When the melt is injected into the cavity with a large volume at high speed and high pressure, the melt rupture phenomenon is easy to occur. At this time, the melt surface appears transverse fracture, and the fracture area is rough inclusion on the surface of the plastic parts to form paste spots. Especially when a small amount of molten material is directly injected into the cavity that is easy to be too large, the melt rupture is more serious, and the paste spot presented is larger.
The essence of melt rupture is caused by the elastic behavior of polymer molten material. When molten material flows in the cylinder, the molten material near the cylinder is subjected to friction from the cylinder wall, resulting in a large negative force and a small flow speed of molten material. Once molten material is injected out of the nozzle, the negative force acting on the tube wall disappears, while the molten material in the middle of the cylinder has a very high flow rate, and the molten material in the cylinder wall is carried and accelerated by the molten material in the center. Because the flow of the molten material is relatively continuous, the flow velocity of the molten material inside and outside will be rearranged to tend to the average velocity.
In this process, the melt will have a sharp stress change will produce strain, because the injection speed is very fast, the stress is particularly large, far greater than the strain capacity of the melt, resulting in melt rupture.
If the molten material encounters a sudden shape change in the flow channel, such as diameter shrinkage, expansion and dead Angle, and the molten material stays and circulates at the dead Angle, it has a different force from the normal molten material, and the shear deformation is large. When it is mixed into the normal flow material, the deformation recovery of the two is inconsistent, and it cannot be closed. Its manifestation is also melt rupture.
From the above, it can be seen that to overcome the trapped melt rupture and avoid paste spots:
The first is to pay attention to eliminate the dead Angle in the flow channel, so that the flow channel as streamlined as possible;
The second is to appropriately increase the material temperature, reduce the relaxation time of the molten material, so that its deformation is easy to recover and close;
The third is to add low molecular weight to the raw material, because the lower the molecular weight of the molten material, the wider the distribution, the more conducive to reducing the elastic effect;
The fourth is to properly control the injection speed and screw speed;
Fifth, it is very important to set the gate position reasonably and choose the correct gate form. Practice shows that the hidden gate (tunnel gate) is more ideal when large point gate is used. It is best to choose the location of the gate to inject the molten material into the transition cavity before entering the larger cavity, do not make the flow material directly into the larger cavity.