Discussion on the characteristics and parameter control of winding drip immersion process
In the dripping process, due to the consideration of the full utilization of the dripping machine, the dripping process is often completed without complete curing, allowing the workpiece to achieve complete curing under the action of future operating temperatures. But generally, the workpiece should be removed from the immersion equipment and then placed in an oven for post-processing to complete the entire curing process. For stator windings, the post-processing time can be shorter, while for rotors with high mechanical strength requirements, the post-processing time should be longer. Practice has proven that the insulation performance of windings that have undergone post-treatment is much better than windings that have not undergone post-treatment.
The immersion process is to continuously drip solvent-free paint that has been precisely measured and can quickly cure onto a preheated rotating winding. The process parameters must be adapted to the drip coating in order to fully utilize its advantages and compensate for some of its drawbacks. The following Ms Based on the characteristics of drip dipping paint, discuss how to reasonably formulate dipping process parameters. The entire process of immersion technology can be divided into four stages, namely preheating, paint dripping, gelation, and curing.
Preheating
The purpose of preheating the winding is to remove moisture from the winding and obtain a suitable temperature for the penetration of paint.
The preheating temperature is determined by the viscosity temperature characteristics and gelation time temperature characteristics of the dripping paint. Generally speaking, the lower limit of preheating temperature should be the temperature at which the viscosity of the paint is easily permeable, and the upper limit of preheating temperature should be the temperature at which it has a faster gelation. During preheating, the winding can be energized for heating, and it usually takes a few minutes to reach the preheating temperature. But for large-scale production, electric heating obviously cannot meet the demand, and most motor manufacturers use preheating ovens.
Drip paint
Dripping paint is an important step in determining the quality of immersion paint. The process parameters for dripping paint mainly include workpiece inclination angle, workpiece rotation speed, dripping paint flow rate, and time.
● Working inclination angle
When coating the winding, the axis of the winding is generally inclined at a certain angle to the horizontal plane, which is called the working inclination angle. This method of dripping paint is called the tilting method. The tilting method can accelerate the flow rate of paint. For windings with thick wire diameter, short iron core, and low slot filling rate, the inclination angle of the workpiece should be smaller to avoid rapid flow infiltration and short paint dripping time, which may cause poor filling; For windings with thin wire diameter, long iron core, and high slot filling rate, the inclination angle of the workpiece should be larger. But for 2P motors with long winding ends, the inclination angle of the workpiece should not be too large, otherwise it will cause the paint content at the winding ends to be too low, resulting in poor surface coverage. The inclination angle of the workpiece can be adjusted and selected according to different specifications between 15 ° and 35 °.
The drop painting process was first adopted using the inclined method, and later the horizontal method appeared, which means that the winding is dropped paint in a horizontal position. The horizontal method can simultaneously drop paint from both ends of the winding to shorten the drop painting time.
The horizontal method equipment is relatively simple for small workpieces. For larger workpieces, the tilting method or a combination of tilting and horizontal methods should be used to facilitate the full penetration of paint into the groove space; Start dripping the paint at a certain angle to make it quickly penetrate. When the lower part starts to flow, quickly level the workpiece and add a certain amount of paint to make the paint evenly distributed and shorten the dripping time.
● Workpiece speed
When dripping paint, the workpiece must rotate continuously to ensure that the winding end receives a uniform amount of paint, and the paint liquid flows along each groove to the lower end of the winding. After the dripping of paint is completed, the workpiece is horizontally placed and rotated to allow the paint to be subjected to centrifugal force, fully filling and penetrating the turns of the winding, and evenly covering the surface of the winding end without loss.
Choosing the appropriate rotational speed is crucial. If the speed is too high, there may be paint peeling, causing the outer circle of the iron core to stick paint, resulting in more groove primer and less groove paint; If the speed is too low, it will cause paint loss and affect the speed of paint dripping. The workpiece speed can generally be selected within the range of 10-50 revolutions per minute. Use lower rotational speed for workpieces with larger diameters; Small diameter workpieces should be rotated at higher speeds.
● Dripping amount of paint
The amount of paint dripping is a key factor affecting the quality of dripping. If the amount of paint dripping is too small, the paint content between the turns and ends of the winding is low, the filling penetration is poor, and the coverage is poor. If the amount of paint dripping is too large, it cannot be completely absorbed, and the excess paint will flow into the inside and outside of the iron core, which not only wastes paint but also increases the cleaning workload. If the structure of certain workpieces is prone to paint accumulation, cracking may occur during curing. For rotor windings, uneven paint adhesion can also affect balance.
The amount of paint dripping is related to the structure and size of the workpiece. It can be determined through experiments. At present, a metering pump is generally used to control the amount of paint dripping, which can ensure that each workpiece receives the same amount of paint to ensure the quality of the workpiece dripping. After determining the amount of paint to be dripped, the dripping speed is determined through experiments, which ensures that even the dripped paint can be absorbed without loss. After determining the amount and speed of paint dripping, the dripping time can be determined.
Gelatinization
At a certain temperature, the solvent-free paint changes from a viscous liquid to an elastic gel like substance, and its properties also undergo a sudden change. This phenomenon is called gelling, or simply gelling.
When gel first appeared, gel was composed of two molecules with different structures. One is the polymer with network structure, which has unique characteristics, so it is called gel. The other is the free gel with small molecular weight, which is enveloped in the network structure of gel.
Under the action of temperature, the gel continues to react, the sol part gradually decreases, the gel part gradually increases, and the elasticity and toughness of the polymer are enhanced, finally the cured product is obtained; Gelatinization causes a qualitative change in paint performance, accompanied by phenomena such as heat release, volume shrinkage, and foaming.
● Heat release phenomenon
The speed of heat release is related to the properties of the paint and the hardening agent used, as well as the gelation temperature and time. The higher the gelation temperature and the shorter the time, the faster the heat release.
The selection of temperature and time for adhesive coating should consider the effect of heat release when dealing with dripping paint. Rapid heat release, high temperature, and rapid molecular reactions can cause explosion, resulting in blistering or cracking of the paint layer. For larger workpieces, due to their large heat capacity, the heat release during gelation generally has little impact; However, attention must be paid to situations where small workpieces or workpiece structures are prone to paint accumulation.
● Volume shrinkage
Paint can cause volume shrinkage during the gelation and curing process. It includes shrinkage caused by intermolecular reactions and shrinkage caused by temperature changes. In general, the shrinkage rate of unsaturated polyester is greater than that of epoxy resin. The gel is too fast, and the shrinkage rate is larger. When the resin shrinks, it can even peel off the paint film of the electromagnetic wire from the wire, causing a short circuit between turns.
In most cases, solvent-free paint has a stronger adhesion to the electromagnetic wire coating than to the wire. Due to excessive contraction force, it is easy to cause cracking on both sides of the groove.
● Bubbling phenomenon
The volatile compounds of solvent-free paint are generally very low, and it should not be easy to produce foaming during the gelation process. However, there are often some volatile diluents or low boiling hardeners in epoxy paint that need to evaporate during the gelation process; Especially when the gelation reaction is intense, the surface paint is already forming a thin film, and the volatile substances inside continue to escape, making it easy to form bubbles. In polyester paint, due to the low boiling point of certain ingredients (such as styrene), foaming may occur. Therefore, after dripping the paint, it is generally desirable to rotate it for a few more minutes and then gel it. The selection of gelation temperature should be such that gelation does not occur too quickly and affect quality at that temperature. The choice of gelation temperature is also related to factors such as the size of the workpiece. The workpiece is large, the gelation temperature should be relatively low, and the gelation time should be relatively long.
Curing
