We are going to apply stop-off paint to the end (about 1½ inches) of a part. We are thinking of putting a 1-inch-wide by 6-inch-diameter RUBBER BAND on the part OD before apply the stop off paint. Then we are going to put the parts (hundreds of them) in our integral quench furnace for carburizing. What do you think would happen to the furnace and quench oil over time? What would this do to our atmosphere?


“Rubber bands” as we think of them are normally made from organic rubber (because it offers superior elasticity). Natural rubber comes from latex, a milky fluid composed primarily of water with a smaller amount of rubber, inorganics, fatty acids and trace amounts of resins, alkaloids, starches, sugars, oils, tannins and gums. As the latex dries out and then burns, hydrochloric acid, hydrogen cyanide and styrene gases may be evolved. All are irritating and poisonous. In some cases, the resin is combustible and burns with a thick, black smoke.

By contrast, “synthetic rubber” can be produced pure or with the addition of specific additives and/or impurities designed to produce certain desired products. These would introduce a host of other undesirable chemicals into the furnace atmosphere. In addition, this rubber is often vulcanized, having sulfur added (sulfur, if present inside a heat-treat furnace, forms a low melting-point eutectic with nickel and causes melting of these nickel-based, high-temperature alloys – fans, radiant tubes, chain guides, etc.)

In either case, I would expect the rubber to shrink on heating and burn off inside the furnace – and more than likely leave a sticky residue or film on the parts. This residue will be carried into the quench tank and wind up in the oil. I would be surprised if parts coming out of the furnace didn’t still have residue on them that might be difficult to remove. Bottom line: I would encourage you to cut these off the parts after making or find a better way to apply your stop-off and avoid putting rubber bands into your integral quench furnace.