Finning (Superficial)

Superficial finning presents as sharp metal fins, about 1 mm high, that appear perpendicular to the surface of the casting. This defect indicates that the primary shell coat has cracked during shelling or dewax and metal fills the crack. This is especially prevalent on round parts. 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 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. See shell causes for heavy finning. Crack did not penetrate shell as severely.
  2. Primary layer(s) crack in shell building process.
  1. See shell cures for heavy finning.

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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