ASSEMBLY PROCESS:DISASSEMBLY

DISASSEMBLY

Challenge for Disassembly

The development of disassembly strategies and technologies has formerly been based on the inversion of assembly processes. The characteristics of components and connections are like a systematic interface between both processes. In addition, assembly and disassembly can have similar require- ments regarding kinematics and tools (Feldmann, et al. 1999). Nevertheless, the most obvious dif- ferences between assembly and disassembly are their goals and their position in the product’s life cycle.

The objective of assembly is the joining of all components in order to assure the functionality of a product (Tritsch 1996). The goals of disassembly may be multifaceted and may have a major influence on the determination of dismantling technology. The economically and ecologically highest level of product treatment at its end of life is the reuse of whole products or components (Figure 47) because not only the material value is conserved but also the original geometry of components and parts, including its functionality. Thus, disassembly generally has to be done nondestructively in order to allow maintenance and repair of the components.

A further material loop can be closed by the disassembly and reuse of components in the pro- duction phase of a new product. In order to decrease disassembly time and costs, semidestructive dismantling technologies are applicable, such as to open a housing.

In most cases, disassembly is done to remove hazardous or precious materials in order to allow ecologically sound disposal or high-quality-material recycling. All types of disassembly technology are used in order to decrease costs and increase efficiency (Feldmann et al. 1994).

In the determination of disassembly technologies, several frame conditions and motivations have to be taken into account (Figure 48). Because of rapidly increasing waste streams, the ecological problems of disposal processes, and the shortage of landfill capacities, the product’s end of life has become a special focus of legislative regulation (Gungor and Gupta 1999; Moyer and Gupta 1997) that requires disassembly and recycling efforts.

Furthermore, the ecological consciousness of society (represented by the market and its respective requirements) and the shortage of resources, leading to increasing material prices and disposal costs, put pressure on the efficient dismantling of discarded products. The increasing costs and prices also provide economic reasons for dismantling because the benefits of material recycling increase as well. Finally, on the one hand, the technological boundary conditions indicate the necessity of disassembly because of increasing production and shorter life cycles. And on the other hand, the availability of innovative recycling technologies is leading to new opportunities and challenges for disassembly technology.

Due to influences in the use phase, unpredictable effects such as corrosion often affect dismantling. Also, the incalculable variety of products when they are received by the dismantling facility has a negative influence on the efficiency of disassembly. Together with the lack of information, these aspects lead to high dismantling costs because of the manpower needed (Meedt 1998).

On the other hand, some opportunities in comparison to the assembly, for example, can be ex- ploited. Thus, with disassembly, only specific fractions have to be generated. This means that not all connections have to be released, and (semi)destructive technology is applicable for increasing effi- ciency.

Disassembly Processes and Tools

The advantages and disadvantages of manual and automated disassembly are compared in Figure 49. Because manual disassembly allows greater flexibility in the variety of products and types as well as the use of adequate disassembly tools, it is generally used in practice. Due to risk of injury, dirt, the knowledge needed for identification of materials, and wages to be paid, manual disassembly is not considered an optimal solution. On the other hand, problems of identification, the influence of the use phase (as mentioned above), damage to connections, and particularly the broad range of products, prevent the effective use of automated disassembly processes (Feldmann et al. 1994). Thus, for most disassembly problems partial automated systems are the best solution.

Along with the combination of manual and automated disassembly, other aspects, such as logistics, flow of information, and techniques for fractioning, storing, and processing residuals, have to be optimized for efficient dismantling.

In addition to the planning of the optimal disassembly strategy, the major area for improvement in the disassembly of existing products is in increasing the efficiency of disassembly processes. Highly flexible and efficient tools especially designed for disassembly are needed.

The function of tools is based on different types of disassembly: (1) destructive processes in order to bring about the destruction of components or joining; (2) destructive processes in order to generate working points for further destructive or nondestructive processes; (3) nondestructive processes such as the inversion of assembly processes (Feldmann et al. 1999).

Figure 50 shows an example of a flexible unscrewing tool developed at the Technical University of Berlin that generates working points in a destructive way. This tool consists of an impact mass that is speeded up in a first step by tangent pneumatic nozzles until a certain angular velocity is reached (Seliger and Wagner 1996).

In a second step, the rotating impact mass is accelerated towards a conical clutch and transmits the linear and rotational impulse to the end effector. Using W-shaped wedges of the end effector, a new acting surface is generated on the head of the screw, avoiding any reaction force for the worker.

Furthermore, the rotational impulse overcomes the breakaway torque and the linear impulse re- duces pre-tension in order to ease loosening. The loosened screw is now unscrewed by a pneumatic drive and the impact mass is pushed to the starting position (Seliger and Wagner 1996). In case a screw cannot be released due to influences in the use phase (e.g., corrosion or dirt), the end effector can be used as a hollow drill with wide edges in order to remove the screw head.

