Which parts of the injection molding machine should the temperature sensor be kept away from?

During the installation of the temperature sensor of the injection molding machine, the following key components and areas should be avoided to ensure the temperature measurement accuracy and stability, and avoid process control deviations caused by improper positioning:
1. Avoid the interference area of ​​the heating system
Heating coil splicing/joint
Component/location: The junction of two adjacent heating coils (usually located at the connection position of the barrel segmented heating section).
Cause: The temperature distribution here is uneven, and there may be local overheating or uneven heat dissipation, resulting in the sensor measuring the value of "mixed temperature" instead of the actual temperature of a single heating section (fluctuation can reach more than ±3°C).
Consequence: The temperature control system misjudged, causing abnormal power regulation of the heating section (such as the actual temperature of a section is too high but not detected, resulting in melt decomposition).
Gap between the heating coil and the insulation layer
Component/location: The area where the outer insulation layer of the barrel is not fully covered (such as the insulation layer is damaged or the installation gap).
Reason: If the sensor is close to such a position, it may measure the "ambient temperature" instead of the metal wall temperature of the barrel, especially during high-speed injection molding, the temperature difference between the melt and the barrel surface can reach 10-15℃.
Consequence: The temperature feedback is delayed and cannot reflect the actual temperature of the melt in time, resulting in uneven plasticization (such as silver threads appearing in PC products).
2. Avoid the dynamic interference area of ​​the screw/barrel
Screw shear zone (behind the mixing section/shear block)
Component/location: The end of the mixing section of the screw, the area within 50mm behind the shear block or the check ring.
Reason: Due to the high-speed rotation of the screw, severe friction heat is generated here, the local temperature is abnormal (may be 20-30℃ higher than the normal plasticization section), and the melt retention time is short, which cannot represent the stable plasticization temperature.
Consequence: The overall temperature is misjudged as too high, resulting in excessive reduction of the power of the heating section, or ignoring the insufficient actual temperature of other areas (such as when processing PVC, overheating in the shear zone causes decomposition but is not detected by other sensors).
Junction between the feeding section and the compression section
Component/location: Transition area between the feeding section (rear section) and the compression section (middle section) of the barrel (usually within 100-150mm from the feeding port).
Reason: This is the area where solid particles transform into molten state, with a large temperature gradient (solid resin and molten material coexist), and the sensor cannot capture stable temperature values.
Consequence: Frequent adjustment of the temperature control system causes temperature fluctuations in the feeding section, causing feed slippage or blockage (such as premature melting and adhesion of PA particles).
3. Avoid the direct impact area of ​​the cooling system
Barrel cooling water pipe/mold cooling water hole
Component/location: Cooling water pipe outside the barrel (within 50mm from the pipe wall), cooling water hole inside the mold (within 30mm from the hole wall).
Reason: Forced convection of cooling water will cause a sudden drop in local temperature, and the value measured by the sensor is lower than the actual melt/mold temperature (for example, during PET injection molding, the sensor display value close to the cooling water pipe may be 15℃ lower than the actual temperature).
Consequences: Failure of mold temperature control, affecting the cooling rate and crystallinity of the product (such as warping of PBT products).
Area directly below the hopper feed port
Components/location: directly below the connection between the hopper and the barrel (within 50mm from the feed port).
Cause: The newly added cold material particles will lower the temperature of this area, and there may be air convection at the feed port, resulting in unstable temperature.
Consequences: Misjudged as insufficient temperature in the feed section, overheating of the heating coil, causing overheating and decomposition of the rear-end molten material (such as common problems during PVC processing).
4. Avoid electromagnetic/signal interference sources
Servo motor, heating coil power cord
Components/location: The sensor signal line needs to be away from the injection molding machine servo motor (especially the high-speed screw motor) and the heating coil high-voltage circuit (susceptible to interference when the spacing is <200mm).
Cause: Strong electromagnetic interference will cause sensor signal distortion and temperature jump (such as a sudden fluctuation of the displayed value by ±10℃).
Consequences: The temperature control system mistakenly triggers an overtemperature alarm or automatic shutdown, affecting production continuity.
Areas with severe mechanical vibration (such as near the clamping mechanism)
Components/Location: Sensor mounting holes near the mold mounting plate and clamping cylinder.
Cause: High-frequency vibration may cause the sensor probe to loosen contact with the barrel/mold, resulting in temperature measurement deviation (contact resistance change causes signal drift).
Consequence: The temperature display value is unstable and the process parameters are difficult to stabilize (such as dimensional fluctuations during precision injection molding).
V. Avoidance of special structural components
Joint of the manifold of the hot runner system
Components/Location: The connection between the hot runner manifold and the nozzle (non-heating area within 5mm from the gate).
Cause: There may be an insulation gap at the joint, causing the local temperature to be lower than the melt temperature, causing gate condensation (such as cold material spots during POM injection molding).
Correct position: It should be installed 2-3mm from the tip of the hot runner nozzle to directly monitor the melt temperature.
Cooling insert of mold core
Components/Location: Beryllium copper cooling insert inside the mold core (within 10mm from the insert surface).
Cause: The high thermal conductivity of the cooling insert will quickly take away the heat, and the value measured by the sensor is lower than the actual temperature of the cavity surface (the temperature difference can reach 8-10℃).
Consequence: Misjudgment of insufficient mold temperature, extended cooling time, and reduced production efficiency.
Summary: Core avoidance principles
Avoid "thermal interference": Do not approach the joints of the heating ring, the shear friction area, and the cold-hot interface (such as near the cooling water pipe).
Avoid "dynamic interference": Stay away from the high-speed shear area of ​​the screw and parts with severe mechanical vibration (such as the clamping mechanism).
Avoid "field interference": The signal line is away from strong electromagnetic sources (servo motors, power lines), and the probe is away from structures where the signal is easily distorted (such as gaps in the insulation layer).
By avoiding the above-mentioned components and areas, the interference of external factors on temperature measurement can be minimized, ensuring that the sensor accurately reflects the actual temperature of the melt/mold, and providing reliable data support for the stable control of the injection molding process. Especially when processing heat-sensitive materials (such as PVC) or precision products, accurately avoiding these locations is the key to avoiding quality defects (such as decomposition, shrinkage, and dimensional deviation).