Porosity in injection molded products refers to cavities formed inside or on the surface of part due to failure of gas to escape during cavity filling, pressure holding, and cooling, or due to intrusion of external gas. Porosity can be divided into three categories based on morphology:
- Surface Porosity: Located on the surface of part, with a diameter of 0.1-2mm and a depth of less than 0.5mm, they are mostly round or oval. They are caused by surface gas failure or rapid melt cooling, resulting in surface closure.
- Internal Porosity: Hidden within part, with a diameter of 0.5-5mm, they are often accompanied by vacuum bubbles (internal pressure less than atmospheric pressure) and are caused by gas accumulation or contraction within melt.
- Bubble Nuclei: Densely distributed, tiny pores (diameter less than 0.1mm) are often caused by gas oversaturation in melt or material degradation.
It is essentially a phase separation between gas and melt. Sources of gas include moisture/ volatiles in material, residual air in mold, entrained air during melt flow, and gas generated by chemical reactions. When gas partial pressure (P_gas) exceeds static pressure of melt (P_melt), gas accumulates and forms pores (formula: P_gas = P_melt + surface tension; P_gas precipitates when it breaks through melt). Microscopically, surface pores have smooth edges (gas escape channels) while internal pores have rough edges (gas encapsulation). Bubble nuclei often appear distributed in a "beaded" pattern.

Porosity and bubbles are result of a multi-dimensional coupling of materials, molds, processes, and equipment. They can be categorized into four main causes, each with its own overlapping impact:
- Uncontrolled gas sources (the most fundamental contributing factor):
- Material side: Hygroscopic materials (PA6, PC, PET) are not fully dried (moisture content > 0.1%), resulting in evaporation of moisture to form water vapor; materials containing volatile components (such as PVC and POM) decompose upon heating, producing gases such as acetaldehyde and formaldehyde; repeated heating of recycled materials causes molecular chain breakage, releasing low-molecular hydrocarbon gases.
- Mold side: Residual air in cavity/runner (not completely vented during mold closing); external air intrusion into ejector pin holes/insert gaps; and excessive volatilization of release agent, generating organic gases.
- Process side: Excessive injection speeds leading to turbulent melt entrainment (air is entrained into the cavity); and insufficient back pressure, which prevents compaction of melt and gas expulsion.
- Mold venting design flaws (key drivers): venting slots positioned away from gas accumulation areas (e.g., not located at the end of melt or at the bottom of a deep cavity), insufficient venting depth (e.g., PA66 requires 0.04-0.06mm, but actual depth is only 0.02mm), venting channel blockage (carbonized material/release agent residue), and improper gate design (e.g., a point gate facing a thin-walled area, causing melt jet entrainment).
- Process parameter out-of-control (explicit trigger): Excessively high injection speed (>100 mm/s) causes turbulent entrainment; excessively high melt temperature (>material decomposition temperature) exacerbates volatile release; insufficient holding pressure (<50% of injection pressure) prevents shrinkage compensation and venting; and excessively short cooling time (ejection before part is fully formed, causing internal gas expansion).
- Equipment deterioration (long-term hidden dangers): Screw wear leading to uneven plasticization (melt contains unmelted particles and trapped gas); check ring seal failure leading to melt backflow (gas enters barrel with backflow); carbonized material remaining in dead corners of barrel (continuous gas generation due to high-temperature decomposition); hydraulic system pressure fluctuations (sudden pressure drop during holding phase, melt contraction and air inhalation).

 

Based on cost of repairing air bubbles, impact on product performance, and probability of recurrence, we recommend systematically addressing them according to following priorities:

Core Logic: Mold is "main channel" for gas discharge and entrapment. Precisely designed venting slots and optimized gate placement can directly block path for gas accumulation and entrapment.
2.1.1 Precision Design and Repair of Mold Venting Systems
- Venting Location Location:
- Identifying Gas Accumulation Areas: Use "short shot method" to mark unfilled areas (such as end of part, bottom of a deep cavity, and root of a rib). These areas are primary points of gas retention.
- Experience-based Judgment: Surface pores are mostly distributed along flow direction, indicating air entrainment near gate; internal pores concentrated in thick wall areas indicate that gas in the center of thick wall has not been exhausted.
- Tiered Venting Design:
- Primary Venting (Melt End/Deep Cavity Bottom): Depth adjusted based on material (0.02-0.03mm for PS/ABS, 0.04-0.06mm for PA66/PC, and upper limit for high-viscosity materials), width 5-8mm, and extension ≥5mm outside mold to prevent gas backflow.
- Secondary Venting (Rib/Pillar Root): Depth 0.01-0.02mm (to prevent flash), width 3-5mm, combined with ejector pin clearance (0.01-0.02mm) to create a "dual venting channel."
- Tertiary Venting (Special Structure): Deep cavities (depth > 50mm) are equipped with permeable steel (PM-35, 38% porosity) or 0.3-0.5mm diameter vent pins (2-3mm spacing) to target areas of minute gas accumulation.
- Gate Optimization: Avoid point gates directly facing thin-walled areas (which can easily cause melt jet entrainment). Instead, use fan gates or overlapping gates to disperse flow direction. Gate size should match injection speed (increasing gate area by 20% during high-speed filling reduces release of volatiles caused by shear heating).
Operational Details:
- Vent Cleaning: Use an ultrasonic cleaner to remove carbonized material/release agent residue (do not use a steel brush to scratch mold surface).
- Vent Verification: After adjustment, observe melt front during injection for "air streaks" (a characteristic of air entrainment). If streaks disappear and there is no flash, venting is effective.
Case Study: Surface of a mobile phone (PC) casing had dense pores distributed along flow direction. Short-shot method showed that end was not filled. Original vent groove was only at the edge (0.02mm deep, insufficient). Adding a 0.04mm deep and 6mm wide vent groove at the end, combined with a fan gate (15% wider), reduced porosity from 25% to below 2%.

Core Principle: By reducing melt turbulence and optimizing gas evacuation timing, gas entrapment and accumulation are minimized, effectively improving porosity defects.
2.2.1 Segmented Control of Injection and Holding Pressure Parameters
- Injection speed: Controlled throughout the entire process at 60-80 mm/s (20%-30% lower than normal process) to prevent turbulent gas entrainment caused by high-speed shearing of melt (measured speed reduction from 100 mm/s to 70 mm/s reduced gas entrainment by 50%).
- Front section (0-50% fill): Medium filling speed (50-60 mm/s) to prevent gas compaction at the front end.
- Middle section (50%-90% fill): Low transition speed (40-50 mm/s) to minimize gas entrapment.
- Final section (90%-100% fill): Extremely low speed (30-40 mm/s) to compensate for shrinkage and avoid turbulence at the end.
- Melt Temperature and Back Pressure:
- Melt temperature: Appropriately reduce melt temperature within material flow range (e.g., from 280℃ to 260℃ for PA6, and from 300℃ to 280℃ for PC) to reduce volatile release (at a PA6 moisture content of 0.1%, amount of water vaporized at 280℃ is three times that at 260℃).
- Back pressure: Increase to 3-5 MPa (for amorphous materials) or 4-6 MPa (for crystalline materials) to compact melt and expel internal gas (increasing back pressure from 2 MPa to 5 MPa reduces melt gas content by 40%).
- Holding and Cooling: Holding pressure = injection pressure * 60%-70%, holding time = part wall thickness * 3 s/mm (extend feeding and venting time); extend cooling time to part wall thickness * 1.5 s/mm (ensure sufficient time for internal gas to escape).
Operational Details:
- For high-volatile materials (such as PVC), an additional "exhaust and hold" stage is required (holding pressure reduced to 30 MPa, holding time 2 seconds) to forcibly expel residual gas.
- Observe product surface: If pinpoint-shaped air holes appear, injection speed is too high, should be reduced to 50 mm/s and proportion of low-speed stage increased.
Case: Bubbles were concentrated in thick-walled area of a certain automotive instrument panel (PP). Original injection speed was 90 mm/s, back pressure 2 MPa, and holding time 5 seconds (wall thickness 2 mm x 2.5 s/mm = 5 seconds). After adjusting injection speed to 70 mm/s (segmented: 60 mm/s in the front section, 50 mm/s in the middle section, and 40 mm/s in the rear section), back pressure was increased to 5 MPa, and holding time was extended to 7.5 seconds (2 x 3.75 s/mm). Internal bubble rate dropped from 18% to 3%.

