Qualification of MIM

Table of Contents

• 13.1 Introduction

• 13.2 The metal injection molding process

• 13.3 Product qualification method

• 13.4 MIM prototype methodology

• 13.4.1 Material selection

• 13.4.2 Prototype production

• 13.5 Process control

• 13.6 Understanding of control parameters

• 13.6.1 Powder characteristics

• 13.6.2 Feedstock behavior

• 13.6.3 Injection molding

• 13.6.4 Debinding

• 13.6.5 Sintering

• 13.7 Conclusion

13.1 Introduction

Metal injection molding (MIM) is a process for forming net-shape or near-net-shape components at a reduced manufacturing cost as compared to machining and at a higher precision level than other forming technologies, such as investment casting. However, the process is fairly complicated, requiring knowledge from various disciplines to ensure that a quality product is manufactured. Knowledge of powder handling, powder sintering, injection molding, powder/polymer rheology, polymer degradation, metallurgy, and so on must be understood and used to ensure a stable process and a quality product. Furthermore, each process step is interactive, i.e. a molding defect might only be detected under certain sintering conditions, thus, characterization of each process is essential to control the MIM process. (See Fig. 13.1.)

Fig. 13.1 Logic diagram to go from concept to production for MIM.

In view of the complexity of the MIM process, an engineer can become lost in all the possible variables that could be implemented to attain a controlled process, and most importantly, a quality product. Often, a high level of process control is required, but at other times it is not. In this chapter, a program is presented for a design engineer to easily qualify a component vendor or a process engineer to qualify and monitor a MIM process. In this way the design engineer can understand the MIM process well enough to make intelligent decisions with regard to the use of MIM in their applications and the process engineer can ensure a controlled process for consistent production.

13.2 The metal injection molding process

To use and to validate a MIM process, an engineer must have a basic understanding of the process. The process must be divided into its subprocess categories. The number of process steps can be as many as nine. The number of steps depends upon the particular technology and the amount of processing that a manufacturer performs. For example, whether the manufacturer purchases feedstock or manufactures proprietary feedstock in-house. The potential process steps are as follows:

1. raw material selecting and monitoring;

2. material blending;

3. feedstock compounding;

4. injection molding;

5. solvent or catalytic debinding;

6. thermal debinding;

7. sintering;

8. secondary operations (coining, machining, heat treating, grinding, surface finish, HIP etc.);

9. inspection and packaging.

Each process step produces a product that feeds the next process step. Thus, process control can be performed on the individual processes and also on the product of each process step to ensure that the entire process is in control. Furthermore, additional processing of out of specification components is a waste of resources and can easily avoided by simply checking the components between process steps. Table 13.1 lists a MIM process sequence and the inputs and output of each sub-process.

Table 13.1 Process input and output products for comprehension and process control

13.3 Product qualification method

Determining if MIM should be used for an application is done by answering two questions. First, is it economically feasible, and second, is it technically feasible? Fig. 13.1 shows a logic diagram that can be followed to fabricate and qualify a MIM component.

One must determine the economics of using the MIM process as opposed to the current fabrication technique or a conventional method for an application. If the economics look undesirable, one must return to a conventional manufacturing technique or make design modifications in material type or part size to make the project economical. Often the size of the part will dictate whether MIM can be used, not only from a technical point of view for removal of the binder, but also because of the powder cost. Typically, parts greater than 300 g cannot be fabricated economically using contemporary MIM technology. After a successful economic analysis, the next step is to look at the technical aspects of the component-specifically the application data such as property requirements and critical dimensions. After these have been defined, the proper material or group of materials is selected for evaluation. These materials are then injection molded and evaluated for properties, such as tensile strength and/or corrosion resistance. Property evaluation can be skipped if a vendor has defined these data in previous work. Satisfactory results for properties justify the fabrication of a prototype mold. "Prototype" is emphasized since these components are produced using low-cost tooling. The prototype components might require secondary operations that would not be required while using production tooling. Application testing is the final test to validate that MIM can be utilized for an application. If application testing is unsuccessful, the development cycle starts again with design modifications and economic analysis, provided there is continued management support.

The above methodology is a philosophy. Variations to this technique may exist for different applications, different amounts of available capital, and also considering time to market for a product. For example, one may choose to go directly to production tooling fabrication and do the development on the production tool. In this way the time to market is reduced; however, the up-front cost and the risk are higher.

13.4 MIM prototype methodology

Following the economic analysis, the next step for any potential MIM application is the evaluation of mechanical properties of potential materials and the production of prototype components. The production of test samples (tensile, fatigue, wear discs) or actual prototypes is done to test a new material for a particular application with the minimum of cost. Property evaluation can be as simple as locating property values in the Metal Powders Industry Federation (MPIF) standards manual, from a MIM component supplier's specifications, or from other literature that pertains to a specific material. Furthermore, mechanical properties can be MIM supplier dependent since all suppliers use different processing methods and raw materials. For example, vacuum sintering of a stainless steel may reduce its corrosion resistance or the feedstock binder may leave a carbon residue that could compromise properties or corrosion resistance of the sintered alloy.

13.4.1 Material selection

Selection of the proper material for a particular application often decides the success of a MIM project. Chapter 2 of this book lists some available metal alloys and their potential applications. Commonly, a number of materials are evaluated for an application and a decision is made based on performance, cost, and input from a MIM component supplier or expert. As a general rule, if a material is available in a fine powder form and can be sinter densified, it can be metal injection molded. A good vendor or MIM design engineer should be able to select the proper material based on application input and thus the time and cost of mechanical property samples fabrication and testing can be avoided.

