Dirt in the grit blasting machine

[Aware]

Hello !

The amount of information on this forum is really huge so hopefully I can get some help here.

In my company we use grit blasting before applying aluminum paint and we have gotten a lot of adhesion problem lately.

I taught right away about the grit blasting process.
The machine was extremely dirty, fill up with fine black dust everywhere.
During 10 years this machine has taken the air from the factory without any filters and has never been cleaned.

We have cleaned it very well (except the pipes and recycling system) and changed all the grit.

Unfortunately after a first test, I tried to wipe the part and the machine with isopropanol: The wipe got dirty again (a little better than before).
Some people from my company say it´s normal to have this dark dust...


Can the process be such that I get a clean part when I take it out the machine (i.e. if I wipe the part with isopropanol the wipe stays white)?

Is it normal to have some pollution in this process (if so, what´s the limit)?

Note that the part is completely clean when it enter the machine.

Roger

Edit: I have requirements to clean the part only with clean pressurized air after the process, however it does not take this dust/dirt away.

[wilfred1963]

A good place to start is to install some filters for the air lines as these can collect oil and other contaminates from the compressor and try replacing the lines from the filters forward if you chose to install filters, I would if for no other reason than to remove that from the list of possible contaminates.

[Gordon]

Hi Roger

to the Surface Engineering Forum.

The key to a successful coating is in the substrate preparation (pun intended )

If you are still picking up dirt from your freshly grit blasted surfaces, then this is probably the cause of your adhesion problems or at least a big contributory factor. The surface should be clean, although inevitably you will get some level of retained/in-bedded grit particles.

I think you are on the right track to solving your problems by paying attention to this vital preparation process. Get the foundations right to start with, as every thing done afterwards is a waste of time/money if its wrong.

wilfred1963 suggestion is a must do.

The blasting process equipment should be dedicated to this type of work only. All too often I hear horror stories of people using the same equipment to do odd cleaning jobs, leaving all sorts of crap behind only to contaminate what follows.

Make checking the quality of the grit blasted surfaces a regular and important part of your QA.

[Aware]

Hi and thanks for your answers.

I completely agree that surface preparation is extremely important.

I have samples that will be analized and I am quite sure to find oil and many others polluants in this dust.
The machine will be plugged to the clean pressurized air system and a filter for oil and water will also be added.


Nowaday, we are using a roughness tester to check the process.

I´d also like to test regurlarly the amount of embended grit.
I am using 150 mesh aluminum oxide, what kind of microscope do I need to perform this test ?

However, I read in some publications that the result also depends of the shape of the parts.
Is it good enough to use test plates (flat) to control the process if we are grit blasting shafts ?
(I believe it is necessary to have some correlation between the test plates and the internal and external surfaces.)

Do you know any others tests that are normally performed to control this process ?
My colleag from plasma spray does several bonding tests but I understood this is quite different for the paint.


Regards.

[MichiganMan]

Abrasive blasting is an inherently dirty process, but if application parameters are controlled properly, then blowing down with compressed air should be able to remove the contaminates after processing. I have a few thoughts for you.

As others have mentioned oil filters are an absolute necessity in the air system. If your blast air has even trace quantities of oil in it, then the result tends to be dust clinging to the part that you can only push around the surface when blowing down, and never get the part clean.

Air dryers are also critical to the system (particulary if the part itself is a ferrous alloy). Are you testing the system air for contaminates prior to blasting?

150 is a relatively fine grade of Al2O3. Are you achieving adequate surface profile? Most aluminum paints that we have worked with in the past require a profile of 2 mils (0.002 in.). If you find that the paint manufacturer calls for a greater profile than you are producing, then go to a coarser grit. If your part has irregular geometry, then I would suggest taking profile measurements at points on the part surface that are most difficult to abrasive blast.

It may also be wise to oil sheen test your reclaim abrasive since this problem is repetative.

[Aware]

I agree that we should be able to remove most of the dust/contaminates with air pressure if the system is free of oil, dirt and moist.
And we do have many ferrous alloy but no water filter.

We are not allowed to use coarser grit than 150 (requirements) and we get a roughness around Ra=2 microns.
Indeed, the Al paint is about 0.002 in (Sermetel W).

Anyway, I knew our biggest problem comes from the air contamination and I´m glad to have a confirmation from you.
Once we´ll get those filters and a clean machine, I believe most of adhesion problems will disapear.

[MichiganMan]

I probably wouldn't trust the coating on 2 micron roughness if it calls for 0.002". There is too much risk of disbondment. The contamination is likely your biggest problem, but you may continue to struggle if you can't get an adequate profile. Consider blasting at higher pressures and/or for a longer period of time to see if deeper profile develops. It is as much art as science to abrasive blast and a little trial and error may be in order if you are still struggling after you get adequate filtration.

[Aware]

Thanks that's good to know !
I will perfom some tests to increase the roughness but what kind of Ra is the best for such a coating ?

After all the manufacturing operations, would I get a better result if I release the stress of the surface by heating the part up until 150°c for an hour (for example) prior to grit blasting ?