1.1 Definition and Microscopic Mechanism of Strain
Strain on injection molded parts refers to surface defects caused by mechanical friction, concentrated shear stress, or insufficient material toughness during cavity filling, holding, or ejection processes, resulting in linear scratches, localized burrs, tears, or even large-scale chipping. Essentially, relative motion between material and mold surface, within material, or within ejection mechanism exceeds interfacial bonding strength, causing microscopic plastic deformation or macroscopic tearing (fibrous tear marks or material delamination visible under a microscope).
1.2 Core Cause Classification and Correlation Logic
Stretch marks are result of multiple factors, which can be categorized into four major, mutually reinforcing causes:
- Poor mold surface condition (the most direct contributing factor): Wear grooves, rust, insufficient polishing (Ra > 1.6μm) on mold cavity/runner surface, and foreign matter adhesion (release agent accumulation/aluminum shavings) lead to a surge in frictional resistance during melt flow (a friction coefficient > 0.3 is prone to causing stretch marks).
- Improper process parameters (a key driver): Excessive injection speed (melt shear rate > 1500s^-1, resulting in internal stress accumulation), excessive holding pressure (overcompaction of material, increasing adhesion to mold during ejection), and low melt temperature (poor material flowability, requiring higher filling pressure, exacerbating friction).
- Ejector system design/maintenance defects (later visible causes): Insufficient ejector pins (localized ejection stress concentration), rough ejector pin surface (Ra > 0.8μm), excessive ejection speed (inertial impact causes product deformation and damage), and lack of ejector pin lubrication (dry friction exacerbates wear).
- Material and equipment issues (hidden risks): Insufficient material toughness (e.g., PC/ABS alloys become more brittle at low temperatures), excessive proportion of recycled material (reduced molecular weight and reduced tear resistance), and screw wear (uneven plasticization, unmelted melt particles, and mold scraping during flow).

 

Based on frequency of mold damage, repair costs, and impact on product appearance, we recommend systematically addressing problem according to following priority:

Core Logic: Minor defects on mold surface (e.g., scratches as deep as 0.01mm) can be amplified into visible damage by melt flow friction, cleaning/polishing is inexpensive and effective quickly.
2.1.1 Accurate Location and Evaluation of Mold Surface Defects
- Tool-assisted: Use "blue lead test" (apply red lead powder to mold surface and observe uniformity of cavity coloration after closing mold). Uncolored areas indicate wear/poor polishing. Use a profilometer to measure surface roughness (target Ra ≤ 0.8μm, ≤ 0.4μm for precision parts).
- Experience-based judgment: Observe location of scratches. If continuous linear scratches appear along flow direction, this indicates longitudinal grooves on runner/cavity surface. Localized, point-like scratches are likely due to foreign matter (such as coked release agent or embedded aluminum chips). Circular scratches around ejector pin indicate a surface defect or insufficient lubrication.
2.1.2 Mold Surface Repair and Protection Measures
- Minor Wear Repair: For scratches with a Ra of 1.6-3.2μm, use 600#, 1200#, and 2000# water-reinforced sandpaper in a stepwise manner, then polish with diamond paste (3μm to 1μm) to ensure that no residual abrasive particles remain on the surface (to avoid secondary scratches).
- Foreign Debris Removal: Avoid using steel brushes to clean mold. Instead, use a copper brush and anhydrous ethanol (soak any release agent residue in a dedicated degreaser and then ultrasonically clean). Remove aluminum chips and copper slag embedded in cavity with a pneumatic chisel; avoid scraping.
- Surface Protective Coating: For areas frequently worn (such as deep cavity entrances and rib roots), spray-coat with diamond-like carbon coating (DLC, thickness 2-3μm). This reduces friction coefficient to below 0.1 and increases wear life by five times.
Operational Details:
- After repair and before mold testing, purge mold surface with compressed air (pressure 0.3 MPa) to ensure there is no residual polishing dust.
- During mass production, clean mold cavity with compressed air every two hours (focusing on runner corners and areas near gate) to prevent mold release agent buildup.
Case Study: A car door handle trim (ABS+PC) suffered a tear. A blue dye test revealed no staining at runner corner (wear groove depth 0.02 mm). After sanding with 1200# wet sandpaper and polishing to Ra 0.6μm, combined with air cleaning every two hours, tear disappeared within five minutes and continued to occur after 500 molds were produced.

