In the insulation resistance testing of hot runner systems, the primary factors influencing measurement results include: temperature, humidity, surface contamination of the test specimen, residual charge, induced voltage, test equipment capacity, and connection methods. These external conditions can lead to readings that are either artificially high or low, potentially resulting in misinterpretation of the results.
1. Temperature (Significant Impact)
Mechanism of Influence: An increase in temperature accelerates ion movement within the insulation material. This weakens the bond between moisture and the insulating material, thereby increasing conductivity and causing the measured insulation resistance to appear artificially low.
Countermeasures:
Whenever possible, conduct periodic testing under identical ambient temperature conditions to facilitate data comparison;
Alternatively, convert measurement values obtained at different temperatures to a standard temperature (e.g., 20°C) for comparative analysis.
2. Humidity and Surface Moisture
Manifestation of Impact: When relative humidity is high, a thin film of water tends to form on the surface of the insulator. This significantly increases surface leakage current, causing the measured insulation resistance value to drop drastically-an effect that is particularly pronounced during foggy or rainy weather, or within humid workshop environments.
Typical Case: If connector terminals absorb moisture, an initially normal resistance reading of 10 MΩ could plummet to below 200 kΩ.
Countermeasures:
Ensure the equipment is situated in a dry environment prior to testing;
Before use, wipe the wiring terminals with anhydrous alcohol to remove any moisture or contaminants.
3. Surface Contamination and Carbon Deposits
Path of Influence: Airborne oil residues, mold release agent remnants, or carbonized plastic particles adhering to the insulation surface can dissolve to form a conductive layer, thereby drastically reducing surface resistance.
Special Note: Even if the internal insulation remains intact, surface contamination can still distort the overall measurement results.
Countermeasures:
Periodically clean the connectors, terminal boxes, and heating element housings;
Select contamination-resistant connectors and install protective covers.
4. Residual Charge (Often Overlooked)
Spurious Effects: If the device under test is not fully discharged after each measurement, the residual charge remaining on the equipment can interfere with subsequent measurements. When the polarity of the residual charge aligns with the output polarity of the megohmmeter, a spurious phenomenon may occur, characterized by a decrease in charging current, a reduction in the absorption ratio, and an artificially inflated reading of the insulation resistance.
Countermeasures:
After the completion of each test, the object under test must be thoroughly discharged to ground; the discharge duration should exceed the charging duration.
For high-capacitance equipment (such as long cables), it is recommended to allow for a discharge period of at least 5 minutes.
5.Induced Voltage
Source of Risk: When performing measurements in the vicinity of energized equipment, capacitive coupling may cause the de-energized test object to carry an induced voltage. This voltage can interfere with the test signal, potentially damage the megohmmeter, or even pose a risk to personnel safety.
Mitigation Strategies:
Prior to measurement, verify that no high-voltage equipment is currently operating in the immediate vicinity.
Utilize shielded cables (connected to the G terminal) in conjunction with proper grounding protection.
Implement electric field shielding measures where deemed necessary.
6. Test Equipment and Wiring Configuration
Impact of Equipment Capacity: If the megohmmeter's output current is insufficient (e.g., <1 mA), the measurement results become highly susceptible to interference. To enhance accuracy, it is recommended to select an instrument with a maximum output current of 1 mA or higher.
Wiring Guidelines:
Employ dedicated, high-insulation test leads, and avoid twisting the conductors together.
Strictly adhere to the proper three-terminal connection method-L (Line), E (Earth), and G (Guard/Shield). In particular, within environments characterized by high humidity or heavy contamination, the G terminal must be connected to effectively eliminate the influence of surface leakage currents.
Tip: To ensure the comparability of test results, it is highly recommended to establish a standardized testing protocol. This involves fixing the test duration, environmental conditions, and operational procedures to minimize interference arising from human factors and environmental variables.
