Here’s how a typical West Coast molding facility with 20 injection presses investigated the downtime impact of water leaks in the plant. Always looking for a competitive edge and a way to reduce costs, plant personnel put together a year-end Defect Costs summary to use as a guide in the tool room, so they could aggressively pursue their goal of reducing unscheduled mold stops.
RUST NEVER SLEEPS
As most mold maintenance audits reveal, water leaks, both internal and external continue to be one of the lead causes of unscheduled mold downtime. This West Coast company suffered no differently. Mold pitting wastes thousands of dollars per year through excessive repair hours and spare tooling, as well as press idle time, scrapped parts, and missed or late orders. Molds and components get replaced prematurely that could have—and should have—made thousands of more cycles and parts, if only specific precautions had been taken to prevent or at least slow down the erosion.
While pinched, cut, or missing o-rings and other assembly oversights share responsibility as root causes for water leaks, rusted and pitted o-ring glands are the main factor, since molds continue to be built and run with no thought to corrosion prevention in either mold design or maintenance practices.
Nourished by heat, pressure, water, and oxygen, rust takes no time off as it slowly eats away our molds. The eroding steel usually goes untreated until the leak is sufficient to stop production. Then, maintenance will be focused only on getting the mold back into production quickly.
HOW TO STOP LEAKS
As discussed here in past columns (see Learn More box), stopping the leak usually involves one or more of the following:
- Stuffing a larger o-ring into the pitted gland.
- Stuffing the gland with two o-rings whose combined cross-sections are slightly larger than the single o-ring being replaced.
- Cleaning up the pits by milling the gland area with an endmill (sharpened with a flat face) and resizing the o-ring.
- Filling the pitted gland with silicone.
- Filling the pits with epoxy.
These are the cheap, short-term fixes. If any of these work—yippee. If not, then we try again—and again.
With critical parts, as in medical products, any kind of water leak that could possibly migrate to the product is cause for shutting down. On other products, I have seen major water leaks simply “redirected” into buckets or an area around the press to keep the mold in production as long as possible. Is this shop-floor ingenuity—or poor molding practice? The answer depends on which culture you were raised in.
So as to not lose sight of reality here, some companies have molds whose yearly volume requirements leave little money to repair a leak professionally and permanently. So the mold is set with rags tied where necessary to absorb and redirect the dripping water, with little thought given to related issues that come with operating in this mode. It is assumed to be a better business decision to “just let ’r leak.”
Then there are higher-volume molds where a misguided decision is made to just deal with the random leaks when they occur, by pulling and performing one or all of the maintenance steps outlined above.
More involved but permanent repairs include grinding plates and tooling (usually involving dimensional restacking), as well as welding, brazing, or nickel plating for future protection. If the leak is unfixable (due to cracked water lines or tooling) you can skip the repairs and just buy a negative-pressure temperature-control unit (TCU) that draws water through the mold rather than pushing it through.
COMMONLY AFFECTED AREAS
In molds, most water leaks occur around static seals where dissimilar metals come into contact, such as a P20 plate against hardened steel tooling such as A2, D2, or S7. This contact sets the stage for electrochemical reactions that form the reddish iron oxide that we know as rust. The oxide is a larger molecule than iron, so it puffs up and cracks, which exposes more bare metal. Mobile oxygen in the metal moves deeper into the base steel.
Cavity blocks and cores are the typical victims, where the walls and bottoms of glands pit and erode until the o-ring can no longer conform to the depth of the pit and water seeps past. The leaks can be enhanced by a molds opening and closing, acting as a virtual pump by slightly moving or shifting tooling with every cycle, and by opening stress cracks that would normally not leak during a static bench test of the circuits.
|Defect Analysis Report|
|Defect Type |
|Internal Water Leak|
LEAK COST ANALYSIS
Is anybody going to automatically send a mold out to get the leaks fixed correctly just because it’s the right thing to do? Not unless there is viable ROI in it, which is why it is important to assign an accurate cost to any mold problem. “Show me the money” is the name of the game for justifying any repair.
Let’s take a look at our West Coast molder’s water-leak costs for last year through its 2008 Defect Analysis report. This company discovered some startling facts about the overall costs of simple water leaks, as seen in the accompanying report.
During the year, 28 mold stoppages occurred for internal water leaks and four more for external leaks (not shown here) where hose lines popped off, were too short, or worn through (which soaks the entire mold rather than just one area). We can see from the Stop Reason Count list that the problem was spread among 14 different molds, two of which accounted for 36% of the total leaks in frequency and cost.
Direct tooling and labor are only part of the cost of mold leaks. Total labor hours (173.25) can also be applied to press idle time, in this case $45/hr, for another $7796.25. Add in 3 hr average for each of the 28 stops—for mold pull, reset, and start-up—and you can tack on another $4200, adding up to $23,468.75 in total costs for water leaks. Total Defect Cost is thus about $4000 each per year for the top two leakers (6169 & 6749). Not exactly coffee money.
|Mold-Stop Reason Count|
Closure Cap “C”
Top Cup “D”
Top Cup “C”
15 ML VHC “K”
18 ML VHC “L”
12 ML VHC “E”
3-Hole Locking Switch
Top Cup “B”
Large Ring “D”
Internal Water Leak
CHOOSE YOUR TARGET
As you can see from this list of leaking molds, the worst actor (6169) accounted for six mold stoppages, so here is a good place to start. Further review of the Corrective Action history revealed the mold had four o-ring groove locations that were pitted and in need of welding and refacing, followed by stripping and nickel plating the entire 18 x 24 x 2 in. mold plate. The cost of this repair was $675 to weld and spot-face the four grooves and another $300 to apply nickel plating, for a total repair cost of $975, making the ROI for this repair about 4 months. Looking at it from this perspective makes the repair decision a no-brainer.
It is not correct to assume that the cost of putting up with random water leaks is less than the cost of eliminating leaks altogether. When you see the total costs of this common problem, it becomes an easily justified decision to fix the mold right. Then you will have one less issue to worry about, and you can proceed on to the next most frequent/costly defects your molds suffer and eliminate them one by one. Before you know it, you will have reduced your costs of unscheduled downtime by 50% or more, which means thousands of dollars in savings, increased capacity, and less stress in the shop and on the molding floor.
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 email@example.com or (419) 289-0281.