| | Technology Talk — Optimizing Component Designs For Successful MIM Applications (Part 1) By Andrew Hanson, Vice President of Sales and Marketing and Steve Perruzza, Vice President of Manufacturing and Director of MIM Operations 
The Metal Injection Molding [MIM] process enables the manufacture of high strength and high volume components with unique and complex geometries. An initial consideration for designing a new part for a given method of manufacture is ultimately final cost. As with any fabrication process, an optimized design is essential for economic consideration and competitiveness. The attributes of the MIM process are best utilized when the end user focuses on form, fit and function rather than the arbitrary "standard" tolerances of an application designed for another method of manufacture. Designing an optimized MIM part requires an understanding of the basic process involved. MIM is a hybrid process combining the material alloy flexibility of powder metallurgy and the shape making capability of plastic injection molding. Today, components are generally less than 0.220 pounds (100 grams) in weight, smaller than a golf ball, and have a wall thickness no greater than 0.25" (10 mm). The smallest applications can be less than 0.3 grams. Furthermore, new binder systems and lower raw material pricing will allow for larger sized parts in the future while micro-molding technology may allow for even smaller ones. Current standard materials for MIM include iron, alloy steel, stainless steel and titanium alloys that exhibit near full density characteristics after sintering. The result produces complex metal components at high volumes with an economic advantage over other competitive methods of manufacture. MIM Part Design
A new component design is the best opportunity for an application to take full advantage of the MIM process attributes. Potential conversions, as well as new applications, require a thorough mutual understanding and agreement on tolerance/feature design change issues. If the geometry can be produced as a plastic component, it more than likely can be molded as an MIM part. Dimensional tolerances can be initially evaluated at 1% of a dimension or a precise as +/- 0.001" [ +/- 0.025mm ] for dimensions smaller than 0.100" [ 0.250mm ].
MIM Tool Design
Detailed tooling requirements should be reviewed and mutually agreed upon for technical particulars (i.e. parting line, gate location, gate blush / vestige, knit lines, etc.). Continued communication during tool construction is an extremely important factor to ensure expectations are met. A "PRP" (Project Resource Planning) document is provided after the initial program kick off. This tracks and manages milestones and addresses APQP (Advanced Product Quality Planning) issues and initiatives. Quality
Actual application requirements and the capability of the MIM process will dictate how the part-specific statistical process control and part inspection will be structured. It is important to understand the form-fit-function of the application and to develop a logical control of critical dimensions. Compounding
Metal powder(s) are combined with a binder system. A variety of binder systems can be used, all of which are designed to transport the metal powder into a desired shape. Consistency of this compounded raw material is critical for control of dimensional tolerances and physical properties of the finished MIM part. AFT uses a polymer/wax binder system which has successfully produced several hundred million components to date. Molding
Shape forming is accomplished in a standard plastic-injection molding machine. Components emerge from the mold approximately 20% (linear dimensions) larger than their final size, but possessing all the details of the finished part. Referred to as "green parts", they have the consistency of a crayon. Green parts are placed on high temperature-resistant flat or contoured ceramic trays for support during subsequent processing. Molding Automation
Molding automation allows for consistency of cycle time and lower labor costs. There are three methods of automation, with "pick" robotic automation being the simplest and most versatile. An "end effector", which is specific for a given mold design, extracts components from the tool and places them on a conveyor belt. Subsequently, an operator places them on setter trays.The "flexible cell" concept offers the ability to run a variety of similar-sized MIM components that have medium annual volume requirements. This system requires an investment above the initial "end effector". In process stations such as weighing and secondary positioning, equipment along with a vision system may be required. This system concept adds a level of savings that can be evaluated on a Return On Investment calculation. The third type of automation — called a "Hard Automation - Dedictated" system-extracts components from the die and precisely places them in specific order on standard or custom setter trays. This system allows for an operator to run numerous machines at a given time. Lower final part costs are realized. Debinding
Extraction of approximately 80% of the binder is required after molding and prior to sintering. Binder removal methods (chemical, thermal or evaporative) depend on the basic binder system used in that particular application. Debinding time increases dramatically with wall thickness over 0.25" ( 10mm). 
Sintering
Sintering is a high-temperature [~25000 F (14000 C)] microprocessor-controlled process in which the residual binder is removed, followed by densification resulting in a high-density, complex-shaped metal component. Secondary Operations
The goal for MIM is net shape. However, if greater dimensional tolerance(s), surface finishing, heat-treating, etc. is required, MIM components can be treated the same as similar composition wrought parts. Sizing can enhance process capability as a bridge between "as-sintered" and precision-machined parts. Hot Isostatic Pressing [ HIP ]
Since MIM components are sintered to high density, exhibiting a non-interconnected porosity micro-structure, they can be hot isostatically pressed [ HIP'ped ]. This process, which has been utilized extensively on aerospace and high-performance investment castings, serves to eliminate residual micro-porosity. The result is a significant increase in ductility and fatigue strength. During the process, components are subjected to high pressure argon gas [ ~ 15 -45,000 psi ] and high temperature [ ~ 1,900 - 2,250 degrees F, depending on material chemistry ]. Under these conditions, the isolated pore walls collapse and diffusion bond together. Subsequently, the density approaches theoretical levels. The HIP process cannot be incorporated into the process plan as an afterthought without final dimensional ramifications. HIP will cause additional shrinkage from the as sintered condition. This could range from 0.5 to 3% depending on "as sintered" density levels. In our next issue featuring the MIM process, we will discuss process and design considerations that aid in MIM design optimization. |
