Sponsored By

April 1, 2002

6 Min Read
What makes for a successful PIM component?

Editor's note: A recognized expert in powder injection molding (PIM), Randall German is Brush Chair professor in materials at Penn State University in University Park, PA. This column is the first in an occasional series on understanding and applying PIM.

Mfg_PIM_Figure1.gif

Figure 1. The concept of effective density is based on looking at the component mass divided by original volume that would be required if it was machined. This component has an effective density of 1.8 g/cu cm.

Powder injection molding (PIM) relies on a thermoplastic polymer blend filled with about 60 percent by volume of a small metal or ceramic powder. This mixture of polymer and powder is injection molded to form a complex shape. Once molding is completed, the polymer (binder) is extracted and the small powder is sintered. Sintering is a high-temperature heat treatment designed to induce densification of the particles. Accordingly, the final product is typically 15 percent smaller than the tooling, but densified to a level where the mechanical and physical properties rival wrought materials.

Although many complex geometries can be fabricated via PIM, only certain component characteristics prove cost-effective. The small powders are more expensive than wrought materials, so there is an initial material cost penalty. Early identification of designs that match well with the PIM technology helps ensure economic success. Typical considerations involve material, properties, component size and shape, tolerances, production cost, production quantity, and design features. For example, PIM excels at forming shapes with dead-ended holes, dovetails, slots, threads, or curved surfaces.

As a starting point in considering PIM, Table 1 summarizes the typical, minimum, and maximum attributes. Some explanation is in order. In practice, there are many technology variants--powder types, binder formulations, debinding techniques, and sintering furnaces. Such variation affects what is possible in terms of each company's capabilities. Accordingly there are significant producer-to-producer differences, largely dependent on the age of the equipment.


Page-60.gif

Capabilities of PIM
Complex net-shape components are the main target for PIM. A gauge of the advantage over competitive technologies is the number of call-outs or dimensions on the engineering drawing. A simple shape has only a few features, while a microcomputer circuit has millions of features; both would be poor applications for PIM. Common PIM successes involve several dimensions--a wristwatch case is one example--that matches well with the technology. The most challenging components in production involve up to approximately 130 dimensional specifications.

A simple way to introduce the geometric design window for PIM is to start with a very simple characteristic, a parameter known as the effective density. Density is defined as mass over volume, and usually is given in g/cu cm (water is 1 g/cu cm and most steels are just less than 8 g/cu cm).

Page-61.gif

For this discussion, let's contrast PIM with machining. The final component is characterized by its geometric aspects, including largest dimension, holes, slots, and other specifications that might include wall thickness. For many PIM parts, the wall thickness is usually small, and might be less than 10 percent of the largest dimension. However, components with aspect ratios (largest dimension divided by the smallest wall thickness) might range up to 120 or more. The blades of scissors are an example where the length is much greater than the thickness, and the thickness is not highly variable. In PIM it is the wall thickness that determines debinding time (time needed to extract the polymer while leaving the particles intact), and is the slow step.

Consider the PIM component shown in Figure 1, which has a mass of 27.4g. If we put an outer box around the component, then we define the volume of starting material that would be required for machining. The final mass divided by the initial volume gives the effective density. In this case, the effective density is 1.8 g/cu cm (27.4g divided by a box of 2.9 by 2.1 by 2.5 cm).

Even this is an underestimate of the starting volume, since it presumes that ideal-sized blocks of steel will be available. In reality there will be more waste and less-than-ideal feed for a machining process. Using this concept of mass divided by the outer envelope volume leads to the bar chart shown in Figure 2, taken from several PIM steel and stainless steel products. Note the high population of components in the 1.5 to 2.0 g/cu cm range (reflecting about 20 percent material utilization).

Mfg_PIM_Figure3.gif

Figure 3. This scatter plot shows how the effective density clusters against the maximum component dimension, with the bulk of current production being smaller objects if the effective density is high.

Another view of the same components can be given as a scatter plot (Figure 3), which shows the characteristic mass and longest dimension for the same samples. The larger components have lower effective densities. These simple plots show that components with many undercuts, holes, slots, and other features requiring mass removal, such as by machining, are ideal targets for PIM. These considerations illustrate how PIM is best suited f
or components with sufficient shape complexity that machining is unattractive.

Today, typical PIM component mass is generally low, in part because the small powders are expensive. Economic justification is strongest when alternative technologies waste time and material. The general production limit for precise components is 250g (.25 lb), although several special processes have evolved to produce precise components of more than 1 kg (2.2 lb), and current production ranges up to 17 kg (37 lb).

The criteria for early identification of PIM candidate components are summarized here in terms of a few key considerations. Several should be familiar to plastics injection molders:

  • Mass/volume. Seek a low effective density, indicating high material loss in machining and possible gains in productivity via PIM. A target would be about 20 percent of theoretical density.

  • Quantity. Tooling and setup costs are not justified for low production quantities. Seek production quantities from 5000 or more a year, preferably at least 20,000 per year.

  • Material. PIM is easier to justify for materials that are hard to machine.

  • Complexity. Seek components that require multiple axes for indexing during machining.

  • Performance. If performance is important, then the properties attained via full density sintering are justified.

  • Surface finish. At no additional cost PIM can deliver relatively smooth surfaces, so use this advantage when possible.

  • Assembly. Look for opportunities to consolidate several parts into a single piece to save on inventory and assembly costs.

  • Novel compositions. Use material combinations that are difficult via traditional processes. For example, wear-resistant stainless steels are possible by adding a small proportion of ceramic particles to the feedstock.

For more information about injection molding powdered metals and ceramics, check out the latest copy of IMMC. Look for more articles on this topic to come in future issues of IMM.

Contact information
Center for Innovative Sintered
Products, Penn State University
University Park, PA
Randall M. German; (814) 863-8025
www.cisp.psu.edu
[email protected]
aIMM Infolink 287

Sign up for the PlasticsToday NewsFeed newsletter.

You May Also Like