Core Principle: Reduce material adhesion and frictional resistance by optimizing melt flow and reducing internal stress.
2.2.1 Segmented Control of Injection and Holding Pressure Parameters
- Injection speed: Controlled throughout the entire process at 60-80 mm/s (lower than burn process) to prevent excessive internal stress in melt due to high shear (measured reductions in speed from 100 mm/s to 70 mm/s and shear rate from 1800 s^-1 to 1200 s^-1 resulted in a 70% reduction in strain rate).
- Holding pressure and time: Adopt a "low pressure, long holding" strategy. Holding pressure = injection pressure * 60%-70% (e.g., injection pressure 100 MPa, holding pressure 60-70 MPa). Holding time = part wall thickness * 1.5 s/mm (e.g., 2 mm wall thickness, holding pressure 3 seconds) to avoid over-compaction leading to ejection and bonding.
- Melt temperature: Increase melt temperature appropriately within material's flow range (e.g., from 230℃ to 250℃ for ABS, from 280℃ to 300℃ for PC). Reduce melt viscosity (from 2000 Pa·s to 1500 Pa·s, reducing flow friction resistance by 30%).
Operational Details:
- For thin-walled products (<1.2mm), injection speed can be slightly increased (80-90 mm/s), but holding pressure should be reduced simultaneously (to 50% of injection pressure).
- Observe surface condition of part: If scratches appear wavy, this indicates unstable melt flow. Reduce injection speed to 50 mm/s and extend injection time.
2.2.2 Lubrication and Additive Optimization
- External Lubrication: For high-friction molds (e.g., aluminum alloy cavities), add 0.1%-0.2% silicone masterbatch to material (reducing friction coefficient by 40%), or apply a water-based mold release agent to ejector pin (spray every 50 molds to avoid excessive flash).
- Internal lubrication: Choose high-flow grades (e.g., ABS with an MI of 25-30g/10min reduces flow friction by 25% compared to 15g/10min).
Case study: A precision gear (PA66 with 30% GF) suffered from strain. Original injection speed was 90mm/s, holding pressure was 100MPa (injection pressure), and melt temperature was 270℃ (low). Adjusting injection speed to 70mm/s, holding pressure was 70MPa, and melt temperature was 290℃. Adding 0.15% silicone masterbatch reduced strain rate from batch rejection (50%) to sporadic, spot-on defects (<1%).

 

Core Logic: Mechanical stress during ejection stage is final cause of strain, requiring optimization in design, lubrication, and operation.
2.3.1 Ejector Pin Layout and Surface Treatment
- Ejector Pin Quantity and Distribution: Increase number of ejector pins (e.g., increase from 4 to 6) based on principle of "≥1 ejector pin per 50 cm² of product area" to reduce stress on each pin (ejection stress reduced from 15 MPa to below 10 MPa).
- Ejector Pin Surface Treatment: For ejector pins φ2-5 mm, use hard chrome plating (thickness 10-15 μm, Ra ≤ 0.4 μm) or TD treatment (carbide layer thickness 5-8 μm, 3x increased wear resistance). For ejector pins φ>5 mm, use nitriding treatment (hardness HV 900-1100).
- Ejector Pin Lubrication Cycle: Lubricate ejector pins with WD-40 every 100 molds (focusing on lubricating gap between pin and ejector hole). Clean any residual release agent from ejector pin surface with alcohol at the end of each shift.
2.3.2 Ejector Action Optimization
- Ejector speed: Segmented control, with a low-speed range (contacting product) of 0.5-1 mm/s and a high-speed range (releasing mold) of 3-5 mm/s, to prevent product deformation and strain caused by impact.
- Ejector stroke: A "double ejection" setting (initial ejection of 2 mm, a 0.5 second pause, and a second ejection of remaining stroke) is set to release internal stress in product and prevent overall deformation.
Case study: A mobile phone midframe (PC) suffered strain, concentrated in ring area around ejector pin.Original ejector pin was φ3 mm (unchrome-plated, Ra 1.2 μm) and had an ejection speed of 5 mm/s. After replacing it with a φ4 mm chrome-plated ejector pin (Ra 0.3 μm) and adding one more ejector pin (total six), ejection speed was reduced to 2 mm/s, and strain rate dropped from 8% to 0.1%.

