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# Thermoplastic material characteristics

The important material characteristics to consider when selecting a thermoplastic material grade are described.

## 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, enabling 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 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 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 will decrease the frozen layer thickness. Decreasing the frozen layer reduces shear stress because flow constriction is less. This results in less material orientation during flow.

## Thermal properties

The specific heat (Cp) of a material is the amount of heat required to raise the temperature of a unit mass of material by one degree Celsius. Essentially, it is a measure of the ability of a material to convert heat input to an actual temperature increase which is measured at atmospheric pressure and a range of temperatures up 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:

• Each row of the table shows the specific heat data at a given temperature.
• T is the test temperature, and the unit of measure is C, Celsius.
• Cp is the specific heat at the given temperature. The unit of measure is J/kg-C, joules per kilogram Celsius.
• The Heating/cooling rate is the rate at which the material was heated or cooled when tested. Typically it is cooled and this is represented by a negative value, usually -0.3333. The unit of measure is C/s, the temperature change in degrees Celsius per second.

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 also shows the thermal conductivity data of the material in tabular format, as follows:

• Each row of the table shows the thermal conductivity data at a given temperature.
• T is the test temperature. The unit of measure is C, Celsius.
• k is the thermal conductivity at the given temperature. The unit of measure is W/m-C, watts per meter Celsius.
• The Heating/cooling rate is the rate at which the material was heated or cooled when tested. Typically this value is zero. The unit of measure is C/s, the temperature change in degrees Celsius per second.

## 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 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 reciprocal seconds and shows the viscosity at the temperature that is specified in parentheses.

## pvT data

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.

## Shrinkage

As plastics cool, volumetric shrinkage causes their dimensions to change significantly. The main factors that affect shrinkage are cool orientation, crystallinity, and heat concentrations.

## Optical properties

Transparent plastic under stress can exhibit stress birefringence, where the speed of light through the part depends on the polarization of the light. Birefringence can result in double images and the transmission of unevenly-polarized light. Some materials are more prone to stress birefringence than others.

## Composite materials

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 to 50 percent of fibers by weight. These are regarded as being concentrated suspensions where both mechanical and hydrodynamic fiber interactions apply. In composites that are injection molded, the fiber orientation distributions show a layered nature and are affected by the filling speed, the processing conditions, and the behavior of the material.

## Environmental Impact

Different materials can have different environmental impacts. The polymer family to which a material belongs to can provide an initial indication of processibility 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. Based on the predicted Injection Pressure and Cooling Time for a suite of part geometries and thickness, an Energy usage indicator has been developed for each material in the thermoplastic material database. This provides an indication of the relative energy requirements to produce a part from any given material.

The Resin identification code and the Energy usage indicator are both stored in the thermoplastic material data.