I. Abstract of the Paper’s Core Research

This study developed a pneumatic aerosol‑jetting platform equipped with an integrated rotary valve, systematically investigated the mechanisms underlying pressure fluctuations within the chamber caused by valve switching, and proposed a densification control strategy based on dynamic regulation of the exhaust flow rate from a virtual impactor. Multi‑group start–stop comparative experiments were conducted using silver nanoparticle ink, yielding the following key findings:

Revealing the Underlying Mechanism of On/Off Delay in Built-in Valves

When the rotary valve is closed, the entire atomization stream is diverted to the pressure‑relief channel, and the chamber pressure returns to atmospheric. Upon reopening, dead zones remain in the chamber, causing a prolonged period for droplet filling and pressure recovery, which directly leads to end‑of‑line breakage and gradual line‑width variations. The pressure sensor enables real-time monitoring of the printing steady state.

Phase densification ratio DR Control parameters

The DR is defined as the mist flow rate during the shutoff period divided by the mist flow rate during the printing period. During the shutoff phase, reducing the exhaust flow rate increases the droplet concentration, and the chamber pressure recovers rapidly at the instant the valve opens. Under the optimal DR of 15, the start–stop waiting time is reduced from 8.5 s to 0.1 s.

Interval-based quantitative partitioning

When DR is too low, pressure recovery is slow and the start of the line is missing; when DR is too high, cavity pressure overshoots, resulting in a large, raised protrusion at the print start. A DR value of 15 yields uniform line width across the entire width, whereas a DR of 21 leads to noticeable material overflow at the beginning of the line.

Composite Motion Compensation Process

Combined with cushioned motion control, it eliminates end‑effector jolts caused by platform acceleration and deceleration; it can reliably print high‑frequency, start‑stop reverse‑fill patterns such as spirals, finger‑capacitor arrays, and surface‑mount antennas.

Long-term stability verification of the equipment

After 6 consecutive hours of operation and 15,000 nozzle‑to‑nozzle switching cycles, no ink buildup was observed within the valve. The system’s stability meets the requirements for high‑volume proofing and small‑batch production, and it is compatible with inks spanning a wide viscosity range of 1–1,000 cP.

II. Complete Aerosol Generation Process Using a Built-in Rotary Valve

1) The pneumatic atomization aerosol main unit integrates a bidirectional built-in rotary shut-off valve and a three‑channel MFC mass flow controller.

2) The silver nanoparticle ink is subjected to ultrasonic homogenization, and the substrate is maintained at a constant temperature of 40°C, securely mounted on a three-axis vacuum stage.

3) Set the baseline flow rates for nebulization, sheath gas, and exhaust, and optimize the DR densification parameters.

4) CAD generates G-code for spiral, interdigital, and inverse‑fill antennas, with a built-in buffer for motion trajectories.

5) The valve assembly undergoes automated high-frequency start–stop printing, and an optical profilometer is used to assess the uniformity of line end-to-end.

6) The pressure acquisition system simultaneously records the pressure variation curve within the chamber, enabling a comparative analysis of the forming performance of different DR processes.

III. Core Advantages of the Three- and Five-Aerosol Control Schemes (Comparison with External Shutter vs. Conventional Pneumatic Shut-off)

No external mechanical interference; compatible with curved and raised PCB substrates.

The built-in valve does not occupy any external space on the nozzle, ensuring a stable focusing accuracy at an optimal spray distance of 1–5 mm, while preventing structural collisions with protruding or recessed components.

No ink buildup on the shutter, significantly reducing equipment maintenance.

The external shading plate, when exposed to prolonged mist droplet adsorption, can lead to overflow contamination; in contrast, the built-in flow‑diverting pressure‑relief design prevents any residual ink buildup.

Simple retrofitting, no additional pressurized air circuit module required.

Unlike commercial pneumatic shutters that require multi‑channel pressure boosting via MFCs, this solution achieves rapid start‑stop operation simply by adjusting the exhaust flow of the virtual impactor, resulting in lower hardware costs.

