Study of a Dot-patterning Process on Flexible Materials using Impact Print-Type Hot Embossing Technology

Summary

Impact print-type hot embossing technology uses an impact header to engrave dot patterns on flexible materials in real time. This technology has a control system for controlling the on-off motion and position of the impact header to create dot patterns with various widths and depths on different polymer films.

Abstract

Here we present our study on an impact print-type hot embossing process which can create dot patterns with various designs, widths, and depths in real time on polymer film. In addition, we implemented a control system for the on-off motion and position of the impact header to engrave different dot patterns. We performed dot patterning on various polymer films, such as polyester (PET) film, polymethyl methacrylate (PMMA) film, and polyvinyl chloride (PVC) film. The dot patterns were measured using a confocal microscope, and we confirmed that the impact print-type hot embossing process produces fewer errors during the dot patterning process. As a result, the impact print-type hot embossing process is found to be suitable for engraving dot patterns on different types of polymer films. In addition, unlike the conventional hot embossing process, this process does not use an embossing stamp. Therefore, the process is simple and can create dot patterns in real time, presenting unique advantages for mass production and small-quantity batch production.

Introduction

Researchers are actively attempting to miniaturize existing devices and displays and increase the flexibility of these devices^1,2. To reduce the width and depth of electrical channels to the micro or nano scale, high-precision technology is necessary. In addition, to increase the flexibility of these devices, the patterns of the electrical channels must be located on a flexible material, such as a polymer film^3,4. To meet these conditions, the study of ultrafine microprocessing technology is actively underway.

Ultrafine microfabrication technology has an advantage in that possible patterning materials include not only highly rigid materials such as iron or plastic but also soft materials such as polymer films. Due to these advantages, this technology is widely used as a core process in various fields, such as communications, chemistry, optics, aerospace, semiconductor, and sensors^5,6,7. In the ultrafine microprocessing field, LIGA (lithography, electroplating, and molding) or micromachining methods are used⁸. However, these conventional methods are associated with several problems. LIGA methods require a considerable amount of time and several process steps to create ultrafine patterns and incur a high cost as well because they need many different types of equipment during the processes. In addition, LIGA methods use chemicals that can pollute the environment.

To address this issue, hot embossing process technology has been spotlighted among ultrafine microprocess technologies. Hot embossing is a technology that creates a pattern on a heated polymer film using a micro- or nanoscale embossing mold. Conventional hot embossing technology is divided into the plate type and roll-to-roll type depending on the shape of the mold. The two types of hot embossing technology are different in terms of the shape of the mold, but these two processes are similar in that the embossing mold presses the polymer film onto a heated plate to engrave a pattern onto the polymer film. To engrave the pattern using the hot embossing process, it is necessary to heat the polymer film above the glass transition temperature and to apply an adequate amount of pressure (~30–50 MPa)⁹. In addition, the width and depth of the pattern change depending on the temperature of the heated plate, the material, and the shape of the embossing mold. Moreover, the cooling method after the patterning process affects the shape of the pattern on the polymer film.

In the conventional hot embossing process, embossing stamps or rollers can be embossed with the desired pattern, and the embossing mold can be used to print the same pattern onto polymer film surfaces continuously. This feature makes this process suitable not only for mass production but also for fabricating devices with soft materials, such as polymer films^{10,11,12,13,14}. However, the conventional hot embossing method can only create the single pattern engraved in the embossing mold. Therefore, when the user wants to make a new pattern or modify the pattern, they must make a new mold to modify the imprinting pattern. For this reason, conventional hot embossing is costly and time-consuming when creating new patterns or replacing existing designs.

Earlier work introduced the impact-type hot embossing process for producing dot patterns with various widths and depths in real time¹⁵. Unlike the conventional hot embossing process, the impact print-type hot embossing method uses an impact header to create patterns on the polymer film. This technology moves the impact header to the desired position with a precision positioning system. A control signal is applied to print patterns at a desired width and depth and at an arbitrary position. The structure of the impact header consists of a mover, a spring, a coil winding, and a core (see Figure 1A)¹⁵. Earlier work confirmed through an analysis and experiment that such an impact header can produce the proper force for hot embossing¹⁶. The protocol of this paper covers the design of the hardware for the impact-type hot embossing process and the control environment for process control. In addition, we analyze the dot patterns on PET film, PMMA film, and PVC film, all of which are processed with the proposed protocol to verify that the impact print-type hot embossing process can create dot patterns with various widths and depths in real time. The results of these experiments are presented below in the results section, confirming that the embossing process can suitably produce ultrafine patterns.

