The commonly used spraying techniques are HOVF spray, plasma spray, arc spray, flame spray, explosion spray and so on, which correspond to various spraying equipment. Today we focus on HVOF and plasma spray equipment.
1、High Velocity Oxygen Fuel spray equipment
There are two primary types we commonly use: HVOF (High-velocity oxy-fuel) and HVAF (High-velocity air-fuel), abbreviated as oil guns and gas guns in our industry, respectively. The main difference lies in their power sources: HVOF uses oxygen with fuels like kerosene, propane, or hydrogen, while HVAF employs compressed air with fuels such as propane or hydrogen. Although their working principles differ slightly, both systems deliver comparable coating quality. In China, HVOF remains more prevalent due to its equipment being easier to operate and maintain.
Many people hype up the superiority of HVAF, but I don't think so. While it does offer some advantages when spraying low-melting-point materials, our most commonly used tungsten carbide coatings show no significant difference. In fact, I'd advise novice spray operators to skip HVAF – it's tough to master without prior experience. Starting with HVOF is actually a more accessible way to get the hang of it.
2、 plasma spray equipment
Plasma devices utilize gases such as argon, nitrogen, hydrogen, and helium (which are too expensive for widespread use) to ionize the gas and generate plasma for energy release. While the fundamental principle of plasma equipment isn't particularly innovative, its power supply system proves crucial. Some claim that plasma flame streams can reach temperatures exceeding 10,000 degrees Celsius, capable of melting all materials. But how do we ensure that all materials are thoroughly melted under such conditions?
The core principle of spray coating equipment involves using various heat sources to melt or semi-melt powder particles, which are then rapidly sprayed onto substrates to form coatings. While all systems utilize different energy sources, cold spraying stands as the sole exception – it employs high-speed airflow to deposit powder onto substrates at lower temperatures. This fundamentally distinguishes physical bonding from metallurgical bonding in coatings. The effective energy delivered by the equipment is the decisive factor in coating formation. It's important to clarify that this "effective energy" doesn't equate to the total power output of the system, as multiple factors influence this process. Let's break down these elements one by one.
1、The total energy of the equipment. For HVOF devices, energy is directly proportional to fuel consumption—the more fuel consumed per unit time, the greater the energy output. In plasma equipment, maximum energy is determined by both power supply and hydrogen gas: higher current intensity and hydrogen concentration directly enhance energy output. This represents the equipment's overall energy capacity, which is just one aspect of its operational parameters. A 50KW plasma system and a 100KW plasma spraying system share identical material processing capabilities, differing only in operational efficiency.
2、The energy of the flame stream. Taking the common JP8000 as an example, the temperature of the flame stream is influenced by oxygen and kerosene. Only when they are properly proportioned can the kerosene burn completely. Typically, setting oxygen at 1850SCFH and kerosene at 6.0GPH achieves optimal flame conditions, and all parameter adjustments are based on this foundation. For plasma, the situation becomes more complex. The energy of the flame stream is specifically allocated for powder melting and acceleration, representing effective energy utilization.
3、The powder feeding process requires maintaining stable delivery to the flame stream's core for complete melting. However, this critical step presents inherent challenges: Since powder particles have a specific size distribution, some inevitably remain unfused and disperse into the air. Effective powder control directly determines coating quality. Yet many spray coating plants neglect this crucial aspect, ultimately compromising production efficiency.
4、The distance in spray coating. Our research has measured both the temperature and velocity of powder particles. When heated and accelerated, the particles reach peak velocity at specific distances. High-performance monitoring equipment allows precise tracking of particle temperature and velocity within the flame flow at different locations. Therefore, maintaining optimal distance control during the spraying process is essential to achieve the best coating performance.
We control the equipment to control the size of effective energy and the distance of spraying, and control every detail of the equipment, such as leakage, oil leakage and other problems. Under the premise of normal equipment, it is meaningful to talk about the performance of coating. A broken equipment can not produce good coating no matter what.
Some of our previous customers process their own equipment, and we will provide a lot of help. We do not mind that our customers become peers, but we only hope that our customers will not fall into the trap when choosing equipment, so that the whole industry can develop.