Analysis of Injection Molded Mobile Phone Back Case

Case sharing: Machine waveform curves that are easily overlooked

The design of injection molds is one of the key steps in manufacturing high-quality plastic products. A good mold design can ensure the quality and production efficiency of plastic products. In this example, we will introduce the injection mold of a phone case as an illustration.

Case introduction
Finished product size: length 145, width 75, height 5 (mm)
Finished product thickness: average thickness 2 (mm)
Sprue system: cold runner plastic
Material used: PC

Analysis focus

As shown in Figure 1, this product is a mobile phone back case with a thinned design at the edge of the camera hole.

During the on-site mold trial, the customer discovered that there was a dent problem on the back of the gate of the product in Figure 2, and there were obvious suture lines in the holes in Figure 3.

Defects seriously affect the appearance quality and cannot pass acceptance. On-site attempts were made to increase the injection rate, injection pressure, and holding pressure, but could not be effectively improved.

Figure 1: Product thickness distribution

Figure 2: Dent appears on the back of the gate

Figure 3: Obvious suture line at the edge of the hole

Analysis of idea

Collect on-site molding machine information, machine waveform curves, trial molding parameter tables, etc. as much as possible to simulate an environment close to on-site trial production. It can be seen from the machine waveform curve in Figure 4 that the injection pressure curve has reached the upper limit of the injection pressure during the filling stage, causing the initial injection speed to begin to decrease rapidly, that is, the actual injection rate is not filled according to the machine setting.

The injection speed in the second half is too low, which means that the heat exchange with the mold during the filling process is accompanied by the lack of rapid replenishment of subsequent hot materials, which can easily lead to serious heat loss of the front-end melt and difficulty in flow. In addition, it can also be observed that after VP switching, the actual pressure curve does not act according to the set high holding pressure, but is releasing pressure. The problem with the machine causes the effect to be opposite to expected.

The mold flow analysis focused on analyzing and verifying the changes in melt temperature under the current abnormal injection process, and considering that the edge of the product hole is thinner than the main plane, whether there is a hysteresis phenomenon, which can further reduce the front-end melt temperature. The welding temperature of the suture often determines its appearance quality. At the same time, pay attention to the shrinkage distribution near the gate during the pressure holding stage.

Figure 4: Abnormal on-site machine waveform curve

Figure 5: Wavefront temperature results and suture line position.

In the results, it can be found that the suture position where the hole is formed is consistent with the actual position. At the same time, it is found numerically that the suture line meeting temperature is low.

Figure 6 shows the melting zone results.

The gate is basically solidified after 1 second of holding pressure. The on-site curve chart shows that the actual holding pressure within 1 second of holding pressure is 135~30MPa.

Figures 7 and 8 show the dent results and volume shrinkage rate during the pressure holding stage (area higher than 4.6%). Due to the premature solidification of the gate, the area near the gate cannot effectively hold pressure, and the post-shrinkage causes dents.

Figure 6: Melt zone results (gate basically solidified)

Figure 7: Comparison between dent results and the actual object

Figure 8: Volume shrinkage higher than 4.6% results

Improvement direction and summary

Through the verification of the above results, the root cause of the dents and sutures is that the actual injection speed is too low, resulting in serious melt temperature loss, causing the suture line to meet at a too low temperature, the gate to be solidified prematurely, and the product to be unable to effectively maintain pressure. For most on-site workers, they often neglect to check the waveform curve of the machine. In fact, the performance of the machine has a huge impact on the process, which directly limits the size of the process window for product molding.

Therefore, in this case, the process end cannot effectively adjust the current product status. The best way to improve it is to check and repair machine problems or replace the machine with one that can support high speed, high pressure, and high response. Secondly, the mold design should optimize the gate and runner size to extend the Holding time and reducing pressure loss.

The above information comes from smartmolding ; if you have any related needs for plastic injection molds, you are welcome to contact us. Plastic Injection Mold JBR (JinBaoRong）Precision Industry Product Co., Ltd.