13-11-2012, 06:22 PM
Casting Defects and Design Issues
castdefect.pdf (Size: 1.4 MB / Downloads: 238)
Porosity due to Gases
•Smooth bubbles
–result from entrapped gases
–solubility in liquid is high, in solid is low, so gas is rejected during cooling
Porosity due to Shrinkage
•Rough bubbles - voids
•Stages
–cooling liquid
–rejects latent heat at melting point
•alloys become slushy - liquid and solid co-exist
–cooling solid
Porosity / Shrinkage Solutions
•Risers allow molten metal to flow into mold to make up for shrinkage
•Design flow so no part freezes early
–large channels
•“Flexible” molds
Pipe Defect
•Due to shrinkage giving rise to a funnel-like cavity
•Solutions
–insulate top (glass wool)
–heat top (exothermic mixture - thermit)
–allow metal to shrink, not hold metal
Design Rules Summary
•Uniform wall thickness
•Flat parting lines
•Gradual thickness transitions
•Draft for removal
–tapers: 0.5 to 2 degrees
•Surface of mold gives surface of part