Question: We run many multi-cavity molds and when there is a complication with a cavity, we just block it rather than fix it. We typically block 20-30% of the mold cavities during production. Is this a bad practice?

My Response: It may be necessary to block a particular cavity if the production demands delay the repair for a short while; but it is difficult to justify the blocking of mold cavities as a standard operating procedure.

When you block 25% of your mold cavities the you must run the mold 33% more cycles to achieve the same amount of production. The main reason a molder uses a high cavitation is to increase productivity due to either customer demands or profit margins. Blocking off cavities generally counteracts the benefits of the high cavitation. This most often results in either a break-even or profit-loss for the production run. In today’s tight market, it is difficult to envision such a company remaining competitive over the long term with such a high margin of loss. In any case, it is critical to institute a repair policy to begin returning the tools to proper operating condition. Without such a plan, your facility will never be prepared for growth as it’s production capacity is hindered by the artificially high production runs.

In the case that a molder must block off a cavity, it is critical that they maintain the same process outputs such as fill time and packing pressure. The only process output adjustments will be a reduction in 1st stage fill weight and final part weight due to the reduce number of cavities. In this case, the molder should schedule the mold to be repaired at the earliest convenience.

-Andy