Analiză sistematică și soluții pentru mărci de arsură în modelarea prin injecție

1、Definition of Burn Marks Defect  

During the injection filling stage, if gas is trapped in the mold cavity, it may combust under high pressure, causing black soot-like residues on the product surface. This defect is commonly known as "air entrapment," "burning black," or "burning," and is easily confused with color mixing, yellow streaks, or black streaks. Its essence lies in gas combustion residues.  

2、Root Causes of Burn Marks: Gas Sources and Exhaust Failure  

01 Gas Sources  

- Air originally present in the mold cavity  

- High-temperature decomposition of materials (due to high moisture content or volatile additives)  

- Volatile gases from scale buildup in the screw/barrel  

02 Key Causes of Exhaust Failure  

✅ Inadequate mold exhaust (accounting for >90% of cases)  

✅ Excessive filling speed → Gas is "sealed off" by high-speed melt  

✅ Excessively high melt temperature → Material decomposition produces gas  

3、Systematic Troubleshooting and Solutions  

01 Injection Molding Process Factors  

1.1 Excessive Filling Speed  

When filling speed is too high, gas is more likely to be trapped in the mold without escaping through exhaust vents.  

Solutions:  

- Temporary: Reduce filling speed.  

- Fundamental: Improve mold exhaust.  

Case: A product showed burn marks at the weld line of the filling end. Reducing filling speed solved the issue. Later, deepening the exhaust vents allowed restoring the original filling speed, achieving faster filling and saving >2 seconds in cycle time.  

1.2 Excessively High Melt Temperature  

High-temperature processing may cause material degradation and generate gaseous by-products.  

Solutions:  

- Check actual melt temperature (use a contact thermometer).  

- Ensure barrel temperature settings are correct.  

- Verify back pressure and screw speed (screw should retract 2–3 seconds before cooling ends).  

1.3 Improper Retraction Settings  

Excessive retraction can suck air into the melt at the nozzle tip.  

Solutions:  

- Inspect and adjust retraction volume.  

- If large retraction is set to control nozzle drool, first check nozzle temperature.  

02 Mold Factors  

2.1 Inadequate Exhaust  

Inadequate mold exhaust is the primary cause of burn marks!  

Solutions:  

- Clean mold exhaust vents (including slides and lifters).  

- Add extra exhaust features.  

- Perform regular maintenance to prevent steel corrosion.  

Typical PA Material Exhaust Groove Design Guidelines (Table format can be added as needed)  

2.2 Mold Contamination  

Contaminants like grease or oil can clog exhaust vents.  

Solutions:  

- Identify and eliminate the pollution source (e.g., hydraulic leaks).  

- Standardize mold lubrication procedures.  

- Avoid placing hydraulic connections on the mold top.  

03 Injection Molding Machine Factors  

3.1 Scale Buildup Issues  

Scale buildup in the screw, end cap, or check valve can cause material degradation.  

Solutions:  

- Establish proper shutdown and cleaning procedures.  

- Check the nozzle-to-end cap/tip fit.  

- Thoroughly clean the barrel before long-term shutdown.  

3.2 Incorrect Screw Design  

Improper screw design generates excessive shear force, leading to material degradation.  

Solutions:  

- Ensure the screw's L/D ratio and compression ratio suit the material.  

- Check if "barrier threads" from scale buildup exist on the screw.  

3.3 Malfunctioning Barrel Heater Temperature Control  

Overheated areas cause local material degradation.  

Solutions:  

- Verify barrel temperature readings.  

- Reduce screw speed or back pressure.  

- Check thermocouple installation.  

3.4 Damaged Components  

Damaged components create high-shear zones or scale buildup.  

Solutions:  

- Inspect and replace damaged screws, barrels, check valves, etc.  

- Remove metal debris from the feeding system.  

04 Material Factors  

4.1 Excessive Moisture Content  

For hygroscopic materials, high moisture content vaporizes, introducing gas into the melt.  

Solutions:  

- Ensure materials are fully dried.  

- Check the operation of drying equipment.  

4.2 Material Type  

Temperature-sensitive materials like PVC and acetal are more prone to burn marks.  

Case: A PVC material showed carbonization marks during trial molding, later found to be due to insufficient formula thermal stability.  

Solutions:  

- Strictly control barrel temperature.  

- Monitor material residence time in the barrel.  

- Ensure no material stagnation zones.  

4.3 Additive Issues  

Color masterbatches and other additives may influence burn mark likelihood.  

Solutions:  

- Use the correct additive dilution ratio.  

- Ensure additives are compatible with the injection material.  

- For dark-colored products, specially inspect hidden areas like rib tips.  

4、Summary and Recommendations  

Follow these steps to resolve burn marks:  

1. Prioritize checking mold exhaust — the most common cause.  

2. Troubleshoot material factors, especially moisture content and material type.  

3. Inspect the injection molding machine for scale buildup, damaged components, etc.  

4. Finally, adjust process parameters (e.g., filling speed, melt temperature).  

Key principle: Do not attempt to mask mold or equipment issues by adjusting processes; address root causes!  

Through systematic troubleshooting and targeted solutions, burn marks in injection molding can be effectively eliminated, enhancing product quality and production efficiency.