When the melt is injected into the larger cavity under high speed and high pressure conditions, the melt fracture is very easy to occur. At this time, the melt surface appears transverse fracture, and the fracture area is coarsely mixed with the surface of the plastic to form the fleck. In particular, a small amount of melt directly into the cavity is easy to over-large, the melt more severe fracture, showing the greater the spot.
The essence of melt fracture is due to the elastic behavior of the polymer melt. When the melt flows in the barrel, the melt close to the barrel is rubbed by the barrel wall with a large negative force. The melt flow velocity Smaller, once the melt out of the nozzle, the negative effect of the wall disappears, and the middle of the barrel melt flow rate is very high, the cylinder wall of the melt at the center of the melt carried and accelerated, due to the melt Flow is relatively continuous, internal and external melt flow rate will be rearranged, tends to average speed.
During this process, the rapid change of stress of the melt will produce strain. Due to the extremely fast injection speed, the stress suffered is particularly large, much larger than the strain capacity of the melt, leading to the melt fracture.
If the melt encounters a sudden shape change in the flow channel, such as diameter shrinkage, expansion and dead angle, the melt stays in the dead corner and circulation, it is different from the normal melt stress, shear deformation larger, when its Mixed into the normal flow of material out, due to inconsistent deformation recovery of the two, can not be bridged, if the great disparity, the occurrence of fracture rupture, the performance of the form is also a melt fracture.
From the above we can see, to overcome the trapped melt fracture, to avoid the Mudou:
First, we must pay attention to eliminate dead ends in the runner, so that the runner streamline as much as possible;
Second, it is appropriate to increase the material temperature, reduce the melt loose time, make it easy to recover and bridge deformation;
Third, add low molecular weight materials in raw materials, because the lower the molecular weight of the melt, the distribution is wider, the more conducive to reducing the elastic effect;
Fourth, proper control of injection speed and screw speed;
Fifth, it is very important to set the gate position and select the correct gate form reasonably, and practice shows that it is more suitable to use enlarged gate and latent gate (tunnel gate) Material first into the transition chamber and then into the larger cavity, do not make the material directly into the larger cavity.