Core Logic: Material dryness and equipment plasticization are long-term risks for porosity, requiring establishment of standardized control procedures.
2.3.1 Drying and Control of Materials Throughout Their Lifecycle
- Drying Process:
- Hygroscopic Materials: PA6 (moisture content <0.1%): 120℃ for 4 hours, PC (moisture content <0.02%): 110℃ for 6 hours, PET (moisture content <0.01%): 140℃ for 4 hours. Use within 4 hours of drying (exceeding this temperature requires secondary drying).
- Volatile Materials: Add 0.1% heat stabilizer to PVC (to reduce acetaldehyde release); store POM in a sealed container after drying (to prevent moisture absorption).
- Recycled Material Management: Recycled material should account for ≤10% (lower than normal processes) and must be dried separately (80℃ for 4 hours) to avoid repeated heating that can break molecular chains and release low-molecular gases.
2.3.2 Equipment Preventive Maintenance
- Screw and Barrel Condition:
- Screw Wear Inspection: Measure screw clearance every 100 tons of production (new screw clearance ≤ 0.15mm, 50mm Φ screw). Replace screw if it exceeds 0.25mm (excessive clearance can cause unmelted particles in melt and trap gas).
- Check Ring Sealability: Red lead powder test: Contact area > 85% (replace if below this value to prevent melt backflow and gas).
- Barrel Cleaning: When changing materials, clean barrel with transition material (such as PE) (at a temperature 20℃ above material's decomposition temperature and hold for 10 minutes) to remove residual carbonized material (which continuously generates gas due to high-temperature decomposition).
Case Study: A precision gear (PA66+GF30) experienced frequent internal bubbles. Investigation revealed insufficient material drying (moisture content 0.15%), screw clearance of 0.3mm (out of tolerance), and a check ring contact area of 70%. After replacing a new screw (with a 0.18mm clearance), increasing drying temperature to 125℃ for 4 hours (moisture content dropped to 0.05%), and replacing check ring, internal bubble rate dropped from 22% to 1%.

Core Logic: Ambient humidity and auxiliary measures can further suppress pores, especially in high-humidity areas or for materials that are easily hydrolyzed.
2.4.1 Environmental and Auxiliary Process Control
- Ambient Humidity: Humidity in workshops producing hygroscopic materials should be controlled at 40%-50% RH (below 60% RH to reduce moisture absorption);
- Mold Release Agent Application: Dilute water-based mold release agent at a ratio of ≥ 1:50 (to reduce volatile content) and apply once every 50 molds (to avoid excessive volatilization and gas generation);
- Secondary Exhaust Process: For thick-walled products (wall thickness > 3mm), apply a 30% injection pressure for 2-3 seconds after hold pressure is completed to compensate for gas expansion during cooling.
Case study: Surface porosity of an appliance casing (ABS) was 15% in a high humidity environment (70% RH). When workshop humidity was reduced to 50% RH and mold release agent dilution ratio was adjusted from 1:30 to 1:60, porosity dropped to 3%.

 

Case 1: Dense Pores on the Surface of a Mobile Phone Middle Frame (PC)
- Symptom: Pinpoint-shaped pores (0.1-0.3mm in diameter) were distributed along product surface along flow direction, with a density of 5-8 pores/cm², resulting in poor appearance.
- Troubleshooting Process:
1. Mold Inspection: Vent depth 0.02mm (PC requires 0.03-0.04mm), point gate (facing thin-walled areas, prone to air jetting and entrainment);
2. Process Inspection: Injection speed 100mm/s (too fast), back pressure 2MPa (insufficient);
3. Material Verification: PC moisture content 0.05% (meets standard), but stored for over 6 hours after drying (slight moisture absorption).
- Solution:
- Mold: Add 0.035mm deep vent grooves at the end, change gate to fan-shaped gate (20% wider);
- Process: Reduce injection speed to 70mm/s (stage-controlled), increase back pressure to 5MPa, and use within 2 hours of drying.
- Result: Surface porosity reduced from 12% to below 1%, and appearance acceptance rate increased to 98.5%.
Case 2: Bubbles inside a PP fuel tank cap
- Symptom: Vacuum bubbles (1-3mm in diameter) appeared in thick-walled area (4mm thickness) of product, causing it to fail burst test.
- Troubleshooting Process:
1. Mold Inspection: No independent venting slots were found in thick-walled area, with venting occurring solely at parting surface (gas entrapment);
2. Process Inspection: Holding time was 5s (4mm wall thickness x 3s/mm = 12s, seriously insufficient), and melt temperature was 240℃ (too high, increasing volatile release);
3. Equipment Inspection: Screw clearance was 0.28mm (out of tolerance), indicating uneven plasticization, resulting in unmelted particles (trapped gas) in melt.
- Solution:
- Mold: Added 0.03mm deep venting grooves in thick-walled area, connected to outside of mold;
- Process: Extended holding time to 12s, lowered melt temperature to 220℃, and added secondary shrinkage (30% injection pressure x 2s);
- Equipment: Replaced screw (with a 0.16mm gap) to ensure uniform plasticization;
- Results: Internal vacuum bubble rate was reduced from 30% to 0.5%, and burst test pass rate was 100%.

Management of product porosity and bubbles requires a systematic approach: mold venting is foundation, process control is the key, material drying is prerequisite, and equipment maintenance is guarantee. Frontline engineers must master core skills such as tiered venting groove design (matching depth and position to material), segmented injection speed control (avoiding turbulent air entrainment), and material dryness testing (moisture content/storage time). This allows them to shift from passive leak prevention to proactive prevention, ultimately achieving long-term control of porosity and bubbles.

 

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