When Assembling Molds, Patience is THE Virtue

Installing and fitting close-tolerance tooling requires patience, a steady hand, attention to detail, and the ability to “read” resistance.

Installing and fitting close-tolerance tooling requires patience, a steady hand, attention to detail, and the ability to “read” resistance. It can take 15 to 20 hr to even get to the assembly stage, so why would anyone want to rush through it? Yet oddly, this is where most mistakes occur.

Re-pulling molds to install left out tooling, tighten bolts, or fix an internal water leak further strains the delicate relationship between the molding and tooling departments. And, as too many molders are painfully aware, one small miscue can result in thousands of dollars in unscheduled downtime, lost product, a damaged mold, and stressed-out employees.

To avoid this, you must develop and meticulously follow procedures to ensure that the hundreds of pieces of tooling will be installed in the proper position, with proper fits (static and dynamic), so that the mold will function as designed for as long as it’s supposed to.

To prepare for assembly, first get the bench cleared of any clutter—cleaning tools, loose paperwork, measuring instruments, etc.

Working in the vertical position means all plates must be standing, braced, and aligned in the proper sequence, and ready to be assembled/installed. All bolts and tooling relevant to position should be placed between the appropriate plates. That way it’s easy to give everything a final pass to confirm that nothing has been overlooked.

Then spray a healthy coat of rust inhibitor on the back (counterbored) side of any plate that has been prone to get especially dirty or rusty during past runs. Medium-duty rust inhibitors that go on clear and dry quickly work great for short runs, such as 250,000 cycles or less, or around 60 days. Use heavier, wax-type inhibitors if long runs are normal, if the mold is subjected to heavy condensation, or if it is to be racked for a long period.

Coat all fountain or bubbler plates and any other plates exposed to internal condensation or heavy contamination. Rust inhibitors also work well on plated or stainless-steel plates because they allow the residue to be ultrasonically removed much faster.

It is critical to protect any exposed molding surfaces of cavity tooling from overspray of the rust inhibitor, as this can cause serious galling or leaching issues. A list of which plates to coat, and a procedure on how to coat them safely, should be noted in the mold-maintenance manual. Once the coating has been applied and dried—only a few minutes—you can begin assembling the mold.



Pinched, twisted, or cut o-rings are responsible for many a repair technician’s bad days. Taking a few precautions during installation will greatly reduce the potential for o-ring damage that may not show up during a bench water or air test, but begins to drip after a few cycles in the press.

  • Verify the correct (print) size of the o-ring.
  • Lubricate the o-ring and the bore the o-rings slide against.
  • Take care not to twist o-rings while stretching them over the heads of tooling and make cone-shaped fixtures to ease this task where necessary (see accompanying photo).
  • Take the time to chamfer or break sharp edges inside plates and internal waterlines that cross over counter-bores.
  • Install o-ringed tooling slowly into bores, giving the o-ring time to collapse. Use only hand pressure wherever possible.
  • If the installation feels wrong, it probably is. Remove the tooling immediately and check the integrity of the o-ring. Don’t waste time by taking a chance that the leak test will be okay.
  • Keep o-rings in good shape by changing them at specified cycle intervals.

Installing tooling into tight-fitting bores and pockets takes patience and the “feel” to detect whether or not the tooling is starting straight, and if the resistance felt is normal. It is very easy to start close-fitting tooling on a bind, assuming it is just a tight fit that needs some extra persuasion, usually in the form of a large hammer.

Tooling with sharp leading edges is especially difficult. An impatient or heavy-handed repair tech might carelessly start the tooling crooked, then continue to drive it further into the bore, hoping it straightens itself out or seats home before it locks up completely. The burr created by this once-sharp tooling can get trapped between the tooling and the bottom or sidewall of the bore, jamming the piece.

The fires from this type of workmanship can be slow to ignite and not immediately recognized. Washed out bores and dinged/damaged and tight-fitting tooling are probable causes for many types of issues that reduce mold performance and inflate labor and tooling costs, but careless workmanship is the root cause.

Another area of assembly that can cause major issues if rushed is when plates containing tooling are installed all in one move to save time. For example, a sleeve retainer plate is bolted to a backing plate with 32 sleeves installed. Then the entire unit is slid onto the mating plate via guide pins and bushings while assuming the sleeves will locate safely into their respective bores. This can work if the plates and any hardened internal tooling are built with generous 30° lead-ins (versus the typical 45°) on every bore. But many molds (especially cheap or older ones) have minimal or no lead-ins, and the critical ends of sharp tooling may be damaged while attempting to insert many pieces into their bores at the same time. If just one sleeve binds or doesn’t start completely, it is difficult to determine exactly which one is causing the plate to stick.

A safer method is to make this assembly with the backing plate off and the 32 sleeves in the retainer plate pushed slightly back out of harm’s way. This will prevent the sleeves from contacting the edges of the bores while you align the leader pins and bushings.

Align the leader pins to the bushings in the retainer plate first, and slide the retainer plate on. This will allow the sleeves to be more accurately and safely slid into their bores one at a time. Another seemingly simple function of mold maintenance is installing and tightening the hundreds of socket-head cap screws (SHCSs) that hold plates and tooling in place. Most problems here can be attributed to differing opinions of what constitutes tight. Too often bolts are tightened by hand or pneumatically to the point of stripping, just to make sure they don’t vibrate loose.

To counter this, some shops require all SHCSs to be tightened to a set torque value, usually determined by the thread/diameter and steel type of the bolt. This is effective, but can really slow up assembly. A more efficient method is to learn what the proper torque feels like through experience, using a calibrated torque wrench as a guide. If you use a pneumatic gun, use the torque wrench to verify the different clutch settings on the gun. Also instrumental is the quality of the SHCSs and Allen wrenches used for installation.


About the Author

Steven Johnson is the operations manager for ToolingDocs LLC, part of the PCIC Group of Companies. Steve also has his own business, MoldTrax in Ashland, Ohio. He can be reached at steve.johnson@toolingdocs.com or (419) 289-0281.