Ejector Mark on injection molded products refers to white marks or tiny protrusions on the surface or edges of product caused by mechanical stress exerted by ejector pins/ejector mechanism during mold release during cavity filling, holding, and cooling processes. Essentially, this is a synergistic failure of "mold release stress and material response":
- Stress concentration: Melt in ejector pin contact area is not fully solidified (surface hardness <80% of bulk), resulting in localized pressure during ejection (pressure >50 MPa), forcing molecular chains into alignment or causing microcracks (depth 0.01-0.05 mm);
- Oxidation blushing: Under stress, molecular chains on material surface break and react with oxygen (such as oxidation of benzene rings in PC), forming a white oxide layer (microscopically visible, with a thickness of 0.5-2 μm);
- Performance impact: Toughness of whitened area decreases (impact strength decreases by 15%-30%). In severe cases, this can lead to poor product appearance (A-side is not permitted) or excessive assembly clearances.

Ejector whitening is result of multi-dimensional imbalances in mold design, process parameters, material properties, and equipment status. It can be categorized into four main, overlapping causes:
- Mold design defects (the most fundamental contributing factor):
- Improper ejector pin positioning (facing cavity plane/thin-wall area), resulting in stress concentration during ejection (pressure > 80 MPa);
- Insufficient draft angle (<0.5°), resulting in high ejection friction (friction coefficient > 0.3), requiring ejector pin to exert greater thrust;
- Insufficient ejector pin count (<0.5 pins per square centimeter), resulting in excessive local ejection pressure (single-pin pressure > 100 MPa);
- Poor venting (gas around ejector pin is not exhausted), resulting in gas compression exacerbating ejection stress (equivalent to "air cushion effect" backlash).
- Out-of-control process parameters (key drivers):
- Excessive holding pressure (>70% of injection pressure) causes excessive melt compression (density increases by 5%-8%), resulting in residual shrinkage stress during demolding.
- Excessive holding time (>3s/mm of product wall thickness) causes increased rigidity (20% increase in hardness) after melt cooling, increasing ejection resistance.
- Insufficient cooling time (<1.2s/mm of product wall thickness) causes insufficient surface curing (hardness <70% of bulk), leading to deformation and whitening during ejection.
- Material properties (hidden risks):
- High-rigidity materials (PC, PMMA) have high molecular chain rigidity (elastic modulus > 2 GPa) and slow stress relaxation (half-life > 24 hours), making white marks difficult to resolve after ejection.
- Low-toughness materials (PA6, POM) have weak stress resistance (elongation at break < 10%), and ejection stress can easily induce microcracks.
- Materials containing volatile matter (e.g., PA6 with a moisture content > 0.1%) can vaporize to form bubbles (stress concentration around bubbles, making ejector marks more noticeable).
- Equipment deterioration (long-term hidden dangers):
- Ejector pin wear (diameter reduction > 0.1 mm) reduces contact area during ejection (pressure increases by 30%), leading to localized stress surges.
- Hydraulic system pressure fluctuations (> 5%) and unstable ejection speed (increased impact loads) exacerbate stress concentrations.
- Excessive plate parallelism (> 0.03 mm/m) and uneven ejector pin force (unilateral pressure deviation > 20%) can lead to localized white marks.

 

Based on cost of repairing whiteout issue, its impact on product appearance, and likelihood of recurrence, we recommend systematically addressing it according to following priorities:

Core Logic: Mold is source of ejection stress. Adjusting ejector pin position, draft angle, and venting system can directly reduce ejection stress concentration.
2.1.1 Precision Adjustment of Key Mold Structures
- Optimizing Ejector Pin Position and Quantity:
- Avoid ejector pins near cavity plane/thin-wall area (≥5mm from edge) and prioritize placement in the center of thick-walled areas (≤10mm from center for wall thickness >3mm);
- Increase number of ejector pins (≥0.8 pins per square centimeter) and reduce pressure per pin (from 100 MPa to below 60 MPa);
- Increase ejector pin diameter from φ2 mm to φ3 mm (increasing contact area by 50%) and reducing local pressure (from 80 MPa to 50 MPa).
- Draft Angle and Surface Treatment:
- Outer draft angle was increased from 0.5° to 1° (inner draft angle increased from 0.3° to 0.8°), reducing friction during demolding (friction coefficient was reduced from 0.3 to 0.15);
- Ejector pin surface was hard chrome plated (Ra ≤ 0.4μm) to reduce friction coefficient (from 0.3 to 0.1), preventing scratches and whitening during ejection;
- Cavity surface was polished to Ra ≤ 0.6μm (reducing demolding friction and reducing stress concentration).
- Exhaust System Improvements:
- Exhaust grooves (0.03-0.05mm deep, 5-8mm wide) were added around ejector pin to exhaust compressed gas (reducing residual gas by 80%);
- Exhaust pins (0.5mm diameter, 2-3mm spacing) were added at the end of main channel to prevent gas backflow during ejection.
Operational Details:
- After mold modification, Moldflow simulation verification is required (ejector stress < 60% of material yield strength, friction coefficient < 0.2);
- For precision molds (such as optical components), ejector pin position tolerance is controlled within ±0.02mm (to avoid stress concentration caused by machining errors).
Case Study: A mobile phone protective case (PC) experienced severe whitening on A-side. Original ejector pin was positioned at the edge of flat surface (pressure 85MPa) with a draft angle of 0.4°. Ejector pin was adjusted to the center of thick wall (pressure reduced to 50MPa), draft angle was increased to 1°, and ejector pin surface was hard chrome plated. Whitening rate was reduced from 40% to below 2%.

