Thin and light tablet computer products are current trend, plastics with glass fiber (PC+GF) and appropriate product design can maintain required rigidity of product. Most tablet computer products are decorated using an in-mold roller (IMR) process. However, IMR process is prone to product defects such as ink flushing and stress marks. Therefore, Acer used design of experiments (DOE) method of Moldex3D expert analysis module to examine problems in IMR process, optimize process conditions and design.

Main challenges faced in this case are "back cover of thin tablet computer product has ink flushing (Figure 1)", "there are obvious stress marks near gate" and "product thickness cannot exceed 0.8mm".

 

Figure 1: There is ink flushing on the back cover of tablet computer product.
In response to above-mentioned challenges, Acer decided to use Moldex3D experimental design method to find high-quality gate design and process. Benefits are as follows:
Successfully optimized gate design, reduced shear stress and solved ink flushing problem;
Thickness of product is reduced by 48%;
Product weight reduced by 40%.

Original product of ultra-thin tablet computer in this case has partial ink flushing at gate position (Figure 2). Purpose of this case is to solve this problem. Acer found through Moldex3D that product had high shear stress (Figure 3).

 

Figure 2: Original gate design
Acer set up 11 sensing nodes at product gate position (Figure 3), then used Moldex3D flow analysis function to detect shear rate at each node, analysis results were compared with experimental results. It was found that shear rate at defective location of product was higher. Therefore, ink flushing problem can be corrected later based on shear rate analysis results.

 

Figure 3: Shear rate analysis results of original design during filling stage
In order to reduce shear stress from gate to mold cavity, Acer first designed 7 gates with different positions and sizes and conducted Moldex3D flow analysis. After comparing shear rate results, 4 more optimal designs were selected.
Then Moldex3D DOE was used to analyze four optimally designed mesh and gate types, two quality factors of "shear stress distribution" and "inlet gate injection pressure value" were set as small characteristics. Find four control factors and four levels according to Taguchi method, and analyze signal-to-noise ratio (S/N Ratio) (Table 1).

 

Table 1: DOE analysis results of quality factors and control factors
According to analysis results in Figures 4 and 5, signal-to-noise ratio shows that material temperature and flow rate are main controlling factors that affect ink flushing phenomenon.
Filling -Shear rate distribution (Mpa)

Factor Response Plot

 

Figure 4: Signal zz noise ratio of shear stress distribution during filling stage
Spure injection pressure value (Mpa)

Factor Response Plot

 

Figure 5: Signal-to-noise ratio of inlet injection pressure value during filling stage
Moldex3D expert analysis module also provides high-quality process conditions (Figure 6). After comparing the worst and best groups, Acer found that the worst group had a significantly steeper shear rate curve at gate edge, while the best group had a fairly flat curve (Figure 7). Acer then changed gate design and remanufactured product based on this analysis result, successfully solving the ink flushing problem (Figure 8).

Figure 6: Comparison of best and worst groups

 

Figure 7: Shear rate curves for the best and worst groups

 

Figure 8: After optimization of product, ink flushing problem has been greatly improved.

Virtual mold trial function provided by Moldex3D can help Acer quickly find high-quality mold designs in a short period of time, and enable processes that were previously considered to produce failed products to produce qualified products. Thickness of lower cover of original tablet computer was 1.3~1.55mm. After optimization to 0.8~0.95mm, weight of product can be reduced by 24%~40%; thickness of product has been reduced by 26.9%~48.4%. This achievement has brought intuitive economic benefits to Acer and also enhanced industrial competitiveness.