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The technology of Microlens array injection molding
Abstract
Injection molding could be used as a mass production technology for microlens arrays. It is of importance, and thus of our concern in the present study, to understand the injection molding processing condition effects on the replicability of microlens array profile. Extensive experiments were performed by varyingprocessing conditions such as flow rate, packing pressure and packing time for three different polymeric materials (PS, PMMA and PC). The nickel mold insert of microlens arrays was made by electroplating a microstructure master fabricated by a modified LIGA process. Effects of processing conditions on the replicability were investigated with the help of the surface profile measurements. Experimental results showed that a packing pressure and a flow rate significantly affects a final surface profile of the injection molded product. Atomic force microscope measurement indicated that the averaged surface roughness value of injection molded microlens arrays is smaller than that of mold insert and is comparable with that of fine optical components in practical use.
1 Introduction
Microoptical products such as microlenses or microlens arrays have been used widely in various fields of microoptics, optical data storages, bio-medical applications, display devices and so on. Microlenses and microlens arrays are essential elements not only for the practical applications but also for the fundamental studies in the microoptics. There have been several fabrication methods for microlenses or microlens arryas such as a modified LIGA process [1], photoresist reflow process [2], UV laser illumination [3], etc. And the replication techniques, such as injection molding, compression molding [4] and hot embossing [5], are getting more important for a mass production of microoptical products due to the cost-effectiveness. As long as the injection molding can replicate subtle microstructures well, it is surely the most cost-effective method in the mass production stage due to its excellent reproducibility and productivity.
In this regard, it is of utmost importance to check the injection moldability and to determine the molding processing condition window for proper injection molding of microstructures. In this study, we investigated the effects of processing conditions on the replication of microlens arrays by the injection molding. The microlens arrays were fabricated by a modified LIGA process, which was previously reported in [6, 7]. Injection molding experiments were performed with an electroplated nickel mold insert so as to investigate the effects of some processing conditions. The surface profiles of molded microlens arrays were measured, and were used to analyze effects of processing conditions. Finally, a surface roughness of microlens arrays was measured by an atomic force microscope (AFM).
2 Mold insert fabrication
Microlens arrays having several different diameters were fabricated on a PMMA sheet by a modified LIGA process [6]. This modified LIGA process is composed of an X-ray irradiation on the PMMA sheet and a subsequent thermal treatment. The X-ray irradiation causes the decrease of molecular weight of PMMA, which in turn decreases the glass transition temperature and consequently causes a net volume increase during the thermal cycle resulting in a swollen microlens [7]. The shapes of microlenses fabricated by the modified LIGA process can be predicted by a method suggested in [7].
The microlens arrays used in the experiments were composed of 500μm -(a 2 × 2 array), 300μm -(2 × 2) and 200μm (5 × 5) diameter arrays, and their heights were 20.81, 17.21 and 8.06 μm, respectively. Using the microlens arrays fabricated by the modified LIGA process as a master, a metallic mold insert was fabricated by a nickel electroplating for the injection molding. Typical materials used in a microfabrication process, such as silicon, photoresists or polymeric materials, cannot be directly used as the mold or the mold insert due to their weak strength or thermal properties. It is desirable to use metallic materials which have appropriate mechanical and thermal properties to endure both a high pressure and a large temperature variation during the replication process. Therefore, a metallic mold insert is being used rather than the PMMA master on silicon wafer for mass production with such replication techniques. Otherwise special techniques should be adopted as a replication method, e.g. a low pressure injection molding [8].
The size of final electroplated mold insert was 30 × 30 × 3 mm. The electroplated nickel mold insert having microlens arrays is shown in Fig. 1.
Fig.1.Moldinsert fabricated by a nickel electroplating (a) Real view of the mold insert (b) SEM image of 200 μm diameter microlens array (c) SEM image of 300 μmdiameter microlens array
3 Injection molding experiments
A conventional injection molding machine (Allrounders 220 M, Arburg) was used in the experiments. A mold base for the injection molding was designed to fix the electroplated nickel mold insert firmly with the help of a frametype bolster plate (Fig. 2). Shape of aperture of the bolster plate (in this study, a rectangular one) defines the outer geometry of the molded part on which the profiles of microlens arrays are to be transcribed. The mold base itself has delivery systems such as sprue, runner and gate which lead the molten polymer to the cavity formed by the bolster plate, the mold insert and amoving mold surface. The mold base was designed such that mold insert replacement is simple and easy. Of course, one may introduce an appropriate bolster plate with a specific aperture shape.