Protocol

1. Fabrication of the impact print-type hot embossing process

1. Make model 1 and combine it with an X-stage (see Figure 1).
   NOTE: It is recommended that Model 1 be made of aluminum to avoid heat being conducted onto the X-stage. Moreover, it is recommended that the length of Model 1 be the distance between the surface of the heat plate and the lowest height of the bearing plate of the Z-stage as the design of Model 1 varies with the size of the heat plate.
2. Combine the X-stage and Z-stage and assemble the Z-stage and Model 2.
   NOTE: Ensure that Model 2 is made of a metal that can endure the heat from the heat plate (e.g., aluminum). Fixing Model 2 to the Z-stage tightly will ensure the capability of the Z-stage to hold the weight of Model 2 and the impact header.
3. Combine Model 2 and the impact header and place the heat plate below Model 1.
   NOTE: Joining the impact header with the lowest position on Model 2 will ensure that the mover reaches the surface of the heat plate. It is recommended to install the heat plate after raising the Z-stage maximally to avoid any contact of the impact head with the surface of the heat plate. Use suitable software to control the stage.
4. Convert the STL files of the film holder (Supplementary File 1 and Supplementary File 2) to GCODE files using suitable software to print the film holder with a three-dimensional (3D) printer.
   NOTE: The software may vary with the 3D printer used, and some environments may support 3D printer environments without GCODE conversion.
5. Use the 3D printer to print the film holder with the GCODE file.
   NOTE: Using a filament (e.g., Z-HIPS) is recommended because less contraction will occur when printing large parts, such as the film holder.
6. Install two film holders onto the end of the heat plate and fix the polymer film onto the film holder, as shown in Figure 1. To ensure that the polymer film is flat on the heat plate, pull the polymer film as much as possible using motion 1 of the film holder (see Figure 1B). To move the polymer film to the side, move the film holder via motion 2 (see Figure 1B).
   NOTE: To fix the polymer film onto the film holder, it is recommended to use a screw. Glue is insufficient to affix the polymer film onto the film holder, and it is best for the detachment of the polymer film after the patterning experiment.

2. Fabrication of the control circuit

NOTE: This process describes the process of constructing the control circuit of the impact header and the X–Z stage.

