Model 9935 Arc Spray Gun

Abstract: The 9935 spray gun of Praxair Daihatsu Co., Ltd. is the key equipment in the CoArcR arc spraying system. This gun embodies the latest technological achievements in the field of arc spraying in recent years. This paper introduces the innovative design of the secondary atomization system from the design point of view; expounds the closed-loop control mechanism of the compressed air in the spray gun.

The arc spray gun is the core equipment of the arc spray system, and the performance of the spray gun directly affects the performance and quality of the coating. Therefore, the development of new high-performance arc spray guns is the focus of the world's major thermal spray manufacturers.

TAFA, a subsidiary of Praxair Surface Technologies, has developed a new, advanced DC servo motor-driven 9935 spray gun. This gun is specially developed for its advanced CoArcR door spraying system. It is also an arc spray gun developed for precise spraying and large loads. It weighs about 9 pounds and can be hand-held or machine-clamped. Its maximum current can reach 350 amps. The design of the gun incorporates the reliable wire feeding technology of the 8830/8835 and the high air flow characteristics of the 9000 gun. The newly designed and improved nozzle assembly has the function of Super ArcJet, which makes the melting and atomization of the wire better. The 9935 spray gun embodies Praxair Daihatsu's research and development capabilities and technology accumulation in arc spraying for more than 30 years. It can be said without exaggeration that the high-tech content of the 9935 spray gun is currently no comparable product. The high-tech elements in the 9935 spray gun are mainly reflected in the following aspects.

1. Innovative secondary atomization system design, integrated super ArcJet
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Since the 1990s, the atomization system of modern arc spray guns generally adopts the secondary atomization method. The so-called "secondary" atomization means that there are two airflows in the gun to atomize the wire droplet. One of them is two independent air paths in the axial direction for the primary atomization and secondary atomization airflow respectively, thus eliminating the complicated external pipelines and forming an integrated super ArcJetR at the tip of the gun. The main airflow, the other is a circular side airflow. Under the joint action of the two airflows, the atomization effect of the spray gun can be greatly improved. The principle of secondary atomization is shown in the figure below.

Schematic diagram of the secondary atomization of the arc spray gun

Secondary atomization is divided into one-in-two-out type and two-in-two-out type. The former is to decompose the compressed air entering the gun body all the way into two strands, axial and annular. The two-input-two-outlet type has two separate channels of gas entering the gun body, one channel directly atomizes the melting zone from the axial direction through the high-speed nozzle, and the other channel enters the gas through the secondary atomizing nozzle to form a lateral (ring-shaped conical ) airflow to carry out secondary atomization on the molten droplets. The two-in-two-out type can adjust the pressure or flow of the primary atomizing airflow and the secondary atomizing airflow respectively according to the needs, so as to obtain the ideal atomization effect. Obviously, the two-in-two-out type is more superior, its coating is finer and denser, its bonding strength is higher, and its quality is better. The 9935 spray gun is a two-in two-out secondary atomization spray gun.

As we all know, Praxair Daihatsu was the first to engage in the research and development of the secondary atomization of the arc spray gun, and achieved the first application results. Its ArcJetR type arc jet component is the secondary atomization component. It can be installed on the front end of the primary atomization spray gun to form a two-in-two-out secondary atomization spray gun. The famous 8835 spray gun is also a spray gun equipped with ArcJetR arc jet components.

The 9935 spray gun has been designed and optimized on the basis of the 8835. Two independent air paths are directly set on the spray gun for primary atomization and secondary atomization airflow, thereby canceling the complicated external pipelines and forming a An integrated Super ArcJet.

The 9935 spray gun can obtain ultra-fine atomization, which can control the particle size distribution of atomized particles. The 9935 spray gun is also equipped with a variety of nozzle structure components, which can generate a variety of different spray beam shapes. Combined with airflow control, it can not only obtain high-speed dense ultra-fine beams, but also obtain ultra-wide spray beams. The coating quality of different metal materials can be optimized and the most appropriate characteristics can be obtained.

2. Monitor the compressed air pressure of the gun head and control the air flow in the gun in a closed loop

Praxair Daihatsu's new generation CoArc arc spraying system has achieved several new breakthroughs in the spraying process. Chief among these is the ability to automatically close-loop control the compressed air pressure at the tip of the 9935 gun.

The 9935 spray gun is designed with a compressed air pressure detection component on the gun head, and can directly convert the pressure signal into an electrical signal on the gun, and feed it back to the CoArc console. This feature gives the operator a tool to monitor the performance of the gun's compressed air performance during the spraying process.

