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-   -   Sandblast warping heat MYTH. (http://www.hotrodders.com/forum/sandblast-warping-heat-myth-76938.html)

MARTINSR 12-24-2005 12:44 PM

Sandblast warping heat MYTH.
 
A fellow forum visiter was nice enough to send me this email a long time ago after I posted about heat causing warpage while sandblasting. Very interesting stuff. If you feel the metal while sandblasting, it actually gets cooler from the air being blown on it!!

I deleted his name not knowing if he would want me to post it.

Brian


Sandblast warping
XXX XXXXX here - I read your post / recommendations against sandblasting body parts and I am in complete agreement. However, the mechanism that creates warpage is not the heat associated with friction of the abrasives hitting the metal. Rather it is the compressive forces induced within the sheet of metal by the heavy abrasive particles hitting the metal surface. You will not see this same warping when light abrasives are used because they lack the kinetic energy required to compress the molecules in the steel sheet thus leaving the sheet in the same stress state as it was in when formed by the dies at the factory.
This is a common misunderstanding, yet this behavior is well understood by some industries and is exploited by those who understand it to improve fatigue life of certain parts (like connecting rods for instance - they are often shot peened to redistribute stresses and place the surface of the rods in compression to reduce fatigue crack propagation).
When you blast one side of sheet metal, that side of the sheet metal surface will build compressive stresses and will physically grow - this places the opposite side of the sheet into tension - this will result in the side being blasted to bow up toward the blasting source.
In order for warping to occur due to thermal conditioning - the metal actually has to be heated up to a point where the molecules reach a transition state where they go from one packing arrangement to another (close cubic packed to close hex packed) - picture a six pack of beer held together with the plastic rings - that is stress relieved / annealed steel. Now - take the cans out and stack them so that each row of cans is offset by half a can diameter - this is a tighter pack - although less friendly from a packaging standpoint at the grocery store. This new denser orientation is the orientation that steel molecules take on when raised to a sufficient temperature (depends on carbon content and alloys) . . . if you raise steel to this temperature and then quench it - you lock the close packing orientation into this dense packing structure and this results in shrinkage (and increased hardness / brittleness).
Using this technique it is possible to take a piece of 1013 mild steel, whack a piece off, heat the piece up and then drop it in a bucket of water, use a water cooled grinder to sharpen it . . . and then you can cut the parent piece of metal with it as it is harder and denser in this condition.
Soooooo . . . . that is the difference between warping / shrinkage of metal due to sandblasting (compressive mechanism - localized molecular packing) or due to thermal influences (gross - macroscopic transition of molecules to a higher density due to re-orientation of molecular packing).
With all this knowledge in hand (or mind) . . . you now also know that thermal conditioning is typically used to shrink metal on a localized basis where sandblasting will always cause metal to grow on a localized basis.
Hope this makes sense . . . thanks for the great writeup!
XXX

BarryK 12-24-2005 01:09 PM

I have read bits and pieces of this in different books over the years.
It was an excellent post.

I guess the question is how much of a sandblast beating would it take to see with naked eye?

Kinda like all solvent base products will have some shrinkage but I guess the trick is keep it small enough so you cannot see it?

Of course I'm one of those wackos that sand blast everything but do it very carefully and a lot of times stop at the factory primer or try to.

Even though risky have in the past cleaned up hard to reach spots on fiberglass and will on the vette I'm working on now but know its risky and requires a lot concentration and very low air pressure.

Once again that was a wealth of information!

Barry

shine 12-24-2005 01:51 PM

barry, type 5 acrylic is quite soft. i use it on vettes. it's best to use it on a metal car first then switch it to glass. it works best at about 30 to 40 lbs of pressure with the media chocked down somewhat so you can go slow. i'm doing a 57 now and the third pass on it should have it nice and clean. plus i have to get rid of a ton of bondo on the rear.

MARTINSR 12-24-2005 02:18 PM

Quote:

Originally Posted by BarryK

I guess the question is how much of a sandblast beating would it take to see with naked eye?

Barry

Barry, are you saying you have never seen a panel damaged by sandblasting? Holy cow, they will warp beyond repair! I had a 1928 Buick Sedan door that I was going to make into a display for a marketing conference. It was super straight and only had some super minor surface rust on the jambs and some funky paint all over. I dropped it off at a sandblaster that I had do things for me for years. I gave it to him along with a couple 32 Ford truck doors. They were pretty rough to begin with so I didn't care much about them. But the Buick door was going to be epoxy primed, and painted with a SS urethane. I described how I wanted these doors just DUSTED. I don't car if all the stuff is off, just blast the jambs good and the outside just a dusting and I would sand the rest off.

I returned to find that his son had done the job. The doors were totally ruined. That Buick door was warped about two inches! No kidding, it would pop in and out like one or two inches. I shrank it with a torch and water just so it would maintain a shape. I then covered it with a LOT of filler, cut it flat with an 8" orbital and poly primed it. The end result was pretty nice, but boy was that thing heavy.

Here is just one of the threads I have seen where someone has ruined a car or panel.

http://www.chevelles.com/forums/show...929#post653929

Brian

BarryK 12-24-2005 05:42 PM

Best I can recall, years ago saw a fender that was destroyed by sandblasting.
I know it can be done for sure.

What I was wondering was although I'm blasting very carefully is if I'm doing damage that you can't see with eye or minor enough to block out with primer.

Not that it really matters if it works out but I would assume any blasting would change the metal somehow or somewhat, just a matter of how much.

