Swelling - Thermoplastic polymers, elastomers and additives

When a molten polymer emerges from a die, the extrudate may swell, so that its cross-section as the die goes is greater than that of the die orifice. For a capillary matrix, the relationship between the diameter of the extrudate and the diameter of the matrix is known as the swelling ratio of the matrix, the swelling ratio or the swelling ratio. For a slit matrix, the relevant ratio is the thickness of the extrudate / depth of the slit.

The swelling of the matrix occurs because the shearing of the melt as it passes through the matrix causes the molecules to spread (with the greatest orientation near the wall of the matrix). Upon leaving the matrix, the molecules tend to coil (back off) resulting in a contraction in the flow direction and an expansion in directions perpendicular to the flow. If an extrudate is cut on the face of the die, it will be seen that the leading edge of the extrudate is convex. This indicates that the greatest contraction in the flow direction is closest to the wall where the shear stress has been greatest.

Decreasing the extrusion speed.

The increase in melting temperature

Increase the length of the parallel die or earths.

Rationalization and / or reduction of the entrance angle of the die.

Increase the extraction ratio

Often, the addition of a lubricant will reduce the swelling of the matrix. The lubricant will cause sliding on the wall and facilitates the flow of polymer through the matrix. The low molecular weight polymer functions as a lubricant and therefore expanding the molecular weight distribution of a material will also reduce the swelling of the matrix.

It is a common industrial practice to compensate for the swelling by stretching the extrudate so that it can pass through a size matrix. However, it must be taken into account that the reduction of the cause causes a molecular orientation, which results in an increase in the force in the flow direction and a decrease in the directions perpendicular to the flow. The shear and wave rates will be higher in the thinner sections. These thin sections can also have a shorter parallel matrix, to ensure that the extrusion rates are constant throughout the cross section, and this will further increase the swelling of the matrix. In the production of tubes and pipes, the situation is further complicated by the fact that the extruded product is usually inflated. In this case, it can be assumed that in the emergence of the matrix, the thickness of the wall will expand adequately to the swell of the shearing matrix used. Subsequently, the thickness of the wall will be reduced proportionally to the amount of inflation given by the diameter ratio of size / outer diameter of the pipe die.

The angle of entry of the matrix is the angle of convergence provided for the molten mass to enter the extrusion matrix. When the molten mass leaves the extruder cylinder, the shape of the fluid melt stream must be changed to that of the extruded form. This transition takes place in the die adapter and it is best to do it in the most gradual and agile way possible. The higher the viscosity of the melt, the more rationalization is required. It is also necessary to maintain the lengths of the flow paths, for each part of the matrix, the same in the aerodynamic region, since this will reduce the subsequent changes of the extruded form.