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

Warpage in injection molded products refers to unintended bending, distortion, or surface undulations in product, either overall or locally, caused by uneven internal stress distribution or shrinkage. Essentially, this is a synergistic failure of material shrinkage, internal stress, and constraints:
- Differential shrinkage: Inconsistent melt cooling rates in different regions (e.g., slow cooling in thick-walled areas and fast cooling in thin-walled areas) lead to differential volumetric shrinkage (ΔV > 0.5%);
- Internal stress accumulation: Hindered shrinkage (e.g., uneven mold cooling or high demolding resistance) induces residual stress (σ > 30% of material's yield strength), driving product deformation;
- Constraint failure: Poor product geometric symmetry (e.g., aspect ratio > 5:1) or lack of support (e.g., thin-walled flat panels) fails to offset deformation caused by internal stress.
Macroscopic manifestations include surface undulations (height difference > 0.2mm), flatness deviations (> 0.1mm/m²), or localized bowing (angular deviation > 1°), seriously impacting assembly accuracy (e.g., uneven gap between phone case and screen) and appearance quality.

Warpage is result of multi-dimensional imbalances in materials, molds, processes, and equipment. It can be categorized into four main, overlapping causes:
- Material properties (the most fundamental contributing factor):
- Crystalline materials (PP, PA6, POM) have large shrinkage differences (Δ shrinkage > 1.5%), resulting in asynchronous volume changes in thick and thin-walled areas during cooling;
- Amorphous materials (PC, PMMA) have uneven molecular chain orientation (crystallinity differences > 10%), resulting in inconsistent shrinkage directions after cooling;
- Excessive proportions of recycled materials (> 15%) lead to molecular chain breakage (shrinkage fluctuations of ± 0.3%), increasing risk of deformation.
- Mold design defects (key drivers):
- Uneven cooling system (e.g., independent cooling circuits for main and branch runners, with a temperature difference >15℃), resulting in large variations in local shrinkage rates;
- Improper gate location (single gate located at the center of long side), long melt flow path (L/D >100), and asynchronous shrinkage at both ends;
- Improper demolding design (uneven ejector pin distribution or insufficient draft angle), resulting in uneven local stress during demolding (stress deviation >20%).
- Out-of-control process parameters (direct trigger):
- Excessive holding pressure (>70% of injection pressure) causes excessive compression in thick-walled areas (density increases by 5%-8%), resulting in greater shrinkage than thin-walled areas after cooling;
- Insufficient cooling time (<product wall thickness * 1.5s/mm) causes incomplete surface solidification (incomplete shrinkage), leading to continued deformation after demolding;
- Excessive injection speed (>100mm/s) causes shear heating at melt front (temperature increase >20℃), resulting in abnormal local shrinkage (Δ shrinkage >1%).
- Equipment deterioration (long-term hidden danger):
- Barrel temperature fluctuations (above ±5℃) cause material shrinkage fluctuations (±0.2%), resulting in inconsistent deformation between product batches;
- Screw wear (clearance >0.25mm) causes uneven plasticization and presence of unmelted particles in melt (impeding shrinkage synchronization);
- Excessive platen parallelism (>0.03mm/m) causes uneven mold force and additional stress during product cooling.

 

Based on cost of repairing warpage, its impact on product assembly, and probability of recurrence, we recommend systematically addressing it according to following priorities:

Core Logic: Mold is "regulator" of shrinkage and stress. By optimizing cooling system, gate layout, and mold release design, we can directly balance product shrinkage consistency.
2.1.1 Precise Adjustment of Key Mold Structures
- Cooling System Zoning Optimization:
- Adopting "conformal cooling" (cooling channels conform to product contours) to reduce local temperature differences (controlled within ±5℃);
- Adding independent cooling circuits to thick-walled areas (such as camera area of a mobile phone housing) accelerates cooling of thick walls (increasing cooling rate from 8℃/s to 12℃/s) and reduces shrinkage difference with thin-walled areas;
- Increasing cooling channel diameter from φ8mm to φ10mm (with a φ50mm screw) to reduce channel resistance (reducing pressure drop by 20%) and ensure cooling uniformity.
- Optimize gate location and number:
- Multi-gate layout (spacing 2-3 times the wall thickness, e.g., 4-6mm for a 2mm wall thickness) balances melt flow path across product (L/D < 80) and reduces differential shrinkage at both ends;
- Place gate at the center of product or on axis of symmetry (e.g., center-feeding a rectangular product) to avoid concentrated shrinkage on one side;
- Reduce length of latent gate to 1-1.5mm (previously over 2mm) to reduce differential cooling at melt front.
- Improvements to demolding and ejection system:
- Increased number of ejector pins (≥ 0.8 pins per square centimeter), reducing pressure per pin (from 100 MPa to 60 MPa) to avoid uneven localized force during demolding;
- Hard chrome plating (Ra ≤ 0.4 μm) on ejector pins reduces coefficient of friction (from 0.3 to 0.1), preventing mold scratches and additional deformation;
- Increased outer draft angle from 0.5° to 1° (from 0.3° to 0.8° on inner side), reducing demolding resistance (friction reduced by 40%).
Operational Details:
- After mold modification, Moldflow simulation verification is required (shrinkage difference < 0.5%, residual stress < 30% of material's yield strength);
- For precision molds (such as optical components), cooling channel position tolerance is controlled within ±0.1 mm to prevent uneven cooling caused by machining errors.
Case study: Flatness of a mobile phone casing (PC) exceeded tolerance (0.3mm/m²). Original gate was located at the center of long side, and cooling system temperature difference was 18℃. Adjusting to a centrally symmetrical dual gate and a conformal cooling system design (temperature difference ±5℃) reduced flatness to 0.05mm/m², achieving a 100% assembly clearance pass rate.

