Inclusions (Ceramic)

Inclusions (Ceramic)Ceramic inclusions present as missing areas of metal in the casting; appearing as small, round, irregular or angular craters, depressions or cavities of various sizes; typically superficial with traces of ceramic and refractory material. This defect indicates that something is present in the mold cavity or the metal that prevents the casting from fully filling to the shell; typically ceramic shell debris, bits of the pour cup, or the inner mold surface broke off into the molten metal during pouring and were left behind on the casting surface.

The causes of this defect are evident in the wax, shell, metal and other portions of the process. To cure these causes, R&R recommends taking the following actions.

Area

Cause

Cure

Wax

  1. Poor wax welds of parts to sprues.
  2. Ash in wax.
  3. Poor wax assembly – undercuts. When patterns are attached to the sprue and the connection is not completely filled or contains undercuts, these undercuts fill with slurry. When the shell is dewaxed, this ceramic extends into the shell cavity and can be dislodged when metal is poured resulting in a ceramic inclusion.
  1. Ensure good wax radii and no undercuts.
  2. Use lower ash wax, improve burnout.
  3. Wax assemblies should include gates with smooth radii, allowing for easy metal flow.

Shell

  1. Cracks in mold.
  2. Shell formed pour cups, jagged rim.
  3. Invested pour cup is brittle, jagged and/or fragile. As a result, the invested pour cup is breaking when the metal is being poured, becoming entrapped as inclusions.
  4. The primary coat layer is spalling.
  5. Inadequately dried shells.
  6. Out of control slurry.
  1. See shell cures for finning.
  2. Use a ceramic pour cup or ensure an even top cut.
  3. Use preformed ceramic cups.
  4. See options under scabbing/delamination.
  5. Verify that the shell is dry before applying additional layers. If necessary, add appropriate equipment or cycle time to ensure the shell is dry between coats.
  6. Check binder solids.

Metal

  1. Poor deslagging.
  2. Dirty revert metal.
  1. Determine proper method and control.
  2. Clean shell material from revert metal.

Other

  1. Housekeeping.
  2. Molds are being fired with the cup upright, entrapping dust or ash from residual wax residue.
  3. Dust particles are trapped in the shell cavity.
  1. Cover molds after dewax and use a filter.
  2. Fire molds with the cup facing down. This eliminates the potential for foreign material to enter the mold cavity and become a metal inclusion later in the process.
  3. If a double-firing cycle (firing/preheating) is occurring, blow the molds out prior to preheat. Consider covering the molds after firing to prevent foreign material from entering the mold cavity. Cleaning the top of the ladle or furnace prior to pouring metal may also eliminate the opportunity to introduce foreign material into the shell.

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