Wide-viscosity inks are universally compatible, and pneumatic atomization is well-suited for high-solids slurries.

Not limited to ultrasonic low-viscosity inks; pneumatic atomization is compatible with metal and ceramic composite inks ranging from 1 to 1000 cP.

Stable under tens of thousands of high-frequency switching cycles, compatible with mass‑production‑grade high‑speed graphics.

0.1-second ultra-fast start-stop, with no defects in reverse‑filling patterns during frequent direction changes, and no clogging of the valve chamber even during prolonged continuous printing.

IV. Five Core Technological Innovations of This Study

Innovation 1: Built-in bidirectional rotary valve aerosol shutoff structure

A mechanical rotary valve is employed to achieve internal flow splitting and shut-off, distinguishing it from pneumatic bypass pressure‑relief schemes; its hardware architecture is simple and readily integrable for retrofitting.

Innovation 2: A Novel Control Theory for Fog-Phase Densification Compared to DR

A dynamic exhaust flow regulation method is established to quantitatively increase droplet concentration during the cutoff phase, thereby eliminating start‑stop delay defects at their root cause—pressure.

Innovative Real-Time Monitoring and Evaluation System for Three-Chamber Pressure

A single-point pressure sensor is sufficient to determine printing steady-state, eliminating the need for multiple high-precision surface‑profile measurement devices to rapidly tune process parameters.

Innovative 4 DR + Buffer Motion Dual-Defect Suppression Scheme

Flow densification eliminates ink starvation at the start, while trajectory buffering compensates for end‑of‑stroke protrusions caused by platform acceleration and deceleration, ensuring consistent line width and height across the entire print.

Innovative Five-Step High-Frequency Reverse Printing Complete Process

It can perform large-area background reverse filling and fabricate complex antenna patterns, with the equipment sustaining tens of thousands of shutter cycles without failure, and is suitable for the batch fabrication of RF and sensor components.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Traditional external shutters occupy spray‑distance space, and curved surfaces or component‑bearing substrates are prone to collisions and interference.

✅ All built-in valves are fully integrated within the print head, eliminating external mechanical obstructions and maintaining the optimal focal working distance.

Challenge 2: The valve’s restart chamber has slow dead-zone filling, resulting in an up to 8-second start‑stop delay and a gradual variation in line thickness.

✅The DR densification strategy increases the concentration of cut-off fog, rapidly restores pressure, and reduces the wait time to 0.1 seconds.

Challenge 3: The DR parameters are improperly set, resulting in either wire breakage at the start or material overflow and protrusion at the starting point.

✅ Quantitative optimization of the DR=15 process window, balancing haze concentration and peak chamber pressure.

Challenge 4: Frequent printing of spiral, interdigitated, and other patterns leads to uneven line widths at the ends due to acceleration and deceleration.

✅ Equipped with a complementary motion‑control buffering algorithm to eliminate excess deposition defects caused by platform start‑stop cycles.

Challenge 5: Commercial pneumatic shutters require multi‑channel pressurized air lines, resulting in high equipment modification costs.

✅ Only the exhaust MFC flow‑control program is modified; no additional gas‑line hardware is required, allowing desktop devices to be upgraded.

VI. Future Diversified Industrial Application Scenarios

1 Mass Production of RF Miniature Patch Antennas

Large-area reverse-fill metal patterns, combined with high-frequency start–stop printing, ensure uniform line widths and maintain low RF loss characteristics.

2 MEMS High-Density Interdigital Sensor Array

Micron‑scale spaced capacitive electrodes enable frequent segmented printing with no filament breakage, adjacent short circuits, or material overflow.

3 Foldable Screen Precision Wiring Module

Small, multi-segment, custom-shaped conductive traces; high-speed printing boosts production line throughput and reduces defect rates.

4 Curved-Surface In-Vehicle Sensing Electronics

Conformal printing on curved, textured housings, with no interference from an external shutter, ensuring a complete and uniform conductive circuit.

