The connection between thermocouple inaccuracy and part defects is often underestimated. When a thermocouple provides incorrect temperature data, the controller compensates in the wrong direction, directly causing a range of observable molding defects. Understanding these cause-and-effect relationships enables faster troubleshooting and more precise corrective action.
Silver Streaks and Splay. When a thermocouple reads lower than actual temperature, the controller overpowers the heater, overheating the melt. Excess heat degrades the polymer, releasing gases that become trapped at the melt front, appearing as silver streaks or splay on the part surface. This is particularly common with moisture-sensitive materials like PC, PET, and PA. If splay appears suddenly on a single zone without changes in material drying, suspect thermocouple drift in that zone-the sensor may be reading 5–10°C low.
Burn Marks and Yellowing. Overheating caused by a drifting thermocouple can also lead to thermal degradation, manifesting as burn marks, yellowing, or brown streaks. Engineering plastics like ABS and PC are especially sensitive; a 3°C excess above the recommended melt temperature can begin to cause discoloration. When burn marks appear in one cavity or one nozzle zone, the thermocouple in that zone is the first place to check.
Short Shots and Incomplete Filling. A thermocouple reading higher than actual causes the controller to reduce power, cooling the melt below its optimal viscosity. The resulting high-viscosity melt fails to fill thin sections or complex geometries, producing short shots. This is often misdiagnosed as insufficient injection pressure, but the root cause may be a thermocouple that has drifted high by 2–4°C.
Flash and Oversized Parts. Conversely, when a thermocouple reads low, the melt becomes overheated and less viscous. The low-viscosity melt can penetrate the parting line, causing flash. Overheated melt also shrinks more during cooling, potentially producing undersized parts, while excessive fill pressure from low viscosity can overpack cavities, producing oversized or heavier parts.
Warpage and Dimensional Instability. Uneven temperatures across zones-caused by some thermocouples drifting and others not-create differential shrinkage. Hotter zones shrink more, colder zones less, leading to warpage. In multi-cavity molds, this appears as cavity-to-cavity weight variation. A temperature difference of just 2°C across cavities can cause measurable dimensional differences.
Gate Freeze-Off Issues. In valve gate systems, the tip thermocouple controls the gate temperature. If it reads low, the tip overheats and the gate fails to freeze, causing stringing or drool. If it reads high, the tip is too cold, leading to premature gate freeze and short shots.
Diagnostic Approach. When a defect appears, compare thermocouple readings across zones. If one zone consistently deviates from others, validate it with a reference probe. The defect pattern-single-cavity vs. all cavities-indicates whether the issue is local (that zone's thermocouple) or systemic (controller or process settings). Documenting defect-thermocouple correlations builds an invaluable troubleshooting reference.
