Design of Injection Mold for NdFeB Magnetic Inductor Based on PA Loading

Time:2023-09-09 19:11:25 / Popularity: / Source:

1 NdFeB magnetic sensor element

As shown in Figure 1, NdFeB magnetic sensor is an E-shaped piece with a total width of 59 mm. Among the three branch forks, length of a single branch fork is 26 mm and thickness is 7 mm. Branch fork has two layers and a single layer thickness is 2 mm. After parts are magnetized, connecting beam is S pole of magnetic pole, three branch forks are N pole of magnetic pole.
Design of Injection Mold 
Figure 1 NdFeB Magnetic Sensor

2 Powder metallurgy molding process of NdFeB magnetic inductor

NdFeB magnetic sensor adopts powder metallurgy forming process route: Fe-Nd-B alloy composition design→master alloy smelting→rapid quenching amorphous thin strip treatment→crystallization to improve coercive force→mechanical ball milling pretreatment powder→binder+ additives (coupling agents, lubricants and antioxidants) → blending with internal mixer → extrusion granulation with screw machine → injection molding magnets → magnetization → magnets. Magnet obtained by this process is isotropic, and target physical parameters of magnet are: remanence Br=0.52~0.71T, internal coercive force Hci=660~750 kA·m-1, maximum magnetic energy product (BH) max=68~ 76 kJ•m-1. Injection molding steps: mold closing → injection → holding pressure (synchronous magnetic field enhanced orientation/pre-molding) → cooling → mold opening and pick-up.

3 Injection material

Injection material is a composite material, and material composition includes magnetic powder, binder, additive, coupling agent, lubricant and antioxidant. Grade of magnetic powder is MQP-13-9, and its physical parameters are shown in Table 1.
Table 1 MQP-13-9 physical parameters
Design of Injection Mold 
Magnetic powder particles are mainly coarser particles of 120 mesh (70%), and finer particles of 200 mesh are supplemented (30%), which is beneficial to reduce void ratio, increase bulk density, and improve fluidity. Content of granulating agent is 0.3%. Adhesive is PA1010 nylon material, because of its low water absorption, good dimensional stability, outstanding wear resistance and self-lubrication, it can be used under conditions of light load, low temperature, large humidity fluctuation, no lubrication or little lubrication. The total content of additives is 11.2%, among which silane coupling agent (3%) is used as coupling agent, 0.5% titanium ester (5%) is used to treat magnet, antioxidant is 1.2%, and lubricant is 2%. Mixing process temperature is 205~215 ℃/6 min.

4 Difficulties in injection molding process

Difficulties of NdFeB magnetic powder injection molding are as follows: ① The higher powder loading in bonded PA1010, the greater flow viscosity of mixture during injection. When loading amount reaches more than 90%, viscosity of mixture is 2 to 3 orders of magnitude larger than that of pure nylon material, and fluidity becomes poor; ②Due to existence of magnetic powder, mixture generates a large friction force on the surface of molded part during flow process, there is friction between magnetic powder particles in material flow, which is easy to generate frictional heat; when injection speed increases, magnetic powder will generate greater shear heat to nylon PA1010, and increase of these heat will easily cause nylon to burn; ③Magnetic powder can’t generate attractive force between mold runner parts and cavity forming parts, otherwise magnetic powder is easy to agglomerate, which is not conducive to cavity filling; ④Push rod cannot use materials that can generate attraction with magnetic powder, otherwise molded part cannot be automatically separated from push rod; ⑤When hard magnetic powder particles in molten PA1010 material are filled at high speed, they are likely to cause greater wear on the wall of cavity, resulting in damage to cavity and premature failure; ⑥ There is no more scientific design reference data for length-to-diameter ratio of screw of injection molding machine. Screw of injection molding machine can only be designed based on experience.          Excessive injection pressure will cause shear overheating. For molding of NdFeB magnetic sensor using PA1010 as binder, according to characteristics of nylon, injection nozzle temperature is set to (265±3) ℃, injection pressure is controlled at about 6 MPa, holding pressure is 4.5-5 MPa, single-stage pressure holding time is 6 s; ⑦ Number of magnetization must be tested and checked before determining number of excitations in mold.

5 Injection mold design

5.1 Cavity Design

According to characteristics of plastic parts, it is proposed to use two-plate molds to realize injection molding of intermediate parts, and molded parts are in the form of inserts. Core of mold design lies in design of cavity plate inserts, as shown in Figure 2. The largest outer contour of plastic part to be formed is used as parting surface, and a single-point gate is used for casting. Latent gate G0 is set at a position away from geometric center of plastic part to be formed, and distance from geometric center is 14.5 mm. Use of latent gate is beneficial to automatic demoulding of flowing waste. Diameter of latent gate is φ1.5 mm, which must be larger than ordinary gate to avoid excessive gate shear force and insufficient cavity filling. Cavity plate insert 1 and core insert 3 are respectively provided with φ8 mm cooling water channels C1 and C2, and cooling water channels are arranged along outer edge of plastic part, so that cavity can be cooled with maximum efficiency.
Design of Injection Mold 
Figure 2 cavity plate design
1. Cavity plate insert 2. Slider insert 3. Core insert 4. Iron core 5. Iron core 6. Iron core 7. Iron core
Considering problem of magnetic attraction, cavity plate insert 1, slider insert 2, and core insert 3 all use non-magnetic material 304 stainless steel, iron cores 4, 5, 6, and 7 use GGrl5 steel magnetic steel. During packing process, magnetic powder needs to have a magnetic field orientation, so that magnetization direction of powder is same as direction of external magnetic field. Coils for orientation are respectively installed on fixed mold and movable mold of mold. Iron cores 4 and 5 are a magnetic path, iron cores 6 and 7 are a magnetic path. When coil is energized, intermediate molding is magnetically guided by 2 sets of iron cores. In order to ensure cooling of intermediate molding after magnetic conduction, cavity plate insert 1 and core insert 3 are respectively cooled by φ8 mm cooling channels. Cooling water rate and cooling time must be optimized and adjusted for process parameters after magnetic properties of intermediate molding are tested.

