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Table 1. Green Sand Molding Defect Chart Defect


Sand or Prepared Green Sand


Shrinkage Veining


Gas defects (blows and pinholes)


Burn-on and burn-in


Metal penetration


Crush, shift, and tear-up


Increase bentonite content


Increase bentonite content


High nitrogen content in sand, core butts


Increase bentonite content


Increase bentonite content


Insufficient mulling


Insufficient mulling


Mulling Moisture Metal Additive


Green sand moisture too high


sand free moisture too high


Green sand moisture too high


Excessive moisture


High free moisture content


surface do not match properly


Stickers and drops


Sand


erosion and inclusions


Hot tears


Hot prepared sand


Bentonite con- tent too high or too low; hot prepared green sand


Lower sodium bentonite content, increase


calcium bentonite


Broken molds, runout


Swells, Expansion


mold cavity dilation


defects (rattails, veining, scabs, spalling)


Friable edges


Low hot strength, increase bentonite content


Increase ben- tonite content


Increase bentonite content


Hot prepared green sand; clay content too low


Insufficient mulling


Insufficient mulling


Insufficient mulling


Green sand moisture too high or too low


High free moisture content


sand free moisture too high


Green sand moisture too low


• Cast iron alloys will dissolve hydrogen and nitrogen.


• Copper-based alloys will dissolve hydrogen and oxygen.


• Steel alloys will dissolve hydrogen, nitrogen, and oxygen. The problem is molten metal


can hold a greater amount of gas in solution than solid metal can. This means large amounts of gas that may dissolve in the liquid metal during melting, pouring, and mold filling


Green


Pouring rate too slow


Insufficient seacoal addition


additions


will be expelled from the metal as it solidifies. During solidification, the dissolved gases will precipitate into tiny bubbles of gas, forming pin- holes in the casting.


Sand Expansion Expansion defects originate, in


part, from the expansion of the sand grains when heated by the metal entering the mold. These defects can cause a positive or negative on the


casting surface. Surface defects are purely cosmetic and typically do not alter the properties of a casting. The hot metal also produces steam in the sand. When the steam reaches the sand where the tempera- ture is less than the boiling point, it will condense, making the sand wet and weaker. The three types of sand expan-


sion defects are rattails, buckles, and scabs.


July 2018 MODERN CASTING | 25


Increase cereal


Low mold hardness


Poor ram or squeeze


Poor ram or squeeze


Poor ram or squeeze


Pattern may flex under loading


Check squeeze pressures, check


bottom boards for flatness


Check squeeze pressures


Check squeeze pressures


Insufficient mulling


Insufficient mulling


Green sand moisture too high or too low


Green sand moisture too low


Hard molds


Poor ram or squeeze


compaction


Decrease mold


Casting design issues


squeeze pres- sures


Check


pattern, or backdraft


Hot sand, cold


Improper equipment usage or maintenance


Improper gating


Pattern core print, cores & parting


Pouring tempera- ture too high


Green


Improper metal


chemistry


Insufficient seacoal addition, add cellulose


Reduce organic additions


Lower organic additive, low density


Hard molds


Poor ram or squeeze


squeeze pres- sures


compaction


Improper equipment maintenance


Low Check


Improper gating; lack of enough mold or core vents


Improper gating


Improper venting


Mold


Mold wall movement


Compaction


Poor ram or squeeze


Sand


Pattern and Core


Molding Equipment


compaction Low


Gating & Venting


Improper gating


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