5 Reasons Thin-Wall Cosmetic Housings Turn White After Welding (Fix Guide)

Why Thin-Wall Cosmetic Housings Turn White After Ultrasonic Welding (Fix Guide)

Thin-wall cosmetic housings are widely used in smart wearables, cosmetic packaging, 

personal care devices, and high-end consumer electronics. However, during ultrasonic welding, 

these thin plastic parts often develop whitening, stress marks, or cosmetic defects, which directly

 affect appearance quality and yield.

This article explains why whitening occurs in thin-wall ultrasonic welding and provides practical,

proven solutions to help manufacturers achieve stable cosmetic quality.

What Is Whitening in Thin-Wall Ultrasonic Welding?

Whitening refers to the localized white or cloudy appearance that forms around

the weld seam after ultrasonic welding.

It is usually caused by excessive internal stress, plastic deformation, or micro-fractures 

in the polymer structure.

For cosmetic housings, even minor whitening is unacceptable, making process control 

especially critical.

5 Key Reasons Thin-Wall Cosmetic Housings Turn White

1). Excessive Collapse Distance

Thin-wall parts have limited material volume.

When collapse distance is not strictly controlled, the material is over-compressed, 

leading to stress whitening and surface deformation.

Common issue:

• Using time-based welding instead of depth-controlled welding

2). Unstable Welding Force (Pneumatic Shock)

Traditional pneumatic systems may apply force too quickly at the start of welding.

This sudden pressure spike causes localized stress concentration, especially in thin 

cosmetic shells.

Typical symptom:

• Whitening appears immediately after horn contact

3). Excessive Initial Amplitude

High amplitude at the start of welding introduces excessive vibration energy into 

a fragile thin-wall structure,exceeding the plastic’s elastic limit.

Risk:

• Surface micro-cracks

• Loss of transparency or gloss

4). Inadequate Fixture Support

Poor fixture design allows thin-wall parts to flex during welding.

This flexing amplifies stress and creates uneven energy distribution at the weld interface.

Result:

• Whitening away from the actual weld seam

5). Narrow Process Window for Thin-Wall Materials

Thin cosmetic housings often use materials like ABS, PMMA, or PC blends, which have 

tight process tolerances.Small variations in material batch or moisture content can easily 

push the process outside the safe window.

How to Fix Whitening Defects: Practical Solutions

1). Use Servo-Controlled Depth Welding

Switch from time mode to collapse distance or depth-controlled welding.

Benefits:

• Prevents over-compression

• Ensures consistent weld height

• Significantly reduces cosmetic stress marks

2). Apply Amplitude Ramp-Up Control

Gradually increasing amplitude at the start of welding allows the material to

soften progressively.

Best practice:

• Low initial amplitude

• Smooth ramp to target amplitude

• Avoid sudden energy spikes

3). Maintain Stable Welding Force

Servo-driven ultrasonic welding systems provide precise and repeatable force control,

eliminating pneumatic shock.

Result:

• Reduced stress whitening

• Improved surface finish consistency

4). Optimize Fixture and Part Support

Fixtures should fully support thin-wall areas close to the weld seam.

Design tips:

• Increase local backing support

• Avoid hollow or floating areas

• Match fixture geometry to part curvature

5). Lock the Process Window with DOE

Perform Design of Experiments (DOE) to identify the safe operating window for:

• Collapse distance

• Welding force

• Amplitude profile

• Hold time

This step is essential for mass production stability.

Why Servo Ultrasonic Welding Is Ideal for Thin-Wall Cosmetic Parts

Compared with pneumatic systems, servo ultrasonic welding offers:

• Precise collapse control

• Smooth force application

• Repeatable energy input

• Better cosmetic surface protection

For high-end cosmetic housings, servo systems significantly improve yield and visual quality.

Summary: Best Practices for Thin-Wall Ultrasonic Welding

To prevent whitening defects in thin-wall cosmetic housings:

• Use depth-controlled servo welding

• Avoid excessive initial amplitude

• Maintain stable welding force

• Improve fixture support

• Validate parameters through DOE

With proper process control, ultrasonic welding can achieve strong joints without compromising 

cosmetic appearance.

Need Help Optimizing Thin-Wall Welding?

If you are experiencing whitening or cosmetic defects in thin-wall plastic housings, our application 

engineers can help you:

• Analyze your current welding process

• Recommend suitable servo ultrasonic welding systems

• Optimize parameters for cosmetic quality

Send us your part drawings or samples，Get a free evaluation.

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