Common faults of hot runner systems in the molding of precision optical components mainly include gate defects, material degradation, material leakage, temperature runaway, and difficulty in material changeover. These faults directly affect the surface quality, light transmittance, and dimensional accuracy of the optical components.
1. Residue protruding or dripping at the gate
Causes: Unreasonable gate structure (e.g., excessively large diameter), improper temperature control, and excessively high residual pressure in the runner after injection.
Impact: Leads to gate stringing, cold sprue, and dripping, affecting the flatness of the optical surface, requiring secondary processing for removal, increasing costs.
Solutions: Use a needle valve type hot runner to achieve mechanical cutting and eliminate stringing; Optimize gate size to avoid excessive length or size; Strengthen cooling of the gate area to prevent heat concentration.
2. Material discoloration, scorching, or degradation
Causes: Excessive temperature, dead zones in the runner, and excessive melt residence time, especially sensitive in optical materials such as PC and PMMA.
Impact: Yellow lines, black spots, and increased haze appear on the finished product, severely affecting light transmittance and appearance yield.
Solutions: Precise temperature control (within ±0.5℃), using a closed-loop temperature control system; Optimize runner design to avoid dead zones and reduce volume; Use low-shear hot nozzles to prevent localized overheating.
3. Severe material leakage (between the nozzle and sprue bushing)
Causes: Damaged sealing elements, burnt-out heating elements leading to uneven thermal expansion, and misalignment between the nozzle and sprue bushing.
Impact: Causes resin leakage, carbonization, and mold contamination, affecting continuous production.
Countermeasures:
Regularly check the condition of the sealing rings and heating elements;
Use leak-proof rings or contact sealing structures;
Ensure that the thermal expansion gap is properly reserved during installation to prevent jamming or misalignment.
4. Abnormal or slow heating of the hot runner
Causes: Inadequate design of the wire channel leading to breakage, short circuit, leakage, or aging of the insulation material.
Impact: The system cannot start normally, prolonging trial molding time and affecting production efficiency.
Countermeasures: Use high-temperature insulation materials and standardize wiring processes;
Regularly check the continuity of the wires to avoid cross-wearing;
Prioritize modular, easy-to-maintain hot runner systems
5. Injection volume shortage or no material injection
Causes: Obstructions in the runner, excessively thick condensate layer, or gate blockage.
Impact: Leads to short shots and insufficient filling, especially prominent in ultra-thin-walled optical components.
Countermeasures:
Ensure smooth flow transition and eliminate dead zones;
Appropriately increase material temperature and hot runner temperature to reduce condensation;
Pay attention to radial temperature difference control when using internally heated nozzles.
6. Poor Material or Color Change
Causes: Unreasonable flow channel design, excessive material retention, improper cleaning methods.
Impact: Long color change cycle, waste of raw materials, and reduced efficiency in switching between multiple product types.
Countermeasures:
Adopt a small-volume, dead-zone-free flow channel design;
Process the flow channel from a single end to avoid orifice misalignment forming material retention zones;
Use new material with similar viscosity and increase injection speed to accelerate cleaning.
