It is important to consider the following characteristics of material when selecting a thermoplastic material grade.
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.
The extent of crystallinity is a function of temperature and time. Rapid cooling rates are associated with lower levels of crystalline content. Lower levels of crystalline content are associated with rapid cooling rates. In injected molded parts, thick regions cool slowly relative to thinner regions, and therefore have a higher crystalline content and volumetric contraction.
The mold temperature is the temperature of the surface of the mold that comes in contact with the polymer. Mold temperature affects the cooling rate of the plastic, and it 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. Additionally, a hotter material decreases the frozen layer thickness. Decreasing the frozen layer means that shear stress is less since the constriction to flow is less. This results in less material orientation during flow.
The Specific heat (Cp) is the amount of heat required to raise the temperature of a unit mass of material by one degree centigrade. It is essentially a measure of the ability of a material to convert heat input to an actual temperature increase. It is measured at atmospheric pressure and a range of temperatures, to the maximum processing temperature of the material.
The Thermal Properties tab of the Thermoplastic material dialog shows the specific heat data in tabular format, as follows:
The thermal conductivity (k) of a material is the rate of heat transfer by conduction per unit length per degrees Celsius. Thermal conductivity is a measure of the rate at which a material can dissipate heat. This rate is measured under pressure and at a range of temperatures. The unit of measure is W/m-C (watts per meter Celsius).
The Thermal Properties tab of the Thermoplastic material dialog box also shows the thermal conductivity data of the material in tabular format, as follows:
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 decreases, indicating a greater ability to flow under an applied pressure. The material database provides a viscosity index for materials, in the Rheological Properties tab, to enable you to compare ease of flow. The viscosity index assumes a shear rate of 1000 1/s and shows the viscosity at the temperature that is specified in parentheses.
Autodesk provides pvT models to account for material compressibility during a Fill or Fill+Pack analysis. A pvT model is a mathematical model using different coefficients for different materials, providing a curve of pressure against volume against temperature.
An analysis based on pvT data is more accurate but the iterations for temperature and pressure at each point in the model increase computational intensity. However, this suits complex models that have sudden and large changes in thickness.
Composite materials contain fillers that are added to polymers for injection molding. Fillers increase the strength of the polymer and help ensure that good quality parts are produced. Most commercial composites contain 10 - 50 percent of fibers by weight. These conditions are regarded as being concentrated suspensions where both mechanical and hydrodynamic fiber interactions apply. In composites that are injection molded, the fiber orientation distributions shows a layered nature. The filling speed, the processing conditions, and the behavior of the material affect the fiber orientation distribution.
As plastics cool, volumetric shrinkage causes their dimensions to change significantly. The main factors that affect shrinkage are cool orientation, crystallinity, and heat concentrations.
Different materials can have different environmental impacts. The polymer family a material belongs to can provide an initial indication of processability and potential recyclability of a material. The Resin Identification code of a selected material is provided to help identify the polymer family.
Minimizing the energy consumption of the injection molding process provides both cost and environmental benefits. An Energy usage indicator has been developed for each material in the thermoplastic material database. The indicator is based on the predicted Injection Pressure and Cooling Time for a suite of part geometries and thickness.
Both the Resin identification code and the Energy usage indicator are stored in the thermoplastic material data.