5.2 Die structure layout

Mold adopts a two-plate mold structure, layout of movable mold is shown in Figure 3. Mold has a layout of one mold and one cavity. A slider mechanism M1 is arranged on the side of movable mold to pull core of slider insert 2; mold base uses 4 guide pillars 8 for guidance, and 4 reset rods 10 for reset. Core insert 3 is fastened and installed in insert groove of movable mold plate 12 with 4 screws 9 . Size of mold base is 280 mm * 250 mm.
Design of Injection Mold 
Figure 3 moving mold layout
3. Core Insert 8. Guide Post 9. Screw 10. Reset Rod 11. Electromagnetic Coil Assembly 12. Moving Mold plate
Layout of fixed mold is shown in Figure 4. Fixed mold is mainly arranged with cavity plate insert 1, locking block 13, inclined guide column 14 of slider mechanism M1, sprue sleeve 15, positioning ring 16, electromagnetic coil assembly 17, guide Set of 18. Sprue bushing 15 is made of non-magnetic material 304 stainless steel. Electromagnetic coil assemblies 11, 17 need to be wired to match magnetic poles.
Injection mold design 
Figure 4 Fixed mold layout
1. Cavity plate insert 13. Locking block 14. Inclined guide post 15. Sprue sleeve 16. Positioning ring 17. Electromagnetic coil assembly 18. Guide sleeve 19. Fixed template

5.3 Mold assembly and working principle

Mold structure is shown in Figure 5. Sprue sleeve sinks into fixed mold seat plate to shorten length of runner. Both pulling rod 21 and push rod 22 are made of non-magnetic material 304 stainless steel. Use spring 27 to reset push plate 23, slider 29 of slider mechanism M1 is installed on movable mold plate, position is limited by limit screw 28, and the two sides of slider 29 are provided with guide bars for sliding out of slider guide. Mold is only opened once (PL side), and the total height of mold closing is 276 mm.
Injection mold design 
Figure 5 Die structure
20. Fixed mold seat plate 21. Pull rod 22. Push rod 23. Push plate 24. Push block 25. Moving mold seat plate 26. Spacer 27. Spring 28. Limit screw 29. Slider
Problems to be paid attention to when assembling mold: ①All moving parts of mold should ensure accurate position, reliable movement, no skewing and sticking, and no movement of fixed parts; ②After mold is closed, parting surface should be closely attached, and fittings of fixed inserts in molding part should be closely attached. If there is a local gap, gap should be smaller than plastic overflow gap; ③After assembly, carry out trial mold acceptance. Demoulding mechanism must not interfere. Molding quality of plastic parts must meet design requirements. Otherwise, mold should be repaired and tried again; ④ Verticality of guide post, guide sleeve and fixed mold installation surface should be designed according to CB/T 12556.
After mold is installed on injection molding machine, its function is realized through following steps.
(1) Mold closing. Mold is clamped on injection molding machine, and barrel of injection molding machine is melted and waits for injection.
(2) Injection. Injection molding machine injects molten plastic into cavity through nozzle, and cuts into pressure-holding state after filling.
(3) Magnetization. During pressure holding process, electromagnetic coil assembly 17 conducts electricity, magnetizes magnetic powder, and completes magnetic pole orientation of plastic part.
(4) cooling. Cooling water cools cavity, and mold is opened after plastic part is cooled to ejection temperature.
(5) Open mold. Sliding block of injection molding machine pulls movable mold downward, mold is opened on PL surface, and plastic part comes out of cavity plate insert 1 and stays on core insert 3. At the same time, inclined guide column 14 drives slider 29 to complete side core pulling.
(6) LAUNCH. After movable mold retreats to a certain position, ejector rod of injection molding machine pushes push plate 23, push plate 23 pushes pulling rod 21 and push rod 22 to push out runner waste and plastic parts, so as to realize complete demoulding of molded plastic parts.
(7) RESET. Push plate 23 resets first, movable mold moves upward until mold is closed at PL surface, and next injection cycle is started.

5.4 Design Discussion

After mold was tested, process parameters were adjusted as follows: For nylon PA1010 binder system, injection temperature of mixture was controlled at 266 ℃, molding injection pressure was 5.8 MPa, holding pressure was 4.88 MPa, and holding time was 5.6 s. The best magnet performance is obtained; mold temperature is preferably 80 ℃, and formed magnet is magnetized twice under action of magnetizing magnetic field to achieve magnet performance design goal.

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