The first mock exam with minimum pressure loss replaced the side gate to solve the problem of broken bars of the cast aluminum rotor. The core shaft twist key was used to complete the laminated skew treatment of the rotor core. The vertical die casting machine was selected and the die casting die with one mold and four cavity rotors were used to raise the placement speed, the smoothness, and the production efficiency of the iron core insert. The rotor die casting die has the advantages of simple and practical structure, reliable operation, low manufacturing cost, convenient operation, and low scrap rate of castings. Next, we go over everything you should know about rotor die casting.

 

Squirrel cage rotor is one of the most important components of AC asynchronous motor. Pure aluminum guide bars are cast in each slot of the rotor. The guide bars are shorted with two end rings at both ends of the iron core to form a short-circuit winding, which is used to generate induced potential and then generate electromagnetic torque. Because the squirrel cage rotor has no winding, the rotor needs no maintenance, low noise and high reliability. If the rotor is seriously broken during the die-casting operation, the output of the motor will be insufficient, the current will increase, there will be periodic swing and the speed will drop, and the stator winding will be burned under the vicious cycle. Therefore, improving the die structure is the fundamental to obtain high-quality cast aluminum rotor.

 

Structure and process analysis of rotor casting

Rotor structure and film processing technology

The material of cast aluminum rotor, end ring and guide strip is cast pure aluminum (al99.8). The rotor core is made of rotor punching (100 pieces) laminated, and the spiral angle of its outer edge is 4.9 º (skew one tooth pitch). There is a 2mm deep and 4mm wide mark groove at the edge of the inner hole of the rotor punching plate, and the head has a sharp angle of 45 º, which is to ensure that the front and back directions are consistent during film processing.

 

The so-called slice trimming is to weigh out 100 punching pieces with the weighing method, pass one end of a fine lead wire of appropriate length through the marking groove and the other end through the guide strip groove closest to the marking groove, and twist the two lead wire heads slightly for two turns with pointed nose pliers. In this way, this group of iron core loose pieces are relatively fixed, and the lead wire is hidden in the 45 º sharp angle of the marking groove without interference with the twist helical key.

 

Structural analysis of die casting die

Calculation of helix angle of twist key

AB = π D outer edge / N number of grooves = 8.575; β= Tan-1ab / AC = 4.9 º, ad = π d inner hole / N number of slots = 3.021, α= tan-1AD/AC= 1.8º

 

Due to the uneven air gap of open slot rotor, the air gap magnetoresistance in the whole circumference is uneven (the slot magnetoresistance is larger than the tooth part), so that the back EMF contains cogging harmonic. When the rotor adopts the inclined slot, it can effectively curb the additional torque generated by the cogging harmonic magnetic field and reduce the electromagnetic vibration and noise. Whether the torsion angle of the rotor core is in place will directly affect the effect of overcoming the cogging harmonic in the back EMF. Therefore, a fast and efficient method of core inclined slot must be sought before rotor die casting.

 

Insert the mandrel matched with the clearance of the inner hole of the punching piece into the inner hole of the punching piece group of the rotor core (pay attention to align the notch with the marking groove of the inner hole of the punching piece), insert the torsion key along the mandrel groove when the shaft extension at both ends is equal, and insert the 2 mm wide oblique convex key of the torsion key into (or tap it gently with a small hammer) the marking groove of the iron core at the same time, so that the torsion work can be completed; The binding lead wire used in cutting and pulling out the slice is used to obtain the rotor core assembly.

 

The traditional rotor die-casting die can be basically divided into two structures. One is the mobile die-casting die often operated by hand in the production of the full vertical die-casting machine, which has the disadvantages of high labor intensity and low production efficiency; Two, the first mock exam machine for horizontal cold chamber die casting machine often adopts the die casting die with multi-cavity side gate feeding. Although the production efficiency is improved, it is difficult to place iron core insert and the casting rate of rotor castings is high, because the pressure loss of side gate is too large to cause breakage.

 

Structure and working principle of full vertical rotor die casting die

The iron core insert is inevitable during rotor die casting, and the die working on the full vertical die casting machine is the most suitable for placing the iron core insert, so the full vertical die casting machine is also called rotor die casting machine. After calculation, the total volume of four rotors is 387 cm3, so the total mass of cast aluminum is 967.5 g < 1kg (maximum mass of aluminum casting); After calculation, the total projected area of the casting is 284 cm2, so zj40-400kn full vertical motor rotor die casting machine can be selected.

 

Structural analysis of die casting die

The initial positioning of the rotor core and the analysis of the gating system. The four-rotor punch groups 17 are respectively placed in the corresponding positioning holes of the split middle ring 11, and then screwed into the positioning stop of the discharge plate insert 18 with a point gate taper hole (at the same time, pay attention to inserting the positioning key into the corresponding groove); The sprue of the die is connected with the cold pressing chamber of the die casting machine Φ For the 60 mm hole, the runner adopts a variant butterfly sheet structure to strengthen the strength of the runner and will not deform under the tensile breaking force with the point gate. It should be pointed out that each rotor adopts uniformly distributed six point gate feeding, which can minimize the pressure loss and greatly improve the yield. The waste of the six conical point gates attached to the lower ring of the rotor casting only needs to be cut off on the lathe.

