Case Study: Eliminating Black Marks on White Plastic Components

Introduction

A Tier-level automotive component supplier was producing white plastic connector parts with strict

appearance requirements.

However, during ultrasonic welding:

• Black marks appeared on the surface

• Defects increased during continuous production

• Multiple machines failed to meet quality standards

 Result: Products could not pass OEM inspection.

Root Cause Analysis

After evaluating the welding process, several factors contributing to black marks were identified:

• Excessive localized heat generation during welding

• Inconsistent pressure control from the pneumatic system

• Material friction at the energy director

• Limited process repeatability

These factors caused slight material degradation on the white plastic surface, resulting in visible discoloration.

Process Optimization Solution

To eliminate the black marks, TIMEAST implemented a servo-driven ultrasonic welding system operating at 35kHz.

Compared with the previous pneumatic setup, the servo system provided:

• More accurate pressure control

• Stable welding speed

• Precise collapse distance monitoring

• Better energy consistency

The optimized welding profile reduced excessive heat buildup while maintaining required weld strength.

Before and After Comparison

Ultrasonic Welding Process Video 

The following video demonstrates the optimized welding process using the servo ultrasonic welding system.

The final welded component achieves a clean surface appearance without black marks while maintaining

reliable welding quality.

Results Achieved

After implementing the optimized welding solution:

• Black marks were successfully eliminated

• Product appearance met customer requirements

• Welding consistency improved

• Production stability increased

• The customer approved the welding process for mass production

RFQ

Q1. Why do black marks occur during ultrasonic welding?

A:  Black marks are usually caused by excessive localized heat, unstable pressure, improper welding parameters, 

or material degradation during the welding process. White and light-colored plastic parts are particularly 

sensitive to these surface defects.

Q2. Can servo ultrasonic welding prevent burn marks?

A:  Servo ultrasonic welding provides more precise control of pressure, speed, and collapse distance. This helps 

reduce excessive heat generation and significantly lowers the risk of burn marks and surface discoloration.

Q3. Is 35kHz better than 20kHz for cosmetic plastic parts?

A:  For small and appearance-sensitive plastic components, 35kHz ultrasonic welding often provides more refined 

energy delivery and lower amplitude, helping achieve cleaner weld surfaces and improved cosmetic quality.

Q4. Which plastic materials are sensitive to discoloration?

A:  White or light-colored materials such as ABS, PC, PC/ABS, and certain engineering plastics are more likely to 

show discoloration, burn marks, or surface defects if welding parameters are not properly optimized.

Q5. How can welding pressure affect product appearance?

A:  Excessive or unstable welding pressure can increase friction and heat generation, leading to surface defects.

 Proper pressure control helps improve weld consistency and maintain a clean product appearance.

Related Ultrasonic Welding Solutions

• Servo Ultrasonic Welding Machine

• 35kHz Ultrasonic Welding Machine

• Automatic Loading & Unloading Ultrasonic Welding Line Case

• Endcap Felt Ultrasonic Welding Case

• Ultrasonic Welding Troubleshooting Guide