Boost Overall Transfer Efficiency

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Preston Lyons
5 years ago
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2 .I TO CUT DOWN ON HIGH-SOLIDS WASTE Boost Overall Transfer Efficiency Arvid C. Walberg Electrostatic Consultants Co. Wheaton, IL T he name of the game when spraying liquid coatings is to apply the exact specified film thickness evenly throughout a part's surface with minimal overspray. Excessive film thickness and overspray are wasteful and cost money. The higher the coating pergallon price, the greater the cost for such waste. It's a rule of thumb that the price of paint tends to be in proportion to the solids (nonvolatiles) content. Therefore, paying strict attention to excessive film thmness and overspray is especially important when applying high-solids Coatings. -- Overall transfer efficiency A measure of the efficiency of applying high-solids coatings, and any liquid coating, can be expressed in a term called overall transfer efficiency (OTE). The term indudes the efficiency of all ' with applying the coating. The major efficiency parameters in applying a liquid coating with automatic electrostatic spray are: basic transfer efficiency voltage output efficiency conveyor loading efficiency empty hanger efficiency horizontal triggering efficiency vertical triggering efficiency spray height efficiency film thickness efficiency reject efficiency manual touchup efficiency operator efficiency OTE depends directly on the efficiency of each parameter and is de rived by multiplying together the decimal equivalents of each parameter efficiency. For example, suppose that the efficiency of each of the 11 parameters is 90% (a decimal equivalent of 0.90). The overall transfer efficiency would be 0.90 multiplied by itself 11 times, or 0.91'. Carrying out the multiplication equals , or 31.3%. In this example, only 31.3% of the paint being applied will deposit at the specified film thickness onto parts being coated; 68.7% will be wasted due to excessive film thickness and overspray. In the example the difference between the average transfer efficiency of all parameters (90%) and the OTE of 31.3% is startling. Multiplying the parameter decimal equivalents instead of averaging them brings out the cumulative effect of all individual parameter efficiencies. To demonstrate the anomaly further, Table 1 gives the OTE for examples where one or more parameters equals 90%, and Table 2, for 50%. It is, indeed, amazing that if each parameter is 50% efficient, only 0.05% of the paint being applied will be deposited at the specified film thickness. In other words, with this supposition 99.95% of the paint will be wasted. It becomes quite obvious that for a high OTE, the efficiency of each parameter must be as perfect as possible. A single parameter with a low efficiency will dramatically lower the OTE. Basic transfer efficiency This term refers to the intrinsic transfer efficiency of a liquid atomizing device. Figure 1 gives the basic transfer efficiency of five types of devices. The comparison assumes that each device directs atomized paint toward the center of a large flat panel or sheet so that overspray and electrostatic wrap around are not possible. The only paint loss that can occur in this assumption Figure 1 Basic Transfer Efficiency I

3 b is rebound from the surface. Rebound loss refers to atomized particles c that are blown away from the target by air currents from the device. Reciprocating disks tend to have the highest basic transfer efficiency because of the total absence of atomizing air. All atomized paint particles would end up on the hypothetical flat panel. High-speed spinning bells use shaping air to help direct the atomized particles toward the target. The shaping air will cause slight rebound, giving the bells a lower basic transfer efficiency than disks. An electrostatic high-volume lowpressure (HVLP) air-atomized electrostatic gun operating at 100,000 V (at the gun electrode) will show only about a 5% loss in basic transfer efficiency 1 ; Figure 2 Conveyor Loading Efficiency 1 1- lcw-+ii+--f Center Spacing I 3 I4 I &12 + f - W+F Center Spacing =loo% --- =8l YO due to rebound. An HVLP gun operates with atomizing air of 10 psi or less. A nonelectrostatic HVLP gun can have a basic transfer efficiency of about 80%. A conventional air-atomked spray gun s basic transfer effidency will be about 50% due to high atomizing air pressures causing considerable rebound. As a general rule of thumb, the higher the atomizing air pressure, the greater the rebound and the lower the basic transfer efficiency. The quality of atomization of highsolids coatings is a topic that needs to be considered separately from basic transfer efficiency considerations. As a genefat rule of thumb, fine atomization will produce smooth coating films; coarse atomization will tend to produce orange peel. Disks, bells and certain types of electrostatic guns have the highest atomization capability. Low atomizing air pressure tends to reduce HVLP atomizing capability. Voltage output efficiency The transfer efficiency of HVLP airatomizing electrostatic guns will rise as the operating voltage is increased. Table 3 compares the increased film thicknesses when the electrode voltage of HVLP electrostatic guns is varied while spraying test panels with a verti- cal reciprocating machine. Everything was held constant so that the amount of paint sprayed at each panel was identical. The only variable was the op erating voltage. After the panels were painted and baked, the dry-film thickness on each was measured. The increase in film thickness represents a rise in basic transfer efficiency. Note that with 100,000 volts, the 1.3 mils deposited is 86% more than with zero voltage. The HVLP guns in the test op erated at less than 10 psi atomizing air pressure at the air cap. The basic transfer efficiency with zero voltage was about 50%; with 100,000 volts, about 93%. The quality of atomization and the resultant smoothness of finish tended to improve in each test as the voltage was increased. Conveyor loading efficiency Perhaps the biggest cause of paint waste in an automatic electrostatic system is poor conveyor loading. Figure 2 compares the conveyor loading efficiency of panels that are spaced in four different ways. The optimum spacing is when the gap between panels equals the width of the panels. As the space between panels is increased, the coating deposition on the panels will decrease. The comparisons Test Number I 1 100, assume that all spray equipment will operate at 0% transfer efficiency when no part of a panel is directly in front of the atomizing device to collect the spray. Even with low-velocity spray, the paint is wasted by applying excess film thickness on adjacent surfaces. Empty hanger eff iciency Empty hangers can be a great source of paint waste. This is especially true for reciprocating disk systems, which cannot turn off the atomization for an empty hanger. Figure 3 compares the spraying of an HVLP airatomizing electrostatic gun with a reciprocating disk. The gun can be triggered off automatically for empty hangers and thus will have an empty hanger efficiency of 100%. If paint cannot be triggered off for empty hangers, the drop in empty hanger efficiency will vary directly with the percent of empty hangers. For ex-