1. Connect the control device that sends the signals (see Table of Materials) to the impact header to control it.
2. After connecting the control device to the impact header, input -3 V and +10 V as control signals into the impact header.
   NOTE: If a +10 V control signal is sent to the impact header (see Figure 1), the mover (impact head) goes down and enters the turn-on state. In this state, the mover hits the polymeric film and engraves the pattern on the polymer film.
   1. Raise the mover to engrave the next pattern after engraving a pattern using the mover of the impact header. To raise the mover (impact head), apply the -3 V control signal.
      NOTE: A negative voltage is input to the impact header to prevent the mover from becoming magnetized by the inner remnant flux of the impact header.
3. If the control device cannot supply a sufficient control signal, use a high-power operation amplifier (e.g., OP-AMP) which amplifies the ~0 V–5 V control signal to ~-3 V–+10 V, as shown in Figure 2, to control the impact header.
   1. First, prepare a dual-channel DC power supply (see Table of Materials). After this step, connect four nodes to provide common ground (GND) nodes to all channels: a positive voltage terminal (V1+) and a ground (GND) terminal for channel 1 and a negative voltage terminal (V2-) and ground (GND) for channel 2. An overall connection diagram is shown in Figure 2.
      NOTE: According to the step described in 2.3.1, positive and negative voltage with different absolute values can be supplied to the operational amplifier (OP-AMP).
   2. Connect the negative voltage terminal of channel 1 (V1-) of the power supply to the negative power supply voltage terminal (Vs-) of the OP-AMP, as indicated by the blue line in Figure 2. Subsequently, input 3 V Vcc voltage to channel 1.
      NOTE: According to step 2.3.1, the 3 V Vcc voltage is supplied as -3 V negative voltage to the negative power supply voltage terminal (Vs-) of the OP-AMP.
   3. Connect the positive voltage terminal of channel 2 (V2+) of the power supply to the positive power supply voltage terminal (Vs+) of the OP-AMP, as indicated by the red line in Figure 2. Subsequently, input 10 V Vcc voltage to channel 2.
      NOTE: According to step 2.3.1, the 10 V Vcc voltage is supplied as +10 V positive voltage to the positive power supply voltage terminal (Vs+) of the OP-AMP.
   4. Connect the +output channel of a control device (Vcon+) to the positive input channel (Vin+) of the OP-AMP, as shown by the green line in Figure 2.
   5. Connect the -output channel of a control device (Vcon-) to the ground (GND) of channel 2 of the power supply, as shown by the black line in Figure 2.
      NOTE: When connecting the (Vcon-) to the ground (GND), it is possible to connect it to one of the terminals connected during step 2.3.1 in addition to the GND of channel 2.
   6. Prepare electric resistance of 1 kΩ and 10 kΩ values in each case and connect them between the red line and black line, as shown in Figure 2.
   7. Connect the terminal between 1 kΩ and 10 kΩ to the negative input channel of the OP-AMP (Vin-), as shown by the purple line in Figure 2.
   8. Pull out the lines from the output channel of the OP-AMP (Vout) and one of the electrical terminals described in step 2.3.1. Connect the lines to the impact header, as shown by the orange line in Figure 2.
   9. Regarding the power supply, set the voltages of channel 1–3 Vcc and channel 2–10 Vcc. Subsequently, generate control signals of ~0 V–5 V from the control device.
      NOTE: The generated ~0 V–5 V control signals will be amplified by the OP-AMP to ~-3 V–+10 V, which is necessary to control the impact header as described in steps 2.2.1 and 2.2.2.

3. Experiment design

NOTE: This section describes the processes of controlling the impact-type hot embossing device and engraving dot patterns onto the polymer film.

