December 27, 2009 – 20:05
If your company uses complex precision metal components, metal injection molding (MIM) can offer significant cost savings and eliminate design restrictions inherent in other metalworking technologies.
Metal injection molding is a young technology, having only been practiced commercially since the mid 1980s. North American industry sales are reported to be near $60 million, and growing at over 30 percent per year. The process is essentially a marriage of the thermoplastic injection molding and conventional powder metallurgy processes. It offers the same level of design freedom for highly configured metal components as is available for plastic parts in the plastic injection molding process.
The technology produces very high-density parts with mechanical properties superior to powder metallurgy, and comparable to wrought materials. It is successfully serving a variety of industries such as medical and dental tools, business machines, power hand tools, industrial equipment, electronics, medical and dental tools, automotive, and sporting equipment.
Typical MIM Process
The MIM process starts by combining fine metal powders with a polymer binder to create a feedstock suitable for injection molding. After the feedstock is compounded, the material is injected into standard plastic injection molds that have been designed about 20 percent larger than the desired final product. An oversized mold is required due to the presence of the binder, which is subsequently vaporized from the molded part in a furnace. This stage of the process is commonly referred to as debinding.
The debound part is sintered at temperatures above 2200 degrees F. This releases the tremendous surface energy stored in the fine-mesh metal powder and fuses the metal particles together, shrinking the part to the final shape and size in a precisely controlled manner. The as-sintered part retains all of its molded features.
Many of the same design principles used for designing plastic parts apply to designing MIM parts. They will exhibit features characteristic to the molding process such as parting lines, gate, and ejector pin marks.
Metal injection molding is a general-purpose technology that applies to a wide range of applications. Its ability to reduce component cost is centered on its ability to produce small, complicated, three-dimensional shapes. Some of the best examples of MIM parts are those that have been designed specifically for the MIM process.
The following part requirements represent applications that can benefit from MIM:
· Tolerances of +/- 0.003 inch per inch or better
· Parts that would normally require four or more machine tool set-ups or cutter tool paths to produce
· Wall thickness to 0.5 inch
· Weights up to 100 grams
· Lengths up to six inches
· Annual quantities of 20,000 pieces and up
· Material density at 97 percent of theoretical
· Material strength near that of wrought materials
· Surface finish of 16 to 32 RMS
As shown in the accompanying graph, MIM saves money for highly complex parts, i.e. parts with at least four machined features. If a component is produced by stamping or die-casting and it meets design and performance requirements, it is probably being produced by the most economical approach for that component. MIM does not compete with screw machined components unless they require two or more secondary operations.
If investment casting are used in the as-cast condition or conventional powder metallurgy parts used without secondary operations, then those processes should be retained. However, if the investment casting or powder metallurgy parts require secondary machining operations, then MIM may offer cost savings. Compared to investment casting, MIM is able to provide a better surface finish and finer feature details.
New Developments
Kinetics customers are indicating that MIM routinely saves them 20 to 70 percent on component costs compared to other manufacturing processes. That’s partly because of the inherent advantages of MIM and partly because Kinetics has advanced MIM technology beyond its standard capabilities.
Kinetics emphasizes a commitment to continued development of process technology, new materials, and quality. The focus is to reduce variability in all aspects of the process, from the selection and compounding of materials to molding and sintering of components. This focus has allowed dimensional capabilities to +/- 0.15 percent in many cases.
Early R&D activities enabled the company to develop a unique debinding and sintering process that controls and maintains carbon levels. This enables the production of MIM-4650 per MPIF Standard 35. This material can be heat treated with a variety of processes, including quench and temper, austemper, and induction hardening, resulting in ultimate tensile strength to 240 ksi and apparent hardness to HRC 50.
Also, Kinetics has developed a continuous feedstock compounding process capable of generating 300 pounds of feedstock per hour. Intensive mixing ensures a consistent dispersion of metal powder particles and binder throughout the feedstock. Process controls monitor rheological properties for molding consistency, feedstock density, and other key characteristics for maintaining precise shrinkage control during the debinding and sintering process.
Designed experiments such as full factorials and fractional factorials have already identified many of the key variables and interactions that are critical to controlling process variation. The experimental research method continues to be used to identify process improvement opportunities.
There are more than 80 parameters including time, temperature, pressure, and velocity specified for each new application. Once established, all molding parameters are electronically recorded by the microprocessor to provide an exact duplication of process conditions for each subsequent customer order. Control charts are established for each product to ensure that part weights are maintained and the process is in control.
A one cubic foot laboratory vacuum furnace is used in the development of all materials, processes, and product applications. The furnace was specifically designed and built at Kinetics to replicate the capabilities of the large, ten cubic foot production furnaces. But the lab furnace includes additional instrumentation to monitor changes in process events.
The availability of the lab furnace eliminates scheduling conflicts with production orders. It also results in shorter development times, with only the resolution of scale-up issues remaining.
Practical Experience
Kinetics is the MIM industry’s fourth largest commercial supplier in North America. The company has assisted customers with component designs that would have otherwise been impossible to produce or economically impractical to produce by other metalworking technologies.
Equipment currently in place at Kinetics provides enough capacity to more than double existing production. The company has five closed-loop, microprocessor controlled, injection molding machines ranging from 55 to 150 tons to meet their production requirements.
They have ten continuous production vacuum furnaces that were specifically designed for debinding and sintering in one operation. This eliminates the oxidation of parts associated with other processes, and the need to handle fragile parts. It also safely removes binders in a precisely controlled manner without hazardous waste or solvents. All furnaces are controlled by microprocessors to ensure part integrity, dimensional control, and process repeatability.
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