Part shrinkage after cooling down.

[Rage2371]

I'm a precision grinder. I grind everything you can think of. Over 25 years in the trade. Just started a job in a HVOF company doing their grinding. The problem I'm having is that the guy spraying 420 SS and other grades of stainless and carbide is giving me inconsistent amounts of stock. Sometimes it's .01 to .05 in carbide. That's very time consuming. He States that he doesn't know how much shrinkage he will have. My main question is, is it the parts being heated up and expansion in the parts (that can be calculated) or is it the material being sprayed that has a lot of shrinkage? I believe the shafts get up to about 225 degrees when finished.

[achmadarifudinh]

I'm a precision grinder. I grind everything you can think of. Over 25 years in the trade. Just started a job in a HVOF company doing their grinding. The problem I'm having is that the guy spraying 420 SS and other grades of stainless and carbide is giving me inconsistent amounts of stock. Sometimes it's .01 to .05 in carbide. That's very time consuming. He States that he doesn't know how much shrinkage he will have. My main question is, is it the parts being heated up and expansion in the parts (that can be calculated) or is it the material being sprayed that has a lot of shrinkage? I believe the shafts get up to about 225 degrees when finished.

Are you already do pre-heating before spray the powder??

[sprayloud]

Mainly its the parts thermal expansion that can be calculated or determined experimentally ( measuring dimensions at elevated and room temperatures and spraying next part accordingly).
If its a spray-fuse coating, then things get tricky as coating shrinks quite a bit during fusing.

[Rage2371]

I'm a precision grinder. I grind everything you can think of. Over 25 years in the trade. Just started a job in a HVOF company doing their grinding. The problem I'm having is that the guy spraying 420 SS and other grades of stainless and carbide is giving me inconsistent amounts of stock. Sometimes it's .01 to .05 in carbide. That's very time consuming. He States that he doesn't know how much shrinkage he will have. My main question is, is it the parts being heated up and expansion in the parts (that can be calculated) or is it the material being sprayed that has a lot of shrinkage? I believe the shafts get up to about 225 degrees when finished.

Are you already do pre-heating before spray the powder??

I asked the operator and he said he doesn't for fear of getting contamination on the part before the coating process.

They coat it with a bond coat then carbide.

[Stephen Booth]

Mainly its the parts thermal expansion that can be calculated or determined experimentally ( measuring dimensions at elevated and room temperatures and spraying next part accordingly).
If its a spray-fuse coating, then things get tricky as coating shrinks quite a bit during fusing.

I agree with this above. You can calculate the thermal expansion of the part (not the coating) and allow for this during the spraying, and the inspection of sprayed coating whilst hot.

if you accurately measure the cold part, and the hot part, you should know the shrinkage.

Sounds like you are in maintenance and repair, so maybe a large variety of diameters and parts sizes, and coating thickness, so its bit more challenging, but your coating operator should provide you with the correct information.

[KevGarty]

This used to drive me nuts when Sprayers would do this!.

Let the part cool before final measured and taken from the booth.

It may take 10-15 mins with compressed air blowing on it to get it to a stable temp/size. Its a margin of the time/cost involved if the part needs re-worked due to insufficient coating thickness.