Core Principle: Reduce residual stress and rigidity in melt during demolding by reducing holding pressure, shortening holding time, and extending cooling time.
2.2.1 Segmented Control of Injection and Holding Pressure Parameters
- Optimizing Holding Pressure and Time:
- Holding pressure = injection pressure * 50%-60% (e.g., injection pressure 100 MPa, holding pressure 50-60 MPa) to reduce melt compression (density increase <3%);
- Holding time = part wall thickness * 2.5 s/mm (e.g., wall thickness 2 mm, holding pressure 5 seconds) to avoid over-curing (hardness < 75% of original part);
- For high-rigidity materials (e.g., PC), reduce holding time by an additional 1-2 seconds (to wall thickness * 2 s/mm) to reduce residual stress.
- Cooling Time and Injection Speed Adjustment:
- Extend cooling time to product wall thickness x 1.5s/mm (e.g., for a 2mm wall thickness, cool for 3s) to ensure sufficient surface solidification (hardness > 85% of bulk).
- Reduce injection speed to 60-80mm/s (a 20%-30% reduction compared to normal process) to minimize premature solidification at melt front (improving surface hardness more evenly).
- Back Pressure and Melt Temperature Control:
- Increase back pressure to 3-5MPa (for amorphous materials) or 4-6MPa (for crystalline materials) to promote close packing of molecular chains (reducing shrinkage by 0.1%-0.2%).
- Increase melt temperature to upper limit of material's flow temperature (e.g., from 300℃ to 310℃ for PC, or from 260℃ to 270℃ for PA6) to reduce viscosity (by 15%-20%) and minimize demolding resistance.
Operational Details:
- For high-toughness materials (such as PP), holding pressure can be appropriately increased to 70% of injection pressure, but cooling time must be extended simultaneously.
- Observe product surface: If ejector mark appears as dense, small bright spots, holding pressure is too high, should be reduced to 50% and holding time shortened.
Case Study: Ejector mark on a certain automotive interior panel (PP) caused assembly clearance deviations. Original holding pressure was 70 MPa (injection pressure 100 MPa) and holding time was 6 seconds (wall thickness 2 mm x 3 seconds/mm = 6 seconds). Holding pressure was adjusted to 50 MPa, holding time was 5 seconds, and cooling time was extended to 3.75 seconds (2 x 1.875 seconds/mm). Ejector mark rate dropped from 35% to 5%.

