POWDER COATING /d

Transcription

1 c E. -- THE KEY TO SUCCESSFUL POWDER COATING /d PgF Michael K. Anderson Binks Industrial Powder Systems Powder Coating 94 Proceedings 193

2 Over the past decade, the powder coating industry has increased at a rapid and steady rate. During this time, companies and personnel have learned that the recovery of powder coatings and the effect associated with it is not as once thought. The more powder coatings are recovered the smaller the average particle size becomes. The smaller the size, the less of an ability the particle has to accept an electrostatic charge and therefore, the less ability to attract to the part. Ultimately, what occurs as powder is continuously recycled is the development of "fines" in the system (fines are powder particles that have a micron size of 10 or less), It is these fines that can cause premature cartridge filter replacement, inconsistent coating thickness, higher coating costs, and several other undesirable occurrences. Today, users of powder coating are looking for new and innovative ways of improving the First Pass Transfer Efficiency (F.P.T.E.) or the amount of powder that is actually applied to the part as compared to the amount of powder that is dispersed from the application device. The higher the F.P.T.E. the less likely the chance for "fines" to develop. Several items have an effect on F.P.T.E.; more than originally thought and all must be evaluated when trying to improve transfer efficiency. 194 Powder Coating '94 Proceedings

3 TYPE OFAPPLICA non EQUIPMENT Powder coating equipment, unlike liquid equipment, has only three basic types of application equipment; standard corona charging guns (automatic and manual), tribo charging guns (automatic and manual), or rotary powder bells. Theoretically, the best type of application device to provide the highest F.P.T.E. would be a manual powder gun. Unfortunately, the ability of a manual operator to coat consistently day after day is highly unlikely. Additionally, a single manual operator can only coat product at slower line speeds. Therefore, the majority of conveyorized powder coating systems utilize either automatic guns, rotary powder bells, or a combination of both. The type of automatic application device that should be used greatly depends upon the size and configuration of the part. Powder guns, which have been used for years, are best for coating parts that have several recessed areas; although they certainly can coat nearly any type of part. Rotary powder bells, a new type of application technology, are excellent for coating large, relatively flat products as well as tubular parts and wire goods. Powder bells, unlike guns, may not be well suited for coating the inside of cabinets, lockers, or other products with several Faraday Cage areas. When determining any type of application equipment, it is highly suggested that testing be done at an equipment manufacturers laboratory. NUMBER OFAPPLICATION DEVICES One area in a powder coating system that can dramatically help increase F.P.T.E. is the quantity of guns or bells. Depending upon the line speed, part size, part configuration, and film thickness requirements, it may be better, for example, to utilize two (2) guns as compared to one (l), even though a single gun may be adequate. The reasons for an increased number of guns is as follows: Powder Coating 94 Proceedings 195

4 1) The higher the powder flow out of an application device the higher the velocity of the coating. As in liquid systems, this coating velocity will generate what is commonly known as bounce back or over spray. If two (2) guns are used instead of one (l), and the powder flow or delivery that the single gun had is divided between the two guns, there will not be nearly the velocity and a softer, gentler pattern will be provided. It is this type of pattern that will allow the coating more time to apply to the product. 2) A commonly over looked or misunderstood area of electrostatic powder coating is that there is a maximum amount of powder that can be effectively charged. All powder application devices today generate either an 80 or 100 KV (kilo volts) electrostatic charge. It is this charge that allows the powder to attract and stick to the product until it is cured. Whether the applicator is 80 or 100 KV, there is a limit to how much powder can be effectively charged and thus applied to the part. A general rule of 50 pounds per hour can be used for a maximum delivery rate for maximum charging; although these type of flow rates are not the most beneficial for high T.E. When evaluating an existing system, or considering a new one, always take into account the number of guns used as compared to the amount of powder being sprayed. When refering to the quantity of application devices, the old saying "more is better" might be correct if high F.P.T.E is desired. 1% Powder Coating '94 Proceedings

