IMM 5/97

We are trying to find a better way to seal plugs and fittings for the oil lines in our high-temperature molds. Oil leakage from these fittings has resulted in part contamination. We use NPT brass plugs and fittings; we've also tried steel. And we've tried several brands of high-temperature thread sealant. Any suggestions?

This issue, at first, seems like a simple one, but one can become justifiably frustrated. This "simple" problem is a troublesome one to resolve. With new engineered materials raising processing temperatures, combined with higher demands for cleanliness, this has become an increasingly common problem. There are three factors that contribute to the leaking of oil lines in high-temp molds during the molding process:

• Poor thread / plug seal upon installation.

• Thermal flexing at the sealing point of the plug and the threaded hold. This occurs from the thermal expansion and contraction during the repeated start-up and shutdown of the tool.

• Vibration affecting the seal point. Because this leakage can lead to part contamination, extra care should be taken.

Considerations during mold construction:

• Threads. The integrity of the threaded hole is critical. Holes should be carefully drilled and reamed, if possible (using a tampered pipe reamer). To prevent spiral leakage, use only NPTF pipe tapes, not NPT. If at all possible, machine the full depth of he thread. Hand tapping to finish the thread to full depth will almost always produce threads that are out of round and unusually slightly out of square, depending on the amount of thread produced by hand. Tap breakage is usually the concern here, but good drilling, reaming, and tapping practices will eliminate most pipe tap breakage. Finally, the threaded hold should be plug gauged for thread depth and size, to ensure correct seal with male fittings.

• Plugs. A pressure-tight taper pipe seal is achieved through full metal-to-metal contact of the mating threads. Ask your pipe fitting supplier if its products are manufactured and gauged to specifications for size, taper, and thread form. Inferior fittings on the market can cause any number of sealing problems.

• Assembly. Teflon, in either tape or liquid form applied to the male fitting, will provide good thread lubrication as the pipe fitting is torqued to proper sealing depth. A variety of pipe sealants and lubricants is available, offering good results in most cases. It is important to remember that taper pipe threads produce a metal-to-metal pressure-tight joint. The use of tapes and sealants simply allows the fittings to reach sealing position without seizing or galling. They also permit fitting removal without thread damage.

Considerations during molding:

• Preventive maintenance. At scheduled maintenance points, look for any oil weeping from the inlet. The likely cause is thermal flexing that occurs between mold storage and start-ups. Vibration is also a cause for plug leakage, but in injection molds, this should be less of a factor.
  Record that oil inlets were checked and also record at what point any leakage was noticed. This data will assist predictive maintenance in the future, and in communicating with your mold builder that integrity of the "tapped hole / proper plug / correct assembly" issue is one that is measurable from one mold to the next.

• Replacement. At the first sign of leakage, replace all bad plugs or fittings. Simply removing and reapplying tape or dope is not sufficient. To create the metal-to-metal pipe seal, threads are deformed during initial assembly. New fittings must be used to reestablish the metal-to-metal taper pipe seal. Conscientiousness to these issues will help control leakage. And because this problem is leading to mold downtime and contaminated parts, the extra care is worth it.

-G. Starkey, Progressive Components, Wauconda, IL (800) 269-6653.

Try using SAE straight-thread plugs that seal with an O ring instead of the threads. Specify Viton O rings for use up to 400F. These plugs and the port dimensions should be available from most hydraulic fitting manufacturers, such as Parker Hannifin Corp. I have not seen these plugs used for high-temperature molds; however, they work very well in hydraulic systems.
-J. Kennedy, Fiskars, Power Sentry Div., Wheaton, MN, (320) 563-4500.

Use steel pipe plugs with a 7/8-inch taper per foot and seal with a Permatex Ultra Copper high-temperature RTV silicon gasket. It's good up to 600F continuous.
-T.Jackson, GH Tool and Mold, Washington, MO, (314) 390-2424. 

First, do not use brass or any yellow metals, which contribute to the oxidation of heat transfer fluid. Use steel parts only. Our recommendation is to use a combination of taping the fitting with high-temperature, anti-seize tape by Milrose, which is effective to 2200F, and applying a thin bead of high-temperature red Permatex RTV sealant over the tape. The sealant is available pretty much anywhere-even auto parts stores carry it.
-M. Pastor, Mokon, Buffalo, NY, (716) 876-9951.

We use brass plugs and stainless steel pipe fittings on all high-temp molds (up to 450F). We seal both with Drip Stop 920 pipe sealant with Teflon (from Hernon Mfg., Sanford, FL, (407) 322-4000). Molds run for years without leaks.
-T. Erwin, Pitney Bowes, Danbury, CT, (203) 790-3647.

We have found steel plugs with Permatex High-Temp Form-A-Gasket (sealant, No. 1372) to work the best. You must let it dry for at least 24 hours.
-B.St. Denis, Rockwell Automation, Allen-Bradley Co., Manchester, NH, (603) 625-8299.


We use Permatex high-temperature RTV silicone sealant, manufactured by Loctite Corp. It is available in 3-oz tubes.
-R. Rudynski, Wells Electronics Inc., South Bend, IN, (219) 287-5941.

There is a product called Astroseal from a company called EPD Technology. This sealant works well in oily, wet, or hot conditions. It should do a great job in this application.
-J. Remmel, Derby Plastics Ltd., Appleton, WI, (414) 738-9979.

The solution to the problems associated with hot-oil mold controllers is to stop using them. When you consider how inefficient oils are at heat transfer and dissipation, and the potential dangers of pumping super-heated fluids around, it is amazing that someone has not invented / developed some alternative approach.

Here's ours: Direct heating of the mold using electric heating overcomes the problem of circulating super-heated fluids (and leaks) and is more energy efficient, saving between 20 and 70 percent of energy inputs.

The cooling of the thermoplastic part can now be carried out with a cool fluid through the existing cooling channels in the mold. For mold temperatures between 95 and 240F, water can be used from the mold shop's central system. For mold temperatures between 200 and 365F, compressed air can be used from the shop's air lines. Molds are in production in Europe using the direct-pulsed electrical heating and compressed air cooling, with mold temperatures of 330F, for PPS and LCP.

Experimental work is being carried out with an Italian hot runner manufacturer molding Teflon, in which mold temperatures of 465F are in use, using a modified version of this system
-R.Evans, R.E. Promotions Services Ltd., Bridgtown, Cannock, U.K., (44)1543 466 585.