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Among injection molded part defects, shrinkage and sink marks are "invisible killers"—they may be hidden in thick-walled areas and difficult to detect, or appear on the surface as unsightly indentations. In mild cases, they affect appearance; in severe cases, they lead to assembly failure. Essence of this problem is insufficient or uneven shrinkage compensation during melt cooling, resulting in localized volume loss. This article, combining real-world cases from automotive and home appliance industries, teaches you how to systematically address this persistent problem from four dimensions: shrinkage mechanism, troubleshooting logic, solution techniques, and prevention system.

 

Core contradiction of shrinkage is: melt shrinks in volume during cooling, but fails to receive sufficient melt compensation in time. This underlying logic needs to be understood from three aspects: material characteristics, process parameters, and mold design.
1. Material Characteristics: "Inherent Setting" of Shrinkage Rate
Different materials have vastly different shrinkage behaviors, which are "genes" of shrinkage:
- Crystalline plastics (such as PP, PE, PA): During crystallization, molecules are tightly arranged, resulting in a high volume shrinkage rate of 1.5%-3.0% (non-crystalline materials like ABS are only 0.5%-0.8%). A trial mold of a car bumper (PP material) showed severe shrinkage in thick-walled area, root cause being high shrinkage rate of PP itself that was not compensated.
- Glass fiber reinforced materials (such as GF-PA66): Glass fibers restrict molecular shrinkage, but localized depressions are easily generated at interface due to stress concentration. Shrinkage in a home appliance bracket (30% GF-PA) was concentrated at the base of reinforcing ribs, related to shrinkage difference at glass fiber-resin interface.
- Hygroscopic materials (such as PC): Moisture evaporation leads to changes in melt viscosity, resulting in more uneven shrinkage during cooling.
2. Process Parameters: "Loss of Control" in Rhythm of Compensation and Cooling
Process settings directly affect compensation efficiency:
- Insufficient holding pressure/too short holding time: Holding pressure is core means of compensation. A mobile phone frame (PC+ABS) showed shrinkage on the back. Analysis revealed that holding pressure was only 60 bar (standard 80-100 bar), and after holding time was reduced from 15s to 8s, insufficient shrinkage compensation problem became apparent.
- Injection speed too slow: Low-speed injection causes melt to cool prematurely, forming a "hard shell" on the surface, preventing material from being supplied through gate during internal shrinkage. A toy casing showed shrinkage on the side wall; increasing injection speed resulted in more continuous melt flow and timely shrinkage compensation.
- Unreasonable cooling time: Uneven cooling leads to asynchronous local shrinkage. A car door panel (PP+EPDM) showed shrinkage at junction of reinforcing rib and main body. Infrared temperature measurement showed that this area cooled slowly (5℃ lower than main body). Extending cooling time reduced indentation.
3. Mold Design: "Inherent Defects" in Shrinkage Compensation Path
Mold structure determines whether material supply "channel" is unobstructed:
- Improper gate location/size: Gate is far from thick-walled area (e.g., gate is located at the other end of thick-walled area of product), requiring melt to flow a long distance for shrinkage compensation, losing pressure due to cooling along way. A power tool casing (ABS) showed shrinkage at thick-walled end; changing gate from side to center of thick-walled area improved shrinkage compensation efficiency by 70%.
- Missing runner/cold slug well: Runner is too thin (e.g., main runner diameter < 6mm) or cold slug well is not designed, leading to mixing of cold material during shrinkage compensation, blocking material supply channel. A medical device casing showed shrinkage; inspection revealed that runner diameter was only 5mm (standard 6-8mm). Increasing diameter resulted in smooth shrinkage compensation.
- Imbalanced cooling system: Local cooling is too fast (e.g., near cooling water holes) or too slow (e.g., no cooling circuit in rib area), leading to asynchronous shrinkage. An air conditioner panel showed shrinkage at rib area; adding cooling water holes to rib area improved cooling uniformity, and indentation disappeared.

