Understanding Plastic Flow Properties

Plastic flowability refers to the ability of plastic melt (or dispersion) to fill the mold cavity under certain temperature and pressure conditions during the molding process. The quality of flowability significantly affects many parameters in the molding process, such as molding temperature, pressure, the dimensions of the mold gating system, and other structural parameters. When determining the size and wall thickness of components, the impact of flowability must also be taken into consideration.

In the industrial context, a standardized approach is adopted to assess the flowability of plastic melts. Common methods for assessing thermoplastic melt include the melt index test and the spiral flow length experiment. The melt index test involves placing the tested thermoplastic material in a standard device and, under specified temperature and constant load conditions, determining the weight of the melt flowing through a standard capillary tube within a defined time (typically 10 minutes). This weight, referred to as the melt index and denoted by the unit g/10 min (commonly symbolized as MI), serves as a parameter for quality control in thermoplastic materials and for establishing process conditions in thermoplastic molding.

For a given type of plastic, the melt index can be used to compare the size of its molecular weight, serving as a quality control measure during production. Generally, for plastics with a consistent chemical structure, a smaller melt index corresponds to a larger molecular weight, resulting in improved properties such as tensile strength, hardness, toughness, and aging stability. Conversely, a larger melt index indicates a smaller molecular weight, contributing to better processability during molding.

The ratio of the distance plastic flows to the end of the mold to the average mold cavity thickness is known as the "aspect ratio" or "flow length ratio." In other words, it represents the distance plastic travels from the nozzle to the end of flow relative to the average mold cavity thickness.

Plastic flowability is closely related to viscosity; lower viscosity corresponds to better flowability, and consequently, longer flow distances. As the flow distance increases, the temperature of the material decreases, viscosity deteriorates, and filling becomes more challenging.

The aspect ratio of plastic flow directly affects the number and distribution of injection points for plastic products, concurrently influencing the thickness of plastic products.

Different plastics exhibit varying aspect ratios, typically with smaller aspect ratios indicating poorer flowability. While calculating the aspect ratio is generally unnecessary for designing small molds, it becomes crucial for larger molds and should not be overlooked.