Part Details Review
| Strength |
Weld lines generally appear on both the inner and outer surface of the molded part. Flow fronts progress via fountain flow in which the polymer chains are drawn out of the core of the thickness and roll out onto the surface of the mold cavity. When the resulting rounded flow fronts meet at a steep enough angle a visible recesses line is left with a "V" shaped cross section.
Weld lines exist even under the visible marks. Weld lines are usually weaker in strength than the rest of the part. This is as a result of:
- Flowing material sweeps gases, air, lubricants and other impurities from the mold surface and deposits them in the welding zone.
- The visible portion of the weld line is shaped like a notch or groove, which acts as a stress concentration point.
- Cool flow fronts reduce the strength of the weld formed at the weld line. Insuring that the flow fronts meet at a higher temperature will improve the bond.
- At a weld zone, two impinging flow fronts tend to align the molecules parallel to the melt front causing incomplete molecular entanglement and bonding.
- Weld line visibility and strength can be improved by providing vents at the weld line to avoid slowing down the flow fronts as air volatiles escape and by insuring that the two flow fronts meet at slightly above the recommended processing melt temperature for the material being molded.
