Had a aluminum decoy weight mold that had a slight gap along one seam. A little lead would leak out and quickly cool. The defect acted like a heat sink. In a narrow area it would let a little lead leak out, would cool down so fast it caused a dam effect and the area below did not fill. My mold had been cast and the defect area was under cast. Have the mold to a machinist who built it up and machined it back.
Dont know if modern molds are cast aluminum are some mix like what we use to calll pot metal.
