Getting the Most out of Airless Spray
如何最有效的使用无气喷涂机--未完待续防腐蚀论坛 @chris1986 原创翻译
paintapplication using airlesse quipmentis, and has been for many years, the methodof choice for large industrial painting projects. Although the industry is awareof the differences in the equipment from various suppliers, some of the basicsof airless application common to all equipment manufacturers are not being effectivelycommunicated to applicators. It is important to the quality and economics of theproject that everyone understands and pays attention to these basics.
Thereare four areas that demand better attention by applicators and paintmanufac-turers. These are:
1) Wornspray tips,
2)Excessive application pressure,
3)Proper tip selection, and
4)Variability in applied film thicknesses.
Anumber of factors, including tip selection, application pressure and appliedthickness variability can affect the quality of an airless spray applicationjob.
Nothingwill waste more paint in less time than a worn spray tip. Most specificationsrequire the painter to apply the coating to a minimum dry filmthickness (DFT). Worn tips increase the average DFT by affecting the
uniformityof the applied paint film. Worn tips also affect quality.
在短时间内没有什么比使用磨损的喷嘴涂装更浪费涂料了。大部分技术指标要求涂层达到最小干膜厚度（DFT：minmum dry film thickness：最小干膜厚度）。使用磨损的喷嘴会使涂装后漆膜的平均DFT升高，也会对质量产生影响。
Atip is completely worn out if the spray pattern is round. The easiest way tounder-stand the dynamics of a tip is to think of the round pattern being theresult of spraying the paint through a circular hole. A proper spray patternis elongated. This isachieved by squeezing the metal on two opposing sides ofthe circular opening. The more it is squeezed, the flatter the fan, hence the widerthe spray pattern.
Inactuality, there are two little “wings” of metal that are used to “squeeze” thefan.The spray pattern becomes rounder and rounder as these pieces of metal areworn away by the abrasive action of the pigments in the paint. As the openingbecomes rounder, the paint is deposited on the sur-face too quickly to control,and the resultant application is usually unacceptable. However, long beforethe “wings” are com-pletely worn away, the tip or nozzle will stop depositing auniform amount of paint across the entire fan width. The amount ofpaint in the center of the fan increases. If the fan is not uniform, appliedDFT variability increas-es and more paint is needed to achieve the minimum DFT.
Thephoto at left shows satisfactory drip from a new spray tip, while the photo atright shows a drip pat-
ternindicating that the spray tip is worn.
Courtesyof Gary Tinklenberg, Corrosion Control Consultants & Labs, Inc.
左边的照片为合格的新喷嘴中漆滴 的图形。 右边为已磨损的喷嘴所喷出漆滴的图形。--腐蚀控制协会、实验室提供。
Thereis a very simple test to determine if a spray tip is worn. Take a piece ofcard-board (or any smooth surface) and mount it in a vertical position. Holdingthe spray gun as steady as possible and in a position to create a horizontalpattern on the surface, apply a short burst of paint.Obviously this will result in an excessive amount of paint beingapplied, and the paint will drip or run down the surface. Examine the drippattern. If the drips are all uniform, evenly spaced and of equal length, thetip is satisfactory. If the drips are longer in the center of the pat-tern, thetip is worn and should be replaced.
Thisvery simple test should be performed at least daily.Why is it that something soeasy and cost-effective is not part of many profes-sional painters’ routines?The answer is sim-ple. Sometimes the true cost of a job is for-gotten. Too muchimportance is placed on the cost of equipment rather than on the cost of paintand labor to apply it.
First let’s consider the cost of paint. Once thesurface is blasted, it generally needs to be painted as quickly as possible.However,worn spray tips can easily result in an average applied DFT of 25microns (1 mil) greater than a coating film applied with new tips.
Thisproblem can result in the application of as much as an extra gallon (3.8liters) of paint to every 500–800 square feet (46–74square meters) of surfacearea, depending on the volume solids of the paint and the configuration of thesurface. The cost of wasting this much paint could certainly pay for a new tip.
