With rapid development of green travel industry, major auto companies have entered new energy field. Motors are core components of new energy vehicles, and their market demand continues to rise. Aluminum alloy motor end caps are an important part of motors. Its stable production quality can ensure supply of complete motors, production and sales of entire vehicle market. Die-casting mold is an important equipment for producing motor end caps. Whether molded parts can be demolded smoothly is an important factor affecting die-casting production, it is also an issue that must be considered in design and development of die-casting molds. Following introduces die-casting forming of aluminum alloy motor end cover. Parts formed during die-casting test mold are easy to be stuck in fixed mold. Reasons are now analyzed.

Aluminum alloy motor end cover is an important structural part of motor. It seals two ends of motor body, fixes position of motor rotor in motor and supports movement of rotor. In addition, it also protects electrical components and circuits of motor, as shown in Figure. Part size: 405.0mm*326.7mm*155.3mm, material is aluminum alloy ADC12, weight is 4.16kg, insert is bearing sleeve.

 

(A) Fixed mold forming part

 

(B) Movable mold forming part
Figure 1 Motor end cover
Motor end cover has structures such as deep cavity, deep ribs, lateral grooves and bosses, shape is more complex. Figure 1 (a) shows complex ribs (different in depth), electrical components and wiring harness installation slots, accessory installation positions, motor positioning semi-circular arcs, etc. Overall structure is irregular, and design is formed on the side of fixed mold; as shown in Figure 1(b), there are insert bearing sleeves and all-around reinforcement ribs, part of motor rotor space, mounting holes, etc. in the center. Overall structure is deep cavity-shaped and has a slightly regular structure, it is designed to be molded on movable mold side.

 

Figure 2 Fracture location
During trial production process of mold for forming motor end cover, when more than 30 pieces were produced, molded casting remained in fixed mold and two parts were broken. As shown in Figure 2, die-casting production could not continue and mold was repaired. Broken parts of molded motor end cover are lateral slider forming part (breaking position 1) and core-pulling forming part (breaking position 2).
Through analysis and research on problem of molded casting remaining in fixed mold, main reason for fracture and failure of molded motor end cover is found. When mold is opened, lateral slider and core-pulling components are in core-pulling process. Due to large clamping force of fixed mold, jamming of insert bearing sleeve, and low strength of molded casting under hot conditions, lateral slider and core-pulling component cannot take out molded casting during core-pulling process, molded casting breaks and remains in fixed mold. In addition, fixed mold ejection mechanism has small ejection force and unbalanced ejection, which causes bearing sleeve and positioning insert to jam.

 

Figure 3 Principle of traditional packing tightening force
Packing force is force produced by packing core or forming part of punch during cooling and shrinking process after molten metal fills cavity during die casting, as shown in Figure 3. Packing force calculation formula:
F=F resistance cosα-F package sinα=Alp(μcosα-sinα)
In formula: F-core pulling force, N; F resistance-core pulling resistance, N; F package-tightening force generated by casting to core after condensation and shrinkage, N; A-section circumference of core forming part wrapped tightly by casting, mm; l——length of core forming part wrapped tightly by casting, mm; p——extrusion stress (tightening force per unit area), aluminum alloy generally takes 10~12MPa; μ— —Friction factor of die-casting alloy to core, aluminum alloy generally takes 0.2~0.25; α——Demolding slope of core forming part.

 

(A) Schematic diagram of casting pressure

 

(B) Principle of actual packing force
Figure 4 Casting pressure and packing tightness
Under normal circumstances, packing force only calculates packing force of core or punch part, and ignores packing force of cavity or concave mold part, which causes distortion of actual calculated packing force. However, in actual die-casting molding process, all go through high pressure and feeding stages. Increasing pressure can avoid trend of volume shrinkage caused by solidification of molten aluminum. In stage of increasing pressure, solidification pressure of molten aluminum is 60~90MPa. This pressure will make all sides of molded casting (see Figure 4(a)) better contact with mold parts (concave surface has a strong affinity). When designing moving and fixed molds, all surfaces with draft angles need to be considered. When mold is opened, there is also a packing force on outer surface (fixed mold side) (see Figure 4(b)). Although it is not as strong as concave surface (moving mold side), it will also affect demolding of molded casting.
Main purpose of design of forming part of movable mold is to make molded casting stay on the side of movable mold under synergy of push-out system of die casting machine. Generally, complex structure design of forming castings is formed on movable mold side, such as reinforcing ribs, bosses (especially higher bosses), concave surfaces, etc. In production, main problem is that mold is fixed and wrapped to form castings. Designer must calculate draft force and push force of cavity and core. General draft angle is 1°~3°, and outside draft angle is 1°. Inside draft angle is 2°~3°. Part with the most complex structure and the most bonding surface is formed on movable mold side. Designer should analyze and consider which side of molded casting is suitable for molding in movable mold.

