The Ford Barn - View Single Post - media blasting

Kevin in NJ · 02-21-2011, 05:38 PM

There is a misconception that sandblasting causes heat which then causes distortion.

We are all familiar with doing hammer on dolly work causing metal to stretch. This is from the metal being squished out.

When you use too much pressure or too much media in the stream you can beat the surface of the metal too much. This causes the surface of the metal to stretch out. Since the surface you are blasting is being lengthened, that surface will bulge out. Now I have to temper this bulge out with the fact that some surfaces may behave differently.

Now lets compare what happens with heat. The surface where the torch is applied get warmer faster then the far side. This means the torch surface will expand towards the torch while being heated. The difference is heated metal will be shrunk once it is heated beyond the plastic state. So the distortion will be different. On the middle of a convex door it will result in the overall curve being reduced.

So now what temp do you need to cause metal to shrink do to heat? Up into the 500* region gets you to the blue color which is where steel shrinks.

What happens when you have rapidly expanding air? It causes cooling. How do you think you put out a match? You are actually moving so much air that there is not enough heat to continue combustion. So exactly where do you get enough heat to significantly heat the surface of the metal?

The distortion is caused by the material beating and expanding the metal on the surface. This causes the metal to expand and make the metal curve towards the expanded metal.

To prevent distortion you need to limit the pressure and limit the amount of sand in the air stream. I have done 4 whole cars from thin French steel to my very rusty cabriolet and never had a distortion from the blasting.
Run 40 lbs or so and use the sand control to limit the amount of sand in the stream to the point at which you just see media in the air stream. I have tried to distort metal by pointing the stream at some test metal for an extended time and it would not move.

Sorry if I step on any toes with my writing.

If anyone can prove me different I am always open to facts. I have only been able to find that heat is not the cause of distortion.

02-21-2011, 05:38 PM	#31
Kevin in NJ Senior Member Join Date: May 2010 Location: South East NJ Posts: 3,398	Re: media blasting - special considerations? There is a misconception that sandblasting causes heat which then causes distortion. We are all familiar with doing hammer on dolly work causing metal to stretch. This is from the metal being squished out. When you use too much pressure or too much media in the stream you can beat the surface of the metal too much. This causes the surface of the metal to stretch out. Since the surface you are blasting is being lengthened, that surface will bulge out. Now I have to temper this bulge out with the fact that some surfaces may behave differently. Now lets compare what happens with heat. The surface where the torch is applied get warmer faster then the far side. This means the torch surface will expand towards the torch while being heated. The difference is heated metal will be shrunk once it is heated beyond the plastic state. So the distortion will be different. On the middle of a convex door it will result in the overall curve being reduced. So now what temp do you need to cause metal to shrink do to heat? Up into the 500* region gets you to the blue color which is where steel shrinks. What happens when you have rapidly expanding air? It causes cooling. How do you think you put out a match? You are actually moving so much air that there is not enough heat to continue combustion. So exactly where do you get enough heat to significantly heat the surface of the metal? The distortion is caused by the material beating and expanding the metal on the surface. This causes the metal to expand and make the metal curve towards the expanded metal. To prevent distortion you need to limit the pressure and limit the amount of sand in the air stream. I have done 4 whole cars from thin French steel to my very rusty cabriolet and never had a distortion from the blasting. Run 40 lbs or so and use the sand control to limit the amount of sand in the stream to the point at which you just see media in the air stream. I have tried to distort metal by pointing the stream at some test metal for an extended time and it would not move. Sorry if I step on any toes with my writing. If anyone can prove me different I am always open to facts. I have only been able to find that heat is not the cause of distortion.