Micro Injection Moulding Research Articles

Investigation on a novel in-mould microcompression system for the precision replication of microlens arrays

As a combination of conventional injection moulding and compression moulding, injection compression moulding is expected to deliver a more uniform distribution of cavity pressure and more precise replication of surface structures. However, processing defects such as flash and design constraints such as thin edges of a product can cause non-uniform compression and imperfect geometric accuracy. The present work proposes a novel in-mould microcompression system, providing high precision compression that is in the magnitude of micrometres during the microinjection moulding process. From the perspectives of geometric accuracy, surface quality and stress birefringence, the influence of the in-mould microcompression system is examined based on conventional microinjection moulding and variotherm assisted microinjection moulding of practical microlens arrays that are designed for light-field applications. The results indicate that the in-mould microcompression can effectively enhance the replication and uniformity in terms of geometric accuracy and surface quality, and that it can marginally reduce residual stresses. As such, this constitutes an innovative solution for the precision replication of optical components.

Effect of Micro-Mold Cavity Dimension on Structure and Property of Polylactic Acid/Polycaprolactone Blend under Microinjection Molding Conditions.

Microinjection molding is a novel frontier polymer processing strategy different from conventional ones. In this paper, three different cavity-sizes of micro-mold tools were firstly fabricated, and the influences of micro-mold cavity dimension on the phase morphology structure, crystallization and orientation, and mechanical performance of the microinjection molded polylactic acid (PLA)/polycaprolactone (PCL) blend microparts were carefully investigated accordingly. The results show that the reduction of the cavity size would result in much higher shear stress field and cooling temperature gradient, which is advantageous to the fibrillation and orientation of PCL-dispersed phase. Consequently, with decreasing the micro-mold cavity dimension from length 26 mm to 15 mm, the interfacial compatibility is improved, significantly increasing number of PCL fibers with smaller diameter are in situ formed in PLA matrix and their orientation degree also obviously increases, which is verified by SEM and 2D-WAXD measurements. The Differential Scanning Calorimetry (DSC) analysis shows that the decrease in cavity dimension causes the enhancement of PLA crystallization property due to shear-induced crystallization, which is reflected by the decreasing PLA cold crystallization temperature and increasing PLA crystallinity (almost doubling that of conventional macropart). As a result, the dynamic/static mechanical property measurements exhibit that with decreasing the cavity size, the storage modulus, and the loss modulus of PLA/PCL blend micropart increase, and the corresponding tensile strength, elongation at break, and Young’s modulus also present an obviously increasing tendency. The related investigations would provide some new spaces and insights for realization of high-performance of PLA/PCL blend micropart.

Ultrasonic nodal point, a new configuration for ultrasonic moulding technology

Ultrasonic moulding is a new technology that uses high power ultrasound to melt and mould thermoplastic polymers to produce samples with mini and micro features. The main feature of this technology is the use of ultrasonic energy as the heating source instead of a conventional injection screw. Even if ultrasonic moulding overcomes some of the drawbacks of conventional mini and micro-injection moulding, it still presents two main limitations that are hindering its widespread applicability: the lack of stability of the process and the difficulty to obtain samples with good mechanical properties for some materials. This article presents a new configuration, called nodal point ultrasonic moulding (NPUSM), to surmount such limitations. This configuration improves the stability of ultrasonic moulding technology and it is capable of processing materials with good mechanical properties. To prove its efficacy, the nodal point ultrasonic moulding configuration is used to obtain the processing window of a polyoxymethylene material and these results are compared with standard ultrasonic injection and conventional injection moulding. The results obtained show that NPSUM configuration improves the capabilities of ultrasonic moulding technologies.

Characterization of process and machine dynamics on the precision replication of microlens arrays using microinjection moulding

Injection moulding has shown its advantages and prevalence in the production of plastic optical components, the performance and functionality of which rely on the precision replication of surface forms and on minimizing residual stress. The present work constitutes a systematic and comprehensive analysis of practical microlens arrays that are designed for light-field applications. Process parameters are screened and optimized using a two-stage design of experiments approach. Based on in-line process monitoring and a quantitative and qualitative evaluation being carried out in terms of geometric accuracy, surface quality and stress birefringence, the replication is shown to relate directly to machine settings and dynamic machine responses. The geometric accuracy and stress birefringence are both largely associated with screw displacement and peak cavity pressure during the packing stage, while surface quality is closely related to cavity temperature. This study provides important insights and recommendations regarding the overall replication quality of microlens arrays, while advanced injection moulding solutions may be necessary to further improve the general replication quality.

Development of 3D printed rapid tooling for micro-injection moulding

The use of additive manufacturing techniques in conjunction with injection moulding is becoming increasingly popular, with financial and time benefits to coupling the techniques. This study demonstrates a systematic development process of 3D printed rapid tooled moulds using stereolithography. A high flexural modulus and elongation were found to increase the likelihood of success of a mould material in the injection moulding process. Success is defined as the mould surviving the process and being capable of producing the desired object successfully. Stereolithography was found to produce high quality moulds when a diagonal print orientation and a scaling factor of 109.3% is employed. The presented technique and systematic workflow is highly suitable for the production of moulds with detailed micro-features. This is of particular interest for rapid tooling for micro-injection moulding for the manufacture of pharmaceuticals and medical devices, where the microstructure directly impacts the performance of the products.

Micro-Injection Molding of Diffractive Structured Surfaces

In recent years, the use of highly functional optical elements has made its way into our everyday life. Its applications range from use in utility items such as cell phone cameras up to security elements on banknotes or production goods. For this purpose, the Leibniz Institute for Materials Engineering (IWT) has been developing a cutting process for the fast and cost-effective production of hologram-based diffractive optical elements. In contrast to established non-mechanical manufacturing processes, such as laser lithography or chemical etching, which are able to produce optics in large quantities and with high accuracy, the diamond turning approach is extending these properties by offering several degrees of freedom. This allows for an almost unlimited geometric complexity and a structured area of considerable size (several tenth square millimeters), achieved in a single process step. In order to introduce diffractive security features to the mass market and to actual production goods, a high-performance replication process is required as the consecutive development step. Micro injection molding represents a feasible and promising option here. In particular, diamond machining enables the integration of safety features directly into the mold insert. Not only does this make additional assembly obsolete, but the safety feature can also be placed inconspicuously in the final product. In this paper, the potential of micro-injection molding as a replication process for diffractive structured surfaces will be investigated and demonstrated. Furthermore, the optical functionality after replication will be verified and evaluated.

A cost-effective process chain for thermoplastic microneedle manufacture combining laser micro-machining and micro-injection moulding

High-throughput manufacturing of transdermal microneedle arrays poses a significant challenge due to the high precision and number of features that need to be produced and the requirement of multi-step processing methods for achieving challenging micro-features. To address this challenge, we report a flexible and cost-effective process chain for transdermal microneedle array manufacture that includes mould production using laser machining and replication of thermoplastic microneedles via micro-injection moulding (micromoulding). The process chain also incorporates an in-line manufacturing data monitoring capability where the variability in the quality of microneedle arrays can be determined in a production run using captured data. Optical imaging and machine vision technologies are also implemented to create a quality inspection system that allows rapid evaluation of key quality indicators. The work presents the capability of laser machining as a cost-effective method for making microneedle moulds and micro-injection moulding of thermoplastic microneedle arrays as a highly-suitable manufacturing technique for large-scale production with low marginal cost.

Functional Analysis Validation of Micro and Conventional Injection Molding Machines Performances Based on Process Precision and Accuracy for Micro Manufacturing

Micro polymer parts can be usually manufactured either by conventional injection moulding (IM) or by micro-injection moulding (µIM). In this paper, functional analysis was used as a tool to investigate the performances of IM and µIM used to manufacture the selected industrial component. The methodology decomposed the production cycle phases of the two processes and attributed functions to parts features of the two investigated machines. The output of the analysis was aimed to determine casual chains leading to the final outcome of the process. Experimental validation of the functional analysis was carried out moulding the same micro medical part in thermoplastic elastomer (TPE) material using the two processes by means of multi-cavity moulds. The produced batches were assessed using a precision scale and a high accuracy optical instrument. The measurement results were compared using capability indexes. The data-driven comparison identified and quantified the correlations between machine design and part quality, demonstrating that the µIM machine technology better meets the accuracy and precision requirements typical of micro manufacturing productions.

Investigation of Interface Thermal Resistance between Polymer and Mold Insert in Micro-Injection Molding by Non-Equilibrium Molecular Dynamics.

Micro-injection molding has attracted a wide range of research interests to fabricate polymer products with nanostructures for its advantages of cheap and fast production. The heat transfer between the polymer and the mold insert is important to the performance of products. In this study, the interface thermal resistance (ITR) between the polypropylene (PP) layer and the nickel (Ni) mold insert layer in micro-injection molding was studied by using the method of non-equilibrium molecular dynamics (NEMD) simulation. The relationships among the ITR, the temperature, the packing pressure, the interface morphology, and the interface interaction were investigated. The simulation results showed that the ITR decreased obviously with the increase of the temperature, the packing pressure and the interface interaction. Both rectangle and triangle interface morphologies could enhance the heat transfer compared with the smooth interface. Moreover, the ITR of triangle interface was higher than that of rectangle interface. Based on the analysis of phonon density of states (DOS) for PP-Ni system, it was found that the mismatch between the phonon DOS of the PP atoms and Ni atoms was the main cause of the interface resistance. The frequency distribution of phonon DOS also affected the interface resistance.

Precision replication of microlens arrays using variotherm-assisted microinjection moulding

Injection moulding is ubiquitous in the production of plastic optical components, the functionality of which depends on minimising residual stress and on precisely replicating surface forms. In this study, variotherm-assisted microinjection moulding is used to manufacture a practical microlens array designed for light-field applications in order to achieve satisfactory geometric accuracy, surface quality and stress birefringence of the microlenses, all of which are difficult to achieve by conventional microinjection moulding. Based on in-line process monitoring and a comprehensive evaluation regarding the quantitative and qualitative aspects of the aforementioned factors, the replication is shown to correlate well with machine settings and dynamic machine responses. Screw movements are found to be crucial in the replication process. Compared to an optimised conventional microinjection moulding condition, the general residual stress level and uniformity of the microlens array area are improved by 5.08% and 88.11%, respectively, when the warm circuit temperature is 135 °C and enhanced by 2.73% and 84.32%, respectively, by a delayed switch from the warm circuit to the cold circuit. It is concluded that variotherm-assisted microinjection moulding can significantly reduce residual stresses while maintaining excellent geometric accuracy and surface quality.

Ultrasonic micromoulding: Process characterisation using extensive in-line monitoring for micro-scaled products

Industry-standard quality management systems such as Six Sigma and emerging Industry 4.0 compliant production processes demonstrate the importance of in-line condition monitoring of manufacturing methods for achieving the highest levels of product quality. Measurement data collected as the process is running can inform the operator about unexpected changes in machine operation or raw materials that could negatively impact production; and offer an opportunity for a process control intervention to stabilise production. However, micro-manufacturing production lines can pose a challenging environment for deploying such systems, since processing events can occur extremely rapidly and in harsh environments. Moreover, the small scale of micro-nano featured components can make sensor installation even more problematic. Recently, ultrasonic micromoulding has drawn attention in niche markets due to its unique advantages for processing thermoplastics as a new micro-manufacturing technology. The process differs from conventional moulding significantly by eliminating the need for a plasticising screw and using direct application of ultrasonic energy to melt the polymer. This offers numerous benefits such as decrease in energy usage, moulding at lower pressures, easier cleaning, and reduced material residence times, the latter which could be beneficial for pharma-grade polymers or polymers with active ingredients. However, very little work has been reported attempting to monitor the process using in-line measurements. This work aims to evaluate the characteristics of the ultrasonic micromoulding process for microinjection moulding of a microneedle array using a range of sensor technologies including: data recorded by the machine controller; a high-speed thermal camera and a cavity pressure transducer. The data has captured the highly dynamic process environment with a high degree of accuracy. The relationship between the process data and dimensional quality of the ultrasonically micromoulded products has been quantified and subsequently implemented as a cost-effective in-line quality assurance method.

Abstract 43: Fully integrated single instrument imaging-based digital PCR platform

Abstract Digital PCR (dPCR) has gained popularity in recent years for cancer research such as rare mutation detection, minimal residual disease, treatment selection, and recurrence monitoring. However, currently available technologies suffer from several limitations such as tedious workflow, long time-to-result, poor multiplexity, and inconsistent reagent digitization that severely hindered its broad adoption. We have designed and manufactured a dPCR platform that is capable of consistent sample digitization, thermal cycling and simultaneous interrogation of 20,000 partitions with walk-away workflow. The instrument integrates a loading and digitization manifold, a thermal cycler and an imaging engine with 5 optical channels into a single benchtop box. The Microfluidic Array Partitioning (MAP) consumable is a micro-injection molded, automation-ready plate that contains 16 individual units that can be loaded simultaneously. Each unit partitions an individual sample into 20,000 fluidically isolated partitions while utilizing >90% of inputted sample. We utilized an oncology standard assay, confocal microscopy, and real-time imaging to validate the dynamic range, sample utilization, and digitization consistency of the platform, respectively. Using the European Breast Cancer Standards ERM-AD623 we quantified replicates that were serially diluted from 1 × 105cp/μL down to 10cp/μL. We demonstrated that the dPCR platform can consistently quantify targets over 5-orders-of-magnitude of concentration with high consistency in both quantification and partition numbers. With confocal microscopy, the reagent inside the partitions are visualized to characterize sample utilization. Using conventional qPCR calibration dye ROX and real-time imaging, we show that over 99% of the partitions can be consistently analyzed for high quantification confidence. With the combination of ease-of-use, fast time-to-result and consistent performance, we believe this novel dPCR platform provides unparalleled usability and capability and will allow dPCR to fulfill its promise to impact cancer patient care. Citation Format: Robert Lin, Andrew Zayac, Steve Gallagher, Lingxia Jiang, Felicia Linn, Paul Hung. Fully integrated single instrument imaging-based digital PCR platform [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 43.

Formation Mechanism of Residual Stresses in Micro-Injection Molding of PMMA: A Molecular Dynamics Simulation.

Injection molding is an economical and effective method for manufacturing polymer parts with nanostructures and residual stress in the parts is an important factor affecting the quality of molding. In this paper, taking the injection molding of polymethyl methacrylate (PMMA) polymer in a nano-cavity with an aspect ratio of 2.0 as an example, the formation mechanism of residual stresses in the injection molding process was studied, using a molecular dynamics simulation. The changes in dynamic stress in the process were compared and analyzed, and the morphological and structural evolution of molecular chains in the process of flow were observed and explained. The effects of different aspect ratios of nano-cavities on the stress distribution and deformation in the nanostructures were studied. The potential energy, radius of gyration and elastic recovery percentage of the polymer was calculated. The results showed that the essence of stress formation was that the molecular chains compressed and entangled under the flow pressure and the restriction of the cavity wall. In addition, the orientation of molecular chains changed from isotropic to anisotropic, resulting in the stress concentration. At the same time, with the increase in aspect ratio, the overall stress and deformation of the nanostructures after demolding also increased.

High Temperature Adiabatic Heating in µ-IM Mould Cavities-A Case for Venting Design Solutions.

Micro-injection moulding (µ-IM) is a fabrication method that is used to produce miniature parts on a mass production scale. This work investigates how the process parameter settings result in adiabatic heating from gas trapped and rapidly compressed within the mould cavity. The heating of the resident air can result in the diesel effect within the cavity and this can degrade the polymer part in production and lead to damage of the mould. The study uses Autodesk Moldflow to simulate the process and identify accurate boundary conditions to be used in a gas law model to generate an informed prediction of temperatures within the moulding cavity. The results are then compared to physical experiments using the same processing parameters. Findings from the study show that without venting extreme temperature conditions can be present during the filling stage of the process and that venting solutions should be considered when using µ-IM.

Microinjection Molding of Out-of-Plane Bistable Mechanisms.

We present a novel fabrication technique of a miniaturized out-of-plane compliant bistable mechanism (OBM) by microinjection molding (MM) and assembling. OBMs are mostly in-plane monolithic devices containing delicate elastic elements fabricated in metal, plastic, or by a microelectromechanical system (MEMS) process. The proposed technique is based on stacking two out-of-plane V-beam structures obtained by mold fabrication and MM of thermoplastic polyacetal resin (POM) and joining their centers and outer frames to construct a double V-beam structure. A copper alloy mold insert was machined with the sectional dimensions of the V-beam cavities. Next, the insert was re-machined to reduce dimensional errors caused by part shrinkage. The V-beam structure was injection-molded at a high temperature. Gradually elongated short-shots were obtained by increasing pressure, showing the symmetrical melt filling through the V-beam cavities. The as-molded structure was buckled elastically by an external-force load but showed a monostable behavior because of a higher unconstrained buckling mode. The double V-beam device assembled with two single-molded structures shows clear bistability. The experimental force-displacement curve of the molded structure is presented for examination. This work can potentially contribute to the fabrication of architected materials with periodic assembly of the plastic bistable mechanism for diverse functionalities, such as energy absorption and shape morphing.

Development of Process Chain for Micro-Injection Molding

In today’s continuously growing demand for components with increasingly smaller dimensions and features, micro-manufacturing is gaining more significance. For the mass production of plastic components with micro-features, injection molding is particularly suitable and still customary in order to keep target costs. Due to high requirements regarding lifetime and resistance to wear, the molds are made of hardened steel. The shaping of these molds involves electrical discharge machining (EDM). This process allows the generation of micro-structures in micrometer range having small inner radii, high dimensional accuracy and extreme aspect ratio independent of the workpiece hardness. As a work tool for EDM, electrodes made of pure copper (Cu) and tungsten reinforced copper (WCu) are commonly used. The shaping of the electrodes is conducted by micro-milling. For an overall understanding of this process chain, the interaction between micro-milling, micro-EDM, and micro-injection molding must be evaluated. This paper provides knowledge on the limits of each process with regard to burr formation, form accuracy, structure size and aspect ratio. Many factors throughout the process chain affect the size of an attainable micro-feature of a final product. The proper selection of the electrode material is a key factor in the process chain. As the feature size of the electrode defines the dimension of the final product shape, the smallest possible structures have to be found during micro-milling. The dimension of the eroded cavity is defined by the feature size of the electrode in combination with the lateral working gap. Eroded cavities with small inner corner radii and steep flanks can be generated when applying flawless and burr-free electrodes. However, using electrodes in inadequate conditions can lead to worse outcomes. The quality and reliability of the final product are determined to a great extent by the design of the injection molding process. Due to the arising vacuum when evacuating the remaining air in the mold, the inflowing melt is being distributed equally. It must be guaranteed that during injection molding, the mold is thermally-controlled. This prevents premature solidification. Through a combination of these two strategies, a form filling rate in micro-cavities up to 100 % is carried out. By deriving an impeccably adapted process chain, a precise micro-molding can be performed. This leads to the capability of manufacturing bars on a final plastic part with a width and height as low as 55 x 100 µm, and a length of 2 mm.

