AMT|Optimizing Design and Material Selection in Powder Injection Molding

The Design Process in PIM

In the realm of Powder Injection Molding (PIM), the design process is pivotal for creating components that meet performance, market, and cost targets. It involves translating concepts into detailed requirements covering form, function, and process at reasonable costs. Key stakeholders include corporate management, marketing, operations, and the design team, each contributing to the final product concept.

Design typically starts with a qualitative statement from marketing, such as: “Our market share has dropped to 5%, and we need a product to compete with Generic, who is beating us with their new offering.” This statement of need is followed by a hierarchical prioritization to outline restrictions and goals. The classic design cycle involves identifying the problem and progressing from problem statement and concept development, through engineering conceptualization, design, and definition. Often, PIM technology is only brought in after the design is advanced, which can limit cost savings and quality improvements.

In many cases, the design is distributed for quotation, and the PIM vendor is completely excluded from the design dialogue. This approach can lead to missed opportunities for optimization and cost reduction. The design team may overlook critical factors that could significantly impact the manufacturability and efficiency of the PIM process. By involving PIM experts early in the design phase, companies can leverage their specialized knowledge to create more efficient and cost-effective designs.

Improving Quality and Lowering Cost

Many examples show that involving the PIM design advisor early in the design process leads to faster, cost-effective, and higher-quality outcomes. An improved version of the design process includes periodic input from the PIM design advisor to ensure manufacturability of the emerging design. This approach not only reduces costs but also improves product quality by addressing key factors that impact processing ease, yields, and costs.

In modern companies, the design process follows a spiral with increasing breadth and detail as the project advances. There is constant input from the PIM process community through the design iterations. Costs incurred during design have minimal impact on the final product cost, but decisions made during the design process significantly affect production costs. Extra consultation during the design phase can save much when the project reaches production.

For instance, a long molding cycle time can be expensive, as roughly two-thirds of the molding expense is time-dependent. If a design change can reduce the molding cycle time, the piece cost will decrease. PIM vendors can provide valuable guidance on assembly, part combinations, material availability, and other factors. They can also advise on tool cost considerations, such as the number of cavities in the mold. While multiple cavities increase the initial tool cost, they reduce variable costs associated with molding by significantly shortening molding times, leading to labor and machine savings.

Early Identification of PIM Candidates

Identifying good candidates for PIM involves considering annual production quantity, shape complexity, engineering specifications, and material requirements. PIM is best suited for production quantities ranging from 5,000 per year to over 100,000,000 per year. The technology works best with at least 10 engineering specifications but no more than 100. PIM materials must be sinterable and available as small powders, typically ceramics, metals, alloys, and cermets.

Good candidates for PIM include components that require materials which are difficult to machine, designs that hinder coolant access during machining, surface finishes smoother than 5 μm but not smoother than 0.2 μm, and designs where considerable mass is removed in machining. PIM is particularly advantageous for components with low effective density, indicating that machining would be time-consuming and wasteful.

A simple decision tree can help determine the suitability of PIM for a given situation. Factors to consider include annual production quantity, geometric attributes, and materials. For example, components requiring high shape complexity, medium to high production quantities, and materials that are difficult to machine are ideal candidates for PIM. Additionally, surface finish requirements can influence the decision, as PIM can be more cost-effective for achieving smooth surfaces compared to machining.

Materials Selection for PIM

A wide variety of materials are available for PIM, including ferrous alloys, common oxide ceramics, tungsten alloys, cermets, cemented carbides, and special materials such as aluminum, precious metals, titanium alloys, and nonoxide ceramics. Modified chemistries are emerging to take advantage of the technology, offering improved properties over traditional manufacturing routes.

When selecting a material for a PIM component, both economics and properties must be considered. Powder availability is a primary consideration, as PIM requires small particle sizes. With over 300 vendors of small powders, almost any material can be custom fabricated for PIM if the price is right. Precompounded powders are often easier to process but are more costly.

