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)
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