For previous reading, please refer to Daily Share: A Comprehensive Approach to Tracing and Systematically Eliminating Flash in Injection M.
In injection molding production, dimensional instability is an "invisible cost killer"—it doesn't directly lead to scrap, but it causes inconsistent product assembly and a surge in customer complaints. Its essence lies in superposition of factors such as shrinkage, orientation, mold deformation during melt cooling and solidification process, combined with process fluctuations, leading to critical dimensions (such as hole diameter, wall thickness, and assembly surface) exceeding tolerance limits. This article, using case studies from automotive and precision electronics industries, teaches you how to thoroughly solve this "chronic problem" through four major modules: shrinkage mechanism, five-dimensional troubleshooting, layered solutions, and long-term closed-loop control.

 

Core contradiction of dimensional instability is "uncontrollability" of volume shrinkage during melt cooling, superimposed with fluctuations in process parameters and mold precision degradation, leading to final dimensions deviating from design values. This requires dissecting underlying logic from five dimensions: material characteristics, process driving forces, mold stability, equipment accuracy, and environmental interference.
1. Material Characteristics: "Natural Fluctuation" of Shrinkage Rate
Shrinkage behavior of material directly determines dimensional stability:
- Crystalline materials have large fluctuations (e.g., PP, PE, PA): Shrinkage rate is significantly affected by cooling rate (e.g., for PP, when mold temperature changes from 30℃ to 50℃, shrinkage rate changes from 1.8% to 2.2%). In mass production of a car bumper (PP), workshop temperature is lower in winter, and mold temperature is 15℃ lower than in summer, resulting in a 0.4% fluctuation in shrinkage rate, causing assembly hole spacing to be out of tolerance.
- Orientation leads to anisotropy (e.g., PC, PMMA): Shrinkage is smaller in melt flow direction (longitudinal shrinkage rate 0.5%-0.8%), and larger in perpendicular direction (transverse shrinkage rate 1.0%-1.5%). Hole diameter of a mobile phone frame (PC) is larger in flow direction (+0.05mm) and smaller in perpendicular direction (-0.03mm) due to orientation, resulting in asymmetrical dimensions.
- Material Batch Differences: Different batches of raw materials have different molecular weight distributions (e.g., molecular weight fluctuations of ±5% between ABS batches), resulting in shrinkage differences of up to 0.2%-0.3%. After changing material for a certain home appliance casing, wall thickness was consistently 0.08mm thicker, root cause being higher shrinkage rate of new batch of material.
2. Process Parameters: "Loss of Control" in Shrinkage and Cooling Rhythm
Process settings determine "completeness" of shrinkage process:
- Insufficient/Fluctuating Holding Pressure: Holding pressure fluctuations > 2% (e.g., from 80MPa to 78MPa), insufficient shrinkage leads to oversized dimensions (e.g., inner diameter of a certain bottle cap is 0.03mm larger on average due to insufficient holding pressure).
- Insufficient Cooling Time: Product is demolded before it is fully solidified (e.g., ABS demolding temperature > 60℃, thermal deformation leads to insufficient shrinkage). A certain tool handle continued to shrink after demolding, and wall thickness was ultimately 0.1mm smaller.
- Injection Speed Variation: Speed fluctuations > 5% (e.g., from 60% to 57%), differences in melt temperature lead to different orientation stresses, resulting in dimensional fluctuations after cooling (e.g., length of a certain connector pin fluctuates by ±0.02mm).
3. Mold Design: "Acquired Damage" Leading to Precision Degradation
Mold condition directly determines "stability" of dimensional output:
- Core/Cavity Wear: Size of slider/core increases after long-term friction (e.g., a Φ5mm core wears to Φ5.02mm, resulting in a 0.02mm larger hole diameter). After producing 100,000 molds, average diameter of tooth tip of a certain precision gear mold was 0.05mm larger due to cavity wear.
- Unbalanced Cooling System: Local areas cool quickly (e.g., sufficient water cooling in rib area, resulting in faster cooling and less shrinkage), while other areas cool slowly (e.g., no water cooling in thick-walled area, resulting in greater shrinkage), leading to dimensional differences in different areas of same product (e.g., in a certain automotive trim panel, thickness of reinforcing ribs is 0.1mm thinner than main body due to uneven cooling).
- Mold fit looseness: Slider gap increases from 0.01mm to 0.02mm, causing product to be "floatingly squeezed" during mold closing, resulting in random dimensional fluctuations (e.g., flatness of assembly surface of an electronic casing changes from 0.05mm to 0.1mm).
4. Equipment Capability: "Fundamental Guarantee" of Stability
Precision of injection molding machine is "cornerstone" of dimensional stability:
- Injection volume fluctuation: Screw wear leads to a difference in injection volume per mold > 0.5% (e.g., gap of a Φ40mm screw increases from 0.1mm to 0.15mm, resulting in an injection volume fluctuation of ±0.3g), causing batch dimensional deviations. For a precision instrument casing, injection volume fluctuations resulted in a weight difference of 0.2g, corresponding to a wall thickness difference of 0.01mm.
- Clamping force fluctuation: Unstable hydraulic system pressure (fluctuation > 3%) causes mold to slightly "expand," resulting in flashing or oversized dimensions (e.g., clamping force fluctuations in a home appliance casing resulted in assembly hole spacing fluctuations of ±0.04mm).
- Temperature control accuracy: Barrel temperature fluctuations of ±5℃ (e.g., PC material temperature changes from 280℃ to 285℃, viscosity decreases by 10%, flow is faster, and dimensions are smaller). For an optical lens, temperature fluctuations resulted in a curvature radius difference of ±0.01mm, leading to substandard optical performance.
5. Environmental Factors: "Hidden Interference" from External Conditions
Changes in workshop environment can amplify dimensional fluctuations:
- Temperature fluctuations: Mold temperature is 10-15℃ lower in winter than in summer, increasing shrinkage rate of crystalline materials by 0.3%-0.5% (e.g., PP dimensions are larger in winter and smaller in summer).
- Humidity changes: Moisture content of PA material fluctuates by ±0.05% (e.g., from 0.1% to 0.15%), reducing melt viscosity by 20%, resulting in faster flow and smaller dimensions.

 

Dimensional instability is easily misidentified as "operator error," requiring a five-step troubleshooting process combining dimensional fluctuation characteristics, process stability, and mold condition:
Step 1: Observe Dimensional Fluctuation Patterns (5-minute quick assessment)
- Batch-wide deviations (too large/too small): Often due to fluctuations in process parameters (unstable holding pressure/cooling) or material batch differences (shrinkage rate variations).
- Random fluctuations (single mold dimensions fluctuating): Often due to mold wear (increased clearance between core/slider) or unstable injection volume (screw wear).
- Fluctuations in different areas of same product: Such as differences in wall thickness direction/flow direction dimensions, often due to orientation or uneven cooling (e.g., asymmetrical longitudinal/transverse dimensions of PC pins).
Step 2: Verify Material Consistency (5-minute test)
- Check batch and shrinkage rate: Compare material batch reports (e.g., PP shrinkage rate standard 1.5%-2.0%, new batch measured at 2.3%, exceeding upper limit).
- Measure moisture content/impurities: For hygroscopic materials (PA, PET), if moisture content fluctuation > 0.05%, use a rapid moisture meter; inspect area with dimensional fluctuations, if impurities (such as material fragments) are present, it may cause localized abnormal shrinkage.
- Material change verification: Test with same brand and batch of material; if dimensional fluctuation decreases, rule out material problems (e.g., after changing material for a PA gear, dimensional fluctuation decreased from ±0.05mm to ±0.02mm).
Step 3: Lock Down Process Stability (10-minute trial and error)
- Fix holding pressure parameters: Use a pressure sensor to lock holding pressure (fluctuation < 2%), e.g., for a mobile phone case, after fixing holding pressure from 70MPa to 70±0.5MPa, dimensional fluctuation decreased from ±0.04mm to ±0.01mm.
- Standardize cooling time: Set cooling time according to "wall thickness × 1s + 2s" (e.g., wall thickness 3mm, cooling time 5s), ensuring demolding temperature is < heat distortion temperature (ABS < 60℃, PC < 120℃). After increasing cooling time of a tool handle from 8s to 10s, problem of insufficient demolding shrinkage was solved, resulting in stable dimensions.
- Stable injection speed: Screw speed was locked using a frequency converter (fluctuation <5%), avoiding variations in melt temperature/orientation caused by speed changes. After reducing injection speed fluctuation of a connector from ±8% to ±3%, pin length fluctuation decreased by 60%.
Fourth step: Check mold accuracy (30 minutes - 1 hour)
- Measure core/cavity dimensions: Use a coordinate measuring machine to check critical dimensions (e.g., a Φ5mm core measured 5.02mm, exceeding tolerance due to wear, and was reground to 5.00mm). A precision gear mold cavity was worn, with a dimensional deviation of 0.05mm, and was restored after electroplating repair.
- Check cooling uniformity: Scan mold with an infrared thermal imager to check cooling temperature difference (e.g., rib temperature 40℃, main body temperature 50℃, temperature difference 10℃; additional cooling channels were added to ribs to control temperature difference within ±2℃).
- Check fitting clearance: Use a feeler gauge to measure clearance between slider and guide rail (standard 0.01-0.015mm). For example, slider clearance of a certain housing was 0.025mm, and after regrinding, it was restored to 0.012mm, eliminating dimensional fluctuations.
Fifth step: Confirm equipment stability (10-minute test)
- Weighing method to measure injection volume: Inject 10 molds continuously and calculate weight fluctuation (e.g., a certain PP part had a weight fluctuation of ±0.3g per mold, standard is ±0.1g, indicating that screw needs to be replaced).
- Calibrate clamping force/temperature: Use a hydraulic gauge to detect clamping force fluctuations (<3%), and an infrared thermometer to calibrate barrel/mold temperature (fluctuation <±2℃). A certain equipment had a barrel temperature fluctuation of ±5℃; after replacing temperature control module, temperature stabilized to ±1℃, and dimensional fluctuations decreased.

