Standardized Production of Internal Thread Plastic Parts: Starting from Five Key Links

Based on Kingsjeng’s nearly 18 years of experience in plastic product manufacturing, stable mass production of internal thread plastic parts relies on controlling five key links: mold design, material handling, process parameters, equipment maintenance, and quality control to form a standardized production process. The actionable methods are as follows:

1. Mold Design: The Core Foundation of Internal Thread Molding (Top Priority)

（1）Selection of Thread Molding Methods (Choose as Needed to Avoid Demolding Defects)

• Detachable Thread Cores/Inserts: Suitable for small-batch, multi-specification products. Manual/mechanical core extraction is required. Ensure tight fitting between the core and plastic part to prevent thread slipping and burrs. The core surface must be polished to reduce demolding resistance.
• Rotary Demolding Mechanism (Recommended for Mass Production): Driven by servo motors or hydraulics to achieve synchronous rotary demolding of threads, avoiding thread deformation and cracking caused by forced demolding. Adaptable to large-scale continuous production. Precisely control rotation speed and demolding rhythm to match the injection molding cycle.

（2）Mold Detail Optimization

• Thread Precision Control: Reserve shrinkage allowance for mold thread dimensions (adjusted according to plastic material shrinkage, e.g., PBT shrinkage 1.7%–2.3%). Control tooth profile and pitch error within ±0.02 mm to avoid material accumulation and shrinkage cavities.
• Vent Design: Install vent grooves (width 0.01–0.02 mm) at the thread end and core root to expel gas during injection molding, preventing bubbles and short shots on the thread surface and ensuring thread integrity.
• Cooling System: Equip the mold with uniform cooling channels near the thread core to ensure even cooling of the plastic part, reduce internal stress, and avoid thread deformation. Adjust cooling time according to part wall thickness (cooling time approximately quadruples when wall thickness doubles).

2. Material Control: Avoid Molding Defects from the Source

（1）Material Selection and Preprocessing

• Prioritize plastic materials with good fluidity and stable shrinkage (e.g., PA, PBT, PC+ABS). For products bearing internal thread loads, add glass fiber reinforcement (e.g., glass fiber-reinforced PBT with ~30% content) to improve thread strength and reduce shrinkage deformation. Avoid easily decomposable and poorly fluid materials to prevent burnt material and short shots during injection molding.
• Material Preprocessing: Thoroughly dry materials before injection (e.g., PBT dried at 140℃ for 26 hours, moisture content controlled below 0.02%) to remove moisture and avoid bubbles and silver streaks affecting thread precision after molding. Seal materials after drying to prevent moisture reabsorption.
• Material Consistency Control: Use materials from the same manufacturer and grade for the same batch production to avoid dimensional fluctuations of threads caused by differences in viscosity and shrinkage between batches. Control the ratio of recycled material to virgin material at 25%–75%, and select impurity-free and non-degraded recycled materials to ensure molding quality.

3. Injection Molding Process Parameters: Precise Adjustment for Mass Production Stability

The core is to determine optimal process parameters via the control variable method, solidify parameters, and implement them strictly to avoid quality fluctuations caused by arbitrary manual adjustments. Key parameters are as follows:

（1）Temperature Control

• Barrel Temperature: Set according to material properties (e.g., PBT controlled at 240–260℃) with multi-stage temperature control (feeding section, plasticizing section, nozzle section) to prevent material decomposition from overheating or poor plasticization from underheating, which affects thread filling. Nozzle temperature should be slightly higher than barrel temperature to prevent melt solidification and blockage.
• Mold Temperature: Controlled at 70–100℃ (adjusted according to material), with a temperature difference of no more than 10℃ between parts. Excessively high temperature prolongs cooling time and causes uneven thread shrinkage; excessively low temperature leads to difficult demolding, brittle parts, and easy thread cracking.

（2）Pressure and Speed Control

• Injection Pressure: Moderate (e.g., 50–100 MPa for PBT). Excessively high pressure causes flash and excessive thread interference; excessively low pressure results in insufficient filling and short shots in threads. The pressure at the nozzle is the highest to ensure smooth melt filling to the thread end.
• Injection Speed: Adopt slow-fast-slowsectional speed to avoid weld marks from high-speed filling (affecting thread strength). Slow down when filling the thread area to ensure full thread profile. For fast-cooling materials such as PBT, appropriately increase the injection rate.
• Holding Parameters: Holding pressure is about 85% of injection pressure, holding time 10–30 seconds. Continuous feeding compensates for plastic shrinkage to avoid shrinkage cavities and depressions in threads. Maintain holding pressure until the gate solidifies to prevent melt backflow.
• Cycle Control: Fix the injection molding cycle (e.g., 60 seconds, adjusted according to part size), with cooling time accounting for 80% to ensure full cooling and solidification of the part and no thread deformation during demolding. Avoid insufficient cooling from an overly short cycle or reduced production efficiency from an overly long cycle.

4. Equipment Maintenance: Ensure Production Continuity and Stability

（1）Injection Molding Machine Maintenance: Regularly inspect the injection and clamping systems to ensure stable screw rotation and pressure without leakage. Use a gradual three-stage screw (length-diameter ratio 15–20), and clean the screw and barrel regularly to avoid accumulated material and burnt material affecting plasticization. Clamping force must match mold size to prevent flash from mold expansion that impairs thread precision.

（2）Mold Maintenance: Regularly clean the mold cavity and thread core to remove accumulated material and oil, check core wear, and replace worn cores promptly (to avoid thread slipping). Lubricate the rotary demolding mechanism regularly to ensure smooth rotation without jamming, synchronized with the injection molding cycle. Apply appropriate surface treatment to the mold according to material properties (e.g., chrome plating for corrosion resistance in flame-retardant PBT molding).

（3）Auxiliary Equipment Maintenance: Regularly calibrate dryers, mold temperature controllers, and cooling systems to ensure accurate drying and temperature control, avoiding incomplete material drying and mold temperature fluctuations caused by equipment failures that affect thread molding quality.

5. Quality Control: Full-Process Supervision and Timely Correction

（1）First-Article Inspection: Produce 3–5 first articles before each batch production, inspect thread dimensions, tooth profile, and surface quality (no bubbles, burrs, deformation), and start mass production only after meeting standards. Record first-article inspection parameters as a reference for subsequent production.

（2）In-Process Patrol Inspection: Sample 5–10 pieces per hour, focusing on thread precision and dimensional consistency. Stop the machine immediately to adjust parameters (e.g., temperature, pressure) or inspect molds and materials if abnormalities (e.g., slipping, short shots, deformation) are found to avoid mass defects. Record patrol inspection data to form a traceability system.

（3）Finished Product Inspection: Conduct full inspection of finished products after mass production, screen out defective products (out-of-tolerance thread dimensions, deformation, cracking, etc.), analyze defect causes, and optimize processes or molds. Store defective products separately to prevent mixing with qualified products.

（4）Continuous Improvement: Establish a continuous improvement mechanism of design—trial operation—problem detection—design optimization. Collect defect data during production, adjust process parameters, mold structure, or material handling methods targeted to gradually reduce the defect rate. Conduct full-staff process training to ensure operators follow standard procedures strictly and avoid quality fluctuations caused by human error.

Key Reminder

The core of mass production is standardization: solidify mold parameters, material specifications, and process parameters to reduce human intervention. Optimize and adjust timely according to production abnormalities to form closed-loop management, which ensures the precision consistency of internal thread plastic parts, improves production efficiency, and reduces the defect rate.