Optical Properties of High-Density Polyethylene in Injection Press Molding for IR System Lenses.
High-density polyethylene (HDPE) is a general purpose type polymer used in materials such as food containers, lids, toys, and buckets , In addition, HDPE is used as a condenser lens for some infrared radiation (IR) systems, such as radiation thermometers, to transmit IR of the 8 to 12 [micro]m region [2-4]. Such lenses are produced by injection molding. When these HDPE lenses are used for IR systems, it is necessary to make them sufficiently thin to provide high IR transmittance. However, thinning them makes them transparent, which makes it possible to view the device interior. If such lenses could be made sufficiently thin for high IR transmittance in an opaque state, they would become useful for other IR systems such as thermal imaging systems.
Injection compression molding (ICM) is useful for thin-wall injection molding. ICM injects the polymer melt into a slightly opened cavity or one with low clamping force. Then the clamping process is conducted during or after injection. This method is used to produce thin-walled parts such as compact disks and optical lenses produced from a transparent amorphous polymer because of transcriptional improvement, uniform pressure distribution, reduction of the residual stresses, and reduction of war-page [5-8]. The method by which polymer melt is injected in the state of the suitably opened cavity is called injection press molding (IPM) , The cavity open distance in the case of IPM is longer than ICM because IPM is used for large products. Therefore, this term "IPM" is used distinctly from ICM.
A multilayer skin-core structure is formed inside of the injection molded parts. The thicknesses of the respective layers are expected to differ according to the mold cavity thickness, material properties, molding conditions, and their position in relation to the gate. Reportedly, differences occur in the molded part properties particularly because of the different thicknesses of the respective layers [10-19]. Furthermore, earlier reports have described that the core layer becomes extremely thin when injection molding is used with a thin-wall cavity [20-22], Semicrystalline polymers such as HDPE examined in this study consist of a crystal phase and an amorphous phase, which have different mechanical properties or optical properties such as transparency by crystallinity in addition to different skin-core structures.
Regarding the thin-wall ICM of semicrystalline polymer, differences reportedly occur in the internal structure, crystallinity, and crystalline structure in molded parts because of differences in the respective polymer molding conditions ,
In our earlier study , to achieve high IR transmittance and low visible ray (VR) transmittance necessary for a high-performance IR lens, an experimental investigation of injection molding and injection press molding was conducted using a disk-shaped mold cavity that was finished to a mirror-like surface. Results obtained for the high mold temperature and long cooling time demonstrated that greater core layer thickness improved the IR transmittance. Simultaneously, higher crystallinity produced lower VR transmittance. Furthermore, when injection press molding was conducted, higher crystallinity produced lower VR transmittance.
A second report  explained a study following up the study described above. Injection molding experiments using molds of different surface roughness were used to elucidate effects of the surface roughness of molded parts on IR and VR transmittance. Results show that the influences of the so-called skin-core structure and crystallinity at high mold temperatures were stronger than the influence of the surface roughness of molded parts. The IR transmittance was higher for thinner skin layers. Furthermore, the VR transmittance was higher in cases of higher crystallinity.
Regarding IPM conducted in our earlier study , this study experimentally investigated the effects on IR and VR transmittance when we changed the press stage conditions in IPM. The changed conditions in the press stage were the cavity open distance and delay time related to the polymer melt flowability. Other conditions of the press stage, such as press speed and press force, were held constant. To investigate the influence of the press condition alone, the other molding conditions were kept constant: cylinder temperature, injection time, cooling time, and mold temperature. Regarding the polymer melt flowability, mold surface roughnesses Ra of two kinds, 0.007 and 0.420 [micro]m, were used considering the influence of the mold surface roughness. Data for IR and VR transmittance of molded parts were evaluated using measurements or observation results obtained for surface roughness, thickness, differential scanning calorimetry (DSC), crystallinity, and the internal structure. Furthermore, to confirm the effects of injection press molding, the distribution of the molded part thickness was evaluated.
Commercially available HDPE (Suntec-HD J240; Asahi Kasei Corp.) was used for this study. The melt flow rate (MFR) was 5 g/10 min at 190[degrees]C. Predrying was not done before molding.
A three-dimensional CAD model that includes the cavity shape and dimensions is presented in Fig. 1. This cavity consists of the entire 4.4 mm height, the transmission part of IR ([phi] 22.72 mm, 0.5 mm thickness), and the hollow cylindrical part ([phi] 24.34 mm outer diameter, [phi] 22.72 mm inner diameter, 3.9 mm height) used at the time of assembly to other parts. Overflow parts are placed equally on the circumference of the hollow cylindrical part. The 3 mm X 1 mm gate was located at the circumference of the cylindrical part. Actually, S and M presented in Fig. 1b, respectively, denote the stationary side and movable side. Plastic mold steel (NAK80; Daido Steel Co., Ltd.) was used as the mold cavity material. The material was finished to different surface roughness values. Surface roughnesses Ra (arithmetical mean roughness) of the mold evaluated at 0.7 mm length were 0.007 and 0.420 [micro]m on the stationary side and movable side together. The surface roughness Ra of the molds was measured using a three-dimensional optical surface profiler (New View 7300; Zygo Corp.).
Injection Molding Machine and Molding Conditions
All electric servo-drive injection molding machines have an injection compression and injection press molding function (J110ELII; The Japan Steel Works, Ltd.). The device used for this experiment had maximum injection speed of 160 mm/s, maximum injection pressure of 188 MPa, clamping force of 1,078 kN, and 35 mm screw diameter.
Injection press molding conditions are presented in Table 1. The values of cylinder temperature, mold temperature, cooling time, and injection time in injection stage were set, respectively, to 200[degrees]C, 80[degrees]C, 15 s, and 0.3 s. In consideration of the flowability for polymer melt, the cavity open distance and delay time in the press stage differed in the range shown in Table 1.
MEASUREMENT AND EVALUATION
IR and VR Transmittance
For this study, IR and VR transmittance were measured, respectively, using an FTIR imaging system (Spotlight 400; PerkinElmer Inc.) and a UV-VIS spectrophotometer (UV-2450; Shimadzu Corp.). Figure 1 shows the measured areas of IR and VR transmittance, respectively, as [phi] 8 mm and 1 mm X 12 mm of the central part in the molded parts. The measured wavelengths of IR and VR were, respectively, 8 to 12 [micro]m and 380 to 780 nm. Transmittance was evaluated as an average of measured values.
Surface Roughness and Thickness of Molded Parts
Surface roughness Ra of the molded parts was measured using a three-dimensional optical surface profiler (New View 7300; Zygo Corp.). The measured locations were, respectively, the near gate, center, and near flow ends of molded parts, which are presented in Fig. 1a as (P1), (P2), and (P3). The measured length of the surface roughness Ra was 0.7 mm. Furthermore, the molded part thickness was measured using a digimatic indicator (ID-C150B; Mitutoyo Corp.). The measured locations were (P1), (P2), and (P3) as presented in Fig. 1a.
Internal Structure Observations
To investigate the internal structure of molded parts through the cross-section, thin slices of 60 [micro]m were cut using a microtome (RM2235; Leica Microsystems GmbH). The cutting location is the broken line part presented in Fig. 1a. Slices were observed using a polarized light microscope (PLM, Eclipse LV100ND; Nikon Corp.) with 90[degrees] cross-polarized light. For this study, the skin-core structure was evaluated as three layers: a skin layer, a shear layer, and a core layer. Each layer thickness was measured from a cross-sectional observation photograph.
Crystallinity and Molecular Chain Orientation
Regarding the size of [phi] 4 mm of these molded parts presented in Fig. 1a as (P1), (P2), and (P3), the heat of fusion [DELTA][H.sub.m] was measured using a differential scanning calorimeter (DSC, DSC8500; PerkinElmer Inc.) that had been calibrated with indium under a flowing nitrogen atmosphere at a heating rate of 10[degrees]C/min. Measurements were taken using the same heating rate of 10[degrees]C/min from 50[degrees]C to 150[degrees]C. Crystallinity ([X.sub.c]) was calculated as
[X.sub.c] = ([DELTA][H.sub.m]/[DELTA][H.sub.c]) x 100%, (1)
where [DELTA][H.sub.c] stands for the heat of fusion values for 100% crystalline polymers. It becomes 293 J/g in HDPE.
