In injection-molded part design, weld lines are often an unavoidable weak point. When two melt fronts meet inside the mold cavity, if the temperature or pressure is insufficient, an “unfused interface” is formed at the junction. On a macroscopic level, it may appear as nothing more than a faint surface line; on a microscopic level, it is actually a “crack” where polymer chains have not fully entangled.

The illustration above reveals a classic piece of engineering wisdom: by adding an overflow pocket in front of the weld line, the melt is allowed to keep flowing. As a result, not only is the strength of the weld line restored, but the surface appearance is also greatly improved.

So what exactly does this “extra bit of volume” change?

When Two Melt Fronts Meet, They Are “Cold and Crashing”
When two melt fronts flow around an obstacle and meet inside a mold cavity, the portions that contact first are often already cooled by the mold wall. At that moment, the meeting angle is close to 180°—a head-on collision. The surface layer is a semi-solid “skin,” while only the core remains slightly fluid. When the two fronts collide, their skins touch but cannot interpenetrate, forming an interface with discontinuous molecular orientation and abrupt fiber realignment. This is the true nature of a weld line.

At this stage, even a temperature difference of just 10 °C can interrupt molecular diffusion across the interface. In glass-fiber-reinforced systems, the situation is even worse: fibers are cut off and reoriented perpendicular to the stress direction, providing almost no load-bearing capability.

The Magic of the Overflow Pocket: Extending the “Fusion Time Window”
As shown in the figure, the overflow pocket is placed in front of the weld line. It may look like nothing more than allowing the melt to flow a little further, but in reality, it delays the freezing of the interface in time. As the melt continues flowing into the overflow pocket, pressure and shear are maintained in the meeting zone, producing three key microscopic effects:

1. Molecular chains at the interface remain mobile
Because the melt does not stop flowing immediately, polymer chains still have time to diffuse into and entangle with each other. This enables true welding rather than mere contact.

2. The surface temperature is “pulled up”
Viscous dissipation generated by continued flow raises the local temperature, delaying vitrification of the skin layer. As a result, the cold interface is reactivated, and the weld line is no longer a cold joint.

3. The flow direction is stretched forward
The overflow pocket changes the meeting geometry from a 180° head-on collision into a forward-moving flow into the pocket. Instead of stopping at the junction, the melt keeps moving. What was originally a 180° impact gradually becomes a 120° → 90° → 60° → 0° transition.

This makes fiber orientation and molecular chain alignment across the interface much more continuous, reducing the formation of weak, fractured layers.

In other words, by simply letting the melt “take one more step,” the overflow pocket buys a few hundred extra milliseconds of activity for the interface—and on the molecular scale, that is more than enough time for chain interpenetration and orientation relaxation to occur.

The Interface Changes from a “Line” to a “Layer”
Under a microscope, the weld line regions with and without an overflow pocket exhibit very different microstructures: