For previous reading, please refer to Solutions and Practical Guide to Glue and Material Overflow Problems on Injection Molding Machines U.

 

Oil stain on injection molded parts refers to residual oily substance that appears on or inside part during cavity filling, holding, and cooling processes of molten plastic. This is caused by thermal decomposition of material, contaminant migration from mold surface, or foreign matter adhering to melt. Essentially, it stems from interaction between contaminants, melt, and mold:
- Material decomposition: Thermal decomposition of materials (such as PVC, POM, and PA) at high temperatures produces low-molecular-weight products (e.g., HCl released by PVC, formaldehyde released by POM, and caprolactam released by PA). These products condense into an oily liquid upon cooling.
- Mold contamination: Residual release agent (e.g., incomplete evaporation of water-based release agent), lubricant (excessive lubrication of ejector pins/guide pins), or carbonized material (residual material in barrel) on mold surface can be flushed into mold cavity during melt filling.
- Melt adhesion: Low melt viscosity (e.g., PP MI = 40g/10min) or excessively fast injection speeds increase friction between melt and mold surface, carrying more contaminants (e.g., mold rust and dust).
Microscopically, oil stains can be seen as unevenly distributed oil droplets (0.01-0.5mm in diameter). Composition analysis reveals presence of material decomposition products (e.g., caprolactam from PA decomposition) or external contaminants (e.g., surfactants in release agent).

Oil stains are result of multiple contamination factors, including materials, molds, processes, and equipment. These can be categorized into four main causes, which amplify each other's defects:
- Thermal decomposition of materials (the most fundamental contributing factor):
- Uncontrolled processing temperature of heat-sensitive materials (PVC decomposes at 160℃, POM decomposes at 240℃) (e.g., PVC processing temperatures exceeding 180℃ can increase decomposition by fivefold);
- Repeated heating of recycled materials (PA6 recycled materials processed at temperatures exceeding 280℃ can release caprolactam three times that of virgin materials);
- Contamination from mixed materials (e.g., mixing PC with ABS, where PC decomposes to produce phenol that contaminates the ABS melt).
- Mold surface contamination (key driver):
- Excessive release agent (water-based release agent sprayed > 1g/m², with residual residue not completely volatilized);
- Lubricant leakage (ejector pin/guide pin grease overflows and enters mold cavity with melt);
- Mold rust (a humid environment causes rust on cavity surface, which migrates with melt).
- Out-of-control process parameters (direct trigger):
- Excessive injection speed (> 100mm/s): Melt shear heat exacerbates material decomposition (e.g., for PP at 250℃, increasing speed from 70mm/s to 100mm/s increases decomposition by 40%);
- Excessive melt temperature (> upper limit of material decomposition temperature), such as PA6 processing temperature > 290℃, which results in significant caprolactam release;
- Excessive holding pressure (> 70% of injection pressure): Over-compression of melt, carrying more mold contaminants.
- Equipment deterioration (long-term hidden danger):
- Carbonized material remaining in barrel (incomplete material replacement, resulting in residual material decomposing and producing oil at high temperatures);
- Screw wear (clearance > 0.25mm), uneven plasticization leading to localized overheating and decomposition;
- Barrel temperature fluctuations (±5℃ or more), with localized temperatures exceeding decomposition threshold (e.g., a certain section of a PVC barrel with a temperature > 170℃).

 

Based on cost of repairing oil stains, their impact on product appearance/performance, and probability of recurrence, we recommend systematically addressing them according to following priorities:

