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Thermal expansion of a nozzle cause by melt plastic flow
will result in size variation, even exceeding mechanical load
limit thus deformation. When design a nozzle, precise
calculation and modification are necessary for assembly length
and operation length, where the operation length is the total
length of the assembly length and the thermal expansion.
Nozzle length is calculated as: /[1 ( )] dF dF L LTT α =+− (mm),
where LF is the mold dimension (mm) in room temperature;
Ld is the nozzle dimension in room temperature; is the
thermal expansion coefficient (for steel, between 20o
C and
200 o
C, is 13×10-6
); Td is nozzle operation temperature (
o
C);
TF is mold operation temperature (
o
C).
The design process of an equilibrium runner is shown
below:
Step1. Determine the arrangement and the structure of
the runner plate by cavity configuration;
Step2. Select an appropriate section shape of the branch-
runner by relevant knowledge of the runner plate;
Step3. Transfer standard runner plate model into model
library;
Step4. Determine the dimensions of the major runner
structure;
Step5. Determine the dimensions of each branch-runners
layer;
Step6. Check the runner dimensions and equilibrium, and
determine the optimum runner section dimensions;
Step7. Modify the standard hot-runner plate model;
Step8. Complete the hot-runner plate model and save it.
IV. KNOWLEDGE CONNECTION OF TEMPERATURE
SIMULATION AND ANALYSIS OF HOT-RUNNER
A. Knowledge Expression of Hot-runner Temperature
Control
Even temperature distribution on hot-runner plate is one
of the key factors to guarantee a successful application of the
hot-runner technology. Therefore, heating elements should be
located on both sides of the hot-runner as evenly as possible.
Thermal expansion factors of the runner plate also need to be
considered. Heating power of a hot-runner plate is calculated
as: Where, P is the required power, kW;
θ is the required
temperature of the hot-runner plate, o
C; assuming temperature
base point is 40 o
C; m is the mass of the hot-runner plate, kg; t
is the hours of heating (from 40 o
C toθ o
C), h;
η is the
efficiency, which is determined by the assembly situation and
the heat insulation situation between the heater and the hot-
runner plate. Usually,
η is between 0.2 and 0.3.
B. Hot-runner Temperature field Simulation Analysis
Hot-runner Temperature field analysis requires the
establishment of a mathematical model of a transient
temperature field. The state of temperature during a plastic
injection process is very complicated, so that in a real hot-
runner temperature field analysis, four aspects of the heat
exchange need to be considered, namely, internal heat
exchange of the melt, heat change between the melt and the
hot-runner system, internal heat exchange within the hot-
runner system, and heat exchange between the hot-runner
system and the coolant. In order to simplify the design model,
these assumptions should be made before analysis:
(1) No variation of plastic melt character, i.e. the density,
the specific heat at constant pressure and thermal conductivity
coefficient are constants.
(2) Heaters on the hot-runner plate have been located on
both sides of the runner plate symmetrically. The heater
evenly heats up along the outside of the runner, and the
ambient temperature of the melt in the runner is evenly
distributed. In all geometrical models, melts form
axisymmetric objects about the runner axes.