【Abstract】According to structural characteristics of parts and molding characteristics of "metal powder + PA resin", a metal powder injection mold was designed. It focused on analysis and research of molded parts, gating system, side core pulling mechanism, fixed distance parting mechanism, demolding system, temperature control system and exhaust system, determined an advanced and reasonable mold structure. By adopting 4 point gates, fixed mold inclined top and fixed mold "T"-shaped buckle lateral core pulling structure, problems of heavy blank weight, difficult feeding and difficult demolding are solved. Results show that dimensional accuracy of product reaches IT5, surface roughness value reaches Ra0.8μm, which all meet design requirements.

Metal Powder Injection Molding is abbreviated as MIM, or Metal Powder Injection Molding. It is an interdisciplinary advanced technology for manufacturing complex and precise alloy parts by injection molding. It uses a special metal powder (micron level) mixed with high-quality macromolecule plastic polymer to form a MIM feed. This feed can provide good fluidity during injection, blank is formed by injection molding. After blanks are efficiently degreased and sintered, density of alloy parts can reach 99% of theoretical density. Metal powder injection molding products have uniform density and good finish. Generally, no subsequent processing is required. Utilization rate of raw materials is close to 100%. It is the most revolutionary near-net molding technology in 21st century. Product shown in Figure 1 is a nickel-added stainless steel metal part in orthodontic frame of medical equipment. Product has a large batch and a complex structure. Surface roughness value is required to reach Ra0.80μm, dimensional accuracy is required to reach IT5 (GB/T1800.1). -2009). Due to difficulty in processing, low efficiency, and large waste of materials using ordinary mechanical manufacturing methods, in order to save costs, metal powder injection molding technology is used.

 

Figure 1 Product structure diagram
a — Front view b — Sectional view c — Three-dimensional view (zoomed out)

Weight of metal powder injection molded products generally does not exceed 250g, but product has a larger size, weighing about 350g, and the largest external dimension: 171.00*67*34.50mm, average wall thickness is 2.2mm, and weight of product is one of difficulties that MIM injection mold structure design must face. In addition, structure of product is complex. There are 4 places where lateral core pulling is required, all of them are on fixed mold side. Fixed mold lateral core pulling is second difficulty that mold design needs to face.

Particle size of nickel-added stainless steel metal powder is 0.2μm. Organic adhesive adopts nylon PA12, which is carrier of metal powder. Its function is to bond metal powder particles to make feed material have rheology and lubricity after heating in barrel of injection machine. Advantage of nylon is that amount is small, it does not react chemically with metals, and it is easy to remove during degreasing. With reference to foreign experience and previous experimental results of our hospital, volume ratio of nickel-added stainless steel and PA adhesive is 8:2. In debinding process, volume of parts will not change during debinding process, but parts will shrink by 13% to 18% during sintering. Therefore, shrinkage rate is much larger than that of a simple PA injection mold. According to feed ratio, mold size uses a 15% shrinkage rate.

In order to solve problem of heavy weight of formed blank, mold adopts a gating system with 4 point gates that feeds materials evenly from cavity. Specification of mold base is LKM Simplified Nozzle Mold Blank 3045-FCH-A130-B90-400, which adopts a built-in fixed-distance parting mechanism. As shown in Fig. 2, mold clamping buckle 47 prevents mold from opening during transportation and causing safety accidents. Mold clamping buckle must be removed during production.

"Nickel-added stainless steel + nylon PA12" feeding material is very sensitive to gap, it is easy to produce flash. Flashing of plastic parts is easy to remove, but flashing of metal parts can cause safety problems like a blade. Therefore, design and manufacturing requirements of MIM injection mold parts are particularly high, dimensional accuracy and fit accuracy must reach IT5 or more. Mold forming parts adopt inlay structure, consisting of movable mold insert 17, movable mold core 18, fixed mold insert 33 and 4 side core pulls. In order to improve rigidity and strength of mold and ensure dimensional accuracy of molded product, parting surface of insert is designed with 4 positioning angles, as shown in Figure 3. Compared with single PA plastic melt, "nickel-added stainless steel + PA" feeding has a feature that it has greater friction on mold cavity, has high requirements on surface roughness of molded parts steel and cavity. Mold molded parts must be used steel with better wear resistance, mold uses die steel S136H, heat treatment hardness is 30-35HRC, surface of cavity is polished to Ra0.4μm to improve fluidity of feed and increase life of mold. High dimensional accuracy, no flash, and super high surface quality, all these requirements must be met by MIM injection molds.

