from any possible grounds. The fluid supply must be enclosed in a caged area with the supply bucket, drum, or tote on an isolation stand. The gates to these cages must be equipped with safety interlocks. When an operator opens the gate to enter the cage, a pneumatically operated ground rod must short the systems" high voltage to ground. This ensures that the operator will not come in contact with a charged waterborne fluid supply. In addition, one of the isolation stand's legs should have a 1,050 megohm bleed resistor installed inside it and attached to earth ground so that when the high voltage is turned off the voltage can bleed off to ground in a timely manner. Despite the fact that these properly confirmed waterborne systems may have safety interlocks and bleed resistors, n e v e r a s s u m e that all of the high voltage has been discharged to ground. Before approaching any of the wetted systems components, always take a secondary ground wire and touch it to all system components to make sure that the system is fully discharged. Failure to do so could result in a painful shock to the operator, Failure to keep the entire system properly isolated from ground can result in a shorting condition. This can potentially short some or all of the high voltage to ground. This can greatly reduce the electrostatic affect which can lead to poor transfer efficiency. Example: A fluid supply hose, of a fluid supply container too close to ground, can short the system out completely or create a high load (high microamp reading) on the power supply which in turn lowers the actual voltage at your applicator. This can significantly reduce transfer efficiency. In addition to keeping all the equipment isolated, the cages (fluid supply) must be kept relatively close to the application equipment. This can result in a significant amount of lost floor space. In many occasions, the amount of floor space it takes to enclose the fluid supply may not be available. In many installations, floor space is extremely valuable and cannot be afforded when lost.
External Charging (Indirect Charging of Material) External charging of waterborne coatings allows the fluid supply to remain grounded. The fluid supply area can remain the same as it was configured for a solvent based coating. Since the paint particles are charged externally, or as some say "indirect," the high voltage does not follow the conductive path through the fluid lines back to ground. The indirect charge of the material is accomplished by placing a probe, which is at high voltage, a few inches away from the gun electrode. This probe creates the electrostatic field to charge the paint particles without coming in direct contact with the waterborne material. Thus, the high voltage does not follow the conductive path back through the fluid lines. With automatic applicators such a rotary atomizers, a ring of probes (6-8) is placed around the applicator a few inches back and away from the rotary bell. This configuration is often referred to as a "Copes" ring. Many U.S. automotive assembly plants have switched to waterborne basecoats and the Copes bells have become widely accepted in the automotive market. Utilizing Copes technology, color changes in the six to ten second range can still be achieved. Unfortunately, of the three common methods of spraying waterbornes electrostatically, the external or indirect charging method is the least efficient. Voltage blocks and isolated systems have been proven to provide higher transfer efficiencies+
Voltage Blocks In recent years, the application of waterborne coatings has become simpler and safer with the development of voltage blocking devices. Voltage blocking devices isolate the spray applicators from the grounded fluid supply. This prevents the high voltage from following the conductive path through the fluid lines back to the ground fluid supply and grounding (shorting) out the system high voltage. These devices can be used to feed both manual and automatic spray applicators. In a
209
handgun situation, only one applicator can be fed from a single voltage blocking device. Where as with an automatic applicator the voltage blocking device can feed multiple applications. This is due to the fact that any and all applicators will be charged back through their fluid lines when connected to one blocking device. Voltage blocking devices eliminate the need for safety cages and interlocks and protect the operator from coming in contact with a charged fluid supply. This eliminates the need for isolation stands and the isolation of the fluid supply from ground. It is now a grounded fluid supply. This can lead to a significant amount of savings in floor space.