5 Laboratory Aerosol Rapid Prototyping Platform

Cost-effectively retrofit existing equipment, eliminating long wait times for tuning across various micro‑patterns and boosting R&D efficiency by several-fold.

Image and Text Guide

Figure 1. Schematic diagram of the AJP system with an internal mechanical rotary-valve shutter and the rotary-valve structure.

a) Complete gas path: N₂ passes through the Atomizer MFC (Q _A =450 sccm) The silver nanoparticle ink is atomized, and the aerosol is conveyed through a Virtual Impactor by the Exhaust MFC (Q _E =435 sccm) Remove small droplets, then pass through the Sheath MFC (Q _Sh =50 sccm) is focused within a 200-μm nozzle; a rotary valve① is inserted between the Virtual Impactor and the nozzle: in the Off position, it vents the aerosol directly through the Vent port, while in the On position, it directs the aerosol toward the nozzle. Three pressure sensors on the tubing—②, ③, and ④—monitor the aerosol delivery line, the outlet of the Virtual Impactor, and the sheath‑gas inlet, respectively. b) The rotary valve is actuated by a pneumatic rotary actuator and sealed at its end face with two Teflon O‑rings; rotating the valve core by 90° switches it between the “On (direct flow to the nozzle)” and “Off (direct venting)” positions.

Figure 3. Measured waveforms of internal pressure at three locations during a single On–Off–On shutter cycle.

a) Fog stream tube pressure P _A , b) Virtual Impactor outlet pressure P _E , c) Sheath gas inlet pressure P _Sh . After Valve OFF, P _A , P _E   It immediately drops to atmospheric pressure (①), while P _Sh   Due to throttling at the nozzle orifice, the pressure drops only to approximately 1.8 kPa (④). After the valve is ON, P _A , P _E   Starting from zero, the system climbs along the exponential curve (⑤⑥); filament extrusion only begins once the “Threshold pressure” (②) is exceeded, and steady-state operation is achieved only when the pressure reaches the “Operating pressure” (③), approximately 2.2 kPa. The time interval between zero and the threshold corresponds to the “Jetting delay.”

Figure 4. Flow paths of the aerosol and sheath gas in the valve–nozzle section under On and Off states.

a) Print (On): The valve spool closes the Vent completely, aerosol Q _mist   All flows downward into the nozzle, with a total nozzle flow rate of Q. _N   = Q _mist   + Q _Sh , Vent flow rate Q _V   = 0; b) Shutter (Off): Rotate the valve spool 90° to bypass the aerosol to the vent, Q _V   = Q _mist , the nozzle is left with only shield gas Q _N   = Q _Sh The gray segment between the valve and the nozzle is the dead zone—when the system is off, this section fills with aerosol but does not dispense; when the system is on, it must be refilled and the pressure raised to the operating pressure before filament extrusion occurs. This is the physical origin of jetting delay.

Figure 5. Microscopic morphology of printed silver lines and corresponding pressure curves under different dwell times, without DR optimization.

a–c) Reverse double lines are printed at equal intervals (the green arrows indicate the printing direction), with waiting times of 8.5 s, 9 s, and 11.5 s, respectively: at the starting end (at point ①), droplet breakage or large beads first appear; in the intermediate transition zone (at point ②), the line width is slightly narrower and the edges appear blurred; only in the steady-state section (at point ③) is a uniform line width on the order of 50 μm achieved. The longer the waiting time, the larger the initial bulge (at point ①). d) Between successive prints, the pressure in the mist‑flow nozzle rises from 0 to approximately 2 kPa, corresponding to a “narrow‑then‑wide‑then‑uniform” evolution of the line. Traditional fixed Q _E =Q _E The scheme with a flow rate of 435 sccm requires a wait time of at least 8.5 seconds; otherwise, the connection will be lost. However, an excessively long wait can result in the formation of large droplets at startup.

