stop off
To fill (a part of a mould) with sand, where a part of the cavity left by the pattern is not wanted for the casting.
stop off: block unwanted cavities in a mould before pouring
In casting, a stop off is the deliberate plugging of a section of the mould cavity so that molten metal will not flow into it when poured. The pattern itself leaves a cavity shaped like the desired final part, but sometimes that cavity has recesses, undercuts, or deep pockets that would either trap metal or create features you do not want in the finished casting. Rather than remake the pattern, the moulder fills these unwanted areas with sand, clay, or a specialized blocking compound before the metal enters the mould.
The most common material for stopping off is bonded sand or clay pressed firmly into the cavity. In precision work, especially at higher volumes, commercial stop off compounds are used: these are heat-resistant putties or pastes made from materials like silica, clay, and binders that cure hard but remain slightly friable so they can be chipped or washed out after casting if needed. The material must withstand the temperature and thermal shock of the molten metal without breaking apart and contaminating the casting or the mould.
Application and technique
Stop offs are packed into the mould cavity by hand or tool before the two mould halves are closed and the pouring begins. The critical skill is packing them tight and smooth so they do not dislodge or create voids where metal can seep behind them. Poor packing leads to metal flash, casting fins, or incomplete mould sections. The boundary between the stop off and the surrounding cavity must be as flush as possible to avoid creating a visible parting line or raised edge on the final casting.
This technique is especially common in medium-volume production and prototype work where the cost of a new pattern does not justify the engineering change. It is also used in complex castings where internal passages or cooling galleries must be blocked selectively, or where a single pattern must produce variants with different features. Foundries doing one-off or small-batch work rely heavily on stop offs to adapt existing patterns to new specifications without the lead time of pattern modification.
The name reflects the practical action: you are stopping off the flow of metal into an undesired part of the mould. Successful stop offs are invisible in the finished part; failed ones show as casting flaws, inclusions, or unwanted protrusions that require grinding or rejection of the part.