Hash Marks
We recently changed to a fast drying enamel. Now, we get definite striations on the painted parts. What is causing this problem and what can we do about it?
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Q. We paint sheet metal parts in our plant. After cleaning and iron phosphatizing, they are painted on a conveyorized dipping line with a one-coat enamel. We recently changed to a fast drying enamel. Now, we get definite striations on the painted parts. These are objectionable to our customers. What is causing this problem and what can we do about it? B. M.
A. Your painted parts are suffering from hash marks. They are the result of differences in film thickness and are caused by a jerky motion of the conveyor chain during part withdrawal from the dip tank. The jerky motion of the conveyor chain would not be as critical on a spray line. This condition worsens when apply a fast drying dipping enamel. It can be eliminated or minimized by adjusting your conveyor chain.
As its parts wear, a conveyor chain stretches and loosens causing uneven motion along its length. The quick fix is to shorten the chain by taking out links or adjusting idler pulleys. However it may be necessary to replace worn parts. I suggest you seek professional help from you conveyor supplier. If he is no longer available, contact suppliers listed under Conveyors, overhead chain on page 283 of the PRODUCTS FINISHING DIRECTORY AND TECHNOLOGY GUIDE or online at www.pfonline.com/suppliers.html.
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