Newbie here, some sandblasting/finishing guidance needed!
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  1. #1
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    Default Newbie here, some sandblasting/finishing guidance needed!

    Hi,

    I'm making small parts and electronic housings; just recently decided to go from prototypes to small production batches but definitely cannot sell my aluminum parts without a good finish. Parts are quite small and outsourcing these to somebody else to make a decent finish is cost prohibitive as these are supposed to be inexpensive parts selling in volumes. Long story short, I'm looking into opportunity to make a good sand blasting setup in my garage if I can get results I want.

    Today I've tried to sandblast a small part outside of my garage using GreenGrit (crushed glass & some sand I guess), 30-60 grit and gravity feed blaster gun. Of course it is a mess without a cabinet and good dust control system but I simply tried to experiment and see what type of finish I can get with this media. I've used this media because I've read that it can produce some luster-like finish on aluminum and also blend out machining marks.

    Well, blending machining marks was a success, the finish texture is also great, however, when I tried to compare my part to a simple Chinese USB hub sitting on my desk, my part appeared to be much darker.

    Please see pictures attached:

    afterblast.jpg

    greengrit.jpg


    Not quite sure how to achieve the same finish, is it just my media? Should I use something else? Or it should be multi step sand blasting process, i.e. blasting it with 60 grit (crushed glass or aluminum oxide) and then blasting with something else to actually whiten it? Meaning I will have to buy two sandblasting cabinets for production work to avoid switching and potentially contaminating media in the same cabinet? Or could it be probably some chemical treatment to a sandblasted part?

    Any input will be appreciated,

    Thank you!

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    Try a vibratory tumbler with some "brightener" after the blasting. You can do a bunch at a time.

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    Quote Originally Posted by m0rg0th View Post
    Hi,
    .....Parts are quite small and outsourcing these to somebody else to make a decent finish is cost prohibitive as these are supposed to be inexpensive parts selling in volumes......

    Thank you!
    a big mistake in thinking. You have slow filthy process but you believe you can make more money doing them in volume yourself?

    I do that all the time.

    The part your comparing to has been anodized.

    If there really is volume, (many hundreds or thousands) you want to either find someone who can make them complete for pennies... or maybe you want to invest in tooling up better.

    If volume is "a couple hundred"... carry on. bast them & home anodize them. it's not hard to do clear well enough for your needs. If you don't anodize them they will turn fingers black etc...

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    Quote Originally Posted by eKretz View Post
    Try a vibratory tumbler with some "brightener" after the blasting. You can do a bunch at a time.
    Can you point me to a good 'brigthener' solution, please? An URL or probably just a chemical name?
    Thank you!

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    Quote Originally Posted by dsergison View Post
    a big mistake in thinking. You have slow filthy process but you believe you can make more money doing them in volume yourself?

    I do that all the time.
    Ha-ha, point taken!!

    Quote Originally Posted by dsergison View Post
    The part your comparing to has been anodized.
    Hmm... And I guess the anodizing color is likely kinda 'silver'? Or whats the right name for that?

    Quote Originally Posted by dsergison View Post
    If there really is volume, (many hundreds or thousands) you want to either find someone who can make them complete for pennies... or maybe you want to invest in tooling up better.
    If volume is "a couple hundred"... carry on. bast them & home anodize them. it's not hard to do clear well enough for your needs. If you don't anodize them they will turn fingers black etc...
    That's actually the case, sorry for misleading you by saying 'selling in volumes'. The actual volume is around 200pcs every couple of months hence I don't see a point to outsource it right now, I don't think it will be economically sustainable. Also, if there will be no growing demand I may want to switch to another product, which will require a product development at first, which, I guess, is also going to be faster and cheaper if I have most of the tools in my possession, hence I'm trying to understand this finishing process

    Well, I guess I also need to take a look into some quick and dirty anodizing setup for my parts, until I will start selling them in some real volumes...

    Thank you for your feedback!

