Flow characteristics of polymers during molding
The flow characteristics of polymers during the molding process refer to the behavior of the polymer melt as it flows under pressure through channels such as the barrel, runners, and cavity. This characteristic directly impacts the filling, holding, and cooling stages of the molding process, determining the dimensional accuracy, surface quality, and internal structure of the plastic part. The flow characteristics of polymer melts differ significantly from those of low-molecular-weight fluids, exhibiting viscoelastic and non-Newtonian properties and being significantly affected by factors such as temperature, pressure, and shear rate. In-depth research on the flow characteristics of polymers is crucial for optimizing molding process parameters and designing appropriate mold structures.
The viscosity of a polymer melt is its most fundamental flow characteristic, manifested as the internal friction between molecular chains during flow and typically measured by viscosity. The higher the viscosity, the greater the melt’s resistance to flow, making it more difficult to fill the mold cavity. Conversely, the lower the viscosity, the better the flow. Polymer viscosity is significantly affected by temperature. As the temperature rises, the thermal motion of the molecular chains intensifies, the distance between molecules increases, the internal friction decreases, the viscosity decreases, and the flow improves. For example, polycarbonate (PC) has a high viscosity and poor flow at 220°C. When the temperature rises to 260°C, the viscosity decreases significantly, allowing it to smoothly fill thin-walled mold cavities. However, different polymers have varying degrees of temperature sensitivity. Polyamide (PA) and polyethylene (PE) are more temperature-sensitive, and increasing the temperature effectively improves their flow. In contrast, polyoxymethylene (POM) and polyvinyl chloride (PVC) are less temperature-sensitive, and excessive heating can lead to degradation. Therefore, during the molding process, the barrel temperature must be appropriately adjusted based on the temperature sensitivity of the polymer to maintain its viscosity within an appropriate range.
Shear rate significantly affects the viscosity of polymer melts, causing them to exhibit non-Newtonian behavior, where viscosity changes with shear rate. Most polymer melts are pseudoplastic fluids, whose viscosity decreases with increasing shear rate, a phenomenon known as shear thinning. For example, polypropylene (PP) has high viscosity at low shear rates. However, when the shear rate is increased (e.g., by increasing the injection speed), the molecular chains align along the flow direction, reducing intermolecular entanglement, resulting in a decrease in viscosity and enhanced fluidity. This shear thinning property is crucial for polymer molding. When filling thin-walled or complex cavities, increasing the injection speed (i.e., increasing the shear rate) can reduce the melt viscosity, ensuring smooth cavity filling. When filling thick-walled areas, reducing the shear rate can avoid flash caused by excessively low viscosity. However, it is important to note that excessively high shear rates can lead to localized overheating of the melt, potentially causing degradation. Therefore, the shear rate must be appropriately controlled based on the cavity structure.
The elasticity of polymer melts is another key characteristic that distinguishes them from low-molecular-weight fluids. This is manifested by the storage of elastic potential energy during melt flow due to molecular chain stretching and orientation. When flow conditions change (e.g., when leaving the runner or entering the mold cavity), this elastic potential energy is released, causing the melt to expand or contract. The most common elastic phenomena include the inlet effect and die swell. The inlet effect occurs when the melt enters a smaller runner from a larger cross-sectional runner (e.g., from a barrel into a nozzle), where the molecular chains stretch and orient due to the sudden contraction, resulting in a pressure drop. Die swell refers to the cross-sectional expansion of the melt after it exits the mold gate due to elastic recovery. The degree of expansion is related to the melt’s elasticity. For example, polystyrene (PS) melts have strong elasticity and a high die swell rate. When molding thin-walled parts, the impact of this expansion on part dimensions must be considered, and gate size must be appropriately reduced to compensate for this expansion. Elasticity can also lead to unstable flow (e.g., melt fracture) during melt flow, manifesting as surface defects such as ripples and roughness. Therefore, the shear rate must be controlled within the stable flow range during molding.
The molecular weight and molecular weight distribution of a polymer also affect its flow properties. Generally speaking, higher molecular weights lead to longer molecular chains, more severe intermolecular entanglement, higher melt viscosity, and poorer flow properties. However, excessively high molecular weights can also increase melt elasticity, making it more susceptible to phenomena like die swell. Polymers with a broad molecular weight distribution, due to their varying molecular chain lengths, are more sensitive to shear rate, resulting in more pronounced improvements in flow properties at high shear rates. For example, low-density polyethylene (LDPE) has a broad molecular weight distribution, making it easier to control flow properties by adjusting shear rates during injection molding. Furthermore, the molecular chain structure (e.g., branches and polar groups) also influences flow properties. Polymers with long branched chains (e.g., LDPE) exhibit better flow and elasticity than polymers with linear chains (e.g., high-density polyethylene (HDPE)). Therefore, when selecting a polymer material, the impact of molecular weight and structure on flow properties should be comprehensively considered based on the molding process and part performance requirements.
The flow characteristics of polymers during the molding process are also affected by mold structure. The cross-sectional shape, size, and length of the runners, as well as the design of the gate, all alter the melt’s flow resistance and shear conditions. A circular runner offers minimal flow resistance, which helps reduce melt viscosity. Increasing runner length increases pressure loss, requiring higher injection pressure to ensure filling. A gate that is too small increases shear rate and may cause melt fracture, while a gate that is too large reduces shear rate, affecting melt fluidity and cooling rate. Therefore, mold design must be tailored to the polymer’s flow characteristics. For example, for polymers with high viscosity and poor flow (such as PC), larger runner and gate dimensions should be used to reduce flow resistance. For highly elastic polymers (such as PS), abrupt changes in runner cross-section should be avoided to minimize the effects of inlet effects and mold swell. By optimizing mold structure and process parameters, the polymer’s flow characteristics can be fully utilized to ensure part quality.
