Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 62 of 331

ditioner or use a rinse additive followed by a deionized water rinse. The following is a representative step-by-step system commonly used in the automotive industry: 1.Tap water prewash, overflowing 20 seconds, ambient. 2. Acid cleaner, 60–90 seconds at 120–140°F. 3.Tap water rinse, overflowing 60 seconds at 130°F. 4. Conditioner or rinse additive, 60 seconds at 130°F. 5.Recirculating deionized water with fresh deionized water in the last rinses, 30 seconds, ambient. It should be noted that the rinse addi tive may also be used in the fresh deionized water supply line. TYPICAL SPRAY WASHER FOR CLEANING PLASTIC The typical washer has the minimum number of stages for proper cleaning and conditioning. Additional stages are sometimes available and can be used for ex- tra rinses or extra cleaning. Washer spray pressure should be in the range 25–30 psi. The conductivity of the recirculating deionized water rinse should be a max- imum of 100 µmho/cm whereas the conductivity of the fresh deionized water should be a maximum of 10 µmho/cm. The parts should be racked in such a man- ner that there is no drainage from one part onto another. This helps to reduce water spotting. From the last rinse stage, the parts will immediately go into an air blow-off. The purpose is to remove all surface droplets to avoid water spotting. Next, the parts should go through a drying oven. The purpose here is to remove mois- ture trapped in the plastic. PROCESS CONTROL For consistent results with minimum paint defects, it is important to maintain constant control of the cleaning and dry-off process. Chemical concentrations should be checked hourly by chemical titration and additions of chemical made immediately as needed. When regular hourly additions are necessary, use of an automatic chemical concentration controller and pump feeder will be helpful. Tap water rinses should be surface overflowed at a rate that turns over the whole volume in the rinse tank every eight hours of production. Also extremely im- portant for the deionized rinse is constant monitoring of the quality of the wa- ter. A conductivity meter should monitor the incoming fresh deionized water, and a meter should monitor the recirculating deionized water in the tank. Parts coming from the dry-off section should be free of water spotting and all visible contamination. It is important that these parts be checked continuous- ly because surface contamination at this point will result in a paint defect. Daily checks should be made for the cleanliness of washer filter screens and for any clogged spray nozzles. In cleaning molded parts, nozzles readily dust. Pe- riodic inspection will prevent localized paint defects on parts due to clogged spray nozzles. 61

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