 

(A) Area of packing force generated on fixed mold side

 

(B) Area of packing force generated on movable mold side
Figure 5 Area where tightening force is generated
Shrinkage of molded casting causes all casting pins, ribs, draft angle surfaces, and bosses to form a tightening force on them. As shown in Figure 5, light-colored surface will form a resultant force, causing casting to stay in mold.
Structure shown in Figure 5(a) has both concave and convex surfaces. It is necessary to analyze complexity of structure and measure contact area. Figure 5(b) has only one boss and a large circumference, draft angle is large, and the others are all small areas.
According to calculation formula of packing force: F=F resistance cosα-F packing sinα=Alp(μcosα-sinα), packing force on fixed mold side is 238.7~368.8kN, and packing force on movable mold side is 145.3-224.6kN. Because 238.7~368.8kN>145.3~224.6kN, probability of molded casting staying in fixed mold is higher.

In process of die-casting mold design, usually parts that are easy to stick to mold are designed in movable mold, because movable mold has an ejection mechanism, but sometimes parts that are easy to stick to mold appear in fixed mold direction, it is necessary to design push-out structure in fixed mold to offset tightness of casting.

 

Figure 6 Independent launch agency
1. Reverse push rod 2. Casting 3. Fixed mold core 4. Fixed mold base 5. Disc spring 6. Limit screw 7. Fixing seat 8. Disc spring cover
Independent push-out mechanism can be designed at a certain position of fixed mold. Reverse push rod is designed on the edge of molded casting. One part of which is in contact with molded casting to push out molded casting. The other part of reverse push rod is in contact with parting surface of movable mold for reset of reverse push rod, as shown in Figure 6.
During mold clamping process, movable mold forces push rod into fixed mold, and disc spring compresses; fixing seat is fixed in fixed mold frame and fixes position of disc spring sleeve; mold opens, disc spring resets, push reset rod to move. Ejection mechanism has a large ejection force and a small ejection stroke, but ejection position is a single-point ejection. Considering that ejection balance needs to be specifically analyzed based on the structure of molded casting. Independent push-out mechanism is suitable for molds with high local tightness of molded castings. In addition, multiple independent push-out mechanisms can also be used in combination for molds with compact structure and no space for pushing plates.

 

(A) Fixed mold structure

 

(B) Fixed mold combined push-out mechanism
Figure 7 Design of push rod of fixed mold and ejection mechanism
1. Fixed mold frame 2. Reverse push mechanism 3. Fixed mold core 4. Disc spring mounting column 5. Disc spring assembly 6. Reverse push plate 7. Fixed plate 8. Reset lever 9. Push rod 10. Guide post 11. Limit post
If local packing force of molded casting is large in fixed mold or overall packing force of fixed mold is large, fixed mold ejection mechanism (combined type) can be used. Resetting and pushing out of push-out mechanism are realized by using a reset lever and a disc spring, as shown in FIG. 7.
In ejection position of molded casting, it is used to push out molded casting; reset rod is designed outside molded casting to reset fixed mold ejection mechanism (combined type). Push plate and push rod fixed plate are equipped with guide sleeves, limit post controls upward movement distance of push plate and push rod fixed plate. When push plate and push rod fixing plate move in place, limit post contacts die-casting machine seat plate; disc spring mounting column fixes disc spring assembly on push rod fixing plate, disc spring is power source for fixed mold ejection mechanism (combined type) to push out casting.
Mold for forming motor end cover adopts above-mentioned fixed mold ejection mechanism (combined type), with 4 reset rods and 8 push rods, 4 sets of disc springs and 4 sets of guide pin guide sleeves; push plate and push rod fixed plate are designed with escape holes for installing water pipes.
When mold is closed, reset lever will push push-out mechanism upwards, disc spring is compressed and stressed; when mold is opened, disc spring loses force of reset rod and is gradually in reset state. Disc spring drives push plate and push rod fixing plate to move downwards, push rod moves downwards with push plate to push out molded casting.
Ejection force of above-mentioned ejection mechanism comes from disc spring. Compared with traditional ejection mechanism using spring, it has advantages of large ejection force and small ejection stroke. Fixed mold ejection mechanism (combined type) adopts 4 sets of guide positioning components, which makes castings to be ejected more stable, safe and reliable. After mold for forming motor end cover adopts above scheme, a fixed mold ejection mechanism (combined type) is added, die-casting production process runs stably. 18,600 pieces of casting produced no overmolding problems of formed castings, which solved overmolding problems caused by large clamping force of fixed mold, jamming of insert bearing sleeve and low strength of formed casting in hot state.