Our goal was to check the influence of a thickness change, even a minimal one, on the filling of a part. The evaluation was made both in simulation and in the actual production of the part, which is a hidden component of a car. Even so, the requirements on the finished product are rather strict and do not allow even slight surface defects.
The part has a constant thickness along the whole circumference, except for a fairly large area obtained by projecting a rectangular shape onto the cylindrical surface, where raised lettering is formed. This area is on the visible side of the cylindrical surface of the part. The thickness reduction is really minimal in absolute terms: just a couple of tenths of a millimetre.
It should be noted that the lettering in this area is not taken into account in the simulation. Contrary to what one might think, modelling such small features does not give a better approximation of reality. The influence on the results would be, at least in this case, irrelevant compared with a considerable increase in calculation complexity and, above all, in meshing work. This explains why the lettering, although present on the real part, is not shown in the pictures.
After the first simulations it immediately became clear that the filling of the part had a problem related to the thickness change, as the following results show.
Simulations were therefore run under different conditions to find a way to fill the mold cavity correctly, acting on the number and position of the gates, on some relevant parameters and on the material used.
Simulation with 2 gates on the moving half
The first simulation used a filled material (glass fibre reinforced nylon) and a standard gate configuration: two submarine gates in the moving half of the mold.
The previous pictures, showing some instants during filling, make clear a non-symmetrical situation at the end of filling caused by the thickness reduction: the last area to fill is concentrated close to it. This is not anomalous given the part geometry, and is not in itself a real problem for the finished product. The problem arises because this asymmetry means that the last area to fill is not on the edge of the part but still inside it, with resulting air traps and possible burn marks caused by air that cannot be vented.
The problem can easily be understood from the previous picture, showing the velocity vectors in the material. At a certain point the velocity increases in the thicker areas, where the material flows more easily than in the thinner end section. As a result, filling closes a small air pocket inside the part. This is also accentuated by the relatively low flowability of the material, due to its filler.
Simulation with 2 gates on the moving half and higher temperatures
Once the problem was identified, the idea was to improve the situation by increasing the overall flowability of the material, to counter the slowdown in the thin end section. This was done by increasing the melt temperature.
The result shows a slight improvement in filling but does not remove the problem. This is because it does not depend on absolute values but on a difference in behaviour between two adjacent areas. Increasing the flowability of the material improved filling in the thin area, but it did so in the rest of the part too, so the same problem remains.
Simulation with 4 gates and higher temperatures
Another solution tested was to act on the gates. In the previous configuration the gates appear to be positioned so as to favour the filling of the wall on the moving half (the thicker one), or so one might think. Different gate layouts were therefore tried. In this case four gates were used.
As the pictures show, filling changed only in the first part of the cylindrical section.
Simulation with 2 gates on the fixed half and higher temperatures
Since four gates did not give the expected results, the two gates pushing the material into the thicker cylindrical section were removed, to favour the thinner upper section as much as possible.
Again, filling changed only in the initial section: after a certain distance from the gates the flow of the material is identical to the previous cases.
Besides the results shown, many other solutions were tried in simulation, including different gate configurations, a different material and different cooling temperature settings. The result was always the same, so these variations are not illustrated.
CONCLUSIONS:
Besides confirming that even a small thickness change can, under certain conditions, have significant effects, a very interesting point is that even when the upper section is favoured with two gates placed directly on it, the flow does not change during the filling of the part. This suggests that, for bodies developed mainly in one direction, when the gate is placed on one of the smaller dimensions, the point chosen on it affects the filling of the part only in the first section.
The problems highlighted were found at sampling: in this specific case the mold geometry had already been defined before the simulation, as shown in the following pictures.
The solution adopted was to remove the thickness reduction, without even considering the changes tested in simulation, since they had proved ineffective; without simulation they would very probably have been tried. This avoided wasting time and money on actually making those changes to evaluate their effects.