Core Principle: Rapidly reduce internal stress and shrinkage differences by controlling shrinkage rate, balancing holding pressure and cooling time.
2.2.1 Coordinated Control of Temperature and Pressure Parameters
- Melt and Cooling Temperature Optimization:
- Raise melt temperature to upper limit of material's flow temperature (e.g., PP from 230℃ to 250℃, PC from 300℃ to 310℃), reduce viscosity (PP viscosity from 1800 Pa·s to 1200 Pa·s), and promote uniform melt flow (shrinkage differences reduced by 30%);
- Control cooling water temperature within recommended range for material (e.g., PP 40-60℃, PC 80-100℃), avoiding localized overcooling (temperature differences <10℃);
- For crystalline materials (PA6), use gradient cooling (first rapid cooling to set shape, then slow cooling to relax stress), reducing cooling rate from 15℃/s to 10℃/s.
- Adjusting holding and cooling times:
- Holding pressure = injection pressure * 50%-60% (e.g., injection pressure 100MPa, holding pressure 50-60MPa) to reduce excessive compression in thick-walled areas (shrinkage differential reduced by 40%);
- Holding time = part wall thickness * 2.5s/mm (e.g., 2mm wall thickness, holding pressure 5s) to avoid stress accumulation caused by excessive holding time;
- Extending cooling time to part wall thickness * 1.5s/mm (e.g., 2mm wall thickness, cooling time 3s) to ensure adequate solidification (shrinkage completion rate > 95%).
- Injection Speed and Back Pressure Control:
- Control injection speed to 70-90 mm/s (20%-30% lower than normal process) to reduce shear heating (temperature increase <10℃) and avoid localized shrinkage abnormalities.
- Increase back pressure to 3-5 MPa (for amorphous materials) or 4-6 MPa (for crystallized materials) to increase internal friction within melt and promote uniform molecular chain alignment (improving shrinkage consistency by 20%).
Operational Details:
- For products with large aspect ratios (such as flat panels), extend cooling time to 2 seconds per mm (wall thickness x 2 seconds per mm) to ensure synchronized solidification.
- Observe direction of product deformation: If product bends toward gate, this indicates insufficient shrinkage near gate, requiring a reduction in holding pressure or an increase in number of gates.
Case Study: A car instrument panel (PP) warped (bend angle 1.5°). Original holding pressure was 70 MPa (injection pressure 100 MPa), and cooling time was 5 seconds (wall thickness 3 mm x 1.5 seconds per mm = 4.5 seconds). Holding pressure was adjusted to 50 MPa, cooling time to 6 seconds, bend angle reduced to 0.3°, and assembly fit met standard.

Core Logic: Material shrinkage and equipment stability are long-term risks of warpage, requiring establishment of a standardized control process.
2.3.1 Material Shrinkage and Uniformity Control
- Material Selection and Modification:
- Add 0.5%-1% nucleating agent (such as talc) to crystalline materials (PP) to refine grain size (reducing shrinkage by 0.5%) and reduce shrinkage variation;
- Add 1%-2% toughening agent (such as MBS) to amorphous materials (PC) to reduce molecular chain orientation (improving shrinkage consistency by 15%);
- Maintain a recycled material ratio of ≤10% (to prevent shrinkage fluctuations of ±0.3% caused by repeated heating).
- Drying and Pretreatment:
- Hygroscopic materials (PA6, PC) must be thoroughly dried (moisture content <0.1%) to prevent hydrolysis and abnormal shrinkage (PA6 with a moisture content >0.1% will increase shrinkage by 0.2%).
- Material storage temperature must be maintained at 25±5℃ and humidity <50% RH to prevent moisture absorption from affecting shrinkage performance.
2.3.2 Equipment Preventive Maintenance
- Screw and Barrel Maintenance:
- Regularly inspect screw wear (gap ≤ 0.15mm for a 50mm diameter screw). Replace screws if out of tolerance (excessive gaps can lead to unmelted particles, hindering shrinkage synchronization).
- Barrel temperature control accuracy must be calibrated (within ±2℃) to prevent temperature fluctuations that may cause shrinkage variations (±0.2%).
- Check ring sealability (red lead powder contact area >85%) to prevent pressure fluctuations caused by melt backflow.
- Pressure and Temperature Stability:
- Hydraulic system pressure stability calibration (fluctuation <5%), replacement of aging seals (e.g., O-rings every 2000 hours);
- Heating coil power check (heating efficiency >95%) to prevent localized temperature deficiencies that can lead to uneven plasticization.
Case: A precision flat plate (ABS) was experiencing repeated warping. Investigation revealed a 15% recycled material percentage (shrinkage fluctuation ±0.4%) and a screw clearance of 0.3mm (out of tolerance). Replacing screw (with a 0.16mm clearance) reduced recycled material percentage to 8%, shrinkage variation to ±0.2%, and warpage from 12% to 2%.

Core Logic: Ambient temperature and humidity, as well as post-processing, can further suppress warping, especially for large-sized or highly crystalline materials.
2.4.1 Environmental and Auxiliary Process Control
- Workshop Environmental Control:
- Production workshop temperature is controlled at 25±2℃ (to prevent excessively cold environments that accelerate material shrinkage), and humidity is maintained at 40%-50% RH (to prevent moisture absorption).
- For large-sized products (such as automotive door panels), a constant temperature and humidity zone (25±1℃, 50±5% RH) is set up to ensure stable material properties.
- Post-Processing Correction:
- Annealing: 80℃ for 2 hours to promote molecular chain relaxation (reducing residual stress by 60% and warpage by 50%).
- Fixturing: After cooling, product is clamped with a fixture (for evenly distributed pressure) to prevent deformation during natural cooling (reducing warpage by 70%).
- Vibration Stress Relief: Low-frequency vibration (50Hz) for 10 minutes is applied to release internal stress (reducing warpage by 40%).
Case Study: A large household appliance casing (PP) was warped (flatness 0.5mm/m²) and workshop temperature fluctuated by ±3℃. A constant temperature zone (25 ± 1℃) was added, and product was annealed (80℃ for 2 hours). Flatness was reduced to 0.1 mm/m², meeting assembly requirements.

 

- Symptom: Product flatness was 0.3 mm/m² (standard ≤ 0.1 mm/m²), and gap with screen during assembly was uneven (> 0.2 mm).
- Troubleshooting Process:
1. Mold Inspection: Single gate located at the center of long side, cooling system temperature difference of 18℃, and uneven ejector pin distribution (20% pressure deviation on one side);
2. Process Troubleshooting: Holding pressure of 70 MPa (too high), cooling time of 5 seconds (insufficient, wall thickness 2 mm * 1.5 seconds/mm = 3 seconds);
3. Material Verification: PC shrinkage difference of 1.2% (due to molecular weight distribution PDI = 3.0).
- Solution:
- Mold: Switched to symmetrical dual gates, conformal cooling system design (temperature differential ±5℃), and increased number of ejector pins (0.8 per square centimeter);
- Process: Reduced holding pressure to 50 MPa, and extended cooling time to 6 seconds;
- Material: Switched to low-PDI PC (PDI = 2.2), reducing shrinkage differential to 0.6%;
- Result: Flatness reduced to 0.05 mm/m², with a 100% assembly clearance pass rate.

- Symptom: Part warped toward gate (angle 1.5°), interfering with metal bracket during assembly (clearance <0.5 mm).
- Troubleshooting Process:
1. Mold Inspection: Gate was located on short side, resulting in a long melt flow path (L/D = 120°), and shrinkage at both ends was not synchronized.
2. Process Inspection: Holding pressure was 80 MPa (too high), and cooling time was 4 seconds (insufficient, wall thickness 3 mm x 1.5 seconds/mm = 4.5 seconds).
3. Equipment Inspection: Barrel temperature fluctuated by ±8℃, and material shrinkage fluctuated by ±0.3%.
- Solution:
- Mold: Moved gate to center and added a cooling circuit (temperature difference ±5℃).
- Process: Reduced holding pressure to 50 MPa and increased cooling time to 6 seconds.
- Equipment: Calibrate barrel temperature (±2℃ fluctuation) to prevent material shrinkage fluctuations;
- Results: Bend angle reduced to 0.3°, eliminating assembly interference.

Managing product warpage requires a systematic approach: "Mold design is foundation, process control is the key, and materials/equipment are guarantee." Frontline engineers must master following:
1. Balancing shrinkage differences (minimizing regional shrinkage differences through mold cooling/gate design);
2. Internal stress control (optimizing holding/cooling parameters to reduce residual stress);
3. Material uniformity management (controlling recycled material ratio and drying process). By shifting from "passive mold repair" to "proactive prevention," we can ultimately achieve a long-term solution to warpage problems.

 

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