The pounding could be heard halfway across the plant. As the clock on the wall raced toward the end of the shift, the repair shop was in high gear assembling several molds—and quickly—that had been pulled for scheduled PMs. It is in this time of the day—appropriately known by some as the 4:00 slam—when short cuts are taken and mistakes get made as repair techs are under the gun to hurry and finish their molds, get them out to production and go home. No fiddling and fitting here … it’s hammer time.
As I approached one bench, I could see the repair tech obviously had a plan. With the cavity plate lying face down, 32 cavities stood complete with o-rings and a light coat of grease, partially perched in their respective bores, waiting to be installed. With a five-pound rawhide hammer in hand, he quickly sighted down the dowel pin in each cavity to its locating slot in the plate. Satisfied the alignment was “close enough” he let fly with the rawhide and drove the first cavity into the bore with one, carnival like, bell ringing swing.
The cavity bottomed out (how could it not) in the bore and without missing a beat (literally), he continued driving the others home in a random “whack-a-mole” fashion until all 32 cavities were seated. I was amazed that only four of the 32 locating dowels broke off.
The retaining plate was then bolted on and the plate hooked up to a portable water table to check for leaks. As I took cover well out of range, not surprisingly, water seeped from several of the abused cavities o-rings.
Like he had done it a hundred times before, the tech marked the leaking cavities, mumbled something about the mold being junk, disassembled the plates and removed the affected cavities as brutally as he had installed them.
Upon inspection, every leaking o-ring had a nice half-moon gash cut out of it at the point the o-ring crossed over the internal water channel.
Style Points Count
As we have discussed before, mold maintenance styles vary in range from fast, freewheeling and furious to excruciatingly slow and over cautious. After a mold has been run, disassembled, troubleshot, repaired and cleaned it pays to stop, clean off the bench, gather your thoughts and regroup before proceeding on with tooling installation and mold assembly.
But when working with grizzled skilled tradesmen, and especially if you are wearing a shirt and slacks, it is not wise to question their technique, unless you are fully prepared to break out your own tools and demonstrate a better way or get wet. But managers/supervisors can, and should, question the results of a technique.
So having whacked a few moles myself, I was pretty sure what his logic was going to be before I asked, so I politely inquired as to the supposed advantages in this particular method of installation. The frustrated repair tech informed me that he had been taught that cavities with o-rings need to be installed quickly, so that the o-ring can “jump” past the water line before it has a chance to expand into the channel and be cut.
He appeared open to suggestions as he was tired of repeating the procedure several times until, simply by luck, the leaks just stop—at least on the bench and under no clamp or injection pressure.
After many years of impromptu behind the bench competitions with dozens of repair techs of varying skill levels debating the best way to perform particular job functions, there are certain clear cut methods that proved effective on troublesome installations and I believe worth mentioning.
- Make certain the o-rings are the correct size. With this type of seal, o-ring cross section should not exceed 15 percent of the total gland depth (including tolerances) to insure you are not trying to compress 10 pounds of potatoes into a five-pound bag.
- If the bore walls are rough and pitted enough to abrade the o-rings (old or neglected tooling or plates or those not made of stainless steel), try using o-rings that give more abrasion resistance (carboxylated version of high-acrylonitrile butadiene copolymer (XNBR). Just keep in mind the effects polar solvents have on Nitrile o-rings—such as softening, swelling degradation, etc. There always is a performance trade-off when changing o-ring elastomers. Your o-ring supplier will know the particulars.
- O-ring lead-ins (chamfers) should be 15 degrees and as wide as the o-ring cross section. Short, 45-degree chamfers will shave o-rings in half.
- Break all sharp edges of intersecting waterlines, counter-bores and dowel alignment slots.
Installing O-rings onto Cavities and Cores
- Apply ample grease (this means to coat completely) the o-ring and especially the bore walls. When not completely coated with grease, o-rings that are twisted during installation will remain twisted. I have observed (and lost money on) o-rings slowly untwisting in their glands after installation.
- Because of varying gland finishes, some o-rings will remain slightly twisted even though they are fully greased. When this happens you can help untwist them by carefully lifting them out of their gland with a small diameter punch (old ejector pins work nicely) and rotating the punch around the cavity several times. This action will help coax an o-ring back into its original molded shape. Be careful not to over-stretch the o-ring. A general rule is never to stretch o-rings over 75 percent of their size.