5 GROUNDING One of the easiest to correct (and most commonly ignored) problems in a powder coating system is the lack of a good and consistent "ground" between the conveyor system and the part. The "grounding" of a part is essential in the electrostatic powder coating process. As a system operates the hooks or racks that are used to hang the parts build up with powder. Mer several cycles through the system, these hooks or racks develop a coating on them similar to that of an insulated wire. This coating isolates the part from coming into direct contact with the hook or rack and thus prevents a good "ground" from developing. The lack of a ground will cause three problems: 1) The more coating on the rack, the less of an ability for a "ground" can occur. This will and does have an effect the amount of powder attracted to the part and the F.P.T.E. is reduced. 2) If the hook or rack is built up with powder so much that no "grounding" of the part can occur, very little if any powder will be attracted to the part. The only powder that may stay on the part will be as of a result of friction or the inherent ground of the part itself. 3) The lack of good ground can also create a safety hazard. If the parts are not grounded, an excessive amount of electrostatic charge will develop on the part, causing it to capacitize. Mer a period of time, the part will discharge this excess by means of an arc or spark. If a grounded operator is within a reasonable distance of the part, the operator may be the discharge point and receive a shock and possible injury. Powder Coating '94 Proceedings 197

6 If the F.P.T.E. in your powder system is poor and it is difficult to coat the products, check the ground. This is easy to do by means of an Ohm meter. A resistance not to exceed 1 megohm should exist to provide a good ground. To assure a good and consistent ground in a powder system there are three things that can be done. 1) Purchase a burn-off oven along with two additional sets of hooks or racks and start a rotation that includes burning off the coating from the hookdracks; 2) Send the hooks or racks out on a regular basis to be striped; 3) If the hooks being used are of minimal expense; replace them on regular basis. LINE SPEED Today, higher line speeds are becoming much more common than they used to be. In the world of Just-In-Time manufacturing, faster line speeds may not be the best in terms of coating utilization. As described earlier, the flow of a powder gun or bell can have a negative effect on transfer efficiency. Faster line speeds require higher flow rates to coat the product. Even if more guns or bells are used at less output, there is still a level at which the line speed is to fast and will effect the F.P.T.E. Experience has shown that the slower the he speed the more advantageous for high application levels. In Europe, for example, where powder coating technology is ahead the United States, maximum line speeds are approximately 18 FPM. If production is such that line speeds need to be considerably higher than what typical systems operate at, and F.P.T.E. is a concern, than there are two choices to consider: 198 Powder Coating 94 Proceedings

7 1) Extend the existing eight hour shift or add another shift and operate the system slower. In the long the run, an additional shift will benefit your company in several areas beside T.E. and the system as a whole will not have to work as hard to generate the desired production. If each piece of equipment is not operated at its maximum, it will last longer and operate more effectively over its life time. 2) If an extended or additional shift is already in existence, or is not a viable option, than the next option is to install a second or additional system; again at a slower line speed. This may seem like an exorbitant option, however, it will not only provide the desired production output and increase the F.P.T.E. but it will also give the company more flexibility and capability than one system, or several systems operating at maximum capacity can provide. A system's line speed can not only effect the F.P.T.E. but several other items. Don't expect your system to operate at its peak efficiency if you are maximizing its capacity. It just won't happen. LINE DENSITY One portion of a powder finishing system that is important to F.P.T.E. is line density or the spacing and hanging between parts. The closer parts can be hung, the greater the chance for the coating to be applied to a part. Spacing between parts should be as close as possible but still allow enough distance so that the parts can travel around a horizontal curve or make inclines and declines without touching (a simple calculation can be done by your conveyor supplier to determine such distances). Line density can also improve (reduce) the line speed. The following example will show how improving line density can reduce line speed and still maintain desired production rates. Powder Coating '94 Proceedings 199