Core Logic: Material toughness and equipment condition are long-term risks of ejector mark, requiring establishment of a standardized control process.
2.3.1 Material Toughness and Purity Control
- Material Selection and Modification:
- For high-rigidity materials (such as PC), add 1%-2% toughening agent (such as MBS) to improve toughness (impact strength increases by 30%) and reduce ejection microcracks.
- For low-toughness materials (such as PA6), select grades with high elongation at break (>20%) to improve stress resistance (whitening rate decreases by 40%).
- Recycled material content should be ≤ 10% (to prevent molecular chain breakage and reduced toughness caused by repeated heating).
- Drying Process Optimization:
- Hygroscopic materials (PA6, PC) should be thoroughly dried (moisture content <0.1%) to prevent moisture vaporization and formation of bubbles (stress concentration around bubbles, making whitening more obvious).
- Use within 4 hours of drying (if exceeded, re-drying is required) to prevent moisture absorption and loss of toughness (PA6 impact strength decreases by 20% after moisture absorption).
2.3.2 Equipment Preventive Maintenance
- Ejector and Hydraulic System Maintenance:
- Regularly inspect ejector pins for wear (replace if diameter reduction > 0.1mm) and ensure contact area (pressure ≤ 60MPa);
- Calibrate hydraulic system pressure stability (fluctuation < 5%) and replace aged seals (e.g., O-rings every 2000 hours).
- Barrel Temperature and Platen Parallelism Calibration:
- Calibrate barrel temperature control accuracy (within ±2℃) to avoid localized overcooling (e.g., temperature < 300℃ in a certain section of a PC barrel);
- Correct platen parallelism (≤ 0.02mm/m) to ensure uniform ejector pin force (single-side pressure deviation < 10%).
Case Study: A precision gear (PA66+GF35) frequently experienced whitening. Inspection revealed ejector pin wear (diameter reduction of 0.15mm) and insufficient material drying (moisture content 0.15%). Replace ejector pin with a new one (Ø3mm diameter) and increase drying temperature to 125℃ for 4 hours (moisture content reduced to 0.05%). ejector mark pin rate dropped from 25% to 1%.

 

Core Logic: Ambient temperature and humidity, along with auxiliary measures, can further suppress ejector mark pins, especially for high-rigidity or precision parts.
2.4.1 Environmental and Auxiliary Process Control
- Workshop Environmental Control: For production of high-rigidity materials (such as PC), 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., PC molds from room temperature to 80℃) to prevent overcooling of melt during initial filling, which can lead to stress concentration during demolding.
- Post-Processing Aids: For high-demand products (such as optical lens holders), add an annealing process (80℃ for 2 hours) to promote molecular chain relaxation (reducing visibility of whitening by 30%).
Case Study: An optical lens holder (PMMA) had an 18% whitening rate in a high humidity environment (65% RH). After workshop humidity was reduced to 45% RH and mold preheating time was extended to 45 minutes, whitening rate dropped to 3% after annealing process was added.

Case 1: Whitening on A-side of a Mobile Phone Middle Frame (PC) Leads to Poor Appearance
- Problem: Ejector pin mark on A-side of product was white (2-3mm in length), leading to customer rejection (no defects allowed on A-side).
- Troubleshooting Process:
1. Mold Inspection: Ejector pin positioned at the edge of plane (pressure 85MPa), draft angle 0.4°, ejector pin surface roughness (Ra 0.8μm);
2. Process Troubleshooting: Holding pressure 70MPa (injection pressure 100MPa), holding time 6s (wall thickness 2mm x 3s/mm = 6s);
3. Material Verification: PC moisture content 0.08% (meets standard), but ejection stress concentration (pressure > 70% of material yield strength).
- Solution:
- Mold: Ejector pin adjusted to center of thick wall (pressure reduced to 50 MPa), demolding angle increased to 1°, and ejector pin surface hard chrome plated (Ra 0.4 μm);
- Process: Holding pressure reduced to 50 MPa, holding time shortened to 5 seconds;
- Result: ejector mark completely eliminated, and appearance pass rate increased from 60% to 99%.
Case 2: Ejector mark on automotive door panel (PP) causes assembly clearance to exceed tolerances
- Symptom: Ejector mark on side B of product was raised (height 0.1 mm), resulting in a clearance exceeding tolerance (>0.2 mm) when assembled with metal part.
- Troubleshooting Process:
1. Mold Inspection: Insufficient number of ejector pins (0.4 per square centimeter), local pressure >100 MPa;
2. Process Inspection: Insufficient cooling time (2 seconds < 2mm wall thickness * 1.5 seconds/mm = 3 seconds), insufficient melt solidification;
3. Equipment Inspection: Hydraulic system pressure fluctuation of 8% (>5%), unstable ejection speed.
- Solution:
- Mold: Increase number of ejector pins (0.8 per square centimeter), reduce pressure per pin;
- Process: Extend cooling time to 3 seconds, shorten holding time to 5 seconds;
- Equipment: Calibrate hydraulic system (pressure fluctuation <3%);
- Result: Ejector mark protrusion height reduced to 0.02mm, and assembly clearance pass rate was 100%.

Management of product white protrusions should follow systematic logic of "mold design is foundation, process control is the key, and materials/equipment are guarantee." Frontline engineers need to master core skills such as optimizing ejector pin position (avoiding stress concentration areas), adjusting demolding angles (reducing frictional resistance), and segmented control of holding parameters (reducing residual stress), shifting from "passive mold repair" to "active prevention" to ultimately achieve a long-term cure for ejector mark problem.