Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 58 of 331

pretreatment PRETREATMENT FOR THE PAINTING OF PLASTICS AND COMPOSITE MATERIALS BY CHUCK SOULE DUBOIS U.S.A. DIV. OF DIVERSEY CORP., CINCINNATI Before one can successfully prepare plastics or composites for painting, there are various factors that must be considered. The identification and isolation of key variables is essential. Because the most effective way to adequately prepare plas- tics and composites may vary from substrate to substrate, it is necessary to con- duct a system overview. This process will provide the basic knowledge of how the system works and what can be expected from that system during operation. For clarity, the system is defined as the mechanical and chemical elements utilized in the preparation of plastics and composite materials prior to painting. The word substrate will refer to various types of plastics or composites, or both. SYSTEM SURVEY The first area for consideration is the system survey. It is vitally important that during the initial stages of the system design you become familiar with the vari- ables that have the greatest impact on the substrate preparation prior to paint- ing. Without a working knowledge of the process and its associated variables, it becomes almost impossible to select the proper pretreatment system. To aid in the performance of the system survey, it is recommended that a flow diagram be generated to detail the variables and their related impacts. A flow diagram is a systematic way of tracking a part through its various process stages. Once you have the basic flow of the process, you can begin collecting more de- tailed information, such as types of soils, oven temperatures, line speeds, paint flash times, and part spacing. These are all key variables in the successful im- plementation of a pretreatment system. The system survey gives a broad understanding of how the parts are handled and produced. The next step is to begin narrowing your focus to more specific elements in the system; however, to successfully implement a pretreatment sys- tem, the focus must not be limited to either the mechanical or the chemical el- ement. Operational success can only be achieved by utilizing the effective syn- ergism between both the mechanical and chemical elements. For clarity, the me- chanical elements refer to the power washer, the conveyor, the ovens, both the dry- off and the paint ovens, as well as the paint booths. The reason for understand- ing the mechanical system is simple. If you closely examine a spray washer, you will find that each stage has at least 50 variables that either directly or indirect- ly affect the outcome of the part being produced. These variables also have a large impact on the effectiveness of the chemicals being used. The spray washer is a vital link in the successful implementation of the chemical cleaning package. How and why is the washer so important? The an- swer can be found in the purpose for the washer, which provides not only a means of application, but also a means of control. To effectively clean a part, it is important to monitor closely time, temperature, concentration, pH, over- flow rates, and pressures. If these variables are closely monitored, effective management of the pretreatment system can be realized. 57

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