Metal Injection Molded Parts

Transcription

1 Metal Injection Molded Parts Metal Injection Molding (MIM) is a powder metallurgy process. he difference between MIM and conventional powder metallurgy is that in MIM, metal powder along with binders is injected under high pressure into the die or mold cavity instead of flowing by gravity. he metal powder used for this is finer than conventional powder. Particles sizes typically of 20 µm in diameter are used. Generally, powder mix contains a much higher percentage of binders and flow agents. Wherein, binders generally include various waxes and thermoplastics. he compounded material is injected with conventional plastic injection molding machines (slightly modified) into metal dies normally having one to eight cavities. he molded component is called a green part. he binders are first driven off either by heating, by solvent extraction, or by a combination of both methods and approximately 80% of the binders are removed. Debinding time is directly related to wall thickness. After debinding, the part is called a brown part which is porous and fragile but size remains the same as the time of molding. he brown parts are then sintered as done in the conventional powder metallurgy. his is done in vacuum or inert-atmosphere furnaces at a temperature of approximately 1300 C depending on the material used for processing. However, sintering temperatures are generally well below the melting point of the material. he entire cycle including debinding normally takes about 24 to 36 hours. ypical characteristics and applications of metal injection-molded parts ypical metal injection-molded parts are small and complex in nature. he process is more advantageous for more complex parts. In this process, high precision is possible to achieve. his method eliminates the constraint imposed on conventional powder metallurgy method i.e. the side wall should be parallel. Very small size parts like smaller than a golf ball in volume, weight in the range of 0.5 and 150 g, and wall thickness less than 6.3 mm can be produced by this method. Multi-functional parts with thin walls, sharp corners, undercuts, cross holes, screw threads, contours, gear segments, and similar features are possible to produce without secondary operations. Densities of parts produced through this process are normally greater than those of conventional powder metallurgy parts, ranging between 95 and 99 % of theoretical maximum density.

2 Important applications of this process are rotors for stepper motors, sear housings for police pistols, hammers and links for printing machines, stainless-steel orthodontic retainers, small hermetically sealable housings for electronic components, shotgun trigger guards, medical camera, computer peripheral, automotive fuel-injector, steering column, seat, and firearm components. Some typical metal injection-molded parts have been shown in Figure M Figure M4.8.1: A collection of typical metal injection-molded parts Materials suitable for metal injection-molded parts Materials suitable for injection molded parts are: low-carbon and alloy steels, stainless steels, nickel alloys, and copper alloys. Also, materials like high-speed steel, titanium and tungsten alloys, cemented carbides, aluminide intermetallics, and ceramics have been attempted in this process. able M4.8.1 provides composition and yield-strength for low-alloy and stainless steels for metal injection.

3 able M4.8.1: Specifications of Low-Alloy and Stainless Steels for Metal Injection (Source: Design for Manufacturability Handbook by James G Bralla, 2nd Ed) Minimum values ypical values ensile properties ensile properties Hardness (Rockwell) Material Designation (condition) Ultimate strength, Yield strength (0.2%), Elongation% Ultimate strength, Yield strength, (0.2%), Elongation% Density g/cm 3 Apparent (direct) Low-alloy steels (MIM material properties) MIM-4600 MIM-4650 MIM-4650 quenched and tempered MIM HRB HRB HRC HRB Stainless steels (MIM material properties) MIM-316L MIMduplex assintered MIM-17-4 PH assintered MIM-17-4 PH solution treated and aged HRB HRB HRC HRC Design recommendations Metal injectionmolding follows many design principles and recommendations applicable to injectionmolding of plastics. Some typical design recommendations are listed below: 1. Uniform wall thickness is recommended. If uniform wall thickness is not possible, then one thumb rule that the thicker wall should not be more than 2 times the thinner wall and make the most gradual transition possible from thick to thin section as shown in Figure M4.8.2.

4 Figure M4.8.2: Gradual from thick to thin section for non-uniform wall thickness 2. Very thick wall is generally not recommended as thick cross-sections reduce the rate of removal of binder. In the design, a general rule of thumb is to keep wall thickness to 6.35 mm or less. If the considered part has sections that are heavier, cores should be added to reduce the cross-section. Figure M4.8.3 illustrates some methods of reducing wall thickness of heavier sections. 3. A draft angle on sidewalls needs to be provided to ensure easy withdrawal of the part from the mold. Angles of 0.5 to 2 per side are recommended for both inside and outside walls. 4. All corners must be provided with generous radii and fillets as much as possible. Unlike sharp corners, rounded corners aid material flow in molds and reduce stress concentrations in molds or in the part. A recommended corner radius of half of the adjacent wall thickness and no less than 0.4 mm is used.

5 Sink Before Coring Redesign Sink Before Coring Extend boss core until t Redesign Sink t Before Coring Redesign Figure M4.8.3: Method to reduce excessive wall thickness. 5. Bosses and ribs are feasible in MIM parts. Ribs provide the strength and rigidity to the parts. Figure M4.8.4 shows suggested designs for bosses, and Figure M4.8.5 illustrates the recommended dimensions for reinforcing ribs.

6 2D D H = 2 to 5 Figure M4.8.4: Suggested design for bosses in metal injection molded parts. ½ to 1 ½ t r = ½ t (0.4m minimum) Figure M4.8.5: Suggested design for ribs inmetal injection molded parts. olerance recommendations Dimensions of MIM parts can be produced with tolerances of ± 0.08 mm to ±0.13 mm. Dimensions of over 25 mm should be allowed plus or minus 0.5 percent of the nominal dimension. Non critical dimensions should be allowed at least ± 0.13 mm or 1 percent, whichever is larger. Sintered surface finishes can be in the range of 0.8 m to 1.1 µm. Surface finishes can be enhanced by standard surface finishing or polishing processes.