Editorâs note: A recognized expert in powder injection molding (PIM), Randall German is Brush Chair professor in materials at Penn State University. This column is part of an occasional series on understanding and applying PIM.
|Figure 1. A simple decision tree suggests how to perform a first screening of candidate components with respect to matches with powder injection molding.|
As a first pass, consider the schematic decision tree shown in Figure 1. One thing to consider is the annual production quantity. Powder injection molding has historically best matched with industrial needs at production quantities from 5000 to 100 million parts/year. These parts range from specialty firearm sights to cellular telephone vibrator weights. If the target production rates are in that range, then it is appropriate to continue with consideration of PIM.
The next factor relates to the engineering specification. Powder injection molding works best where there are at least 10 specifications (dimensions, locations, surface finish, and such) on the engineering drawing or definition. But the process struggles when the complexity and constrictions exceed more than 100 callouts. Further, it struggles when tolerances become too tight (Â±.1 percent) on more than a few dimensions. Components are in production by PIM outside this window, but they are the exceptions. For example, one crash avoidance sensor mount for luxury automobiles is in production using PIM with 130 dimensional specifications. In other cases, critical dimensions are machined after sintering.
|Figure 2. A Venn diagram suggests some of the current justifications for PIM based on high production quantities, high performance, and shape complexity, with various PIM products shown in each of the intersections with these three concerns. Cemented carbide sandblast nozzles and water jet cutting nozzles or other wear-resistant, complex shapes are an idealized convergence of these three concerns.|
If a small powder is available (diameter smaller than 20 Âµm), then sintering becomes the next concern. In many cases, small powders can be sinter densified without extraordinary processing cycles, but many require compositional shifts for easier sintering. For ceramics, this usually means small concentrations of additives to enhance sintering. A common example is the addition of .1 percent magnesia (MgO) to alumina (Al2O3).
For metals, sinterability usually means the powders have low contents of ingredients that prove reactive, especially the strong oxide formers, reactive metals, volatile elements, and toxic materials. This usually means PIM compositions avoid beryllium (toxic and easily oxidized), mercury (toxic and volatile), lead (toxic and volatile), manganese (strong oxide former and both the metal and oxide are volatile), zinc (volatile), sodium (reactive), magnesium (reactive and strong oxide former), aluminum (strong oxide former), tantalum (reactive), diamond (unstable during sintering), oxides of metals such as indium and tin (unstable during sintering), and titanium (reactive and strong oxide former).
This is not to say these are impossible to process by PIM, since several have been processed successfully. But the problems that arise with these ingredients are usually best avoided by using more inert compositions.
Another problem is with lower-melting-temperature materials, where other technologies are very effective. Generally, materials that melt at temperatures greater than 1000C (1832F) are more successful by PIM. One reason for this is that lower-melting materials prove easier to process using diecasting, machining, or other fabrication routes where there is adequate tooling for low-temperature forming. But as the melting temperature increases, then problems with technologies geared to lower-temperature materials increase, creating more interest in PIM. Consequently, even though PIM aluminum (melting temperature of 660C or 1220F) and other lower-melting-temperature alloys such as brass have been demonstrated, they still are not commercially successful.
|Figure 3. Machining is relatively inexpensive for rough surfaces and setup dominates cost, while for smooth surfaces cost is dominated by the machining time. For PIM to compete against machining, it is important to seek smooth surfaces and situations in which there is considerable mass removal needed to generate the final shape.|
If all of these simple tests are passed, then probably the component is a candidate for PIM, except for one final barrier: How much does it cost? In simple terms, the application dictates how much can be paid for a component. It is widely recognized that consumer products tend to migrate toward the low cost of plastics. Conversely, PIM is a favorite for higher-performance metallic and ceramic products used in medical or dental devices, defense and aerospace systems, sporting goods, appliance and industrial components, hand tools, business machines, watches, sensors, cutting tools, automotive engines, electronic packaging, or marine equipment.
These applications share similar requirements: good performance (as measured by resistance to high service stresses); resistance to wear, corrosion, and high temperatures; good thermal and electrical conductivity; high density; or excellent magnetic response.
Although these criteria might seem constrictive, PIM has succeeded in thousands of applications. As indicated in Figure 2, success comes from the coincidental concerns over shape complexity, production quantities, and performance. To help realize the applications, this Venn diagram indicates some PIM applications that intersect with each of these areas. In addition, surface finish and final properties are often cited as reasons for using PIM. Hard materials prove difficult and expensive to grind or machine, so applications that require materials with poor machinability or applications that require difficult-to-machine geometries are better candidates for PIM.
Other factors that impact the identification of good candidates include tolerances and surface finish. For rough surfaces, the machining cost is dominated by setup, but for smooth surfaces machining costs associated with the longer time of machining dominate, as illustrated in Figure 3. Thus, from the perspective of machinability, the following attributes provide an incentive to use PIM:
Today, most common engineering materials are available via PIM. Although a wide range of materials can be processed, in the end a few dominate the field because of widespread use and low raw material cost:
For these materials, there is an ample supply of powders, the powder cost is relatively low, and sintering is well established.