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    DO NOT use crushed glass or sand as media, especially not without a blasting cabinet. The silica produced will kill you eventually. Blasting creates a lot of very fine dust and should be done carefully. My cabinet is evacuated constantly while blasting using a HEPA filtered vacuum and I wear a P100 respirator as well. Better to be safe than to spend your last days in a hospital gasping for air...

    Aluminum oxide media is not much more expensive than crushed glass and will last a lot longer.

    You may want to try glass bead media at low pressure to get a brighter, smoother, finish. Aluminum is soft and beads will 'peen' the surface easily and produce a nice finish. Don't use glass beads above about 50PSI as you will start to break down the media and produce dust. At low pressures glass beads will last a long time and will produce a really nice surface finish. Whether or not bead blasting is enough to cover tool marks in your application really just depends on how deep your tool marks are, some optimization of the machining process might be necessary.

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    you will not get a bright pearly looking finish by blasting with sand, that is about the worst thing you can do to aluminum parts (especially if you will anodize them later), you'll get slightly better results with glass beads, but that is also a bad option due to beads turning into dust that then gets pounded into the soft aluminum - that is the main reason your parts are gray (silicone contamination) when compared to chinese anodized finish

    best results wiyh blasting will be with stainless steel shot (~0,5-0,8mm diameter), carbide balls will also work, post blasting the parts still may look grayish, but that will be due to al powder embedding, which will clean up to "white" matte surface after even a short caustic etch

    chinese parts are either blasted or special etchant being used to achieve the matte surface texture, and most likely a short bright dip (chemical polishing) to give them a slight luster and then anodized

    tumbling with green pyramids for 1-2 hours + simple caustic etching + anodizing will produce a very nice result aswell

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    Quote Originally Posted by aarongough View Post
    DO NOT use crushed glass or sand as media, especially not without a blasting cabinet. The silica produced will kill you eventually. Blasting creates a lot of very fine dust and should be done carefully. My cabinet is evacuated constantly while blasting using a HEPA filtered vacuum and I wear a P100 respirator as well. Better to be safe than to spend your last days in a hospital gasping for air...
    Yep, I was aware about this and of course I was wearing P100 when I was trying sandblasting outside. I'm also considering purchasing a sandblasting cabinet with some good dust collection system, not only I'm concerned about my health but I also don't want any abrasive contaminants around my Haas!



    Quote Originally Posted by aarongough View Post
    You may want to try glass bead media at low pressure to get a brighter, smoother, finish. Aluminum is soft and beads will 'peen' the surface easily and produce a nice finish. Don't use glass beads above about 50PSI as you will start to break down the media and produce dust. At low pressures glass beads will last a long time and will produce a really nice surface finish. Whether or not bead blasting is enough to cover tool marks in your application really just depends on how deep your tool marks are, some optimization of the machining process might be necessary.
    I read this article How to Finish Aluminum – JDS Labs Blog before I tried to sandblast my part and was kinda inspired I can get a decent finish but it seems like I a) used a wrong media b) used a media at much higher PSI

    Looks like I have to try to follow your advise and change the media to glass beads (machining marks are quite minor already, but some of them cannot be reached by a tumbling media hence another reason why I decided to give sandblasting a try)

    The whole thing with anodizing is quite unexpected but, oh well, looks like I should try it as well.

    Thanks Aaron!

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    Quote Originally Posted by jz79 View Post
    best results wiyh blasting will be with stainless steel shot (~0,5-0,8mm diameter), carbide balls will also work, post blasting the parts still may look grayish, but that will be due to al powder embedding, which will clean up to "white" matte surface after even a short caustic etch
    Hmm, that's strange, I'm not an expert but I read somewhere that one should avoid using steel shots as abrasive media for aluminum because it is too abrasive... Well, anyway, are there any special requirements for a sandblasting cabinet in order for it to be able to handle steel shot media? Like, much stronger walls, or some special paint? Thanks!