Core Logic: Material toughness and equipment plasticization are long-term potential risks of injury, requiring standardized management and control.
2.4.1 Full Lifecycle Management of Materials
- Toughness Control: Preheat the PC/ABS alloy in an oven (60℃ for 2 hours) at low temperatures (<15℃) to improve material toughness (notched impact strength increases from 5 kJ/m² to 8 kJ/m²).
- Recycled Material Control: Recycled material content should be ≤ 15% (lower than scorching process) and must be dried separately (80℃ for 4 hours) to avoid molecular weight chain breakage and decreased tear resistance due to repeated heating.
- Additive Verification: New batches of material must undergo a "strain sensitivity test" (coating mold surface with 0.5μm alumina particles and observing degree of strain after injection. If strain exceeds standard, adjust process or replace material).
2.4.2 Preventive Maintenance of Equipment
- Screw-Barrel Clearance: Regularly inspect (every 100 tons of production). New screws must maintain a clearance of ≤0.15mm (50mm Φ screw). Screws exceeding 0.25mm require replacement (excessive clearance can cause unmelted particles in melt to scrape mold during flow).
- Check Ring Tightness: Test with compressed air (pressure 0.5MPa, hold for 3s, leakage <0.1L/min) to prevent melt backflow and uneven plasticization.
- Barrel Cleaning: When changing materials, clean barrel with transition material (such as PP) (at a temperature 20℃ above material's decomposition temperature and hold for 10 minutes) to prevent residual material from carbonizing and falling off, potentially scratching mold.
Case Study: A home appliance housing (PP+EPDM) was frequently scratched. Inspection revealed a 30% recycled material content (a 40% decrease in molecular weight) and a screw clearance of 0.3mm (out of tolerance). After replacing screw (with a 0.18mm clearance), reducing recycled material ratio to 10%, and adding a transition material cleaning step, strain problem was completely resolved.

Case 1: Linear Strain on Entire Length of an Automotive Instrument Panel Trim (ABS+PC)
- Symptom: Continuous scratches parallel to flow direction appeared along the entire length of product (200mm), with a depth of 0.03mm, causing abnormal noise during assembly.
- Troubleshooting Process:
1. Blue Dan Test: Runner was not colored throughout. A profilometer measured runner surface Ra of 2.5μm (design requirement: Ra ≤ 0.8μm), confirming mold wear.
2. Process Adjustment: Injection speed was reduced from 100mm/s to 70mm/s, and melt temperature was increased from 240℃ to 260℃. Strain was reduced but not eliminated.
3. Ejector Inspection: Ejector pin Ra was 1.0μm (not meeting standard), and lubrication cycle was excessively long (every 200 molds).
- Solution:
- Mold: Polish runners with diamond paste to an Ra of 0.5μm and apply a DLC coating.
- Ejector: Chrome-plated ejector pins (Ra of 0.3μm) and lubricated every 50 molds.
- Result: Strain was completely eliminated, with no recurrence after subsequent production of 1,000 molds. Product qualification rate increased from 75% to 99.5%.
Case 2: Circular strain around ejector pin of a precision bearing seat (PA66)
- Symptom: Radial tear marks appeared around ejector pin hole, resulting in a 15% scrap rate during mass production.
- Troubleshooting Process:
1. Ejector pin inspection: φ2.5mm ejector pin was not chrome-plated, with a surface Ra of 1.5μm. Ejection speed was 4mm/s (too fast).
2. Process troubleshooting: Holding pressure (injection pressure) was 110MPa, resulting in excessive ejection stress.
3. Material Verification: Recycled material content was 25%, molecular weight decreased by 35%, and tear resistance was insufficient.
- Solution:
- Ejector: Replaced with a 3.5mm chrome-plated ejector pin (Ra 0.4μm), reduced ejection speed to 1.5mm/s, and added a secondary ejector.
- Process: Reduced holding pressure to 70MPa and increased melt temperature from 260℃ to 280℃.
- Material: Reduced recycled material content to 10%, and increased material preheating (70℃ x 2h).
- Result: Reduced scrap rate to 0.2%, and extended ejector pin life from 5,000 to 20,000 molds.

 

Core issue of product strain lies in friction and stress balance at "material-mold-process" interface. This requires a thorough investigation of "mold first, process, then ejector/material" process. Key priorities include controlling mold surface accuracy (Ra ≤ 0.8μm), melt flow stability (shear rate < 1500s⁻¹), and ejector stress control (single ejector force < 10MPa). Frontline engineers must master skills such as blue-dan testing, surface roughness measurement, and ejector lubrication cycle management. This allows them to shift from passive mold repair to proactive prevention, ultimately achieving long-term control of strain issues.