Core Logic: Mold is source of contaminant migration. Thoroughly cleaning mold surface and optimizing demolding and lubrication systems can directly block path for contaminants to enter cavity.
2.1.1 Deep Cleaning of Mold Surfaces
- Routine Cleaning:
- After shutting down, wipe cavity, runner, parting surface with a copper brush and anhydrous ethanol (do not use a steel brush to scratch the mold surface) to remove release agent residue, rust, and carbonized material.
- For hard-to-clean areas such as deep cavities and ribs, clean using an ultrasonic cleaner (40kHz frequency, 100W power) with a mold-specific cleaning agent (such as a neutral degreaser) for 10-15 minutes.
- Anti-pollution Coating:
- Spray a fluorine coating (0.5-1μm thickness) on easily contaminated areas (such as parting surfaces and ejector pinholes) to reduce surface energy (contact angle > 110°) and minimize release agent/lubricant adhesion.
- Regularly inspect coating integrity (every 5,000 molds) and recoat worn areas.
Operational Details:
- After cleaning, mold must be purged with compressed air (pressure 0.3 MPa) to ensure no residual cleaning agent (to prevent secondary contamination).
- Before the first production run on a new mold, a "blank mold test" (no injection molding, 50 dry strokes) must be performed to verify mold cleanliness (no oil residue).
Case Study: A toy shell (PVC) had extensive oil stains on its surface. Original mold had residual release agent (sprayed at 1.5 g/m²) on parting surface, and lubricant leaked from ribs. After deep cleaning, a fluorine coating was sprayed, reducing release agent dosage to 0.5 g/m² and oil stain rate from 40% to below 2%.

Core Principle: Rapidly reduce oil stain generation and migration by reducing risk of material decomposition and melt's ability to carry contaminants.
2.2.1 Optimization of Temperature and Pressure Parameters
- Melt Temperature Control:
- Processing temperatures for high-temperature-sensitive materials (PVC) are reduced to lower decomposition temperature (e.g., reducing PVC from 180℃ to 165℃ reduces decomposition by 70%).
- Processing temperatures for recycled materials are 10-15℃ lower than those for virgin materials (e.g., reducing PA6 recycled material from 280℃ to 265℃ reduces caprolactam release by 50%).
- Temperature deviations in each barrel section are controlled within ±2℃ (to avoid localized over-temperature decomposition).
- Injection speed and holding pressure adjustment:
- Control injection speed at 60-80 mm/s throughout the entire process (a 20%-30% reduction compared to normal process) to reduce shear heating (actually, a speed reduction from 100 mm/s to 70 mm/s reduced melt temperature rise from 15℃ to 5℃).
- Holding pressure = injection pressure × 50%-60% (e.g., injection pressure of 100 MPa, holding pressure of 50-60 MPa) to reduce contaminant carryover caused by melt compression (for every 10 MPa pressure reduction, contaminant carryover decreases by 15%).
Operational details:
- For easily degradable materials such as PVC and POM, add a "cooling reflux" stage (after holding pressure ends, reverse injection at 20% injection pressure for 2 seconds) to remove decomposition products from barrel.
- Observe product surface: If oil stains appear as a continuous film, mold is severely contaminated and requires more cleaning. If oil stains appear as dotted droplets, material is primarily decomposing and processing temperature needs to be lowered.
Case study: Surface of a certain automotive interior panel (POM) was densely stained with oil. Original melt temperature was 240℃ (close to its decomposition temperature of 240℃), and injection speed was 90 mm/s. Adjusting melt temperature to 225℃ and reducing injection speed to 70 mm/s reduced oil stain rate from 35% to 5%.

Core Logic: Material purity and equipment condition are long-term risks of oil stains, and standardized control procedures must be established.
2.3.1 Material Selection and Pretreatment
- Material Selection:
- Prefer low-decomposition materials (e.g., for PVC, choose high-polymerization grades with a decomposition temperature > 170℃; for PA, choose grades with low caprolactam release);
- Recycled materials should be ≤ 10% (to prevent accumulation of decomposition products from repeated heating), and should be dried separately (80℃ for 4 hours);
- Avoid mixing different materials (e.g., when mixing PC and ABS, add a compatibilizer to prevent interfacial decomposition).
- Drying Process Optimization:
- Hygroscopic materials (PA6, PC) must be thoroughly dried (moisture content <0.1%) to prevent moisture from participating in decomposition reaction (when moisture content of PA6 is >0.1%, amount of caprolactam released increases by 20%).
- Use within 4 hours of drying (if material is dried beyond this time, it must be dried again) to prevent further decomposition after moisture absorption.
2.3.2 Equipment Preventive Maintenance
- Barrel and Screw Cleaning:
- When changing materials, clean barrel with a transition material (such as PE) (at a temperature 20℃ above material's decomposition temperature and hold for 10 minutes) to remove any residual carbonized material.
- Regularly (every 2000 hours of production), disassemble screw and clean clearance between screw flights with a copper brush (to remove any residual carbonized material). Replace screw when wear clearance exceeds 0.25mm.
- Temperature and Pressure System Calibration:
- Calibrate barrel temperature control accuracy (within ±2℃) to avoid local overheating (e.g., a section of a PVC barrel with a temperature >170℃);
- Calibrate hydraulic system pressure stability (pressure fluctuation during hold phase <5%) and replace aging seals (e.g., O-rings every 2000 hours).
Case Study: Housing of a precision instrument (PA66) was frequently stained with oil. Investigation revealed insufficient drying of material (moisture content 0.15%) and residual carbonized material in barrel (PA66 decomposition produces caprolactam). After replacing drying equipment (dew point ≤ -40℃) and cleaning barrel, oil stain rate dropped from 25% to 1%.

