How to produce Qualified Plastic Threaded Products?

Backed by Jingsheng's 16-Year plascit Injection Molding Expertise

The core of producing qualified plastic threaded products lies in addressing four key issues: dimensional accuracy, thread integrity, defect-free (no flash, shrinkage, or cracking), and assembly compatibility. It requires closed-loop control throughout the entire process from "design → material selection → mold design → injection molding process → inspection". Below is the detailed implementation plan covering key links and common pitfalls:

I. Preliminary Design: Avoiding Thread Defects from the Source

Thread design directly determines the feasibility of subsequent production and must balance "plastic characteristics" and "assembly requirements":

1. Thread Structure Design (Key Pitfall Avoidance)

• Thread Profile Selection: Prioritize standard thread profiles (e.g., Metric, UNC/UNF imperial, BSP pipe threads) and avoid custom profiles (difficult to process and control precision). For plastic threads, coarse threadsare recommended (fine threads are prone to breakage and injection filling difficulties) .
• Dimensional Reservation: Shrinkage Compensation: Plastics shrink during cooling, so thread dimensions must be enlarged according to the material's shrinkage rate during design. For example, PP has a shrinkage rate of 1.5%-2.0%, so the outer diameter of an M10 thread should be designed as 10.15-10.20mm to meet standards after cooling .
• Stress Concentration Avoidance:
  • Add 3-0.5mm chamfersat the start and end of threads (prevent material shortage during injection and thread chipping during assembly);
  • Add R0.1-R0.2mm fillets at thread roots (replace right angles to reduce cracking caused by shrinkage stress).
• Thread Length Control:
  • Effective length of external threads ≤ 1.5 times the thread diameter (e.g., M8 external thread ≤12mm). Excessive length causes demolding difficulties and thread deformation;
  • Effective length of internal threads ≤ 2 times the thread diameter. A relief groove (width ≥0.5mm) is required for longer lengths to facilitate core demolding.
• Demolding Direction Optimization: Threads must be designed along the mold opening direction (avoid precision deviations from complex side core-pulling structures). For threads not aligned with the opening direction, prioritize "split cores" over angle pins (side core-pulling easily causes thread misalignment).

2. Overall Product Design Coordination

• Reserve sufficient wall thicknessaround threads (at least 1.5 times the thread height. e.g., M6 thread height 0.81mm, surrounding wall thickness ≥1.2mm) to prevent thread deformation during cooling shrinkage;
• Avoid thick-walled areas near threads (thick walls cause shrinkage marks that indirectly deform threads). Hollow out thick sections or add ribs to disperse stress.

II. Material Selection: Matching Thread Strength and Injection Characteristics

The "rigidity, shrinkage rate, and fluidity" of plastic materials directly affect thread quality. Selection should be based on product application:

1. Priority Material Selection List (Sorted by Requirements)

2. Material Preprocessing Requirements

• Hygroscopic materials (PA, PC, ABS): Must be dried before injection (PA: 80-100℃ for 4-6h; PC: 120℃ for 2-4h). Otherwise, internal thread bubbles and reduced strength will occur;
• Glass fiber-reinforced materials: Use "glass fiber-specific screws" to prevent screw wear, material dispersion unevenness, and local thread strength deficiency.

III. Mold Design: Key to Thread Precision

The mold is the "master template" for thread forming, focusing on "thread core/cavity precision, demolding method, and venting system":

1. Thread Core/Cavity Machining (Precision Benchmark)

• Machining Precision: Mold thread precision must be 1-2 grades higher than the product (e.g., product grade 6H requires mold grade 5H). Use CNC thread milling + precision grinding (avoid taper errors from lathe machining);
• Surface Treatment:
  • Internal thread cavities: Polish to Ra ≤0.2μm to reduce plastic adhesion and prevent thread damage during demolding;
  • External thread cores: Nitride (hardness ≥HRC 60) for glass fiber materials to prevent dimensional drift from core wear.

2. Demolding Method Selection (Prevent Thread Deformation)

• Manual Demolding: Only for small-batch, large-diameter threads (e.g., M20+). Design 1°-2° draft angles to avoid thread chipping from forced prying;
• Automatic Demolding:
  • Standard threads: Use "rotary demolding mechanism" (hydraulic motor-driven core rotation). Control speed at 5-10r/min (excessive speed breaks threads);
  • Fine/precision threads: Use "forced demolding" (only for elastic materials like PP/PE). Require draft angle ≥3° and thread height ≤0.5mm (otherwise thread deformation occurs).

3. Gate and Vent Design

• Gate Position: Avoid direct alignment with threads (melt impact causes local weld lines and reduced strength). Prioritize side gates or pin gates with size 0.8-1.2 times the product wall thickness;
• Vent System: Must add vent grooves (width 0.1-0.2mm, depth ≤0.03mm) at thread ends. Unvented gas causes thread material shortage and burning .

4. Mold Temperature Control

• Stable mold temperature (±2℃):
  • Crystalline materials (PP, PA, POM): 40-80℃ (improve crystallinity and thread strength) ;
  • Amorphous materials (ABS, PC): 20-50℃ (avoid uneven shrinkage and dimensional deviation);
• Independent temperature control for thread areas (separate water channels) to prevent uneven shrinkage from local temperature differences.

IV. Injection Molding Process: In-process Control to Avoid Defects

Injection parameters must match materials and molds, focusing on "melt temperature, injection pressure, holding pressure, and cooling time":

1. Key Process Parameter Settings (General Reference)

2. Common Process Issues and Solutions

• Thread Material Shortage (Incomplete Profile): Increase melt temperature by 10-15℃, raise injection pressure by 10-20MPa, enlarge gates/vents;
• Thread Flash (Burrs on Crests): Reduce injection/clamping pressure, minimize mold gap (≤0.02mm), clean mold parting surfaces;
• Thread Shrinkage Marks (Root Depression): Extend holding time by 5-10s, increase holding pressure, lower melt temperature (avoid excessive shrinkage);
• Thread Deformation (Inclined Profile): Extend cooling time, optimize demolding mechanism (avoid forced demolding), reduce mold opening speed.

V. Inspection and Quality Control: Ensuring Qualified Delivery

Establish a three-level inspection system: "first article inspection → in-process inspection → outgoing sampling inspection", focusing on key thread indicators:

1. Core Inspection Items and Tools

2. In-process Quality Control

• First Article Inspection: Use CMM to test thread pitch diameter, major diameter, and lead error before batch production;
• Inspection Frequency: Sample 5-10 pieces hourly, verify dimensions with thread gauges, adjust parameters (temperature/holding pressure) for deviations;
• Batch Traceability: Record material batch number, mold ID, and process parameters for unqualified product tracing and root cause analysis.

VI. Common Unqualified Causes and Quick Solutions

Conclusion

The core logic for qualified plastic threaded products is: design adapting to plastic characteristics (avoid stress, reserve shrinkage), mold ensuring precision (fine machining, good demolding), process achieving stable forming (control temperature/pressure, sufficient cooling), and inspection implementing closed-loop verification (check dimensions, test assembly).

For mass production, it is recommended to conduct "small-batch trial molding" (50-100 pieces first) to verify thread dimensions and assembly compatibility before optimizing molds and process parameters, which significantly reduces the risk of batch unqualified products.