Cleaning Plastics

Ask an Expert From: Products Finishing,

Posted on: 4/1/2004

Question: We are a third tier automotive supplier for plastic parts like grills, hubcaps and other trim parts.

Question:

We are a third tier automotive supplier for plastic parts like grills, hubcaps and other trim parts. The parts are molded out of ABSand/or ABS/PC plastic. Ninety percent of what we do is paint, the rest is a chrome plate process. Since we don’t mold or chrome the parts in house, we are wondering if there is a way to wash the parts prior to our painting process. For more than 11 years we have just used DI air and blown the parts off one at a time just prior to painting it. Now it seems our customers want to know why we don’t wash the parts. Does anyone else wash plastic parts for this type of application? My experience in plastic is in a case where it would need washing, that hand wiping it with denatured alcohol has worked but that is not really feasible for large runs of parts. R.H.

Answer:

Part of the need to provide a pretreatment would be due to the type of paint you are using and the performance requirements it is expected to meet. A more “forgiving” paint like a solvent-based system, would require less pretreatment than water-based or powder.

Also, I am not sure how close you are to meeting the performance requirements of your coating. If you are meeting the specifications of your customers, I am not sure why they are asking you to add additional steps to your manufacturing process that they will wind up paying for in the end. If you are falling short of the customer expectations, that would likely justify the investment into additional pretreatment equipment. You mentioned that currently, in some situations, you do experience the need to “pretreat” your parts by hand wiping with alcohol, which indicates that you must have some means to differentiate a clean from a dirty part, and the effect that has on your overall paint process.

The deionized air you are currently blowing your parts off with will do no more than remove particulate contamination from the surfaces. To actually remove surface contamination from processing (for instance, mold release agents), it is more common to use an acidic cleaning solution that also contains surfactants. The acid along with the surfactant is usually sufficient to remove many processing agents from the surface of the plastics. The acid is usually phosphoric acid and most often, this step is performed using a spray washer. Following the cleaning step will be a rinse. Usually a tap water rinse is sufficient if you have at least moderately good water quality. The final step in the pretreatment is usually a rinse that incorporates a rinse aid. The purpose of the rinse aid is to eliminate water beading and spotting by allowing the water to shed uniformly off the otherwise hydrophobic surface. The rinse aid may be reactive such that it actually turns the surface of the plastic from hydrophobic to hydrophilic. All of these steps could be automated with a spray wash system. If a larger-scale capital investment is not feasible, it may be possible to find the right chemistry in a spray-wand type of system that could allow separate steps to the process. I would suggest looking into the Products Finishing web site for a list of potential suppliers (www.pfonline.com/suppliers.html). Select Cleaning and Pretreatment Chemicals then Cleaning Chemicals, aqueous for a listing.

 



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