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Thermal simulation of Mold Temperature Control under Induced Heating

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Applications

Introduction

In injection molding tools, the common employment of Electromagnetic surface induction heating instead of conventional volume heating, is due to many advantages: it provides a rapid selective heating time and a reduced cooling time.
Although many studies have discussed the influence of induction heating rate by major factors, such as coil design, number of coil-turns, working frequency, and heating distance, few studies have investigated other crucial factors, such as the thickness of the heated target and the position of the induction coil. In this study, the effects of the design and the position of the used inductors were investigated to control the surface mold temperature. To achieve such investigation, an electro-thermal simulation using EMS software is conducted to study the High-Frequency Proximity Heating.

What is high-frequency proximity heating

High-frequency proximity heating is frequently used to rapidly heat injection molds. Its principle is founded on the proximity effect between a pair of parallel mold plates (Cavity and core) facing each other with a small gap and forming a high-frequency electric close circuit. High frequency current will then flow at the inner surfaces of each mold insert, thus rapidly heating the mold surfaces by skin effect. The figure below shows a schematic demonstration of the studied phenomenon.

Schematic illustration of the High-frequency proximity heating principle

Figure 1 - Schematic illustration of the High-frequency proximity heating principle [1].

Problem description and design

The studied model consists of two parallel square mold inserts with three circular cross-section channels supporting the current conductors. The inductor design selected in the simulation, shown in figure2, is made of 6 copper tubes hollowed from inside for the cooling need.
Table 1 contains the detailed dimensions of each component.

3D CAD design of the studied model

Figure 2 - 3D CAD design of the studied model.

Table 1 - Components` dimensions

Component Part Dimensions (mm)
Mold plates Length 100
width 100
thickness 32
Mold gap 1
Channel Diameter 16
Channel depth 4
Distance between channels 25
Coil Outer diameter 16
Inner diameter 8

Simulation Setup

The main goal of this analysis is to compute the temperature distribution across each mold plate surface to achieve a better temperature control during the heating mold process. Thus, AC magnetic module of EMS is used coupled to the transient thermal study to model the induced heating treatment.
The following 4 steps are needed for the simulation setup.

1.Select the appropriate materials

The mold plates are made of Stainless steel N700. The corresponding thermal and magnetic properties are detailed in table 2.

 Table 2 - Material properties
Part Material Density  
(début de style de taille 12px K g divisé par m au cube fin de style)
Magnetic permeability  Electrical resistivity 
(?m)
Thermal conductivity 
(W/m. K)
Specific heat capacity 
(J/Kg. K)
Coil Copper (Cu) 8940 0.99 1.71 E-07 400 392
Mold plates Stainless steelN700
(ASTM-A564)
7900 200 7.1 E-07 16 470

2.Electromagnetic Inputs 

The inductor coils are defined as solid coils supporting a maximum current of 600 A rms and a frequency of 70 kHz.

Applied current input for each mold plate
Figure 3 - Applied current input for each mold plate.

3.Thermal Inputs

The mold plates are pre-heated with an initial temperature of 40°C. A thermal convection is applied on the air body at ambient temperature of 25°C with a coefficient set to 10 W/m²C.

4.Meshing

A mesh control was applied to both mold surfaces that are exposed to the proximity heating effect. A fine mesh is needed in these surfaces since eddy currents are mostly located in them.

Meshed model 
Figure 4 - Meshed model

Results

After 15 s of induction heating, the simulation revealed the results shown in the figures below.
The induced current density is visualized in Figure 5. It achieved its maximum across the heated surfaces (figure 5b) between the core and cavity mold plate, which is in a good agreement with the Reference [1] results.

Current density distribution across the heated mold after 15s
Figure 5 - Current density distribution across the heated mold after 15s.
 

EMS allows to compute and visualize the temperature distribution on the induced surfaces, which is showing a good balanced distribution having an average value varying between 80°C and 90°C.

Temperature distribution after 15s of heating 
Figure 6 - Temperature distribution after 15s of heating.
 

A comparison between the obtained and the reference [1] (Experimental and simulation) temperature results at the mold surface center, shows a good coincidence between them. Which confirms the experimental results.

Table 3 - Comparative table between EMS and reference [1] results.
Results Experiment-Ref [1] Simulation-Ref [1] EMS
Temperature at cavity plate center (°C) 83 83.4 83

The figure 7 shows a second comparison between the experimental [1] and EMS simulation results taking into account the temperature variation at the center of the cavity surface plate versus time.

Temperature variation versus time for both Experimental [1] and EMS results 
Figure 7 - Temperature variation versus time for both Experimental [1] and EMS results.

Conclusion

The obtained results show that temperature distribution and its uniformity on the mold plate surfaces were improved by the High-Frequency Proximity Heating (HFPH). They are in a good agreement with experimental measurements. Therefore, application of HFPH can rely on the estimated EMS simulation tool.

Reference

[1]- Chen, Shia-Chung, et al. "Mold temperature control using high-frequency proximity effect induced heating." International Communications in Heat and Mass Transfer 39.2 (2012): 216-223.