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Preparation for the Pinch-off

Tubulation I.D. must be totally free of contamination at the weld point. Sonic or mechanical cleaning rather than chemical cleaning prior to pump down yields the best cold-welds. The O.D. of the tubing should be polished with 320-grit emery cloth to remove any oxide crystals. Oxides are harder than the base metal and can interfere with the cold-welding process.

The lapped tungsten carbide anvil inserts of the press must be cleaned before each pinch-off; often a very thin ribbon of work-hardened metal remains on the anvil insert and must be removed. Any contamination pressed into the metal at the weld point can injure the cold-welded seal. The pinch-off is a cold extrusion and a lubricant can be used to aid the material flow. A clean #10 machine oil works well on most metals.

During the pinch-off, hand or hydraulic pressure must be applied evenly until the tubing severs suddenly. Any interruption of this process while the material is in a plastic state will result in an incomplete cold weld. Hydraulic pumps, whether hand-powered, air booster or electric, provide reliable service and assure the best results. Leaks are likely to occur if the pinch-off phase is incomplete and the tubing needs to be wiggled apart.