The common causes of partial discharge in hot runner systems include: the presence of air gaps or voids within the insulation material or at interfaces; non-uniform electric field distribution (such as the "tip effect"); moisture ingress or aging of the insulation; poor contact at interfaces between different dielectric media; and residual impurities resulting from the manufacturing or assembly processes. Among these factors, air gaps and electric field concentration are the two primary triggers.
1. Air Gaps or Void Defects (Most Common Cause)
If tiny bubbles or voids exist within the insulation layers of heating rods, cables, or connectors-given that the dielectric constant of air is significantly lower than that of solid insulation materials-the electric field strength within these air gaps will rise sharply under the influence of an applied electric field. This causes the gap to reach its dielectric breakdown strength first, thereby triggering partial discharge.
Typical Scenarios: Insufficient silicone potting; air entrapment during the injection molding of heating rods.
Manifestations: Partial discharge is detected immediately upon installation of a new system, or discharge levels are abnormally high during the initial stages of operation.
Key Mechanism: Once the gas within the air gap becomes ionized, it forms tiny electrical arcs that continuously erode the surrounding insulation material, gradually expanding the scope of the defect.
2. Non-Uniform Electric Fields (The "Tip Effect")
At the edges of conductors, at burrs on wiring terminals, or at the sharp corners of metallic foreign objects, the electric field becomes highly concentrated, leading to "corona" discharge.
Typical Locations: Heating rod electrode cores lacking chamfering; metallic debris lodged inside connectors; uneven or jagged cuts in cable shielding layers.
Manifestations: Discharge events typically occur near the voltage peaks, and the Phase-Resolved Partial Discharge (PRPD) pattern exhibits a characteristic "rabbit-ear" distribution.
Physical Principle: According to the formula for electric field strength, the smaller the radius of curvature, the higher the field strength; this makes it highly susceptible to exceeding the local dielectric breakdown threshold.
3. Insulation Moisture Ingress or Aging
The intrusion of moisture or the long-term thermal aging of materials leads to a degradation of insulation performance. This results in a reduction of dielectric strength in localized areas, making them prone to partial discharge even under normal operating voltages.
Moisture Ingress Mechanism: Under the influence of an electric field, water molecules decompose to generate gas, thereby creating new channels for gas bubbles.
Aging Mechanism: Mica layers delaminate and silicone gel cracks, forming minute gaps where partial discharge can occur.
Typical Manifestations: A simultaneous decline in insulation resistance values and the presence of partial discharge; this is frequently observed in high-humidity environments or in equipment that has exceeded its service life.
4. Poor Interfacial Contact
At the interface between dissimilar materials (e.g., between a ceramic substrate and a copper electrode, or between silicone gel and a metal housing), if the bonding is weak or minute gaps exist, a "floating potential" zone may form, triggering surface discharge.
Commonly Found At: The edges of DCB (Direct Bonded Copper) substrates, and the mating surfaces between heating rods and their mounting holes within molds.
Risk Factors: Thermal expansion and contraction cycles can exacerbate interfacial separation, causing the discharge phenomenon to progressively worsen.
5. Contamination by Impurities or Foreign Objects
Metal shavings, fibers, or dust introduced during manufacturing, assembly, or maintenance processes can act as "seeds" for discharge when subjected to a high-voltage electric field.
High-Risk Practices: Failure to properly clean the surrounding environment when replacing heating rods on-site, or the use of substandard potting compounds containing trapped air bubbles.
Manifestations: The discharge is stochastic in nature and may fluctuate in intensity in response to vibrations or temperature variations.
Industry Case Study: A plastic injection molding facility experienced persistent partial discharge-resulting in the burnout of three heating rods within a three-month period-due to residual copper shavings trapped inside the electrical connectors.
