Thermoplastic Materials
This topic describes some important material characteristics to consider before selecting a material grade. The material that you select will depend on the characteristics that you require.
Crystallinity The crystallinity of a material identifies the state of the polymer at processing temperatures, and can range from amorphous to crystalline states. Amorphous polymers are devoid of any stratification, and retain this state at ambient conditions. Crystalline polymers have an ordered arrangement of plastic molecules, allowing the molecules to fit closer together. Therefore, they are more dense than amorphous polymers. The rate of crystallinity is a function of temperature and time. Rapid cooling rates are associated with lower levels of crystalline content and vice versa. In injected molded parts, thick regions cool slowly relative to thinner regions, and therefore have a higher crystalline content and volumetric contraction.
Mold and melt temperature The mold temperature is the temperature of the mold where the plastic touches the mold. Mold temperature affects the cooling rate of the plastic, and cannot be higher than the ejection temperature for a particular material. The temperature of the molten plastic is the melt temperature. Increasing the melt temperature reduces the viscosity of a material, which also reduces shear stress. This results in less material orientation during flow. Additionally, the material is hotter which decreases the frozen layer thickness. Decreasing the frozen layer means that shear is less since the constriction to flow is less.
Viscosity The viscosity of a material is a measure of its ability to flow under an applied pressure. Polymer viscosity is dependent on temperature and shear rate. In general, as the temperature and shear rate of the polymer increases, the viscosity will decrease, indicating a greater ability to flow under an applied pressure. The materials database provides a viscosity index for materials, in order to compare ease of flow. It assumes a shear rate of 1000 1/s and indicates the viscosity at the temperature specified in brackets.
PVT data To account for material compressibility during a flow analysis, Autodesk provides PVT models. A PVT model is a mathematical model using different coefficients for different materials, giving a curve of viscosity against pressure against temperature. An analysis based on PVT data is more accurate but computationally intensive—through iterations for temperature and pressure per node. However, this makes it particularly suited to complex models that have sudden and large changes in thickness.
Shrinkage As plastics cool, there is a significant change in their dimension, due to volumetric shrinkage. The main factors that affect shrinkage are cool orientation, crystallinity, and heat concentrations. A Shrinkage Summary Report is provided for each material in the Autodesk Moldflow materials database that contains shrinkage data.
Composite materials Composite materials contain fillers that are added for injection molding. Adding a filler to a polymer can increase the strength of the polymer and ensure that good quality parts are produced. Most commercial composites contain 10-50% fibers by weight, which are regarded as being concentrated suspensions, where both mechanical and hydrodynamic fiber interactions apply. In injection-molded composites, the fiber orientation distributions show a layered nature and are affected by the filling speed, the processing conditions and material behavior.