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Vacuum! That is the question!

Discussion in 'Tools of the Trade/Accessories' started by archimede, Dec 18, 2018.

  1. archimede Active Member

    Country:
    Italy
    Hi everyone, so far I have printed the pieces in resin with the pressure. But I would like to start printing with the vacuum. I have all the equipment, but I'm trying to print a wheel like the one in the picture, but the piece is not perfect.
    So if anyone would like to explain, I will be grateful.
    I'd like to know:
    1) How many shores must have silicone rubber;
    2) If the mold must be bivalve or monovalve;
    3) how the mold should be made, that is if it has to have a sprue with a single entry hole or more for air outlets or other.
    In short, all the information you need.
    Thank you

    Attached Files:

  2. 1969 A Fixture

    Country:
    United-Kingdom
    I normally go for a medium shore hardness for most casting, I find the harder the rubber the less chance of distortion with the casting and less slip on the parting line of the mould.
    I have cast spoked wheels before and found you can get away with one sprue at the top which will work fine for vacuum casting, I always cast with the smallest sprue possible so there is less clean up of the parts. You can pour the resin into the sprue and the air will vacuum out through the same sprue once vacuum is applied. When vacuum casting always vacuum until bubbles are forming, allow to bubble for about 30 seconds, release pressure slowly back to atmospheric pressure then repeat again. Also adding a small drop of Acetone to your resin when mixing will help eliminate air bubbles, you only need about 1:30 ratio of Acetone to resin, it just breaks surface tension of the resin so bubbles escape and break easier. It is important to release the pressure slowly once vacuuming is complete to prevent air being drawn down into the mould when the resin sucks back down.

    Steve
  3. archimede Active Member

    Country:
    Italy
    Steve, thanks for the kindness and the excellent explanation. Do you make it bivalve or monovalve mold?
    1969 likes this.
  4. 1969 A Fixture

    Country:
    United-Kingdom
    I would use monovalve, if you have problems you can always cut a second valve into the rubber (y)

    Steve
    Uruk-Hai likes this.
  5. archimede Active Member

    Country:
    Italy
    Ok, for the wheel you have the spokes but I have to force a bivalve or cut the monvalve. Do you agree?
  6. 1969 A Fixture

    Country:
    United-Kingdom
    All you need is to attach one sprue to the top of the wheel, this will allow you to pour the resin and will allow the air/bubbles to escape.
    Uruk-Hai likes this.
  7. archimede Active Member

    Country:
    Italy
    (y) Thank you Steve!
  8. archimede Active Member

    Country:
    Italy
    Excuse me Steve, I'll show you the result. I scrupulously followed what you told me, but the result is always the same. The piece does not come as you see in the photo.

    Attached Files:

  9. 1969 A Fixture

    Country:
    United-Kingdom
    Hi Fabio,

    Can you tell me what resin you are using, you need a very low viscosity resin to overcome the air bubbles in the thin spokes. Also if the air bubbles are forming in the same place each time then cut a vent in the mould next to those spokes, that may solve the issue.
    I also notice the spokes are distorted, are you pulling the part from the mould too early and not letting the resin set fully?
    Cheers
    Steve
    Uruk-Hai likes this.
  10. archimede Active Member

    Country:
    Italy
    Yes, I changed before it hardened, I know.
    Here is the technical data sheet of the resin I use.

    Attached Files:

  11. 1969 A Fixture

    Country:
    United-Kingdom
    I cannot see the Viscosity on the data sheet, a low viscosity resin will always help when casting small or this parts, high viscosity is OK for large castings that have open moulds or large pour sprues/vents.
    Uruk-Hai likes this.
  12. archimede Active Member

    Country:
    Italy
    Hi Steve, yes, in the data sheet there is the viscosity of both the resin and the hardener. In the attached photo I marked them in red.;)

    Attached Files:

    1969 likes this.
  13. 1969 A Fixture

    Country:
    United-Kingdom
    OK I see that now, looks OK to me.
    Steve
  14. archimede Active Member

    Country:
    Italy
  15. Uruk-Hai PlanetFigure Supporter

    Country:
    Sweden

    Dont know if theres problems with language and translation. But that seems rude towards, Steve?

    Cheers
    Janne Nilsson
  16. archimede Active Member

    Country:
    Italy
    no no, it's the wrong translation !!!
    Uruk-Hai and 1969 like this.
  17. archimede Active Member

    Country:
    Italy
    I'm asking Steve for advice, to get his opinion
  18. archimede Active Member

    Country:
    Italy
    sorry, sorry!
  19. archimede Active Member

    Country:
    Italy
    Janne unfortunately, when translated, there are errors of interpretation. I'm not the person who offends friends. I'm sorry.
  20. Tommi A Fixture

    Country:
    United-Kingdom
    From the looks of the pictures it does not seem to be pulling hard enough on the vacuum. I need to pull it to the Max of 1bar and let it bubble away for about 30 seconds before letting the air back in slowly. If you have a shallow pour reservoir then this will become a problem. Wrap parcel tape around the top of the mould to build a wall then over fill the mould so that the vac bubbles reside in the overfill of resin before letting the air back in.

    Tommi
    1.jpg 2.jpg
    Uruk-Hai and 1969 like this.

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