Characterization of manufacturability of microstructures for micro-injection moulding of micro devices using star patterns

The replication accuracy of micro features plays a very important role in polymeric micro devices, such as microfluidic and micro optics. However, there is no effective way at the moment for the assessment of manufacturability of such microstructures. This study develops a new method of using a star pattern with a continuous change of width and aspect ratio to characterize the manufacturability of microstructures. The designed patterns have a width from 20 microns to 320 microns with an aspect ratio from 2.5 to 0.156. Concentric markers are designed on the star patterns for the easy identification of achievable feature accuracy under various processes. It is found that the dry etching and electroforming cause a depth error of less than 5%, while the maximum error from injection moulding is 2.8% where the channel aspect ratio is less than 1. However, when the aspect ratio of protrusion microstructures is higher than 1 with a width of 50 µm, insufficient filling occurs under nominal injection moulding conditions with relative error more than 10%. The designed star patterns are proved to be an easy and fast way to determine achievable feature accuracy for polymer micro replication processes, including injection moulding and hot embossing.

Design of Ultrasonic-assisted Microinjection Mold and Cavity Pressure Measurement System

Design of Ultrasonic-assisted Microinjection Mold and Cavity Pressure Measurement System

Correlating nano-scale surface replication accuracy and cavity temperature in micro-injection moulding using in-line process control and high-speed thermal imaging

Micro-injection moulding (μIM) stands out as preferable technology to enable the mass production of polymeric components with micro- and nano-structured surfaces. One of the major challenges of these processes is related to the quality assurance of the manufactured surfaces: the time needed to perform accurate 3D surface acquisitions is typically much longer than a single moulding cycle, thus making impossible to integrate in-line measurements in the process chain. In this work, the authors proposed a novel solution to this problem by defining a process monitoring strategy aiming at linking sensitive in-line monitored process variables with the replication quality. A nano-structured surface for antibacterial applications was manufactured on a metal insert by laser structuring and replicated using two different polymers, polyoxymethylene (POM) and polycarbonate (PC). The replication accuracy was determined using a laser scanning confocal microscope and its dependence on the variation of the main μIM parameters was studied using a Design of Experiments (DoE) experimental approach. During each process cycle, the temperature distribution of the polymer inside the cavity was measured using a high-speed infrared camera by means of a sapphire window mounted in the movable plate of the mould. The temperature measurements showed a high level of correlation with the replication performance of the μIM process, thus providing a fast and effective way to control the quality of the moulded surfaces in-line.

Efficient and Precise Micro-Injection Molding of Micro-Structured Polymer Parts Using Micro-Machined Mold Core by WEDM.

In this paper, micro-structured polymer parts were efficiently and accurately fabricated by micro-injection molding using a micro-structured mold core machined by wire electrical discharge machining (WEDM). The objective was to realize low-cost mass production and manufacturing of micro-structured polymer products. The regular micro-structured mold core was manufactured by precise WEDM. The micro-structured polymer workpieces were rapidly fabricated by micro-injection molding and the effects of the micro-injection molding process parameters on replication rate and surface roughness of micro-structured polymers were systematically investigated and analyzed. It is shown that the micro-structured polymer can be rapidly and precisely fabricated by the proposed method. The experimental results show the minimum size machining error of the micro-structured mold core and the maximum replication rate of micro-formed polymer were 0.394% and 99.12%, respectively. Meanwhile, the optimal micro-injection molding parameters, namely, jet temperature, melt temperature, injection velocity, holding pressure and holding time were 195 °C, 210 °C, 40 mm/min, 7 Mpa and 5 s, respectively. The surface roughness Ra at the groove bottom and top of the micro-structured polymer workpieces achieved minimum values of 0.805 µm and 0.972 µm, respectively.