||Measure the Height of the Tree before Investing A good practice for preventing investment blowout is to measure the height
of the trees before investing. The maximum tree height should be between
0.5 inch and (for larger trees with more metal) 0.75 inch shorter than the
fill level of the flask. It's also important to leave a 0.125 inch gap between
the top of the investment and the top of the flask if you are using benchtop
vacuum casting with a solid flask. With perforated flasks cast in a vacuum
chamber or with centrifugal casting, the flasks can be filled completely
and the tree height lengthened accordingly.
||Dip the Tree in a Debubblizer Unless you are mixing the investment and filling the flasks in a vacuum,
dipping the tree in a debubblizer is recommended . This reduces the surface
tension on the patterns, which helps to prevent air bubbles from clinging
to the surface and forming nodules on the casting.
|| Properly Position the Tree in the Flask How the tree is positioned in the flask for casting is very important
in ensuring investment strength. There should be ample space-generally
a minimum of 0.25 inch (6 mm)-between the outermost part of the pattern
and the side wall of the flask
The positioning will also help to calculate the
amount of time you have before the flask has cooled and your castings
are in danger. Most gypsum-based investments have a heat transference
rate of about 400°F (204 C) per minute/mm. If the investment surrounding
the tree is 6 mm thick, you will have about 1.5 minutes before the temperature
at the outermost portions of the pattern drops 100°F (38 C). If you
have a 200°F (93 C) window for casting, you will have about 3 minutes
to complete the casting before the temperature drops below that limit.
If you have 12 mm of investment between the pattern and side walls, you
will have about 6 minutes to complete the casting.
||Carefully Measure and Monitor the Investment When it comes to investing, following manufacturers' recommendations for
mixing and measuring is critical. Powder should be measured by weight
and fluid should be measured by volume.
Safety is a consideration when mixing investment.
You should wear a good particle mask that seals tightly to your face.
Wearing a paper dust mask is only marginally better than wearing no protection
at all; it provides a false sense of security.
After carefully measuring and achieving the correct
water-to-powder ratio, the investment must be mixed thoroughly to form
a creamy slurry (photo 4). Carefully follow the manufacturers' directions
for mixing times and temperatures. Keep in mind that the water and powder
temperatures will affect mixing time. The hotter the materials, the shorter
the mixing time.
Note: Investment has a shelf life. Over time-between
six months and one year-the investment can absorb humidity from its surroundings
and become ruined. It's important to store investment according to the
manufacturer's instructions. One tip: Never store investment directly
on a concrete floor, since concrete holds moisture.
Aside from problems with humidity, the silica
begins to settle out of the investment powder after a while. If not re-mixed
periodically, the investment formula can change enough to cause major
problems in casting. The most common problems associated with this are
fragile investment and poor surface texture on castings.
||Vacuum the Investment The mixed investment must be vacuumed to remove trapped and suspended
air. When vacuuming is complete, carefully pour the investment into the
flask and fill it to the top of the tree. Place a rubber sleeve over the
top of the flask and vacuum the flask (photo 5). The investment will rise
and boil during vacuuming as the trapped air expands and rises. After
vacuuming, add a little more investment to top off the flask to the proper
level, as described earlier, and set it aside to gloss off (solidify). Before loading the flasks into the oven, care
should also be taken when removing the rubber. Improper handling can cause
small amounts of investment to chip off the surface, thereby causing inclusions
in the casting.