Atlas of Casting Defects
Negative
Generally a smooth sided irregular negative shape of indefinite size. Inclusions caused by ceramic material are usually more angular and may contain bits of embedded ceramic.
Variable
Most obvious on external surface of casting where the “inclusion” prevented the alloy from filling the mold cavity to the shell surface.
Dirt, Non-metallic inclusions, Ash
Visual Inspection
Foreign material in the mold cavity which can originate either from the mold or from outside the mold. The shape of the defect aids in determination of the source
| Area | Possible Cause | Potential Correction |
|---|---|---|
| Wax | Junction between wax gate and sprue not completely sealed (undercuts). Ceramic fills the undercut during shelling and breaks off during dewax or pouring. |
Improve gating technique to eliminate undercuts. Make sure wax joints (parts to sprue) are sooth and complete . |
| Wax | Filter breakage | |
| Wax | Ash in wax – as in wax typically floats in the molten alloy and is present on the topside of castings |
Test ash content of wax |
| Wax | Incomplete soluble removal |
Confirm soluble leaching process and inspection is adequate |
| Wax | Filler settles out in areas that do not drain during dewaxing |
Reconfigure gating design to improve wax removal. Add wax bleeder |
| Shell | Cracks in mold and ceramic bits get into mold cavity |
See Finning |
| Shell | Poor mold patching technique – patch enters mold |
Improve patch technique |
| Shell | Poor adhesion of shell layers to one another |
See Spall |
| Shell | Ceramic debris entering mold after dewaxing (poor housekeeping) |
Cover the pour cup after dewaxing (allow moisture from the mold to escape). Store the mold cup down. Wipe rim of pour cup before turning up-right |
| Shell | Ceramic breaking loose inside the mold during dewaxing |
Vacuum or wash out mold after dewaxing |
| Shell | Slurry floods pour cup during shell building |
Cover pour cup, immediately rinse slurry out. Remove all dried ceramic prior to dewaxing. Coat the inside of the pour cup with wax prior to shelling to aid in slurry removal from cup during dewax. |
| Shell | Ceramic material from jagged lip/edges on in house shell built pouring cups is broken off during handling, burnout or casting |
Use a preformed ceramic pour cup or assure in house shelled cup is uniform and robust on top edge |