Masking Plugs: Cost Effective?

Q. We powder coat automotive braces that have two weld nuts per brace. The weld nuts do not have any Teflon coating because the attaching bolts, which have an adhesive patch,?are hand-started, and the assembly plant does not want any interference during installation.

We don’t plug the threads because of the cost of the plugs and the manpower required to insert and remove them. We’ve tried to position the parts so they get the smallest amount of powder spray at the nut area, but it’s not 100% effective. We’ve also designed racks that try to mask the threads with pins, but because the contact point now becomes the nut, we pull the powder?right to the nut and the pins can’t be effective because they still allow?some powder to enter the thread. This has resulted in having to re-chase 100% of the threads. Is there an alternative coating that can be used in the weld nut that’s not detrimental to the assembly and will absolutely not allow powder to stick to the threads? Are there any other alternatives to masking the threads with plugs that would be less costly? M. H.

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A. Let me try to understand what you are saying: It is cheaper to chase the threads after coating than to plug them with masking plugs before coating. That defies logic and convention. People use masking plugs to eliminate the need to remove the coating from the threads all the time. You can even design an automatic insertion and removal device at a separate work station to perform this activity. This would be preferable to chasing the threads afterward.

Using the hanger to mask the intended surface works great, unless we are talking about threads. Threads will be masked at their peak by the hanger pin but coated at their root (at least the first thread would be coated). So this method can be problematic if you cannot tolerate any coating, even if it is just the first thread root.

Positioning the part to minimize the coating within the threads is a good idea, but will not be 100% effective, as you have already noticed.

Using a tapered masking plug is the preferred method, as they are normally inexpensive and can be reused many times before you have to toss them away. These devices are 100% effective if you choose the correct design and use them properly.
Using sacrificial materials, like high temperature grease, to mask the threads can lead to all sorts of problems. The grease will get viscous (melt) under the cure oven temperatures and drip onto the powder coated surface during cure. This will cause all sorts of problems and is not recommended.

This is one occasion where time-tested methods are the best solution. It’s always exciting being unconventional, but also risky. This is one case where the risk is unacceptable.