- Instead of “rolling” an o-ring over cooling ribs to get it into its gland, fabricate cones that go over these ribs allowing for much easier and quicker installation (see photos).
Installing Cavities into Bores
- Start the cavities into the bores by hand and line up any flats or dowels. This can be accomplished more accurately with a small square, but most do it by eye, and practice makes you better.
- Make sure the cavities are straight and not on a bind. I am not a fan of using dead-blow or soft hammers here or anywhere you need to be able to “feel” how tight a fit or cavity is and to gauge how much you are actually moving (or binding) the cavity or piece of tooling. I like to make hammers from CRS (cold rolled steel) because the steel is softer than store bought hammers and they don’t chip and flake like aluminum, brass or bronze. Also as a rule of thumb, a cavity must be inserted at least a ¼ of the diameter into a bore before the bore walls will effectively guide it. For example, a cavity two inches in diameter needs to be installed at least ½” into its bore. Until the cavity reaches this ½” wall length, you run a higher risk of binding or cocking the cavity, and creating a bur on the bore wall that can cut an o-ring or get trapped under it.
- When started straight and properly greased, cavities that are four inches in diameter or less are many times able to be installed with simply hand pressure. If you can’t install by hand, then use light persuasion “gradually,” allowing the o-ring time to compress in the gland. Moving slowly increases the ability of the o-ring to slide past a water channel or other transitions, as again the rubber will compress and slip past these edges instead of being cut from a single swift movement. As a bonus, it is much easier, and less damaging to mold plates and tooling when the cavity can rotate slightly allowing the locating dowel to “seat” itself into the slot when installed gradually and not binding.
Pitted Glands: Desperate Moves in Desperate Times
When glands get pitted and cause slow leaks, the right repair is to send the plate or tooling out and have the gland welded or plated and re-cut to print dimension. In some cases, you can re-cut pitted glands to take the next larger size o-ring. Unfortunately, when production is on the line, there is not always time to do this, so other measures—or dare I say it, band-aids—are used to get by until the glands can be repaired properly. The challenge then is how best to fill these pits and stop the leak. There are several methods that I have used with success and though not a professional fix they still work in a pinch.
Rubber silicon (almost too easy)
Silicone works well in filling pits and stopping leaks. But unless you like unflattering nicknames, don’t get into the habit (as many do) of using it anywhere you spring a leak or think a gland looks pitted. Silicone will effectively glue plates and tooling together making disassembly and clean-up very difficult.
Precautions to take:
- The glands must be thoroughly cleaned (ultrasonic works best) and free of grease/oil.
- You must wait until the silicone sets up (usually four to eight hours) before applying water pressure or risk cleaning out half cured silicone and more leaks. You can speed the curing process by applying some heat (200 deg. max) if working on the manifold side of the mold.
- The cured silicone is hard to remove from the glands if the process must be repeated, as when removing a cavity for replacement or inspection. And you must remove all traces of cured silicone before new can be applied.
- If too much silicone is applied, it will squeeze out into bubbler (fountain) return channels where it sets up and restricts water flow, or worse, breaks off and ends up lodged inside a fountain tube.
Two-part epoxies work well in filling glands (I prefer JB Weld) and are more permanent, but they too have issues:
- The glands must be glass bead blasted to remove all traces of grease or contamination for good adhesion.
- Make a miniature trowel from a small rod that fits the gland area so that you can evenly spread the epoxy for a thin and uniform coat.
- If the coating isn’t as smooth as you would like after it has dried, you can spot face the epoxy in a mill or if you are a true mechanical surgeon, using a dremel and a small wheel (fine) stone.
Finish the Job
Always perform a “final check” on a mold before releasing to production. There is no better means to prevent unnecessary mold pulls than to discover the problem while the mold is still benched.
Once you are satisfied with the installation, apply water and check it out. Take it to max line pressure, usually about 90 psi. then back down to standard press pressure of 45 to 60 psi. Don’t be alarmed if water initially begins to drizzle out from around a cavity. As water pressure plays with the o-ring, it will seep as it untwists, pushes out the grease and finds a seat. Light and careful tapping on tooling with a small hammer will send vibrations through the tooling that will speed up this process.
Putting the hundreds of pieces of a mold back together after it has been strewn across the shop is a process that requires focus and patience. To keep our businesses home, maintenance must continue rising to higher levels of craftsmanship by eliminating or greatly reducing the number of maintenance “oversights” that cause unscheduled downtime, such as unnecessary water leaks.