Thin-wall injection molding of high-density polyethylene for infrared radiation system lenses

Abstract

<jats:title>Abstract</jats:title><jats:p>High-density polyethylene (HDPE) lenses are used for infrared radiation (IR) systems, such as radiation thermometers to transmit the IR of the 10 μm region. High IR transmittance and low visible ray (VR) transmittance are necessary for IR system lenses. This experimental investigation of thin-wall injection molding was conducted using 0.5 mm cavity thickness with a disk shape, finished to a mirror-like surface. As factors affecting transmittance, we evaluated the thickness, surface roughness, crystallinity, internal structure, and molecular chain orientation of molded parts, which were produced using four HDPE melt flow rates (MFRs). The changed molding conditions were mold temperature and holding pressure. Results showed that the thin-molded parts had higher IR transmittance. The thin-molded part was obtained with the smallest MFR of 5. Furthermore, the VR transmittance decreased when the molecular chain orientation in the molded parts was small and the crystallinity was high. The small orientation and high crystallinity were obtained simultaneously with the largest MFR of 42. Therefore, it was impossible to obtain high IR transmittance and low VR transmittance simultaneously by a change of MFR. This study confirmed that surface roughness and crystallinity do not affect transmittance.</jats:p>

Influence mechanism of injection process parameters on iridescent pattern defects on optical molded parts

“…In the pre-experiments, six key injection process parameters were selected based on previous work. [17][18][19][20][21][22][23][24][25] All achievable levels of each injection process parameter were selected according to the type of polymer material, previous work, and simulation results. [17][18][19][20][21][22][23][24][25] From the analysis results of the pre-experiments, it can be obtained that holding time and cooling time have almost no effect on the iridescent patterns because they have a small effect on the retardation and transmittance, which is consistent with the previous work and simulation results.…”

“…[17][18][19][20][21][22][23][24][25] All achievable levels of each injection process parameter were selected according to the type of polymer material, previous work, and simulation results. [17][18][19][20][21][22][23][24][25] From the analysis results of the pre-experiments, it can be obtained that holding time and cooling time have almost no effect on the iridescent patterns because they have a small effect on the retardation and transmittance, which is consistent with the previous work and simulation results. Therefore, the holding time and cooling time are fixed, and the four main process parameters of mold temperature, melt temperature, injection speed, and holding pressure, which significantly affect the iridescent patterns, were chosen as the experimental factors in the paper.…”

“…The results showed that the increase of holding pressure was favorable to improving the uniformity of the refractive index distribution. Kaneda et al [23][24][25] investigated the effects of mold temperature and holding pressure on the transmittance of polyethylene lenses and concluded that the mold temperature and holding pressure affected the transmittance mainly by changing the degree of orientation and the crystallinity of part. Furthermore, the transmittance decreased as the orientation decreased and the crystallinity increased.…”