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

Sink marks in injection molded products refer to depressions (surface shrinkage) or tiny voids (internal shrinkage) that form on or within surface of a part due to localized melt shrinkage that is not effectively compensated during cavity filling, holding, and cooling processes. This results in a decrease in material density and volume. Essentially, it is a physical process characterized by an imbalance in shrinkage and feeding:
- Cooling shrinkage: After melt stops flowing from gate, core temperature continues to drop (amorphous materials shrink 1%-3% from Tg to room temperature, and crystalline materials shrink 1%-2% from melting point to room temperature), resulting in volume reduction.
- Insufficient feeding: Holding pressure/time is insufficient to push subsequent melt into shrinkage area, or gate solidifies prematurely, blocking feeding path.
- Gas interference: Residual air in cavity or volatiles (such as moisture and low molecular weight compounds) accumulate in shrinkage area, exacerbating localized volume loss (space occupied by gas is equivalent to "false shrinkage").
Microscopically, surface shrinkage cavities appear as "craters" (with raised edges and a concave bottom) with a depth of 0.02-0.1mm. Internal shrinkage cavities are spherical or elliptical cavities with a diameter of 0.01-0.5mm, often accompanied by material degradation (e.g., yellowing around shrinkage cavities in PA6).

Shrinkage is result of a multi-dimensional imbalance in material properties, mold design, process parameters, and equipment status. It can be categorized into four major, overlapping causes:
- Material properties (basic trigger):
- High-shrinkage materials (PP shrinkage 1.5%-2.5%, PA6 shrinkage 1.2%-2.0%) do not match feeding capacity;
- Crystalline materials (PE, PP, PA) have large crystallization shrinkage during cooling (for example, PP crystallinity increases from 50% to 70%, resulting in a 1.2% volume shrinkage), requiring higher feeding requirements;
- Material contains volatiles (for example, PC moisture content >0.02%, PA6 moisture content >0.1%), which vaporize and occupy shrinkage space (1g of water vaporizes, causing its volume to expand 1700 times, equivalent to "excessive shrinkage").
- Mold design defects (key drivers):
- Improper gate location/size (e.g., gates far from thick-walled areas, resulting in a long feeding path; gates that are too small result in significant holding pressure loss);
- Uneven cooling system layout (thick-walled areas cool slowly, leading to delayed shrinkage, while thin-walled areas cool quickly, resulting in premature solidification and blocking feeding);
- Improper thick-wall/rib design in product structure (e.g., wall thickness > 4mm and no process bosses, resulting in concentrated shrinkage and no feeding channels).
- Out-of-control process parameters (direct trigger):
- Insufficient holding pressure (<60% of injection pressure) or too short holding time (<wall thickness * 2s/mm), failing to compensate for core shrinkage;
- Excessive injection speed (>100mm/s), causing premature solidification of melt front and blocking feeding path;
- Excessively high melt temperature (>material decomposition temperature), exacerbating volatile release (e.g., volatile release at 240℃ for PP is three times that at 220℃), resulting in "gas shrinkage voids."
- Equipment deterioration (long-term hidden danger):
- Screw wear (clearance > 0.25mm) leading to uneven plasticization and presence of unmelted particles in melt (concentrated shrinkage around particles);
- Failure of check ring seal (melt backflow) leading to a sudden drop in pressure during holding phase (holding pressure fluctuation > 10%);
- Low barrel temperature control accuracy (above ±5℃), resulting in localized melt overheating and decomposition, producing gas (e.g., PVC decomposition produces HCl gas, which occupies shrinkage space).