Shrinkage and indentation are easily confused with gas entrapment and short shots; it is necessary to quickly identify direction based on defect characteristics.
Step 1: Observe location and shape of shrinkage (3-minute assessment)
- Thick-walled/ribbed areas: Often due to material shrinkage + insufficient packing pressure (e.g., thick-walled end of a PP bumper).
- Near gate: May be due to premature pressure holding switch (melt not fully filling before switching to holding pressure).
- Randomly distributed depressions: Often due to insufficient material drying (moisture causing abnormal shrinkage) or process fluctuations (e.g., fluctuating material temperature).
Step 2: Verify process parameters (10-minute trial adjustment)
- Increase holding pressure: Gradually increase holding pressure (by +10 bar each time) and time (by +2s each time), and observe whether shrinkage is reduced. For a home appliance casing with shrinkage, holding pressure was increased from 60 bar → 80 bar → 100 bar. When it reached 90 bar, depressions basically disappeared, indicating insufficient holding pressure.
- Increase speed/decrease material temperature: Increase injection speed (especially during filling stage of thick-walled areas), or lower barrel temperature (to reduce premature cooling). For a mobile phone case with shrinkage, increasing injection speed by 20% resulted in more timely melt replenishment.
- Extend cooling time: Extend cooling time separately for areas with uneven cooling (such as ribs) (e.g., from 10s → 15s), and observe whether shrinkage is synchronized.
Step 3: Check mold design (30 minutes - 1 hour)
- Measure gate size: Use a projector to measure gate cross-sectional area (e.g., for ABS, a gate diameter of 6-8mm is recommended). If it is too small, hole needs to be enlarged.
- Analyze cooling circuit: Use an infrared thermal imager to scan mold surface and find areas with slow/fast cooling (temperature difference > 5℃ requires adjustment). For a car trim panel with shrinkage, thermal image showed that rib temperature was 3℃ higher than main body. After adding cooling channels, temperature difference was reduced to 1℃.
- Simulate CAE analysis: Use Moldflow to simulate filling + cooling process, and visually see if packing path is obstructed (e.g., if pressure loss from gate to thick-walled area is > 30%, gate needs to be modified).
Step 4: Verify material and drying (5-minute quick check)
- Measure moisture content: Crystalline materials (such as PA) that are not properly dried are prone to hydrolysis, leading to abnormal shrinkage. A PA gear showed shrinkage, with a moisture content of 0.15% (standard ≤0.1%). Problem was alleviated after drying.
- Material verification: Test molding with dried material of same grade. If shrinkage is reduced, material problem is ruled out.

Based on troubleshooting results, make targeted adjustments:
- If main cause is process:
- Increase holding pressure/time (e.g., from 80 bar to 100 bar for PP, time from 10s to 15s).
- Increase injection speed (especially during filling stage of thick-walled areas) to ensure timely melt replenishment.
- Extend cooling time (for areas with slow cooling) to balance shrinkage in different parts.
- If main cause is mold:
- Modify gate: Move gate to center of thick-walled area (e.g., add a submerged gate at thick-walled end of a car bumper).
- Add cold slug wells/enlarge runners: Increase runner diameter from 5mm to 6mm, or add cold slug wells (8-10mm in diameter) in thick-walled area.
- Optimize cooling: Add cooling channels (8-10mm in diameter) in rib/shrinkage area, or adjust cooling water flow rate (e.g., from 5L/min to 8L/min) to accelerate cooling.

 

Preventing shrinkage and sink marks requires a system that spans the entire product design, mold development, and trial molding verification cycle:
1. Product Design Stage: Predicting Shrinkage Risks
- Avoid abrupt changes in wall thickness: Transition between thick and thin walls should be smooth (e.g., R≥1mm) to reduce localized shrinkage concentration. A home appliance casing originally had a wall thickness that abruptly increased from 2mm to 6mm, resulting in severe shrinkage; after mold modification, it was changed to 2mm→4mm→6mm (R=1.5mm), and shrinkage disappeared.
- Mark areas prone to shrinkage: Circle thick-walled/reinforcement rib locations on drawings to prompt mold manufacturer to strengthen venting/cooling design.
2. Mold Development Phase: Matching Shrinkage Requirements
- Gate Design: For thick-walled areas, prioritize using submarine gates/fan gates (large shrinkage compensation area), avoiding point gates (short shrinkage compensation distance).
- Cooling System: Optimize cooling circuit using mold flow analysis software (e.g., separate cooling channels for ribs) to ensure uniform cooling (temperature difference ≤ 2℃).
3. Mold Trial and Verification Phase: Establishing a "Shrinkage Database"
- Record Key Parameters: Document optimal holding pressure/time, cooling time, and other parameters for each product, for direct use in subsequent production.
- Short Shot + Weighing Method: During mold trials, perform a 50% short shot and mark flow front; then fill mold completely and weigh product. If weight deviation is >2%, it indicates insufficient shrinkage compensation and requires adjustment.

Core of shrinkage and sink marks is an imbalance between "shrinkage and compensation." Troubleshooting should focus on material shrinkage rate, process compensation efficiency, and mold design rationality. Prioritize adjusting process (low cost, quick results), then optimize mold (to solve fundamental problem). The key to long-term error prevention is "design prediction + parameter standardization + experience accumulation," ultimately achieving "no shrinkage during mold trials, and stable production."
Core Mantra: Check thick-walled areas first for shrinkage, insufficient holding pressure is most common; try increasing pressure and extending time, check gates and cooling; design to avoid sudden changes in wall thickness, and archive data to prevent recurrence.