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Possible Cause Excess mold coating slows solidifi cation.


Defect: Pinhole Porosity in Heavier Section Possible Cause





Suggested Action Clean the mold and recoat with a thinner layer. Check the general


condition of the mold coating. Excessive buildup of spray, excessive roughness, spray chips and more than four consecutive shifts of produc- tion are good indications the mold is overdue for blast and spray.





Possible Cause Excessive agitation occurs when


transferring molten metal to the holding furnace.


Suggested Action Avoid fast pouring rates. Pour at a


steady rate with minimum metal fall. Flux to remove suspended dross.


Possible Cause Excessive pouring temperature


overheats the mold, slows solidifi - cation and increases gas porosity.


Suggested Action


If pouring temperature must be high to avoid misruns, check causes and suggested actions for misruns.








Melt temperature is excessive. A damp ingot or wet oil scrap was in the melt.


Suggested Action Check melting practice. Metal


temperature checked with a pyrometer should not vary more than 20 degrees with the furnace readout. If the level of metal in the furnace is more than four inches below the “full” level, then an inaccurate reading may be expected. If it is a full furnace, the thermocouple may need to be replaced. De-gas if necessary.


Defect: Shrinkage Porosity 


Possible Cause Casting exhibits lack of


directional solidifi cation, and proper temperature gradients are not established.


Suggested Action Change gating so solidifi -


cation proceeds directionally toward the riser. Check the thickness of the mold coating, increasing it on feeders and thinning it where more chill is required. If the condition per- sists, increase feeder size and provide special chilling. Mold temperature control may be needed. Cooling lines located below the surface of the tool can circulate air or water to cool the mold, or heating elements can be used on areas that need to be kept warmer longer.


 


Possible Cause Heavy section is not receiving


enough feed because it is isolated between thin sections.


Suggested Action If the heavy section cannot be on the


parting line, try to improve feeding by redesigning sections and using more generous fi llets or ribs. If these are not practical, provide special chilling.





Possible Cause Pouring temperature is too high.


Suggested Action If pouring temperature must be high to


avoid misruns, check causes for misruns. February 2012 MODERN CASTING | 41


Possible Cause Alloy composition.


Suggested Action If shrink continues after cor-


recting the other problems listed, check for proper grain size of the metal being used. If the grain size readout does not fall within the permitted range, try a grain refi ner. Check the chemical analysis of the alloy against specifi cations. Consider using a diff erent alloy. Sometimes it is permissible to use slightly gassy metal. A small amount of hydrogen gas replaces shrinkage defects with fi ne porosity that is uniformly distributed and harmless.


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