▁via ▁holes ▁in ▁the ▁core ▁surface ▁which ▁cause ▁only ▁the ▁surface ▁of ▁the ▁core ▁to ▁hard en ; ▁the ▁remaining ▁sand ▁is ▁then ▁just ▁dump ed ▁out ▁to ▁be ▁used ▁again . ▁For ▁example , ▁a ▁cold - box ▁sand ▁casting ▁core ▁b inder ▁is ▁sod ium ▁sil icate ▁which ▁hard ens ▁on ▁expos ure ▁to ▁carbon ▁dio x ide . ▁ ▁Special ▁bind ers ▁are ▁used ▁in ▁air - set ▁s ands ▁to ▁produce ▁core ▁at ▁room ▁temperature . ▁These ▁s ands ▁do ▁not ▁require ▁a ▁gas ▁catal yst ▁because ▁organ ic ▁bind ers ▁and ▁a ▁cur ing ▁catal yst ▁are ▁mixed ▁together ▁in ▁the ▁sand ▁which ▁initi ates ▁the ▁cur ing ▁process . ▁The ▁only ▁dis adv antage ▁with ▁this ▁is ▁that ▁after ▁the ▁catal yst ▁is ▁mixed ▁in ▁there ▁is ▁a ▁short ▁time ▁to ▁use ▁the ▁sand . ▁A ▁third ▁way ▁to ▁produce ▁room ▁temperature ▁cores ▁is ▁by ▁shell ▁m old ing . ▁ ▁The ▁term ▁no - b ake ▁s ands ▁can ▁refer ▁to ▁either ▁the ▁cold - box ▁process ▁or ▁air - set ▁process . ▁ ▁Other ▁consider ations ▁To ▁increase ▁the ▁strength ▁of ▁cores ▁internal ▁w ires ▁and ▁ro ds ▁can ▁be ▁added . ▁To ▁enh ance ▁col laps ibility , ▁st raw ▁can ▁be ▁added ▁to ▁the ▁middle ▁of ▁the ▁core ▁or ▁a ▁h ollow ▁core ▁can ▁be ▁used . ▁This ▁attribute ▁is ▁especially ▁important ▁for ▁steel ▁casting ▁because ▁a ▁large ▁amount ▁of ▁shr ink age ▁occurs . ▁ ▁Ex cept ▁for ▁very ▁small ▁cores ,