Where the
world gathers for metal finishing
Q&As since 1989


-----

Powder coating peels off, when a bolt is tightened on a punched hole in sheet metal door

We are a manufacturer of sheet metal enclosures,cabinets et all,
We are facing a problem in which the powder coating near the punched hole peels off while a bolt or screw is tightened during assembly of the components, the adhesion at other places (center of the door) seem to be good as they endure the scratch test,what are the possible ways to prevent or rectify the problem from occuring ?

Vaikunth Bashyam
sheet metal fabricator - Coimbatore, Tamilnadu, India
2006

---

First of two simultaneous responses --

Just based on the description I might ask what your final rinse is. One possibility(there are many, but you need to give up a little bit more info on your process) is that you are not using purified water, like R/O or DI, in your final rinse tank. You can have fairly good adhesion in the middle of the product becauwse the solids in the final rinse water will run to the bottom edge of the part, and then the dryoff oven will bake them on. Even if you are not able to see it, there is now a contaminate between your substrate and paint which can cause the problems that you mentioned, making the assumption that the holes are punched close to the edge(i.e. holes for hinges, door latches, etc.) and you are hanging the parts holes down for drainage.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
2006

---

Second of two simultaneous responses -- 2006

Poor adhesion is usually due to one or both of the following:
Dirty substrate.
Make sure the metal is clean.
Uncured powder.
Ensure that the oven is at the correct temperature and that the components are in the oven long enough.
If you are zinc phosphating, this can also be a problem area.
Try using the crosshatch test; we use this on every job as one of the means to test adhesion.

Dale Windridge
coatings shop - Dane Valley Road, Broadstairs, Kent, United Kingdom