This PDF document is a sample chapter from the book Item Code BK93PUB11

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1 This PDF document is a sample chapter from the book Item Code BK93PUB11 here to order on line or complete and print out the order form document. Then mail, fax, or phone in your order. Society of Manufacturing Engineers Copyright 1993

2 THE IMPACT OF PRODUCT DESIGN ON THE FINISHING SYSTEM PRODUCT PLANNING AND DESIGN Until recently, a wall existed in organizations between those involved in product planning and design and those responsible for producing the product. Yet, manufacturing engineers knew only too well the serious impact product design had on manufacturing. This was and is especially true with product finishing. This chapter highlights some of the product design factors which affect the design of the finishing system. Those with the appropriate responsibility should see to it, in the interest of manufacturing efficiency and excellence, that product planning and design always include the contribution of manufacturing engineers and practitioners. Product Size Although the manufacturing engineer can do little to alter the size of the product, he or she should be aware of the impact of size on the finishing system design. A common mistake is to equate the problems of size to those of production rates. A large product does not necessarily dictate a low production rate and the system design factors implicit in it. The greatest impact of product size is on equipment size. Be it a washer, a spray booth or an oven, the equipment must house the product. Further, the This chapter was prepared by John Stauffer, Stauffer Associates (Product Planning and Design); Bob Collins. Metal Finishing Services, Inc. (Product Substrate Material), and Greg Taylor, General Motors of Canada (Production Requirements).

3 building must house the equipment. For very large products, the equipment can become the building with tunnels interconnecting the various process buildings. When choosing whether to increase equipment height or length to accommodate large products, choose the former. In most cases, height increases cost less. It also has little impact on floor space requirements. Very small products require proportionally smaller equipment. The handling method or conveyor system is greatly impacted by product size. For example, a ship is not moved during its construction until it can move by flotation. As soon in the process as automobiles or trucks can roll on their own wheels, they usually do so. For some large products, while movement between process steps is practical, movement within is not. All steps in spray pretreatment can be done in a single chamber with the product stationary. This is done with multiple solution tanks connected to the chamber by valves and timers. Another familiar example of static processing is the batch oven. Extremely small parts do not lend themselves to individual handling and are either mass processed (as in dipping or tumbling). or are automatically handled by mechanical devices such as feeders and positioners when individual care is required. Often, a "chain-on-edge"-type or belt-type conveyor is indicated rather than an overhead monorail. System design focuses on equipment and its layout for best flow. But when product size is large, pay special attention to floor space requirements for marshalling and storage of work in process. Provide adequate space for the required buffer stocks. Product WeightlDensity Most finishing processes involve a time-temperature requirement. As product density increases, pay attention to this. Heat sources in pretreatment tanks and curing and drying ovens are sized in part to replace heat lost to the product. With heavy parts consisting of cast iron or large structural sections, heating requirements increase significantly. Where complete heating of the product is impractical, methods such as radiation curing (or its combination with convection curing) often can be employed, heating the coating rather than the entire product. As with heating. cooling is of increased eoneern with massive parts. Thus, heating methods which do not thoroughly heat the part are of interest. Catalyzed coatings which do not require a thermal cure may solve both heating and cooling problems on massive ware. As product weight increases. personnel safety is a concern. The use of guards, fences. and access restriction can be employed. Material handling equipment must not only safely carry the higher loads, but must operate reliably over time without undue wear. Extremely lightweight parts pose special handling problems in spray pretreatment and spray painting. The parts must be secured to prevent the force of the

4 spray from blowing them off the handling device. Magnets or spring clips are often effective in securing small or lightweight parts. Product Shape The product design must specify the material used for the part and its shape. It must also specify the finish and the area of the part which it must cover. Varied finish or film thickness may be specified for different portions of the product. Surfaces exposed to view and to the effects of sunlight and weather often require a better finish than those which are not. Subsequent handling or processing often requires a special finish or thickness in selected areas. Choice of the finishing process must address these special coverage requirements. Coating application processes divide almost totally into either spraying or immersion. Part configuration often dictates the choice between these two. When overall coating uniformity is of deciding importance, one of the immersion processes will be the choice. The exception would be the case of extreme large product size, although if production volume warrants, very large immersion tanks can be constructed. Spraying is preferred where selective coating (such as the exterior of a part) is required. On occasion. where a primer and a top coat are specified. the primer may be applied by immersion and the top coat applied only to exterior appearance areas by spraying. Certain product features send caution messages to the finisher. Cup-shaped areas are of major concern. These areas may trap pretreatment solutions in the spray zones or dip tanks, carrying solution from one stage to another. These cupped areas waste and contaminate pretreatment solutions and will rarely receive a good quality coating. Further, solution is typically carried into the drying oven and fails to dry completely. In immersion painting, cupped areas are intolerable for obvious reasons. Where cupped areas cannot be avoided, parts should be oriented to prevent collection of solutions or drainage holes should be provided. Deeply recessed areas are difficult to paint by most spray methods. A classic example is the inside bottom of a wastebasket. Air, entrained in the paint spray, collects in the deep recesses and prevents the entrance of sufficient spray. This phenomenon is experienced to a lesser degree in spraying most case goods. If spraying must be employed, "airless" spraying may provide the solution. Electrostatic spraying, while dramatically more efficient than conventional spray processes, exacts special disciplines of the designer. The "Faraday Cage Effect" describes the phenomenon whereby charged paint particles are attracted most strongly to exterior edges and corners and are literally repelled from the interiors of narrow recesses and sharp interior corners. Where exceptional exterior and edge coverage are important, the "Faraday Cage" can be an asset, but where complete overall coverage is required. it can be an enemy. Rede-

