Hot Runner Not Heating: Troubleshooting Guide

A hot runner system failure to heat is a common and disruptive issue in injection molding, directly causing material flow interruptions, part shortages, or complete product scrap. Effective troubleshooting must follow a "Simple to Complex, Electrical to Mechanical" sequence to systematically identify and resolve problems in four core areas: power supply, temperature control system, heating elements, and mechanical contact structures.

1. Electrical Power & Wiring Check

Before attempting any mold disassembly, prioritize eliminating power supply and wiring issues, as these are the most frequent causes of heating failures.

Power Supply Verification: First, check if the main workshop circuit breaker or the dedicated hot runner circuit breaker has tripped. If tripped, investigate potential overloading or leakage; inspect the wiring for short-circuit signs before resetting. Additionally, confirm that the supply voltage matches the hot runner system’s rated voltage (typically 220V single-phase or 380V three-phase, refer to the heating element nameplate for accuracy).

Terminal & Connection Inspection: Examine power cables for damage, aging, or breakage. Disassemble the terminals connecting the temperature control box to the mold, checking for looseness, oxidation, or burning. Oxidized or loose terminals increase contact resistance, blocking normal current transmission. Use sandpaper to clean oxidized terminals and re-tighten the wiring; replace severely burned terminals and re-crimp the wires if necessary.

Continuity & Short-Circuit Test: In a power-off state, use a multimeter in resistance mode to measure both ends of the power cable. An infinite resistance reading indicates an open circuit, while a near-zero reading accompanied by cable heating signals a short circuit—replace the entire cable in either case.

2. Temperature Controller  Check

The temperature control box acts as the "command center" for hot runner heating; malfunctions here will directly prevent heating commands from being executed.

Display & Fault Code Check: Observe the temperature controller display for error codes (e.g., "E1" for open thermocouple, "Overload" for excessive current). Refer to the manufacturer’s manual to interpret specific codes. If the controller shows no display at all, first check if its internal power is connected and if the fuse is blown; replace the fuse with the same specification before retrying.

Parameter Setting & Output Test: Confirm that the set temperature on the controller is reasonable (matching the melting temperature of the plastic being processed) to avoid no heating due to a set temperature lower than room temperature. Switch the controller to manual mode (supported by most models) and use a multimeter in current mode to measure the output terminal. A stable current output indicates the controller is functioning normally; no current output means the internal modules (such as solid-state relays or the main control board) are faulty and require repair or replacement.

Thermocouple (Temperature Sensor) Check: The thermocouple is responsible for feeding back the actual hot runner temperature; an open circuit or poor contact will trigger a controller fault and stop heating. Disconnect the thermocouple wiring while power is off and measure its resistance with a multimeter—normal thermocouples have a fixed resistance value (varying slightly by model). An infinite reading indicates a broken thermocouple that needs replacement with the same model. Also, ensure the thermocouple is firmly inserted into the hot runner’s temperature sensing hole; clean any carbon deposits or debris in the hole to ensure accurate temperature measurement.

3. Heating Element Check

Heating elements (cartridges or bands) are the core heat-generating components and are prone to breakage, short circuits, and aging under long-term high-temperature operation.

Visual Inspection: Remove the hot runner mold’s heating cover and check for obvious signs of melting, bulging, or carbonization on the heater cartridges or bands. If a large amount of plastic residue adheres to the surface, it will insulate heat and reduce efficiency—clean the residue before retesting.

Resistance Test: Measure the resistance of the heating element terminals with a multimeter and compare it to the standard resistance value (found on the element nameplate or hot runner manual):

Infinite resistance: Internal breakage of the heating element; replace with a same-specification cartridge or band.

Near-zero resistance: Short circuit of the heating element; replace immediately to avoid damaging the temperature controller.

Resistance deviation >10%: Aging of the heating element, leading to reduced heating efficiency; replacement is recommended to ensure stable heating.

Fit Check: Excessive gaps between the heating cartridge and the installation hole in the hot runner will cause poor contact and low heat transfer efficiency, resulting in "controller showing heating but no actual temperature rise in the hot runner." Replace the cartridge with a properly sized one or fill the gap with thermal grease to enhance heat conduction.

4. Mechanical & Structural Check

If no issues are found in the first three steps, inspect the hot runner system’s mechanical structure or special conditions.

Manifold Deformation Check: Long-term high temperatures or mold opening/closing impacts may cause slight warping of the hot runner manifold, preventing tight contact between heating elements and the manifold. Use a dial indicator to test the manifold flatness; if the deformation exceeds 0.1mm, professional calibration or replacement is required.

Blockage & Cold Slug Check: Severe blockages in the hot runner nozzle or channels will trap cold material around the heating elements, blocking heat transfer to the melt and potentially burning the elements due to local overheating. Disassemble the hot runner system, clean charred plastic and cold slugs from the nozzles and channels, then reassemble and test.

Insulation & Grounding Check: Damage to the heating element’s insulation layer will cause leakage, triggering the protection mechanism of the temperature controller or workshop leakage protector to cut off the heating power. Use a megohmmeter to measure the insulation resistance between the heating element and the manifold—normal resistance should exceed 2MΩ. If the resistance is too low, replace the heating element and check if the mold is properly grounded.

5. Verification & Prevention Measures

Verification Steps: After troubleshooting, set the temperature controller to 50℃ below the melting temperature for preheating, observing if the temperature rises stably. Once stable, gradually increase to the process-set temperature and check that all hot runner areas heat uniformly without local overheating or temperature lag.

Prevention Measures: Regularly clean carbon deposits and charred plastic from nozzles and channels to avoid blockages; inspect and re-tighten terminals and heating elements after each production batch to prevent loosening; calibrate the temperature controller periodically to ensure control accuracy; avoid long-term high-temperature dry-firing of the hot runner system without material to slow heating element aging.