Optimizing Component Designs For Successful MIM Applications
(Part 2 of 2)

By Andrew Hanson, Vice President of Sales and Marketing and Steve Perruzza, Vice President of Manufacturing and Director of MIM Operations 

Process and design considerations are broken down into two subgroups that address aid in designing a part for Metal Injection Molding [MIM]. Although separate, they should be addressed in conjunction with each other when designing a new part. 

Process Considerations 

Material   
As with conventional powder metallurgy, elemental powders can be blended together to create unique alloy-material properties. Incorporating a custom material generally requires a development program. To design a robust part for MIM, one of the basic materials listed below is recommended. 

Molding    
The first consideration is the number of cavities. Generally, one to eight cavities are employed based upon annual volume requirements, with up to 24 cavities based on size and geometry. The cavities and subsequent mold design must be well-balanced. An unbalanced tool can affect fill and green weight capability from cavity to cavity. 

The next consideration is automation. For high volume parts, it is recommended for the sake of efficiency to remove parts from the molding machine using robotic automation. Both the mold and the parts need to be carefully designed to allow a robot to grasp parts without damage and position them on a conveyor or staging tray. Automation has proven to enhance part quality by reducing the variability of operator handling and also by allowing for consistent cycle times. Custom tooling would include a custom-designed and manufactured "end effector" to extract parts from the mold.

Finally, there is tooling design. To minimize MIM part costs and optimize downstream processing, tool design is essential. Different designs may dictate longer or shorter cycle times. Each part is unique in geometry and should be considered on an individual basis. This will determine the most robust design to optimize cycle time. Continuous customer contact using the "PRP" with engineering is critical during the tooling fabrication and initial sampling phase.

Debinding
Wall thickness is an equally important consideration during the design stage of the process. Thick sections should be avoided as they create problems similar to those in plastics, such as "sink". In addition, thick sections can greatly increase binder-removal time, adding to the cost of production. Coring (details designed in the direction of die pull) the part design to form relatively uniform walls helps reduce this process time and produce an overall better MIM component.                                                                                   

Sintering      
Part size — or footprint — is the cost driver for this process. Whether the part is sintered in a continuous or vacuum furnace determines the number of parts per tray or given area. Consider a ring shape. Since parts cannot touch during this operation, a ring shape (staged flat on trays) leaves a large amount of empty space. If components can nest within themselves or be staged in a manner that will utilize the entire staging surface, lower costs may be achieved for this process.               

Secondary Operation(s)
The essence of MIM is to attempt to utilize all of the attributes of the process for a net shape component. However, this is not always possible. Sizing is a very common method of calibrating components to optimize certain dimensional variation. All other common forms of metalworking can be employed to MIM parts to bridge the gap between the MIM net shape process capability and functional requirements.          

Inspection       
Capability studies, ease/frequency of measurement, agreement on standardized measurement techniques and PPAP / First Article requirements are all cost drivers for any given part. These must be thoroughly reviewed, agreed upon up-front, and included into the "PRP" / Control Plan & FMEA. Implementation of functional gauges tests is highly recommended as an element of advanced production quality planning. 

Part Design Considerations

Parting Line              
All parts must determine the location on the component that a parting line will be acceptable.  Agreement will be required on the maximum parting line flash that will be allowed. It is not unusual to have parting line flash up to .007" (.177mm) from tools that have run over 100,000 shots. When possible, it is recommended to design a parting line that is flat on all non-flat surfaces (example below). When considering parting line location it should be noted that edges can be natural locations. Tumbling and deburring can remove parting line flash easily.

Ejector Pin Location(s)    
Once the parting line location has been established, placement of the ejector pin(s) can be determined. Typically, these need to be positioned so actuation will eject the part straight out of the cavity, avoiding any possible drag marks or damage. Some alternatives to ejector pins are ejector sleeves around holes and ejector plates. All of these concepts are very similar to plastics tooling.

