Calculation of Heat Transfer Coefficient at Polymer-Mould or Polymer-Air-Mould Interfaces

Figure 1: Heat transfer coefficient apparatus

A novel heat transfer coefficent apparatus has been developed (Figure 1). The polymer specimen is sandwiched by two circular plates that are parallel, Figure 1. The top ‘cold’ plate acts as a heat sink, the bottom ‘hot’ plate being temperature controlled by electric heaters. Both the lower and upper plates house heat flux sensors and thermocouples. A Labview^® data logging system is used to log the outputs from the lower and upper heat flux sensors and thermocouples. Good thermal contact with the specimen can be maintained across a wide range of specimen thicknesses as the instrument has an adjustable upper plate.

The baseline thermal resistance R_Baseline for the instrument was determined, with the instrument set to the test temperature, by placing the upper and lower plates in contact with each other. Instrument thermal resistances due to plate conductivities and interface heat transfer coefficients are accounted for using this baseline.

The test specimen was then inserted between the 'hot' and 'cold' plates and the 'hot’ plate was set to the required test temperature. The apparatus was left for at least 4 hours to achieve steady state conditions and test temperature assessed by the constancy of the temperature and heat flux readings.

The heat transfer coefficient h (W/(m2 K)) at the interface of two surfaces in contact is given by:

where T₁ (K) is the temperature at the ‘hot’ interface side and T₂ (K) is the temperature at the ‘cold’ interface side and q is the heat flux across the bottom plate. Typical values for heat transfer coefficient at polymer-air-mould interfaces are given in Figure 2:

Figure 2: The effect of increasing thickness of an air gap on the heat transfer coefficient of a polymer-air-metal interface:
a comparison of measured and calculated (using thermal conductivity values for air) values 

Background (page 3)