The next step of the design process is to analyze the amount of stress that may develop within each of the prospective mold core inserts based on the results of the injection molding process simulation. This step is incredibly important because the plastic pressure within the mold cavity can be very high, and the placement of the cooling channels is often close to the surface of the mold inserts. If the resulting stresses are too high, it can create fractures within the core inserts which may cause the coolant to leak or the inserts to break. In either case it will reduce the overall life of the mold insert. The use of both flow simulation and FEA stress analysis allows the designer to analyze and identify any potential failure points before the mold is actually built.
The Twelve Channel Core (TCC) study showed high stresses developing at the interface of the inlet channel that feeds the primary cooling channel. The compressive stress in the core due to the plastic pressure approached 400 Mpa which is higher than desirable and may result in a short life span of the conformally cooled insert. High stresses can result in cracks developing within the steel over time. Therefore, the designer will need to determine a design stress for each project based on the type of mold steel, expected life of the insert, and manufacturing method.
For the Four Helical Channel (FHC) design, stresses were even higher at the exit channels for each of the cooling circuits, approaching 500 Mpa. The high stress in this region highlighted a thin steel condition created at the location where the helical cooling channel and exit channel intersected. This region would need to be redesigned if this option is to be further developed.
Next was the Eight Loop Core (ELC). The maximum stress for this core looked much better at around 174 Mpa. It is important to note, that at the top of the core, there are different distances for the heat to travel, so the temperature uniformity might not be perfect, but subtle temperature differences are common within injection molds and are not necessarily a bad thing. A bigger concern with this design option was the amount of support structure that would be required within the cooling channels. The support structure would then have to be drilled out of the inside of the mold, adding extra steps and complicating the build.
The final design was the Two Helix Core (THC). The stress simulation showed stresses of around 264 Mpa, which is higher than the eight loop core, but still under the design stress determined by the mold designer. This core also provides the most uniform mold steel temperature of the four and will not require additional machining operations to remove support structures.
The next step of the process is to compare the cooling efficiency of each mold.