by Steve Johnson, ToolingDocs LLC
As most mold maintenance audits reveal, both internal and external water leaks continue to be the front running causes of unscheduled mold downtime. Pitting – the simple act of electrons from one steel component migrating to another – wastes thousands of company dollars per year through excessive repair hours, additional tooling costs, press idle time, scrapped parts and missed or late orders. Molds and components have been prematurely replaced when they 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.
Rust never sleeps
Working 24/7 and nourished by heat, pressure, water and oxygen, rust takes no time off as it slowly eats away at molds. The eroding steel that is the result of this electrochemical reaction usually goes untreated until the leak is sufficient to stop production. Only then is a spotlight shined on the problem, and questions arise. From there, the focus of maintenance revolves around what is necessary to quickly get the mold back into production. In other words, focus is thrown onto stopping the leak versus stopping the rust.
Band-aiding the leak is usually handled by
- Stuffing a larger o-ring into the pitted gland
- Stuffing two o-rings into the gland, whereupon the combined cross-sections are slightly larger than the single o-ring
- Filling the pits with epoxy (J/B Weld) or
- Filling the pitted gland area with silicone sealer (most popular).
These are cheap and short-term fixes that don’t solve the problem, causing repair technicians to try again and again for a solution.
Commonly affected areas
In molds, most water leaks occur around static seals where dissimilar metals, such as a P-20 plate, contacts hardened steel tooling, such as A2, D2 or S7. This dissimilar contact sets the stage for the formation of Fe203, the reddish form of iron oxide that we know as rust. The oxide is a larger molecule than iron, so it puffs up and cracks, which exposes bare metal. Mobile oxygen in the metal moves deeper into the base steel, continuing destruction and creating the “rust lives” mythology.
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 mold’s opening and closing, acting as a virtual pump by slightly moving or shifting tooling with every cycle. This opens stress cracks that would normally not leak during a static bench test of the circuits.
Read the full article, which includes cost evaluation of mold repairs, in the Spring 2012 issue of Plastics Business, due in May.
Steven Johnson is operations manager for ToolingDocs, a provider of mold maintenance training and consultation based in Ashland, OH. He designed and developed MoldTrax™, a documentation software system for tracking mold performance and maintenance. To learn more, call 800.257.8369 or visit www.toolingdocs.com.