Ultrasonic Hot Embossing Research Articles

Coupled thermomechanical finite element analysis of ultrasonic hot embossing process

Ultrasonic Hot Embossing of polymers is one of the most attractive manufacturing methods of high-quality and complex microparts needed for biological and chemical processes. A little simulation work has been done to study the deformation mechanism and the required load in the Ultrasonic Hot Embossing of polymethyl methacrylate (PMMA). This paper developed a 2D coupled thermo-mechanical finite element model to simulate the forming behavior of the ultrasonic embossing process of PMMA, optimize the mold microfeature corner, and predict the embossing load as well. VDISP ABAQUS subroutine has been used to describe the downward motion of the mold assisted with ultrasonic vibration. The Arbitrary Eulerian-Lagrangian re-meshing technique has been implemented to refine the deformation zone during simulation to avoid divergence due to localized excessive deformation. Embossing load, temperature, stresses, strains, and energy dissipation have been recorded and analyzed. Simulation results revealed that the coupled thermo-mechanical FE Model efficiently captures the deformation behavior and Embossing load. It also proved that a Mold microfeature corner of 20 μm filet radius is recommended to adequately fill out the area around the mold and maintain uniform temperature and strain distributions.

Dynamic Mechanical Properties of PVC Plastics in the Formation of Microstructures with Novel Magnetostrictor

Molding in thermoplastic polymers using ultrasonic hot embossing technology is promising due to its high precision reproducibility. To understand, analyze and apply the formation of polymer microstructures by the ultrasonic hot embossing method, it is necessary to understand dynamic loading conditions. The Standard Linear Solid model (SLS) is a method that allows analyzing the viscoelastic properties of materials by representing them as a combination of springs and dashpots. However, this model is general, and it is challenging to represent a viscoelastic material with multiple relaxations. Therefore, this article aims to use the data obtained from dynamic mechanical analysis for extrapolation in a wide range of cyclic deformations and to use the obtained data in microstructure formation simulations. The formation was replicated using a novel magnetostrictor design that sets a specific temperature and vibration frequency. The changes were analyzed on a diffractometer. After the diffraction efficiency measurement, it was found that the highest quality structures were formed at a temperature of 68 °C, a frequency of 10 kHz, a frequency amplitude of 1.5 µm and a force of 1 kN force. Moreover, the structures could be molded on any thickness of plastic.

Replicability of process conditions of ultrasonic hot embossing for micropattern fabrication on thermoplastic substrates

Abstract This study replicated a molded plastic substrate with micropatterns by a mold insert employing ultrasonic hot embossing. A plastic substrate and mold insert were combined and heated above the plastic’s glass transition temperature, and then the softened plastic was allowed to flow into the micropattern of the mold insert making use of pressure applied by conventional techniques. A longitudinal ultrasonic wave was added to the ultrasonic hot embossing process. Groove-shaped micropatterns were formed on the surface of the Ni mold insert, and their dimensions were a width of 2 μm and a depth of 200 nm. Polypropylene and poly(methyl methacrylate) were selected as the embossed materials. This study determined the replication and surface properties of the plastic substrate by various process parameters (delay pressure, sonotrode pressure, holding pressure, delay time, ultrasonic duration time, and holding time) of ultrasonic hot embossing.

Rapid calcification propensity testing in blood using a temperature controlled microfluidic polymer chip.

Phosphate toxicity is a major threat to cardiovascular health in chronic kidney disease. It is associated with oxidative stress, inflammation and the accumulation of calcium phosphate commonly known as calcification in soft tissues leading to functional disorders of blood vessels. An improved calcification propensity test for the assessment of phosphate toxicity was developed, which measures the velocity of calcium phosphate mineralization from colloidal precursors in vitro. This so called T50 test measures the transformation from a primary into a secondary form of nanosized colloidal plasma protein-calcium phosphate particles known as calciprotein particles. The T50 test in its previous form required a temperature controlled nephelometer and several hours of continuous measurement, which precluded rapid bed side testing. We miniaturized the test using microfluidic polymer chips produced by ultrasonic hot embossing. A cartridge holder contained a laser diode for illumination, light dependent resistor for detection and a Peltier element for thermo control. Increasing the assay temperature from 37°C to 75°C reduced the T50 test time 36-fold from 381 ± 10 min at 37°C to 10.5 ± 0.3 min at 75°C. Incorporating sputtered micro mirrors into the chip design increased the effective light path length, and improved signal-to-noise ratio 9-fold. The speed and reproducibility of the T50 chip-based assay run at 75°C suggest that it may be suitable for rapid measurements, preferably in-line in a dialyser or in a portable microfluidic analytic device with the chip inserted as a disposable cartridge.

Friction embossing

A new fabrication process for micro structures from thermoplastic polymers is presented. Two layers of thermoplastic polymer are placed onto a tool with micro structures. By a friction welding machine, the polymer surfaces are rubbed against each other generating friction heat and melting the polymer. The polymer adapts to the surface shape of the micro structures on the tool. Then, the movement of the machine stops and the polymer cools down and hardens in the shape of the micro structures. The entire process is finished within a few seconds and the required investment costs are in the range of some 10,000 € to approximately 200,000 €. Compared to ultrasonic hot embossing, samples with larger overall dimensions can be fabricated and the heat distribution has been proven to be more homogeneous.

Ultrasonic welding for the rapid integration of fluidic connectors into microfluidic chips

We introduce a variety of biocompatible fluidic connectors that can be integrated into microfluidic chips by ultrasonic welding. Commercially available barbed fittings and dispensing needles with Luer lock fittings were integrated between two chip components ensuring a fluidic in-plane contact. In addition, straight Luer lock fittings in combination with ultrasonic hot embossing, 3D printed thermoplastic connectors with Luer lock and barbed fittings were integrated out-of-plane. The integration was successful without clogging any fluidic channels.Depending on the connector type, the pressure tightness differs. Dispensing needles showed the lowest pressure tightness of only 1.14 bar. However, all other connector types were pressure tight to at least 3.75 bar.The main advantage of the integration technique of ultrasonic welding is the rapid implementation of individual connectors adapted to the required situation—for prototypes as well as for large-scale production. Moreover, multiple connectors can be integrated simultaneously in just one single step. This provides a user-friendly and stable connection of commonly used connector types such as barbed or Luer lock fittings for microfluidic applications.

Screwed micro fluidic connections fabricated by ultrasonic hot embossing and welding

Recent connection methods to micro fluidic chips, such as gluing tubes and hoses into orifices or pressing a gasket on them have been studied to improve the common problem to establish a connection of the macroscopic world to a fluidic micro system. In this study, micro fluidic chips from polycarbonate plates with screwed connections have been fabricated by ultrasonic hot embossing and welding, both as in plain and out of plain arrangements. To fabricate threads in the polymer plates, an M3 screw was ultrasonically welded into a hole in the chip and the screw was turned out after the process. The tightness of micro channels and fittings was proven by pumping ink and yeast cells through the micro channels and by applying air pressure of up to 700 kPa, respectively. The process introduced here is especially advantageous because sealing micro channels and welding in micro fluidic screwed connections is done in a single fabrication step.

Development of thermoplastic films for ultrasonic manufacturing of printed circuit boards

AbstractConventional production processes of PCB involve a high amount of complex production steps and the use of hazardous chemicals. Furthermore, a great effort is required for their recycling. Ultrasonic hot embossing of coextruded films, filled with electrical conductive fillers, could be an economic and ecological alternative for the production of PCB. This study focuses on the identification of suitable filler–matrix combinations for the isotropic and anisotropic conductive film layers. Hence, monolayer films with different fillers and filler contents are extruded. Conductivities of 8.31 and 12 S/m were measured in cross and machine direction, respectively, for a polypropylene‐film with 10 wt.‐% of CNT. This is two magnitudes below the required 535 S/m. With 70 mS/m, the conductivity in thickness direction is 115–170 times lower. This is caused by process‐related surface layers with low filler content. For steel fibers, no electrical conductivity can be measured.

Measurement of temperature and pressure distribution during ultrasonic processes by sensor foils from polyvinylidene fluoride

Micro and nano structures and systems are generated in polymer surfaces with cycle times of a few seconds by ultrasonic processes such as ultrasonic hot embossing, welding and thermoforming paving the way for a variety of new products. However, measuring temperature and pressure during these processes is very difficult because the polymer is enclosed between an anvil and a sonotrode. Temperature and pressure distribution during ultrasonic processing now have been measured inside of a stack of thermoplastic polymer layers by sensor foils from polyvinylidene fluoride, 55 µm in thickness. The measurements are based on the piezoelectric and pyroelectric effect of polyvinylidene fluoride allowing to achieve resolutions in temperature and pressure of up to ± 1 °C and ± 0.5 kPa, respectively. The achieved resolutions in time and in normal and lateral direction are approximately 1 µs, 60 µm and 1 cm, respectively. The maximum temperature inside a foils stack that could be measured was 73 °C because the sensor foils lost sensitivity when heated up more. Every single oscillation of the polymer was measured as a pressure change. The difference in temperature change and ultrasonic pressure amplitude measured in lateral direction below a sonotrode with outer dimensions of 8 × 12 cm are approximately 12 °C and 4 kPa, 25 and 60%, respectively, indicating the width required for process windows of ultrasonic processing. Moreover, phase shifts are measurable and thus analysis of oscillation characteristics of sonotrodes were investigated.

