Why Plastic Parts Crack or Turn White During Ultrasonic Welding

Introduction

Ultrasonic welding is widely used for joining thermoplastic components in industries 

such as automotive manufacturing, medical devices, electronics, and packaging. 

The process provides fast cycle times, strong weld joints, and excellent repeatability.

However, manufacturers sometimes encounter common defects during ultrasonic welding,

especially plastic cracking and stress whitening around the weld area. These issues can 

affect both product appearance and structural integrity.

Understanding why these defects occur is essential for optimizing the welding process

and improving production reliability.

This article explains the most common causes of cracking and whitening during ultrasonic

welding and provides practical solutions to prevent these problems.

1. What Is Stress Whitening in Ultrasonic Welding

Stress whitening occurs when plastic materials experience excessive localized stress during

ultrasonic welding.

Under high mechanical vibration and pressure, the polymer structure inside the plastic begins

to stretch and form microscopic voids. These microscopic changes scatter light and create a 

white or cloudy appearance on the surface.

Stress whitening is commonly observed in materials such as:

• ABS

• PC (Polycarbonate)

• PMMA

• Acrylic

Typical whitening areas include:

• near the weld interface

• around ribs or thin sections

• near energy directors

Although whitening may not always reduce structural strength, it often affects product 

appearance and indicates that the welding parameters may be too aggressive.

2. What Causes Plastic Parts to Crack During Ultrasonic Welding

Plastic cracking is a more severe defect than whitening and often indicates excessive

stress or improper welding conditions.

Cracks may appear:

• along the weld line

• near sharp corners

• around molded features

• at the edge of the welded part

Several factors can cause cracking during ultrasonic welding.

3. Excessive Ultrasonic Energy

One of the most common causes of whitening and cracking is excessive ultrasonic energy.

If the welding amplitude or welding time is too high, the ultrasonic vibration generates 

excessive heat at the weld interface.

This can lead to:

• material degradation

• excessive melt flow

• internal stress buildup

Over-welding can also cause the plastic material to collapse or deform, increasing the 

risk of cracking.

4. Improper Joint Design

Joint design is one of the most important factors affecting ultrasonic welding quality.

Poorly designed joints can lead to uneven energy distribution and stress concentration.

Common joint design problems include:

• oversized energy directors

• sharp internal corners

• insufficient alignment features

• poor wall thickness transitions

Proper joint design helps concentrate ultrasonic energy exactly where melting should occur.

Common joint structures include:

• energy director joints

• shear joints

• tongue and groove joints

  ultrasonic welding joint design

5. Material Compatibility Issues

Not all plastics respond equally to ultrasonic welding.

Some materials are more prone to cracking or whitening due to their molecular structure.

For example:

Amorphous plastics

• ABS

• PC

• PS

These materials generally weld easily.

Semi-crystalline plastics

• PP

• Nylon

• POM

These materials melt quickly and require more precise parameter control.

If the welding energy is not properly controlled, semi-crystalline plastics are more 

likely to crack.

6. Excessive Clamping Pressure

Welding pressure also plays a critical role in ultrasonic welding quality.

If the clamping force is too high before the plastic melts, the material may experience 

excessive compressive stress.

• This can lead to:

• deformation

• whitening

• cracking

Proper pressure control ensures that the plastic melts first before significant mechanical

 deformation occurs.

7. Poor Injection Molded Part Design

Injection molding design can also influence welding performance.

Plastic parts with thin walls or sharp corners often experience stress concentration during

 ultrasonic welding.

Design improvements may include:

• adding fillets to reduce stress concentration

• maintaining uniform wall thickness

• optimizing rib structures

Good part design significantly improves welding reliability.

8. How to Prevent Cracking and Whitening in Ultrasonic Welding

Manufacturers can prevent welding defects by optimizing several key factors.

Optimize welding parameters

Adjust parameters such as:

• amplitude

• welding time

• pressure

• hold time

Improve joint design

Use appropriate joint structures that concentrate ultrasonic energy efficiently.

Select suitable plastic materials

Choose plastics with good ultrasonic weldability.

Conduct welding sample testing

Testing helps determine the best parameters before mass production.

9. Sample Testing Helps Identify Welding Problems Early

Because plastic formulations and part designs vary, welding performance can differ 

significantly even for the same material type.

Many manufacturers perform sample welding tests before purchasing ultrasonic 

welding equipment.

Professional testing can evaluate:

• weld strength

• sealing performance

• production feasibility

TIMEAST provides ultrasonic welding sample testing services to help manufacturers 

identify welding issues and optimize welding parameters.

Send your samples for ultrasonic welding testing.

10 Recommended Ultrasonic Welding Equipment

Different plastic parts require different ultrasonic welding machines depending on size and material.

• Servo Driven Ultrasonic Plastic Welding Machine

• 20kHz 3000W Ultrasonic Welding Machine

• High Precision Ultrasonic Welding Machine

11. Conclusion

Plastic cracking and stress whitening are common defects during ultrasonic welding, 

usually caused by excessive energy, poor joint design, improper materials, or excessive

welding pressure.

By optimizing welding parameters, improving joint design, and validating the process

through sample testing, manufacturers can significantly improve welding quality and 

production stability.

Understanding these factors helps engineers achieve strong, reliable, and visually 

acceptable welds.

• Recommended Machines →

• Sample Testing →