How to Repair Minor Hot Runner Cracks

As we spoke before about severity determination of hot runner fractures and modification of repair techniques, small hot runner cracks means shallow micro-cracks less than 1/3 rd of the wall thickness, spread only on the outer surface in non stressed places. Repair can be done via the following conventional procedure:

I. Pre-treatment

Full Crack Removal : Grind the cracked region out entirely with an angle grinder until you reach the unbroken metal substrate. • Perform penetrant test to confirm no crack points remain.

Overall preheating and temperature control: The entire hot runner is preheated to 200-300 ℃ to lower the temperature difference between the welding region and the substrate to reduce the instantaneous welding stress.

II. Welding

Choosing the welding wire correctly: Choose a welding wire that is the same composition as the hot runner substrate, for example S308L welding wire for 304 stainless steel.

Low heat input welding: Control of weld deposition in each pass is provided by multi-layer, multi-pass TIG welding at low current. Each pass is followed by a light hammer blow on the surface of the weld with a pneumatic hammer for real time relief of some of the welding stress.

III. Postprocessing and Validation

Stress relief heat treatment: The weld is put into the heat treatment furnace within 12 hours after welding is completed, and kept at 600-650℃ for 2-3h, and then cooled to room temperature slowly, so that the residual stress can be entirely eliminated.

Final check and verification: The restored region is flat with the surrounding area. Penetrant testing was used to determine that no new microcracks were present. A stepwise heating and cooling cycle test is performed. Once no irregularities are found, the weld can be put into service.

This method prevents the formation of secondary cracks and extends the service life of the repaired hot runner up to 60%-80% of a new hot runner.