 

Analysis of demoulding mechanism

The success of demoulding is related to the success or failure of the mold. There are 4 molds in total × 6 places Φ For 2-point gate, the rotor casting can be taken out only after the point gate and the butterfly runner are broken. The die adopts a discharge plate discharge mechanism driven by the return force of the upper die.

 

Working principle of die casting die

Adjust the die casting machine to make the upper die reach the upper limit position; Put the four rotor punching components that have been inserted into the assembly of the twisted mandrel into the four-hole positions of the split middle ring 11 respectively, and screw them into the hole of the discharge plate insert 18 as a whole after tightening Φ 2300 + 0.05 stop and insert the positioning key 22 into the positioning groove of the discharge plate insert 18 at the same time; First, put the asbestos paper cup that has been filled with sufficient molten pure aluminum into the bottom of the runner insert 19 Φ 60 mm hole. Start the die casting machine to make the upper die go down and the upper die cavity 8 Φ The 2300 + 0.050 stop is introduced into the upper end of the rotor punching group and continues to press down. At the same time, the discharge plate 14 also slides along the second guide column until the upper die is pressed down to the bottom to complete the die closing. Start the injection punch of the injection cylinder to conduct upward injection, and the molten pure aluminum flows through the flow channel insert 13 Φ 60 mm sprue Φ 24 of 220 mm butterfly runner and discharge plate insert 10- Φ The 2 mm point gate is injected into the ring cavity at the rotor end and the inclined groove of the iron core guide bar, and the mold is opened after pressurization, solidification and cooling (at this time, the pressure injection punch with pull hook is still in its original position at the end of injection);

 

Start the upper die upward. When the gasket 4 fixed at the end of the discharge pull rod 2 (4 in total) pulls the discharge sleeve 5 (4 in total), the discharge sleeve 5 pulls the discharge plate 14, When the discharge plate 14 rises strongly (because the 60 º barb of the injection punch reversely pulls the gate waste to ensure that it will not rise due to the tensile force of the point gate), it breaks the joint surface of the associated runner insert 12 Φ 2-point gate, continue to go up to the set limit position. Since the taper hole of the point gate is provided with a 3 ° slope, it is only necessary to tap the side of the split middle ring 11 radially with a wooden hammer to loosen it slightly, and then the four split middle rings 9 and four cast aluminum rotors with gate can be removed respectively; At this time, the mandrel 10 and the twist key 12 are still in the inner hole of the rotor. Because the mandrel 10 is in gap fit with the inner hole of the rotor, it is only necessary to hold the outer edge of the rotor and knock any end of the mandrel 10 with a small hammer. The mandrel 10 will exit quickly after being knocked axially. After the mandrel 10 falls, slightly hit the head of the twist key 12 radially, and it will fall with it. So far, the perfect cast aluminum rotor can be obtained. Repeat the above operation to enter the next die casting cycle.

 

Characteristic analysis

1. The first mock test is the first fake exam that dies.

 

2. Point gates are used to address the drawbacks of traditional side gate feeding, which is prone to strip breakage and undercasting of the reverse end ring. Because the inner gate has a 9-mm-long taper expansion area like the top gate, it is not as near to the casting as a standard point gate, which reduces the pressure loss significantly. In order to boost the feeding dynamic pressure transmission capacity and get better results, the number of point gates on each rotor was raised from the initial design of four to six after the mold test.


3. Four petal closed middle ring construction is used for the rotor punching, which not only aligns the four iron cores, but also prevents overflow at the open slot on the rotor outer edge. A significant role in emissions is also played by the space between the four middle rings that are closed at the top and bottom by petals.


4. In order to prevent the gate waste butterfly shunt channel from being dragged out when the discharge plate pulls the point gate off, a 60 o hook is fitted at the injection punch’s end.


5. Transform the outer edge slot’s direct skew into the inner hole mark’s indirect skew. The secret to successful twisting tooling is calculating the spiral angle of a 2mm wide oblique convex key at the mark groove of the inner hole to be 1.8 o by twisting a pitch (4.9 o spiral angle) at the outer edge.


6. Torsion key 12 has a divided construction with mandrel 10. Despite the twist key 12’s oblique key being in the rotor oblique key slot, it does not prevent the mandrel 10 from exiting after being struck by the axial direction after the die casting is done and the components are extracted. In order to bring the mandrel 10 down, all it needs to do is to bump against the head of the twist key 12 a little bit radially. An inherent construction of the mandrel 10 and twist key 12 will make it difficult to screw the twist key 12 smoothly into the keyway of the punching group.


7. Disturbances and deformations can arise easily when the point gate is damaged and separates from the runner owing to inadequate strength, however the total strength of the butterfly runner overcomes this disadvantage.