One question per week: Basics of polymers – What are the factors that affect the extrusion swelling effect of polymer materials?

Melt viscosity, a property of the resin itself. . . . . . . .

The phenomenon in which the extrudate expands after exiting the die, resulting in a cross-section larger than that of the die, when the polymer melt is extruded through a small hole, capillary, or slit. Quantitatively, it is expressed by the expansion ratio. Extrudate swelling is common in polymer melts, as the polymer melt becomes oriented due to flow within the die, and the molecules re-curl as they exit the die orifice. The swelling of the extrudate increases as the shear rate increases, and then decreases after reaching a maximum value. An increase in molecular weight and other factors that can increase entanglement (such as an increase in long branches) will both lead to an enlargement of the extrudate.

Extrudate swelling is common in polymers, resulting from the orientation of the polymer melt due to directed flow, followed by the re-curling of the molecules. The main influencing factors are molecular weight and its distribution, as well as the length of the branches in the molecule.

Factors affecting the effect: An increase in temperature reduces the expansion effect; an increase in speed increases the expansion effect. An elongation of the capillaries reduces the expansion effect, while an increase in stiffness also reduces the expansion effect

The inherent structure of the polymer – with its many branches – results in significant flexibility; the flow rate of the fluid is also affected by the length of the mold gate

Generally speaking, the extrusion swelling ratio of polymer melts decreases as temperature increases, because the mobility of polymer molecular chains increases with rising temperature, leading to increased viscous dissipation of the deformation energy stored in the melt during flow, which in turn weakens its elastic properties.

The extrusion swelling of polymer melts during the extrusion process can be classified into different mechanisms, including Newtonian swelling, swelling caused by instantaneous elastic recovery, swelling due to inelastic factors, and delayed swelling (or residual relaxation swelling). Regarding Newtonian swelling, S. Middleman and others have conducted studies and analyses showing that at a Reynolds number of Re G6, the Newtonian swelling ratio is Bnewt = 1.06. The swelling caused by instantaneous elastic recovery occurs after the polymer melt exits the die; it is dependent on the melt’s relaxation time. At the moment the extrudate leaves the die, its swelling value is low; over time, this swelling value increases gradually, and after a certain period, it reaches a relatively constant level. Regarding the causes of extrusion swelling, it is generally believed that there are two reasons: one is the elastic recovery of tensile deformation caused by population convergence; the other is the elastic recovery of shear deformation caused by the difference between shear stress and normal stress. If the length-diameter ratio of the capillary is very small, the former is the main factor; if the length-diameter ratio is large, the latter is the main factor, due to the gradual relaxation of the elastic deformation along a relatively long distance within the capillary. The swelling caused by inelastic factors is mainly due to thermal effects, internal forces, surface tension, etc. Delayed swelling refers to the slow swelling that occurs after an immediate rapid swelling, with the degree of swelling decreasing gradually. The first aspect of Newtonian swelling has been resolved: the swelling caused by inelastic factors is minimal and is often ignored when calculating the swelling of polymers during extrusion. Research on the extrusion swelling of polymer melts focuses primarily on the effects of instantaneous elastic swelling and delayed swelling on melt extrusion swelling.

The viscoelasticity of the resin matrix, the processing temperature, melt pressure, etc.