For previous reading, please refer to Solutions and Practical Guide to Warpage Problems in Injection Molding Machines Under 800T.

Burning in injection molded parts occurs when molten plastic is overheated during cavity filling process, leading to thermal decomposition or oxidative degradation of material, ultimately forming black carbonized material on the surface or inside part. This defect is essentially caused by breakage of material's molecular chains under high temperature, high shear, or prolonged retention, resulting in carbon-containing residues (visible as black granular accumulation under a microscope).

Burning is result of multiple factors and can be categorized into four interrelated causes:
- Poor venting (the most direct cause): Gases (air/moisture/decomposition gases) at the end of cavity cannot be exhausted. Compressed gasses cause a sudden increase in local temperature (reaching above material's decomposition temperature).
- Improper process parameters (key drivers): Excessive injection speed (shear heating surge), excessive barrel temperature (direct material decomposition), excessive back pressure (excessive melt shearing).
- Mold design flaws (long-term hidden dangers): Undersized/remote gate (melt shear overheating), high flow channel resistance (flow stagnation), uneven cooling (localized high temperature).
- Material and equipment issues (hidden risks): Moisture absorption/excessive impurities (exacerbated decomposition), screw wear (poor plasticization/stagnation), check ring failure (pressure fluctuation).

 

Based on frequency of burning issues, difficulty of troubleshooting, and impact on production efficiency, we recommend systematically addressing them according to following priorities:

Core Logic: Poor venting directly leads to heat generation from gas compression and is the most common cause of burning. Adjustments are low-cost and quick to produce results.
2.1.1 Accurately Positioning Vent
- Tool-assisted: Use "short shot method" (injecting 50% of melt) to mark unfilled areas, which are high-risk areas for burning (such as end of part, root of rib, and bottom of a deep groove).
- Experience-based judgment: Observe location of burn—if a "snake-shaped burn line" along flow direction is most severe at the end, it indicates insufficient venting at the end; if burn marks are concentrated on the back of rib/rib, it indicates a lack of venting at the root.
2.1.2 Tiered Design and Adjustment of Vents
- Primary Vent (end of cavity): Depth 0.03-0.05mm (lower limit for PP/PE, higher limit for ABS/PC), width 5-8mm, and extending ≥5mm outside mold to prevent gas backflow.
- Secondary Vent (root of rib/column): Depth 0.02-0.03mm (to prevent flash), width 3-5mm, combined with ejector pin clearance (0.02mm) to form a "double vent channel."
- Three-stage venting (special structure): Deep cavities/slender ribs are equipped with additional breathable steel (PM-35, 38% porosity), or venting pins with a diameter of 0.5-1.0mm (3-5mm spacing).
Operational Details:
- Cleaning old venting grooves: Use a copper brush and alcohol to remove carbonized matter and release agent residue (avoid scratching mold surface with steel brush).
- Gradually adjust depth: Increase by 0.005mm each time (e.g., 0.03mm to 0.035mm) to prevent flash.
- Verifying effect: After adjustment, observe whether scorching has subsided. If not, check whether venting grooves are blocked (a compressed air backflush test can be used).
Case: A home appliance panel (ABS) was scorched. Short shot method showed that end was not filled. Original venting grooves were only at the edge (not end). A new venting groove with a depth of 0.04mm and a width of 8mm was added at the end. Using ejector pin clearance for venting, scorching was eliminated within 1 hour.

Core Logic: Directly suppress material decomposition by reducing shear heat and shortening high-temperature dwell time.
2.2.1 Segmented Injection Speed Control
- Front Stage (0-70% fill): High-speed filling (80-100 mm/s), leveraging melt's kinetic energy to quickly fill cavity and reduce cold front (which can easily cause local pressure fluctuations).
- Middle Stage (70%-90% fill): Medium-speed transition (40-60 mm/s), reducing shear rate (target: <800 s⁻¹) to avoid frictional heating.
- Final Stage (90%-100% fill): Extremely low holding speed (5-10 mm/s), solely to compensate for shrinkage and prevent heating at the end due to excessive compression (actually, reducing final stage speed from 20 mm/s to 5 mm/s reduced end temperature by 30-50℃).
Operational Details:
- Adjust based on product wall thickness: Thin-walled products (<1.5mm) can have a slightly higher speed (8-10mm/s) at the end section; thick-walled products (>2mm) require a lower speed (3-5mm/s).
- Observe melt: If "snake-like turbulence" appears at melt front during injection, speed is too high and needs to be reduced.
2.2.2 Coordinated Optimization of Barrel Temperature and Back Pressure
- Temperature Setting: Upper limit is material's decomposition temperature (e.g., 250℃ for ABS, 300℃ for PC). Homogenization section temperature should be 20-30℃ higher than melting point (e.g., PC melting point 220℃, homogenization section 250-260℃).
- Gradient Cooling: Reduce temperature of nozzle, front section, middle section, and rear section by 5-10℃ (e.g., nozzle 280℃, front section 275℃, middle section 270℃, rear section 265℃) to reduce stagnation.
- Backpressure control: 1.5-2.5 MPa for amorphous materials (PP), 3-4 MPa for crystalline materials (PA). If burning is accompanied by poor plasticization, backpressure can be slightly increased (no more than 3 MPa) and screw speed can be reduced.
Case study: A charger casing (ABS) was burning. Original injection speed was 80 mm/s throughout the entire process, end temperature was 265℃ (super decomposition temperature). Adjusting speed to "80 mm/s in the first section → 50 mm/s in the middle section → 5 mm/s in final section" reduced end temperature to 240℃, and burning disappeared.