Table 1 + Figure 6. Pressure curves and print lines under the two typical DR conditions after introducing the “densification ratio DR”

Table 1 presents three algorithms: Conventional (DR=1, wait time of 8.5 s, non-uniform lines), Optimized (DR=15, Q _E '=215 sccm, wait 0.1 s, line is uniform), Overshoot (DR=21, Q _E '=100 sccm, wait 0.1 s, non-uniform line). a,b) P when DR=15 _A   The pressure curve rises cleanly from 0 to approximately 2 kPa without overshoot, with a line width of 42 μm at both the start and steady-state regions, exhibiting uniformity. In c and d, when DR = 21, the pressure curve peaks at about 2.3 kPa before decaying, resulting in an overshoot; the line width at the start is stretched to 93 μm, while the steady-state width remains at 42 μm, leading to the formation of a bulge at the beginning.

Figure 8. Curves showing the variation of jetting delay and line width with DR under different gold‑spraying flow rates.

a) At four Atomizer flow rates (450/500/550/600 sccm), the jetting delay decreases rapidly with increasing DR: at DR = 1, the delay is approximately 8 s (①); when DR ≥ 15, the delay is reduced to about 0.1 s (②); further increasing DR no longer results in additional reduction of the delay (③). b) Q _A At 450 sccm, the initial line width increases with increasing DR, starting from approximately 15 μm at DR = 1, while the end line width remains constant at about 42 μm. When DR approaches 15, the initial and end line widths converge (②), marking the optimal process window. Once DR exceeds roughly 22, the initial line width is over‑etched to exceed 90 μm (③). In conclusion, DR ≈ 15 corresponds to the optimal switching process characterized by a 0.1‑second dwell time and consistent initial and end line widths.

Figure 9. Helical line and interdigitated capacitor under Buffered Motion control: (a) Center‑cut line with DR = 1, (b) Uniform pattern with DR = 15, (c) Line‑width–time curve, (d) IDC

Spiral printing starts from the center: when DR = 1, the central starting point suffers from insufficient material supply, causing the line to widen progressively (Fig. 9a); at DR = 15, the spiral’s width remains uniform around the entire circle (Fig. 9b). Quantitative analysis in Fig. 9(c) shows that the black line at DR = 1 increases from 10 μm to 42 μm in 2.5 s, whereas the red line at DR = 15 stays consistently at 43 μm throughout. In Fig. 9(d), an interdigitated capacitor (IDC) printed using the optimal DR combined with buffered motion exhibits a macroscopic scale of 5 mm and, under microscopy at 200 μm, clean, well-defined finger edges with no bulging at the start.

Figure 10. Shutter frequency adaptability: (a) Test pattern definition; (b) L/S ratio at four frequencies—0.25, 0.5, 1, and 2 Hz.

Test pattern: steady velocity of 5 mm/s with a duty cycle of 0.5; theoretically, the printed segment L is equal in length to the interval S. Due to jetting delay, the actual printed segment is shorter, so the L/S ratio is defined to quantify this effect. At 0.25 Hz, L/S = 0.95, close to the ideal value; at 0.5 Hz, it is 0.90; at 1 Hz, 0.85; and at 2 Hz, 0.71—higher frequencies leave insufficient time for the mist to recover within the short intervals, resulting in progressively shorter effective printed segments. This demonstrates that the internal rotary‑valve design can maintain high fidelity at shutter frequencies up to 1 Hz.

Figure 11. Reverse printing demonstration: (a) dummy component for the patch antenna, (b) SCH letters

Reverse printing, or “negative‑image printing” where the background is printed first and the lines themselves are left unprinted, is highly sensitive to edge overprinting—any slight deviation in the starting point can accumulate on the background, resulting in uneven line widths. For a patch antenna measuring 30 mm × 15 mm and an SCH lettering element measuring 20 mm × 10 mm, continuous printing for 6 hours yielded approximately 15,000 shutter cycles, with no wetting or deposition residues inside the valve; microscopic examination reveals uniform edge line widths and no stringing.