Thanks Thanks:  0
Likes Likes:  0
Needs Pictures Needs Pictures:  0
Picture(s) thanks Picture(s) thanks:  0
Results 1 to 12 of 12
  1. #1
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default Pitting and pinholes in cast

    I was wondering about how to minimise pitting in a cast. A recent project of mine ended up with a fair amount of tiny holes, almost like freckles or skin pours.

    What causes it?
    How can you prevent it?
    How do you clean it up?

    I'll try and get a picture posted if my phone cooperates.

  2. #2
    Join Date
    Dec 2005
    Location
    South Australia
    Posts
    1,358

    Default

    Some more information may be helpful, e.g. type of metal, you casting set up, procedure you used.

  3. #3
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    Quote Originally Posted by China View Post
    Some more information may be helpful, e.g. type of metal, you casting set up, procedure you used.
    Of course. I'm working with bronze, and doing lost wax casting. I use ceramic shells rather than investment. The pieces I cast tend to be quite small - 3kg or less.

  4. #4
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    As you can see from the pictures, the surface of the metal is very rough. They were all cast at the same time, and while the flaws are present on all, it's worse on a few and hardly noticeable on others. That makes me suspect it is a temperature issue. I cast them all at the same time and I'm wondering if it was the metal cooling while I was pouring that caused this somehow.



    IMG-20210617-WA0000.jpgIMG_20210620_134602_964.jpg

  5. #5
    Join Date
    Dec 2005
    Location
    South Australia
    Posts
    1,358

    Default

    That is not a really bad cast, however I would start by making sure you moulds are are absolutely dry the tiniest amount of steam will cause this, also that there are no impurities in your metal, impurities can burn off and leave voids, are you using a flux

  6. #6
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    Quote Originally Posted by China View Post
    That is not a really bad cast, however I would start by making sure you moulds are are absolutely dry the tiniest amount of steam will cause this, also that there are no impurities in your metal, impurities can burn off and leave voids, are you using a flux
    I wasn't using a flux. The shells should have been dry they had been baked in the furnace, but they had been sat on my shelf in my studio for a month or so. I'm not sure if any condensation or moisture could have built up during that time. I was careful to not get any debris in the shells, covering the opening up with tinfoil and giving them a blast of air before use. The ingots seemed pure to me too. I didn't have any slag floating on top or anything.

    These frogs need to be recast as they are missing their back toes. An issue with venting that seems easy enough to fix. My gut tells me that the pitting is an issue with temperature. I don't think my metal was hot enough when I poured. You can see in the picture the front centre frog looks nice and smooth, while the first rear row frog is quite rough. I can't be sure, but I'm guessing they were the first and last to be cast respectively. I'm going to try for a hotter metal next time and also try heating the shells before hand so the metal doesn't cool as quickly. Hopefully that will give better results.

    On the chance that I do end up with flaws like this, is there any way to fix them after the fact? Filing them out doesn't seem like an option and a tig welder seems like overkill for such small holes.

  7. #7
    Join Date
    Dec 2005
    Location
    South Australia
    Posts
    1,358

    Default

    Could well be temperature might be a case of experimenting, filling the holes tid would be the best you could also use oxy/ acetylene

  8. #8
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    Pinholes.jpg

    I've recast the frogs I was making. The picture is from right after knocking off the mould.
    While a much better cast than the last batch, the pinhole flaw issue is still present. It seems to only be affecting the lowest parts of the figurine, the tip of the nose, eyes and back.

    I ensured that there was no moisture in the mould before casting this time. They were glowing orange from being in the furnace before I buried them in sand and covered the spout with tinfoil while I melted the metal. It's possible that there are some impurities in the metal. One of the frogs in particular looks like it had a bit of sand in the cast, but the rest look a little different from that. So I'm still not sure what is causing the issue or how to correct it. It would be great if there was a way to buff the flaws out, but finding info on metal chasing techniques is difficult as it defaults to 'repousse' which is a little different from my needs.

    I'm considering creating a sort of reservoir for the next batch. If the lowest points of the model are where the flaws are showing, then my thought is to create a new lowest point that I can cut away afterwards.

  9. #9
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    Here's my idea for sprueing up. Hopefully it should help.

    The round chamber at the bottom should catch any heavy particulates as well as provide a unimportant place for any oxygenation caused by the metal splashing around to happen. The channel backfills to the branch and flows into the mould smoothly. Air escapes out the upper vents and exits from the spout.




    Uq9-1BZRTg6yj73LnA4qtQ.jpg

  10. #10
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    After some more research, it seems that the problem I'm having is to do with gas porosity leaving bubbles in the metal. I'm working on methods to de-gas my metal before pouring and will report back on my findings later. In the meantime however, I've worked out a technique for cleaning up the metal using a multitool.

    Burrs.jpg

    First bit is a burr, used to hammer the metal flat and push it into the cavities. I then use the rasp and sanding bits to smooth everything out. If the hole is too deep, then I use the cutting disk to strip a layer of skin away, then repeat the filing process.


    Before and After:

    before.jpg

    after.jpg

  11. #11
    Join Date
    Dec 2005
    Location
    South Australia
    Posts
    1,358

    Default

    Going by your drawing, I would not have the bottom chamber and eliminate the three section sprue, you don't want "Backfilling" the metal needs to enter the mould quickly, run some radial gas relief lines from the edges of the casting, only need to be thin 2mm, it will mean a bit more to cut off and clean up, much easier to cut off an finish than to fill and finish.

  12. #12
    Join Date
    Apr 2021
    Location
    UK
    Age
    39
    Posts
    11

    Default

    Quote Originally Posted by China View Post
    Going by your drawing, I would not have the bottom chamber and eliminate the three section sprue, you don't want "Backfilling" the metal needs to enter the mould quickly, run some radial gas relief lines from the edges of the casting, only need to be thin 2mm, it will mean a bit more to cut off and clean up, much easier to cut off an finish than to fill and finish.
    The last casting had adequate venting and had just as many air bubbles. They appeared at the lowest points of the mould, so running a vent underneath the pattern might not be a bad idea.

    My thoughts are about how liquid reacts when poured. Take a glass of water and pour it from one cup to another. As soon the liquid hits the other glass it splashes and bounces around, oxygenating the liquid. Same thing happens with molten metal. There should be ways to minimise that effect however.

    Stirring the metal gently before pouring to release any trapped air in the crucible.
    Creating a trough to pour in to (might cool too quickly though).
    Tapering the sprue so the metal flows with less velocity.
    Creating a rounded impact point to lessen the splash back.

    I'm spit balling a bit, but I think these are ideas worth exploring.

Similar Threads

  1. WTB-Cast Small Cast Iron Lapping Plate
    By Betnorm in forum WANTED TO BUY
    Replies: 4
    Last Post: 19th Nov 2019, 01:01 PM
  2. Where to buy cast round
    By morrisman in forum METALWORK GENERAL
    Replies: 22
    Last Post: 3rd Dec 2015, 12:55 PM
  3. Cast iron VS Cast steel lathe compound slide
    By Ueee in forum METALWORK GENERAL
    Replies: 10
    Last Post: 23rd Dec 2012, 11:40 PM
  4. Cast Iron
    By RayG in forum METALWORK GENERAL
    Replies: 17
    Last Post: 30th Apr 2011, 02:27 PM
  5. How to fix cast metal
    By JackD in forum METALWORK GENERAL
    Replies: 19
    Last Post: 9th Jan 2010, 10:50 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •