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Home Technical Articles Injection Molding Process Optimization and Mechanical Performance of PVC Fittings ——Quality Control Based on Orthogonal Experiments

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Injection Molding Process Optimization and Mechanical Performance of PVC Fittings ——Quality Control Based on Orthogonal Experiments

/ Post by by 孙飞虎

1. Research Background


PVC fittings, critical connectors in plastic pipe systems, suffer from 8.7% defect rate due to poor injection molding:


  • Weld lines reduce pressure strength by 25% (GB/T 18742.3-2017)
  • Uneven cooling causes size deviation >0.5mm
  • Material degradation shortens aging resistance by 30%

2. Core Issue: Process Parameter Mechanism

2.1 Key Process Parameters


Moldflow simulation and Taguchi method identify four 主控参数:


Parameter Range Performance Impact
Melt temperature (℃) 180-200 Degradation and weld quality
Injection pressure (MPa) 80-120 Filling and residual stress
Packing time (s) 10-30 Shrinkage and dimensional stability
Mold temperature (℃) 30-50 Crystallinity and cooling uniformity

2.2 Failure Modes


Failure Type Process Cause Fitting Location
Weld lines Incomplete melt fusion Tee branches, elbow inner sides
Warpage Uneven cooling rate Large fittings (dn≥110mm)
Stress cracking Residual stress > yield strength Socket joints

3. Process Optimization Experiments

3.1 Orthogonal Test Design


L9(3⁴) test for dn110mm 90° elbows, testing tensile strength, Vicat temp, and weld line strength:


Test No. Melt Temp (℃) Injection Pressure (MPa) Packing Time (s) Mold Temp (℃) Tensile Strength (MPa) Vicat Temp (℃) Weld Line Strength (MPa)
1 180 80 10 30 42.5 78.2 38.7
2 180 100 20 40 45.8 81.5 42.3
3 180 120 30 50 44.1 83.1 40.5
4 190 80 20 50 47.2 85.3 45.1
5 190 100 30 30 49.6 87.5 48.2 (Optimal)
6 190 120 10 40 46.8 84.7 43.9
7 200 80 30 40 43.6 82.4 39.2
8 200 100 10 50 44.9 80.9 41.1
9 200 120 20 30 45.3 81.2 42.7

3.2 Optimal Parameters


Range analysis reveals:


  • Melt temp 190℃ (degradation <0.5%)
  • Injection pressure 100MPa (filling time <3s)
  • Packing time 30s (shrinkage ≤1.0%)
  • Mold temp 30℃ (cooling time -20%)

4. Mold Structure Optimization

4.1 Gate Design


Fan gate (Figure 1) vs. pin gate:


  • Width increased from 5mm to 15mm
  • Depth reduced from 2mm to 1.2mm
  • Vent slots (0.05mm depth) added

4.2 Cooling System


Conformal cooling (3D-printed mold) vs. straight channels:


  • Temperature difference reduced from 12℃ to 3℃
  • Warpage decreased by 45%
  • Cycle time shortened by 18%

5. Quality Control Solutions

5.1 Internal Production Standard


Based on GB/T 18742.3-2017:


Parameter GB Requirement Optimized Standard Test Method
Weld line strength (MPa) ≥80% of base material ≥90% Tensile test (GB/T 8804)
Dimensional deviation (mm) ±0.8 ±0.5 CMM measurement
Residual stress (MPa) - ≤25 X-ray diffraction

5.2 Process Monitoring


"Three-Stage" control:


  1. Preheating: Mold temp stable ±2℃, moisture <0.01%
  2. Injection: Pressure 波动 <5%, melt temp deviation <3℃
  3. Cooling: Combined chiller and mold temperature controller

6. Production Validation


dn110mm elbow production comparison:


Index Traditional Process Optimized Process Improvement
Defect rate (%) 8.7 2.1 -76%
Pressure strength (MPa) 4.2 5.1 +21%
Production efficiency (units/h) 45 58 +29%

7. Advanced Technology Extensions

7.1 Smart Injection System


AI process optimization algorithm auto-adjusts parameters, improving adaptability by 35%.

7.2 Low-Carbon Injection


Chemical foaming agent (AC) reduces material usage by 15% and carbon emissions by 22%.


Conclusion
This paper establishes a quality control system through orthogonal experiments + mold optimization + process control. As a professional supplier, we provide:
✅ Custom injection processes for dn20-dn200mm fittings
✅ Fitting mechanical testing services
✅ Injection mold design and modification


Keywords: PVC fittings, injection molding, orthogonal experiment, mechanical performance, GB/T 18742

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