Achieving High Quality Surface Finishes with Simulation Analysis

In the last few years, there has been a trend towards thinner and lighter tablets. Proper product design and fiber reinforcement materials (PC+GF) are key factors for reducing the weight while retaining required product stiffness and strength. In-mold roller (IMR) processes are usually used to decorate tablet products, but potential defects such as stress marks and ink wash-out are likely to occur in the IMR process.

Acer used a Moldex3D DOE (Design of Experiment) analysis to analyze possible defects and obtain optimum gate designs and process conditions.

Challenges

  • Clear stress marks around the gate
  • Strict requirement for thickness (not more than 0.8 mm)
  • Ink wash-out on the thin-wall injection molded base case (Figure 1)

The ink wash-out problem on the base case.

Figure 1. The ink wash-out problem on the base case.

Solutions

Moldex3D DOE can be used to achieve optimum gate design and process conditions.

Benefits

  • Reduce the weight of the product by 40%
  • Identify the best gate type to avoid ink wash-out and reduce shear stress
  • Considerably reduce the wall thickness by 48%

Case Study

The purpose of this study was to solve the stress marks and ink wash-out in an ultra slim tablet. Acer detected high shear stress in the design through Moldex3D analysis (Figure 3).

The original gate design.

Figure 2. The original gate design.

Acer fixed 11 sensor nodes onto the gate (Figure 3) and ran Moldex3D Flow analysis to verify the sensors’ shear rate. Following this, Acer then compared it with the experimental results and found that the problematic areas have a higher shear rate. Thus, the results of shear rate analysis might serve as an indicator that could help reduce ink wash out.

The shear rate result of the original design during the filling process.

Figure 3. The shear rate result of the original design during the filling process.

Acer designed seven sets of gates with different dimensions and locations, and ran Moldex3D Flow analysis with the goal of reducing the shear stress that had developed across the mold cavity and the gate. Four designs were selected as the optimal process designs after comparing the results of shear rate.

Acer then carried out a Moldex3D DOE analysis to study the different types of gates and meshes in the four selected designs and subsequently set two quality factors: the sprue pressure and the shear rate during the filling (the smaller, the better). Based on the Taguchi method, four levels and four control factors were chosen for examining signal/noise (S/N) ratio (Table 1).

The DOE method analysis data of the quality factors and the control factors

Table 1. The DOE method analysis data of the quality factors and the control factors

According to the simulation results demonstrated in Figures 4 and 5, the S/N ratios signify that the flow rate and the melt temperature are the main control factors of the ink wash-out issue.

The S/N Ratio data of shear rate distribution at filling stage.

Figure 4. The S/N Ratio data of shear rate distribution at filling stage.

The S/N Ratio data of sprue injection pressure at filling stage.

Figure 5. The S/N Ratio data of sprue injection pressure at filling stage.

Moldex3D Expert also helped identify the best run using the optimum processing parameters (Figure 6). When Acer compared the worst and the best run, it found that, in the worst run, the shear rate curve of the gate area was extremely steep, while the curve of the best run was quite smooth (Figure 7). Based on these results, Acer modified the gate design and then reworked the design, effectively solving the problem of ink wash-out (Figure 8).

A. Mesh B. Melt Temp. C. Mold Temp. D. Max Flow Rate Shear Stress
(Mpa)
Spure injection pressure value
(Mpa)
Worst Run f-1 300 50 25 2.40365 87.3878
Best Run d-3 320 60 30 0.432175 65.6163

Figure 6. Comparison of the worst and best design runs.

Comparison of shear rate curves between the best design and the worst. one.

Figure 7. Comparison of shear rate curves between the best design and the worst.

The ink wash-out problem has been successfully reduced.

Figure 8. The ink wash-out problem has been successfully reduced.

Results

Moldex3D's virtual molding technology allowed Acer to achieve optimal mold designs in the shortest time possible. In a study, the thickness of the base case of the tablet decreased from 1.3–1.55 mm to 0.8–0.95 mm. The weight of the product decreased by 24%~40%, and the thickness of the product was reduced by 26.9%~48.4%. Using Moldex3D solutions, Acer is now able to avoid costly mistakes and gain a competitive advantage.

This information has been sourced, reviewed and adapted from materials provided by Moldex3D.

For more information on this source, please visit Moldex3D.

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