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Modelling Heat Transfer in Polymer Processing

 

Modelling can help predict cycle times in injection moulding, which can save companies money if predictions of reduced cycle times can be realised. A reliable model with reliable data are needed to achieve this goal.

Figure 1 shows how thermal conductivity values affect the "time to freeze" of an example part using Moldflow. A 15% change to the thermal conductivity data led to a 13% change in "time to freeze". A recent study at NPL showed that uncertainties in thermal conductivity of around 16% can be expected. Massive uncertainties (up to 50%) can result from taking data from the literature without taking account of differences in temperature, pressure or grade. NPL have also shown that uncertainties can be reduced by a factor of two (down to 8%) by performing measurements under pressure. This illustrates how important it is to reduce uncertainties in thermal conductivity data as much as possible to obtain reliable simulations of processing.

Modelling can also help make sure that a measurement does not disrupt what is being measured - a particular issue in the measurement of temperature in processing and in the measurement of thermal conductivity of plastics.

In the example in Figure 2 a thin layer representative of a metallic thermocouple is shown to affect the temperature within a specimen of polypropylene by more than 10oC over a significant proportion of the material as it cools. This is because the metallic layer has a much higher thermal conductivity than polypropylene and consequently acts as a significant heat transfer path for heat loss from the polypropylene.



Figure 1. The relationship between time to freeze a T-piece, as predicted by Moldflow, and thermal conductivity of the polymer (red points: standard thickness moulding, blue triangles: half-thickness moulding)


Example of therMOL simulation of a PP pipe

Figure 2. The effect on the temperature field of an insert, representative of a thermocouple, in a polypropylene specimen: temperature difference with and without the insert after 700 seconds cooling from 250 oC (using TherMOL, an NPL software package). TherMOL Software

 

Updated 7 May 2010