For example, in stainless steels, the 316L composition (Fe-19Cr-9Ni-2Mo, in weight percent) is frequently used due to its combined strength and corrosion resistance. Modified compositions with higher levels of chromium, molybdenum, or silicon can enhance sintering performance and provide superior properties compared to castings, forgings, or machined products. However, these modified compositions may be more expensive, so a balance between cost and performance is essential.

Properties of PIM Materials

PIM products typically achieve properties comparable to those obtained from other manufacturing routes. For metals, the tensile properties are a primary concern, while for ceramics and cemented carbides, the focus is on rupture properties. PIM products often match the densities and properties of cast products, with typical densities ranging from 96 to 100% of theoretical values.

PIM has emphasized mechanical properties, with products showing strong correlation to handbook values. For example, stainless steel with the 174 PH composition (AISI 630) is one of the most widely used PIM alloys. When sintered and heat treated, the PIM product typically has a yield strength of 980 MPa (142 ksi), comparable to wrought products. However, the sintering step in PIM produces an annealed microstructure, which may lack the strength of forged products without post-sintering heat treatment. To address this, some PIM materials undergo additional heat treatment to enhance mechanical properties.

Besides mechanical properties, PIM products also exhibit competitive machinability, thermal behavior, wear resistance, and corrosion resistance. These properties make PIM suitable for a wide range of applications, from medical devices to automotive components. The clean processing environment of PIM contributes to higher purity products compared to some competitive processes, further enhancing their performance in demanding applications.

Physical, Chemical, and Thermodynamic Properties

The chemical, physical, and basic thermodynamic attributes of PIM materials typically match the values found in handbooks. Much attention has been devoted to learning how to make powders to tight compositional specifications and then how to densify the powders without contamination. PIM products can be higher in purity than those attained with competitive processes due to the microstructure access by the process atmosphere, where hydrogen and vacuum prove very effective in removing impurities from throughout the PIM body during heating.

Many PIM products are not sintered to quite 100% density. The densities attained in PIM are often slightly below theoretical (typical range is 96 to 100%). Since most castings have 2 to 4% residual porosity, PIM matches well with the densities and properties of cast products. Other properties, such as crystal structure, heat capacity, elastic modulus, melting temperature, and Poisson’s ratio, are the same as obtained with wrought products.

Mechanical Properties

More than other attributes, PIM has emphasized mechanical properties. For metals, the tensile properties are a primary concern, while for ceramics and cemented carbides, the focus is on rupture properties. Some work has been performed on creep, fatigue, and fracture toughness.

Powder injection molded products have essentially the same mechanical properties as attained with conventional technologies. For example, Figure 3.10 shows the measured yield strength for PIM metals and alloys compared against the handbook values for the same compositions. This plot shows a strong correlation between the PIM and handbook strengths, with a statistically significant correlation of 0.953. Note that 40% of the alloys are stronger as PIM products. The standard error of the estimated PIM strength is only 104 MPa (15 ksi) if the handbook strength is known.

Stainless steel with the 174 PH composition (AISI 630) is one of the most widely used PIM alloys. It consists of approximately 17% Cr, 4% Ni, 4% Cu, and low concentration alloying additions of Mn, Si, and Nb or Ta. When sintered and heat treated to the H1025 condition (aged at 1025°F or 552°C), the PIM product typically has a yield strength of 980 MPa (142 ksi), but ranges between vendors from a low of 965 MPa (140 ksi) to a high of 1040 MPa (151 ksi). As illustrated in Figure 3.11, when compared to a cast product, this range is slightly higher and comparable to wrought products.

Conclusion

Optimizing the design process and material selection in Powder Injection Molding is crucial for enhancing manufacturing efficiency and reducing costs. By involving PIM experts early in the design phase, companies can achieve significant cost savings and quality improvements. PIM offers a versatile and efficient solution for producing complex components across various industries.