Furthermore, a slice every 0.01 mm was made from the surfaces of molded parts of the stationary side using an ultraprecision machine (AHN60-3D; JTEKT Corp.). Then the distribution of the crystallinity in the thickness direction was evaluated using the method described above. When molecular chain orientation occurs in the molded parts, reportedly two peaks are visible in the curve obtained using DSC analysis: the melting point of isotropic crystals and the melting point of oriented crystals [14, 23, 26]. Therefore, for a slice taken every 0.01 mm, the degree of orientation in each layer was evaluated using the obtained DSC curve.
RESULTS AND DISCUSSION
Measurement Results of IR and VR Transmittance
Figure 2 presents measurement results of IR and VR transmittance for molds of different surface roughness. Regarding the IR transmittance depicted in Fig. 2a and b, when the cavity open distance was set to 0.3 or 0.5 mm, it showed a tendency to be greater with an extended delay time, but this tendency was not apparent at 1.0 mm. Furthermore, comparison of Fig. 2a and b revealed no great difference; no influence of the mold surface roughness was found. Figure 2c and d present VR transmittance measurement results. The same tendency was obtained at cavity open distances 0.5 and 1.0 mm. The minimum value was obtained at delay time 0.3 s. At the cavity open distance of 0.3 mm, when the mold of Ra 0.007 [micro]m was used, the minimum value was obtained at delay time 0.5 s, but no constant tendency was apparent when the mold of Ra 0.420 [micro]m was used. It was approximately constant. When Fig. 2c and d were compared, because no large difference in the VR transmittance occurred, the influence of the mold surface roughness was not confirmed as it was with IR transmittance.
Effects of Surface Roughness of Molded Parts on IR and VR Transmittance
Figure 3 presents measurement results of surface roughness Ra of molded parts. Figure 3a and b and Fig. 3c and d respectively present results obtained when molds of Ra 0.007 [micro]m and Ra 0.420 [micro]m were used. These measurement results are the means of measurement values for the locations presented in Fig. 1a as (P1), (P2), and (P3). When the mold surface roughness was high, the surface roughness of molded parts was high, but no great difference was apparent in the IR and VR transmittance depicted in Fig. 2, as described above. Therefore, regarding molding conditions used for this study, results show that the surface roughness of molded parts did not affect the transmittance.
Effects of Molded Parts Thickness and Crystallinity on IR and VR Transmittance
When light density ([I.sub.1]) becomes light density ([I.sub.2]) by the optical path length (l), the Lambert-Beer law is established. Absorbance (A) can be expressed as
A = log([I.sub.1]/[I.sub.2]) = [epsilon]cl, (2)
where [epsilon] stands for the molar absorptivity ([cm.sup.2] x [mol.sup.-1]) and where c signifies the molar concentration of a material causing absorption (mol x [cm.sup.-3]). Actually, A is proportional to the sample thickness and density [27, 28],
Figure 4 presents the thickness of molded parts produced under cavity open distances of 0.3, 0.5, and 1.0 mm when the delay time was changed using different mold surface roughness. These measurement results are the means of measurement value for locations presented in Fig. 1a as (P1), (P2), and (P3). When a mold with surface roughness Ra of 0.007 [micro]m was used, the molded part thickness became thick. The cavity depth was measured using a depth micrometer (DMC; Mitutoyo Corp.), which showed that it was approximately 0.04 mm deeper than the cavity with surface roughness Ra of 0.420 [micro]m. We regard this as an important factor. When a mold of different surface roughness was used, the transmittance showed almost no difference, even when the thicknesses of molded parts differed. Furthermore, the molded parts tended to become thick by setting of a longer delay time. However, the transmittance depicted in Fig. 2 shows no constant tendency overall. Therefore, the molded part thickness did not affect the IR and VR transmittance for the range of 0.065 mm examined in this study,
Furthermore, the polymer density depends on the crystallinity. The crystallinity measurement results are presented in Fig.
