For conventional CAE results, only sutures and encapsulation are directly linked to cosmetic defects. Others such as flow marks, impressions, etc. do not have corresponding results, so this article will use CAE analysis and field experimental results to share with readers how to judge such appearance defects.

Before that, we have to figure out how these defects occur? What are factors related to it? In client projects I have participated in, I sometimes encounter some interesting phenomena. If you are a product designer, when you are told that your product has appearance defects, you may wonder whether mold design is not done well or molding parameters are not adjusted properly. If you are a mold designer, you may think that product design is unreasonable and mold test parameters are incorrect; if you are a mold tester and you fail to adjust them for a long time, you may think that it is a problem with mold design and product design. Engineers in each position have their own technical theories, and final result is to lead to T1, T2, T3...
In fact, from perspective of product design, mold design, and on-site mold testing, factors that affect product appearance are basically not single. We can refer to contents of Table 1.

Table 1: Factors affecting product appearance
From Table 1 we can see that many common appearance problems are not necessarily caused by a single factor. And our focus is not to shift blame to others, but to find real problem and solve it. Quality of finished products is interrelated with design, materials, and molding parameters. If design is more reasonable, then molding window during mold trial will be wider; if there are complete and scientific mold trial standards and equipment with good performance on site, we can still find correct molding parameters even if design structure is complex and molding window is narrow. CAE is a very important tool in scientific and digital design. If you are a CAE engineer, your problem may be how to judge risk of appearance through results and avoid repeated mold modifications and test trials. This is what this article will focus on.

Advantage of computer mold trial is that it can easily simulate plastic flow process no matter how big product is. By looking at flow results alone, we can determine many problems such as: suture lines, flow stagnation, trapped air, flow imbalance, etc. As shown in Figure 1, flow results can directly determine suture line formed due to thicker flesh on both sides, resulting in melt flow competition. As can be seen from actual sample on the right side of Figure 1, suture lines are in same position.

 

Figure 1: Flow results of suture lines caused by structure and actual sample
As shown in Figure 2, this product has 3 points of pouring. From flow results, we can judge that due to design of number of pours, there will eventually be multiple melt junctions, and customer will also need to increase cost of later spraying.

 

Figure 2: Flow results of suture line caused by amount of pouring and actual sample
Through flow results, you can also directly judge whether thickness distribution of product is reasonable. As shown in Figure 3, due to thin thickness of circular area, when melt passes through this area, flow resistance will increase, so it will tend to flow from both sides of circle, thus forming a back-packing phenomenon. This type of flow behavior can easily lead to color differences in appearance of product.

 

Figure 3: Flow stagnation and encapsulation caused by uneven meat thickness

Shearing results may be a result that CAE users often overlook. I believe readers who have done CAE have seen viscosity curve of material (Figure 4). This is a viscosity curve for a common thermoplastic material. Molten plastics are basically non-Newtonian fluids, which means that viscosity of liquid changes with shear rate.

 

Figure 4: Viscosity curve of thermoplastic materials
Shear force and shear rate can be explained by Figure 5. Figure 5 can be seen as two parallel boards, and middle is filled with molten plastic. At this time, when a rightward force is applied to upper wooden board, speed and force will be transferred from one layer to another in a "relay race", and each transfer will produce a speed difference. In addition, melt has different heat dissipation effects in mold cavity and is divided into a skin layer, a shear layer and a flow center layer. Therefore, there will be a "Fountain Flow" phenomenon in which melt is fast in the middle and slow on both sides.

 

Figure 5: Shear force transfer & fountain flow diagram
After understanding these principles, we can know that based on "fountain flow" behavior of melt, when shear is more severe, speed difference of melt will be greater. Differences in speed and temperature of different laminar flows will be amplified, eventually become defects in appearance. Variable mold temperature technology (RHCM), also known as rapid cooling and rapid heating technology, can improve most appearance defects such as suture lines, flow marks, gloss, color difference, etc., mainly because it changes "fountain flow" behavior of original melt under high temperature, allowing melt to flow more smoothly in mold cavity and temperature of melt welding to be more uniform. So after understanding way melt flows, let's take a look at an example.
For example, cause of stress marks is mainly due to increase in temperature of shear layer, which may soften and melt previously solidified skin layer again, or even burst aligned skin layer to form chromatic stress marks. In CAE, although phenomenon of softening and melting of skin layer cannot be shown, it can be manifested as high shear stress and high flow front temperature. Use these results to determine whether there is a risk of stress marks on product's appearance.

 

Figure 6: Shear stress results and actual sample
In addition to stress marks, there are also flow marks, color differences, and injection marks that cannot be judged by direct results on CAE. We can all use results of shear rate, velocity vector, and wavefront temperature to determine whether there are drastic speed differences and temperature differences, and thereby determine whether there is a problem with design or process. As shown in Figure 7, there is a problem with a circle of traces that look like flow marks. Looking at velocity vector results through CAE, you can see that green area is where velocity is high, and blue area is where velocity is low. Drastic changes in velocity here risk causing appearance defects.

 

Figure 7: Actual sample and velocity vector results
Another example is problem of white marks and color difference on exterior surface. As shown in Figure 8, product always produces a white mark on exterior surface above gate. Through on-site experiments, it was verified that this defect is related to mold temperature and shooting speed. When only adjusting mold temperature, there is a chance to improve defects, but there are still slight white marks (bottom left in Figure 8). If speed is reduced at gate and shear rate is reduced, defects can be perfectly eliminated (bottom right in Figure 8) ).

 

Figure 8: Experimental results of multi-stage setting of mold temperature and shooting speed
Through shear rate results in CAE (Figure 9), settings of firing rate and gate speed reduction were simulated, and results were similar to actual results. That is, the more drastic speed change of appearance surface, the greater risk of producing such appearance color difference.

 

Figure 9: Mold flow analysis results of one-stage shooting speed and gate speed reduction

Many customers will ask a question: Are there fixed judgment standards for these CAE results? For example, shear rate value should be within a certain range before there will be no appearance problems, and wavefront temperature should be within a certain range before there will be any problems. There are rumors on Internet about maximum shear rate values of different plastics. Is it true that as long as CAE results are lower than that value, there will be no appearance problems? Answer is obviously no. CAE results mentioned above such as shear rate, velocity vector, wavefront temperature are all judged through indirect methods and comprehensively, unlike suture results, which can be judged directly. Take stress marks as an example: thickness of molded product, speed of injection speed, as well as temperature of mold and material will all affect shear layer. If your shear rate is high, but mold temperature and material temperature are also controlled at a high and uniform level, or if thickness of product is not very different, and design transition is reasonable, actual injected sample may not have appearance problems.
Therefore, in addition to using software, CAE users should also have certain product design, mold design and molding process capabilities. This will help each CAE engineer establish his or her own result judgment standards. For example, standards for 3C products and home appliances are definitely different. As a tool, CAE software has different values for different users. I hope this article can give some inspiration to engineers. If there is anything wrong, please feel free to discuss it.