Core Logic: Ambient temperature and humidity, along with auxiliary measures, can further suppress oil stains, especially for high-temperature-sensitive materials or precision parts.
2.4.1 Environmental and Auxiliary Process Control
- Workshop Environmental Control: For production of high-temperature sensitive materials (such as PVC), maintain a temperature of 25±2℃ (to prevent ambient heat radiation from overheating melt) and a humidity of 40%-50% RH (to prevent moisture absorption).
- Mold Preheating: For cold-start molds, preheat to process temperature 30 minutes in advance (e.g., PVC molds from room temperature to 50℃) to prevent overcooling of melt during initial filling, which could lead to decomposition.
- Post-Processing Aids: For products with a high risk of oil stains (such as PA gears), add an extraction process (wiping surface with anhydrous ethanol) to remove residual low-molecular-weight substances (oil stain removal rate >90%).
Case Study: An optical lens (PMMA) had an oil stain rate of 18% in a high humidity environment (65% RH). After workshop humidity was reduced to 45% RH and mold preheating time was extended to 45 minutes, oil stain rate dropped to 3% after addition of extraction process.

 

- Symptom: A continuous oil film (thickness 0.05-0.1mm) covered product surface, resulting in poor appearance (A-side is not permitted).
- Troubleshooting Process:
1. Mold Inspection: Residual release agent (spraying amount 1.5g/m²) on parting surface, and lubricant leakage at ribs;
2. Process Troubleshooting: Melt temperature 180℃ (close to PVC decomposition temperature of 160℃), injection speed 100mm/s (too high);
3. Material Verification: PVC contains impurities (decomposition product content 0.3%).
- Solution:
- Mold: After deep cleaning, spray-coat with fluorine, reduce release agent dosage to 0.5g/m², and repair lubricant leaks.
- Process: Reduce melt temperature to 165℃ and injection speed to 70mm/s.
- Material: Replace with high-purity PVC (decomposition product content <0.1%).
- Result: Surface oil stains completely eliminated, and appearance acceptance rate increased from 60% to 99%.

- Symptom: Oil droplets with a diameter of 0.1-0.3mm appeared at base of rib, covering >30% of area, resulting in an excessive assembly clearance.
- Troubleshooting Process:
1. Mold Inspection: Lubricant overflowing from ejector pin hole (grease dosage exceeded standard);
2. Process Inspection: Holding pressure 70 MPa (injection pressure 100 MPa), holding time 5 seconds (wall thickness 2 mm x 2.5 seconds/mm = 5 seconds, insufficient);
3. Equipment Inspection: Carbonized material remaining in barrel (PA6 decomposes to produce caprolactam).
- Solution:
- Mold: Adjust ejector pin grease dosage (from 0.5 g/stroke to 0.2 g/stroke), and add guide pin dust covers;
- Process: Reduce holding pressure to 50 MPa and extend holding time to 7.5 seconds (2 x 3.75 seconds/mm = 7.5 seconds);
- Equipment: Clean barrel of carbonized material and replace worn screw (clearance 0.18 mm);
- Result: Point-like oil drop rate decreased from 25% to 2%, and assembly clearance pass rate was 100%.

Management of product oil stains requires a systematic approach: mold cleaning is foundation, process temperature control is the key, and materials/equipment are guarantee. Frontline engineers must master core skills such as deep mold cleaning (fluorine coating), material decomposition temperature control (PVC < 165℃), and regular equipment maintenance (barrel cleaning). This allows them to shift from passive oil removal to proactive prevention, ultimately achieving a long-term solution to oil stain problems.