A major problem in dismantling is in opening housings quickly and efficiently in order to yield optimal accessibility to hazardous or worthy materials. Especially with regard to small products, the effort in dismantling with conventional tools is very often too high compared to the achieved benefit. To solve this problem, a flexible tool, the splitting tool (Figure 51), has been developed (Feldmann et al. 1999).

The splitting tool has specific levers and joints that divide the entry strength or impulse (e.g., by a hammer or pneumatic hammer) into two orthogonal forces or impulses.

Through the first force component with the same direction as the original impact, the splitting elements are brought into action with the separating line of the housing in order to generate acting surfaces. The strength component set normally to the entry impact is used simultaneously as sepa- ration force. This way, screws are pulled out and snap fits are broken.

Thus, in general the housing parts are partially destroyed and torn apart without damaging the components inside. Using specially designed tools for disassembly allows unintentional destruction of other components, which is often an effect of dismantling with conventional tools such as hammers and crowbars, to be avoided.

The examples given show the basic requirements for disassembly tools: flexibility concerning products and variants, flexibility concerning different processes, and failure-tolerant systems.

Applications

Conventional tools such as mechanized screwdivers, forceps, and hammers are still the dismantling tools generally used. The use of tools, the choice of disassembly technology, and the determination of the disassembly strategy depend to a large degree on the knowledge and experience of the dis- mantler because detailed disassembly plans or instructions are usually not available for a product. This often leads to a more or less accidental disassembly result, where the optimum between disas- sembly effort and disposal costs / recycling gains is not met. Many approaches have been proposed for increasing the efficiency of disassembly by determinating an optimal dismantling strategy (Feld- mann et al. 1999; Hesselbach and Herrmann 1999; Gungor and Gupta 1999).

Depending on the actual gains for material recycling and reusable components, the costs of disposal, and legislative regulations, products are divided into several fractions (Figure 52). First, reusable components (e.g., motors) and hazardous materials (e.g., batteries, capacitors) are removed. Out of the remaining product, in general the fractions ferrous and nonferrous metals and large plastic parts are dismantled so they can be recycled directly after the eventual cleaning and milling processes.

Removed metal–plastic mixes can be recycled after the separation in mass-flow procedures (e.g., shredding). Special components such as ray tubes are also removed in order to send them to further treatment. The remainder must be treated and / or disposed of.

As mentioned in Section 5.2, the economic margin of dismantling is very tight and the logistic boundary conditions—for example, regarding the sporadic number of similar units and the broad variety of product types—are rather unfavorable for automation. Depending on the flexibility of automated disassembly installations, three strategies can be used.

Some standardized products (e.g., videotapes, ray tubes of computer monitors) can be collected in large numbers so that automated disassembly cells especially designed for these products can work economically.

Greater flexibility from using PC-based control devices and specially designed disassembly tools, as well as the integration of the flexibility of human beings allow the automated dismantling of a certain range of discarded products in any sequence. Figure 53 shows an example of a hybrid dis- assembly cell that was realized on a laboratory scale. One of the integrated robots, used for unscrew- ing actions, is equipped with a flexible unscrewing tool that can release various types of screws. The second robot, using a special multifunctional disassembly and gripping device, is able to split com- ponents, cut cables, and remove parts without changing the device.

Another strategy for automated disassembly is the enhancement of flexibility using sensors that detect automatically connection locations and types. By an evaluation of the sensor data, the re-

spective tools are determined and controlled (Weigl 1997). This strategy in general leads to high investments that have to be compensated for by disassembly efficiency.

Entire Framework for Assembly and Disassembly

Mostly in regard to maintenance and reuse of components, but also in regard to optimal disassembly planning, an integrated assembly and disassembly approach should be considered—just with the design of a product (DFG 1999). Five stages of integration can be distinguished (Figure 54).

1. Information is exchanged between assembly and disassembly. Data on, for example, connection locations and types ease substantially the disassembly of discarded products.

2. Recycled material, such as auxiliary materials, can be delivered from disassembly plants to the production plants.

3. The reuse and reassembly of components, which first requires disassembly, is already practiced with many products. Nevertheless, the integrated consideration of both assembly and disas- sembly is important for efficiency.

4. The exchange of staff requires the proximity of assembly and disassembly facilities. At this stage, the knowledge and experience of the workers can be used for both processes and thus provide major benefits in efficiency.

5. The highest stage of integration is the use of a device for both assembly and disassembly.

Examples are repairing devices for printed circuit boards.

The possible level of integration is determined mainly by the design of a product. Thus, an overall approach that looks at all phases of the product’s life cycle is required.