5. These measurement results are the means of measurement values for locations presented in Fig. 1a as (P1), (P2), and (P3). No clear tendency was apparent in the variation of the crystallinity by changing the delay time, but the crystallinity decreased because of the large cavity open distance. Results of our earlier investigation show that the VR transmittance depends on the crystallinity. However, the VR transmittance shows no constant change by the difference in the cavity open distance, as depicted in Fig. 2c and d. Therefore, the IR and VR transmittance are considered to be affected by changes that occur in the internal structure as a result of the different press conditions.
Effects of Molded Part Internal Structure and Crystallinity on IR and VR Transmittance
Figure 6 portrays a cross-sectional observation photograph of molded parts obtained by changing the delay time at cavity open distance of 0.5 mm when a mold of Ra 0.007 [micro]m surface roughness was used. The observation locations are three points presented in Fig. 1a as (P1), (P2), and (P3). Differences in the skin layer, shear layer, and core layer were not clear for any photograph. However, the boundary of the layer with the knife mark and without the knife mark was judged when the thin slices were produced using a microtome, as presented by the broken line in Fig. 6. Therefore, in this study, we evaluated the skin layer from the surface of molded parts to the boundary and the shear-core layer inward from the boundary. The measurement results of shear-core layer thickness and total thickness in molded parts are also shown in these photographs. The means of these measurement results of three locations are presented in Table 2. Results of our earlier investigation  demonstrated that the IR transmittance increased concomitantly with the decrease of the skin layer thickness. Therefore, when the IR transmittance of molded parts obtained at the cavity open distance of 0.5 mm presented in Fig. 2a is compared with the skin layer thickness presented in Table 2, the IR transmittance increased concomitantly with decreased skin layer thickness, as shown also in our earlier investigation. However, regarding the VR transmittance depicted in Fig. 2c and d, correlation with the skin layer and shear-core layer thickness was not apparent.
Effects of Crystallinity and Molecular Chain Orientation on IR and VR Transmittance
Results demonstrated that VR transmittance was independent of crystallinity as described above, and as depicted in Fig. 5. Furthermore, the correlation of VR transmittance and skin-core layer thickness was not confirmed. Therefore, regarding molded parts produced under the same injection press molding conditions with molded parts portrayed in Fig. 6, the distribution of crystallinity in each layer sliced to 0.01 mm in the thickness direction was evaluated. These results are presented in Fig. 7a-d as the depth from the molded parts surface of stationary side. For the delay time of 0.3 mm shown in Fig. 7b, more uniform crystallinity was obtained. Figure 6 shows that this result has strong correlation with the state of birefringence; it is more uniform at delay time of 0.3 s. However, the effect on VR transmittance was indistinct.
The characteristic shapes of DSC curves of three kinds obtained from these measurements are presented in Fig. 8. Each curve shows three forms of approximately one melting peak (Type I), two unclear melting peaks (Type II), and two clear melting peaks (Type III). In these DSC curves, the ratio of layers obtained for Type III for the whole layer is presented in Fig. 9. When two peaks appear, the molecular chain orientation occurs [14, 23, 26]. Therefore, this Fig. 9 shows the ratio of the layer where strong orientation occurred for the total thickness of the molded part. Comparison of Fig. 9 with the VR transmittance at the cavity open distance of 0.5 mm depicted in Fig. 2c shows a corresponding tendency, except for the delay time of 1.0 s. This result revealed that the VR transmittance depends on the molecular chain orientation. Results of this investigation show that the molecular chain orientation becomes least at 0.3 s delay time.
Regarding the light scattering intensity in the oriented polymer, Norris and Stein have shown the Rayleigh ratio R([theta]) in the following equations as a function of the angle between the direction of incident light and the direction of the scattered light based on Debye-Bueche theory:
[mathematical expression not reproducible] (3)
k = 2[pi]/[lambda] (4)
s = 2sin([theta]/2) (5)
[gamma](r) = exp(-r/a) (6)
Therein, [bar.[[eta].sup.2]] stands for the mean-square dielectric constant fluctuation, [[lambda].sub.0] and [lambda] respectively represent the wavelengths of light in vacuum and in the medium, and [gamma](r) signifies the correlation function describing the correlation between fluctuations for vacuum elements separated by distance r, as reported by Debye and Bueche. Furthermore, a is the correlation distance [29-31].