8 Desired Production: Part Hanging Arrangement: 12" wide x 12" high x 6" deep 10,000 Parts Per Eight Hour Shift One per pendant-18'' Centers Calculation A 10,000 Parts / 8 Hrs per Shift = 1250 Parts per Hour 1250 Parts per Hour /60 Minutes per Hour = 20.8 Parts per Minute 20.8 Parts per Minute * 1.5 (18" centers / 12" per foot) Hanging Centers = FPM Now, utilizing the same production rate, the hanging arrangement will be re-configured to allow the conveyor speed to be slowed down by 67%! Desired Production: Part Hanging Arrangement: 10,000 Parts Per Eight Hour Shift Two per pendant-12'' Centers Calculation B 10,000 Parts I 8 Hrs per Shift = 1250 Parts per Hour 1250 Parts per Hour /60 Minutes per Hour = 20.8 Parts per Minute 20.8 Parts per Minute * 1.0 (12" Centers I 12" per foot) Hanging Centers = 20.8 FPM 20.8 FPM / 2 Parts per pendant = 10.4 FPM Required Line Speed. As shown in the above examples, proper line density or part hanging can provide two benefits towards high F.P.T.E. First, the parts are closer together which will provide less open space for the powder to miss the parts. Second, the line speed has been slowed down by 67% which will prodde a better chance for the powder to attract to the part since lower powder flow rates will be required. Powder Coating '94 Proceedings

9 BOOTH AIR FLO w In the current market, there are basically two different types of powder booth air flows available. These types of booths are: Side-Draft Powder Recovery Boofh Down-Draft Powder Recovery Booth The comparison of Side-Draft vs. Down-Draft Powder Booths has been one of minimal discussion since the mid 1980 s. Nearly all manufacturers of powder booths have chosen the side-draft design as a standard and have provided down-draft powder booths only in specialty applications. The Pros and Cons of both technologies are as follows: Down-Draft Powder Booths The main advantage to down-draft powder booths is the amount of plant floor space this type of design requires. Since the collector and final filter chamber are underneath the spray enclosure, the floor space required is limited only to the size of the spray enclosure itself Additionally, it was determined that the down-flow of air would be better from the stand point of color change time. The down-flow of air, in combination with gravity, helps evacuate the air-borne powder faster than a side-draft powder booth does, therefore helping prevent powder from building up on the walls, floor, and ceiling of the booth. This did,and still does, reduce the amount of time required to perform a color change. However, as powder costs have increased and fbrther evaluation and investigation has been done pertaining to the quality of finish recovered powder provides, companies are looking to increase the first pass transfer efficiency or the amount of powder that is applied to the part the first time it is sprayed. There are several items that effect F.P.T.E., one of the most important ones being the air flow of the booth. Powder Coating 94 Proceedings 201

10 A down-draft booth has a tendency to reduce transfer efficiency as it assists the gravitational pull of the powder to the floor of the booth. Side-Drafl Powder Boo6hs As stated above, one of the most important factors to increase First Pass Transfer Efficiency is the air flow of the booth. Unlike down draft powder booths, side-draft powder booths help overcome the effect gravity can have on the amount of powder applied to a part. The sprayed powder has a tendency to remain air-borne longer than a down-draft powder booth and "follow" the parts as they travel through the booth. This type of air flow increases the dwell time for the powder to attract to the part and thus increase F.P.T.E. Not only is the direction of the air in a powder booth an important consideration as it relates to transfer efficiency, but the controlling of the air is just as important. When a powder booth is new and there is no powder in the cartridge filters, the air flow within the booth is very aggressive. This is due to the motodfan running at its maximum speed and thus generating the maximum amount of exhaust aidstatic pressure that the fan is capable of. A very aggressive air velocity within the booth will be generated since there is no resistance (powder) in the filters that can slow the air down. This type of arrangement is not the most optimum as it relates to transfer efficiency. The faster the air flow within powder booth, the faster the powder will be evacuated from the spray enclosure and it will be literally be "pulled away" from the part. 202 Powder Coating '94 Proceedings

