How to Determine Maximum Mold Cycle Counts

At my first production meeting with my new employer, I was told, “We want you to establish a preventive maintenance program that is based on maximum cycle counts for all our molds.” Then, before I could launch into an explanation of how best to determine maximum cycle counts, I was hit with the follow-up: “So, how many cycles do you think our molds can safely run before we need to clean them?”

Leaving my crystal ball at home and unable to get my x-ray vision working, there was no way to answer this question with any real accuracy. Unfortunately, that was not what they wanted to hear. The group of PhD’s surrounding the table looked on with minor disgust, assuming this should be an easy question for a “mold guy” to answer. They were also disappointed because they would not be able to leave the meeting with an immediate production scheduling plan that would help molds run better and eliminate some tooling damage they had suffered in the past from molds being over-run.

 

NO GENERIC NUMBERS

Not wanting disappoint on the first day, I threw out a standard rule of thumb used when pressed for max cycle counts: 250,000 cycles is typically a good starting point for the average injection mold, but it can vary greatly—up or down—depending upon the following production characteristics and mold design features:

• Resin type, additives, required temperature, corrosiveness, abrasiveness, and flashing tendency.
• Residue (off-gas) type—powdery, flaky, gummy (tar-like), or oily consistency.
• Vent locations, depth, and finish.
• Vent dump size, location, and configuration (ring, channel, blind pockets, etc.)
• Static vs. dynamic (moving-tooling) vents.
• Tooling plating type and condition.
• Tooling tolerances and running fit.
• Any inaccessible internal bushings or other actuating components.
• Internal condensation level and mold-plate steel type.
• O-rings and other internal water-seal leakage history.
• Specific hot- or cold-runner issues.

Another critical element when determining maximum cycle counts is knowing part defect frequencies and locations that can be affected by many of the above criteria. Part defects “localized” by unbalanced filling or steel variations must be recognized and considered when setting maximum cycle counts. But generally speaking, anytime you over-run a mold, you increase the risk of molding bad parts through burns, shorts, weld lines, or dirty or flashed parts. Mold off-gassing or residue will end up someplace if it can’t escape through clean vents.

 

IN REAL LIFE

Let me say first that stopping and pulling molds for scheduled preventive maintenance is good practice—period. Unfortunately, it not always possible if you want to keep your customers happy.

Obviously, if a mold was pulled for a good cleaning every time its maximum cycle number lit up, those responsible for mold maintenance would be grateful. With scheduled maintenance, it is much easier to more accurately gauge the labor hours required to disassemble, clean, and return the mold to production, knowing that the tooling has not been worn prematurely with gummed up close fits. “No surprises” in mold repair is always a good thing for schedulers and for the tooling and labor budget. But in the real world this seldom happens because “Production rules.”

Molders are customer driven—as they need to be—when it comes to running a mold to complete an order. If a couple more hours, shifts, days, or even weeks are required to complete an order, the mold will run—or lock up trying.

So why bother setting max cycle counts if they are not adhered to? Although we may not always be able to clean every mold every time it needs it, timely cleaning is critical with some molds because of the damage that can occur when maximum cycle counts are ignored. When steel is gone—it’s gone. So the challenge in the shop is to know max counts for molds along with a “safe range” in which molds should be pulled if possible, but may be run longer without undue damage if there’s danger of missing the production schedule.

Molds that are subjected to many, short production runs are also in danger of premature wear if the tool room doesn’t track total mold cycles over a series of short orders. Without a tracking system, it’s easy to damage tooling because cycle counts get lost in the day-to-day affairs of pulling and setting lots of molds.

 

AN EXPENSIVE LESSON

Over-run molds create situations that require more abrasive cleaning methods, which can be hard on tooling and drastically reduce usable life. For instance, one particular 16-cavity medical mold had a sleeve/core setup with a typical radial vent-ring groove (0.060 wide x 0.001 in. deep) at the bottom of the part around the core, which fed into three flats (0.250 x 0.060 in. deep) that ran down the core lengthwise into a radial vent dump 0.125 wide x 0.125 in. deep. The part vented fine up to about 150,000 cycles, (or 20 days, or 480 hours), which was typically how long it took to fill up the vent dumps under normal processing conditions.

