Finning (Heavy)

Heavy finning presents as sharp metal fins, several mm high, that appear perpendicular to the surface of the casting. This defect indicates that the ceramic shell has cracked during shelling or dewax and metal fills the crack. This is especially prevalent on edges. Hoop stress caused by cylindrical parts.

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 a stress point for a shell crack.
  2. High melting point/viscous wax.
  3. Runner wax out of control (runner wax melt point > part wax).
  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).

Shell

  1. Weakness of ceramic shell.
  2. Too brittle of shell.
  3. Low green permeability.
  4. Poor shell buildup.
  5. 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.
  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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