Click to go to the MTU ECDM page

Annually, nine million automobiles in the United States are discarded. Washing machines, refrigerators, telephones, radios and the host of other products of our civilization fill our sanitary land-fills. The ecological consequences of this are enormous. Some suggest that manufacturers should be responsible for solid waste disposal. What does this mean for manufacturing in the United States? This means that manufacturers and designers must plan for some form of demanufacturing of a product. This is the problem set forth to be answered by the industrial world.

-- Walter W. Olson, John W. Sutherland

Links to Other Interesting Stuff

Design in Manufacturing Page

Comments? Concerns? Questions? Please E-mail me at

Research has been done in many areas, but the focus of the work on this page is aimed at understanding how physical attributes affect the demanufacturing of discrete parts.

Basic Definitions

Possible post-life uses: Reuse, Recycling, Remanufacturing, Disposal
The ability to use an old part as a new part by remachining or reprocessing
Bounding Volume
The basic cylinder that encloses the part

The development of the Reprocessability Index (RPI) was initially started as a look into understanding how the physical attributes, like shape, affect the recyclability of a discrete part. Advances in this work have shown that volume, surface area, features, stress, and moving contact zones are factors that may affect the reprocessability of the part. By the creation of an index system that evaluates these factors, an understanding of the design effort can be observed. Overall, by linking this aid with current DFX strategies, the manufacturing of the part should be quite sizeable. The following table outlines the metrics of the RPI. The next table details what the RPI values mean to a designer.

Reprocessability Index for Rotational Parts

RPI TermMethod to Obtain Terms

RPI Evaluation

< 7Part is reprocessable and has distinct possibilities for use as another function.
7 - 13Part has limitations for Reprocessability.
13-16Reprocessability of the part is severely restricted.
> 16Part is most likely not reprocessable.

Theoretical RPI usage

Click on any of the above parts to get the RPI terms

(These parts are samples from the D-Class Classification and Coding System)

Click here to to calculate the RPI of a part

RPI Definitions Area

R1 Development

The first term, R1, is created by dividing the volume of the bounding volume by the volume of the actual part. R1 provides insight into how much material is in the actual part as compared to the solid primitive shape. This term indicates if a part has many volume removing features associated with it, or was created with extremities that increase the size of the bounding volume.
Return to RPI Table

R2 Development

The second term, R2, is the surface area of the actual part divided by the surface area of the bounding volume. R2 represents the complexity of the part. This ratio quantifies the difference between the bounding volume and the actual part, but can be influenced by features or overall shape. Generally more complex parts have less potential for reprocessing.
Return to RPI Table

R3 Development

The third term in the RPI model, R3, compares the minimum usable volume to the part volume. Since a small part requires more precise tooling and fixturing, a volume of 100 cm^3 (6.1023 in^3) was established as the minimum practical volume. This value may be changed to reflect market and technology conditions. Any volume under this minimum level is denoted as unreprocessable and is given an infinite value. R3 provides information about the size of the part in relation to other parts.
Return to RPI Table

R4 Development

The fourth term, R4, is a specific analysis of blind holes, through holes, slots, and keyways in a part. Holes, keyways, and slots reduce the structural strength of a part and thus make a part more likely to fail in use. In addition, parts with these features will have inherent difficulties being converted unless these features match exactly with the new part to be created. It must be understood that when the RPI is used by the manufacturing engineer at concept, a proposed follow on part may not yet be known or defined. Extreme features such as holes and slots, severely limit any potential ability to create a new part from the existing part. These features represent an important amount of volume that is removed from the core of the part and may reduce the reprocessability. The most critical feature is a through hole. The blind holes and keyways are features that have less impact, but are still considered to reduce reprocessability. To account for these features, the volume of the part (V) is divided by the volume of the part (V) minus the volume of the feature (Vf). Each feature of these types is analyzed in this manner and then summed. The summation indicates the possibility of features that cause unreprocessability. If the part has substantially less volume than the features within it, a negative value of R4 may result. The use of the absolute value will account for this negative term, but will maintain the value’s influence in the RPI equation. Typically, when the ratio is negative, the R4 value will be relatively large indicating that these specific features undermine the reprocessability of the part. As a more complete understanding of reprocessable parts is developed, other features can be added to the recognizable feature list.
Return to RPI Table

R5 Development

The effects of stress originate from several sources. Stress and cyclic stress are two sources for concern in the evaluation of the reprocessability of a part. Fatigue effects have yet to be studied in evaluating the reprocessability and are not further discussed here. The stress evaluated for the R5 term is based on the maximum design stress (sDS) for the part. Typically, a low design stress will not cause any reprocessability problems because the stress has not caused plastic or brittle failure. As the design stress approaches the yield limit of the material, the chances of the part being reprocessable are reduced. The R5 term is arranged so that an increase in maximum design stress will cause an increase in the unreprocessability of the part.
Return to RPI Table

R6 Development

The final term of the RPI, R6, evaluates wear. Since wear occurs predominantly on surfaces in moving contact, an evaluation of wear is based on the relative magnitude of moving contact surface area.
Return to RPI Table


Once all of the terms have been found, the complete equation is assembled. By using this equation, the design may be evaluated by the RPI model to determine manufacturing reprocessability.
Return to RPI Table

Comments? Concerns? Questions? Please E-mail me at