EL ANÁLISIS DESDE LOS EXPERTOS

The quality of the injection molded components is strongly dependent on process conditions. For this research, the optimal process conditions were obtained with the aid

of DOE (Design of Experiments) and DEA (Data Envelopment Analysis) methods according to basic measurement results.

The mold inserts were made of copper nickel C715 (www.farmerscopper.com) and the design was a plano lens with diameter of 50 millimeters and thickness of 3 millimeters. After initial tuning of the process, five parameters in different levels were set up for a full factorial design of experiments. Seventy-two experiment conditions were listed in Appendix C.

For the plano lens, both the diameter and the thickness of the injection molded part were measured. A micrometer was opted for diameter measurement and a precision indicator was used for thickness measurement on specific position. A precision scale was used for part weight measurement. Every part was measured and the average and standard deviations were calculated for each group under the same process condition.

The measurement data (total weight and its standard deviation) were processed using ANOVA (analysis of variance) built in MINITAB and DEA to obtain the effects of the process variables and the optimal conditions (Refer to Appendix D and Appendix E). From ANOVA results, melt temperature, mold temperature, packing pressure were found to be the most important variables and cooling time, packing time were less important. The conclusion from ANOVA results just narrowed down the critical process parameters for the following study. From DEA results, four process conditions (Condition 12, 27, 32

mold temperature 65 °C (150 °F), cooling time 40 sec, packing pressure 76.3 MPa (35%) and packing time 7.5 sec) was chosen for the following study. The result for total weight vs. standard deviation by DEA is shown below and four optimal conditions are marked on Figure 3.1.

Total Weight vs Standard Deviation

0.0000 0.0050 0.0100 0.0150 0.0200 0.0250 0.0300 0.0350 0.0400 20.1 20.2 20.3 20.4 20.5 20.6 20.7 20.8 20.9 21

Total Weight (grams)

S tan da rd D e vi a tio n

Figure 3.1: DEA method for total weight vs. standard deviation

These basic measurement results provide qualitative analysis and show the process consistency and comparison of the rough dimension of the molded lens to the design. However, they cannot be used to assess the part optical performance due to the rough measurement and different merit functions. The main benefit from these results is

to narrow down the critical process parameters and choose one optimal condition for following studies.

3.2 Surface Geometry and Part Thickness

The surface geometry accuracy on a lens is critical to its optical performance. The deviation of the molded surface with the design surface will introduce unwanted aberrations in an optical assembly. Each single surface geometry and part thickness need to be measured accurately and the aberration from the geometry can be estimated for the following compensation scheme. The surface geometry and part thickness can be measured by two LVDTs (linear variable difference transformers) mounted on the 350 FG machine (Figure 3.2(b) as viewed in the Z direction). The axial movement accuracy of the 350 FG machine is only several nanometers, much higher than the molded surface geometry deviation. The two LVDTs are coaxially mounted and the molded part surface is perpendicular to the direction between two LVDT tips. This setup provides the accuracy for thickness measurement as shown in Figure 3.2.

(a) 350 Machine Frame (b) LVDT setup

Figure 3.2: Thickness and surface measurement setup

The molded lens is held on the machine main spindle and the LVDT setup is installed on the Z slide (as shown in Figure 3.2 (b)). Since the valid measurement range of LVDT is only ±100 µm, the spindle and Z slide moving position for each measurement point should be preset on the estimated spot to prevent probe from over-traveling. The geometry of each surface can be obtained from single surface LVDT measurement data on the same side after removing the tilt error. The thickness can be obtained from both surface LVDT measurement data for the corresponding pair of points. By modifying the part holder and keeping a constant environment condition (temperature, noise etc), the measurement repeatability were maintained to less than 0.4 µm.

Only selected non-ferrous materials can be machined by diamond turning process to optical quality without polishing, so in this dissertation research aluminum and copper

nickel alloy were chosen to fabricate mold inserts. The thermal properties of the mold insert materials are listed in Table 3.1.

Copper Nickel C715 Aluminum 6061 T6 CTE*, linear 250 °C 16.2 µm/m-°C 25.2 µm/m-°C Specific Heat Capacity 0.380 J/g-°C 0.896 J/g-°C Thermal Conductivity 29.0 W/m-K 167 W/m-K

* Coefficient of Thermal Expansion

Table 3.1: Thermal properties of mold insert materials

Due to the different material thermal properties, even under same process condition, the molded parts by using different mold insert materials also will be different in final shape such as part thickness P-V (peak to valley) value. The thickness of the molded plano lenses from nickel inserts and aluminum inserts but under same process condition was measured in the locations listed in Table 3.2. The center of the molded lens was set as origin of the coordinate system and Y axis and Z axis were in the same direction as the machine coordinate system. The measurement area for surface geometry and thickness was limited by the dimensions of the lens holder and LVDT probes, so the area that is close to the edge of the molded lenses could not be measured by current measurement setup. For this study, the radius of the measurement area was 12 mm to avoid the interference between the lens holder and the probes.

Location Y (mm) Z (mm) 1 0 -12 2 -6 -6 3 0 -6 4 6 -6 5 -12 0 6 -6 0 7 0 0 8 6 0 9 12 0 10 -6 6 11 0 6 12 6 6 13 0 12

Table 3.2: Thickness measurement locations

The measurement results from nickel inserts and aluminum inserts are shown in Figure 3.3. The thickness P-V value of the molded plano lenses with copper nickel C715 inserts was about 20% less than that with aluminum 6061 inserts. Since aluminum is easier to machine, aluminum was again chosen as main mold insert material for the following experiments in this dissertation research. The measurement results from different mold materials also show the same tendency for the thickness distribution on the molded lens, therefore, the conclusions from the following experiments which were based on aluminum inserts can be applied for the molded lenses with copper nickel inserts.

Figure 3.3: Thickness measurement comparison between the molded lenses from nickel inserts and aluminum inserts

With the aluminum flat mold inserts, the P-V value of the thickness deviation of the molded plano lens is about 7 µm from the first experiment round under condition 12 (melt temperature 210 °C (450°F), mold temperature 65 °C (150 °F), cooling time 40 sec, packing pressure 76.3 MPa (35%) and packing time 7.5 sec) on all measurement locations. Figure 3.4 shows the thickness measurement results. From the figure, it can be seen that the square area was measured due to the constraints for measurement point selection. The lens compensation scheme that would be implemented was based on this measurement result.

Figure3.4: Thickness distribution on the molded lens