Blocking Mold Cavities

Blocking off mold cavities will quickly cause you to quickly lose the profit margin on your existing production. Just 1 out of 8 cavities will require the mold to operate 12 percent longer to meet the same production goals.

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.
  1. Brent left a comment on 2012/11/19 at 11:17 pm

    Blocking a cavity can also upset the fill balance and pattern of multi-cavity mold
    Andy in next to last sentence,shouldn’t it read “final shot weight” rather than “part weight”? Each individual part should weigh as before, no?
    I enjoy your informative blog
    Keep on Molding

  2. Andy Routsis left a comment on 2012/11/20 at 9:56 am

    Good Point Brent,
    There are many factors which are affected by blocking off cavity, the intent here was to impress the financial losses. These aspects are more likely to convince management to act on correcting the situation. I have seem companies with all their molds running at low cavitation, yet claim their margins are too close to stop production for repairs. In reality, such companies are losing money in many ways by not repairing the tooling. Also, you are correct, I meant to type ‘shot weight’.

  3. Brent left a comment on 2012/11/20 at 6:56 pm

    Andy, I worked in a closure place that had a situation as described. So much lost production due to blocked cavities. Blocked cavities equals lost time, something that can never be regained!
    The place had a full tool room also. I went to another place that never allowed a blocked cavity, and they didn’t have Tool makers.
    Love your forum!

  4. Andy Routsis left a comment on 2012/11/21 at 9:48 am

    Thank you for contributing Brent.

  5. Pam left a comment on 2016/02/22 at 11:32 am

    Does anyone have a good process for blocking cavities? It is unavoidable at times and we experience issues with the cavities coming unblocked which causes more quality issues and downtime.

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