13.4.2 Prototype production

A MIM prototype is a MIM component that has been fabricated using the MIM process; however, the tooling that is used is much lower in cost than production tooling. A prototype tool can cost less than one quarter the cost of a production tool since expensive mold components such as slides and cams are not used. Additionally, the lead-time for functional components can be more than cut in half. Difficult features are machined in as secondary operations, since fewer than 1000 prototype components are typically produced. The tooling can also be made with easy-to-machine metals such as P20 and unhardened H13. Aluminum can also be used; however, this metal is easily dinged and mauled during prototype development. It is often valuable to have the mold vendor work with materials that they are accustomed to machining or EDMing.

13.5 Process control

Once a MIM prototype or production component has passed initial qualifications for use, the next step is to ensure that a process is sufficiently in control for a particular application. Minimum process control is required if the specifications are broad and a significant amount of process control is required if the specifications are narrow. This section breaks down the MIM process to make it possible to determine which process control needs are required for a specific MIM process to achieve the required specifications at the minimum of cost.

To analyze the MIM process for process control, the process must be divided into its sub-process categories. Each of these sub-processes can be controlled to ensure a more repeatable process; however, the more a process is monitored, the costlier is the overall process. Therefore, an engineer must balance between cost and control to ensure that the MIM process is profitable for a particular application. For example, if a company is producing aerospace or medical components, where the exposure for a catastrophic failure is high, there must be a high level of process documentation and control. A company that manufactures products that have less financial exposure for catastrophic failure would require less. The general concept is to produce the best possible product with the least amount of process monitoring.

Table 13.2 lists each of the process steps for MIM, and also the parameters that could be controlled for that process step. Although there are many potential parameters to control in the MIM process, not all need to be controlled. Application and process type define the required control. Table 13.3 lists a comparison of process control auditing for two levels of control. One process control would be considered minimum in both cost and effort, whereas the other provides precise control for precision and high performance components.

Table 13.2 Potential parameters to control for the PIM process

Table 13.3 Continued

13.6 Understanding of control parameters

The following section is devoted to describing the different process controls and the reason for their use. These can also be used in the process set-up and qualification stage to ensure a stable process and reviewed periodically or when a problem arises.

13.6.1 Powder characteristics

Chemistry

Chemistry monitoring is most critical for materials that are sensitive to carbon level and oxygen level; however, other elements such as chromium for stainless steels may be important to monitor. Carbon level is required for materials where it is important for properties and heat treatment; the most common are tool steels, low-alloy steels, and martensitic stainless steel. In these materials, carbon level affects dimensional stability, sintered density, and mechanical properties. Also, materials sensitive to carbon embrittlement, such as titanium, should have the carbon monitored. Oxygen monitoring is important for materials such as titanium because it affects the elongation. Also, oxygen in combination with silicon in stainless steels may affect the elongation by the formation of silica particles.

Powder size and size distribution

Powder size and size distribution affects mixture viscosity and injection molding. As particle size increases, mixture viscosity decreases. This affects the molding process consistency. Also, sintered density and mechanical properties are affected by the powder size. As particle size decreases, the sintering response increases. Therefore, variability in particle size affects the part dimensions, part density, and mechanical properties.

13.6.2 Feedstock behavior

Density (pycnometer, Archimedes)

Density is a direct measure of the ratio between the powder and the binder. Improper feedstock density affects sintered size (shrinkage), mixing viscosity, and molding.

Feedstock viscosity

Improper feedstock viscosity will result in variability during molding and part quality.It also can be an indication of improper raw materials, degraded raw materials, degraded feedstock, and poorly mixed feedstock.

13.6.3 Injection molding

Component mass

Component mass variation will result in sintered component dimensional variation.The component mass variability may be the result of the feedstock preparation step or of a molding process variation. If the feedstock was mixed incorrectly, the mass of the part could be high or low, resulting in high or low sintered dimensions. Variability in the switchover position or hold pressure of molding can cause mass variability and sintered component dimensional variability. Furthermore, the molding operation is a constant-volume process, thus, any variation in mass will result in a variation in sintered dimensions.

13.6.4 Debinding

Weight loss

Understanding the amount of binder in the component and the rate at which it is removed is critical for defect-free components. Also, if the binder is not removed correctly, excess carbon may result in the component, and affect the final mechanical properties of the components.

13.6.5 Sintering

Select component dimensions

Dimensional variability shows the most effect after sintering. As the temperature and gas flow in the furnace varies, so do the dimensions. Therefore, knowledge of the dimensions and how they vary is important to understand the sintering process and to maintain a controlled process. Furthermore, variability in component mass at molding manifests itself as variation in component dimensions at sintering. The variability of all the previous process steps will be exacerbated in the sintering step and show up as component dimension variability.

Component mass

The mass of a component can change if there is an alloying element that vaporizes in the furnace during sintering. For example, the loss of chromium in stainless steel during vacuum sintering is well documented.

X-ray

Voids and cracks are easy to detect using X-ray equipment. This could be used for the set-up of critical parts for medical or aerospace applications.

Mechanical testing

Often, the components can be put through function testing for strength or evaluated for hardness on the actual component. Another method to monitor the sintering process is to have test specimens sintered with the components and evaluate the test samples for strength, elongation, or some other test.

13.7 Conclusion

A method to qualify metal injection molding for an application has been presented.This was done for both product and process. A thorough evaluation of process controls and monitoring that can be carried out on a MIM process has been laid out. Also, a rationale for the selection of the best process control for a particular application has been presented. In general, one should monitor only the most critical parameters that are dependent upon the application and its specifications.