When the melt is injected into the cavity with large volume under high speed and high pressure, it is very easy to produce melt fracture. At this time, the melt surface appears transverse fracture, and the fracture area is roughly mixed with the surface of the plastic part to form paste spots. Especially when a small amount of melt is directly injected into the cavity which is easy to be too large, the melt fracture is more serious and the paste spot is larger.

The essence of melt fracture is due to the elastic behavior of high polymer melt. When the melt flows in the barrel, the melt near the barrel is rubbed by the barrel wall, the stress is large, and the flow speed of the melt is small. Once the melt is injected from the nozzle, the negative force acting on the pipe wall disappears, and the melt flow rate in the middle of the barrel is very high, The melt at the barrel wall is accelerated by the melt at the center. As the flow of molten material is relatively continuous, the flow velocity of internal and external molten materials will rearrange and tend to the average velocity.

In this process, the melt will undergo rapid stress change and strain. Due to the extremely fast injection speed, the stress is particularly large, which is far greater than the strain capacity of the melt, resulting in melt fracture.

If the molten material encounters sudden shape changes in the flow channel, such as diameter shrinkage, expansion and dead corner, the molten material stays and circulates at the dead corner. It is different from the stress of normal molten material, and the shear deformation is large. When it is mixed into the normal molten material for injection, it cannot be bridged due to the inconsistent deformation recovery of the two. If there is a great difference, fracture will occur, Its manifestation is also melt fracture.

It can be seen from the above that to overcome the trapped melt fracture and avoid paste spots:

First, pay attention to eliminate dead corners in the flow channel and streamline the flow channel as much as possible;

Second, properly increase the material temperature and reduce the relaxation time of molten material, so that its deformation can be easily recovered and bridged;

Third, low molecular weight is added to the raw material, because the lower the molecular weight of the melt, the wider the distribution, the more conducive to reducing the elastic effect;

Fourth, properly control the injection speed and screw speed;

Fifthly, it is very important to reasonably set the gate position and select the correct gate form. The practice shows that the latent gate (tunnel gate) is ideal when using the expanded point gate. The position of gate shall be selected after the molten material is injected into the transition cavity and then into the larger cavity. Do not make the flowing material directly enter the larger cavity.

180 ton plastic injection molding machine

Fuhrung plastic equipment supply high quality 180Ton Plastic injection molding machines. Main features of 180 ton injection mold machine is as below:
-Clamping unit: 1800 kN =180 Ton
-Tie bar: 520*470 mm
-Injection weight: 326grams with B screw Dia. 45