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| Heat Transfer Modes--three ways of exchanging energy |
| Published:2009-8-3 21:27:34 |
There are three ways of exchanging energy between objects of different temperatures：
Conduction
Conduction is solid phase contact heat transfer．Conduction is the primary way energy moves through plastic sheet and metal molds．
Three thermal properties are important in conduction：
·Density．
·Specific heat，heat capacity or enthalpy，and
·Thermal conductivity．
These properties have been reviewed．In addition，thermal diffusivity is important in time-dependent heat conduction．Thermal diffusivity was also reviewed．
Convection
Convection is fluid phase contact heat transfer．Throughout the thermoforming process．the sheet contacts ambient air．
Energy is transferred when the air temperature difiers from the sheet temperature．Energy transfer depends on the extent of air movement．As expected．energy transfer is low in quiescent air and relatively high when the air is actively moved across the plastic surface．
The proportionality between thermal driving force or temperature differential and the amount of heat transferred is called the
onvective heat transfer coefficient．Convection is important when water mist or fog is used to cool the free surface of a
formed Otherwise．the effect of convection on overall heat transfer is secondary to conduction and radiation．
Radiation
Radiation is electromagnetic energy interchange between an energy．Source or hot element and an energy sink or cold
element．Radiation pervades nature．Electromagnetic energy is usually characterized by the wavelength of the energy.X．
rays and gamma rays are characterized by very short wavelengths．Ultraviolet rays have wavelengths 1ess than 0.4um．
Visible light wavelength range is 0.4to 0.7um．Near infrared wavelength range is 0.7to about 2um．Far infrared wavelength
range is from about 2um to 8um．Longer wavelength electromagnetic energy includes microwave，short wave radio freq
uency，long radio frequency and ultrasonic frequency．
Heated metal or ceramic surfaces are used throughout thermoforming to radiantly heat plastic sheet．The majority of energy transfer takes place in the 2um to 8um wavelength range，or the，far infrared region．There are several aspects of
radiant heat transfer that req uire careful attention．For example．the efficiency of energy transfer depends on the relative
abilities of the source and sink to transfer
energy efficiently．Absorptivity and emissivity are terms used to describe this efficiency．For most thermoforming
applications．the energy interchange is between the heater surface and the plastic sheet surface．In some cases．energy
is transmitted into or through the polymer．In addition．even though the sheet is sandwiched between the heaters．the
interchanging elements are not infinite in extent．As a result，the efficiency of energy interchange depends on geometric
factors as well as material properties．
Efficient heating and cooling of thermoplastic sheet depends on the balance between the rate of energy input to the
sheet surface and the rate of energy conduction from the sheet surface to the centerline．There are two classic cases of
time-dependent conduction that illustrate this． |
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