The part examined belongs to a kit for a shower slide rail.
The part had already been in production for some time when this work started. The starting problem was the presence of some surface defects, visible above all after the chrome plating the part undergoes.
First, the influence of the machine parameters on the part was analysed, trying to solve the defect through continuous adjustments. Then changes to the part geometry were tested, evaluating their effects by means of process simulation.
Using suitable process parameters, many parts were molded and analysed, and the following were found:
- Air traps and burn marks:
This defect appears as a "Y"-shaped mark. The shape is given by the flow fronts meeting in the last area to fill, where the air therefore cannot vent from the mold. The simulation shows the exact position of this defect!!!!
- Flash:
Thanks to simulation these problems can be predicted too!!!!!
- Visible weld lines:
Numerical simulation predicts weld lines and their condition!!!!
- Mold imbalance:
The mold produces two parts per cycle; by observing some short shots it was possible to notice a filling imbalance. Simulation shows this imbalance very clearly!!!!!
- Wrong sizing of the feed system:
The picture shows unwanted jumps in the path of the melt during filling. A sink mark can also be seen, caused by the large change in size between sprue and adjacent runner. With simulation the feed system can be correctly sized to prevent these anomalies!!!!
- Visible mark at the gate:
With simulation we can check that the gate size is adequate, avoiding gates larger than necessary!!!!
After many tests it was concluded that the defect could not be reduced by changing the machine parameters alone. The use of simulation was therefore essential.
The simulation results shown focus on solving the problem that caused parts to be rejected. All results not directly related to it have therefore not been included in this short description of the study.
First, the accuracy of the software analysis was evaluated: the values previously used on the machine were entered and the results obtained from the program were compared with the real parts. The first comparison was between the short shots and the filling result obtained from the simulation; here are the comparison images:
Next, the position of the weld lines was analysed:
It should be noted that the shape of the weld lines shown in the simulation is affected by the mesh, so they appear jagged. The relevant information from the simulation is not so much their actual shape as their position, which in this case is correctly defined.
Finally, the air traps were observed:
Not all the air traps reported by the simulation are reliable; those shown on the radius are due to the mesh definition.
Next, several analyses were run with modified part geometries, trying to hide the defect as much as possible. The ideas developed in simulation were of two kinds:
- The first simulations varied the thickness on the side opposite the gate, trying to move the defect outwards. However, this shift was not sufficient compared with the large thickness changes made.
- The second geometric change was the introduction of an internal overflow to collect the air and therefore the defect. The overflow was placed exactly where the defect appeared.
In simulation this idea immediately looked good, with a clear improvement in part quality. After evaluating the results, it was decided to adopt this solution to solve, or at least improve, the defect.
The parts molded with the new geometry did improve, even though the defect was not completely eliminated; the "Y" mark and its groove are much less pronounced and negligible compared with the chrome plating to be applied to the part. In the end, parts of excellent quality were obtained, with no more rejects.
After chrome plating the surface defect is completely hidden; the slight groove is visible only to an expert eye and in certain lighting conditions.
Simulation was therefore a useful aid to mold design, making it possible to improve production without wasting time.