Sponsored By

Tooling Corner: To block, or not to block?

March 7, 2006

5 Min Read
Tooling Corner: To block, or not to block?

How do I block off a cavity?? is in one sense a silly question, yet in another it is perfectly logical. Experienced folks in the industry would never ask a question like this in the first place. They?d say you?re a moron for even considering it. But, a layer of experienced staff has been retired/downsized (dumbsized) and management has seen fit to bring in ?Scooter,? a kid with a plastics degree from a snooty East Coast university. When asked ?How are you this morning?? he is likely to answer, ?I?d like to see more data on that.? As the Texans might say, the kid is ?all hat and no head.?

Let?s look at the facts. Assume you have an eight-cavity mold. You?ve quoted a run of the mold based on some machine hourly rate, productivity higher than 99.5%, with the normal cost-plus-10% material markup, along with whatever regrind allowance has been specified. Profit is based on productivity: salable pieces produced per hour. Another very important fact to remember is that you cannot recover lost time or lost money. Time = money and money = time.

Scooter has been informed (for this example) that the part in cavity number 5 in an eight-cavity mold got stuck. The part and cavity got crushed when the machine closed with full clamp. For this example we?ll ignore the fact that the setup allowed this damage to occur because the mold protection system was set too high, disabled, or ignored. Here?s his logic: Block off the mortally wounded cavity and run on seven cavities to complete the run and meet the JIT delivery deadline. Therefore, asking how to block off a cavity is a logical and valid question.

Losing Double

First let?s examine our definition of profit with a blocked cavity: Going from eight to seven cavities has reduced the productivity by 12.5%. We?ve been polite enough not to ask the customer for a price increase due to our mistake. Unless we?ve managed to sucker our customer into an obscene profit margin, we?re probably more profitable to mail him an envelope full of $20 bills than we would be by finishing the job because we?re losing a serious amount of profit we?ll never recover.By losing money we?ve also lost time. To make the same amount of pieces will now take longer. This means not only did we lose profit because of our price but also the cost increased. The additional time it took to complete the run is stolen from other jobs that could be making profit. Therefore the time lost is really a double loss.


Now to the technical side: Unfortunately, people do block off cavities. But, for some reason, they never take into account that the shot size got proportionately smaller from this action. It is like trying to push 8 quarts into a 7-quart basket. It first unbalances the mold?more force will be exerted at the peak of injection on one section of the mold and not on another because of the blockage. This is why blocked cavities usually result in an increase in flash on the remaining cavities.

Increases in scrap also prolong the run time. Unbalanced molds come out of alignment. You can see the wear on the guide pins by running unbalanced molds. Over-packing not only causes flash, it also changes molding stresses. Molding stresses determine dimensions. Our off-spec parts also increase run times until this situation is remedied. This means you?ve also messed up your Cp (process) and CpKs (process constants).

Over-packing also causes difficulty in mold opening and part ejection. If one part is stuck in the mold and you closed up on it, this is an invitation to close on another stuck part. Another question: How do you know that the damage that occurred did not affect slides, cores, rack and pinions, and so on? If we damage a second cavity, or a third, when will Scooter get it?

Not to Block

Let?s look at some alternatives:

  • Why not call the customer and ask if you can short ship the order, get the mold fixed, then ship the balance; or, ship this order on an expedited basis at your cost?

  • Pull the mold, get it fixed, and hope you can meet the shipping deadline?

Now let?s examine some positive alternatives:

  • When the mold was originally specified, did anyone think of purchasing spare cavities? The usual number is 10% over the normal cavity count. This means you can merely swap out the damaged cavity for the new one and send out the damaged cavity for repair or replacement. If you didn?t do it initially, you could still do it now. A spare qualified cavity on the shelf would mean minimum downtime before you?d be back running with a full complement of cavities.

  • When you were finally faced with the option of blocking off the cavity did you do the calculations of how much you?d lose by blocking the cavity off, balanced against pulling the mold/cavity and getting an expedited repair done? Which would be cheaper (keep in mind you?ll have to fix (pay for) the damage anyway)?There is also the issue of pride in workmanship (not yours, the mold builder?s). You?ve pounded something into the gate, then removed it and sent the mold off for repairs. Little did you know that pounding something in then removing it has opened the gate size. Mold builders are not machinists, they are artists, and you ruined his perfectly balanced gating system. You violated his perfect cavities. He?d rather you drove up on a motorcycle, took his 16-year-old daughter on a date to a roadhouse, and brought her home drunk with a tattoo than mess with ?his? mold. Yes, mold builders are very wary of you ruining their masterpieces. Also, you?ll now have to requalify a mold you have permanently unbalanced. This makes for ugly relations with your mold builder and a lot of work that could have been avoided.

So, when and how do you block a cavity? Don?t!! Fix the mold. There are rare instances that can recommend this practice, but I?ll keep them secret along with the technique for not losing money or destroying the mold. If you block a cavity you might as well burn your paycheck. At least that way your company won?t lose too much money.

Bill Tobin is a frequent contributor who also publishes a monthly newsletter on injection molding productivity. To be added to this private mailing list, contact him at the address below.

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like