The rheology of injection molding plastics

The rheology of injection molding plastics

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

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.

The density of plastic be changed during injection molding processing

injection molding products

During injection molding processing, the density of the raw material will be changed with the temperature and compression process. When the temperature is high, the density is small; When the degree of compression is large, the density is large.

While injection molding is finished, if the injection molding parts can be contracted slowly and adequately during cooling. The final density of the injection molding product should return to its stable density at room temperature, and If the cooling is rapid, the result may be the insufficient density of injection molding parts, finally, the resulting in a significant deviation between the actual product size and the design size.

The density of plastic be changed during injection molding processing

Despite considerable improvement in quality standards, variations exist in the Chinese market. 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.

The density of plastic be changed during injection molding processing

The melt viscosity of plastic is mainly related to the activity of the plastic molecular chain and the length of the molecular chain. In addition, it is influenced by the following factors: 

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. 

Degree of compression:

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.

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