Andy:
In reply to your question I have quoted some more information from the
article that might answer
some questions that people may have:
"Fine-tuning the process:"
"Not surprisingly, many problems were encountered during the development
of the freeze cast process.
Mold design was particularly challenging, because water expands as it
freezes. Fortunately, ice exhibits the same linear expansion repeatability,
if frozen at the same temperature. However, the expansion is
multidirectional (I take this to mean that water is isotropic - GW).
Therefore, the elastomer mold must be designed to provide enough extra
volume to absorb multidirectional forces. Moreover, the thickness and
consistency of the elastomer must be correlated with the different modules
for the different shapes of the castings".
"After conducting hundreds of tests and measurements, problems involving
the morphology of castings and the induced stresses were resolved. A system
was developed in which several basic geometries were designed, to which
certain semi-constants could be applied in the design of a specific mold. A
major breakthrough came when directional solidification was viewed in terms
of expansion rather than shrinkage".
"Another major problem was how to prevent the cracking caused by
stresses induced in the ice pattern by constricted freezing. This was
studied and attacked from three different directions: mold design, the
freezing medium's velocity and temperature, and air content in the water/ice
system. It was discovered that
extremely low temperatures trigger cracking, particularly when the patterns
are stripped at higher temperatures. This problem was adressed by
"conditioning" the ice, just as wax is conditioned prior to being injected.
Conditioning involves controlling the air content of the ice and regulating
additives (amount and chemistry) in the water".
"Warmer mold temperature was found to be crucial for fast, clean
stripping of rubber molds. Parting sprays that do not freeze, pre-chilling
the water solution, and higher temperatures are used for fast-stripping of
the molds".
"Air bubbles trapped on the surface of the ice were a recurring and
persistent problem, in spite of molds designed to permit air bubbles to
escape. Unfortunately, these bubbles would be reproduce on the surface of
the casting. This problem was minimized by controlling turbulence and
counter pressure within the mold, through proper mold design and venting. A
vacuum was also applied in a sealed chamber, prior to and during the
mold-filling procedure. A combination of these methods, along with special
additives for the water, finally eliminated the air bubbles".
Fascinating process!!
Hope this additional information helps.
Best Regards,
Glenn Whiteside
>In a message dated 99-04-05 19:39:55 EDT, SiderWhite@worldnet.att.net
writes:
>
><< Then water
> (with "special" additives to minimize air bubbles) is poured into the
rubber
> mold and frozen. >>
>Water expands when it freezes. Is it isotropic? Then you need to compensate
>for the metal shrinkage as it cools. Sounds to me, it might take a few
>iterations to get the numbers dialed in.
>Andy Scott
>Lockheed Martin Aerospace
>
>For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
>
For more information about the rp-ml, see http://ltk.hut.fi/rp-ml/
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