Flexible insert for microinjection compression molding of polymer microlens arrays

18 September 2017
Han-Xiong Huang, Heng Xie, and An-Fu Chen
A new method of microinjection molding, based on the use of a flexible insert combined with a porous plate, enables the fast and efficient fabrication of polymer microlens arrays with tunable heights.

Microlens arrays (MLAs) are widely used in many fields due to their unique optical properties, small element size, and high integrity.1 To satisfy the demands of a number of applications, a range of studies have been carried out with the aim of fabricating precise microstructured arrays for, e.g., sensors,2, 3 light sources,4 and optical-fiber interconnects.5

The high productivity and excellent reproducibility enabled by microinjection compression molding (μ-ICM) makes it an ideal process for the rapid fabrication of polymer MLAs.6 However, to enable the preparation of MLAs with different geometrical morphologies (e.g., diameters, heights, and arrangements), a number of different mold cavities or inserts are required. This necessitates the use of a significant amount of material, and complicates the production process. A low-cost and effective method for manufacturing mold inserts would make the preparation of MLAs using μ-ICM more economically viable.

In our work, we have developed a low-cost method—based on μ-ICM—for the preparation of MLAs.7 Using a porous plate made of stainless steel and flexible poly(ethylene terephthalate) film, we assembled a flexible insert for MLA fabrication: see Figure 1(a). This insert replaces the negative features (i.e., the concave cavities) of the mold, thus removing a step in fabrication, reducing the amount of material required, and simplifying production.


Schematic showing our molding process for the fabrication of microlens arrays (MLAs). (a) The flexible insert is mounted on the cavity surface of the mold. (b) The polymer melt is injected into the cavity. Pressure causes a slight deformation in the flexible film, leading to the creation of semi-spheres in the polymer (i.e., at the locations of holes within the porous sheet). (c) Compression causes enhanced arc-like profiles, leading to the creation of an array of microlenses. The height of the MLA can be tuned by changing the compression force. (d) The MLA is demolded.

Our MLA molding process comprises several steps. First, we inject the polystyrene (PS) melt into the mold cavity (i.e., the melt-filling stage). In this step, the flexible film (on top of the porous plate) is deformed slightly by the pressure of the melt. As such, arc-like profiles are created in the gaps provided by the microholes of the porous plate: see Figure 1(a) and (b). These profiles are enhanced as some of the melt is squeezed further into the microholes by a compression force (in the mold-compression stage): see Figure 1(c). The melt in the mold cavity is then cooled down, and the finished MLA is removed: see Figure 1(d). In this process, the melt temperature, injection rate, and mold temperature are kept constant, and only the compression force is varied. As the compression force changes, so too does the melt pressure in the mold cavity. Thus, the height of the MLAs can be adjusted by varying the compression force.

One important parameter of an MLA is its geometric-morphology uniformity, which has a significant effect on the optical properties of the lenses. As is evident in Figure 2(a), the microlenses (with a diameter of about 190μm and a pitch of about 320μm) are distributed in an orderly and periodic way across the molded surface. This distribution pattern indicates that the microholes in the porous plate have been successfully replicated on the PS surface via our μ-ICM technique. To characterize the geometric-morphology uniformity of the lenses, we analyzed the height of the lenses in the MLA. To do so, we acquired a 3D surface profile of a microlens—see Figure 2(c)—and we intercepted cross-sectional curves through the highest point of its surface profile: see Figure 2(d). We thus found that the cross-sectional profile of the microlens forms an arc, which agrees with the result observed using scanning electron microscopy: see Figure 2(b). Furthermore, we investigated the effect of compression force on the geometric-morphology uniformity of fabricated MLAs. The MLAs were prepared using μ-ICM under four compression forces (from 90 to 120kN, in steps of 10kN). Our subsequent measurements of the mean values and standard deviations of the heights of the microlenses—listed in Table 1—show that the mean height increases with the compression force. This result suggests that MLAs with different heights, and acceptable height uniformities, can be molded successfully by using a flexible insert (i.e., without the need for complicated microlens-mold manufacturing processes).


Geometric characteristics of a single lens in the MLA, molded under a compression force of 110kN. (a) Scanning electron micrograph of an MLA. A (b) magnified side view, (c) 3D surface profile, and (d) cross-sectional profile of a single microlens from the array.

Mean values and standard deviations of the heights obtained in the MLAs molded under four compression forces (90, 100, 110, and 120kN).

Compression force (kN)90100110120
Mean value (μm)61.6165.1574.7986.28
Standard deviation (μm)2.271.411.361.60

Next, we investigated the imaging ability of our MLAs. A schematic of our experiment is shown in Figure 3(a). We positioned a mask (with an annulus gap) between a white light source and an MLA. We then placed an optical digital microscope (with an image acquisition and transmission device) behind the MLA, to capture the images in the microlenses. As shown in Figure 3(b), a sharp and annulus white band can be clearly seen in each microlens, demonstrating that our μ-ICM MLAs achieve a favorable geometric-morphology uniformity and imaging capability. Finally, we measured the surface roughness of the lenses in the array by using scanning probe microscopy. Figure 4 shows the surface roughness over an area of 25μm2 at a position close to the apex of the microlens. We measured the average surface roughness of this area to be 13.2nm, meaning that the microlens surface exhibits good optical smoothness.


Characterization of the imaging ability of the MLA molded under a compression force of 110kN. (a) Schematic of the imaging measurement system. White light travels through a mask with an annulus gap and then passes through the MLA. The image obtained by the MLA is then captured with a digital microscope. (b) Image obtained using a digital microscope. An inset at the top right shows the magnified view from one lens.


Scanning probe microscopy image showing the surface roughness of the microlens, which was molded under a compression force of 110kN, measured over an area of 25μm2.

In summary, we have developed a method for assembling a flexible insert—for use in the creation of MLAs by μ-ICM—using an easily available porous plate and a flexible polymer film. We molded the MLAs using a microinjection compression mold and mounting the flexible insert within the cavity. With this approach, microlenses with a diameter of about 190μm and a pitch of about 320μm were periodically distributed across the large surface. We found that the height of the MLAs molded using one flexible insert is adjustable within a certain range (from about 61 to 86μm) by altering the compression force (between 90 and 110kN). All of the MLAs (i.e., molded under the four compression forces) exhibit a favorable geometric-morphology uniformity and good optical properties. In the next stage of our work, we will design mold inserts for the fabrication of MLAs with controllable distribution and height.


Authors

Han-Xiong Huang
South China University of Technology

Heng Xie
South China University of Technology

An-Fu Chen
South China University of Technology


References

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DOI:  10.2417/spepro.006949