    Quote Originally Posted by jz79 View Post
    tumbling with green pyramids for 1-2 hours + simple causticchinese parts are either blasted or special etchant being used to achieve the matte surface texture, and most likely a short bright dip (chemical polishing) to give them a slight luster and then anodized etching + anodizing will produce a very nice result aswell
    I've tried tumbling already, and while it is working OK in general there are some spots which cannot be reached by a tumbling media... Its quite good in corner rounding and I will continue to use it for this particular purpose but for surface finishing (btw one product consist of 7 different aluminum parts, different sizes and shapes) it simply doesn't work that well...

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    Good to hear you've got the safety side figured out! And yes keeping abrasives away from the CNCs is definitely a good idea!

    Things to think about when buying a blast cabinet:
    - Don't go cheap on nozzles. Most machines come with ceramic or steel nozzles and they're pure crap. Changing nozzles or losing abrasive flow to a worn nozzle just wastes time. Spend the extra money and make sure you get a carbide nozzle as they last a very long time!
    - Try looking for a cabinet that has a stationary mounted gun and a foot pedal to activate it. Holding the gun in one hand and manipulating parts with the other hand gets exhausting very quickly if you have any number of parts to do. Getting a stationary gun and a foot pedal allows you to have both hands free to move the parts around, especially important when blasting smaller tricky parts that need to have a nice even finish like your ones!

    You could look into a 'tumble blasting' setup, but from what I've seen they are not as even as blasting by hand, might work ok for your parts though as they're relatively small. That will be pricey, although you might be able to home-brew something inside a regular cabinet if you have a fixed position gun.

    Any decently built cabinet should be able to handle most media, you might find yourself having to change nozzles and so on to match the size of the media. I've run glass beads, aluminum oxide and steel shot in my cheapo POS cabinet with no problems at all. My cabinet has a home-brewed stationary gun in it. I blast parts for finish every day and I'm planning on upgrading my cabinet soon.

    Not sure what your compressor situation is but you'll probably find you need to upgrade once you start blasting more parts. I have a 5HP two stage piston compressor that makes about 20CFM of air and it gets hot pretty quickly when blasting. I bought a 10HP Kaeser screw compressor used that is waiting to be hooked up, looking forward to having it running.

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    Quote Originally Posted by m0rg0th View Post
    Hmm, that's strange, I'm not an expert but I read somewhere that one should avoid using steel shots as abrasive media for aluminum because it is too abrasive... Well, anyway, are there any special requirements for a sandblasting cabinet in order for it to be able to handle steel shot media? Like, much stronger walls, or some special paint? Thanks!




    I've tried tumbling already, and while it is working OK in general there are some spots which cannot be reached by a tumbling media... Its quite good in corner rounding and I will continue to use it for this particular purpose but for surface finishing (btw one product consist of 7 different aluminum parts, different sizes and shapes) it simply doesn't work that well...
    with "stainless steel shot" I mean stainless steel balls - they are non-abrading small round balls, nothing special is needed in regards to the blasting equipment, of course they do remove some aluminum, like from sharp corners, that will turn into a dust and slowly contaminate the blasting media, and gets embedded in the surface of part making them look gray after blasting, but even short etching prior to anodizing will remove it very quickly, it is similar to what Aaron suggested with glass beads - it will peen the surface, not chop it up like sand or other "sharp" media, and higher pressure can be used - blasting will go A LOT quicker when compared to glass beads at 50 psi, and most important - no silicone contamination of the aluminum (glass dust is mostly silicone)

    here are couple pics of what silicone contamination looks like after anodizing, the parts are wet, hence the gloss, but they are deep matte, one pic shows clean part, the other one where there was a lot of dirt embedded (gray patches) - this doesn't happen at all if stainless steel shot is used

    glass-bead-clean-part.jpgglass-bead-dirty-part.jpg

    p.s. and only stainless steel can be used, not regular, that will contaminate aluminum with Fe and cause lots of problems

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    this leaves a beautiful finish on Aluminium perfect for what you are after
    Don


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    Im quite familiar with both wet and dry bead blasting,and all I can think is compared to the Haas churning out a part every few seconds ,it s going to take five minutes to individually blast each bit.There must be quicker more efficient way,vibratory cleaning ,tumbling with pins,ultrasonic slurry ?

