As an introduction to this subject, it must be understood that this condition is caused by the inner-relationships of the following:
- Type of design being cast
- Sprue/gate size
- Location of article on tree
- Solidification rate of metal in relation to:
A. Metal temperature
B. Flask temperature
C. Type of alloy being used
Hot tearing is seen as a clear crack, occurring usually on the sides of rings or toward the top of the ring near the setting. In most cases there will be only I crack. The crack will usually appear on several pieces of a tree. The crack is usually in the same area of these pieces. In most cases, it is an identical spot. Most of the time the location on the tree is toward the bottom.
The cause of hot tearing is the relationship between the rate of solidification of the alloy, the flask temperature and type of design including spruing. Unless the crack appears at the lower portion of the ring, we can usually rule out gate problems.
Hot tears occur when the molten metal fills the ring cavity but does not solidify in an even manner. A section of the ring begins to solidify before another, and a pulling apart takes place leaving a crack. This crack may be noticed after breakout, but usually shows in finishing. The crack usually appears after:
- Chemical treatment
- Heating, such as during soldering
When the hot tears appear usually the alloy is accused of being the problem. Many times switching to another alloy solves the problem because different alloys have different solidification characteristics. Due to the fact that deoxidized alloys are susceptible to cracking from many factors, it is most difficult for the caster to determine what the cause may be.
NOTE: All alloys are subject to hot tears.
In our opinion most of the hot tear problems are related to overheating of metal or flask.
We recommend the following:
- Never heat metal so it is smoking or fuming. In addition to causing too much oxide to be produced, the molten metal will stay liquid too long allowing hot tearing to take
place. White gold may fume a small amount depending on alloy and equipment used for melting.
- Flask temperature should be as low as possible. Flask should only be hot enough to allow all pieces to fully form. This does not mean to drastically reduce temperatures if you
see I or 2 cracks. We only suggest possibly reducing flask temperature in 50 degree steps to see if problem disappears.
- Leave at least 1 inch from button of tree to the last row of rings to avoid a large pull-back effect.
- Gate using round gates when possible. This avoids angles at the gate which will solidify unevenly. Extra gating may be necessary for some designs due to changes from heavy to thin sections.
Once flask temperature is determined for style and weight of ring, metal temperature must be controlled. The type of alloy used will determine how fluid the metal is, controlling the solidification rate. High fluidity alloys such as United's ULTRALOY and 200 Series and non-deoxidized alloys will be adversely affected by overheating conditions. Alloys such as United's 4Ooand 6OOSeries will be less affected by over heating, due to their solidification characteristics. However all alloys are subject to this effect.
When conditions are controlled that flask temperature, metal temperature and placement of rings on the tree are proper, the design must be looked at for symmetry. if the gate is too heavy, tears may occur at or near the gate. If the design has changes from thinner to thicker areas, tearing usually occurs at the side to upper shoulder. On rings with very light settings, tearing may appear on one side where the setting gallery begins, or at the very center of the middle stone. It must be noted that insufficient temperature of flask may mimic this problem, although a lack of cavity fill should also be present at the lower portion of the tree. Cold metal and poor vacuum would also cause cracking; but again the lower part of the tree should not fill properly.
Centrifugal casting may enhance the hot tear effect by the way in which the flask is filled with the metal. The solidification rate is not uniform due to the directional nature of the centrifuge. On the positive side, flask size is usually smaller, allowing the metal to travel a shorter distance.
In summary, hot tears can occur when any condition related to cavity filling is not proper. Regular maintenance is of major importance. An oven overheating by 100 degrees may cause hot tearing. Beware of electric melting units that seem to get hotter (melt faster) on consecutive melts. This may overheat the metal. Casting is still an art, we must use our eyes and our heads to identify changes (flask color, metal fuming, oxide build-up, etc.) if quality and efficiency are to be maintained.
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