Formed blank has two outside undercuts and two inside undercuts, both of which need to be designed with a lateral core-pulling mechanism. Two inner core-pulling distances are both 1.7mm. Because core-pulling distance is short and inner space is small, mold can only adopt inclined top side core-pulling mechanism, as shown in cross-sectional view A-A in Figure 2c. Also, because inner undercut position is formed by a fixed mold, fixed mold can only be used for inclined top, which is difficulty and key point of mold design. Inclined roof 10 is fixed on inclined roof fixing plate 37 and inclined roof bottom plate 36 in fixed mold, as shown in sectional view B-B in Fig. 2b. Because fixed mold side does not have thrust of ejector rod of injection molding machine, fixed mold inclined top fixed plate can only be pushed out by spring 38 and reset by reset rod 35, number is 4. In addition to inclined roof 4 and 10, driving part spring 38 and reset rod 35, inclined roof lateral core pulling mechanism also includes an inclined roof fixing plate 37 and an inclined roof bottom plate 36. Pushing distance of inclined top fixing plate should be strictly controlled within 25mm to prevent two inclined push rods from interfering with each other when they are pushed out. Inclination angle of two inclined roofs should not be too large, and this model is taken as 7°.

 

 

Figure 2 Structure diagram of injection mold
a — —Moving mold arrangement diagram b — —B-B section view c — —A-A section view d — —Fixed distance parting mechanism e — —M-direction view f — —N-direction view g — —D-D section view

1. Fixed mold seat plate 2. Stripping plate 3. 12. T-shaped clamping block 4. 10. Inclined top 5. Pin 6. Inclined top base 7, 25, 26. Support column 8. Inclined top base 9. Pin 11 Gate sleeve 13. Fixed template 14, 28, 30, 38. Spring 15, 32. Wear block 16, 31. Fixed mold slider 17. Movable mold insert 18. Movable mold core 19. Movable mold 20, 44 .Guide post 21. Push rod fixing plate 22, 45, 46, 49. Guide sleeve 23. Push rod bottom plate 24. Movable mold seat plate 27, 35. Reset rod 29. Side core pull 33. Fixed mold insert 34. Fixed Template inserts 36. Inclined top bottom plate 37. Inclined top fixing plate 39. Nylon plug 40. Push tube 41. Positioning block lower 42. Positioning block upper 43. Limit nail 47. Clamping buckle 48. Small pull rod

 

Figure 3 Three-dimensional view of mold forming parts
a — —3D view of movable mold insert b — —3D view of fixed mold insert
Two outer undercuts are also formed by a fixed mold. To ensure appearance quality, fixed mold is used for side core pulling, as shown in Fig. 2c, Fig. 2b, Fig. 2e, Fig. 2f, and Fig. 2g. Core parts of mechanism are fixed mold slider 16, fixed mold slider 31, T-shaped buckle locking block 3, and T-shaped buckle locking block 12. T-shaped buckle locking blocks 3 and 12 are both driving parts and locking parts, locking angle is 18°. During core-pulling process, locking block is always in T-shaped groove of slider, so there is no need to design slider positioning parts.

Mold adopts a point gate pouring system, and mold must use a three-plate mold base. There are 3 parting surfaces when mold is opened, of which there are two on fixed mold side. In order to ensure that runner aggregate can be automatically demolded and side core pulling mechanism of fixed mold completes core pulling before moving and fixed molds are opened, opening sequence and opening distance of three parting surfaces of mold should be strictly controlled, so mold must be designed with a fixed-distance parting mechanism. Mold adopts a built-in fixed-distance parting mechanism, which is composed of a small pull rod 48, a limit nail 43 and a nylon plug 39. Detailed structure is shown in Figure 2b and Figure 2d. Nylon plug 39 ensures that fixed mold plate 13 and movable mold plate 19 are finally opened, small pull rod 48 ensures that mold opening distance between fixed mold plate 13 and stripping plate 2 is 145mm. Inclined top 10, fixed mold slider 16, and fixed mold slider 31 successfully complete lateral core pulling. Limit nail 43 ensures that opening distance between stripper plate 2 and fixed mold base plate 1 is 12mm, this distance can ensure that pull pin is separated from flow channel condensate.
Due to larger specific heat of metal powder, mold absorbs more heat than ordinary nylon injection molds during molding process, so temperature control system design is more difficult. According to shape of product, fixed mold adopts 1 straight cooling water channel, and movable mold adopts 3 cooling water channels, of which 1 is a straight cooling water channel, and 2 is a "straight water pipe + a partition well" cooling water channel. Diameter of water channel is taken as ϕ 12mm, diameter of well is taken to be ϕ 20mm. Figure 4 is a schematic diagram of mold temperature control system.