Once the vent dumps were full, the residue had nowhere to go and was forced past the flats between the core and sleeve, where the resulting sliding action of the core and sleeve during ejection turned the typical red, powdery residue into a black, gummy substance that quickly wore down the softer—and more expensive—core. This left flash on the bottom of the part. The mold builder would not permit our tool room to increase the size of the vent dump on the grounds that it would compromise sleeve/core alignment during part fill and ejection.

 

RUN IT ANYWAY

Our particularly aggressive production manager made the controversial decision to run the mold an extra week (50,400 more cycles) to gain additional uptime. When the mold was finally pulled, nine of the 16 cavities had flash close to or greater than the specified maximum (0.003 in.), requiring us to send the cores back to the builder to be welded, ground, and plated. The sleeves were so gummy that the ejector plate could not be moved by hand and required considerable force with a 5-lb mallet to drive the cores out from the sleeves.

Through ongoing diligent inspections during repairs, we found that even a few hours over the maximum cycle count caused measurable excess wear. We also found that if the mold over-ran its cycle limit, the cores required abrasive cleaning (“Scotchbriting”) to remove the ground-in residue. If this cleaning was performed incorrectly, a repair tech could easily “round-over” the sharp, leading edge of the sealing diameter of the parting line. That in turn would cause flash, setting up yet another Tool-Room-vs.-Process debate over the root cause of the flash. But if the mold were cleaned during the “safe range” of cycle counts, the residue was quickly and easily removed ultrasonically, causing no additional wear whatsoever. So from that day on, this mold had a mandatory max count of 150,000 cycles—no exceptions.

An expensive lesson indeed, but a common problem that highlights the importance of setting accurate numbers for maximum cycle counts on specific molds.

 

INCREASING RUN TIME

On the other hand, there are many molds that can easily run more than 250,000 cycles between cleanings, which also need to have that number validated. We had one 48-cavity jump-thread cap mold that we started out at 250,000 cycles max count and found—after steady count increases of 100,000 per run—that it could actually run up to 1,500,000 cycles before it needed cleaning! The residue on this mold was contained in non-critical static vent dumps that posed no threat to moving tooling or shut-offs.

For non-tooling-trained administrators, it is sometimes difficult to understand why molds that run similar resins have such different reactions to extended cycle counts. This is why the effects of process variations need to be monitored and understood so we can maximize run time without additional component wear. Over-maintaining molds wastes money, too—and increases the chance for mold damage during repairs.

 

BASIC DATA REQUIRED

What you really need to know to set maximum mold cycles can only be determined through close visual inspection of mold plates and tooling after a production run. You also need accurate answers to a few production-related questions that will dictate maintenance requirements:

• Date and time the mold was started.
• What press did it run in.
• Who started it.
• What configuration is the mold running.
• Date and time the mold was stopped.
• Who stopped it.
• Why was it stopped (scheduled or unscheduled stop reason).
• How many cycles did the mold run.

The following are also very helpful when evaluating mold residue and wear levels:

• Was the production run interrupted by stops for changeovers, unscheduled breakdowns, or weekend downtime?
• Were repeated ejector cycles required to release the parts from the mold?
• Was the mold properly and regularly serviced in the press during the run?
• Were the cycle time and processing parameters consistent with past runs?
• Are any part defects related to flash in a vented area?

Molds and tooling are not getting cheaper and thus can significantly reduce a company’s profit margin through off-the-cuff decisions to run molds longer just to fill a truck—or an empty corner of a warehouse. “Efficiently producing quality parts on time” is the mantra of every molder. You cannot live this mantra by allowing production requirements to dictate maximum cycle counts in every situation.

 

About the Author

Steven Johnson worked as a toolmaker for 26 years, rebuilding and repairing multicavity molds for Calmar Inc. and then as mold-maintenance engineer for Hospira Inc., a medical device manufacturer. Today, he is the maintenance systems manager for Progressive Components and has his own business, MoldTrax in Ashland, Ohio, which designs and sells software for managing mold maintenance (www.moldtrax.com). He can be reached at steve@moldtrax.com or (419) 289-0281.