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material removal of two previously mentioned milled groove
methods are the same and equal to the bounding box volume of the
whole core inserts in Fig. 2. However, the second method can
reduce the volume of removal if the parallel and zigzag types of
insert are formed by bending a sheet of soft metal material with an
appropriate thickness corresponding to milled groove. Subsequently,
these inserts are fixed on a plate with similar grooves patterns, and
then the whole insert block is milled to obtain the final shape. It is
noticed that there can be three types of milled groove conformal
cooling channels layout: parallel type, zigzag type and spiral type.
This paper mainly focus on the parallel and zigzag type because it is
easier to form the parallel inserts using sheet metal as previously
mentioned for reducing the extra machining cost of U-shape
conformal cooling channels. In this case, the control of mold
temperature can be done by adjusting the position of cooling
channels closer to or farther from the surface of molded part in Z
direction. This method is more convenient for automatic
optimization as the number of variables is reduced.
3. Physical and mathematical modeling cycle-averaged temperature approach is used for mold region and
transition analysis is applied to the molded part.4,6,17
The general heat conduction involving transition heat transfer
problem is governed by the partial differential equation. The cycle-
averaged temperature distribution can be represented by the steady-
state Laplace heat conduction equation. When the heat balance is
established, the heat flux supplied to the mold and the heat flux
removed from the mold must be in equilibrium. Figure 4 shows the
sketch of configuration of cooling system and heat flows in an
injection mold. The heat balance is expressed by equation:
0 mce QQQ ++= (3)
e Q are the heat flux from the melt, the heat
flux exchange with coolant and environment, respectively.
The heat from the molten polymer is taken away by the coolant
moving through the cooling channels and by the environment
around the mold’s exterior surfaces. The heat exchange with the
coolant is taken place by force convection, and the heat exchange
with environment is transported by convection and radiation at side
faces of the mold and heat conduction into machine platens. In
application, the mold exterior faces can be treated as adiabatic
because the heat lost through these faces is less than 5%.
6,18
In fact, the total time that the heat flux transfers to coolant
should be cycle time including filling time tf, cooling time tc and
mold opening time to. By comparing the analysis results obtained by the analytical method using the formula (6) and the analysis
result obtained by commercial flow simulation software, the
formula (6) under-estimates the heat flux value. On the contrary, if
tc in (6) is replaced by the sum of tf
, tc and to, the formula (6) over-
estimates the heat flux from the mold exchanges with coolant. The
reason is that the mold temperature at the beginning of filling stage
and mold opening is lower than other stages within a molding cycle.
The under-estimation or over-estimation is considerable when the
filing time and mold opening time is not a small portion compared
to the cooling time, especially for the large part with small
thickness. For this reason, the formula (6) is adjusted approximately
based on the investigation of the mold wall temperature of
rectangular flat parts by using both practical analytical model and
numerical simulation. From the formula (11), it can be seen that the cooling time only
depends on the thermal properties of a plastic, part thickness, and