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Old 12-16-2011, 07:21 PM
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Originally Posted by oldred
Just a note about blasting sheetmetal since this has come up a bunch of times,

It's a misconception that sandblasting heats a part and warps it, the warpage is caused by the peening effect not from heat. I mention this because many parts have been ruined from warpage because steps were taken to prevent heating when heat is not the problem, high pressure does however contribute to warpage but blasting at direct angles is the biggest culprit. About two years ago this subject came up and a couple of us decided to prove or disprove this heat myth so using an IR thermometer we monitored the temperature on an old truck hood while sandblasting the opposite side. Starting with a temperature of 68 to 70 degrees before blasting we expected to see little change as the blasting started but surprisingly it actually dropped about 4 to 6 degrees! I would guess the cooling effect would be from the air flow but whatever the reason the temperature on the backside of the metal being blasted actually cooled slightly even with the metal warping. The real reason fro the warpage is the sand acting as millions of tiny peening hammers that displace metal causing the surface to "grow" and thus to distort, this effect is cumulative and attempting to keep the part cool by whatever means will not prevent it from warping.
Great info!
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