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Molders say: prevention is the best medicine for injection molds.

Processors tell how scheduled maintenance keeps their molds healthy and part quality in top shape.

Maybe nothing lasts forever, but no molder wants expensive injection tools to age prematurely. Unfortunately, the effects of temperature extremes, pressure, moisture, and even the polymer itself can too easily band together to wear out a mold before its time. These unavoidable enemies of tooling eventually shut molds down for good, though not necessarily before producing scores of inconsistent shots and sending your mold on repeated trips to the repair shop. On the bright side, you can minimize the ravages of a mold's environment with a simple prescription from some successful molders. Just practice prevention, they say. Instead of waiting until something goes terribly wrong, develop a regularly scheduled maintenance regimen for molds.

Prolonging mold life takes more than a coating of rust inhibitor or other maintenance products, say molders and toolmakers who are sold on the idea of preventive maintenance. It's not so much what you put on a mold, they say, it's when you do it. "You have to come up with a schedule and stick to it. That's the important thing," advises Jeffrey Lotz, molding supervisor at Crown Cork & Seal Co., a beverage-cap maker in Sandston, Va.

One toolmaker, John Von Holdt Sr., president of Plas-Tool Co. in Niles, Ill., estimates that a mold commonly expected to produce 4 million shots could actually last for up to 20 million shots, "if it's taken care of properly." So strong are his company's recommendations for preventive maintenance that its mold guarantees actually hinge on a customer's adherence to a regular program. If the molder doesn't perform complete cleanings at Plas-Tool's specified intervals--say, every 1000 hr or so--all bets are off as far as the guarantee goes.


Perhaps the trickiest part of a maintenance program is actually developing the schedule, one that spells out what to do and when to do it. The molders we talked to agree that it's nearly impossible to put a finger on hard-and-fast guidelines because of the wide range of products, polymers, presses, and process variables. Many molders handle a wide range of jobs within their plants, creating a need for many different maintenance schedules individualized for specific molds.

Custom medical and electronics molder Nypro Inc. of Clinton, Mass., requires "a custom-tailored program" for each and every mold, according to director of engineering John Casali. That level of individualization arises because the maintenance issues differ with each job. On a pipette-tip mold, for instance, part straightness is a chief virtue. "Even the slightest core shift can cause the parts to warp," says Casali. So part of the preventive maintenance procedure in this case includes almost daily measurements to make sure the core hasn't shifted out of line. Other visual inspections identify obvious problems such as wear, clogged vents, worn leader pins, or obstructed water lines.

By contrast, molders with just a few products can determine a nominal schedule from experience with a given family of molds. At Crown Cork & Seal, Lotz says complete cleaning of the company's beverage-top molds take place every 460,000 shots or about once every eight weeks. Between these complete mold scrubbings, 15-min cleanings with the mold in the press and visual inspections of mold components are performed weekly.

One strategy that could help molders develop mold-maintenance schedules is to keep a maintenance and repair history for each tool. Later on, this information can serve as a predictive tool for setting new cleaning and repair timetables for other molds. As a side benefit, molders say they use the historical data when quoting to identify jobs that have a high potential for mold maintenance and repair costs.

These histories would include detailed descriptions and costs of any repair and cleaning work done to a mold, according to Ed Jolley, tool-room manager at Mack Molding Co., a custom molder in Cavendish, Vt. Using a card-file system, Mack has stored such information for some time. Nowadays, however, the computer may prove a molder's natural ally in keeping track of mold maintenance.

Indeed, all the molders cited here report efforts to create computerized databases of maintenance information. The software need not be specialized. Instead, Lotz notes, mold-maintenance information can be organized with commonly available database software. For Crown Cork & Seal, mold and press maintenance information already go into a database with the help of a Fox-pro software package from Microsoft Corp. Nypro and Mack Molding, meanwhile, are in the midst of switching to computerized mold databases.


Production goals--or the notion that they aren't compatible with preventive maintenance--are perceived by some manufacturers as an obstacle to sticking with a maintenance schedule. But preventive maintenance doesn't have to entail a production sacrifice at all. Instead, it can carry unexpected productivity and quality benefits, molders say.

For about two years, the Sandston plant of Crown Cork & Seal has had a program of regularly scheduled visual inspections and cleanings, some of which require the mold to be taken off the press. "We don't let production goals get us off our maintenance schedule here," Lotz says. The resulting benefits have been well worth it. The maintenance program has bumped up mold-cavity utilization from 85% to 96%. That means far fewer cavities are blocked off because they produced bad shots. Moreover, cycle times have improved 26% overall, thanks to the current state of mold cleanliness.