Summary Of the three methods discussed for spraying waterbornes electrostatically all have their advantages and disadvantages. The end user must decide as to which method is best suited for their application. Voltage blocks are the simplest and can be used with any type of fluid supply, but up front cash can sometimes be a factor in the mind of the decision maker. Isolated systems can be cheaper on most occasions, but can also take up a lot of valuable floor space. Isolated systems are also the least safe and may be impractical when your fluid supply is a remote paint kitchen. Although indirect charging may be the least efficient of the three methods discussed, it may be the most practical in some applications. For example, in automotive assembly plants where a large paint kitchen is involved or extremely fast color changes are necessary. Voltage blocks and isolated systems have been proven to provide higher transfer efficiencies.
ELECTROSTATIC PROCESS FOR PLASTICS & OTHER NONCONDUCTIVE SUBSTRATES The ideal application for the use of electrostatics is metal because the only thing that needs to be done to spray electrostatically is to connect a ground wire to the product: however, when you try to electrostatically spray a nonconductive substrate, such as plastics, it must be made conductive. There are several ways of making the part being coated or the application conductive. The most common of these being: 1. Build a bracket of grounded metal and place the nonconductive part between the applicator and the conductive fixture. (The charged particles will see the ground and be drawn to the part being coated. Examples for utilizing this method of technology would be the coating of fabrics, paper or other thin structures.) 2. Certain materials become conductive with heat. Materials such as glass, rubber products, and some plastics may be heated until they are conductive and electrostaticaIly sprayed while warm. 3. All nonconductors, such as wood, rubber, plastic and glass, may also be treated with chemical sensitizers. These are generally hydroscopic chemicals that attract moisture onto the surface of the product to create conductivity. Controlled concentrates of the sensitizer may be applied by dipping, wiping, spraying or a mist chamber. After treatment, the part becomes conductive when exposed to adequate humidity such as a humidity chamber or high ambient humidity (70% relative humidity). Sensitizers are nonfilm-forming liquids. 4. Another method of making the part conductive is by using a conductive primer. The conductive primer can be applied to the substrate by conventional means, thus allowing the top coat to be applied electrostatically. Conductive primers may be sprayed, dip coated, flow coated, or molded in. The reason for making nonconductive parts more acceptable to an electrostatic charge is to utilize the most efficient process with the highest quality finish at the most minimal cost. By utilizing the electrostatic process, you will achieve each of these benefits.
210
Table II. Paybaek in Dollars per Year (225 Working Days) Paint Usage (gal/day) 5 10 15 20 25 30 35 40 45 50 100 150 200 250 300 350 400 450 500 550 600 651) 700 750 800 850 900 950 1,000
Trans2fer Efficiency Improvement 4.00%
8.00%
12.00%
16.00%
20.00~
675 1,350 2,025 2,700 3,375 4,050 4,725 5,400 6,075 6,750 13,500 20,250 27,000 33,750 40,500 47.250 54,000 60,750 67,500 74,250 81,000 87,750 94,501) 101,250 108,000 I 14.750 121,500 128,250 135,000
1,350 2,700 4,050 5.400 6,750 8,100 9,450 10,800 12,150 13,500 27,000 40,500 54,000 67,500 81.000 94,500 108,000 121,500 135,000 148,500 162.000 175.500 189,1)00 202.500 216,000 229,500 243,000 256,500 270,000
2,025 4,050 6.075 8,100 I 0,125 12,150 14,175 16.200 18,225 20,250 40,500 60,750 81,000 ] 01 +250 121,500 141,750 162,000 182,250 202,500 222,750 243.000 263,500 283,500 303,750 324,000 344,250 364,500 384,750 405,000
2,700 5,400 8, I 00 10,800 13.500 16,200 18,900 21,600 24,300 27,000 54.000 81.000 108,000 135.000 162,000 189,000 216.000 243,000 270.000 297,000 324,000 351,000 378,000 405.000 432,000 459,000 486.000 513,000 540,000
3,375 6,750 10,125 13,500 16.875 20,250 23,625 27,000 30.375 33,750 67,500 101,250 135,000 168,750 202,500 236,250 270.000 303,750 337,500 371,250 405,000 438,750 472,500 506,250 540,000 573,750 607,500 641,250 675,000
Cost of paint: $15/gaI.
