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Dave Parvin
Joined:
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16-Mar-2005
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Started On : 18-Mar-2010 at 11:02:17 PM, #Views : 6009
Topic Subject : Re: vacuum pump for air traps in finger tips?? |
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Adele, I certainly hope that you didn't feel that I was talking down to you when I asked that you read the two articles. But the truth is that what you are asking about is a little complex. It would take me at least several days of typing to try to cover it in detail. As much as I want to explain about vacuums and pressure, I just don't have time to start from scratch. But if you read the articls, then we are on the same page.
The problem is that pouring plaster into an alginate mold and then pulling a vacuum around it WILL NOT remove the air from the fingertips. It will only make things far worse:
1. The vacuum will greatly enlarge the volume of the air in the fingertips.
2. Any air bubbles in the plaster will increase in volume and most will not escape from the plaster.
3. Air in solution in the plaster will form additional bubbles.
4. Any bubbles in the alginate will expand and and some will dimple the surface of the plaster.
One's next thought might be to pressurize the mold with the wet plaster in it. This will sort of work. The volume of the are in the fingertips will for sure decrease and it is possible perhaps even disappear. Any bubbles in the plaster will be reduced in volume and some will completely disappear. But I can promise you that any bubbles in the alginate will also collapse and your cast hand will appear to be covered with zits.
Here is the process I use, simply and briefly stated.
I cast the hand or foot with an alginate mold. I mix the plaster in a "Whip Mixer" which mixes it in a vacuum eliminating bubbles in the plaster,. I never, however, use plaster for the finished product. I use the plaster to make a silicone rubber mold in which I can cast additional copies in Forton MG or resin usually with metal powder. These are pressure cast to eliminate bubbles. But it is essential that the the rubber for the secondary mold to have been de-aired in a vacuum chamber and allowed to cure under pressure. (See my article about making a secondary mold for a hand). While making the secondary mold may seem like a lot of trouble, it allows me to make additional copies for other family members.
Vacuum an pressure usage are two of the things I cover in detail in my workshops. I have having another workshop from April 12 to the 16th.
It is after midnight and I can hardly stay awake... Hope this helps. If you have any more questions, please don't hesitate to ask.
Dave
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