An Investigation of Robust Tooling for Microinjection Molding

Abstract

The advancement of microinjection molding (μIM) as a mass production technology for components with nano/microscale features primarily depends on durable tooling to meet the increasing demand for such products. Traditional tooling made of materials such as steel, silicon (Si), and nickel (Ni) each presents distinct limitations. Steel, for instance, cannot integrate features smaller than its grain boundaries, whereas Si and Ni-based inserts often fail prematurely under high-throughput manufacturing demands. These challenges have led to the adoption of bulk metallic glass (BMG) as a durable and robust alternative for tooling, capable of enduring over 20,000 cycles as reported in the literature. BMG is increasingly employed to fabricate micro/nanostructured tooling for the μIM process. This research investigates the performance of tooling in the μIM process using BMG-based, Si-based, and Ni-based inserts, with a particular emphasis on the robustness evaluation of BMG-based tooling. The investigation involves the fabrication process of microfeatures on three BMG-based inserts, with different geometries, utilizing the focused Ion Beam (FIB) technique. It emphasizes the durability and performance of BMG-based tooling in comparison to traditional Si-based and Ni-based tooling, where performance limitations were addressed during this work. The project includes the enhancement of a previously developed model for the apparent elastic modulus of molded micropillars to address certain limitations that occurred during this research. Additionally, various aspects were investigated to enhance the scientific understanding of the μIM process. Key findings from Moldflow simulations and experimental trials identified key factors impacting the filling behavior and replication quality of microfeatures including molded part and microfeatures geometries, and processing parameters. The simulations indicated a significant deflection towards the ejection direction and less radial deflection, attributed to the proximity of the sprue to the micro-featured area. This resulted in lower aspect ratio features filling rapidly but cooling slowly, which may have contributed to various phenomena of micropillars failure observed during the inspection of molded micropillars. Experimental work with BMG-based inserts of low aspect ratio demonstrated excellent replication quality despite challenges such as potential crystallization and the instantaneous disintegration of titanium-based coatings, which showed no substantial damage to the structural integrity of the inserts. Durability testing exceeded 1,000 and 5,100 molding cycles for BMG-I, and BMG-II inserts, respectively. The BMG-I insert exhibited degradation of replication quality over molding cycles, occurring alongside surface morphology changes due to potential crystallization from elevated heating of the mold assembly. It showed a degradation rate of 0.428 nm/cycle. Additionally, the BMG-II insert exceeded 5,100 molding cycles regardless of dynamic conditions arising from various molding issues during the process. This insert showed a degradation rate of 0.0492 nm/cycle. It showed a better degradation rate compared to the previous insert and those reported in the literature with a value of 0.115 nm/cycle. Although dynamic conditions introduced process variability during molding trials, process parameter adjustments to adapt to these conditions introduced some degree of uncertainties in the height measurements of molded micropillars. Consequently, microcavities depth measurements were adopted for more reliable performance evaluation. Overall, both BMG-based inserts demonstrated robust performance, especially the BMG-II insert, which is projected to last over 83,000 molding cycles based on a simple linear fit using only two data points. In contrast, Si-based inserts with low aspect ratio microfeatures showed consistent failure patterns, potentially originating from nanoscale scallops created on the sidewalls by the deep reactive ion etching (DRIE) process. These scallops acted as high-stress concentration points, potentially initiation cracks. Additionally, Ni-based inserts with low thickness failed prematurely under high injection pressures but preserved microfeature geometry to some extent. Other inserts successfully withstood more than 150 molding cycles without visible structural damage. Larger replicated microfeatures exhibited better replication quality due to reduced flow resistance, which enhanced cavity filling, whereas smaller microfeatures showed reduced quality due to premature solidification. The replication of high aspect ratio micropillars using thermoplastic polyurethane (TPU) was prone to defects such as stretching and directional collapses. The processes of filling and stretching of micropillars were influenced by processing parameters such as packing pressure and cooling time. Lip-like features were commonly observed on low aspect ratio micropillars using polystyrene (PS), indicating contact between micropillars and the sidewalls of the microcavities. This observation was confirmed by numerical investigations of various ejection conditions. The presence of these features was notably more distinct when the substrate rotated counter-clockwise with minimal radial movements, and also when rotated clockwise but with increased radial movements. Numerical investigations of the demolding process demonstrated that changes in radial movement and rotational values drastically affect micropillar integrity and stress distribution. This dissertation advances the understanding of BMG-based tooling as a robust solution that could enhance the manufacturing process in high-volume production settings. It provides valuable insights into the interactions between polymer melts and tooling under dynamic conditions, contributing to manufacturing practices and tool design. Additionally, the research explores the impact of process variability on tooling performance throughout molding cycles to provide insights into key challenges and opportunities for improvement.