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Mar 06 2006

Mold Maintenance: Predictive or Preventative

Published by WJT Associates at 5:50 pm under management,Tooling

Maintenance is designed to extend the life of a mold. There are those who do it religiously on afixed schedule and those who believe it can be predicted by a computer model. The conflictbetween the customer and supplier is usually WHO pays and HOW MUCH and how or why isthis expense justified. Somehow this is usually divided among each with less than the Wisdom ofSolomon.Anytime you want to start a fight in a staff meeting put this topic on the table for opendiscussion, sit back and watch the feathers fly. Kinda fun.What most people miss, especially in mold maintenance is the endgame is the same for eitherphilosophy: Customers pay for molds of a given lifespan then routinely expect them to last asmuch as 3X longer. Maintenance is designed to extend the life of a mold. The conflict between

the customer and supplier is usually WHO pays and HOW MUCH. Somehow this is usuallydivided among each with less than the Wisdom of Solomon.Preventative Maintenance is something you most obviously do on your car: You change the oil,have all the fluids checked and topped off, and replace the brake linings when necessary.Predictive maintenance on a car might be consistently checking the oil level and the amount of dirtin it, the thickness of what is left of the brakes, and the amount of antifreeze and level of thecooling BUT doing nothing in terms of replacement until it hits some predetermined limits.Which is better? I tend to be a supporter of preventative maintenance. In my mind I have morefaith in my mechanic than I do in some manual provided by the car industry because it is in theirbest interest that I buy more cars, not maintain the one I own.I had a wonderful discussion recently with a PhD type about Predictive Mold Maintenance. He

was overflowing with left-handed fatigue equations of steel to determine when the plates shouldbe stress relieved, friction and abrasive math models showing how slides and gears mechanismswould wear, the action of how lubricants would forestall premature wear out and he evenproduced a study about smashing down parting lines and vents because of over-clamping of amold. To further bury me in Techno-Babble, he went on to describe the abrasive effect of certainmaterials on mold steel and the electrolytic action of scale build up in waterlines that would posean insulating affect on mold cooling. If I was a grad student in a snooty East Coast university, Iwould immediately re-title my thesis and go about making a computer model of all this andpresent it at a plastics conference as the ultimate predictor of mold wear and therefore a predictivemaintenance program. Then Id copyright it, and sell it to the millions of True Believersbecoming a millionaire full professor before I was 29 years old.More than 30 years ago I derived a multivariate equation that described all this and immediatelyabandoned it because it had no basis in reality: What if the windows are left open and the plant isfull of (abrasive) dust? What if a percentage of runs are abusive while the majority are run at theminimum acceptable molding conditions to extend the life of the mold? What is the effect ofshort runs versus long runs? How to we know the steel is hardened to specification or made out ofsome funny or equivalent alloy from China? What if the mold is run in a geographic area wherethe water is so hard you feel like you are being sandblasted when you take a shower or an area thathas spring-fed naturally soft water or it is acidic or caustic? What if the molder had enough brainsto individually connect each of his machines to earth ground (hole drilled in the floor with acopper rod pounded down to the water table with a copper strap to the machine) so that theanode/cathode effect of scale buildup is minimized? The PhDs hypothesis, like most models,lived in a perfect world; and injection molding is rarely perfect.

However there is some good to be had from this kind of analysis: Analysis of any kind forces youto consider the variables and make some determination of their functional upper and lower limits.Preventative mold maintenance makes an assumption that things should be constantly checked,

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repaired and replaced (depending on their criticality) as a function of how many cycles orproduction runs. The reason preventative maintenance is so popular is its minimaldocumentation. If something is worn, you fix it; if broken, it is replaced, if plugged youclean/unplug it. The decision to replace or rework is usually experiential and tends to err on earlyrepair/replacement rather than waiting too late. Whatever data is collected is more in the form of amold history.Predictive Maintenance is an exercise in data tracking and data base decision making. There areseveral wonderful spreadsheet based programs already available that allow the number nerds anddata dogs to measure everything, input it into a computer or write it down, and draw theappropriate Least Squares trend lines and other predictive charts. This is a labor intensiveprogram that requires the diligence of a monastic friar to make sure it is both current and accurate.When time and personnel allow (read: the abuse of summer interns) these are excellent programs,but like most labor intensive record keeping programs they get quickly pushed aside for the moreexpedient programs of keeping production running.Predictive Maintenance is excellent for at least three things:(1) If you are managing a group of tools all producing the same product on an ever increasingproduction demand schedule (such as disposable medical products) and your job is to not onlyfulfill the current production needs but also maintain the tooling set, retire old molds and bring

new tooling on line in a seamless fashion to keep production running. When Ive been in thesesituations Im measured and graphed productivity as well as the cost of maintenance. Quite likethe cattle farmer who measures each of his herds weight gain against food consumption balancedagainst the market price of beef; he makes the decision to maximize his profit. In my caseproductivity (profit) determined the life of the tool.(2) Highly mechanized or complex molds the best example being ultra-high cavitationunscrewing cap molds require a high amount of maintenance to keep running on a full cavity set.These lend themselves to Predictive Maintenance (and FMEA failure mode effect analysis)models again with the endgame being to keep the molds running by interdicting premature failuresthat usually occur in the mechanisms.(3) If you employ new materials or mechanisms such as a novel self-lubricating wear plates or the

use of liquid nitrogen tempering to improve steel toughness, it is good to gather data doing a side-by-side comparison to see if these are truly improvements to the state of the art in yourapplication.

The best solution for the debate between Predictive and Preventative Maintenance is toincorporate both in your operation. Preventative Maintenance should be followed with almost areligious fervor on every mold. It will keep quality and profits up, while keeping costs down.Predictive Maintenance should guide your decisions on when to stop the refurbishment processand begin replacing cavities or the entire tool.

My attitude has always been Were here to make profit not technology breakthroughs. Measure,

log, plot and analyze those things that are important, but dont make a fetish or career out of it.Your Predictive Maintenance philosophy should be an adjunct to Preventative Maintenance. Bothof these are contributors to profit.

Think about it.

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