此问题在施工中会对结果产生影响。根据油漆的固形物百分比和表面配置，这个问题会导致每喷涂500 - 800平方英尺(46 - 74平方米)的面积,要额外多用1加仑(3.8升)的油漆，被浪费的这些油漆的成本都可以用来买一个新的喷嘴了。
Mostapplicators are aware that low pres-sure in spray equipment can causeprob-lems, but they are not aware or do not believe that high pressures canalso cause problems. It is rare to see applicators adjusting spray pressures asa matter of routine, even when starting a new project with a new paint. But toomuch pressure can be costly and can affect quality.
Thetail pattern in this photo is a result of too little applicationpressure.--Courtesy of Gary Tinklenberg, Corrosion Control Consultants &Labs, Inc
The proper pressure is affected by sever-al variables, includingtemperature, paint vis-cosity, batch-to-batch variability, hose length, andresistance to flow. Since it is impossible to know the consequences of allthese variables each time paint is applied, a simple field method is needed toarrive at the proper pressure. If there is insufficient pressure, the spraypattern will result in “tails” (Fig. 3). A “tail” pattern is evidence of an incompletefan. In other words, as the spray gun is moved through a normal stroke, thereis a line of paint at the top and bottom of the spray pattern that is separat-edfrom the rest of the pattern. If tails are present, the pressure must beincreased to the point where the tails disappear.
Againthe question must be asked: why is something so simple not a part of a typicalpainter’s daily routine? There are several rea-sons, but the most important isthat the consequences of higher than necessary pressures are not completelyunderstood.
One ofthe important characteristics tout-ed by the manufacturersof airless spray equipment is its improved transfer efficiency compared toconventional spray, as noted in“On the Differences Among Spray Systems, ”JPCL , July, 1999. Transferefficiency refers to the percentage of sprayed paint that actually sticks tothe surface. However, transfer efficiency is reduced by excessive pressure. Thepaint has excess energy when it reaches the surface, causing it to bounce offrather than stick to the surface. Excessive pressure wastes paint and increasestip wear, the problems of which have already been discussed.
Highspray pressure also increases the prevalence of dry spray by partially dryingthe paint droplets before they reach the sur-face. Dry spray, in turn, canresult in poor film formation because the coating will not be smooth andcontinuous. When this hap-pens, the dry spray needs to be sanded out,and a new layer of coating applied. The cost can be very high.
Settingthe proper pressure seems so easy, but it is so often ignored. An applica-torcan simply turn the pump to the maxi-mum pressure and spray. It will work — notideally — but it will work. Dry spray is one of the most common coating defectsencountered with spray application, and it is one of the simplest things tocorrect.
Theproper tip is determined by the viscosi-ty of the material, the size of theobject to be painted and sometimes the particle size of the raw materials usedto produce the paint.
However,before proper tip selection can be discussed, it is necessary to discuss howtip sizes are designated. There are two important characteristics to airlesstips. The first is the orifice size of the tip. This repre-sents the diameterof the hole if it were a perfect circle; thus, the larger the orifice, thelarger the hole in the tip. The second is fan width. Different manufacturersuse different methods of denoting fan width. In some cases the fan width isdesignated by the angle of the material as it exits the tip. A 20-degree fan isquite narrow, while an 80-degree fan results in a wide pattern. A tip designatedas 4-19 would be one with a 40-degree fan and a 0.019 thousands of an inchorifice.
One ofthe first rules in tip selection is that the thinner (lower viscosity) thematerial that is to be applied, the smaller the orifice size needed. Accordingto equipment manu-facturers, the best rule is to use the widest fan and thesmallest orifice size that is prac-tical.选择喷嘴方法不二法则是：涂料原料越细，则选择越小尺寸的孔口。设备制造商实践证明喷嘴选择最好的方法是使用最宽的扇面和最小的孔口尺寸。
It isessential to select the proper tip for the viscosity of the paint being used,the area of surface being
coatedand the particle size of the raw materials used to produce the paint. Courtesyof Graco Inc.