When r becomes large by molecular chain orientation, the Rayleigh ratio becomes small because [gamma](r) in Eq. 6 becomes small. Therefore, it is shown that the transmittance increases because of low scattering. This calculation result corresponds to the result found from this study that VR transmittance increases when the molecular chain orientation is large.
Furthermore, because the DSC curve of the molded parts surface was obtained, results for curves of Type I and Type II presented in Fig. 8 depict that the orientation was small. This result demonstrates that the skin layer has a small orientation. Absorption of IR is known to occur because of intermolecular vibration. This frequency v, which depends on the molecular structure, is expressed as the following Eq. 7:
v = 1/2[pi]c [square root of (f/[mu])] (7)
Therein, c and f respectively represent the velocity of light and the force constant. Also, [mu] is the reduced mass of this system, as defined by the following Eq. 8:
[mu] = [m.sub.A] x [m.sub.B]/[m.sub.A] + [m.sub.B] (8)
This intermolecular vibration is inferred from results of Fig. 7 to occur only in a thin skin layer having smaller molecular chain orientation and low crystallinity. Furthermore, v in the skin layer becomes the same value because the only polymer used for this study was HDPE. Therefore, the IR transmittance is expected to depend on the skin layer thickness. Most of the IR in the shear-core layer is thought to be transmitted because this vibration occurs only with difficulty when crystallized by molecular chain orientation.
Distribution of Thickness in the Melt Flow Direction
Figure 10 presents measurement results for the molded part thickness when the delay time was changed at the cavity open distance of 0.5 mm using a mold of different surface roughness. The measurement locations were (P1), (P2), and (P3), as presented in Fig. 1a. The thickness became uniform by setting a longer delay time. Particularly, results showed that mold surface roughness affected the molded parts thickness uniformity because it became a constant value when using a mold with large surface roughness.
To obtain high IR transmittance and low VR transmittance necessary for a high-performance IR lens as a target HDPE, injection press molding was conducted with different cavity open distances and delay times considering the polymer melt flowability. The effects of these conditions on IR and VR transmittance of molded parts were assessed experimentally. The disk-shaped mold cavity has 0.5 mm thickness. Regarding the flowability of the polymer melt, two mold surface roughnesses Ra were used, 0.007 and 0.420 [micro]m, considering the influence of the mold surface roughness. As factors affecting transmittance, we evaluated the surface roughness, thickness, differential scanning calorimetry (DSC), crystallinity, and internal structure of molded parts. Results of this investigation show that the surface roughness and thickness of molded parts did not influence IR or VR transmittance. Furthermore, when the skin layer having a small molecular chain orientation became thin, the IR transmittance rose by setting of a longer delay time. When the molecular chain orientation in the shear-core layer was small, the VR transmittance decreased. The molecular chain orientation in the shear-core layer changed depending on IPM conditions. Conditions sufficient to obtain the minimum value were found from this investigation. As a result of having evaluated the distribution of the molded parts thickness, it became uniform by setting a longer delay time. Furthermore, because a constant value was obtained when a mold of large surface roughness was used, the mold surface roughness was shown to affect the uniformity of the molded parts thickness.
In two of our earlier reports, injection molding changed the mold temperature and cooling time using a mold having mirror-like surface and surface roughness of three kinds. Furthermore, as described in this report, injection press molding with changed cavity open distance and delay time was done based on two of our earlier investigations. The main results obtained for injection molding and injection press molding obtained using a 0.5 mm thick cavity with disk shapes are described below.
* IR transmittance becomes higher with a thin skin layer having small molecular chain orientation and low crystallinity.
* VR transmittance becomes lower when the molecular chain orientation in the shear-core layer is small.
To obtain a skin-core structure as described above:
* For injection molding, the mold temperature and cooling time are set, respectively, as higher and longer. In such cases, it is not necessary to finish a mold in the mirror-like surface because the influence of the surface roughness of molded parts becomes extremely small when the mold temperature is high.
* For injection press molding, the IR transmittance increases when the delay time is particularly long. However, the VR transmittance increases simultaneously. In these investigation results, the most suitable values of cavity open distance and delay time were, respectively, the same values as the cavity thickness and injection time.