Crown's program has also helped improve part quality by standardizing the molding process--something that was elusive before the current program went into place. Lotz explains, "The molds were never in a uniform state of cleanliness before, which made it difficult to standardize pressures, temperatures, and cycle times."

Likewise, preventive maintenance goes hand in hand with part quality at Nypro. "In searching for zero defects, you can't wait until a problem occurs," says Casali. Many of the company's customers have quality standards so stringent that running a dirty mold or one with wear problems simply is out of the question. Flash specifications, for instance, have commonly dropped from a max. of 0.005 in. in years past to 0.002 in. today, according to v.p. of engineering Rick Hoeske. "Some of our customers are looking for zero flash," he says. "So we have to do more cleaning and checking for mold wear to maintain their standards."

With some programs, preventive maintenance ironically has led to molds staying on the press longer. For example, weekly reviews of mold condition help Mack Molding draw an accurate bead on mold problems before they turn serious--and expensive. Under its collaborative approach, the plant, tool room, quality, and molding managers jointly determine the condition of every operational mold by examining the week's final shots. "Together, we get a good feeling of what needs to be done with a mold so we can accurately write any necessary work orders," Jolley notes. "We don't end up tearing down the mold unnecessarily."

In the past, by contrast, the causes of any part-quality problem might have been incorrectly attributed to the tooling, when process variables were the real culprit. A mold might even have been sent to the shop unnecessarily, Jolley says.

During Mack's quality meetings, the group looks for telltale signs that maintenance time has come--things like incorrect part dimensions, parting-line or knock-out pin flash, and fill problems. And though the part may tell almost the whole story, the group of managers also considers observations on mold performance from press operators who get a good view of the molds in action.


A final, but key, element shared by these three molders is that they all try to anticipate problems and build low-maintenance features into their new molds. All concede that the strategy costs more going in, but the payback comes from a long-lasting tool that runs consistent shots.

"We look for features that allow us to maintain the mold at a consistent level of performance," notes Nypro's Casali. So the company has gone to more expensive molds, oftentimes investing up to 10% more up front for stainless-steel plates and sometimes cavities and cores as well. "Everything that's not stainless steel is at least nickel-plated," says Casali. And he adds that Nypro has been experimenting with a number of coatings, titanium nitride (TiN) for one, that allow grease-free operation. "We still haven't found any that work in every situation."

An important aspect of Crown's maintenance program aims at minimizing downtime when the inevitable problem does occur. Favoring a single source for its tooling, the company keeps a large inventory of interchangeable parts like knock-out pins, bushings or even whole cavities. This way, even a 64-cavity mold with damaged cavities can be brought back on line, completely cleaned, with all cavities working, within 32 hr. "We just replace any damaged cavities with a spare to get the mold ready quickly," he says. The damaged tooling would then be fixed later.

A Few Tips for Healthier Molds

When we asked experienced toolmakers and molders with successful mold-maintenance programs what advice they had for other molders, responses always seemed to come back to scheduling rather than to specific ways to clean or store molds--since these procedures seem by necessity to vary greatly from molder to molder. Nevertheless, we learned ways to avoid a few mold-maintenance pitfalls that many molders seem to fall into:

* Toolmakers targeted water lines as being especially needful of attention, because even the tiniest amount of scale can really dampen mold performance. According to Al Fosco of chiller and water-treatment supplier Conair Tempro Inc. in Elgin, Ill., scale just 0,006-in. thick on a molds water lines can cut its cooling capacity by 6%. And the numbers go up geometrically from there. To combat scale encroachment, molders need a "four-pronged strategy," Fosco says. Both open- and closed-loop cooling systems should include filtration, conductivity control, an automatic feed system for scale and corrosion inhibitor, and automatic biocide feeding.

* Hot runners can constitute another problem area when operators don't correctly monitor the system's temperature zones. If the temperatures go too high, especially during cycle interruptions, polymer degrades in the manifolds, explains Bob Duffy of Polymer Cleaning Technology (PCT) Inc., Sommerville, N.J. He should know. PCT performs contract cleaning of injection mold components. Using a fluidized-bed system, the company removes the carbon leftovers of degraded polymer on hot-runner manifolds, breaker plates, and even inserts. "We get most of our work from residual polymer problems. That's what we address."

* As for rust-prevention sprays, they shouldn't go on a hot mold or moist mold if you expect them to work, notes Herb Wilhelm of D-M-E Co. in Madison Heights, Mich., which offers its own line of mold-maintenance products.
COPYRIGHT 1992 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:includes related article
Author:Ogando, Joseph
Publication:Plastics Technology
Date:Dec 1, 1992
Previous Article:Why thermoforming is looking better than ever.
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