Gating             
Location of the gate is one of the most critical aspects for MIM, as it is in plastics design. Consider that the gate will be visible in the finished part and that the area around the gate periodically has flow lines as well as cold flow imperfections that will be clearly visible. The gate vestige will generally be +/- .005" from the surface it is gated into. Good design practice is to gate into the thickest section of the part. Other factors, such as the potential for jetting and mold flow, will also need to be addressed on a part-by-part basis.

Removing the gate vestige is another consideration. This can be done either manually or automatically in the mold by using a sub-gate or three-plate design. When removing a gate manually, a special trim tool — "De-gater" — is required to produce a uniform surface, which will add additional startup tooling costs.  

Staging Surface            
Components must be supported during sintering due to their relatively fragile state and the high temperature stresses induced by shrinkage during the sintering process (approx. 17-25% linear).  When designing a part for MIM, keep in mind flat surfaces are ideal for staging. If there is not a flat surface, consideration must be given to support for special high temperature ceramic fixtures. An unsupported surface will sag and affect dimensional capability. Therefore it is essential that those types of features are supported to their final dimensions. Staging and furnace furniture development for certain components are developed empirically once the tooling has been completed and first samples have been molded. 

Wall Thickness and Coring           
When part design allows, a consistent wall thickness throughout the part will help the entire MIM process. Debinding and subsequent sintering processes will be more uniform, distortion will be held to a minimum and savings in material will result. Wall sections of .03-.25" [ .75 — 6.25mm ] are practical for MIM parts.

Draft Angles         
Angles of 0.5 — 2 degrees are typical for MIM parts. When possible, draft should apply to holes, outside edges and coring. It is possible to produce part features with less draft, however draft will allow for a more robust process. 

Personalization          
Logo(s), company name, part name and other surface design(s) can be added to the tooling for a nominal charge, depending on the surface specified. Once set in the tooling, all parts will have that customized detail which typically does not increase the part price. Keeping these details — whether surface-positive or negative — in the direction of pull of the tool will minimize tooling maintenance costs. 

Once a preliminary MIM part design is complete, the following checklist can be used to expedite the economic suitability review: 

• Target / Market price
• Material options
• Actual part weight
• Annual usage
• Start of production
• Proto-type or production tooling
• Gate Location
• Ejector pin and parting line locations
• Complete application description
• Functional test expectations
• Standard measurement techniques
• Identify critical dimensions
• Process capability requirements
• Special packaging

Successful applications for MIM or virtually any other manufacturing process will always have an economic constraint as its cornerstone. In evaluation of MIM, one must realize that it is not a process for all parts. Open early target / market pricing and capability discussion will save substantial time for all. Potential conversion candidates may be investment castings, machined components or possibly combining two or more components into one MIM friendly design. The best designs are MIM designs from the start. These designs take full advantage of the process and its capabilities.  

Metal Injection Molding Technology's Future
As successful applications and education expand the knowledge, acceptance and possibilities of the MIM process, new parts and applications will continue to emerge. Larger, heavier components produced in volume will have a positive economic impact on raw material volume and cost that in turn will drive the competitive edge for MIM ahead. Smaller components manufactured via micro-molding technology may open an entirely new market segment for parts less than 0.10 grams. Initial applications for these micro components appear to be well suited to the medical market. Furthermore, sinter bonding of similar and dissimilar materials remains largely untapped. Possibilities to use less expensive (larger grain powder) material may offer advantages from a cost perspective for components not requiring the high density characteristic of conventional MIM. The result could be three dimensional shape making capability with material properties similar to conventional powder metallurgy.

MIM has clearly become a reliable, production-viable process for the medium to high volume production of complex shaped components. Continued development of published industrial material standards, end-user based education, research and development, automation and increased recognition of successful applications will continue to drive metal injection molding's impressive growth. Successful applications will always involve a strong customer/supplier relationship maximize the benefits of the use of Metal Injection Molding. 

To read Part 1 of Optimizing Component Designs For Successful MIM Applications, click here.

Advanced Forming Technology
7040 County Road 20
Longmont, CO 80504 USA
303.833.6000
www.pcc-aft.com