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    Member dharmic's Avatar
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    Default Help a n00b - polishing aluminium moulds?

    First mould making attempt. Aluminium (6061) billet, for hand injection of cold 2 pack polyurethane resin charged with glass powder. Did a trial run with a 3D printer and it plays ok, trial run milling soft pine worked ok but the pine mould tore to bits trying to get the part out.

    Exhibit A - go on, have a laugh (I know I did when I came back to it this morning, in between the "start again..." tears). The bottom half of the mould still needs to be milled.

    Off the mill, finished with 2mm ball nose set with 0.05mm (0.002") scallop height:


    After a couple of hours with SIEG X2 driving an umbilical with die grinders and then a Dremel with assorted grinders, wire wheels and brushes and then polish.


    There are a few drag marks in there because I stuffed the CAM settings on a couple of operations and didn't set the retract plane high enough. The general level of sheen and thumbnail roughness seems ok but all that pitting and distortion is a product of my crappy workmanship and possibly tool selection.

    Give an idea of scale, those holes are 6mm aka 1/4". There's not a lot of room to get sandpaper or stones in there.

    I'm ok with writing this one off as a learning experience. It looks generally flattish but with too many little gouges, I'm guessing from one of the wire wheels. I don't really want to oversize the part so chasing those gouges is probably going to wreck the mould.

    Help please
    I'd like the next attempt to be a little better!

    Any advice on how fine I should be shooting that final mill pass and what common hardware shop stuff I can use to clean up the tooling marks without pitting the soft aluminium surface?

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    Default Re: Help a n00b - polishing aluminium moulds?

    Try mothers polish and scotch brite

    Sent from my Z958 using Tapatalk



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    Default Re: Help a n00b - polishing aluminium moulds?

    Sorry, should have updated - I changed tack and bought a little sandblasting cabinet. Really like the matte finish and it holds silicone oil in place well, preventing it from beading away from the metal.

    Thanks, though, jchal3 - I'll bear in mind those two if I do another part that wants a bright finish.



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    Default Re: Help a n00b - polishing aluminium moulds?

    I have achieved a mirror finish with the two. Just an FYI it works. Glad you worked out your issue.

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    Default Re: Help a n00b - polishing aluminium moulds?

    Looks to me like your scallop height is much more than 0.05mm. I personally prefer to use a scallop height of 0.01-0.02mm. Takes forever to mill but I prefer the mill to do the work over me having to do a lot of sanding. Also, I stopped using any sort of rotary power tools to polish molds as pitting is almost unavoidable with them. I suggest to completely sand/polish by hand unless you can afford a reciprocal mold polishing tool (1000$+).
    Just looking a the picture, it looks like you rounded off the edges of the parting line. This will cause a lot of flash and therefore make the mold unusable.



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    Default Re: Help a n00b - polishing aluminium moulds?

    Thanks cordvision. I did haul the other half of the mould back to 0.015 and it made a huge difference. I get it about the power tools - I just don't have the patience vs the finish requirements. And yes, the output from this mould does need some hand finishing around the parting line because yes, I managed to round off the plane. Bugger.

    A bunch of valuable lessons learned for the next iteration of the mould, thanks guys.



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    Default Re: Help a n00b - polishing aluminium moulds?

    In your blasting cabinet, you want glass bead. FYI, just in case you bought one at one of the chinese importers, such as harbor fright, make sure you get actual bead. I bought some there about ten years ago, and it was real bead. Got some in the last year or so, and the same part number is now crushed glass, even though the description says bead. It leaves a bad finish, and even some of the glass shards get embedded into the aluminum.

    I can tell you a few things from your photo, though. First, don't rush. It looks like you were rushing. Expect a mold like that to take over 12 hours to polish correctly without losing your corners. If you touch it with a wire wheel, then you are basically saying you don't care what it looks like, so you might as well forget polishing it.

    So, how do you do it without rushing? Do NOT push hard. Most folks rush sandpaper, meaning they push real hard because it gets it done faster. If you want it to look good and keep dimensions, the lighter you press, the better your part will come out--it just takes longer. The reason is because when you push hard, the sand paper doesn't only contact the "high spots" that you want to remove. Also, you want to start with fine sandpaper. I'd only consider something as rough as 220 if the machine job was real crummy.

    Also, a 2 thou scallop is ENORMOUS for a mold! I use 3-5 thou stepovers, depending on the job, or a max scallop of under half a thou. Yes, the finish pass takes most of a day. But then you can remove those scallops with a few minutes and some 600 grit paper. It all depends if you'd rather have your machine do the work or do it yourself.

    Oh, yeah...don't polish the parting line! Just machine it to seal well.



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    Default Re: Help a n00b - polishing aluminium moulds?

    Thank you tenaja. You're absolutely right - I did rush the heck out of it. I went in with die grinders and wire wheels and deformed the shape and dug holes and scratched it and, no surprise, ended up with an apprentice's first piece if that apprentice was left to his own devices and not supervised throughout.

    The sandblasting cabinet I bought was not bead, it was garnet sand. For the other half of the mould I seriously cut back on the stepover / scallop height to make life easier. A couple of spots with obvious scallop/layer lines still copped it with a die grinder on the rotary tool but a lot more gently this time. The blaster took care of the rest. I was going to start polishing it from here but got lazy and just loaded the mould with silicone oil as a release agent and it came out with a texture on the surface that I quite like.





    So, for this time around, I'm calling it done.

    Next time, though, I now know: much finer on the machining, much finer on the sandpaper, a light touch and leave the Dremel hanging on the wall. Thanks!



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    Default Re: Help a n00b - polishing aluminium moulds?

    I've been working on some samples and the only other thought I have would be to leave .025" axial allowance on the parting line, so after blasting, you could fly cut the parting plane smooth.



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    Default Re: Help a n00b - polishing aluminium moulds?

    That's not a bad idea - of course, in this case the parting plane isn't a plane so it needs to be re-mounted and re-finished with an itty bitty ball mill or something. The flashing off this mould isn't too big a drama, only making a few items so a box cutter knocks the flashing off and it's ok.

    Still. If I do it again I won't bother polishing. And now that I have a touch probe on the machine I can relocate the pieces pretty well and easily, so the sequence of mill with allowance, sand blast to remove tooling marks, then re-load and finish the parting surfaces in the router might work well.



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Help a n00b - polishing aluminium moulds?

Help a n00b - polishing aluminium moulds?