Fig. 2. Mold base and mold insert used in the injection molding experiment
The injection molding experiments were carried out with three general polymeric materials – PS (615APR, Dow Chemical), PMMA (IF870, LG MMA) and PC (Lexan 141R, GE Plastics). These materials are quite commonly used for optical applications. They have different refractive indices (1.600, 1.490 and 1.586 for PS, PMMA and PC, respectively), giving rise to different optical properties in final products, e.g. different foci with the same geometry.
The injectionmolding experiments were performed for seven processing conditions by changing flow rate, packing pressure and packing time for each polymeric material. Furthermore, same experiments were repeated three times for checking the reproducibility. It may be mentioned that the mold temperature effect was not considered in this study since the temperature effect is relatively less important for these microlens arrays due to their large radius of curvature than other microstructures of high aspect ratio. For high aspect ratio microstructures, we are currently investigating the temperature effect more closely and plan to report separately in the future. Therefore, flow rate, packing pressure and packing time were varied to investigate their effects more thoroughly with the mold temperature unchanged in this study. Table 1 shows the detailed processing conditions for three polymeric materials. Other processing conditions were kept unchanged during the experiment. The mold temperatures were set to 80, 70 and 60 _C for PC, PMMA and PS, respectively.
It might be mentioned that we carried out the experiments without a vacuum condition in the mold cavity considering that the large radius of curvature of the microlens arrays in the present study will not entrap air in the microlens cavity during the filling stage.
Table 1. Detailed processing conditions used in the injection molding experiments
Case
Flow rate (cc/sec)
Packing time (sec)
Packing pressure(MPa)
1
12.0
5.0
10.0
2
12.0
5.0
15.0
3
12.0
5.0
20.0
PS
4
12.0
2.0
10.0
5
12.0
10.0
10.0
6
18.0
5.0
10.0
7
24.0
5.0
10.0
PMMA
1
6.0
10.0
10.0
2
6.0
10.0
15.0
3
6.0
10.0
20.0
4
6.0
5.0
10.0
5
6
7
6.0
9.0
12.0
15.0
10.0
10.0
10.0
10.0
10.0
PC
1
6.0
5.0
5.0
2
6.0
5.0
10.0
3
5
6.0
6.0
9.0
5.0
10.0
15.0
5.0
6
5.0
5.0
7
12.0
5.0
5.0
4 Results and discussion
Before detailed discussion of the experimental results, it might be helpful to summarize why flow rate, packingpressure and packing time (which were chosen as processing conditions to be varied in this study) affect thereplication quality. As far as the flow rate is concerned, there may exist an optimal flow rate in the sense that too small flow rate makes too much cooling before a complete filling and thus possibly results in so-called short shot phenomena whereas too high flow rate increases pressure fields which is undesirable.
The packing stage is generally required to compensate for the volume shrinkage of hot molten polymer whencooled down, so that enough material should flow into a mold cavity during this stage to control the dimensionalaccuracy. The higher the packing pressure, the longer the packing time, more material tends to flow in. However, too much packing pressure sometimes may cause uneven distribution of density, thereby resulting in poor opticalquality. And too long packing time does not help at all since gate will be frozen and prevent material from flowing into the cavity. In this regard, one needs to investigate the effects of packing pressure and packing time.
4.1 Surface profiles
Figure 3 shows typical scanning electron microscope (SEM) images of the injection molded microlens arrays for different diameters for PMMA (a) and different materials (b). Cross-sectional surface profiles of the mold insert and all the injection molded microlens arrays were measured by a 3D profile measuring system (NH-3N, Mitaka).
(a)Injection molded microlensarrays (PMMA) (b) Injectionmolded microlenses of 300 μmdiameter for different materials
Fig. 3. SEM images of theinjection molded microlensarrays and microlenses
As a measure of replicability, we have defined a relative deviation of profile as the height difference between the molded one and the corresponding mold insert for each microlens divided by the mold insert one. The computed relative deviations for all the microlenses are listed in Table 2.
