How to prevent plastic melt from rupturing during injection molding?
When the melt is injected into a cavity with a large volume under high-speed and high-pressure conditions, the melt fracture phenomenon is easily generated. At this time, the surface of the melt is laterally fractured, and the fracture area is coarsely mixed with the surface layer of the plastic part to form a paste. In particular, when a small amount of molten material is directly injected into an easily oversized cavity, the melt fracture is more serious, and the appearance of the paste is larger.
The nature of the melt fracture is due to the elastic behavior of the polymer melt. When the melt flows in the barrel, the melt near the barrel is rubbed by the wall of the barrel, the force is large, and the flow rate of the melt Smaller, once the melt is injected from the nozzle, the force of the tube wall disappears, and the melt flow rate in the middle of the barrel is extremely high, and the melt at the wall is accelerated by the melt at the center, due to the melt The flow is relatively continuous, and the flow velocity of the inner and outer melts will be rearranged to average speed.
During this process, a sharp stress change in the melt will cause strain, and the injection is extremely fast, and the stress is particularly large, far greater than the strain capacity of the melt, resulting in melt fracture.
If the melt encounters a sudden shape change in the flow path, such as diameter shrinkage, enlargement, and dead angle, the melt stays and circulates at the dead corner, which is different from the normal melt, and the shear deformation becomes larger when it is When it is mixed into the normal flow material, the deformation recovery of the two is inconsistent and cannot be bridged. If the disparity is large, the fracture fracture occurs, and the form of the fracture is also melt fracture.
It can be seen from the above that it is necessary to overcome the rupture of the trapped melt and avoid the occurrence of faint spots:
First, we must pay attention to eliminate the dead angle in the flow channel and make the flow channel as streamlined as possible;
The second is to appropriately increase the temperature of the material, reduce the relaxation time of the melt, and make its deformation 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 melt, the wider the distribution, the more beneficial to reduce the elastic effect;
The fourth is to properly control the injection speed and screw speed;
The fifth is to set the gate location reasonably and choose the correct gate form. This is very important. Practice shows that the use of enlarged gates and latent gates (tunnel gates) is ideal. The position of the gate is preferably selected after the melt is first injected into the transition chamber and then into the larger chamber. Do not allow the flow to directly enter the larger chamber.