▁break ▁down ▁as ▁the ▁material ▁shr inks . ▁Moreover , ▁they ▁must ▁be ▁easy ▁to ▁remove ▁during ▁sh ake out . ▁Good ▁ref ractor iness ▁is ▁required ▁as ▁the ▁core ▁is ▁usually ▁surrounded ▁by ▁hot ▁metal ▁during ▁casting ▁or ▁m old ing . ▁A ▁smooth ▁surface ▁finish . ▁Min imum ▁generation ▁of ▁g ases ▁during ▁metal ▁pour ing . ▁ ▁Types ▁Many ▁types ▁of ▁cores ▁are ▁available . ▁The ▁selection ▁of ▁the ▁correct ▁type ▁of ▁core ▁depends ▁on ▁production ▁quantity , ▁production ▁rate , ▁required ▁precision , ▁required ▁surface ▁finish , ▁and ▁the ▁type ▁of ▁metal ▁being ▁used . ▁For ▁example , ▁certain ▁met als ▁are ▁sensitive ▁to ▁g ases ▁that ▁are ▁given ▁off ▁by ▁certain ▁types ▁of ▁core ▁s ands ; ▁other ▁met als ▁have ▁too ▁low ▁of ▁a ▁mel ting ▁point ▁to ▁properly ▁break ▁down ▁the ▁b inder ▁for ▁removal ▁during ▁the ▁sh ake out . ▁ ▁Green - s and ▁cores ▁ ▁Green - s and ▁cores ▁makes ▁casting ▁long ▁narrow ▁features ▁difficult ▁or ▁impossible . ▁Even ▁for ▁long ▁features ▁that ▁can ▁be ▁cast ▁it ▁still ▁leave ▁much ▁material ▁to ▁be ▁mach ined . ▁A ▁typical ▁application ▁is ▁a ▁through ▁hole ▁in ▁a ▁casting . ▁ ▁D ry - s and ▁cores ▁The ▁simplest ▁way ▁to ▁make ▁a ▁dry - s and ▁cores ▁is ▁in ▁a ▁dump ▁core ▁box , ▁in ▁which ▁sand ▁is ▁pack ed ▁into ▁the ▁box ▁and ▁sc rap ed ▁level ▁with ▁the ▁top . ▁A ▁wood ▁or ▁metal ▁plate ▁is ▁then ▁placed ▁over ▁the ▁box ,