4 ample, with 5% empty hangers, about 5% of the total paint is wasted, and the empty hanger efficiency is 95%. This will be true even if the paint is not thrown into the spray booth but collects on the edges of the neighboring parts. This paint is not available to paint additional parts. In the worst possible scenario, the edge buildup on adjacent parts may cause runs and sags that will reject the two adjacent parts. If these two adjacent parts are lost, the empty hanger efficiency may have to be charged with a paint waste of 15%, thereby dropping the empty hanger efficiency to 85%. The two adjacent rejects can also be treated separately under "reject efficiency." The empty hanger efficiency would remain at 95% in the example. Horizontal triggering efficiency Horizontal triggering efficiency refers to the capability of the coating system Efficiency I Figure 7 Paint Film Thickness Efficiency cv"l- (I I

0.r to trigger on and off at the beginning and end of each job run. Reciprocating guns and bells have a great advantage over reciprocating disks in this regard.

5 0.r to trigger on and off at the beginning and end of each job run. Reciprocating guns and bells have a great advantage over reciprocating disks in this regard. Figure 4 compares the horizontal triggering efficiency of a recpirocating disk system with spray devices. Paint flow to a reciprocating disk can obviously be tumed on and off. However, the paint must be tumed on when parts begin entering the spray booth and tumed off as the last part exits. Since the paint is sprayed from the 360- degree circumference of the disk, a substantial amount of paint is wasted at the beginning and end of each run. The amount of empty conveyor can total at least 30 ft. This compares with only 2 ft with a gun or bell system. Vertical triggering efficiency Paint wasted at the tops and bottoms of reciprocator strokes is affected by the vertical triggering efficiency. A full-stroke vertically reciprocating elec trostatic system can be divided into two basic types: those that trigger the spray device off during reciprocator reversal and those that continue to spray during the reversal. A reciprocator must of necessity come to a complete stop during the reversal. This throws an excessive amount of paint at the top and bottom of the stroke. If the device continues to spray during the reversal, precision control is no longer possible. When spraying during the reversal, at least 5% excess paint is sprayed at the top and bottom of the stroke. This drops the vertical triggering efficiency to 95% on go-in.-high parts and to 90% on 30- in.-high parts, as shown in Figure 5. A spraying device with a proper vertical triggering system can be turned off during the reversal and will produce a vertical triggering efficiency of 100%. The triggering or reversal points must be adjusted accurately by the operator of both types of systems to minimize paint loss. Paint sprayed too far above or below the target is wasted. This specific loss should be charged to operator efficiency. Spray height efficiency Being able to adjust the spray height with a high degree of precision is nec- essary for high spray height efficiency. Figure 6 compares a full-stroke reciprocating electrostatic system with vertically stacked spray devices, either spray gu ns or bells. Reciprocating electrostatic systems with either adjustable stroke or easily adjustable spray height can be rated at 100 /~ efficiency when spraying parts of constant height. The stacked spray devices, on the other hand, cannot be adjusted with any great precision for the height of the part. The spray height is adjustable in 15-in. or greater increments by selecting the number of spray devices to be used. The spray height efficiency will vary from 50% to 100% under these conditions, depending on how closely the spray pattern height fits the product height. Film thickness efficiency If a paint specification calls for a 1-mil thickness, paint applied in excess of 1 mil represents wasted paint. A 2-mil film thickness would be equivalent to 50% film thickness efficiency in this example. One manufacturer calculates that each 0.1 mil of extra film thickness adds over $100,000 to its annual paint costs. Figure 7 illustrates how a spray pat-

tem affects film thickness. A round or doughnutshaped spray pattem cannot apply a uniform film thickness when this pattem is swept across a surface.

