The Hidden Culprit: The Back-Pressure Valve
USB cable overmolding issues：In the world of cable manufacturing, theory is important, but experience is king.
Today at the Longyd workshop, our technical team encountered a stubborn challenge during the pilot production of a new USB cable order. Despite hours of adjusting temperatures and holding pressures, the PVC overmold kept showing persistent surface bubbles.
It was a classic “bottleneck” moment—until we looked at the problem from a different angle.
Why Temperature & Holding Pressure Weren’t Enough
The technician initially followed the standard troubleshooting protocol:
- Lowering the temperature to prevent material decomposition.
- Increasing holding pressure to try and “crush” the bubbles.
However, the bubbles remained. This is a common trap in Vertical Injection Molding. When standard adjustments fail, the root cause usually isn’t in the mold cavity, but inside the injection barrel itself.
The Power of Back Pressure
When the issue was reported to me, I suggested one specific change: Increase the Back Pressure.
The result? The problem was solved almost instantly. #### Why did this work? Back pressure is the pressure applied to the back of the screw as it rotates to draw in new material. Here is why it was the key to eliminating those USB bubbles:
- Expelling Trapped Air: High back pressure forces the PVC melt to be more compact. It squeezes out the air that gets caught between the granules during the feeding stage. Without enough back pressure, that air travels through the nozzle and ends up as a bubble on your USB plug.
- Improved Melt Uniformity: It ensures the PVC is thoroughly mixed and at a consistent density.
- Eliminating “Silver Streaks”: By densifying the melt, we prevent the “unstable flow” that causes those annoying silver lines on the surface.
What is your “secret weapon” for handling stubborn injection molding defects? We invite you to exchange ideas and share your expertise with LONGYD through our blog comments.
www.lyd123.com e-mail:nick.xu@lyd123.com
At Longyd, we prioritize quality in every step of our process. Check out our custom industrial USB cable solutions here.
