Plastic injection molding process is also a rheological process for polymers. Understanding rheological characteristics of plastics suitable for injection molding can provide a deeper understanding of root causes of anomalies and product quality defects during injection molding process, leading to better solutions for practical production problems. Significance of studying polymer rheology lies in: ① It can guide polymerization to produce polymers with excellent processing properties. ② It is important for evaluating polymer processing properties, analyzing processing process, selecting appropriate processing conditions, and guiding formulation design. ③ It provides guidance for design of processing machinery and molds.
We have now gathered some theoretical knowledge relevant to actual injection molding production to equip injection molding practitioners with theoretical knowledge. Please stay tuned for relevant content.
1. Rheological Theory
Rheology studies processing characteristics of materials, such as deformation and flow, after being subjected to stress. These include shear rate, shear viscosity, viscoelasticity, viscous heat, and extensional viscosity. Most molten plastics exhibit pseudoplastic behavior. According to power law, n < 1, when plastics are subjected to shear stress, their viscosity decreases as shear rate increases. This phenomenon is known as shear thinning in polymer materials. A common plastic property indicator provided by manufacturers is melt index (MI). MI value for general plastics ranges from approximately 1 to 25. A higher MI value indicates a lower viscosity and a lower molecular weight; conversely, a lower MI value indicates a higher viscosity and a higher molecular weight. MI value is simply a point on plastic's shear viscosity curve. (Note: Viscosity units are 1 cp = 0.001 Pa·s, cp = centipoise, Pa = N/m²). Other factors that influence plastic properties include molecular weight and distribution, molecular orientation, glass transition temperature, and additives.

 

(1) Molecular weight and molecular weight distribution
One of characteristics of plastics is that they have a large molecular weight. Molecular weight distribution curve and polymerization method and conditions have a close impact on molded products produced. The larger molecular weight, the higher glass transition temperature Tg. Mechanical properties, heat resistance, and impact strength are all improved. However, viscosity also increases with increase in molecular weight, making processing difficult. In terms of molecular weight distribution, short molecular chains affect tensile and impact strength, medium molecular chains affect solution viscosity and low shear melt flow, and amount of long molecular chains affects melt elasticity.
(2) Glass transition temperature (glass transition temperature, Tg)
This means that polymer chain begins to move with large links, that is, it breaks away from rigid glass state and begins to become more ductile. Size of Tg has a great influence on properties of plastics, so it is often an important indicator for judging properties of plastics. In glassy state, it shows a hard property similar to glass, but in rubbery state, it becomes a softer rubber property.

(3) Molecular orientation

Original properties of plastic materials will change with external factors and forces. For example, viscosity of polymer melt (indicating flow resistance of material) increases with increase of molecular weight, but decreases with increase of temperature. Furthermore, molecular orientation caused by high shear stress acting on material will also reduce viscosity of plastic melt.

(4) Influence of additives, fillers, and reinforcing materials on polymers Including stabilizers, lubricants, plasticizers, flame retardants, colorants, foaming agents, antistatic agents, fillers, and reinforcing materials, etc. can be used to change or improve physical and mechanical properties of plastics.

Note:
1. Shear rate (shear deformation rate)
Shear deformation rate refers to rate of shear deformation of an object when it is subjected to external force. Shear deformation refers to change in relative displacement between layers inside object, which is usually measured by shear strain. Shear deformation is ratio of relative displacement of layers inside object, that is, magnitude of shear deformation of object. Shear deformation rate can be used to describe speed of shear deformation of a material under action of external force.
2. Shear viscosity
This is ratio of shear stress to shear rate under steady-state flow conditions. Its unit is Pascals-second (Pa*S). Shear viscosity is a measure of intramolecular friction of a liquid and a specific reflection of viscous flow properties of an object. Shear viscosity is often used in contrast to extensional viscosity. Generally, term "viscosity" refers to shear viscosity.
3. Viscoelasticity
This is combined viscosity and elasticity of a fluid.
During processing, polymers typically transform from solid to liquid (melting and flowing) and then from liquid to solid (cooling and hardening). Therefore, under different processing conditions, polymers exhibit solid and liquid properties, namely, elasticity and viscosity, respectively. However, due to long-chain structure of polymer macromolecules and gradual nature of their motion, deformation and flow of polymers cannot be purely elastic or purely viscous. Plastics exhibiting dual characteristics of elastic solids and viscous fluids in their response to stress are called viscoelastic.
4. Viscoelasticity
Shear heating (viscoelasticity) is principle of shear heating, which occurs when, under shear force, molecules within a material slide relative to each other. Frictional heat generated by this friction causes temperature to rise.
5. Extensional Viscosity
Extensional viscosity is ratio of tensile stress on cross-sectional area of polymer perpendicular to flow direction to tensile strain rate when polymer melt flows out of any type of pipe.