 

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

Core Logic: Mold is "physical boundary" between shrinkage and feeding. Adjusting gate location, cooling system, product structure can directly balance shrinkage-feeding relationship.
2.1.1 Precision Adjustment of Key Mold Structures
- Optimizing Gate Position and Size:
- Gate should be located in the center of thick-walled area (e.g., for a product with a wall thickness of 3mm, distance between gate and thick-walled area should be ≤10mm). This shortens feeding path (flow length/wall thickness ratio <80:1).
- Increasing gate size (point gate diameter from φ0.8mm to φ1.2mm, latent gate width from 1.0mm to 1.5mm) reduces holding pressure loss (increasing gate cross-sectional area by 50% reduces pressure loss by 40%).
- Avoid having gate directly facing thin-walled area (e.g., for a wall thickness <1.5mm, angle between gate and thin-walled area should be >45°). Use fan-shaped or overlapping gates to distribute feeding pressure.
- Cooling system zoning design:
- Add independent cooling circuits to thick-walled areas (wall thickness > 2.5mm) (reducing water temperature by 10℃ increases cooling rate by 30%), delaying cooling of thick-wall center (enabling synchronized cooling of thick and thin walls, with a temperature difference of <10℃);
- Reduce cooling circuit density in thin-walled areas (wall thickness <1.5mm) (increasing spacing from 50mm to 80mm) to prevent overcooling and premature solidification that could block shrinkage feeding;
- Maintain a distance of 1.5-2 times channel diameter from mold cavity (e.g., 12-16mm for an 8mm diameter channel) to ensure uniform cooling.
- Product structure and processability improvements:
- Add process bosses (2-3mm height, 5-8mm width) to thick-walled areas (wall thickness > 4mm) to distribute concentrated shrinkage to bosses (boss shrinkage rate is consistent with base to avoid surface concavity);
- Rib design must match base wall thickness (rib thickness ≤ 60% of base wall thickness, rib height ≤ 3 times rib thickness) to reduce concentrated shrinkage at rib root (rib shrinkage is 1.5-2 times that of base).
Operational details:
- After mold modification, use Moldflow simulation for verification (thick-wall area packing rate > 90%, cooling time difference < 5%);
- For precision molds (such as optical components), maintain a gate size tolerance of ±0.02mm to avoid insufficient packing due to machining errors.
Case study: A large shrinkage cavity developed in thick-walled area (4mm thickness) on the back of a PP appliance housing. Original gate was located in thin-walled area (1.5mm thickness), and cooling circuit only covered surface. Gate was relocated to center of thick wall (4mm wall thickness), an independent cooling circuit was added to thick-wall area (water temperature was reduced by 15℃), and a process boss (2mm height) was added to thick-wall area. This reduced shrinkage rate from 35% to below 2%.

Core Principle: By optimizing holding pressure strategy, reducing shrinkage rate, and extending feeding time, risk of localized shrinkage not being fed was reduced, resulting in rapid improvement of shrinkage defects.
2.2.1 Segmented Control of Injection and Holding Pressure Parameters
- Optimizing Holding Pressure and Time:
- Holding pressure = injection pressure * 60%-70% (e.g., injection pressure 100 MPa, holding pressure 60-70 MPa) to ensure that melt can be pushed into thick-walled areas for feeding.
- Holding time = product wall thickness * 2.5 s/mm (e.g., 2 mm wall thickness, holding pressure 5 s; 4 mm wall thickness, holding pressure 10 s) to extend feeding duration (for every 1 mm increase in wall thickness, increase holding time by 2.5 s).
- For crystalline materials (e.g., PP), increase holding time by an additional 1-2 s (to a maximum of 3 s/mm wall thickness * wall thickness) to compensate for crystallization shrinkage (the higher crystallinity, the greater feeding requirement).
- Injection speed and melt temperature adjustment:
- Control injection speed at 60-80 mm/s throughout the entire process (a 20%-30% reduction compared to normal process) to prevent premature solidification at melt front (measured speed reduction from 100 mm/s to 70 mm/s delayed gate solidification time by 30%).
- Raise melt temperature to upper limit of material's flow temperature (e.g., from 220℃ to 240℃ for PP, from 260℃ to 280℃ for PA6) to reduce melt viscosity (from 1800 Pa·s to 1200 Pa·s) and improve shrinkage flowability.
- For high-volatile materials (e.g., PA6), control melt temperature below lower limit of its decomposition temperature (below 280℃) to minimize volatile release (avoiding "gas cavitation").
- Back Pressure and Cooling Time Control:
- Increase back pressure to 3-5 MPa (for amorphous materials) or 4-6 MPa (for crystalline materials) to increase internal friction within melt and promote close packing of molecular chains (for every 1 MPa increase in back pressure, shrinkage decreases by 0.1%-0.2%).
- Extend cooling time to 1.5 seconds per mm of product wall thickness (e.g., 3 seconds for a 2 mm wall thickness) to ensure sufficient cooling of core layer (reducing core temperature below glass transition temperature to prevent subsequent shrinkage).
Operational Details:
- For high-shrinkage materials (such as PP), adopt a "stepped pressure hold" (60 MPa for 3 seconds in the first stage, 40 MPa for 2 seconds in the second stage) during holding phase to avoid interruptions in feeding caused by sudden pressure drops.
- Observe product surface: If shrinkage cavities appear as dense, small dimples, this indicates insufficient holding pressure. Increase holding pressure to 70% of injection pressure and extend holding time.
Case study: Localized shrinkage porosity was observed on the surface of an automotive interior panel (ABS). Original holding pressure was 60 MPa (injection pressure 100 MPa) and holding time was 5 seconds (wall thickness 2 mm x 2.5 seconds/mm = 5 seconds). Adjusting holding pressure to 70 MPa and extending holding time to 7.5 seconds (2 x 3.75 seconds/mm = 7.5 seconds) reduced shrinkage porosity from 25% to 3%.

