1. Stripped Threads (Thread Failure)
This is the most typical quality problem, manifesting as the screw being unable to be tightened or loosening after being screwed in. Common causes include:
Insufficient Torque Bearing Capacity of the Substrate: For example, the hole wall of the plastic part is too thin, or the sheet metal hole has low strength, causing damage to the internal threads.
Excessive Screw Torque: The torque applied by the power tool exceeds the material's breaking torque, causing stripping.
Inappropriate Screw Thread Profile Design: The thread angle is too large or the lead is mismatched, resulting in poor engagement.
Repeated Assembly and Disassembly: Self-tapping screws in plastic generally fail after more than 8 assembly and disassembly cycles because the plastic has been repeatedly cut and lost strength.
2. Cracking of Mounting Holes in Plastic Parts
During assembly, plastic parts are prone to cracking along weld lines or at stress concentration points at the root. The main reasons include:
Low Weld Line Strength: The connection at the material flow confluence during injection molding is not strong, making it prone to cracking under stress.
Insufficient or Uneven Wall Thickness: The risk increases significantly when the screw stud wall thickness is less than 1.0mm, with localized thinner sections being more prone to breakage.
Material Issues: Use of degradable materials, excessive sprue material, or abnormal moisture content (e.g., dimensional changes in PA6GF30 after water absorption).
Design Defects: Lack of root fillet transition or reinforcing ribs lead to stress concentration.
3. Difficulty in screwing in or breakage
Screws jamming or breaking during screwing is common in thin metal sheets or hard materials:
No pre-drilled holes or holes with too small a diameter: Increases screwing resistance, causing the screw to break.
Excessive speed: High-speed rotation of power tools generates overheating and inertial impact, causing torque overshoot.
Excessively high material hardness or brittleness: For example, carbon steel screws that have not undergone heat treatment are prone to brittle fracture.
4. Surface Quality Issues
These affect appearance and corrosion resistance. Common problems include:
Uneven or peeling plating: Insufficient plating thickness (<6μm), leading to rapid rusting during salt spray testing.
Burnt, cracked, or misaligned threads: Poor thread rolling process damages threads, affecting assembly smoothness.
Head groove wear: Stripped cross-shaped threads prevent tightening.
5. Substandard Materials and Performance
Inferior materials are frequently used:
Material mismatch: Labeled as 304 stainless steel but actually 201, with poor corrosion resistance.
Insufficient hardness: Not meeting the 10.9 standard (320-370 HV10), affecting connection strength.
Hydrogen embrittlement risk: Lack of hydrogen removal treatment after galvanizing may lead to delayed fracture during long-term use.
6. Whitening or loosening after assembly
Whitening: White stress marks appear on the surface of the plastic part, caused by excessive torque or insufficient wall thickness leading to localized plastic deformation.
Loosening and abnormal noise: Insufficient clamping force or lack of anti-loosening design under vibration conditions leads to connection failure.