Shine thanks for the tip, going to try and locate some.

shine 12-25-2005 10:03 AM

T6 is the softest. i may drop down to it for these early vettes i'm doing. i have a 54 next and i'm sure it will be a joy :) it's been repainted 4 or 5 times. anyway i think you'll like the soft plastic.

BarryK 12-25-2005 07:20 PM

Quote:

Originally Posted by shine
T6 is the softest. i may drop down to it for these early vettes i'm doing. i have a 54 next and i'm sure it will be a joy :) it's been repainted 4 or 5 times. anyway i think you'll like the soft plastic.

Now not asuming the name has any meaning! (Shine)
But if you want to come and help I just happen to have clear moonshine and brown moonshine in the garage of course this is used to start hard to start cars.

shine 12-25-2005 07:33 PM

after the navy i can smell whisky and get sick. sure , i should be able to take a day off in oh say 2008 :)

krusty 12-26-2005 07:21 AM

when i started a new job in a panel shop they were doing a 65 mustang, the owner had told me it was in perfect condition no rust no bog(bondo) no dents it was perfect

when i looked at it i could`nt beleive it it was f*'d buckled to the s***house even the frame under the boot warped and started protruding through the skin making high spots in the shape of the frame in the outer skin

i get people telling me all the time there going to get there car blasted and i always tell them not to do it some listen some dont the ones that dont always regret it

they always tell me the blaster said he wont buckel it and he`s going to do it slow and garentee`s he wont buckel it and when they do buckel it they say it had bog(bondo) in it or it was like that before i started

in 14 years of doing this job i have never seen a panel not damaged from blasting unless of course it was an engine bay or something

all i say to the people who want to blast, good luck and dont bring it to me when it`s *******'d

crashtech 12-26-2005 11:09 AM

My first job in 1984 was at a deburring company working as a media blaster. It amazed me how big an effect the media could have on even relatively thick sheetmetal. I was doing some 3'x4' 1/8" thick punched aluminum instrument panels the customer wanted blasted prior to anodizing. Blasting one side would produce dramatic curvature of the panel away from the side being worked, so both sides had to be blasted quite evenly at a fair distance to help the panel regain some semblence of flatness. it was very clear to me then that the media was peening and stretching the surface of the metal!

h0trod389 12-27-2005 01:28 AM

electrolsis?
 
why do people sand blast?
because its easier?
because it gets the metal clean?
sandblasting is good for somethings,
and others shouldnt be touched with it. IMO
now ive never read up on this, because its worked well for me.
i used a plastic 33 gallon garbage can, filled it with a box of arm n hammer washing soda, and water.
i ran 4 electrodes on the perimeter of the can.
all hooked together with jumper cables to the + side of my 12 V battery charger.
and the negative to a rod in the center suspended the
parts i wanted clean into the middle of the can, being sure not to complete the circut.
was so much easier than sand blasting, and alot less mess.
supposed to be environmentally safe, and the results rocked!
if you had a pool, you could do a car body in your back yard!
check out my dash that i left in overnight. and spent 15 min with a wire brush.
http://www.lemanz.net/1930olds/50dash.JPG

krusty 12-27-2005 06:53 AM

to remove the rust off that dash just mix molasass and water in a plastic drum and dip your panels in it for a few days

shine 12-27-2005 07:30 AM

keep in mind electrolysis is the 3rd ingredient in rust. it's the one you cant stop with a coverup. rust lives " in " the metal. what you see on top is rust poop. :)

Lost in NJ 12-27-2005 08:57 AM

There is some very important information left out of all these posts.

What pressure, relative sand density and angle was being used in each case?

I have done 3 cars, yes full cars by sand blasting. I have not been able to find a single panel with warpage. On the other hand, we paid someone to sandblast and the panels were warp to crap.

What is the difference?

Well the guy who we paid blasted at 100lbs and with the sand valve wide open. You have a lot of particles moving at high speed. Lots of energy to move metal.

We run a Model B engine with a compressor head. It runs at idle and we have it set to give about 40 lbs of pressure at the pressure blaster. Then we only crack the sand valve open until we just start to see sand coming out. At these settings you must be patient, but on the other hand I did some thin Frence car panels with NO warpage (I also kept the noozle at and angle to the surface).

Now if you open the valve up more than you are increasing the amount of energy your beating the panel with. So even at 40lbs you might be able to warp a panel if the sand valve is wide open. I never tried this to see what would happen.

So yes if you want to be impatient and blast at high pressures with lots of sand in the stream you are gona warp some panels.

You can safely blast sheet metal if you pay attention and run lower pressures and keep the sand valve minimumly open. This will take a lot longer and is more effective (or faster) at removing rust in the pits than other methods.

So will those who swear that blasting will always warp panels because it has happened to them, speak up and truthfully tell us the whole story. What pressure were you using and how wide open was the sand valve.

MARTINSR 12-27-2005 11:17 AM

This thread is not a "How to sand blast", sure there are ways you "can". This thread is about WHY sandblasting will warp sheet metal.

It is also the reason why I will say over and over until I am blue in the face, DO NOT SAND BLAST SHEET METAL PANELS. I am not speaking to everyone, as you point out YOU have done it. You took the chance, or practiced on something or whatever and can pull it off. I warn others DO NOT SAND BLAST SHEET METAL PANELS because the learning curve is VERY steep. I don't want to see some newbe ruining his car because he reads somewhere that it "can" be done.

You see, as some of these other posts say, you don't cause a "Crap, I messed up the panel I have more work to do" kind of damage. You cause a "HOLY #$^@ I RUINED THE PANEL AND HAVE TO TOSS IT IN THE GARBAGE!" kind of damage.

Brian


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