Morphological evolution of HDPE parts in the microinjection molding: Comparison with conventional injection molding

A Review of Microinjection Moulding of Polymeric Micro Devices

Polymeric micro devices are gaining huge market potential in broad areas of medical devices, diagnostic devices, drug delivery, and optical applications. Current research is focusing on developing functional polymeric micro devices on a mass-production scale. Microinjection moulding is a promising technique suitable for fabricating polymeric micro devices. This review aims to summarise the primary achievements that have been achieved in various aspects of microinjection moulding of polymer micro devices, consisting of micro parts and micro surface structures. The relationships of the machine, process, rheology, tooling, micro/nanoscale replication, morphology, properties, and typical applications are reviewed in detail. Finally, a conclusion and challenges are highlighted.

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

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.

Optical Properties of HDPE in Injection Molding and Injection Press Molding for IR System Lenses Part II: Mold Temperature and Surface Roughness Effects on Injection Molding

An experimental investigation of injection molding was conducted to assess the high infrared radiation (IR) transmittance with an opaque state (low visibility ray (VR) transmittance) necessary for IR system lenses as a target high-density polyethylene (HDPE) IR transmission material. In our earlier investigation, experimental investigations of injection molding and injection press molding were conducted using the mold cavity of a disk-shaped finished mirror-like surface. For high mold temperature and long cooling time, the increased core-layer thickness improved the IR transmittance. Simultaneously, the VR transmittance decreased because the crystallinity became higher. Furthermore, when injection press molding was done, the VR transmittance decreased because the crystallinity increased. Using a 0.5-mm-thick mold cavity with disk shapes having different surface roughness, an injection molding experiment was conducted for this study while changing only the mold temperature of 20 to 80°C. Results show that when the mold surface roughness became high, the surface roughness of molded parts became high, too. Thereby the IR and VR transmittance were decreased. However, when the mold temperature was high, the influences of the so-called skin-core structure and crystallinity were stronger than the influence of the surface roughness of molded parts. The IR transmittance increased because of the decrease of the skin layer. Furthermore, the VR transmittance decreased because of a simultaneous rise of the crystallinity. The minimum value of obtained VR transmittance was 9.3% at mold temperature of 80°C using a mold of the highest surface roughness. This molded part reached an opaque state. Furthermore, a higher IR transmittance of 64.9% was obtained. When injection molding was conducted using a mold of a mirror-like surface with the same conditions as those in this investigation in our earlier investigation, the obtained IR and VR transmittances were 65.4% and 0.6%, respectively. Consequently, when the mold temperature was high, a few differences were found with transmittance of the molded parts using the mold of the mirror-like surface. Results showed that no difficulty arises when a mold having higher surface roughness is used at a higher mold temperature.

Optical Properties of HDPE in Injection Molding and Injection Press Molding for IR System Lenses

To obtain the “high infrared radiation (IR) transmittance and low visible ray (VR) transmittance” necessary for a high-performance IR lens, cavity thickness and molding conditions were investigated experimentally for disk-shaped high-density polyethylene (HDPE). When injection molding and injection press molding were done using different cavity thicknesses, results showed that the molded part using the thinner cavity thickness maintained a semitransparent state. Simultaneously, they showed that higher IR transmittance was obtained. For high mold temperature and long cooling time, the increased core-layer thickness improved the IR transmittance. The VR transmittance decreased because crystallinity became higher. Furthermore, when injection press molding was conducted, the VR transmittance decreased because crystallinity became higher. IR transmittance and VR transmittance were obtained respectively as 65.4% and 6.4% when injection press molding was conducted at higher mold temperatures with longer cooling times.

In situ nano-fibrillation of microinjection molded poly(lactic acid)/poly(ε-caprolactone) blends and comparison with conventional injection molding

During microinjection molding, there are highly oriented PCL nanofibrils in situ formed, while during conventional injection molding, there are oriented microfibrils in situ formed.

A novel morphology development of micro-injection molded isotactic polypropylene

The appearance of a stripe morphology in MIM-iPP was induced by the optical path difference of the polarized light on the polymer crystal.