Metal Finishing Guide Book

2012 Organic Finishing Guidebook Issue

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Page 61 of 331

of reaction injection molding (RIM), internal mold release (IMR), reinforced RIM, sheet molding compound (SMC), thermoplastic olefin (TPO), and vari- ous other thermoplastic alloys. In these operations, the most common conta- minants are mold release, sanding dust, shop soils, material handling soils, machining soils, and fingerprints. Mold releases are of two basic types — external or internal. External mold re- leases are usually either soaps or waxes. Water-soluble, soap-based mold releas- es are the easiest to clean and are the least harmful to paint adhesion. Some wax and oil mold releases are not water soluble and may require higher tempera- tures for cleaning in a spray washer. Poor paint adhesion can result from too much mold release being left on the substrate. This typically happens on corners or sharp areas of the part, where it is difficult to release from the mold. One may suspect this when poor paint ad- hesion occurs in the same location on many parts. Also, plugged spray nozzles in the power washer can sometimes cause this same result. IMR is a compound in the molded plastic that surfaces in the molding op- eration to help release the part from the mold. These are proprietary com- pounds of the plastic supplier. IMR-RIM and most SMCs contain IMR. With IMR, the need for time-consuming applications of external mold release is de- creased significantly, and molding rates have increased; however, with the in- troduction of IMR-RIM, modification of the cleaning chemicals may be re- quired to achieve paintable parts. CLEANING COMPOUNDS To ensure that contaminants, such as mold release compounds, shop soils and handling soils, will not hurt the paintability, the substrate must be cleaned pri- or to painting. Solvents and various aqueous cleaning chemicals have been eval- uated for the cleaning of molded plastic substrates. Solvent cleaning, however, is not strongly pursued because of environmental limitations with respect to use and disposal. In aqueous cleaning, the primary process makes use of acidic cleaners. There are four general categories for plastics cleaners: acidic cleaner, phos- phated; acidic cleaner, reduced phosphate level; acidic cleaner, nonphosphated; and alkaline cleaner. The advantages and limitations of each are given in Table I. The bottom line is that an effective aqueous cleaning chemical will contain pro- prietary ingredients suited for the cleaning of common contaminants as well as shop and handling soils. PREPAINTED CONDITIONING OF NONPRIMED PLASTIC After the cleaning step, a further conditioning of the substrate is sometimes necessary to aid in the paintability of the cleaned part. This conditioning step is especially helpful when paint defects are due primarily to water spots and pin- holes. The conditioning chemical is an acidic material that is specially formulated to further prepare the substrate prior to a final rinse of deionized water. As an alternative, a rinse additive may also be used to minimize "spotting" problems. THE OPTIMUM CLEANING SYSTEM The optimum cleaning system for virgin plastic that is to be painted is a two-step system. First, clean off the contaminants — the significant contaminant is usu- ally mold release. Second, condition the plastic for good paintability with a con- 60

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