We know that the performance of the compressed air in the gun depends on many parameters such as the pressure of the compressed air, the flow rate, the geometry and structure of the nozzle assembly. If the flow of compressed air in the gun is an isentropic process, then the combined effect of these parameters determines the hydrodynamic characteristics of the air in the gun. However, after the compressed air enters the spray gun, it passes through the arc zone of the spray gun. Because the discharge of the arc will generate a large amount of heat energy, this will cause part of the gas to ionize and release heat. After the gas is heated, it will produce a sharp volume expansion, and a high-pressure and high-temperature zone will be generated in the gun, and the thermodynamic properties of the gas will also change accordingly. The enthalpy of part of the air will increase, and the isentropic process will be destroyed. It can be seen that the flow of compressed air in the gun is a more complicated process. Only by grasping the key parameters that can control the flow of compressed air can the difficulty of closed-loop control be solved.
Praxair Daihatsu has conducted a lot of in-depth research on this, and based on the experience gained on JP-5000 and JP8000, it has been determined that the pressure of the "gun head" directly affects the jet velocity of the arc flame, so that the "gun head" will eventually be Head” pressure was set as the only valid key parameter. Combining the monitoring of the pressure at the end of the spray gun with the automatic control of the power supply current (spraying rate), the closed-loop control of the arc spraying process is truly implemented.

In order to directly control the arc voltage on the gun, the spray gun is also equipped with a voltage measuring circuit on the gun, so that the CoArc control system can directly control the arc voltage on the gun without considering the length of the power cable.

3. Driven by DC servo motor, precise push-pull wire feeding

The 9935 gun is equipped with a reliable and accurate DC servo motor, which is driven by the servo motor drive power supply on the CoArc type console. The reliable driving device verified on the 8830/8835 spray gun is installed on the gun, and the gear reducer in it reduces the output speed of the driving motor to the speed range of the wire feed wheel, so that the wire of Φ1.6mm can be used within the power supply. Spray within the current range. The feed speed of the wire can be adjusted from 0 to 1079m/hr (3540ft/hr) and controlled by closed loop. The advantages of using DC servo motor drive are:
Precise control of spraying rate;
With closed-loop control function, it can be linked with the wire pushing mechanism;
The ability to achieve a consistently low spray rate on most wire materials allows for more precise control than today's generation of spray guns.
The 9935 spray gun, like other arc spray guns designed and manufactured by Praxair Daihatsu, produces serrated indentations on the upper and lower sides of the wire when the wire passes through the wire feed wheel. The actual electrical contact (rubbing against the tip wall) as the wire passes through the tip occurs at a 90 degree angle to the indentation. Compared with other companies' spray guns, this design can greatly reduce the wear of the contact tip and maintain a more stable arc voltage.

An important reason for the 9935 spray gun to be equipped with a DC servo motor is to achieve synchronous wire feeding with the push-type wire feeding mechanism on the CoArc console. Thus realizing push/pull wire feeding over a long distance (15.2 meters).

Push-pull wire feeding must ensure that the speed of the push and pull is exactly the same. Seemingly simple is actually very difficult. First of all, because the wire is processed by drawing and has undergone a certain surface treatment, the surface conditions are different, and the oil and dust adhered to the surface will affect the forward torque obtained by the wire. and the size of the resistance. Secondly, even though the forward torque obtained by the wire materials is the same, their moving states in the guide wire tube are different and constantly changing. Therefore, a DC servo motor is selected as the pull-type wire-feeding motor on the gun. When pushing/pull-type wire-feeding, the servo motor that can be precisely controlled is the dominant factor, and the push-type wire-feeding mechanism provides a boosting torque, thereby realizing synchronization. wire feed.

The 9935 spray gun embodies Praxair Daihatsu's more than 30 years of experience in arc spraying, and the 9935 spray gun also embodies the latest scientific and technological achievements in arc spraying technology. It has a precise spray rate and a large spray load.

The 9935 spray gun is innovative in design. Two independent air paths are set in the gun for primary atomization and secondary atomization airflow respectively, forming an original two-in-two-out secondary atomization structure. Integrated Super ArcJetR. The 9935 spray gun is advanced in technology and has established a control mode for monitoring the compressed air pressure of the "gun head". For the first time, the closed-loop control of the spraying process has been implemented for arc spraying; The proven wire feeding technology, combined with the advantages of the large air flow of the 9000 spray gun, fully guarantees the reliability and practicability of the equipment.