 

Figure 4 Schematic diagram of mold temperature control system
a — —Fixed mold temperature control system b — —Moving mold temperature control system

In metal powder injection mold, demoulding and cooling are both key issues. After lateral core pulling is completed, formed blank is finally pushed away from movable mold by push rod and push tube. Among them, there are 3 push rods with a diameter of 6mm, 8 push rods with a diameter of 4mm, 4 push rods with a diameter of 3mm, and 4 push rods with a diameter of ϕ 2mm. There are two specifications of push tube, including 2 ϕ 3.5* ϕ 5.5*160mm and 4 ϕ 2.2*ϕ 4.5*160mm. As shown in Fig. 5, in Fig. 5, except for push tube, all push rods are used. This combined demolding mechanism is sufficient and effective to ensure that blanks are safe and stable without deformation during demolding.

 

Figure 5 Location map of mold release system

MIM injection mold is very different from ordinary injection molds in terms of depth of exhaust groove. Ordinary injection molds, according to their different molding plastics, depth of venting groove is generally 0.02～0.06mm, while depth of venting groove of MIM injection mold is generally between 0.0025～0.005mm, flashing will occur if depth exceeds this depth. A good feed has very low shrinkage during subsequent solidification process. In order to strengthen shape retention of molded product as much as possible, a large amount of metal powder filler can be used, and amount is often close to 70% of volume. In order to obtain good fluidity of high-filled feed, low molecular weight plastic PA is used to make MIM feed have high flash sensitivity, similar to characteristics shown by many filled nylon materials.

(1) Mixing. Nickel-added stainless steel metal powder and nylon PA12 are uniformly mixed in a volume of 8:2 to obtain a feed.
(2) Injection molding. Feed is put into barrel of injection molding machine and heated to 150℃ to become a viscous paddle-like substance, which is injected into mold cavity through a point gate gating system under high pressure. Controlling molding parameters such as injection temperature, mold temperature, injection pressure, and holding time is essential to obtain stable green body quality.
(3) Cool and solidify. After feed material fills cavity, it is pressure-maintained and cooled to solidify into a blank, then mold is opened under pull of injection molding machine.
(4) Open mold. Under action of fixed distance parting mechanism, mold is opened from parting surface I, parting surface II and parting surface III in sequence. When parting surface I is opened, mold completes three tasks: ①Gating system is separated from condensate and plastic part; ②T-shaped crimping blocks 3 and 12 drive fixed mold sliders 16 and 31 to pull outer core of fixed mold; ③Spring 38 pushes fixed mold inclined top fixing plate 37, then pushes inclined top 4 and inclined top 10 to complete inner core pulling of fixed mold. Mold opening distance of parting surface I is 145mm, which is controlled by a small pull rod 48. When parting surface II is opened, mold completes outer core pulling, at the same time, stripper plate 2 pushes runner aggregate away from mold to realize automatic stripping of mold. Opening distance of parting surface II is 12mm, which is controlled by limit nail 43. When parting surface III is opened, movable mold plate 19 and fixed mold plate 13 are separated, formed blank is separated from fixed mold cavity. After mold opening stroke is completed, push rod pushes molding blank out of mold to complete one injection molding.

(1) Compared with other metal molding methods, metal powder injection molding can produce products with more complex shapes, with high efficiency and no material waste. It is a typical precision manufacturing and green manufacturing technology. However, size of product is subject to certain restrictions, generally not exceeding 250g. Molded product weighs 350g, which is a breakthrough and successful attempt for metal powder injection molding.
(2) "Nickel-based stainless steel + PA" feeding material has poor plasticity compared with pure nylon PA. It is very risky for mold to use inner and outer sides of fixed mold to pull core at the same time. This success is also a major breakthrough in bold innovation of mold structure.
(3) Mold structure is advanced and reasonable, mold trial is successful once. After mold is put into production, it runs safely and smoothly. Molding cycle is 28s, and daily output can reach 2,300 pieces. After degreasing and sintering, dimensional accuracy of formed blanks reached IT5 (GB/T1800.1-2009), and surface roughness value reached Ra0.8μm, which all met design requirements.