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

Problem

In ESL housings, wearables, and glossy cosmetic shells, whitening/flash often causes scrap or rework. 

Even if the joint is strong, appearance kills the project.

Why it happens?

A. Over-collapse: depth not controlled, energy keeps pushing after melt.

B. Force spike: pneumatic systems surge at contact, stressing thin walls.

C. Amplitude too high at start: surface overheats before fusion stabilizes.

D. Horn/fixture mismatch: uneven support creates local stress.

E. Process window too narrow due to material or molding variation.

Fix (recommended servo approach)

Step 1. Use servo depth mode and limit collapse to 0.05–0.20 mm for thin walls (<1.0 mm).

Step 2. Start with constancy amplitude (e.g., 60% → 100% in 0.2–0.5 s).

Step 3. Set force closed-loop to keep pressure stable through welding.

Step 4. Verify fixture support; add local ribs or vacuum support if needed.

Step 5. Run DOE to lock a window: energy ±10% / depth ±0.02 mm.

Proof

In a smartwatch housing project, TEC servo ultrasonic welding system reduced whitening defects by 28%

and improved cosmetic yield from 87.1% to 98.8%.

Next step

Send your part drawing/material/thickness. We provide a free weld report and parameter window in 48h.