Material Applications

Material Characteristics

This topic describes some important material characteristics to consider before selecting a thermoplastic material grade. The material that you select will depend on the characteristics that you require.

Crystalline
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.

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 Centigrade. It is essentially a measure of a material's ability to convert heat input to an actual temperature increase. It is measured at zero pressure and a range of temperatures, or averaged across the temperature range of 50°C to the material's maximum processing temperature.
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. In many cases, the table consists of one row.
• T is the test temperature. 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. It is essentially a measure of the rate at which a material can dissipate heat. It is measured under pressure and at a range of temperatures, or averaged across the material's melt temperature range. The unit of measure is W/m-C, Watts per meter Celsius.
The Thermal Properties tab of the Thermoplastic material dialog shows the thermal conductivity data in tabular format, as follows.

• Each row of the table shows the thermal conductivity data at a given temperature. In many cases, the table consists of one row.
• 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 will decrease, indicating a greater ability to flow under an applied pressure. The material 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, Mold flow 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.

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

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.


Polymer Material Grades Represented:-