6 tem affects film thickness. A round or doughnutshaped spray pattem cannot apply a uniform film thickness when this pattem is swept across a surface. A flat fan-shaped spray pattem provides far more precise film thickness control. The fan-shaped pattem is rated at 100% when applied to the desired film thickness. Reject efficiency Reject efficisncy is the percentage of good parts made. Table 4 shows the relation between percent rejects and reject efficiency. The capability of electrostatic spray systems to provide wraparound can contribute to rejects. The wraparound can build excessive paint film on edges, causing wrapsag. Edge buildup is a m4or cause of rejects. To minimize wrapsag, it may be necessary to cut back on the paint delivery to the automatic system and increase the load on the touchup sprayers. However, this will lower touchup efficiency. Ncwniform application of paint by an electrostatic system increases the touchup requirement. A hand sprayer m t apply paint to light areas with- out repainting heavy areas at the same time. This can cause runs and sags in the heavy areas. Nonnally this problem is charged against the hand sprayers, but it should be charged against the automatic system. Manual touchup efficiency Products that cannot be sprayed 100% automatically with an electrostatic system will require manual touchup. This will drop OTE substantially. If a conventional nonelectrostatic hand spray gun is used for touchup, the operating efficiency will normally be W? or less. The touchup efficiency should increase to 50% or better when an electrostatic hand gun is used. Modem electrostatic hand guns typically cannot apply high-solids (over 60% volume sdids) efficiently. They usually require cutting the paint to achieve adequate visoosity and resultant film quality. The painting efficiency is computed in Table 5 for different amounts of required manual touchup. The automatic system efficiency and hand gun efficiency are combined to produce manual touchup efficiency. The efficiency of the electrostatic system is computed at 100% since an overall transfer efficiency factor for the automatic system is included in the calculation elsewhere. None of the elements of efficiency can be duplicated. This will lower the computed efficiency. An HVLP automatic electrostatic system, for example, provides superior penetration into recessed areas. This will reduce the required manual touchup in comparison with a disk or bell system. This results in substantial paint savings in the touchup booth and cuts overall paint costs. A reciprocating disk system with three touchup manual sprayers painting folding chairs was replaced recently with an HVLP automatic electrostatic system. Two of the touchup sprayers were eliminated. Savings in rejects ran about $100,000 a year, and overall paint consumption decreased by 30%. Production speed and quality of finish has improved. However, the previous waste was caused by the touchup sprayers and not by the disk system. Operator efficiency The operator of a painting system plays a very important role in paint application efficiency. The operator is responsible for seeing that all other parameters of efficiency are maintained at the highest possible levels. When the operator does everything right, the op erator efficiency is 100%. Operator efficiency improves with experience and training. The importance of experience and training is illustrated in the fdlowing example. An experienced painting system op erator quit a large company and went to work elsewhere. The company ap plies expensive fluoropolymers at $60 to $300 a gal onto aluminum extruslons. The first month after his departure, paint consumption costs increased by $35,000. The company solved the problem by rehiring the experienced painting operator at a substantial salary increase. Three systems compared The efficiencies of the 11 categories are compared in Table 6 for three types of automatic electrostatic application systems. The efficiencies in each cate- gory are quite high and are amhut as good as is possible to achieve. Hdwever, multiplying the categories of efficiencies for each system reveds some startling OTE results. The OTE will drob further if additional manual touchup is required. Reducing rejects will save substantially. An electrostatic gun used for manual touchup can sometimes bring a big improvement. Note that the conveyor loading efficiency is shown as 90%. This level of efficiency is very difficult to achieve and requires close supervision and careful production planning. This factor can greatly increase paint waste if not watched carefully. Making an analysis When making an efficiency analysis on a given system, great care must be taken to avoid duplicating or overlap ping categories. This would tend to introduce emrs that would show up on the low side. Obviously, OTE cannot be oversimplified without introducing large errors. A relatively complex formula is needed. This article merely attempts to break such a formula down into simple, easily understood categories. Additional factors need to be considered in addition to the 11 categories. Color-change efficiency must be considered if several color changes are to be made per day. Reducing colorchange time from 4 to 1 min with only 10 color changes a day increased production capacity by over $1 million a year in one instance. When buying new equipment, it is essential to determine the capability of applying high-solids or other environmental coatings. It is best to avoid the strong possibility that the new equip ment will have to be replaced within a year or two if it cannot apply an environmental coating. Some environmental codes already specify that paint be applied at 65% transfer efficiency. Equipment that can do this is available. Spray equipment that is capable of applying 75% volume solids coatings is highly limited. If the paint cannot be properly atomized, the applied film thickness will increase greatly and substantially increase paint consumption. Inadequate atomization will also lower the quality of finish. A Reprinted from May 1990 issue of INDUSTRiAL FiNlSHiNG HITCHCOCK PUBLISHING COMPANY ALL RIGHTS RESERVED

TUNE-UP YOUR SYSTEM THE RANSCRAFTED WAY

Ransburg TECHNICAL MANUAL (Replaces IL-204-D) March - 2013 TUNE-UP YOUR SYSTEM THE RANSCRAFTED WAY IMPORTANT: Before using this equipment, carefully read all instructions in this manual. Keep this Technical