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ABSTRACT: In thermoplastic injection moulding, part quality and cycle time depend strongly on the cooling stage. Numerous strategies have been investigated in order to determine the cooling conditions which minimize undesired defects such as warpage and differential shrinkage. In this paper we propose a methodology for the optimal design of the cooling system. Based on geometrical analysis, the cooling line is defined by using conformal cooling concept. It defines the locations of the cooling channels. We only focus on the distribution and intensity of the fluid temperature along the cooling line which is here fixed. We formulate the determination of this temperature distribution, as the minimization of an objective function composed of two terms. It is shown how this two antagonist terms have to be weighted to make the best compromise. The expected result is an improvement of the part quality in terms of shrinkage
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and warpage. 21511
1 INTRODUCTION
In the field of plastic industry, thermoplastic injection
moulding is widely used. The process is composed of
four essential stages: mould cavity filling, melt packing,
solidification of the part and ejection. Around seventy
per cent of the total time of the process is dedicated to
the cooling of the part. Moreover this phase impacts
directly on the quality of the part [1][2]. As a
consequence, the part must be cooled as uniformly as
possible so that undesired defects such as sink marks,
warpage, shrinkage, thermal residual stresses are
minimized. The most influent parameters to achieve
these objectives are the cooling time, the number, the
location and the size of the channels, the temperature of
the coolant fluid and the heat transfer coefficient
between the fluid and the inner surface of the channels.
The cooling system design was primarily based on the
experience of the designer but the development of new
rapid prototyping process makes possible to manufacture
very complex channel shapes what makes this empirical
former method inadequate. So the design of the cooling
system must be formulated as an optimization problem.
1.1 HEAT TRANSFER ANALYSIS
The study of heat transfer conduction in injection tools is
a non linear problem due to the dependence of
parameters to the temperature. However thermophysical
parameters of the mould such as thermal conductivity
and heat capacity remain constant in the considered
temperature range. In addition the effect of polymer
crystallisation is often neglected and thermal contact
resistance between the mould and the part is considered
more often as constant.
The evolution of the temperature field is obtained by
solving the Fourier’s equation with periodic boundary
conditions. This evolution can be split in two parts: a
cyclic part and an average transitory part. The cyclic part
is often ignored because the depth of thermal penetration
does not affect significantly the temperature field [3].
Many authors used an average cyclic analysis which
simplifies the calculus, but the fluctuations around the
average can be comprised between 15% and 40% [3].
The closer of the part the channels are, the higher the
fluctuations around the average are. Hence in that
configuration it becomes very important to model the
transient heat transfer even in stationary periodic state.
In this study, the periodic transient analysis of
temperature will be preferred to the average cycle time
analysis.
It should be noticed that in practice the design of the
cooling system is the last step for the tool design.
Nevertheless cooling being of primary importance for
the quality of the part, the thermal design should be one
of the first stages of the design of the tools.