1. Install a stage-control program (e.g., Micromove) to control the X-stage and Z-stage using a control computer (PC).
2. Install DAQ driver software to detect the control device on the control PC that controls the impact header and install an operating program (e.g., MATLAB) to control the control device.
3. After installing the software, construct the hardware environment as shown in Figure 3A to conduct the patterning experiment.
   1. Install the X stage, Z stage, impact header, film holder, and heat plate as shown in Figure 3A to construct the hardware environment.
   2. Fix the polymer film onto the film holder and adjust the position of the polymer film using motions 1 and 2 (see Figure 1B) to fix the film flatly.
      NOTE: To keep the film flat while adjusting direction 2, the locations of the two film holders should be parallel. To make the film flat on the heat plate, it is recommended to adjust the film holder by lowering the position according to direction 1, as shown in Figure 1B.
   3. After fixing the polymer film, adjust the temperature of the heat plate to heat the film above the glass transition temperature.
      NOTE: Each type of film has its own glass transition temperature. Therefore, it is recommended to adjust the temperature of the heat plate to its own glass transition temperature after checking the material properties of the film in the corresponding datasheet.
4. After setting the hardware, put the control circuit together as shown in Figure 3B to control the stage and the impact header.
   1. Prepare the PC, control board, power supply, and OP-AMP to construct the control environment as shown in Figure 3B. Connect the devices as shown in Figure 2 and then connect the computer to the control board.
   2. Enter the 3 Vcc and 10 Vcc values into an OP-AMP through channels 1 and 2 of the power supply respectively, as described in step 2.3.9.
5. Control the stage and impact header using the control computer.
   1. Adjust the initial position of impact header by controlling X and Z stages using the stage control program.
      NOTE: While adjusting the initial position of the impact header, ensure that there is no collision between the impact header and the heat plate. If the position of the Z-stage is too low, the mover will collide with the heat plate, damaging both the mover and the heat plate. If there is damage to both devices, it will hinder the creation of fine patterns on a polymeric material.
   2. Using the operating program, generate a 5 V control signal from the control device. According to steps 2.3.1–2.3.9, the OP-AMP will amplify the 5 V control signal to +10 V, turn the impact header on, and engrave the patterns on the polymer film.
   3. Now generate a 0 V control signal from the control device using the operating program. According to steps 2.3.1–2.3.9, the OP-AMP will amplify the 0 V control signal to -3 V and turn the impact header off.
      NOTE: The mover of the impact header will be raised, waiting to engrave the new pattern.
   4. Move the X-stage into position to engrave the next pattern.
   5. Engrave patterns 3x on the polymer film by repeating steps 3.5.1–3.5.4 sequentially.
   6. Lower the Z-stage 10 μm from the initial position and execute step 3.5.5, counting the number of Z-stage moves. When the number of Z-stage movements exceeds three, move the X-stage to the initial position and raise the impact header maximally by moving the Z-stage.
      NOTE: Changing the height of the Z-stage will ensure adjustments in the depth and width of the dot pattern.
6. Detach the polymer film from the film holder and measure the width and depth of each pattern using a confocal microscope (see Table of Materials), as shown in Figure 4A.
   1. Before starting the measurement process, select the magnification value of the microscope and use the direct observation mode initially to adjust the scanning position of the polymer film. After adjusting the position by means of direct observation, fix the polymer film and change the scanning mode to the laser scanning mode.
      NOTE: When using the confocal microscope, using an acrylic panel is recommended to fix the sample, as shown in Figure 4B.
   2. Using the laser scanning mode, measure the depth and width of the dot pattern.
7. Repeat steps 3.3.2–3.6.2 after changing the type of film.
   NOTE: Considering the glass transition temperature of each type of film, set the temperature of the heat plate before placing each film on the heat plate. In this study, the glass transition temperature of PVC film is 100 °C; for PMMA film it is 95 °C, and for PET film it is 75 °C.

Results

The impact print-type hot embossing process is a process that can be used to engrave dot patterns onto a polymer film in real time, as shown in Figure 1. This process can resolve the issues of the high cost and long times for pattern replacement associated with the existing hot embossing process. A control circuit was constructed, as shown in Figure 2 (see steps 2.3–2.3.9), using the DAQ, OP-AMP, and power supply to carve patterns on various types of polym...

Discussion

In this study, we implemented the impact print-type hot embossing process and engraved dot patterns with various widths and depths onto a range of polymer films in real time. Among the protocol steps, two steps should be critically considered among all steps. The first is the setting of the temperature of the heat plate (step 3.3.3), and the second is the setting of the initial position of the impact header (step 3.5.1). In step 3.3.3, if the temperature of the heat plate is too high, it becomes difficult to form a patte...

Materials

Name	Company	Catalog Number	Comments
0.3mm High Quality Clear Rigid Packaging PVC Film Roll For Vacuum Forming	Sunyo	SY1023	PVC film / Thickness : 300µm
Acryl(PMMA) film	SEJIN TS	C200	PMMA film / Thickness : 175µm
Confocal Laser Scanning Microscope: 3D-Topography for Materials Analysis and Testing	Carl Zeiss	LSM 700	3D confocal microscope / Supporting Mode : 2D, 2.5D, 3D topography
DAQ board	NATIONAL INSTRUMENTS	USB-6211	Control board for two stage and impact header / 16 inputs, 16-bit, 250kS/s, Multifunction I/O
DC Power Supply	SMART	RDP-305AU	3 channel power supply / output voltage : 0~30V, Output current : 0~5A
L511 stage	PI	L511.20SD00	Z-stage / Travel range : 52mm
Large Digital Hotplate	DAIHAN Scientific	HPLP-C-P	Heatplate / Max Temp : 350ºC
M531 stage	PI	M531.2S1	X-stage / Travel range : 306mm
Mylar Polyester PET films	CSHyde	48-2F-36	PET film / Thickness : 50µm
OPA2541	BURR-BROWN	OPA2541BM	OP-AMP / Output currents : 5A, output voltage : ±40V