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Troubleshooting-Alloys
Brittle Alloy - Caused by
overheating alloys during casting. Adjust fuel mixture so that the
dark blue inner cone is approximately ˝ inch long. Lower noble,
high palladium or platinum containing alloys are particularly
susceptible to oxidation and extra care should be exercised in their
melting and casting. Overheated alloys can result in large grain
size castings and may contain excessive quantities of impurity
elements absorbed from the torch flame. BACK TO TOP The melting, casting or recasting of buttons can cause the vaporization of certain components of the alloy and thus slightly change its composition. Each time the alloy is melted, a quantity of the interstitial elements (carbon, hydrogen, and oxygen) are absorbed from the torch flame. These elements tend to migrate and concentrate in the grain boundaries. Excessive amounts of interstitial elements result in brittle grain boundaries. Compensation by adding new alloy will assure that the metal retains its composition and reduce the probability of heat tears. Using a separate crucible for each type alloy metal and insuring that the crucibles are not discolored or glassy will assure proper casting. BACK TO TOPInsufficient Burnout of the Investment - Be sure to follow the instructions for the investment allowing sufficient time at temperature to assure all the investment is at equilibrium with the furnace temperature. BACK TO TOP Inadequate Spruing Techniques and Waxing too Thin - Sprues and runner bars must be of sufficient gauge to assure that the molten alloy will flow freely during casting. Too narrow a sprue gauge, too long a sprue, or pattern walls too thin may cause insufficient flow to all parts of the casting resulting in voids and potential porosity. Sprues should be placed at the thickest portion of the wax-up and the reservoirs 1.5-2mm (1/4 inch) from the wax-up. Wax thickness of at least 0.3 to 0.5 mm should be used to assure a more complete casting.BACK TO TOP Check spring of the casting machine and apply extra windings for lower density alloys. Check the balance of the arms periodically to assure smooth flow of the alloy during casting. The suggested number of windings depends on the condition of the spring and type of alloy used. High gold, dense alloys can typically be cast with as little as 2 windings in the casting machine. High palladium content alloys that have lower densities require 3 or more windings. BACK TO TOP
Insufficient Heating of the Metal
- The metal must be sufficiently fluid to effectively cast. If
the temperature is too low, the metal may partially solidify before
completely filling the cavity. Typically, molten alloys should be
white hot for proper casting; however avoid overheating the alloy at
all costs (see Porosity in Casting). Gases Forming During Casting - Gas formation in alloys can occur because of a variety of issues.
Overheated Alloys - Overheated alloy remains fluid for too long a period of time, causing excess gases to form between the investment and the alloy, resulting in a suck back. BACK TO TOP Improper Quantity of Alloy Used - There is a minimum and maximum alloy ratio. The minimum amount should be equal amount of alloy in the button as in the casting. The maximum is three times the alloy in the button as in the casting. If the button is too large, the alloy will stay molten too long causing suck back. BACK TO TOP
Improper Spruing -
Sprues and runner bars must be of sufficient gauge to
assure that the molten alloy will flow freely during casting. Too
narrow a sprue gauge, too long a sprue, or pattern walls too thin
may cause insufficient flow to all parts of the casting resulting in
voids and potential porosity. Sprues should be placed at the
thickest portion of the wax-up and the reservoirs 1.5-2mm (1/4 inch)
from the wax-up. BACK TO TOP Periodically calibrate burn-out ovens to confirm temperatures. Use the suggested burnout temperature(s) listed in the alloy technique sheet. When using multiple rings, add an additional 10 minutes or more for each ring in the oven. BACK TO TOP
Insufficient Sprue Size -
Small sprue
gauges and long runner bars can cause the entrapment of air and
decomposition products resulting in porosity. Too narrow a sprue
gauge, too long a sprue run, or pattern walls too thin may cause
insufficient flow to all parts of the casting resulting in voids and
potential porosity. Sprues should be placed at the thickest portion
of the wax-up and the reservoirs 1.5-2mm (1/4 inch) from the wax-up. BACK TO TOP
Too Much Old
Alloy Used - Most alloys require that fresh alloy be added to
the button when casting. This will reduce the affect of overheated
alloy or contamination that might arise from reused alloy. BACK TO TOP |
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