Crimping Coated Parts
Q. I have a sheet metal part (cylindrical ring) that is 1 ½ inches in diameter and ¼ inch high. The sheet metal is 0.03 inch in thickness. After powder coating, the edge of the part needs to be crimped about 0.008 inch by 85°. A problem we have is that the powder coating cracks after the crimping.
My coater had many trials, but it was still not stable, with about 50% of the parts cracked after crimping. I would like to know if there is a powder coating suitable for crimp or bending operations afterward, or if it is possible that our process or parameters were not right. If the powder coating really cannot meet such requirements (crimping or bending without surface cracking), what other surface treatments would you recommend? Your help would be greatly appreciated. D.W.
A. Powder coatings have been successfully formulated and applied to many metal substrates that are formed afterward. The radii of these bends can be very small, up to a “zero T bend” where the metal is bent back completely onto itself. That doesn’t mean that all powder coating formulations and all powder coating job shops have the capability to provide this functionality.
Your success is directly tied to the following issues:
Powder Formulation: You first have to specify, test, and procure a powder formula that has been created with excellent flexibility properties. Numerous powder formulators can provide such a coating, but only after you have discussed all of your coating performance requirements in great detail with them. This requires a well-written and cogent coating specification detailing all your requirements and listing methods of application and curing. You will then collect several samples of coatings from some of the 63+ suppliers of powder coatings in the North American market. Next, you must verify their submissions by testing on your product to ensure success! Often, the powder formulator may perform this testing for you under ideal laboratory conditions. Select only those coatings that have proven successful and ensure that your custom coater has been informed where to purchase the approved material.
Powder Application: Even if you have the best forming powder formulation, it will never work up to your expectations if it is not applied properly. For crimping and forming applications, it is imperative that the powder adhere properly to the substrate. For this to happen, the substrate must be scrupulously clean. The substrate must also have some surface roughness (tooth) for the coating to bond to. This can be accomplished by either mechanical or chemical means. Mechanical means include media blasting, sanding or wire brushing. Chemical means include iron phosphate, chromate or non-chrome sealers, other conversion coatings and etchants.
After cleaning, the powder must be applied to the correct film thickness. Too thin and the coating will not entirely cover the surface. Too thick and the coating will become brittle and less flexible. Finally, the coating must be fully cured. Uncured or over-cured powder coatings can be very brittle, making post forming impossible.
Crimping Tooling: Some care with the crimping tooling is also required for success. The tooling can’t gouge the coated surface and must have clearance to allow for the coating to move with the metal.
The speed of the crimping operation can also affect the coating. For instance, slower die speeds will allow the coating to bend without mechanical shock that can cause cracking. Often, applying a little heat during the bending operation can also help. Die lubricants can also help keep the coating intact during the bending and crimping
Powder coatings are some of the best materials to post-form. The appliance industry has post-formed powder-coated blanks for years. The dryer door insert has been post-formed after powder coating with a draw up to 6 inches by several appliance companies. Therefore, I expect that your crimping application can work successfully, as well. Just take some care with which powder you use, how it is applied, and how the part is crimped afterward, and you will enjoy similar success.
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