Diameter( μm)
Relative deviation (%)
1
2
3
4
5
6
7
PS
200
300
500
-7.62
5.86
2.38
-7.59
2.03
-0.38
2.08
2.86
0.51
-5.56
5.61
1.47
-8.66
6016
1.47
-11.44
4.29
1.47
-9.47
5.73
1.95
PMMA
200
300
500
7.20
5.77
-0.66
1.31
5.60
-1.62
-3.88
6.45
3.98
-5.80
5.95
2.80
-0.97
5.95
-0.72
-8.53
6.68
-0.90
4.86
-2.62
-0.72
PC
200
300
500
23.02
6.20
-0.93
16.05
4.96
5.09
16.87
2.66
-1.86
19.66
4.53
1.88
33.97
4.78
6.96
18.67
1.79
2.43
-2.94
4.15
-1.55
It may be mentioned that the moldability of polymeric materials affects the replicability. Therefore, the overall relative deviation differs for three polymeric materials used in this study. It may be noted that PC is the most difficult material for injection molding amongst the three polymers. The largest relative deviation can be found in PC for the smallest diameter case, as expected. In that specific case, the largest value is corresponding to the low flow rate and low packing pressure. Packing time in this case does not significantly affect the deviation. The relative deviation for PS and PMMA with the smallest diameter is far better than PC case.
Table 2 indicates that the larger the diameter, the smaller the relative deviation. The larger diameter microlens is, of course, easier to be filled than smaller diameter during the filling stage and packing stage. Microlenses of larger diameters were generally replicated well regardless of processing conditions and regardless of materials. The best replicability is found for the case of PS with 500 μm diameter. Generally, PS has a good moldability in comparison with PMMA and PC.
It may be mentioned that some negative values of relative deviation were observed mostly in the smallest diameter case for PS and PMMA according to Table 2. In these cases, however, the absolute deviation is an order of 0.1 μm in height, which is within the measurement error of the system. Therefore, the negative values could be ignored in interpreting the experimental data of replicability.
Surface profiles of microlens of 300 μm diameter are shown in Figs. 4 and 5 for PC and PMMA, respectively. As shown in Fig. 4, the higher packing pressure or the higher flow rate results in the better replication of microlens for the case of PC, as mentioned above. Packing time has little effect on the replication for these cases. For the case of PMMA, the packing pressure and packing time have insignificant effect as shown in Fig. 5; however, flow rate has the similar effect to PC. It might be reminded that packing time does not affect the replicability if a gate is frozen since frozen gate prevents material from flowing
into the cavity. Therefore, the effect of packing time disappears after a certain time depending on the processing conditions.
Fig.4a–c(leftside).Surfce profiles of microlens (PC with diameter (/) of 300 μm). a effect of packing pressure, b effect of flow rate, c effectof packing time
Fig.5a–c.(rightside)Surface profiles of microlens (PMMA with diameter(/) of 300μm). a effect of packing pressure, b effect of flow rate,c effect of packing time
4.2 Surface roughness
Averaged surface roughness, Ra, values of 300 μm diameter microlenses and the mold insert were measured by an atomic force microscope (Bioscope AFM, Digital Instruments). The measurements were performed around the top of each microlens and the measuring area was 5 μm · 5 μm. Figure 6 shows AFM images and measured Ra values of microlenses. PMMA replicas of microlens have the lowest Ra value, 1.606 nm. It may be noted that AFM measurement indicated that Ra value of injection molded microlens arrays is smaller than the corresponding one of the mold insert. The reason for the improved surface roughness in the replicated microlens arrays is not clear at this moment, but might be attributed to the reflow caused by surface tension during a cooling process. It may be further noted that the Ra value of injection molded microlens arrays is comparable with that of fine optical components in practical use.
a Nickel mold insert, b PS, c PMMA, d PC
Fig. 6. AFM images and averaged surface roughness, Ra, values of the mold insert and injection molded 300 μm diameter microlenses.
4.3 Focal length
The focal length of lenses can be calculated by a wellknown equation as follows:
where f, nl, R1 and R2 are focal length, refractive index of lens material, two principal radii of curvature, respectively.For instance, focal lengths of the molded microlenses were approximately calculated as 1.065 mm (with R1 0.624 mm and R2