Finning

finningFinning presents as sharp metal fins that appear perpendicular to the surface of the casting and vary in severity from superficial (about 1 mm high) to heavy (several mm high). This defect indicates that a portion of the shell has cracked during shelling or dewax and metal fills the crack. Superficial finning typically occurs when the primary shell coat has cracked and is especially prevalent on round parts due to hoop stress. Heavy finning typically occurs when the entire ceramic shell has cracked is especially prevalent on edges.

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

Area

Cause

Cure

Wax

  1. Parting line was not removed and serves as stress point for a shell crack.
  2. High melting point/viscous wax.
  3. Runner wax out of control (runner wax melt point > part wax).
  4. Wax pattern not clean.
  1. Remove all part line indications.
  2. Change wax or increase green permeability of the shell.
  3. Evaluate melt points and adjust as warranted (drop melting point of runner wax).
  4. Check on pattern cleaning.

Shell

  1. Weakness of ceramic shell.
  2. Too brittle of shell.
  3. Low green permeability.
  4. Poor shell buildup.
  5. Primary layer(s) crack in shell building process due to temperature variation during drying.
  6. Finished shell temperature not controlled.
  1. Increase strength – add polymer and fiber (check on backup slurry specification and control).
  2. Reduce refractory loading – add polymer.
  3. Increase green permeability.
  4. Evaluate evenness of shell buildup, draining times and positions, stucco picking up.
  5. Evaluate and correct drying conditions.
  6. Keep finished shell in controlled environment.

Metal

   

Other

  1. Slow loading of dewax.
  2. Insufficient heat capability in dewax process (improper dewax operation).
  1. Load dewax unit quickly to reduce exposure to heat.
  2. Pressurize to 80 psi in less than 10 seconds. Depressurize from maximum pressure to atmospheric gradually over about 2 minutes (evaluate autoclave operating parameters).

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