Thanks Thanks:  0
Likes Likes:  0
Needs Pictures Needs Pictures:  0
Picture(s) thanks Picture(s) thanks:  0
Results 1 to 8 of 8
  1. #1
    Join Date
    Nov 2011
    Location
    Darwin
    Posts
    1

    Question Pressure or Vacuum?

    G'day all,
    Thank you for this great and informative forum. I'm new to this casting caper, and am a little confused about casting under pressure or vacuum. Can someone please explain to this newbie, what situations lend themselves to casting under pressure or vacuum.
    Thanks

    Ned

  2. # ADS
    Google Adsense Advertisement
    Join Date
    Always
    Location
    Advertising world
    Posts
    Many





     
  3. #2
    Join Date
    Apr 2005
    Location
    Nerang Queensland
    Age
    66
    Posts
    10,766

    Default

    Vacuum can tend to form more bubbles than it removes before the resin goes off when casting, so I only use it for stabilization. Care is also needed with vacuum, resin boiling if too much vacuum, and expansion of the liquid resin.

    Pressure is used when casting to force the resins into every nook and cranny of worthless wood type casting. Heating the resin slightly also helps with bubbles. PR resins also shrink and the pressure can help making it stay in contact with the WW wood

    Hope this helps explain.

    Cheers
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

  4. #3
    Join Date
    Nov 2006
    Location
    US
    Age
    65
    Posts
    1,131

    Default

    Neil.. you might have been cutting and pasting and forgot where you were!

    Pressure compresses the air bubbles. Some say it will compress them so they
    can't be seen. (I'm still not convinced of that one) Warming the resin makes it
    thinner, so bubbles don't stick around. Imagine shaking a bottle of honey (cool
    resin) and then imagine shaking a bottle of water (warm resin) and you can imagine
    what the air bubbles will do.

    Vacuum pulls the air out of the wood. Vacuum alone will not fill the voids, you need
    to release the vacuum so the resin gets sucked in when the pressure is equalized.

    Since I don't do stabilizing, I don't use vacuum any more. And since I started warming
    the resin so that it gets watery (and any air bubbles just rise to the top) I don't use
    pressure either. Next month will be two years since I ditched the pressure pot and
    I don't miss it. And I do cast PR with wood.. no pressure and no vacuum.
    Here's a pic from the PITH: (he got his pen yesterday, so I can post the pic! )
    maker of the original ResinSaver mold

  5. #4
    Join Date
    Apr 2005
    Location
    Nerang Queensland
    Age
    66
    Posts
    10,766

    Default

    Quote Originally Posted by NewLondon88 View Post
    Neil.. you might have been cutting and pasting and forgot where you were!
    Guilty - I have amended text
    Neil
    ____________________________________________
    Every day presents an opportunity to learn something new

  6. #5
    Join Date
    Nov 2006
    Location
    US
    Age
    65
    Posts
    1,131

    Default

    well, I could point it out, as I have never made a mistake.
    maker of the original ResinSaver mold

  7. #6
    Join Date
    Oct 2007
    Location
    Horsham Victoria
    Posts
    901

    Default

    Quote Originally Posted by NewLondon88 View Post

    Pressure compresses the air bubbles. Some say it will compress them so they
    can't be seen. (I'm still not convinced of that one)

    )
    I am convinced this is not so. I have put my casting under 60psi and have regularly got pockets of bubbles down the bottom of the mould. With I do I find the best way to go is both vacuum and then pressure, next best option is stir and pour carefully and slowly so as not to create bubbles or air pockets and then pressure.

  8. #7
    Join Date
    Apr 2011
    Location
    McBride BC Canada
    Posts
    3,543

    Default

    I was looking around for a thread on safety gear for turning (first time next week!) and found this one. I have some PhD experience with resins and casting for both light and electron microscopy.

    1. Gases dissolve more readily in cold liquids than in warm liquids (warm beer/cold beer).
    2. Gases compress under pressure. Bubbles seem to disappear. Think of scuba divers and decompression stages to avoid the bends. The problem is that the little bubbles coalesce into big bubbles and they won't disappear by decending another 20 meters.
    3. In mixing, don't whip the resin around as that can only incorporate air. Gentle application of vacuum to a liquid resin mix can be done to out-gas the soup.
    4. Repeated (important process) gentle application of vacuum to specimens will eventually pull out most gas voids in the structure. Each release allows ambient atmospheric pressure to push the resin into voids a little further. Might have to repeat this step 5-10 times.

  9. #8
    Join Date
    Dec 2011
    Location
    Klein, Texas
    Posts
    77

    Default Still learning but this is my process

    I am still trying to figure this mess out as well but this is whhat I do and it seems to be working.
    1. I have some 2oz Silicone "pinch cups" that I picked up from amazon to mix in. They are great because you pop them inside out and the old resin comes right out and I dont have to worry about them melting. I pour a small amount of resin in there and add my power. I mix until the powder is disolved. I then pour the rest of the resin in the cup and mix it around.
    2. Hot Water - I place a jar of water in the microwave to heat it up. I pour about 3/4" into an aluminum (disposable) Loaf Pan and place my cup of resin in there. I wont take long before you see the bubbles rise to the top. Warming it thins the resin allowing the bubbles that were trapped in the thick resin to raise.
    3 Vacuum - I used to then place the cup in my vacuum pot and pull it under a vacuum to remove any and all remaining bubbles. I skipped this step this time, lets see if it causes a failure.
    4. MEKP - Add it and stir SLOOOOOOOWLY so you dont introduces any more bubbles.
    5. Pour - Pour it into your mold using what ever process you use (IE. let it thicken then pour, pour and let thicken in the mold......etc)
    6. Pressure - If casting a meduim such as Wood or Oatmeal I place it under pressure to fource the resin down and around the wood. I dont know science but what I feel happens is the resin displaces the air and is heavier than the air so the air is forced up and the resin down. This will give you a blank that will have all or most of its bubbles at the top of the resin. For this reason I pour a little extra resin so I can easily cut this off.


    Again, this is what I do but I am big time still learning and have a lot to learn yet about timing my pours but we will get there, with each others help.

    Quick Edit, I dont place my resin under vacuum once I introduce the medium as it will pull the air out the the wood or whatever leaving it in the resin as it starts to thicken. Only vacuum the resin by itself.


    Thanks and good luck.

    Chris Burgess
    Up and over in Texas

Similar Threads

  1. Vacuum Pump - suitable for Vacuum Clamp?
    By John H in forum CNC Machines
    Replies: 5
    Last Post: 17th January 2011, 08:10 AM
  2. My Vacuum/Pressure Set-up
    By dai sensei in forum CASTING & STABILISATION
    Replies: 16
    Last Post: 8th November 2009, 12:10 PM
  3. Could this be used as a pressure pot
    By madcraft in forum CASTING & STABILISATION
    Replies: 3
    Last Post: 3rd June 2009, 11:02 PM
  4. Dust extractor vacuum pressure drop
    By digitalis49 in forum DUST EXTRACTION
    Replies: 16
    Last Post: 9th June 2008, 11:01 PM
  5. Pressure Pump - pressure switch setting?
    By Woody Allan in forum PLUMBING, ELECTRICAL, HEATING, COOLING, etc
    Replies: 4
    Last Post: 4th January 2006, 04:12 PM

Tags for this Thread

Posting Permissions

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