5 signing corners with generous radii makes the effect less severe. Parts with narrow grooves. fins, or louvers are also susceptible to the "Faraday Cage" effect. PRODUCT SUBSTRATE MATERIAL Product substrate design should be considered very carefully when a finishing system is being proposed. If you have only one substrate such as cold rolled steel in sheet form, cleaning. pretreatment, coating, and curing could be quite different from a system that must handle aluminum, cold rolled steel, galvanized steel. plastic, and other types of substrate material. Therefore, it is very important at the onset to carefully consider the material substrate and product size and shape during the planning stages of the finishing system. Pretreatment If multiple substrates arc to bc processed, the chemical supplier should be aware of all of the substrates to be used and will be able to makc rccommendations as to the types of chemicals rcquired to properly clean and pretreat the substrates. This will dictate the type of material that should be used for the construction of the prctrcatment machine and such things as number and length of stages. types of rinses and seals, etc. The material and design of the product also dictate the length and number of stages. the type and number of spray nozzles, and the size of pumps. The size of the pump and valving is important because of the impingement factor. The heating of the process solutions will also be a faetor for certain substrates. Some types of plastics must not be heated beyond 140 to 150 F (60 to 65.S C), while metal substrates will withstand greater temperatures. This factor should be discussed with the chemical supplier. Drying The drying operation is often affected by the substrate composition. Drying before finish application is very important. Unless parts are electrocoated or another form of dipping is used, parts must be dry before they cnter the coating equipment. Some plastics cannot he heated above 140 to 150 F (60 to 65.S C) but must be completely dry before coating. Therefore, longer drying times and/or mechanical methods of removing water must often be used on plastic parts. The mechanical methods include blowing the parts with clean air, shaking, and using a rinse to facilitate water removal before drying with heated air. Finish Application The type of coating or application method is often determined by the substrate composition of the part to be coated. While various factors help

6 determine the type of coating to be used, the method of applying these coatings should be investigated to ensure that the proper method is used. There are many ways to apply coatings to substrate. each having its own set of advantages and disadvantages. Investigate each one to determine the best method for your operation. Curing The curing of the coating is also an important element of the total finishing system, and substrate material is critical to curing oven design. As previously mentioned, plastic substrates carry their own set of limitations. The temperature is very important not only to the curing cycle, but also to the effect of the temperature on the substrate. There are numerous methods of curing coatings on the market today, each with its own advantages and disadvantages. Investigate all of the methods available before deciding on one. Because someone else is using a particular method does not mean that it is the best or only way. Many advances have been made in curing coatings in the last five years, and what was "state-of-the-art" a few years ago, may be second best today. Infrared curing is gaining in popularity each year. Vapor curing is making some progress in the industry. Direct-fired convection curing and indirect-fired convection curing are only two of the convection processes used today. High-intensity convection is proving to be a valuable tool in curing some coatings. Combination infrared and convection curing is also being used. All of these methods have a place in the finishing industry and they should be investigated and considered whenever a new system is under consideration. Do not limit yourself to old technology. Ask questions. check with others in the industry who are coating similar products, and seek advice from equipment and material suppliers in the industry. PRODUCTION REQUIREMENTS The reason for designing a new paint finishing system is often a current production requirement for a product. A modification to an existing finishing system is usually due to a future production requirement such as an upcoming product change or an anticipated increase in the quantity being produced. This section discusses three aspects of production requirements during design for modifying a system or installing a new system. These aspects are current and future needs and the expected life of the product. All aspects must be considered together during design because they are interrelated. Current production is usually referred to as the current year and the upcoming year. Current production requirements for a finished product are usually considered first when building a new system or modifying an existing one. Some of the production requirements which must be considered include:

7 Quantity of product required per hour, day, or year. Product size and shape. Type of finish required. Cost allowed to finish the product. Number of production personnel involved. Floor space required for other production operations such as packaging, loading and unloading conveyors, and removal and cleaning of conveyors, fixtures, or hangers. Employee facilities such as offices and lunchrooms. Current and Future Requirements Current requirements will result in the basic design of the system or the planned modificat ion. Future production requirements include any years beyond the upcoming year. The number of future years used in design considerations is limited only by the degree of comfort felt in predicting the requirements. In predicting future requirements, many factors such as current trends in design and anticipated government legislation in areas such as the environment must be considered. It is usually less expensive to design to accommodate a larger product, a faster line speed, or an extra paint color during the initial system installation or major modification than to make many small modifications every year to accommodate every change in production requirements. Once the system is operational and consideration has been given to future requirements. it is usually found that this also provides system flexibility. A major change in product size or production rates may not necessitate modification in a flexible system. even if this change was not anticipated in the original future requirements design. Product Life The product life factor can affect system design considerations in many ways. If a product is going to have a life of one to three years, the system design may accommodate this product very well. Unless the next product is known in advance, however, the finishing system may not accommodate the new product. In a case where the product life is very short, and the future product or products are not known, it is best to ensure that the finishing system has a maximum amount of flexibility built into the initial design. The amount of flexibility which can be built in is usually limited by the amount of capital funding available. If a product is going to last only one to three years, and the future is unknown, it is possible to purchase finishing equipment designed to last less than 10 years and significantly reduce the initial capital investment. It is always best to estimate the life span of the product and use this as one of the major items to be considered in justifying decisions during design.