Effect of a pressurized cavity on the replication of micro-patterns with injection molding

3.1 Experimental setup

The all electric injection molding machine SE50D (Sumitomo Heavy Industries, Tokyo, Japan) was used for experiments. We used the high hardness urethane O-ring for mold sealing in Figure 2. The pressurization machine is composed of the regulator, booster, check valve, relief valve and the system controller. It fills the inside of the mold cavity at a constant pressure (4 MPa) with a check valve. It exhausts air out of the mold with a relief valve if the air pressure is higher than 4 MPa. Due to the limit in the pressure booster, the cavity air pressure had been tested only up to 4 MPa. In addition, we measured the pressure of the cavity air with a pressure sensor 601A (Kistler, Winterthur, Switzerland) as shown in Figure 3. As it takes about 1 s to fill the mold cavity with pressurized air, the experiments were carried out after 2 s of air supply. The whole setup and the process are illustrated in Figure 4.

Figure 2:

Configuration of the mold: (A) mold; (B) structure of mold and overall schematic of melt flow and cavity air; (C) cross sectional view of the mold with dimensions of air runner and air flowing gap.

Figure 3:

Air pressure measurement of the mold cavity.

Figure 4:

Illustration of the experimental setup and the process in this study.

3.2 Experimental conditions

The experiments were performed with two geometries of patterns, namely a rib line pattern and a microscopic line pattern. For the rib line pattern, the size of the cavity was 100×100×2 mm. The dimensions and the cross-sectional shape of rib are shown in Figure 5A. Air pressure of a 4 MPa was applied, and the injection speed was set to be 100 mm/s. In order to evaluate the replication quality during the filling stage, the short shot molding test was conducted with 90% charge.

Figure 5:

Dimensions and configuration of patterns: (A) rib pattern; (B) microscopic line pattern; (C) configuration of the patterns relative to the flow direction; (D) overall size and shape of base plate with micro-patterns.

For the micro-sized line patterns, the cavity air pressure was also set to 4 MPa. Three injection speeds were used, namely, 100, 150 and 200 mm/s, respectively. The size of the cavity was 100×50×2 mm. The pattern width was 60 μm and the line patterns were separated by 120 μm. The geometry of the microscopic line patterns is shown in Figure 5B. For the microscopic line pattern, the replication quality was evaluated using both the short shot and the full shot. The short shot molding tests were conducted with a 90% charge as before. For the full shot test, the filling process was followed by the packing process. A packing pressure, which was 70% of the main injection pressure, was applied for the full shot test.

The molding conditions are shown in Table 1. The polypropylene (PP) was used for all the experiments. The barrel temperature was set to be 235°C and the mold temperature was 40°C. Both the line patterns were configured perpendicular to the main cavity flow (see Figure 5C). The location for measuring the height of the pattern is the center point of 30 mm away from the gate as shown in Figure 5D. For the purpose of comparison, all the experiments were repeated under same conditions without pressurizing the cavity air.

4.1 Rib pattern

The filled heights for the mold with a rib pattern were measured for the two different cavity air pressures, namely, 0 MPa (no pressure) and 4 MPa. The comparison of filled heights after the short shot molding is shown in Figure 6. When no pressure was applied to the cavity, the filled height of the rib was 1.7 mm. For the cavity air pressure of 4.0 MPa, the filled height was increased to 2.8 mm. It is noted that the amount of charge for cavity air pressure of 4.0 MPa was 7% <0 MPa. The amount of charge should not be different under the same injection conditions. However, the charging amount was reduced with the cavity air that makes inaccurate weighing of the injection molding machine. Therefore, we could not exactly match the end position of flow front for 0 MPa and 4 MPa in short shot test. Despite the flow front of the 4 MPa traveled less than the 0 MPa, the height of rib-pattern was higher as represented in Figure 7. Therefore, we expect more enhancement of replication if we could make equal weighing of 0 MPa and 4 MPa.

Figure 6:

Comparison of rib pattern heights between the samples molded with cavity air pressures of 0 MPa and 4 MPa.

Results of short shot tests with 90% charge.

Figure 7:

Comparison of end position of flow front and pattern heights with cavity air pressures of 0 MPa and 4 MPa.

4.2 Microscopic pattern

The height of the patterns was measured with a laser scanning digital microscope (OLS4100, Olympus). Heights of five microscopic patterns were measured for each of five molded articles.

The replication of patterns was observed to have improved with an increase in the injection speed as shown in Figure 8. This is due to the fact that a higher pressure is required for faster injection speed and higher filling pressure results in better replication. It is noted that the decrease in the melt flow speed leads to the lower shear rate and eventually results in the increase of viscosity due to the shear-thinning effect. However, under the condition employed in this study, the increase of the viscosity was found to be not big enough to change the overall flow pattern.

Figure 8:

Pattern heights with different injection speeds for short and full shot tests.

Measured values for the microscopic pattern for cavity air pressures of 4 MPa and 0 MPa.

For short shot experiments, the replication of patterns with the air cavity pressure improved when compared with the non-pressurized cases. There was a 48% increase in the pattern height for the injection speed of 50 mm/s (injection pressure of 34.3 MPa). When the injection speed was increased to 150 mm/s (injection pressure of 49.0 MPa), the increase in the pattern height became smaller. As mentioned above, the injection pressure increases with the injection speed, while the cavity air pressure was fixed at 4 MPa. As a result, the replication did not improve because of the smaller influence of the cavity pressure relative to the injection pressure.

The full shot experiments were also performed for each injection speed. The improvement in the replication is similar to the short shot test results as shown in Figure 8. Samples made from the full shot tests exhibited better replication than the samples by short shot tests. The increase in the pattern height with application of cavity air pressure was greater at lower injection speed, as in the short shot tests.

The improvement of pattern height in accordance with the ratio of the cavity air pressure against the injection pressure is shown in Figure 9. As can be seen, the replication improved with increasing cavity air pressure to the injection pressure ratio. There was less improvement in the replication when the ratio was small.

Figure 9:

Improvement in the pattern height with the ratio of 4 MPa air pressure to the injection pressure.

Measured values for the microscopic pattern for short and full shot tests.