If main cause is process parameters:
- Optimize holding pressure curve: Use "segmented decreasing holding pressure" (e.g., 80% pressure for 5s → 60% for 8s → 40% for 10s) to reduce orientation stress and avoid dimensional fluctuations.
- Dynamically compensate for cooling time: Adjust cooling time according to ambient temperature (increase by 2-3s in winter) to ensure consistent demolding temperature.
If main cause is mold problems:
- Repair/replace worn parts: When core/slider wear > 0.005mm, repair by grinding or electroplating (e.g., if a Φ5mm core is worn to 5.02mm, electroplate to remove 0.02mm).
- Balance cooling system: Add cooling channels (diameter Φ8-10mm) in areas with slow cooling (e.g., thick-walled areas), or adjust cooling water flow rate (e.g., from 5L/min → 8L/min) to ensure a temperature difference of < 2℃.
If main cause is equipment capability:
- Replace worn parts: Replace when screw/barrel gap > 0.1mm (e.g., a Φ40mm screw with a gap of 0.15mm; after replacing with a new screw, injection volume is stable).
- Upgrade control system: Install a closed-loop injection volume controller (real-time adjustment of screw speed to maintain injection volume fluctuation < 1%).

1. Design and Mold Trial Stage: Embedding "Stability Genes"
- Material selection: Prioritize materials with low shrinkage rates (e.g., POM copolymer with shrinkage rate fluctuation < 0.2%), or add nucleating agents (e.g., adding a nucleating agent to PP reduces shrinkage rate fluctuation from ±0.3% to ±0.1%).
- Mold cooling CAE analysis: Use Moldflow to simulate cooling uniformity, ensuring a temperature difference ≤ 2℃ to avoid local shrinkage differences.
- Dimensional stability mold trial: Continuously produce 100 molds, record key dimensional fluctuations (target within ±0.02mm), and optimize process/mold if tolerance is exceeded.
2. Production Phase: Dynamic Monitoring and Regular Calibration
- Establish a "Dimension-Process" database: For each product, record "process window during stable production" (e.g., holding pressure 80±0.5MPa, cooling time 10±0.5s), and quickly adjust when abnormalities occur.
- Regular equipment maintenance: Calibrate injection volume (fluctuation <1%), clamping force (fluctuation <2%), and temperature (fluctuation <±1℃) monthly. Inspect mold wear (slider/core clearance) every 50,000 cycles.
3. Anomaly Handling: Rapid Localization and Closed-Loop Correction
- Fluctuation root cause tree: Develop a "Dimensional Instability Troubleshooting Flowchart," verifying each item according to "fluctuation pattern → material → process → mold → equipment," locating root cause within 2 hours.
- Preventive maintenance plan: For easily worn parts (such as sliders and cores), set a replacement cycle (e.g., inspect wear every 200,000 cycles, mandatory replacement every 500,000 cycles).

 

Essence of dimensional instability is a dynamic imbalance of "shrinkage-compensation-mold state-process fluctuation." Troubleshooting requires a five-dimensional approach: materials (shrinkage rate/batch), process (holding pressure/cooling), mold (wear/cooling), equipment (injection volume/temperature), and environment (temperature/humidity). Prioritize stabilizing the process (low cost, quick results), then repair mold (addressing accuracy degradation), and finally ensure equipment reliability (addressing stability shortcomings). The key to long-term closed-loop control is "design prediction + dynamic monitoring + regular maintenance," ultimately achieving "zero dimensional fluctuation, zero assembly problems."
Core Mantra: For dimensional fluctuations, first observe pattern; for batch deviations, check process; for random fluctuations, repair mold; equipment calibration cannot be neglected; design and material selection control shrinkage; data archiving prevents recurrence.