The Harm of Thermocouple Measuring Head Carbon Deposition and Full-Cycle Cleaning Standard Process

During the long term high temperature operation of hot runner thermocouple measuring heads, constant accumulation of carbonized plastic residues in form of dense carbon layers blocking the heat conduction would happen. Most of the maintenance workers just clean carbon deposits in the case of visible temperature deviation, but they do not have regular graded cleaning plans and standardized operation methods, resulting in the long-term heat transmission barrier of the measuring head, delayed temperature response and frequent parameter adjustment. This chapter explores the formation mechanism of carbon deposition, the graded judgment standards for cleaning, the safe cleaning operation flow, and the measures for post-cleaning protection, so as to realize the regular maintenance of carbon removal and prolong the service life of thermocouples.

1. Formation mechanism of measuring head carbon deposit and performance hazards

The basic raw ingredients for injection molding, such as PP, ABS, PPS, PA, are made up of polymer chains, flame retardants and lubricants. At the high temperature of hot runners over 280℃, a little quantity of molten plastic will breakdown and form carbon black particles, which are adsorbed on the surface of the thermocouple measuring head. Nano ceramic anti-stick coating can slow down the adsorption of carbon, but long-term accumulation will still produce multilayer carbon deposits. The carbon layer has a very low thermal conductivity and provides 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 result has consistent faults such as wire drawing, black spots, and uneven shine.

As the carbon deposits get thicker, the measurement drift value increases day by day. A 0.1mm carbon layer will bring about negative deviation of 1~1.5℃ persistently; when the thickness is beyond 0.3mm, the deviation can be over 3℃, and the dimensional tolerance of the product will be out of control significantly. Also, the hard carbon layer will scratch the anti-stick coating while cleaning, hasten the peeling of the coating and create a vicious cycle where the damage of the coating results in faster carbon deposition. When plastic decomposes, it produces acidic compounds. These acidic substances will be encapsulated by carbon deposits, which corrode the metal substrate of the measuring head, generating microscopic pits. This further increases the adhesion of carbon residues and greatly shortens the overall service life of the thermocouple.

2. Three-level standard for visual appraisal of carbon deposition in routine patrol

Level 1 mild carbon deposition: Light gray carbon dust is deposited in a thin coating on the surface of the measuring head, no hard caking, completely blown off with low pressure nitrogen. No visible temperature drift, the response speed is normal. Maintenance requirement: incorporate nitrogen blowing cleaning in the daily mold closing inspection item, no need to detach thermocouple for deep cleaning, shorten the cleaning cycle by 3 days.

Level 2 moderate carbon deposition: Dark gray partial hard carbon coating appears on sensing head, nitrogen blowing can not take it off entirely and alcohol wiping is necessary to clean thoroughly. Temperature has a steady drift of roughly 0.8℃ each week. The product sometimes contains little black spots. Maintenance requirement : during next planned mold downtime, arrange disassembly and comprehensive cleaning and after cleaning examine the integrity of anti-stick coating.

Severe carbon deposition, level 3: Thick black carbon deposits on more than half the measuring head surface, hard and difficult to clean, local coating fallen off, exposed metal substrate. The temperature drift is more than 2℃, the temperature curve changes noticeably and the batch defective items are produced continually . Maintenance requirement: Stop production immediately for disassembly and cleaning, then re-spray anti-stick coating after cleaning and verify whether the hot runner temperature is set too high to reduce plastic breakdown.

3. Standardized carbon cleaning technique with grading

Light on-mold nitrogen auxiliary cleaning (disassembly not necessary, daily routine)

0.2MPa low pressure dry nitrogen pistol, line the air nozzle with the measurement hole of the thermocouple, blow continuously for 30 seconds, blow away the loose carbon dust. It is illegal to have high-pressure nitrogen above 0.5MPa, which will impact and scrape the anti-stick coating on the surface of the measuring head. 2. Blow, use a magnifying lens to check the surface state of the measuring head, and fill in the carbon deposition grade in the daily inspection form. This operation can be performed during mold cooling waiting time after shutdown, without further loss of downtime.

Medium Disassembly Alcohol Cleaning (Planned Down Time Maintenance)

Step 1: Power off completely and cool the hot runner; wait until the temperature of the measuring head is below 50℃. Then slowly loosen the positioning screw and remove the thermocouple out horizontally to avoid carbon deposits dropping into the measuring hole.

Step 2: Soak the lint-free cloth with isopropyl alcohol, softly 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 the remaining alcohol and carbon debris with nitrogen, set the thermocouple in a ventilated area to dry thoroughly for 10 minutes, examine the coating for peeling and scratches.

Severe carbon deposit chemical soaking cleaning damage thick carbon block and coating

Step 1: After disassembly, remove the measuring head of the thermocouple, and use neutral ceramic coating remover to peel off the remaining coating and carbon block. Soak for 12 minutes, and the hard carbon layer will fall off automatically.

2. Scrub with soft copper brush gently to remove remaining Carbon slag, rinse with IPA alcohol and bake in continuous temperature oven with 120℃ for 2 hours to remove solvent residues completely.

3. Polish the surface of the measuring head with 1200# polishing paste to remove corrosion pits, blow off polishing dust, then spray and cure nano ceramic anti-stick coating in accordance with the re-coating process standard.

Step 4: Reinstall on the mold after fully cured and execute a high temperature test to ensure that the anti-carbon deposition performance is restored.

4. Anti-carbon deposition preventive measures after cleaning

After each cleaning, a thin layer of high temperature thermal conductive paste should be uniformly spread on the measuring head, which may fill the microscopic pits on the metal surface, lower the adhesion point of carbon deposits and accelerate the efficiency of heat transmission. Optimize the hot runner process parameters to prevent long-term temperature settings from exceeding the plastic suggested processing temperature by more than 20℃, therefore decreasing the decomposition rate of plastic raw materials and carbon generation.