Ultrasonic fabrication of micro fluidic channels from polyether ether ketone (PEEK)

Micro channels from polyether ether ketone (PEEK), 1 mm in width and depth and 1.6 cm in length, have been fabricated by ultrasonic hot embossing and ultrasonic welding. Micro channels from PEEK enable constructing chemically inert microfluidic systems suitable for applications at temperatures of more than 200 °C. Besides this, a new design can be realized within one working day, investment costs are only a few 10,000 €, and the cycle time of fabricating a microfluidic system is a few seconds. Maximum temperature and pressure difference over a micro channel are not limited by the material properties of semicrystalline PEEK but by difficulties in producing them and by their change of morphology during manufacturing. To ultrasonically fabricate a microchannel two ultrasonic processes need to be combined. The channel itself is fabricated in a first ultrasonic hot embossing process and then sealed with a lid foil in a second ultrasonic welding step. This paper shows that, PEEK changes its morphology when it is ultrasonically hot embossed to produce a microfluidic channel. Moreover the degree of crystallinity varies according to the local position in the micro channel. It turned out that successful sealing of a micro channel by ultrasonic welding is facilitated by energy directors from semicrystalline PEEK. Therefore, semi-finished PEEK channels need being exposed to a temperature of at least 170 °C for 30 min before ultrasonic welding. This way, the amorphous PEEK is recrystallized resulting in hardened polymer micro structures. Such a heat treatment is also necessary after welding to achieve the desired stability at high temperatures. Tests show that the PEEK channels remain tight at a pressure difference of up to 700 kPa at a temperature of up to 220 °C.

Ultrasonically manufactured microfluidic device for yeast analysis

We present a polycarbonate-based microfluidic device that was rapidly manufactured by ultrasonic hot embossing and welding. The fabrication of microstructures using ultrasonic processing allowed the embossing of micrometer-sized structures in polymer films by molding from a master die; this process was completed within seconds. The short manufacturing time required using this process and its ease of reproducibility allows rapid prototyping of custom-made microfluidic devices. Through ultrasonic fabrication, disposable microfluidic devices were newly designed and manufactured within a working day. The feasibility of these devices was demonstrated by cultivating yeast cells. The cells remained viable within this system for at least 22 h. Enhanced green fluorescent protein (eGFP) expression was initiated by providing the cells with a supply of the inducer galactose. The present study not only shows the potential of microfluidic devices fabricated by ultrasonic processing but also discusses their capability for use in microbial analysis.

Heat generation and distribution in the ultrasonic hot embossing process

Microstructures are fabricated from polymer foils in seconds by a new process called ultrasonic hot embossing. This process allows to realize new designs within a day and is suitable both for large- and small-scale production. The required investment costs are in the order of some 10,000 ź. In this paper, heat generation and distribution in ultrasonic hot embossing is investigated with piezo-electric and pyro-electric foils from the polymer polyvinylidene fluoride (PVDF) placed into a stack of polymer layers. This way, the influence of pre-structures on and powders between polymer layers is revealed.

Ultrasonic fabrication of micro nozzles from a stack of PVDF foils for generating and characterizing microfluidic dispersions

Nozzles with circular cross-section and a diameter varying in axial direction have been fabricated in a microfluidic channel from polyvinylidene fluoride as chemically resistive thermoplastic polymer. Smallest diameter and length of the nozzle are approximately 150 µm and 3.4 mm, respectively. The nozzle and the entire channel system have been fabricated from two halves generated by ultrasonic hot embossing and bonded to each other by ultrasonic welding. Alignment during ultrasonic welding was assisted by a fit of energy directors and an accuracy of 35 and 10 µm in normal and lateral direction, respectively, was obtained. Thermoplastic molding of the two halves of the channel structures was performed by ultrasonic hot embossing with a cycle time of a few seconds. The development was significantly accelerated by milling the tools directly into aluminum plates. This way, new designs were realized within a day. The micro nozzles have been proven generating liquid/liquid dispersions of different flow patterns as a function of flow velocity and Capillary number.

Ultrasonic Welding of Chemical Optical Sensors Supporting O2, pH and CO2 Imaging in Microfluidic Systems

Chemical optical sensors for the two-dimensional imaging of three different analytes (O2, pH, CO2) have been integrated in microfluidic systems at up to three positions simultaneously by ultrasonic welding. The systems are a 48 well titer plate and a diffusion channel. The channel and the holder for the sensor foils have been fabricated by ultrasonic hot embossing. The non-invasive fluorescence read-out was performed with the VisiSens TD imaging system. The selec-tive sensing capability of the optodes in the microfluidic channel has been proven. Masters for ultrasonic hot embossing can be fabricated inexpensively in various formats. Typical cycle times for production are in the order of a few seconds which allows low-cost production even in low quantities.

Ultrasonic Hot Embossed Polymer Microreactors for Optical Measurement of Chemical Reactions

AbstractUltrasonic hot embossing of polymers is an alternative to reduce fabrication costs of microreactors. An ultrasonic welding machine is used to melt a stack of thermoplastic foils and adapting them to a short‐time milled aluminum mold showing the inversed design of the desired microfluidic cavities. Two micromixers were fabricated this way providing a low degree of axial dispersion and pressure loss. Stability analysis is successfully performed for a wide temperature range and high pressure. Mixing of colored aqueous solutions and neutralization reactions are implemented to both systems for defined volume flow rates and optically investigated via microscope. Reaction progress is automatically determined with a MATLAB script by reference to the consequential color change of the neutralization reaction with a color indicator. Typical mixing characteristics are identified for both mixers.

Tools for ultrasonic hot embossing

Ultrasonic hot embossing has been developed in recent years enabling quick and flexible fabrication of micro structures from thermoplastic polymers. This paper describes many possibilities of the fabrication, use, and limits of tools for this process. Tools have been produced by milling, drilling, silicon etching, photo and X-ray lithography, and electroplating and by combining these processes.

Position detection and remote controls powered by micro whistles

This paper describes the design and detection of mechanically actuated micro whistles not equipped with any electronics. When actuated, the micro whistle generates an ultrasonic tone above the audible range. A set of three or more microphones in the near record this tone and the position of the whistle is calculated from the differences of the arrival times of the acoustic signals at the microphones. In experiments the positions of whistles were measured in a 2m×2m area with an accuracy of less than 10cm. Besides this, micro whistles have been employed as remote controls such as switches for room light or air conditioning.The micro whistles have been fabricated by ultrasonic hot embossing of thermoplastic foils at low-cost and their overall size is approximately 1cm. The paper also describes how to overcome obstacles such as accidentally generated ultrasonic noise, frequency shifts due to changing activation forces of the whistles, and distinguishing the signals of different whistles.

Polymer circuit boards fabricated by ultrasonic hot embossing

In this paper, a novel polymer circuit board fabricated by ultrasonic hot embossing is presented. Conductive paths are gene­rat­ed by ultrasonically embossing interruptions into a continuous metal layer coated onto a plastic substrate, and all electronic components are mounted onto the board in a single ultrasonic process step. With this process both the fabrication of a board and the interconnection of electronic components can be achieved in seconds. Overall dimensions of the board are determined by the size of the sonotrode of the ultrasonic machine. As a demonstrator a multivibrator circuit was built up from five resistors, two LEDs, two transistors and two capacitors. The multivibrator was tested successfully.

Slug length monitoring in liquid–liquid Taylor-flow integrated in a novel PVDF micro-channel

A polyvinylidene fluoride (PVDF) micro-sensor channel with integrated gold wires was developed and manufactured by ultrasonic hot embossing and welding. The sensor channel was employed to detect the slug length distribution in a liquid–liquid Taylor-flow by conductivity measurements. The sensor channel was also tested regarding its usability for the detection of blockages by hot wire anemometry.

Electromagnetically force balanced polymer accelerometer

An acceleration sensor from polymer has been developed which balances a proof mass by magnetic forces. The sensor is fabricated from a polyimide membrane with conductor paths from gold patterned by photolithography and etching, a frame manufactured by ultrasonic hot embossing, and permanent magnets fixed to the frame. Except the conductor path and permanent magnets, all components are made of polymers on a planar substrate, and then the frame is kinked forming the desired three-dimensional structure. In a first try, a sensitivity of 0.46 V/(m/s²) was achieved, and cross axis sensitivity error was less than 3 %.