Can you recommend a pretreatment cleaning process for laser cut parts? The laser cut edge leaves an oxide area that causes adhesion problems. Often the cured powder coating will chip off these edges when they come into contact with each other. We have a four-stage washer using alkaline cleaning and phosphatizing. Most (98%) of our parts are HRP&O steel, with the remaining 2% being CR and aluminum. We also have heard that if we cut with nitrogen on the laser it will reduce the oxidation of the material; however, nitrogen is also 60% more expensive than oxygen. D.B.
Well you've got the first thing done...understanding the problem. Now let's try to fix it. Everything you said is correct, as far as what the laser is doing to the parts and that using a nitrogen shielding gas will eliminate the oxide. Besides changing the gas, you can remove the oxide by grit blasting, sanding, grinding, or wire brushing the edge before pretreating the parts. This can be done automatically using a sanding machine or a machine for grit blasting. If the size or shape of your products do not lend themselves to be automatically cleaned, you may look for a manual method to do the same thing, if your production quantities will allow you enough time.
As far as pretreatment chemicals that can be used to clean this oxide, you will have to look to pickling. This method uses citric or other acids to pickle the metal parts prior to applying the phosphate coating. The only problem you have is that this method won't work in a four-stage washer, unless you eliminate the alkaline cleaner and opt for a combination cleaner/phosphatizer. This can be a problem since a combination cleaner/phosphatizer will not remove as heavy a soil load or deposit as much phosphate as a separate alkaline cleaner and phosphatizer. You should work this out with your pretreatment chemical supplier, but the final process would be pickling-rinse-combination cleaner/phosphatizer-rinse. Remember to always test your product's performance to known standards before you make changes to your process. Otherwise, you and your customers may be very disappointed with the results.