Mould polishing and texture advice

[Robin Hewitt]

I am planning to make some moulds using Alumec 89, it's like that 7075 T6 the Yanks rave about but takes a deeper temper, or so I am told.

Right now I am all theory and no experience. I thought about bead blasting to get that sparked look. I think sand blasting is a no-no on iron because it is the perfect key for rust unless you bead blast afterwards, I find sand blasted iron is rusty before I get it home even on a hot sunny day. I presume you could protect your shut-offs while blasting and cut the gates after maybe?

OTOH Alumec lasts longer if you anodise which includes a dunk in dilute nitric to remove any extant oxidation before it goes in the sulphuric. Anodising could negate any previous texture. I actually have hopes it might smooth the surface somewhat in those hard to reach spots. Bit worried about whether the ejector pin sleeves are going to fit if I can't block their holes off while it's in the tanks, shellac?

Regarding someone's earlier comment, I believe the draft angle is supposed to protect the finish on ejection.

I did buy a die grinding pencil. They are air driven at enormous rpm and come with a 3mm collet. Haven't played with it yet, not really sure what goes in it, the bottle of oil seems to have emptied itself and the instructions have dire warnings about what happens without.
Whatever, a sparked finish is now passe, the Law of Bling says polish the crap out of it

[Jonathan]

Anodising could negate any previous texture. I actually have hopes it might smooth the surface somewhat in those hard to reach spots. Bit worried about whether the ejector pin sleeves are going to fit if I can't block their holes off while it's in the tanks, shellac?

From the anodising I've done I can confirm that you get the same finish after anodising as before. The parts are always slightly smaller - it's hard to say how much but bearings wont have the proper fit after anodising. That's easily solved by pressing some nylon into the bores and the same tactic works well for protecting threads. People block areas of a part off so they can anodise them a different colour to the rest, so it can't be that hard. Probably just need something resistant to sulphuric acid and sodium hydroxide.