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Why Do Whitening (Fogging), Cracks, or Flashing Occur After Welding?

In the ultrasonic plastic welding process, improper control of energy, pressure, or structural design may result in defects such as whitening (fogging), cracks, or flashing after welding. Here are the common issues and their causes:

1. Whitening (Fogging)

Appearance:
White, foggy marks or areas around the welded zone, often seen in materials with high aesthetic requirements or transparent materials (e.g., PC, PMMA, ABS).

Possible Causes:

  • Excessive ultrasonic energy or welding time: This causes material to stretch too much or damages the molecular structure.

  • Too high welding pressure: Leads to internal stresses within the plastic.

  • Too high amplitude: Causes rapid deformation of the material.

Improvement Strategies:

  • Lower the amplitude or welding energy.

  • Shorten the welding time to prevent excessive energy.

  • Adjust welding pressure to ensure uniform melting and cooling.

  • Use Energy Mode to precisely control the energy input.

👉 Tip: Using a Closed-Loop Ultrasonic Welding System (e.g., T-sonic iS Series, iX Series) with energy precision output can effectively improve the situation.

2. Cracking

Appearance:
Micro-cracks appearing immediately or after a period, either at the weld area or nearby.

Possible Causes:

  • Excessive energy: Leads to thermal shock or thermal aging of the material.

  • Too fast cooling after welding: Results in residual internal stress.

  • Poor impact resistance or ductility of the material.

  • High moisture content (e.g., nylon that hasn’t been dried).

Improvement Strategies:

  • Reduce amplitude and welding energy.

  • Extend the holding time to allow the weld zone to cool completely.

  • Confirm material is dry and suitable for ultrasonic welding.

  • Reassess whether the material grade is appropriate for ultrasonic welding.

3. Flashing (Excess Material)

Appearance:
Excess melted plastic protruding from the weld zone, affecting appearance or assembly.

Possible Causes:

  • Excessive welding energy or pressure: Causes an overflow of molten plastic.

  • Poor design of the energy director (weld line): Incorrect size or positioning leads to poor energy concentration.

  • Insufficient space for plastic flow at the welding interface.

  • Poor fixture rigidity: Leads to displacement during welding.

Improvement Strategies:

  • Lower welding pressure and amplitude.

  • Redesign the energy director’s height and angle.

  • Add a flash trap design to catch excess material.

  • Improve fixture accuracy and rigidity.

👉 Tip: A Closed-Loop Ultrasonic Welding System with a variable pressure actuator can help effectively mitigate these issues.

4. Poor Energy Director Design Leading to Uneven Melting

Possible Issues:

  • Too low energy director height: Fails to concentrate energy effectively.

  • Too high energy director height: Causes localized overheating and excessive flashing.

  • Misaligned energy director: Leads to incomplete welding.

Improvement Strategies:

  • Redesign the energy director’s dimensions based on material properties.

  • Ensure proper alignment of the energy director with the welding interface.

  • Use Collapse Mode to precisely control the melt flow.

5. Incompatible Materials for Ultrasonic Welding

Possible Issues:

  • Thermosetting resins cannot melt and bond effectively.

  • Rubber or high-elasticity materials absorb vibration energy rather than focusing it at the welding interface.

  • Large melting point differences between materials make it difficult to fuse them effectively.

Improvement Strategies:

  • Confirm the material is a thermoplastic that is suitable for ultrasonic welding.

  • Test material compatibility or consider alternative bonding methods.

6. Poor Mold/Fixture Alignment

Possible Impact:

  • Uneven force distribution: Leads to localized overheating or insufficient welding.

  • Workpiece movement: Causes discontinuities in the weld line.

Improvement Strategies:

  • Improve fixture positioning accuracy and rigidity.

  • Ensure welding pressure is applied uniformly and perpendicularly.

  • Check for fixture-induced resonance that might affect welding quality.

Key Points

White, cracked, or flashed areas in ultrasonic welding are usually caused by excessive energy, pressure, poor structural design, or fixture instability. By optimizing parameters, redesigning energy directors, and using a Closed-Loop Ultrasonic Welding System, welding quality can be significantly improved, leading to better consistency and stability in mass production.

👉 Tip: The Closed-Loop Ultrasonic Welding System (e.g., T-sonic iS Series, iX Series) can effectively ensure energy precision, improving the overall welding process and enhancing product quality.

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