Does the installation position of the injection molding machine temperature sensor affect the test results?

The installation position of the temperature sensor of the injection molding machine has a direct and critical impact on the test results. Improper installation will lead to temperature measurement deviation, response delay or signal interference, which will affect the plasticization quality and process stability. The following are specific impacts and installation requirements:
I. The main impact of the installation position on the test results
1. Temperature measurement area deviation: uneven temperature distribution at different positions of the barrel
Barrel segment temperature characteristics:
Injection molding machine barrels are usually divided into feeding section (rear section), compression section (middle section), and metering section (front section). The temperature design of each section is different (such as low temperature in the rear section and high temperature in the front section). The sensor needs to be installed in the core temperature measurement area of ​​the corresponding section (generally the middle of the barrel or the position specified by the manufacturer).
Error example: If the rear section sensor is installed in the cooling area near the feed port, the displayed temperature will be lower than the actual melt temperature, causing poor feeding or screw wear.
2. Contact depth and fit: affecting heat conduction efficiency
Insufficient insertion depth:
If the sensor probe is not fully inserted into the barrel temperature measuring hole (it must be at least 2/3 of the barrel wall thickness or reach the depth specified by the manufacturer), it will result in:
The measured temperature is the barrel surface temperature, not the internal melt temperature (especially during high-speed injection molding, there may be a 5-10℃ temperature difference between the barrel surface and the inside);
Response delay (such as fast heating during heating, but the actual melt temperature rises slowly, resulting in insufficient plasticization).
Poor contact:
There is a gap between the sensor and the inner wall of the temperature measuring hole (no thermal grease is used or the installation is loose), which will cause temperature measurement lag due to air insulation (the displayed temperature fluctuates greatly and cannot reflect the melt state in real time).
3. External interference: ambient temperature and electromagnetic influence
Close to heating/cooling components:
If the sensor cable is close to the heating coil (causing local overheating) or the cooling water pipe (local overcooling), the displayed value will drift due to ambient temperature interference (such as sudden temperature changes when the heating coil leaks or the cooling water flow rate changes).
Electromagnetic interference area:
The installation position is close to electromagnetic sources such as motors and relays, and shielded cables are not used or the grounding is poor, which will cause signal noise (the temperature display value jumps irregularly).
2. Key requirements for correct installation position
1. Axial position: Accurate positioning according to barrel segmentation
Follow the equipment design drawings: the sensor should be installed in the center of the temperature measuring hole of each section of the barrel (usually located in the middle of the heating ring, avoid being close to the edge of the heating ring or the junction of two sections of the heating ring);
Avoid flow dead angle: In the front section where the screw back pressure is large, the sensor needs to face the melt flow direction to ensure that the main channel temperature (rather than the retained material temperature) is measured.
2. Radial depth: Ensure that the probe is in full contact with the barrel body
Insertion depth standard:
For cylindrical barrels: The probe needs to be inserted to 1/2~2/3 of the barrel inner diameter (for example, if the barrel inner diameter is 50mm, the probe needs to be inserted 25-35mm);
For barrels with insulation layer: The insulation layer needs to be penetrated and directly contact the inner wall of the metal barrel (avoid measuring the temperature of the insulation layer);
Fixation method: Use special nuts or ferrules to tighten, and ensure that the sensor and the temperature measuring hole are coaxial and non-inclined (can be calibrated with a square ruler).
3. Cable processing: Reduce environmental interference
High temperature protection: The cables near the barrel need to be covered with high temperature resistant silicone tubes/ceramic sleeves (temperature resistance > 200℃) to avoid aging and short circuit of the insulation layer;
Shielding and grounding: Thermocouple cables need to use twisted pair shielded wires, and the shielding layer is single-ended grounded (connected to the metal shell of the injection molding machine), and stay away from the power line (spacing > 10cm).
4. Auxiliary measures: Improve temperature measurement accuracy
Fill with thermal conductive material: Apply high-temperature thermal conductive silicone grease (such as models with a temperature resistance of more than 300°C) in the gap between the sensor and the temperature measuring hole to eliminate air gaps and improve heat conduction efficiency;
Check the sealing regularly: Under high temperature and high pressure environment, the temperature measuring hole may produce an oxide layer or material accumulation due to vibration. It is necessary to clean the hole wall with fine sandpaper every quarter to ensure close contact between the sensor and the barrel.
3. Typical problems and consequences of improper installation
Installation problems Abnormal test results Impact on injection molding process
The probe is too shallow (contacting the barrel surface) The displayed temperature fluctuates greatly and lags behind the actual melt temperature Insufficient plasticization (low melt temperature) or overheating (set temperature is too high), resulting in defects such as flash and silver wire
Installed at the junction of two heating rings The displayed temperature is a mixed value of the temperatures of adjacent sections The process parameters are chaotic (such as insufficient actual temperature in the front section, resulting in excessive injection pressure)
The cable is not shielded or poorly grounded The temperature display value jumps irregularly (more than ±5℃) The temperature control system frequently starts and stops heating/cooling, resulting in increased energy consumption and unstable product size
The sensor is tilted/eccentric The temperature measuring hole is partially in contact, and the temperature difference is greater than 3℃ The melt temperature is uneven, resulting in stratification, burning or cold material spots
4. Verification method of installation position
Infrared temperature measurement comparison method:
Use a high-precision infrared thermometer (accuracy ±1℃) Measure the barrel surface temperature near the sensor installation point and compare it with the temperature controller display value. The deviation should be less than ±2℃ (if it exceeds, check the insertion depth and fit).
Step response test:
Manually increase the temperature setting from 150℃ to 200℃, and record the time for the displayed temperature to reach the target value (normal response time <2 minutes). If it exceeds 5 minutes, it may be poor contact or insufficient depth.
Actual temperature detection of molten material:
Quickly sample the molten material after shutdown, measure it with a portable melt thermometer, and compare it with the sensor display value (the deviation should be less than ±3℃, otherwise the installation position needs to be adjusted).
Summary: Installation position is the basis of temperature measurement accuracy
The installation position of the temperature sensor directly determines whether it can accurately reflect the actual temperature of the molten material inside the barrel. It is necessary to strictly follow the principle of "accurate axial positioning, sufficient radial depth, close contact and no interference". Improper installation is a common source of failure of temperature control systems and product defects. It is recommended that after equipment commissioning or sensor replacement, the effectiveness of the installation position must be verified by actual measurement to ensure the reliability of process parameter control.