The rheology of injection molding plastics

injection molding products

The rheology of injection plastics refers to the science of studying the flow and deformation characteristics of plastics during injection. Rheology is the study of material flow and deformation behavior. Rheology of injection plastic studies the flow and deformation behavior of plastic under high temperature and high pressure injection molding machine.

Plastic will be dissolved by heat or the pressure provided by the injection molding machine, the elasticity and viscosity of plastic be changed. While liquid plastic is in the flow, the plastic will change its molecular chain’s entanglement state and direction due to tensile stress.

Affected by the rheological properties of plastics, the plastic dissolution rate, deformation rate (or tensile rate) increase and decrease. Therefore, injection molding is a relatively copy of the process; The injection molding master must know skills about the best processing environment of various materials. Of course, the advanced processing equipment and the precise injection mold are also the critical factors affecting the quality and efficiency of injection molding products.

What is the rheology of injection molding plastic?

PLA Materials

The rheological behavior of plastics refers to the relationship between the stress, deformation, deformation, and viscosity of plastics during processing. 

During the injection process, the plastic must pass through the nozzle of the injection molding machine into the mold and fill the cavity in the mold. Therefore, the rheology of injection plastic studies the flow performance of plastic in this process, including the flow speed, flow pressure, extrusion force, expansion coefficient, deformation temperature and so on.

The effect of rheological properties of plastics on the quality of injection molding parts

Blow Molding

If the rheological property of plastic is not good, it may lead to defects in injection molding parts, such as bubbles, shrinkage holes, cracks and so on. Therefore, in the rheological research of injection plastic, it is necessary to consider the type, composition, processing conditions and other factors to select the best injection parameters, to ensure the quality of products and the improvement of production efficiency.

To avoid costly errors, make sure to select a reputable manufacturer that has a history of exporting plastic molding products outside China. Visit the factory or send a local quality-control inspector to inspect the manufacturing process for initial evaluation. If possible, ask for samples.

What is Melt viscosity?

plastic materials

Melt viscosity, also known as fluid viscosity, refers to the resistance of a melted polymer material to flow under an applied force or stress. It is an important parameter in the field of polymer processing and determines how easily the polymer can be processed, shaped and formed.

The melt viscosity of a polymer material is affected by various factors such as temperature, pressure, shear rate, molecular weight, molecular weight distribution, and the presence of additives or fillers. Generally, the higher the temperature, the lower the viscosity, and the easier the polymer can be processed. Conversely, the higher the pressure or the slower the shear rate, the higher the viscosity, and the more difficult it is to process the polymer.

Melt viscosity can be measured using different methods, such as capillary rheometry, rotational rheometry, and melt flow index (MFI) testing. Capillary rheometry measures the pressure required to force a polymer melt through a small-diameter capillary under a constant shear rate, while rotational rheometry measures the torque required to rotate a circular disk in contact with the polymer melt. MFI testing measures the flow rate of the polymer melt through a capillary under a constant load.

Melt viscosity is an important parameter to consider in the design and optimization of polymer processing operations such as extrusion, injection molding, and blow molding. The appropriate melt viscosity is critical to ensure the desired product quality, processing efficiency, and overall economics of the production process.

What are the factors affecting melt viscosity?

The activity of plastic molecular chains

The activity of plastic molecular chains usually refers to their degree and ability to participate in chemical reactions. Plastic molecular chains can change their activity through a series of chemical reactions, such as introducing functional groups at the end of the chain, conducting cross-linking reactions or breaking reactions.

The introduction of terminal functional groups can be achieved by different methods, such as in situ polymerization or post-modification reactions triggered by catalysts. These terminal functional groups can provide the chemical reaction basis for the subsequent functional modification, so as to achieve the control of physical and chemical properties on the surface or inside the material.

Crosslinking reactions can be achieved by introducing a crosslinking agent between or within the molecular chains. These crosslinkers can react with functional groups on the molecular chain to form chemical bonds, thus increasing the cross-linking between molecules and improving the strength and rigidity of the material.

The breaking reaction can be achieved by introducing a chain breaking agent into the molecular chain. These chain breaking agents can react with the bonds on the molecular chain, causing the molecular chain to break, thus reducing the molecular weight and molecular weight distribution, and thus affecting the physical properties and machining properties of the material.

It is very important to control the activity of molecular chains in plastic processing and manufacturing. By adjusting the activity of molecular chains, the material properties can be regulated and optimized, and the high performance plastic materials can be prepared to meet the specific requirements.

The length of the molecular chain

The length of the chain refers to the number of monomers in the plastic chain, which is known as the degree of polymerization. The length of molecular chain is one of the important factors affecting the physical properties and processing properties of plastics. Generally speaking, the longer the length of the molecular chain, the higher the physical properties of the plastic (such as strength, stiffness, heat resistance, etc.), but the processing properties (such as fluidity, fusibility, etc.) will be reduced.

The length of the molecular chain can be controlled by the polymerization conditions, such as the concentration of monomer, the type and amount of catalyst, reaction temperature and pressure, etc. In the polymerization process, with the extension of reaction time, the length of molecular chain will gradually increase. In industrial production, reaction conditions and reaction time can be controlled to achieve the desired molecular chain length.

In addition, for synthetic plastic materials, the length of molecular chains can be affected by the way of processing. For example, processing methods such as stretching, extrusion and injection molding can change the arrangement and direction of plastic molecular chains, thus affecting their physical properties and processing properties. In addition, plastic molecular chains can also be modified by heat treatment, radiation and other ways to change their length and properties.

Melt temperature:

Te higher the temperature of the injection molding machine, the lower the plastic viscosity. 

Degree of compression:

Under compression, the free volume becomes smaller, and the viscosity increases. 

As the shear rate of the injection molding machine increases, the viscosity of plastic will be increased, and the shear force is proportional to the shear rate. When the shear rate is close to zero, the fluid exhibits Newtonian fluid properties (viscosity is not affected by the shear rate).

Molecular Orientation

The limited by the flow field, the molecular chain of the molten liquid produces a difference in the direction of stress, which results in different properties and shrinkage rates of the formed products in different directions.

Generally speaking, differences in molecular orientation, density distribution, and cooling rate will affect the shrinkage rate of the product, and the difference in shrinkage rate of plastic products in different parts is the biggest killer of product deformation.


Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.