Troubleshooting Hot Runner Leakage in Plastic Molds
2026-02-12 10:33:48
Plastic Molds
Hot runner leakage is one of the most costly, disruptive, and dangerous failures in injection molding. It can cause seized molds, burned heaters, damaged manifolds, cracked cavities, and extended unplanned downtime. Over 90% of leaks occur at sealing surfaces, nozzles, clamping screws, flanges, or heater and thermocouple holes. Leakage must be diagnosed in order from external to internal, simple to complex, to avoid unnecessary disassembly and secondary damage. Quick and correct diagnosis directly reduces production loss and maintenance cost.
How to Identify Hot Runner Leakage
True hot runner leakage shows typical and recognizable symptoms: burnt plastic flowing from parting lines, ejector plates, or support gaps; solidified plastic buildup inside the mold or around the manifold; sudden short shots, flash, or inconsistent filling; unstable temperatures or heater alarms; and hard resistance, unusual noise, or locking during mold opening. If any of these appear, stop machine, shut off material, and lower temperature immediately to prevent further damage. Continuing production under leakage conditions will almost certainly lead to severe mold damage.
Nozzle Tip LeakageThe most common location of leakage is the nozzle tip, where the hot runner contacts the mold cavity. Check for worn, collapsed, or damaged sealing surfaces, mismatched R angles, insufficient contact with the cavity insert, or inadequate thermal expansion compensation. Long-term production wear, improper installation, or incorrect mold processing are common causes. Repair or replace the nozzle tip and refit the mold seat to ensure tight, stable sealing under high temperature and pressure.
Manifold‑to‑Nozzle LeakageLeakage in the manifold chamber often leads to massive plastic accumulation and widespread mold contamination. Causes include loose clamping screws, damaged or dirty sealing surfaces, missing or deformed gaskets, and uneven loading during assembly. Remove the top plate, clean the manifold area thoroughly, replace worn or damaged gaskets, and evenly torque the nozzle screws diagonally to ensure uniform pressure. This type of leakage often affects multiple nozzles at the same time.
Flange and Locating Ring LeakageLeakage at the machine nozzle interface usually results from misalignment, worn sealing surfaces, damaged insulation pads, or loose mounting bolts. Incorrect nozzle center alignment is a frequent cause during mold installation. Realign the nozzle center accurately, replace damaged seals and insulation pads, and evenly tighten mounting screws to ensure stable connection between the injection machine and hot runner system.
Heater and Thermocouple Bore LeakageHidden leakage often occurs at heater or thermocouple holes due to oversized bores, loose sealing plugs, or failed high-temperature seals. Molten plastic can enter and wrap around heating elements, causing burnout, temperature failure, and short circuits. Check sealing plugs, replace damaged components, and ensure proper fitting to avoid seepage. This type of leakage is difficult to detect from the outside and often only found after heater failure.
Insufficient Preload and Height Mismatch
Systematic leakage happens when the overall hot runner height is incorrect or preload is insufficient. Thermal expansion creates gaps between components, causing multiple leaks simultaneously. This is a design or processing issue rather than a component failure. Measure total height accurately, adjust support pillars or spacers, and ensure proper preload during assembly to compensate for thermal expansion.
Standard Troubleshooting ProcedureFirst, perform an emergency shutdown and cool the system to safe handling temperature. Clean the exterior to trace the leakage path and identify the general area. Remove the top plate and inspect the manifold for plastic accumulation. Check nozzle clamping, sealing surfaces, and gaskets. Examine heater and thermocouple bores for seepage. Verify total height and preload. Repair or replace damaged parts, then test manually before resuming production.
Prevention and Quick SolutionsDo not inject further once leakage is detected. Always clean sealing surfaces during assembly to remove dust, debris, and residual plastic. Tighten screws diagonally and evenly to avoid uneven pressure. Calibrate nozzle centering for new molds before production. Conduct regular maintenance of heaters, screws, seals, and wiring. Prevention is far more effective than repair for hot runner systems.
ConclusionMost hot runner leaks result from poor sealing, insufficient preload, improper assembly, or component wear. Following a structured inspection sequence allows quick and accurate location of leaks. Timely diagnosis and proper repair minimize downtime, mold damage, and production loss. Establishing regular inspection and maintenance procedures can effectively prevent most hot runner leakage issues and ensure stable long-term operation.
