This article elaborates a new design approach that aims at designing products with built-in disassembly means to be activated at the end of a product’s life. Two projects explored in this area are the design of a new class of joints that can be detached by the application of localized heat, and the design of assemblies that can disintegrate via a domino-like process triggered by the removal of one or only a few fasteners. The detachable joints (heat-reversible locator-snap systems) allow easy, non-destructive, and clean detaching of product enclosures. They consist of locators and snaps molded on the internal surfaces of thin-walled product enclosures. During disassembly, thermal expansion of the enclosure walls constrained by locators and the temperature gradient along the wall thickness are exploited to realize the deformation required to release the snaps. In self-disintegrating assemblies, the relative motions of components are constrained by the locators integral to the components, in such a way that the removal of one or few fasteners would cause the self-disintegration of the assembly in a desired sequence.
There was a time when the end of a product's life did not matter greatly to the designer. As long as the product was safe and reliable over a desired period of time, business would prosper. The expected end for every Model T or television set was a one-way trip to the junkyard or perhaps to the bottom of a body of water.
Today, however, society's views of the proper way to treat land and water have changed. From a strictly economic standpoint, land, materials, and water have all become too valuable to be treated like that.
What's more, the regulatory environment is increasingly requiring manufacturers to consider the disposal as well as the creation of their products. Perhaps the most widely spread rules of this nature are those in the European Union governing the disposal of vehicles and electronics.
For social and legal reasons, design practices must change to accommodate more sustainable practices, in which engineers have an additional product-design goal—an economical end of life, or design for disassembly. Economic feasibility of an end-of-life (EOL) scenario of a product is determined by the interaction among retrieval cost, the value of the retrieved components or materials, and the regulatory requirements on them. While meeting regulatory requirements is obligatory regardless of economic feasibility, EOL decision making is often governed by economical considerations.
Compared to shredding followed by sorting and separation, disassembly has advantages in being able to non-destructively retrieve reusable components and to allow separation of target materials with far less contamination. However, disassembly is more expensive than shredding, especially in developed countries, since it must rely largely on manual labor due to the uncertainties in conditions of the end-of-life products in a waste stream.
Even if a component has high recycling value or high environmental impact, therefore, it may not be economically justifiable to retrieve it via disassembly if doing so requires excessive cost.
Since the cost of manual disassembly depends largely on the number of fasteners to be removed and of components to be reached, grabbed, and handled, it is highly desirable to use joints that can disengage with minimum labor, part damage, and material contamination, and to retrieve high-value or highimpact components by removing fewer fasteners and components.
These thoughts motivated my research team to develop a concept of product-embedded disassembly, a new approach to design for disassembly that aims at designing products with built-in disassembly means to be activated at the end of a product's life. Two of the projects in this area that we have explored are the design of a new class of joints that can be detached by the application of localized heat, and the design of assemblies that can disintegrate via a domino-like process triggered by the removal of one or only a few fasteners.
Heat-Reversible Snap Joints
The detachable joints, which we call heat-reversible locator-snap systems, allow easy, non-destructive, and clean detaching of product enclosures. They consist of locators and snaps molded on the internal surfaces of thin-walled product enclosures. The assembly process is analogous to regular locator-snap systems. During disassembly, thermal expansion of the enclosure walls constrained by locators and the temperature gradient along the wall thickness are exploited to realize the deformation required to release the snaps.
The heat-reversible locator-snap systems allow easy access to the internal components during repair and maintenance, protect internal components from destruction during disassembly, and minimize the amount of incompatible materials during material recycling. While the concept is simple, designing such a locator-snap system for an arbitrary enclosure and parting line geometry would require the examination of numerous alternatives for locators, snaps, and heating areas.
We have formulated an optimization problem to find the locations, numbers, and orientations of locators and snaps, and the numbers, locations, and sizes of heating areas, all of which realize, in harmony, the release of snaps with minimum heating, while satisfying motion and structural requirements. Screw theory is utilized to precalculate the set of feasible types and orientations of locators and snaps that are examined during optimization, and a multi-objective genetic algorithm is used for solving the optimization problem. We have applied it to an automotive frame-panel assembly and an enclosure assembly for consumer electrical products.
Although effective for disassembly, the design was at first subject to accidental opening. We later addressed this issue with double latching, “lock-and-key” snaps that require the displacement of snaps within a certain range to disengage, and multiple snaps that require heating multiple locations at different temperatures to disengage.
We have applied the lock-and-key heat-reversible locator snap systems for the bezel that can hold the LCD panel of a flat-screen television. The Waste Electrical and Electronic Equipment Directive of the European Union requires removal of the panel before the disposal of a TV set.
We have also investigated computational synthesis of self-disintegrating assemblies, where relative motions of components are constrained by the locators (tabs, slots, lips, rests, etc.) integral to the components, in such a way that the removal of one or few fasteners would cause the self-disintegration of the assembly in a desired sequence, much like the domino effect.
In a conventional assembly, components A, B, and C are each fixed with a separate fastener. With high labor cost for removing fasteners, it may be economically practical only to disassemble part A for reuse and send the remainder to a landfill. This end-of-life scenario is obviously not ideal from either the economic or the environmental viewpoint.
In a self-disintegrating assembly, on the other hand, the motions of B and C are constrained by locators on components. As such, the removal of the one fastener that holds part A (called a trigger fastener) also activates the domino-like self-disassembly pathway. When A is removed, B is free, and then C.
Since no additional fasteners need to be removed, B and C can be efficiently disassembled, allowing the recycling or reuse of all components. This scenario can be economically and environmentally better than the one for the conventional assembly.
The concept of self-disintegrating assembly can be applied to a wide variety of products, since it requires no special tools, materials, or actuators to implement. The embedded disassembly sequence depends on the component layout within an enclosure, and on the types and layout of locator features on components. Both considerations affect total disassembly cost and in turn the economics of the end-of-life scenario. This fact poses a unique mathematical challenge in designing products with a built-in disassembly pathway for an optimal end-of-life scenario.
The use of part-locating features such as tabs, slots, and lips in a product assembly is not a new idea. In fact, it is a common design practice that has been widely adopted in many electromechanical products to enhance the ease of assembly.
However, its application has been limited within the context of local redesigns of a small subset of components that facilitates assembly or sometimes disassembly of these components. We generalized this design practice and mathematically formulated an optimization problem of simultaneously synthesizing part-locating features and special layouts of components (which in turn determines feasible disassembly sequences) in an entire product assembly, for an optimal balance between disassembly cost and environmental impact. Given component geometries, the algorithm simultaneously determines the spatial configuration of components, locators, and fasteners, and the end-of-life treatments of components and subassemblies, such that the product can be disassembled for the maximum profit and minimum environmental impact through recycling and reuse via a domino-like “self-disassembly” process. A multi-objective genetic algorithm is utilized to search for Pareto optimal designs in terms of satisfaction of the distance specification among components; efficient use of locators on components; profit of the end-of-life scenario, and environmental impact of the EOL scenario. We have successfully applied the developed algorithm to a desktop PC assembly, where some components have high reuse values. Compared to the original design, we have demonstrated that few locating features and rearrangements of certain components can greatly facilitate an economical and ecological EOL scenario with the expense of minor manufacturing cost.
End-of-life issues are relatively new considerations for most product designers, and so procedures to deal with those issues are not as advanced as many other aspects of design practice. We are hoping to do what we can to change that. For various reasons—economic, regulatory, social, and ecological—pressures to reuse components and to recycle materials, and the means to do so economically, will grow in importance in the years ahead.
In Greater Depth
Research discussed in this article is described in more detail in the following papers.