How can the plastic melt be prevented from breaking during injection molding?

How can the plastic melt be prevented from breaking during injection molding?

When the melt is injected into a relatively large cavity at high speed and high pressure, melt fracture is very likely to occur. At this time, transverse cracks appear on the surface of the melt, and the fractured areas are roughly embedded in the surface layer of the plastic part, forming smeared spots. This is particularly severe when a small amount of melt is injected directly into an oversized cavity, resulting in more pronounced smearing.The essence of melt fracture arises from the elastic behavior of the polymer melt. When the melt flows inside the barrel, the melt near the barrel wall experiences friction and resistance, causing it to move more slowly. Once the melt is extruded from the nozzle, the resistance from the barrel wall disappears, while the melt in the center of the barrel flows much faster. The melt near the walls is accelerated by the central flow. Because the melt flow is relatively continuous, the velocities of the inner and outer melt layers rearrange and tend toward an average flow rate.During this process, the melt undergoes rapid stress changes, generating strain. Due to the very high injection speed, the stresses experienced are extremely large, far exceeding the strain capacity of the melt, leading to melt fracture.

If the melt encounters sudden changes in the shape of the runner, such as diameter contraction, expansion, or dead corners, the melt tends to stay and circulate in the dead corners. The stresses on it differ from those on the normal melt, and it undergoes greater shear deformation. When it mixes with the normal flow and is injected, the differences in deformation recovery cannot be reconciled. If the difference is significant, breakage occurs, which manifests as melt fracture. From the above, to overcome difficult melt fracture and avoid flow patterns or streaks:1. Pay attention to eliminating dead corners in the runner and make the runner as streamlined as possible;2. Appropriately increase the material temperature to reduce the relaxation time of the melt, making its deformation easier to recover and reconcile;3. Add low molecular weight substances to the raw material; the lower the molecular weight and the broader the distribution of the melt molecules, the more it helps reduce elastic effects;4. Appropriately control injection speed and screw speed;5. Properly set the gate position and choose the correct gate type, which is quite important. Practice shows that using enlarged spot gates or submerged gates (tunnel gates) is ideal. The gate position is best chosen so that the melt first enters a transition cavity before flowing into a larger cavity, preventing the flow from entering the larger cavity directly.