COST SAVINGS Transfer Efficiency/Paint Savings The cost savings associated with the use o f electrostatic e q u i p m e n t can be realized in m a n y different areas. The most obvious savings is in paint usage. With the increase o f high-solids, plural-component, and base/clear finishes, it is not unrealistic to pay $100 per gallon for these coatings. C o n s i d e r i n g this cost. it is crucial that the coating is applied to the product as efficiently as possible, With a conventional air spray gun, r o u g h l y 15 to 4 0 % of the paint sprayed f r o m the g u n is applied on the part. This is k n o w n as transfer efficiency. The r e m a i n i n g 60 to 85% is lost in the filters or left as overspray on the floor a n d walls. Conventional H V L P g u n s are m o r e efficient than conventional air spray guns. H V L P g u n s will typically yield transfer efficiencies of 30-60%. Electrostatic g u n s can obtain even greater transfer efficiency. An electrostatic air spray g u n is normally in the 4 0 to 8 0 % transfer efficiency range. This m e a n s you c a n coat twice as m a n y parts with an electrostatic air spray gun, c o m p a r e d to a nonelectrostatic air spray g u n given the s a m e quantity of paint. As with noneleetrostatic guns, H V L P t e c h n o l o g y s h o w s significant i m p r o v e m e n t in transfer efficiency. The same holds true with electrostatic H V L P t e c h n o l o g y as well. In some cases, electrostatic H V L P has obtained efficiencies as high as 90%.
211
Table III. Maintenance Costs Estimated in Dollars A[,~l);'oxmtate Paint Usage Gall &O 5 10 15 2(I 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 10('1
TE
Gall day
Waterwetsh Chemi~ol U~age TE (65~ )
TE (75%)
1,250 104 64 2,50('1 208 129 3,75(I ~ 12 193 5,(XX) 416 257 6.250 521 322 7.5('10 625 386 8,75('1 729 450 10.000 833 515 I 1.250 937 579 12,500 1,041 644 13,75('1 1.145 708 15,000 1,249 772 16,250 1,353 837 17,500 1,457 901 18.750 1.562 965 20,000 1.666 1,030 2 t,250 1,770 1.094 22.5(X) 1,874 1.158 23,750 1,978 1.223 25.000 2.082 1.287 Note: Note: 251) days/yr I ~)~'/ib of overspray Estimated $ 1.4 I/Ib of chemical
Wowru'a.dl Slud,,¢e Retnot'ol TE (65q) 295 591 886 1.181 1.477 1.772 2,067 2,362 2,658 2,953 3.248 3,544 3,839 4,134 4,430 4,725 5,020 5,315 5,61 I 5,906 Note: 400 Ib/drum $ I00.(X)/ disposal drum
TE (75%) | 83 366 548 731 914 1,097 1,280 1.463 1.645 1,828 2,01 1 2.194 2.377 2,559 2.742 2,925 3,108 3.291 3,473 3.656
Drr Filter Divposal Co~t TE (65%)
TE (75q)
1.023 632 2,045 1.265 3,068 1,897 4,091 2.53(I 5.113 3.162 6.136 3,795 7.159 4.427 8,181 5.060 9,204 5,692 10,227 6,325 I 1,249 6,957 12,272 7.590 13.294 8,222 14.317 8,855 15.34('1 9.487 [6,362 [().120 17,385 10.752 18,408 I 1.385 19.43('1 12,017 20,453 12.650 Note: 60 tilter~,/drum $200.(X)/ drum o f tilters
D 0 1.71wr Usage IF. (65q)
TE (75q)
307 190 6,14 380 920 570 1,227 760 1.534 950 1.841 1.141) 2.148 1,330 2.454 1.519 2,76 l 1.709 3,008 1,899 3,375 2,089 3,682 2.279 3,988 2,469 4,295 2.(~59 4.602 2.849 4.909 3.039 5,216 3,229 5,522 3,419 5,829 3,609 6.136 3.799 Note: S 1.00/fihcr
transfizr efficiency.
Typically, cost justification is obtained fi-om paint cost savings alone. Its typically enough to cost justify the purchase of the electrostatic applicator. Table ll displays the dollar figure in paint savings that can be achieved by slightly increasing transfer efficiency.
VOC Reduction Another savings area is emission reduction. With tederal and local regulations becoming tougher by the day, VOC (volatile organic compound) emissions has become a major issue. We arc constantly trying to reduce the amount of VOCs enfitted into the atmosphere. By increasing transfer efficiency you lower VOC emission. (See Fig. 8.) This is a result of more paint being applied on the part and less paint being deposited into the booth filters or atmosphere, Many states, such as California, now mandate that you use either H V L P or electrostatic technology to qualify for a permit to install a new spray booth. A manufacturing facility is permitted to emit a specified amount of VOCs (in tons) per year. It" the tonnage limit is exceeded, strict fines are enforced. These fines can easily exceed thousands of dollars. As a result of these laws many companies have invested in electrostatic finishing equipment to comply with V O C regulation.
D e c r e a s e d M a i n t e n a n c e Costs As stated earlier, when transfer efficiency increases, the amount of overspray deposited into the spray booth decreases. This means spray booths that previously required weekly cleanings and filter changes may now require biweekly maintenance. Water-wash booths that previously consumed 55 gallons of chemicals in one month may now use only 30 gallons. Not 212
10
20
[::!:::ilili i :i:ii::!:i:3:.] 50%
30
40
SO
60
70
80
90
100
67%
75%
80%
83%
86%
88%
89%
90%
33%
50%
60%
67%
71%
75%
78%
80%
25%
40%
50%
57%
63%
76%
70%
20%
33%
43%
50%
56%
60%
44%
50%
25%
33%
40%
13%
22%
30%
IllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllU[llll 14% i:~i~.~iiiiiii~i!i~i~iiiii~iii~i~.~.~i~i~i~.~ii~
~ ~
i.~.~.~`~.~.£~.~:.~.~.~.~.~.~;.~.~.~.~5~.~.~.~.~.%.~.~.~.~.~.~.~:~`..~5.~.~%.~.~£~.~.~.~:.~.~.~A~.~.~$.~.~:~.~.~.;~;.~.~.~.~.'~.~.~.~.~?5.~.~`3.~.~.~.~.~`~.~.~I.~.~.`.~.~`..
Fig 8. Emission reduction fi'om installation ~'new equipment. Etample: If era'rent TE is 40~ and new equipment is rated at 90%. the emission redaction would be 56%. only is the cost to purchase these materials reduced, but the cost to dispose of them is decreased as well. Dirty booth filters and contaminated booth water must typically be disposed of as hazardous waste. In recent years, the cost associated with the disposal of hazardous waste has sky rocketed. Not only is there a direct cost reduction, such as filters, chemicals and disposal (see Table Ill), there is also an indirect savings in labor cost, due to the tact that booth maintenance can easily consume on an average up to 8 man-hours per week.
Better Coverage/Improved Quality Electrostatic finishing has many other benefits in addition to cost savings. Application time may be reduced with the aid of electrostatic wrap. Electrically charged paint particles can change direction and be deposited on the top. bottom, and sides of the part when sprayed from one direction. Depending on part size and configuration this wrap-around may sufficiently coat all of the product at once, eliminating the need for additional passes. When using a nonelectrostatic gun you must point the gun at every area that requires paint, if you miss a particular angle it will not be painted. With electrostatic applicators, the wrap-around may coat these areas producing a more uniform finished product. As a result of a more uniform finish, many manufacturing facilities have experienced a lower reject rate in their production. In the case of on-site furniture refinishers or appliance refurbishers, it would be virtually impossible to paint without electrostatic finishing equipment.
213