How to solve the appearance defects caused by melt fracture?

Melt rupture/fracture is one of the elastic phenomena of polymer melts, which means that when the polymer melt is extruded from the die through the runner, if the shear rate is greater than a certain limit value, unstable flow is often generated, and the surface of the extrudate is unstable. The results might be irregularities or the appearance of bamboo, spiral and other distortions, as well as fragmentation and fracture.

The reason for melt fracture is that when the material flows into the die, due to the excessive shear rate, the flow velocity is too large, and the parallel flow cannot be formed, but an unstable flow is formed instead.

Or because the melt viscosity is too high and the internal stress relaxation time is too long, the elastic strain exhibited by each point of the melt is different, so that the extrudate is distorted or fractured during the elastic recovery process.

In addition, melt fracture can also occur when the angle of flow into the die is not appropriate or the die flow path has a dead angle.

Phenomenologically, extrusion melt fracture can be classified into two categories.

The fracture is characterized by a rough surface first, and when the extrusion shear rate exceeds the critical shear rate, the melt fracture occurs, showing a random fracture shape. Materials belonging to this category are mostly polymers with branched or large side groups, such as polystyrene, styrene-butadiene rubber, branched polydimethylsiloxane, and the like.

The stress concentration effect of LDPE melt flowing near the extrusion die and the entrance area, the stress is mainly concentrated in the entrance area of the die, and the streamline of the entrance area shrinks in a typical trumpet shape, and there is a circulation at the dead corner of the die or eddy currents (pictured below).

The characteristics of melt fracture are rough surface first, and then with the increase of shear rate, regular distortion, such as bamboo-shaped and spiral distortion, gradually appears. When the shear rate is high, random fracture occurs. Materials belonging to this category are mostly linear molecular polymers, such as polybutadiene, ethylene-propylene copolymer, linear polydimethylsiloxane, polytetrafluoroethylene, and the like.

For HDPE type melt, the stress concentration effect when flowing near the extrusion die and the entrance area is not mainly in the entrance area of the die, but occurs near the inner wall of the die, and there is no dead angle circulation in the entrance area of the die (as shown in the figure above).

In fact, this classification is not strict enough, and the melt fracture behavior of some materials does not have these two typical characteristics.

The shear rate at which melt fracture begins to form is called the critical shear rate τ c.

As the temperature increases, the critical shear rate of melt fracture increases. As the temperature increases, the melt viscosity decreases and the shear stress decreases.

τ c · Mw=K (Mw is the average molecular mass, K is a constant.)

That is, high molecular weight melts are more prone to melt fracture than low molecular weight melts.

The lower the viscosity, the higher the critical shear rate for melt fracture to occur.

Extrusion quality can be significantly improved by tapering the die inlet, which allows for no distortion of the outer extrudate to occur much faster than the critical point (there may be some internal distortion, though).

When the surface of the material produces a non-smooth surface, the pressure will generate tiny vibrations. By testing the tiny vibration of the pressure, the equipment can detect the uneven surface of the material without the human eye being able to observe it.

As shown in the figure above, the structure diagram on the left is the test diagram of the traditional pressure sensor. The installation position is in the barrel. The traditional pressure sensor is used, and the sampling frequency is low, about 10Hz.

The structure on the right shows the installation and test layout of the new high-frequency pressure sensor in the unstable flow test accessory. The new pressure sensor is installed in the belt die to directly test the pressure change of the melt in the die. The ultra-high frequency pressure sensor is used with a frequency of 20000Hz, which is 2000 times of the traditional pressure sampling frequency. Therefore, the tiny pressure can also be collected.

It is known that the extrusion fracture behavior of polymer melt is a manifestation of the elasticity of the melt, so all factors that can affect the elasticity of the melt will work.

These factors can be roughly divided into three categories: one is the shape and size of the die; the second is the technological conditions of the extrusion molding process; the third is the properties of the extruded material. Therefore, the improvement methods of melt fracture mainly include the following:

1. According to the nature of the polymer itself, change the size of the die and appropriately increase the aspect ratio of the die, so that the disturbance in the entrance area can be relaxed in the die, which is beneficial to the stability of the extrusion process. The die setting length can be adjusted. For example, for LDPE, the die setting length should be reduced, and HDPE should be increased.
2. Turn the entrance angle of the die into a rounded corner to reduce stress concentration. The key to improving the overall distortion of the extrudate is to reduce the flow stress concentration effect in the die inlet area, reduce the disturbance of the extensional flow field and eliminate various secondary flows.
3. Add processing aids. Such as adding carbon black or silica to improve the extrusion distortion and melt fracture behavior of SSBR (styrene butadiene rubber).
4. Moderately increase the processing temperature. For example, increasing the temperature is effective in improving both the overall distortion and the surface distortion. An increase in melt temperature and a decrease in viscosity result in a shorter relaxation time, resulting in an improved appearance of the extrudate.

For instance, the industrial solution to the melt fracture of modified plastic extrusion products includes to: streamline the flow of the die head, increase the temperature of the die setting zone, run the screw at a low speed, reduce the molecular weight or polymer melt viscosity, increase the cross-sectional area of ​​the outlet runner, or the use of external lubricants, etc.