Thesize of the object being painted must also be considered when selecting a tip.On a project in a fabrication shop, for instance, a crew was painting steelcomponents fabri-cated from 3-inch by 3-inch (7.5-cm by 7.5-cm) angle iron.They were using a 50-degree fan. When asked why, their response was verysimple: “It’s what we always use, and besides it’s all we got.” This tipresulted in the use of about two gallons (7.5 litres) of paint and about 10minutes of labor per component. But most of the paint was being sprayed intothe air due to the length of the spray pattern. Obviously, reducing the fanwidth to a 10- or 20-degree fan could have reduced paint waste and consumptionby at least half. (It is interesting to note that, in this case, thefabrication shop could have further reduced its waste and saved money by usingconventional application methods. The cost of the paint wasted by usingair-less spray was more than the cost of the additional labor required by theuse of con-ventional equipment.)
Finally,sometimes the paint itself has par-ticles of sufficient size that require spraytips with a minimum opening size. Opening size is different from orifice size.The actual opening of an airless tip is in the shape of an eye. The orificesize is the width of the eye (the x direction). The height of the eyedetermines the fan width. The smaller the height of the eye, the longer the fanpattern will be. Think of it as simply squeezing the same size round pattern aspreviously described. Sometimes, it is possible to solve plugging problems dueto particles in the paint simply by reducing the fan width, since the openingdid not have to be “squeezed” as tight. Given the same size orifice, the narrowerthe fan is, the larger the actual opening.
Variabilityin Applied Film Thicknesses
Airlessspray frequently results in coatings applied with more variation in thicknessthan allowed in product data sheets. This prob-lem can be addressed in twoways. First, users must become aware of normal film thickness variation;second, the product data sheets must list achievable limits.
Today’scoatings are being formulated to lower volatile organic compound (VOC)con-tent. This often results in higher volume solids. At the same time,information on film-thickness tolerance on product data sheets has notnecessarily changed.
Thisexample provides a good explana-tion. A project stipulated that the material beapplied in accordance with the manufac-turer’s product data sheet, requiringthe 65% volume solids coating to be applied at 40–65 microns (1.5–2.5 mils)DFT. The owner assumed these were a minimum and maximum and insisted that theapplicator meet this “requirement.” This was a very large project and the onlypractical way to apply the coating was with airless equip-ment. With theairless equipment used, the minimum applied DFT to achieve hiding of the surfacewas actually 50 microns (2 mils). Therefore, 50 microns became the minimum, not40 microns (1.5 mils). At 65% volume solids, the minimum applied WFT would be75 microns (3 mils). Using 75 microns as minimal WFT and a variation of 50 micronsWFT, the overall range is computed as 75 to 175 microns (3 to 7mils), or 125±50 microns (5 ±2 mils).Thus, the expected applied variation in DFT based on65% volume solids is 50 to 115 microns (2 to 4.5 mils), where DFT=% vol-umesolids x WFT.
Areading of 125 microns (5 mils) would be common given complicated shapes. Thisis the best that can be expected! Even under the best of conditions, anapplicator cannot apply coatings to a maximum vari-ability of only 10 microns(0.5 mils) DFT from the average, as was required by this example. Thisconclusion is based on a review of lab data both from paint compa-nies and from independent labs.
Coatingsmanufacturers and owners must realize that while the percent solids by vol-umeof paints has been increasing, the abili-ty of applicators to maintain ±50microns WFT — at best — has not changed. Manufacturers must provide realisticthick-ness ranges on product data sheets and owners must have realisticexpectations for the ranges in the field. Increased volume solids also meansthat coating application by airless spray has become more exacting andapplicators must be aware.
Thequality and economy of airless spray application can be improved by attention tothe equipment and properly setting the
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