Core Logic: If the first two steps are ineffective, address flow retention and shear hotspots at mold design level.
2.3.1 Optimize Gate Design
- Position Adjustment: Gate should be directly opposite thick wall area (such as base of product's ribs) to ensure melt flow from thick wall to thin wall (lower flow resistance and lower end shear).
- Size Matching: Point gate diameter = wall thickness x 0.8-1.2 (e.g., 1mm wall thickness, 0.8-1.2mm diameter); Side gate width = wall thickness x 1.5-2.0 (avoid overheating due to shearing caused by too small a gate).
- Shape Improvement: Use fan gates or lap gates to disperse flow direction and reduce concentrated shear at the end.
2.3.2 Runner and Cooling System Optimization
- Runner Smoothness: Polish to Ra ≤ 0.8μm (mirror EDM) to reduce frictional resistance (a runner with Ra = 1.6μm has a melt temperature 15-20℃ higher than a runner with Ra = 0.8μm).
- Balanced Cooling: Add conformal cooling channels (water temperature 20-40℃, flow rate ≥ 5L/min) near burnt area to achieve localized cooling (mold temperature difference < 5℃).
Case: Original side gate diameter of a PC electronic watch case was 0.7mm (wall thickness 1.2mm), with a shear rate of 1800s⁻¹, resulting in end burns. After switching to a 1.0mm diameter fan gate, shear rate dropped to 1000s⁻¹, eliminating burns.

Core Logic: Material stability and equipment status are hidden risks of burns, requiring long-term control.
2.4.1 Full-Process Material Management
- Drying Control: Hygroscopic materials (PA66) should be dried in a dehumidifying dryer (dew point -45℃, 120℃ x 4 hours). Use within 4 hours of drying (re-drying is required if drying time exceeds limit).
- Impurity Filtration: A magnetic rack is installed at barrel inlet to absorb metal chips, and mixer is made of stainless steel to prevent rust contamination.
- Recycled material limit: ≤ 20%, crushed and passed through an 80-mesh screen (to remove particles > 0.2mm).
2.4.2 Preventive Maintenance of Equipment
- Screw-barrel clearance: ≤ 0.2mm for new screws (60mm Φ screw). Check every 500 tons of production (measured with a feeler gauge). Replace if clearance exceeds 0.3mm.
- Check ring seal: ≥ 85% contact area (lower than this value) using a red lead powder test (replacement required).
- Barrel dead corner cleaning: Clean thread grooves at flange joints monthly with a copper brush to prevent accumulation of carbonized material.

 

Case 1: Burnt End of Automotive Interior Panel (PP)
- Symptom: Continuous burn lines, 5-8mm in length, appeared at the end of product (edge ​​of a 150mm flat plate).
- Troubleshooting Process:
1. Short-shot positioning: End was not filled, confirming incorrect exhaust position (original exhaust was at the edge).
2. Checking venting grooves: No venting at the end, only a 0.02mm deep groove at edge (too shallow).
3. Process Adjustment: End-stage speed was reduced from 15mm/s to 5mm/s, and end-stage temperature was reduced from 250℃ to 235℃, but burn line was not completely eliminated.
- Solution: A 0.04mm deep and 8mm wide venting groove was created at the end, combined with ejector pin clearance for venting.
- Result: Burning was eliminated within 1 hour, and yield rate increased from 85% to 99%.
Case 2: Burning of Connector (PA + 30% GF) Bone
- Symptom: Black spots appeared at the base of product bone (2mm thickness), resulting in a 10% scrap rate in mass production.
- Troubleshooting Process:
1. Venting Inspection: No venting grooves at the base of bone, and breathable steel was clogged (glass fiber debris + mold release agent).
2. Process Adjustment: Screw speed was reduced to 40 rpm (from 60 rpm) and back pressure was 2 MPa (from 4 MPa), reducing the burn rate to 3%.
3. Mold Optimization: Breathable steel was cleaned and ultrasonically cleaned, and a 0.02 mm deep, 3 mm wide venting groove was added at the root of bone.
- Result: Scrap rate was reduced to 0.5%, with long-term stability.

Product burn is result of multiple factors, including materials, processes, molds, and equipment. Troubleshooting sequence should be "venting first, then process, then mold/equipment," combined with a "tiered control + data verification" approach. Frontline engineers must master core skills such as short-shot positioning, melt temperature monitoring, and breathable steel maintenance, shifting from firefighting to preventive management to ultimately achieve a systematic solution to burn problem.