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Ryo Kaneda (iD),(1) Toshihiro Takahashi, (1) Masayasu Takiguchi, (2) Motoharu Hijikata, (2) Hiroshi Ito (3)
(1) Yamagata Research Institute of Technology, 2-2-1 Matsuei, Yamagata, Yamagata 990-2473, Japan
(2) Yamagata Factory, Chino Corporation, 1515 Midarekawa, Tendo, Yamagata 994-0002, Japan
(3) Graduate School of Organic Materials Science, Yamagata University, 4-3-16 Jonan, Yonezawa, Yamagata 9928510, Japan
Correspondence: H. Ito; e-mail; firstname.lastname@example.org
Caption: FIG. 1. Cavity shape and measuring location.
Caption: FIG. 2. Effects of injection press molding conditions on IR and VR transmittance of molded parts produced with cavity open distances of 0.3, 0.5, and 1.0 mm when the delay time was changed using mold surface roughnesses Ra of (a), (c) 0.007 [micro]m and (b), (d) 0.420 [micro]m.
Caption: FIG. 3. Effects of injection press molding conditions on surface roughnesses of molded parts produced with cavity open distances of 0.3, 0.5, and 1.0 mm when the delay time was changed using mold surface roughnesses Ra (a), (b) 0.007 [micro]m and (c), (d) 0.420 [micro]m.
Caption: FIG. 4. Variation of thicknesses of molded parts produced under cavity open distances of 0.3, 0.5, and 1.0 mm when the delay time was changed using mold surface roughnesses Ra (a) 0.007 [micro]m and (b) 0.420 [micro]m.
Caption: FIG. 5. Variation of crystallinity of molded parts produced under cavity open distances of 0.3, 0.5, and 1.0 mm when the delay time was changed using mold surface roughnesses Ra (a) 0.007 [micro]m and (b) 0.420 [micro]m.
Caption: FIG. 6. Cross-sectional photograph of near gate, center, and near flow end in molded parts obtained at cavity open distance of 0.5 mm with different delay times (a) 0.0 s, (b) 0.3 s, (c) 0.5 s, and (d) 1.0 s using mold surface roughness Ra of 0.007 [micro]m, dimensions shown in the photograph are the layer thickness evaluated as the shear-core layer and the whole thickness of molded parts.
Caption: FIG. 7. Distribution of crystallinity for thickness direction in molded part produced under delay time (a) 0.0 s, (b) 0.3 s, (c) 0.5 s, and (d) 1.0 s at cavity open distance of 0.5 mm when using mold surface roughness Ra of 0.007[micro]m.
Caption: FIG. 8. Characteristic shape of DSC curve obtained by different layer sliced on thickness of 0.01 mm in molded parts produced with 0.5 mm cavity open distance and 0.0 s delay time when using mold surface roughness Ra of 0.007 [micro]m.
Caption: FIG. 9. Variation of the ratio of layer obtained with clear two melting peaks in DSC curve for the thickness direction in molded parts produced under different delay times at cavity open distance of 0.5 mm when using mold surface roughness Ra of 0.007 [micro]m.
Caption: FIG. 10. Distribution of molded parts thickness produced under different delay times of 0.0, 0.3, 0.5, and 1.0 s at cavity open distance of 0.5 mm when using mold surface roughnesses Ra (a) 0.007 [micro]m and (b) 0.420 [micro]m.
TABLE 1. Injection press molding conditions. Cavity open distance (mm) 0.3, 0.5, 1.0 Delay time (s) 0.0, 0.3, 0.5, 1.0 TABLE 2. Comparison of skin layer and shear-core layer thicknesses under different delay times at cavity open distance of 0.5 mm when using mold surface roughness Ra of 0.007 [micro]m. Whole Shear-core Skin layer Delay thickness layer thickness thickness time (s) (mm) (mm) (mm) 0.0 0.48 0.40 0.08 0.3 0.49 0.41 0.08 0.5 0.48 0.40 0.08 1.0 0.50 0.44 0.06
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|Author:||Kaneda, Ryo; Takahashi, Toshihiro; Takiguchi, Masayasu; Hijikata, Motoharu; Ito, Hiroshi|
|Publication:||Polymer Engineering and Science|
|Date:||May 1, 2018|
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