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    Quote Originally Posted by D Nelson View Post
    this leaves a beautiful finish on Aluminium perfect for what you are after
    Thanks, Don, I will definitely give it a try!!! Cheers!

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    Quote Originally Posted by aarongough View Post
    I blast parts for finish every day and I'm planning on upgrading my cabinet soon.
    Wow, Aaron, your input regarding sandblasting cabinets is invaluable, thanks A LOT! You've just saved me couple of days (if not weeks!) dedicated to sandblasting cabinets research!

    Quote Originally Posted by aarongough View Post
    Not sure what your compressor situation is but you'll probably find you need to upgrade once you start blasting more parts.
    Don't want to spoil this thread so I've sent you a PM re my compressor situation

    Thank you!

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    It sure if anyone has mentioned this but....glass bead items we have a vapor home with glass bead that uses high volume of water and low air pressure to carry the glass bead slurry.



    YouTube

    Could build your own but we bought a large setup for about $7000. Certainly could buidkbit yourself without all the features for under $1000

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    Quote Originally Posted by m0rg0th View Post
    ....I've tried tumbling already, and while it is working OK in general there are some spots which cannot be reached by a tumbling media... Its quite good in corner rounding and I will continue to use it for this particular purpose but for surface finishing (btw one product consist of 7 different aluminum parts, different sizes and shapes) it simply doesn't work that well...
    With all due respect, that just means the process needs more development. Good surface finishes are the point of most vibratory deburring.

    Agreed, interior corners are a problem. Small media get farther into the corner and the remainder will come out bright. But usually mill tool marks won't be noticed, just a slight color perception difference. If anodised afterward, the caustic dip will dull the corners a bit, enough for an acceptable finish. But don't leave it in the caustic too long or the grain will show after anodizing.

    If tumbling for only 1-2 hrs (my usual tumbling time target, too) the surface finish of the part going in has to be fairly smooth. A corner radius on all tooling is essential. If you can take a swipe over the surface with 300 grit and not see any tool marks after, 1-1.5 hrs in the tumbler should be adequate. That's for a Sweco 3cuft tumbler using 3/4" and smaller green or red tetrahedron shaped polyester media. My experience is that tub and drum type tumblers are slower, so YMMV.

    A vibratory tumbled and anodised finish (black, if acceptable to the customer, otherwise clear) is my default finish for aluminum. Much smoother and sexier than any other, IMO.

    The best part about the process is that the machine finishes 30 or 40 pieces without a person having to do them, one.....at.....a....time..... I can't imagine anything more painful than blasting them individually, trying unsuccessfully to get an even finish/radius on every feature and surface while maintaining dimensions and knowing a machine could be doing it faster, better and healthier and I could be doing something else.

    Tumbling is an art. Took quite a few tries to get just this one process right and I had help. Keep at it, you'll get it. When you've got that down, you can take on anodizing.

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    Tumbled finish on aluminum is ugly. Glass bead however is not....

    All Apple products are glass bead before anodize


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    it would be more correct to say - getting tumbled finish look right on aluminum is much more difficult than blasting with media, but once tumbling and anodizing is sorted (mainly how to etch given material), then tumbling is very repeatable, dry blasting result is very operator mood dependent

    that wet blasting setup looks something worth looking into, but parts being wet can cause corrosion problems with aluminum if they are not dried off fairly quickly - this is from tumbling experience, wet parts were carefully stacked in plastic boxes after tumbling by the operator, with a paper towel in between layers so that layers of parts wouldn't rub, and 2 days later I received them, paper towel between layers still wet, anodized, and nearly all parts had corrosion spots (tiny black deep craters, very noticeable on undyed finish) mostly on surfaces in contact with the soaked paper towel, long etch prior to anodizing didn't help, stripping reanodizing also didn't help, which means the corrosion was quite deep, wiping down parts after tumbling and letting them air dry before packing into boxes solved that issue

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    Tumbling lends to other issues like powder residual of the soap concentrate isn't correct or little to no water in the process.

    Both tumbling and vapor glass bead leave the parts wet and need to be dried.


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