In injection molding, polymer material is heated to a molten state for processing. At this time, melt can be regarded as a continuous medium, a flow field is formed in certain parts of machine, such as screw, barrel, nozzle and flow channel of mold cavity. In flow field, melt undergoes stress, time, and temperature combined to deform or flow. In this way, flow of polymer melt is closely related to certain geometric parameters and process parameters of machine.
Polymer melt in laminar flow state can be divided into Newtonian and non-Newtonian fluids according to its molecular structure and processing conditions. Their rheological properties will not be introduced in detail.

Generally, shear stress increases as shear rate increases, while viscosity decreases as shear rate or shear stress increases.
The stronger dependence of shear viscosity on shear rate, the faster viscosity decreases with increase of shear rate. This kind of polymer is called a shear polymer. This kind of shear thinning is an inherent characteristic of polymer. However, degree of shear thinning of different polymers is different, and understanding this is of great significance to injection molding.

When polymer melt leaves mouth of flow channel, diameter of melt flow is larger than diameter of outlet of flow channel. This phenomenon is called mold release expansion effect.
It is generally believed that this is the expansion effect caused by viscoelastic effect of polymer. Viscoelastic effect affects expansion ratio. Temperature, shear rate and flow channel geometry can all affect expansion effect of melt. Therefore, expansion effect is a reflection of elasticity in melt flow process, this behavior is related to shear stress action of macromolecules along flow direction and normal stress action perpendicular to flow direction.
In pure shear flow, normal effect is small. The greater normal effect of viscoelastic melt, the more obvious mold release expansion effect.
Influence of runner; if length of runner is very short, mold release effect will be affected by entrance effect. This is because melt that enters gate section has to flow, and flow is in an unstable period of speed redistribution. If gate section is very short, melt flow will exit quickly, effect of shear stress will suddenly disappear, velocity gradient will also be eliminated, macromolecules will curl up and produce elastic recovery, which will intensify mold release expansion effect.
If runner is long enough, elastic strain energy has enough time for elastic relaxation. At this time, main reason for effect of mold release expansion is shear elasticity and normal effect when flow is stabilized.

Shear rate has three rheological zones: low shear rate zone. Polymer chain entanglement that is destroyed at low shear rate can be restored in time, so it exhibits Newtonian characteristics with constant viscosity. In mid-shear zone, as shear rate increases, polymer chain segment entanglement is smoothly opened and it is too late to recover.
This helps to stop relative movement between chain segments and reduction of internal friction. Melt viscosity can be reduced by two to three orders of magnitude, resulting in shear thinning. In high-shear zone, when shear rate is very high, viscosity can be minimized and it is difficult to maintain a constant. Entanglement of macromolecular segments has been all straightened under high shear, showing properties of Newtonian fluids. If shear rate is increased again, an unstable flow will appear. This unstable flow will form an elastic turbulent melt, melt will have ripples, and rupture phenomenon is an important sign of melt instability.
When shear rate reaches elastic turbulence, not only will melt not continue to thin, it will thicken instead. This is because melt is broken.

Mechanism that viscosity depends on temperature is that there is a correlation between molecular chain, "free volume" and temperature. When it is below glass transition temperature, free volume remains constant, macromolecular chains begin to vibrate as temperature increases. When temperature exceeds glass transition temperature, large chain segments begin to move, free volume between chain segments increases, force between chain segments decreases, and viscosity decreases. Different polymer viscosity has different sensitivity to temperature.

During injection of polymer melt, whether it is in pre-molding stage or injection stage, melt must be subjected to combined action of internal static pressure and external dynamic pressure. In pressure-holding and feeding stage, polymer generally has to withstand pressure of 1500~2000kgf/cm2, and precision molding can be as high as 4000kgf/cm2. Under such high pressure, free volume between molecular segments must be compressed.
As free volume between molecular chains is reduced, closeness of macromolecular segments increases intermolecular force, that is, viscosity increases.
When processing temperature is constant, compressibility of polymer melt is greater than that of general liquid, and it has a greater impact on viscosity. Due to different compression rates of polymers, sensitivity of viscosity to pressure is also different; the greater compression rate, the greater sensitivity.
Viscosity of polymer will increase due to increase in pressure, which can have same equivalent effect as lowering melt temperature.

Under normal circumstances, viscosity increases with increase of molecular weight. As molecular weight increases, molecular chain segments increase. The slower center of gravity of molecular chain moves, the more opportunities for offsetting relative shift between chain segments, the greater flexibility of molecular chain, the more entanglement points increase, difficulty of chain release and slippage, which increases assist in flow process, also increases time and energy required.
As molecular weight increases, polymer flow is reduced, which makes injection molding difficult. Therefore, some low-molecular substances, such as plasticizers, are often added to high-molecular-weight polymers to reduce molecular weight of polymer, so as to reduce viscosity and improve processing performance. .