Long-term high-temperature operation of hot runner thermocouple measuring heads will continuously accumulate carbonized plastic residues, forming dense carbon layers that block heat conduction. Many maintenance personnel only clean carbon deposits when obvious temperature drift occurs, lacking regular graded cleaning plans and standardized operation steps, resulting in long-term heat transfer barrier of the measuring head, delayed temperature response and frequent parameter adjustment. This chapter analyzes the formation mechanism of carbon deposition, graded cleaning judgment standards, safe cleaning operation flow and post-cleaning protection measures, realizing regular carbon removal maintenance to extend the service life of thermocouples.
1. Formation mechanism and performance hazards of measuring head carbon deposition
The core raw materials for injection molding such as PP, ABS, PPS, PA contain polymer chains, flame retardants and lubricants. Under the high temperature of hot runners above 280℃, a small amount of molten plastic will decompose and produce carbon black particles, which are adsorbed on the surface of the thermocouple measuring head. The nano ceramic anti-stick coating can slow down carbon adsorption, but long-term accumulation will still form layered carbon deposits. The carbon layer has extremely low thermal conductivity, forming a thermal resistance barrier between the mold metal and the thermocouple sensing point. The controller reading will always lag behind the real mold temperature, the PID heating power adjustment is delayed, the nozzle temperature is prone to overshoot, and the product has consistent defects such as wire drawing, black spots and uneven gloss.
With the thickening of carbon deposits, the measurement drift value increases day by day. A carbon layer of 0.1mm thickness will produce a fixed negative deviation of 1~1.5℃; when the thickness exceeds 0.3mm, the deviation can reach more than 3℃, and the product dimensional tolerance will be seriously out of control. In addition, the hard carbon layer will scratch the anti-stick coating when cleaning, accelerate the coating peeling, and form a vicious cycle of faster carbon deposition after coating damage. Acidic substances produced by plastic decomposition will be wrapped in carbon deposits, corroding the metal substrate of the measuring head, forming tiny pits, further increasing the adhesion of carbon residues, and greatly shortening the overall service life of the thermocouple.
2. Three-level carbon deposition visual judgment standard for regular patrol
Level 1 slight carbon deposition: A thin layer of light gray carbon dust adheres to the measuring head surface, no hard caking, and can be completely blown off with low-pressure nitrogen. No obvious temperature drift occurs, the response speed is normal. Maintenance requirement: add nitrogen blowing cleaning in the daily mold closing inspection item, no need to disassemble the thermocouple for deep cleaning, shorten the cleaning cycle by three days.
Level 2 moderate carbon deposition: Dark gray partial hard carbon film appears on the sensing head, nitrogen blowing cannot remove it completely, and alcohol wiping is required to clean thoroughly. The temperature has a slow drift of about 0.8℃ within a week, the product occasionally has tiny black spots. Maintenance requirement: arrange disassembly and full cleaning during the next mold planned shutdown, and check the integrity of the anti-stick coating after cleaning.
Level 3 severe carbon deposition: Thick black block carbon deposits cover more than half of the measuring head surface, hard and not easy to wipe off, local coating has fallen off, metal substrate exposed. The temperature drift exceeds 2℃, the temperature curve fluctuates obviously, and batch defective products appear continuously. Maintenance requirement: immediately stop production to disassemble and clean, re-spray anti-stick coating after cleaning, and check whether the hot runner temperature is set too high to reduce plastic decomposition.
3. Graded standardized carbon cleaning process
Light on-mold nitrogen auxiliary cleaning (without disassembly, daily routine)
Prepare 0.2MPa low-pressure dry nitrogen gun, align the air nozzle with the measuring hole of the thermocouple, and blow continuously for 30 seconds to blow away loose carbon dust. It is forbidden to use high-pressure nitrogen above 0.5MPa, which will impact and scratch the anti-stick coating on the measuring head surface. After blowing, observe the surface state of the measuring head with a magnifying glass, and record the carbon deposition grade in the daily inspection form. This operation can be completed during the mold cooling waiting period after shutdown, without additional downtime loss.
Moderate disassembly alcohol cleaning (planned shutdown maintenance)
Step 1: Complete power cut and cooling treatment of the hot runner, wait for the measuring head temperature to drop below 50℃, slowly unscrew the positioning screw and take out the thermocouple horizontally to avoid carbon deposits falling into the measuring hole.
Step 2: Fold lint-free cloth soaked with isopropyl alcohol, gently wipe the surface of the measuring head repeatedly, soften and strip the thin carbon film, do not use metal scraper to scrape hard to prevent coating damage.
Step 3: Blow off residual alcohol and carbon debris with nitrogen, place the thermocouple in a ventilation place to dry completely for 10 minutes, check the coating for peeling and scratches.
Step 4: Wipe the residual carbon deposits in the mold measuring hole with alcohol cloth, blow dry and coat a thin layer of thermal conductive paste, reinstall the thermocouple and adjust the positioning screw torque to the standard value.
Severe carbon deposit chemical soaking cleaning (coating damaged and thick carbon block)
Step 1: After disassembling the thermocouple, peel off the residual coating and carbon block on the measuring head with neutral ceramic coating remover, soak for 12 minutes, and the hard carbon layer will automatically fall off.
Step 2: Scrub gently with soft copper brush to remove residual carbon slag, rinse with IPA alcohol, and bake in a constant temperature oven at 120℃ for 2 hours to completely remove solvent residues.
Step 3: Polish the measuring head surface with 1200# polishing paste to remove corrosion pits, blow off polishing dust, and carry out nano ceramic anti-stick coating spraying and curing according to the re-coating process standard.
Step 4: After the coating is fully cured, conduct a high-temperature test to confirm the anti-carbon deposition performance is restored before reinstallation on the mold.
4. Post-cleaning anti-carbon deposition protection measures
After each cleaning, evenly coat a thin layer of high-temperature thermal conductive paste on the measuring head, which can fill the tiny pits on the metal surface, reduce the adhesion point of carbon deposits, and accelerate heat transfer efficiency. Optimize the hot runner process parameters appropriately, avoid long-term temperature setting exceeding the plastic recommended processing temperature by more than 20℃, reduce the decomposition rate of plastic raw materials and carbon generation. Strengthen the mold exhaust maintenance, clean the exhaust groove every week to discharge the decomposed volatile gas in time, reduce the concentration of carbon black particles around the measuring point.
For production lines with easy carbon deposition such as flame retardant PA and PPS, shorten the disassembly cleaning cycle from one month to two weeks, and increase the frequency of daily nitrogen blowing inspection. Record each cleaning time, carbon deposition grade and coating state in the thermocouple lifecycle management system, track the carbon deposition speed change, and adjust the maintenance cycle dynamically according to the actual production load.
5. Forbidden operation in carbon cleaning
It is forbidden to use steel wire balls, metal blades and other hard tools to scrape carbon deposits, which will scratch the anti-stick coating and form irreversible pits on the metal substrate. Do not use strong corrosive solvents such as acetone for long-time soaking, which will corrode the compensation wire outer jacket and accelerate wire aging. Do not install the thermocouple before the alcohol or cleaning agent is completely dried, residual solvent will vaporize at high temperature to form new carbon pollution, and may cause short circuit hidden danger inside the measuring hole.