11 In recent years, control systems have been developed that automatically control the speed of the motor,and thus the fan, electronically. This type of system controls the air flow within a powder booth by automatically increasing or decreasing the speed of the motodfan to maintain a constant operator programmed inlet air velocity as it relates to the amount of powder loading on the cartridge filters. This type of control system prevents initial high air flow velocities, since the motor is not operating at its maximum speed, and maintains a constant inlet air velocity. Controlling booth air flow has proven to be a benefit in several areas; one of the most important being F.P.T.E. The "softer" and slower the air flow within a powder booth, the better the opportunity for the powder to "follow" the parts and,therefore, the higher the transfer efficiency. GUN R~GGERING The triggering on and off of liquid spray guns has been used for years. This was done primarily to reduce the amount of overspray which meant less VOC emissions and lower coating costs. Powder coating systems, however, commonly did not use gun trigger systems due to the ability to recover oversprayed powder and re-spray it; thus coating utilization approaches 95%. Most companies determined that there was no need to invest in systems to trigger guns on and off if, eventuuh'y, nearly all of the powder will be applied. However, companies today are realizing that these type of systems can provide a better operation and improve F.P.T.E.. Additionally, gun triggering systems have become considerably simpler to use, more reliable, and much more cost effective than in the past. Powder Coating '94 Proceedings 203

12 The following reasons provide justification for gun triggering systems. 1) As stated earlier, the more powder is recycled and re-sprayed, the smaller the powder particles become. The smaller the particle size, the less of an ability the particle has to receive an electrostatic charge and be applied to a part. The end result can be the development of fines in the system (powder particles having a size of 10 microns or smaller). The triggering of guns, not only when parts are present, but also in the vertical plane, will improve the F.P.T.E and reduce the recycling of powder which will reduce the development of fines with in the system. 2) Many companies today have determined that it is cost prohibitive to reclaim oversprayed powder. With companies operating on a Just-In-Time type of manufacturing, and several colors being required by marketing departments, there simply is not the time to complete several color changes per day. Additionally, many powders, such as metallics, textures, and acrylics, do not lend themselves well to recovery. Gun triggering systems can dramatically reduce the costs associated with spray to waste systems. 3) The powder coating of large products such as metal furniture, filing cabinets, and other similar products are excellent candidates for gun triggering systems. Manufacturers of these type of products require the inside, as well as the outside, to be coated, yet will not accept high mil builds at the edges of the products. The triggering of the guns at specific points can improve the F.P.T.E. by spraying the majority of the coating inside of the product which will have more of an ability of "absorb" the coating. 204 Powder Coating '94 Proceedings

13 Most gun triggering systems are operated by a Programmable Logic Controller or PLC. The PLC is linked to a series of photo eyes at the entrance of the powder booth, each of the power supplies that control the guns, and some type of conveyor sensor. When a part passes in fiont of an array of photo-eye(s) the part size is determined along with the part location in the system. The PLC then counts either the number of inputs it receives from the conveyor sensor or starts a timer to determine when the guns should be turned on as well as which guns need to be used to coat the product. This type of scenario may sound sophisticated, but in today's world of high-tech electronics and controls, it is very simple. Additionally, systems of this type, as compared to conventional systems used in liquid systems, are much more accurate, reliable, and flexible, as well as value priced. Powder Coating '94 Proceedings 205

14 SUMMMY First Pass Transfer Efficiency is an area in day to day operations that many companies in today's powder coating field are addressing. The previousiy mentioned areas all effect F.P.T.E., not to mention numerous others, and all need to be taken into consideration if a high F.P.T.E is desired. In addition, several of the above suggestions can be implemented into existing powder coating systems and are not limited only to new or future systems. Powder Coating '94 Proceedings

15 BIOGRAPHY Michael K Anderson is the National Sales (e Marketing Manager for Binks Industrial Powder Systems; A Division of Binks Manufacturing Company. He has been involved in the pow& coating industry for over eight years and in the finishing industry for over ten years. He has been involved in the development of the latest control systems used in the powder coating industry along with a variety of other projects. A member of the Powder Coating Institute and the PCI sub-committee on application systems, he is also a member of the Northern Illinois Chapter Chemical Coater!y Association. Anderson attended California State University at Fullerton where he majored in Business with a Marketing emphasis. Powder Coating 94 Proceedings 207