Effects of gate design and cavity thickness on filling, morphology and mechanical properties of microinjection mouldings

Abstract

Miniaturized parts weighing single or tens of milligrams represent a large category of microinjection moulded products. Both miniaturization and extreme processing under microinjection moulding subject material to high shear rates and high cooling rates, and cause the same material to have different morphologies and final properties when used in conventional injection moulding. It also makes mold design challenging. This study investigates the effects of micro gate design (opening and thickness) and cavity thickness (100-500μm) on filling, morphology and the mechanical properties of miniaturized dumbbell parts. It is found that a reduction of gate size has two conflicting effects, namely, increased shear heating increases flow length, and increased cooling rate reduces flow length. Filling increases significantly with an increase of cavity thickness. In addition, the thickness of the skin layer reduces from ~70% to ~10% when part thickness increases from 100μm to 500μm. This oriented skin layer determines molecular orientation and broadly influences Young's modulus, elongation and yield stress. Natural aging at room temperature induces an increase of modulus and yield stress, and a decrease of strain at break. The mechanical properties of microinjection moldings is significantly different from those of conventional injection moldings and measurement at microscale is required for miniaturized product design. 1 Effect of Gate Design and AbstractMiniaturized parts weighing single or tens of milligrams represent a large category of microinjection moulded products. Both miniaturization and extreme processing under microinjection moulding subject material to high shear rates and high cooling rates, and cause the same material to have different morphologies and final properties when used in conventional injection moulding. It also makes mold design challenging. This study investigates the effects of micro gate design (opening and thickness) and cavity thickness (100-500μm) on filling, morphology and the mechanical properties of miniaturized dumbbell parts. It is found that a reduction of gate size has two conflicting effects, namely, increased shear heating increases flow length, and increased cooling rate reduces flow length.Filling increases significantly with an increase of cavity thickness. In addition, the thickness of the skin layer reduces from ~70% to ~10% when part thickness increases from 100μm to 500μm. This oriented skin layer determines molecular orientation and broadly influences Young's modulus, elongation and yield stress. Natural aging at room temperature induces an increase of modulus and yield stress, and a decrease of strain at break. The mechanical properties of microinjection moldings is significantly different from those of conventional injection moldings and measurement at microscale is required for miniaturized product design.

Microinjection molding of polypropylene/multi‐walled carbon nanotube nanocomposites: The influence of process parameters

“…The properties of PP/CNT nanocomposite were not available in the materials database for Moldflow. In this scenario, a substitution could be chosen from the software database to perform simulation analysis . Then, a replacement of unfilled PP (Adstif HA740N, Basell Polyolefins North America), which has a similar structure to that of the PP used in this study, was selected for evaluating the distribution of possible maximum shear rates within microparts.…”

“…Then, a replacement of unfilled PP (Adstif HA740N, Basell Polyolefins North America), which has a similar structure to that of the PP used in this study, was selected for evaluating the distribution of possible maximum shear rates within microparts. The actual processing conditions were extracted from injection molding machine and were employed as the inputs to Moldflow to represent the actual molding conditions .…”

“…Polymers and polymer‐based composites play an important role in these areas due to their superior properties, such as light weight, easier processability and excellent resistance to corrosive environments when compared with metals . Therefore, microinjection molding (μIM) of polymeric products has gained considerable research interest due to its suitability for mass production of high precision micro‐components at relatively low cost .…”

Electrical and morphological properties of microinjection molded polypropylene/carbon nanocomposites

“…The slight discrepancy could be attributed to the presence of higher shear rates in the microinjection molding (µIM) process, which largely favors the orientation of CNT in the flow direction. The difference of skin/core ratio could be another contributing factor as Zhang et al . found that there is a remarkable increase of skin/core ratio with a decrease of part thickness in µIM.…”

Dynamic local temperature control in micro‐injection molding: Effects on poly(lactic acid) morphology

“…Consequently, the materials experience high stresses and shear rates. This thermomechanical situation could have an influence on the nucleation and growth rate of crystalline entities as far as to material degradation .…”