best way to speed up mold polishing
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  1. #1
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    Default best way to speed up mold polishing

    I am making "molds" and perhaps also "extrusion tools" - but for a material that is sort of cross between putty and cement.

    While I can machine flat surfaces very very smooth (like anybody else) getting the convex and concave surfaces ultra smooth requires a lot of post machining work.

    These are not plastic injection molds, but issues of sticking and flow still apply, so similar principles apply.

    I have finishes that we are going to try soon, achieved with just lots of sanding with various grits of wet-dry sandpaper.

    I am finding that polishing stones don't seem to work any better (I have some different ones coming.)

    Do folks have a notion of the highest return-on-time tool to try next? (For example is an NSK with the reciprocating polishing handle the bees knees, or is it something only useful for special applications? Is there some particular flap disk, flap cylinder, etc. that I should likely try?)

    So far these molds are aluminum...

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    Consulting " Mold polishing made easy" by Martin McGrath
    ,he states aluminum cannot stand rough treatment. Gesswein makes a super-soft stone such as the #7002....If you need a higher polish, try yellow diamond with a brush, then felt, then hand rub.
    I got the book from Gesswein.

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    Our toolrooms used to use Diprofil reciprocating polishers, but these were most useful for thin slot features or features with poor access on mould tools. Other than that, it was pretty much stoning then reducing grades of wet & dry paper at 90 degree overlaps on the change.
    A thought just occurred - is this a high pressure or low pressure process for the moulding? If it's low pressure, and it's not a perfectly replicated shiny surface you're after, then you might actually find that a bead blasted surface will give you a good release. Flow shouldn't be affected in any way by this.
    The same wouldn't apply to extrusion

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    I thought this is where "extrude hone" comes in....

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    Check these guys out, and try to get it all done in the machine: XEBEC(R) Deburring Technologies

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    Most mold shops I am familiar with send their molds out for polishing.

    Tom

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    Thanks - new knowlege for me in the replies!

    I will note that this project is for a charity, and it's a kind of experiment (testing refractories to make longer lived combustion chambers in the high carbon gas load of a charcoal domestic cook stove used in the developing world.) - so there is no "send it out to a polishing shop". [Though that's an appealing idea!]

    (Also, if you think "the real answer is to electrify all those places with cheap reliable electricity so people could use electric and induction cooktops" - you'd be right, but don't like hold your breath for this obviously correct solution.)

    I will get that book, and reminded of xebec, seems like it's time to try it. (Getting even the teenist tinest milling marks off takes for frilling ever... if just does that....)

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    digger - how elaborate is an extrude hone setup? (I'm doing ultra small quantities...)

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    Quote Originally Posted by bryan_machine View Post
    digger - how elaborate is an extrude hone setup? (I'm doing ultra small quantities...)
    I have no idea, I just know of the process.

    FWIW "silly putty" is part of the process, they even list it on the website

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    Having spent a few years polishing for water pipe fitting 'dies'.... there really isn't a shortcut. If you want to call it that (a shorcut), it is getting the absolute best finish out of machine before you start the polishing process. Starting with a 220 grit polishing stone (make sure it is a polishing stone so it wears away, not the same as like an india stone), you need to get a uniform surface with this grit. Then move to a 300-320 and get all the tool/stoning marks from the 220, then on to 400 and continue until you can get to a 'coarse' diamond paste, then repeat once or twice with finer diamond pastes until the finish is what you need/want.

    I think the key is to make sure for example when moving from the 220 to 300 stone you get ALL the stone/polish marks out because if you don't, and move onto the 400 grit lets say, it is just going to "highlight" the areas you missed with the 300 stone. So you might end up with a nice finish from a distance, but on closer inspection you have scratches left over. Felt bobs on a air grinder with the paste work well if you have the room.

    McMaster-Carr

    Notice there are different grades. If you don't have experience with them, I would buy one or two of each grade to see what you like / what works best for you.

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    Quote Originally Posted by bryan_machine View Post
    digger - how elaborate is an extrude hone setup? (I'm doing ultra small quantities...)
    One example of extrusion honing is the finishing of the interior of aluminum intake manifolds. The manifold is bolted to a fixture plate and the slurry forced through the interior channels with a specialized piston pump. The slurry is captured at the exit side of the manifold and returned to the sump. The part requirements are a closed channel for the slurry flow ,a constant cross section area passing through the channel, a pressure tight inlet port, and the ability to contain the pressure of the pumped slurry without damage.

    A less expensive alternative would be the Okamoto Aerolap hone machine and abrasive. This would be suited for finishing open mold surfaces in small quantities.

    Or one could fill the mold with a finishing media, close the inlet and exit ports, and bolt the mold to a paint can shaker table. This last method satisfies the requirements of cheap, fast, and do it yourself.
    Last edited by Robert R; 11-12-2019 at 08:39 PM.

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    Quote Originally Posted by bryan_machine View Post
    Thanks - new knowlege for me in the replies!

    I will note that this project is for a charity, and it's a kind of experiment (testing refractories to make longer lived combustion chambers in the high carbon gas load of a charcoal domestic cook stove used in the developing world.) - so there is no "send it out to a polishing shop". [Though that's an appealing idea!]

    (Also, if you think "the real answer is to electrify all those places with cheap reliable electricity so people could use electric and induction cooktops" - you'd be right, but don't like hold your breath for this obviously correct solution.)

    I will get that book, and reminded of xebec, seems like it's time to try it. (Getting even the teenist tinest milling marks off takes for frilling ever... if just does that....)
    Can you pawn this off on students at one of the local colleges or community colleges? Oops, sorry, I meant to say "Can you involve some undergraduate researchers from a local college or community college to evaluate the best method of polishing these parts and what level of polish is the best option economically"

    In all seriousness, it might be worth talking to one of the engineers without borders chapters. I think UW has one, Seattle U might as well.

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    OK, the AFM process won't work because in at least one state, the "mold" is closed on one end. (It will get rammed full, then have that end removed, and then attempt to extrude the molded part in a usable form to go into a curing oven.)

    As for "fill it with fine media and put it in a shaker" .... "hmmmmmmm"

    (Have to make a lid, but that's no big thing...)

    That said I've already orderd some of the other things suggested to read/try etc.

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    Quote Originally Posted by bryan_machine View Post

    (testing refractories to make longer lived combustion chambers in the high carbon gas load of a charcoal domestic cook stove used in the developing world.)
    CO reacts with any iron in the refractory causing it to expand and disrupt the matrix.
    Start be selecting refractory products with a minimum iron content.


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