Mold trial run is the key link to verify hot runner temperature control stability, but cold junction offset faults induced by junction box heat radiation, humid air and wiring errors rarely show obvious abnormal alarm codes during trial production, only producing subtle temperature reading deviation that technicians easily ignore. After mass production is officially launched, the deviation gradually expands, triggering batch product dimensional over-tolerance, color difference and melt degradation defects. The concealment mechanism of cold junction offset in trial runs and targeted rapid detection methods can screen hidden cold junction interference before mass production.
The core concealment characteristics of cold junction offset during trial runs: First, short trial production time cannot accumulate deviation superposition. Mold trial run usually only lasts 1–3 hours, the junction box has not absorbed enough heat radiation from the hot runner manifold, and the cold junction temperature only rises slightly by 3–5°C, generating a small offset of 2–4°C, which is within the allowable process adjustment range, so technicians will only fine-tune the set temperature value instead of checking the cold junction interference source. After 24 hours of continuous mass production, the junction box internal temperature rises steadily to 38–45°C, the cold junction compensation error expands to 7–12°C, and the adjusted process parameters completely lose balance, forming mass defective products. Second, trial run ambient temperature is stable and dry, without seasonal high humidity and temperature fluctuation interference factors in formal production workshops, so water vapor-induced pin oxidation cold junction drift will not appear during short trial runs, and only breaks out in rainy and hot summer mass production. Third, trial run mold wiring is newly installed, plug pins are clean without oxide powder, and poor contact cold junction offset caused by pin corrosion will not occur temporarily; after weeks of production, oxide powder accumulates on pins to generate intermittent offset faults that cannot be reproduced in trial run tests.
Four hidden inducing factors that cannot be fully exposed during trial runs:
1. Junction box unreasonable layout close to high-temperature manifolds: Short-time heating cannot make the box reach stable high temperature, cold junction compensation error is tiny and difficult to perceive; long-term continuous heat radiation will continuously raise the cold junction temperature and amplify deviation.
2. Unshielded thermocouple cables laid parallel to heater power lines: Short trial run operation has low heater power load, weak electromagnetic radiation intensity and slight signal offset; full-power continuous production will strengthen interference and lead to severe cold junction reading distortion.
3. Double-end grounding of shielding braided layers forming ground loops: The trial run injection molding machine and temperature control cabinet are not fully loaded with other equipment, the workshop ground potential difference is small, and the ground loop induced offset is inconspicuous; multi-mold simultaneous mass production will enlarge the potential difference and generate fixed temperature drift.
4. Thermocouple plugs without waterproof sealing in humid workshop environments: Trial run time is short, water vapor cannot condense on pin surfaces; long-term overnight shutdown makes water vapor accumulate inside the junction box to corrode pins and trigger offset.
Four rapid detection methods to expose cold junction offset hidden dangers during mold trial runs, without waiting for mass production fault outbreaks:
1. Extended constant temperature holding test: After the trial run conventional parameter test, set all hot runner zones to 300°C and hold constant temperature for 4 consecutive hours, regularly record the temperature reading of each thermocouple every 30 minutes. If the reading of a certain zone gradually decreases by more than 4°C as the holding time extends, it proves that the junction box absorbs manifold heat radiation to generate cold junction offset, requiring heat insulation transformation of the box before formal production.
2. Cold junction artificial cooling contrast test: Use a small cold air fan to blow the mold junction box continuously for 20 minutes, observe the change of temperature controller readings. If the displayed temperature rises significantly after cooling the plug terminals, it confirms that the original cold junction temperature is too high and there is compensation offset; normal fault-free thermocouple readings will not produce obvious changes after cooling the junction box.
3. Single-end grounding shielding inspection test: Temporarily disconnect the grounding wire of the shielding layer at the controller end and observe whether the temperature reading deviation is reduced. If the reading becomes stable after disconnection, it proves that double-end grounding forms a ground loop cold junction offset interference source, which needs to be modified to single-end grounding.
4. Cross wiring exchange test: Swap the thermocouple plugs of two adjacent heating zones with consistent actual melt temperature, observe whether the temperature deviation transfers to the corresponding channel following the plug. If the deviation moves with the plug, the fault source is cold junction contact or junction box interference; if the deviation stays in the original heating zone, the problem lies in the probe hot junction drift or installation gap.
After completing the four sets of trial run cold junction hidden danger tests, all offset interference sources can be eliminated in the mold modification stage before mass production, avoiding huge batch scrap and mold shutdown losses caused by delayed cold junction offset faults after formal production line launch.