Core Logic: Material shrinkage and equipment plasticization conditions are long-term potential risks for shrinkage porosity, necessitating establishment of a standardized control process.
2.3.1 Material Shrinkage Matching and Control
- Material Selection and Modification:
- Low-shrinkage materials should be prioritized (e.g., block copolymers for PP, with a shrinkage of 1.2%-1.8%, 30% lower than homopolymer PP);
- For high-shrinkage materials (e.g., PA6), add 0.5%-1% nucleating agent (e.g., talc) to refine crystals (reducing crystallinity by 10%-15%) and reduce crystallization shrinkage;
- Proportion of recycled material should be ≤ 10% (to prevent molecular chain breakage and shrinkage fluctuations), and recycled material should be dried separately (80℃ for 4 hours).
- Drying Process Optimization:
- Hygroscopic materials (PA6, PC) should be thoroughly dried (moisture content <0.1%) to prevent moisture evaporation and formation of "gas shrinkage" (air bubbles occupying space, which is equivalent to shrinkage);
- Use within 4 hours of drying (if exceeded, re-drying is required) to prevent abnormal shrinkage after moisture absorption (PA6 shrinkage increases by 0.3%-0.5% after moisture absorption).
2.3.2 Equipment Preventive Maintenance
- Screw and Check Ring Maintenance:
- Regularly inspect screw wear (gap ≤ 0.15mm for a 50mm diameter screw). Replace screws if they are out of tolerance (excessive gaps result in unmelted particles in melt and concentrated shrinkage around particles).
- Check ring tightness test (red lead powder contact area > 85%). Replace if it falls below this value (to prevent melt backflow and a sudden drop in holding pressure).
- Barrel Temperature and Pressure Calibration:
- Calibrate barrel temperature control accuracy (within ±2℃) to prevent localized overheating and decomposition gas production (e.g., PVC decomposition produces HCl gas, which occupies shrinkage space).
- Hydraulic system pressure stability calibration (pressure fluctuation during holding phase < 5%). Replace aging seals (e.g., O-rings every 2000 hours).
Case study: A precision gear (PA66+GF30) frequently exhibited shrinkage cavitation. Investigation revealed insufficient material drying (moisture content 0.15%), a screw clearance of 0.3mm (out of tolerance), and a check ring contact area of 70%. A new screw (with a clearance of 0.18mm) was replaced, and drying temperature was increased to 125℃ for 4 hours (moisture content dropped to 0.05%). After replacing check ring, shrinkage cavitation rate dropped from 22% to 1%.

