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Lowering the Average Surface Roughness of Hardcoated Aluminum Casting

Q. I am interested in means of lowering the average surface roughness of a hardcoated aluminum casting without losing hardness / wear resistance. Target roughness is less than 10 µm Ra on a 50 µm thick hardcoat.

Kirk Cooper
- Troy, Ohio, USA
2001

A. Your questions concerns both hardness and surface finish. I have a partial answer for you. If you want to improve hardness as well as surface finish, your best method is probably a mass finishing system using steel or SS ball media. This is similar to shot peening, which improves surface hardness, but without the dimpling effect. The only problem is, I am not sure you can achieve that much surface improvement as you want or need using this process. You might want to consider a 2 step operation using a new dry organic media shape first.

AF Kenton
retired business owner - Hatboro, Pennsylvania
2001

A. What is the surface finish before hardcoating? If you can improve the surface before hardcoating, this will probably make your target surface easier to reach afterwards. A good finishing media that you might also consider is a very hard and dense ceramic (140 lb/cu ft). There are some available that have light abrasives for cut and polish. This type of media used in conjunction with a fine powdered abrasive could work very well.

Arnold Walker
- Independence, Kentucky
2001

A. Depending upon the initial Ra of the raw part, there are numerous ceramic, plastic, and synthetic medias capable of reducing your finish below a 10 Ra. I suggest you send parts to a process lab that has the necessary surface reading equipment to run samples with the different types of media and take the Ra readings for you.

Eric M Hurley
- Hamilton, Ohio
2001

A. Kirk,

Typically hard coat anodize will raise the surface finish of anodize 2-1/2 to 3 times the starting Ra. First you need to begin with a very low Ra. I have developed a process that will only increase the Ra by 1-1/2 times. With a very low starting Ra, this process and some of the other suggestions listed you should not have a problem reaching your requirement.

David H [last name deleted for privacy by Editor]
- Georgia, USA
2006