1. The role of paint nozzles
The paint nozzle is the second most important part of the Spray Gun, which controls the delivery of the paint liquid. The function of the paint nozzle is to control the flow of paint and direct the paint into the atomizing air stream. The paint nozzle also provides a valve seat for the valve needle, so that the valve needle can cut off the passage of paint.
The material of the paint nozzle is determined by the nature of the paint to be sprayed. For the standard paint with general performance, the quenched alloy steel nozzle can be used, but if the paint is corrosive, such as water-based paint, etc., it should be made of stainless steel. of the nozzle. For highly abrasive coatings containing various abrasives, tungsten carbide-lined nozzles should be used.
2. Selection of paint nozzle diameter
For different types of paint, three factors are mainly considered when selecting the matching paint nozzle aperture, namely the viscosity of the paint, the amount of spraying work and the spraying flow rate of the paint.
(1) Viscosity of the coating Fluid with low viscosity also has low resistance when flowing, so the nozzle diameter used can be smaller, and the hydraulic pressure used can also be lower; while the coating with high viscosity has large flow resistance, so in order to obtain sufficient flow velocity, it is necessary To use a large diameter nozzle and higher hydraulic pressure. Generally, the caliber of the paint nozzle is between 0.5~5.0mm, the caliber of 0.5~0.7mm is used for spraying of colorant, shellac and other easily atomized low-viscosity paints, and the caliber of 1.0~1.8mm is used for nitro paint, synthetic resin For paint spraying, the caliber of 2.0~2.5mm is used for the spraying of paints with high viscosity such as orange paint, and the caliber of 3.0~3.5mm is used for the spraying of viscous paints such as plastisol and anti-stone impact paint.
(2) The spraying flow rate of the paint requires a larger amount of paint to be sprayed, and a larger nozzle diameter, otherwise, more paint cannot be delivered per unit time. There are two basic methods to control the spray flow: one is to change the aperture of the spray hole, and the other is to change the pressure applied to the coating. Changing the aperture of the spray hole is a more convenient method. For ordinary coatings, the hydraulic pressure at the mouth of the Spray Gun should generally not exceed 0.2MPa. Excessive hydraulic pressure will produce excessively high spraying speed of the coating, making it impossible for the air nozzle to perform a good atomization treatment on it, so it should be selected as much as possible. The principle of large caliber and as low hydraulic pressure as possible to meet the flow requirements.
In order to determine the paint flow in the pressure feeding mode, the actual measurement method that can be used is to first close the air nozzle, then trigger the trigger to inject the paint into a container with volume scale, and use a stopwatch to record the different paint nozzle diameters and paint pressures. Under the spray flow rate (m1/min). But this method is cumbersome, and some empirical charts are also provided in some materials for people to choose. As shown in Figure 4-14, it is an empirical map to guide people how to choose the appropriate nozzle diameter when they know the viscosity of the paint, the spraying air pressure and the flow rate of the paint. As shown in Figure 4-15, the paint flow data is marked at the bottom of the figure, and the unit is (oz/min) (1 oz=28.3495g). The graph is divided into three parts, respectively representing the three viscosity states of the paint (that is, the kinematic viscosity measured with the No. 4 Ford cup is 15s, 20s, and 50s). All the measurement conditions in the figure are carried out within the hydraulic pressure range of 8~181b/in2 (55.2~124kPa). If it is known that the viscosity of a paint is 20s (measured by No. 4 Ford Cup), and the spraying flow rate is 26 ounces/min, then you can look up the point along the vertical line from the point where the coordinate flow rate is 26 ounces/min at the bottom, and find that in the viscosity The middle area of 20s (measured by No. 4 Ford Cup) intersects with the three boxes of 0.046in, 0.052in, and 0.059in, indicating that there are three calibers of paint nozzles that can be selected, and they can achieve the desired level by cooperating with different hydraulic pressures. paint flow. It is a good choice to choose the middle 0.052in, because the selected hydraulic pressure should be the middle value of the minimum hydraulic pressure and the highest hydraulic pressure. Therefore, it is unwise to choose the smallest caliber of 0.0461n, because the highest hydraulic pressure needs to be used at this time, and it is not appropriate to choose a larger caliber of 0.0591n, because the corresponding hydraulic pressure used at this time is small, so the middle size of the caliber and moderate hydraulic pressure should be selected. hydraulic.
For suction (siphon) spraying, the flow of paint can be adjusted by adjusting the aperture of the air cap and the paint flow control device on the Spray Gun, while for pressure-feeding spraying, it can be realized by adjusting the aperture of the paint nozzle and the hydraulic pressure applied to the paint. Adjusting the flow control device also has a certain limit. When the control knob is adjusted to the maximum, it has reached the limit of its maximum diameter. When adjusting the hydraulic pressure, experiments have shown that for ordinary paint sprayed by pressure delivery, when the atomizing air is turned off to make the paint spray out in a solid straight line, the jet flow should start to show a downward trend at a distance of 90cm from the nozzle. If the drop point is farther than 90cm, it means that the jet velocity is too high, and the hydraulic pressure should be reduced at this time.
(3) The amount of spraying work is generally small or complex workpieces should use small-caliber nozzles to make the spraying flow small and easy to obtain good spraying effects. Smaller nozzles should also be used in occasions that require higher atomization performance. In the case of small spraying flow, the paint is more likely to be atomized by compressed air, and in the case of heavy spraying work, a larger nozzle should be selected to ensure sufficient spraying flow and atomization effect.