 

Core Logic: Ambient temperature and humidity, along with auxiliary measures, can further suppress shrinkage cavitation, particularly for materials with high shrinkage rates or precision parts.
2.4.1 Environmental and Auxiliary Process Control
- Workshop Environmental Control: For high-shrinkage materials (such as PP), workshop temperature is controlled at 25±2℃ (to prevent uneven melt cooling due to ambient heat radiation) and humidity is maintained at 40%-50% RH (to prevent moisture absorption).
- Mold Preheating: For cold-start molds, preheat to process temperature 30 minutes in advance (e.g., PP molds are heated from room temperature to 60℃) to prevent overcooling of melt during initial filling, which can lead to concentrated shrinkage.
- Post-Processing Aids: For products with high stress shrinkage (such as optical lens holders), an annealing process (70℃ for 2 hours) is added to promote molecular chain relaxation (reducing shrinkage by 0.2%-0.3%).
Case Study: An optical lens holder (PC) exhibited a shrinkage rate of 15% in a high humidity environment (65% RH). After workshop humidity was reduced to 45% RH and mold preheating time was extended to 45 minutes, shrinkage rate was reduced to 3% after annealing process was added.

- Symptom: Shrinkage cavities with a diameter of 0.5-1mm appeared in thick-walled area (3mm thickness) on the back of product, concentrated around camera bracket.
- Troubleshooting Process:
1. Mold Inspection: Gate was located in thin-walled area (1.2mm thickness), feeding path was long (flow length/wall thickness ratio 120:1), and cooling circuits in thick-walled area were spaced 50mm apart (too dense, resulting in localized undercooling).
2. Process Troubleshooting: Holding pressure was 60MPa (injection pressure 100MPa), and holding time was 5s (3mm wall thickness x 2.5s/mm = 7.5s, insufficient).
3. Material Verification: Moisture content of PC+ABS was 0.08% (meeting standard), but shrinkage was concentrated in thick-walled area (1.8%).
- Solution:
- Mold: Adjust gate to center of thick wall (wall thickness 3mm), increase gate diameter to φ1.2mm, and adjust cooling circuit spacing in thick wall area to 80mm;
- Process: Increase holding pressure to 70MPa and extend holding time to 9s (3*3s/mm = 9s);
- Material: Add 0.5% talcum powder (to reduce crystallization shrinkage);
- Result: Backside shrinkage was completely eliminated, and appearance acceptance rate increased from 75% to 98%.

- Symptom: Dense, small dimples (0.1-0.3mm in diameter) appeared along base of ribs on product surface, covering >40% of surface area, resulting in an excessive assembly clearance.
- Troubleshooting Process:
1. Mold Inspection: Rib thickness was 70% of main wall thickness (exceeding 60% standard), and there was no chamfer at rib base (causing stress concentration).
2. Process Inspection: Injection speed 110mm/s (too high), holding time 5s (wall thickness 2mm x 2.5s/mm = 5s, insufficient).
3. Equipment Inspection: Screw clearance 0.28mm (exceeding tolerance), uneven plasticization resulting in unmelted particles in melt (concentrated shrinkage around particles).
- Solution:
- Mold: Adjust rib thickness to 55% of main body wall thickness, and add a 0.2mm chamfer at rib base to reduce shrinkage concentration.
- Process: Reduce injection speed to 70mm/s and extend holding time to 7.5s (2 x 3.75s/mm = 7.5s).
- Equipment: Replace screw (with a 0.16mm clearance) to ensure uniform plasticization.
- Results: Surface pit rate decreased from 30% to 2%, and assembly clearance pass rate reached 100%.

Managing product shrinkage requires a systematic approach: mold design is foundation, process control is the key, and materials/equipment are guarantee. Frontline engineers must master core skills such as gate location optimization (matching material shrinkage), cooling system zoning (balancing thick-wall and thin-wall cooling), and staged holding control (extending shrinkage-feeding time). This allows them to shift from passive mold repair to proactive prevention, ultimately achieving a long-term solution to shrinkage problems.