The part examined is a cover for a medical device. It must meet fairly high aesthetic requirements, since it is constantly visible during use. The mold studied is a family mold containing both of the two mating cover parts.
The study was divided into several phases. First, simulation was used to choose between different gate locations and to evaluate the machine parameters involved. Then, after a "real" surface sink mark problem was found, a dedicated simulation was run to solve it.
The study is briefly described below, without dwelling on its initial parts.
PHASE 1 - Gate design:
Analyses were carried out on the single parts to identify the best areas in terms of flow balance. Moldflow has specific results for this search.
The following picture shows the areas that can be used as gate locations.
Light blue shows areas where gates cannot be placed because of slides and moving parts, while green shows areas where, for geometric and construction reasons of the submarine gating, gates cannot be placed.
The yellow dots refer to the first gate area used in simulation, while the red dots show the final location chosen after evaluating both the simulation results and the aesthetic and construction constraints of the project.
This choice is mainly due to the fact that the areas marked by the yellow dots produced weld lines in very visible areas, even though those weld lines were found to be less severe than the ones obtained with the chosen gates (red dots). The choice can also be evaluated from the following pictures.
In conclusion, the gates marked by the red dots were chosen for the following reasons:
- The flow balance during cavity filling is much better distributed.
- The weld lines form in more hidden areas, even though they are more severe than with the gates marked by the yellow dots, as shown by their formation temperatures (temperature at flow front).
Filling is fairly well balanced, as shown both by the "fill time" result and by the result showing the pressures inside the mold. There is actually a slight imbalance between the two cavities, which was not considered relevant.
To complete this first phase, carried out alongside the design, the gate dimensions were also evaluated by testing different solutions. An optimal configuration was then chosen, also taking into account the aesthetic constraints on the part. As for the shear stress levels found (stress of the material during flow), the bosses where the gates are connected could be considered rather small in terms of molecular orientation and residual stress in the surrounding area but, because of aesthetic constraints (sink marks on the opposite surface), it was decided not to enlarge them. In addition, no "shear rate" values high enough to suggest material degradation were found.
PHASE 2 - Mold check:
To obtain more reliable results, a complete analysis was used, including the cooling simulation. This also made it possible to check the runner layout and to indicate the coolant temperatures to be used in production.
The runners turned out to be oversized, which increases the cycle time by many seconds, as was also found when molding the parts.
This has a direct effect on production costs, since it increases production times, even considerably, above all for large quantities. This was also confirmed in production, where at ejection the runners were found to be not very solid.
PHASE 3 - Sink mark defect:
After mold sampling started, a problem was found on one of the two parts: an area with a pronounced sink mark. The defect was then checked with the simulation too, using a result specifically created to identify the areas where sink marks may occur.
The previous picture shows the "sink index" result, which represents on a colour scale from blue to red the areas with increasing probability of surface shrinkage. As can be seen, there is an area, highlighted in red, in the corner near the circular hole: exactly where the defect actually appears.
Since the sink marks in these areas were considered unacceptable in production, even though they were in a relatively hidden area, it was decided to modify the runners to reduce the problem. The mold was therefore modified as shown in the next picture.
Before modifying the mold, simulation was used to check the proposed solution. The clearest result came from comparing the "sink index" results of the initial and modified solutions.
The colour scale is the same in both pictures, as are the parameters used for the process simulation.
To conclude this phase, the mold balance in the new configuration was checked and found to be very good, as shown for example by the pressures in the two cavities in the following pictures. A further check was also made on the weld lines. It was therefore possible to verify that the tested change did not cause other problems.
