Edited by David S. Hotter
Boothroyd Dewhurst Inc.
With much of the focus these days on cost, engineers should consider the effects of manufacturing, assembly, and end-of-life recovery early in the concept stages of product development — before committing to detailed designs and manufacturing processes. By incorporating the principles of design for manufacturing and assembly, designers can reduce part counts, shorten time-to-market, simplify assembly, improve quality, and reduce overhead. And, as a sign of the times, plan for the environmental impact after a product reaches the end of its useful life.
Two software packages simplify design analysis by simulating manufacturing, assembly, and recycling processes and the costs associated with each: Design for Manufacturing and Assembly (DFMA) and Design for the Environment (DFE). Together these programs help optimize assembly and disassembly and prevent used goods from ending up in landfills.
More than ever U.S. manufacturers are concerned that products made today will soon be subject to takeback programs, as is the case in countries such as Germany and the Netherlands, and soon to be in Japan. Some computer and automobile manufacturers are already preparing for take-back programs by making products easier to disassemble and recycle. The goal is to save money as well as win market recognition ahead of government regulations. Conforming to such imminent legislation also prepares companies for global markets, by designing one product that can be sold worldwide.
Design for Manufacturing and Assembly consists of two separate modules for manufacturing and assembly processes. Design for Assembly (DFA) lets engineers analyze the overall efficiency of a design based on each individual component’s contribution to the complete assembly. Besides quantifying assembly time and labor costs, it helps users simplify the structure and reduce both part and assembly costs.
The software uses assembly-time standards based on extensive research that has been confirmed by years of industrial use. Users can customize the standards databases for company-specific parts and operations. DFA computes the efficiency of assembly steps and offers suggestions for redesign options. Design teams end up with several concepts, each with its own dimensions, materials, and manufacturing processes.
Selecting the best alternative usually means comparalternaing costs. This is where the Design for Manufacturing (DFM) module comes into play. DFM evaluates the cost trade-offs between different manufacturing processes and materials. The DFM module analyzes common processes such as injection molding, sheet metalworking, die casting, and powder metallurgy.
The DFMA package works best when applied at the concept stage of product design, where at least 70% of the final product cost is fixed. At such early stages concurrent engineering teams use the quantitative tools to put dollar figures on various design options or proposals. DFMA can also be used to benchmark competitors’ products and to monitor vendor quotes.
With worldwide environmental regulations on the rise, manufacturers must find ways to turn their products green, before legislation is enacted that could put them in the red. By designing products with disassembly and recycling in mind, manufacturers can make it easier to recover and recycle parts and material, to cut costs as well as benefit the environment.
Designed for Environment (DFE) software simulates disassembly of products at end-of-life and identifies costs and environmental impacts of various designs. DFE also works together with DFA to improve the disassembly process. Users can edit assembly lists to form groups of items that won’t be taken apart, but instead recycled or disposed while still assembled.
The software analyzes the financial return of disassembly, disposal, reuse, and recycling, and influence each process has on the environment. The result is a summary of financial impact at each stage of disassembly, calculated from the difference between what it costs to disassemble each item and the value of the recovered item, less disposal costs.
A value-assessment metric developed at TNO Institute of Industrial Technology, a major research establishment in the Netherlands, quantifies environmental impact. The metric, called MET (materials, energy, toxicity), assigns points based on how disassembly and recycling processes affect the earth’s resources (See MACHINE DESIGN, January 16, 1997, pg. 106).
REVAMPING AN IP
To show the benefits of DFMA and DFE software, an instrument panel (IP) from a production truck was evaluated for potential redesign. The results included alternative designs that make the IP easier to manufacture, assemble, and disassemble, using existing product specifications. The goal was to identify methods that would make it easier to manufacture the IP, not to design a new one.
The final results showed that the current design, consisting of a plastic facing supported by a metal structural frame, could be more efficiently manufactured, assembled, and recycled by using two injection-molded shells to form the main structure, as well as secure several subassemblies. The front structural molding, a horizontal rib design, provides the needed lateral bending stiffness and strength. To help maintain stiffness and strength along the horizontal rib, cantilever snap fits are limited to the vertical ribs.
Foam and rubber gaskets applied around the snap-fit subassemblies provide a snug fit in the IP and eliminate squeaks. In addition, extra gasketing around the instrument cluster replaces a separate rubber sheet and five staking operations by filling the gap around the steering column.
The back molding contains channels that form air ducts when ultrasonically welded to the front molding. The channel running along the back of the molding supplies air to four vents and boosts bending strength and stiffness, particularly around the glove compartment opening. The front and back IP moldings replace a four-piece ultrasonically welded duct, plus a support structure made from steel tubes and stampings.
Molding the IP from plastic offers many opportunities to consolidate parts. For instance, the back of the IP contains channels to support wire harnesses, replacing separate clips attached to the IP molding and steel structure. To minimize disassembly steps required for recycling, subassemblies such as the cigarette lighter/power outlet unit, glove box, cup holders, and air vents were molded from the same polymer as the main structural moldings.
The DFMA software evaluates the assembly and helps engineers identify what operations eat up the most time and money. The existing IP consists of 28 subassemblies and 314 parts, requiring 482 assembly steps, ranging from attaching electrical connectors to applying adhesives. It takes an estimated 63 min to complete these tasks, which at a burdened labor rate of $42/hr, results in a total assembly cost of $44/IP.
The software shows there are 41 necessary items, including the instrument cluster, wire harnesses, and other subunits, which were not evaluated for further disassembly. Once categorized, it is easy to see that the cost to assemble these necessary components is overshadowed by the cost for separate fasteners, and processes such as welding, staking, and adhesive bonding.
DFMA software also quantifies assembly bottlenecks, such as parts not easy to align during assembly and those that require extra effort because of tight fits or interference from other components. These difficult-toassemble components boost assembly time and increase overall assembly costs. More important, they can increase defects and quality deficiencies.
The redesigned IP is assembled in 121 steps compared to the 482 steps in the current design; the number of separate fasteners was cut from 123 to 23. Miscellaneous items were reduced from 173 to 39 parts and, of these, seven are washers and gaskets used to provide a tight fit between the snap-fit assemblies in the front structural molding. The majority of other miscellaneous items eliminated in the redesign come from the four air-deflector assemblies. In theory, an entire deflector assembly can be molded in a single spherical part, so all deflector parts except the case are considered redundant.
Separate operations were reduced from 130 to 15. The remaining operations include ultrasonic welding, a staking operations, and thirteen steps to route wire harness and snap-fit connectors on the back of the IP.
The bottom line from reducing assembly operations and difficult tasks brings assembly costs down to $8.52/IP compared to $44.12.
One of the most significant assembly cost savings comes from replacing steel tube and sheet metal structures with injection- molded structural components and trim. Using ABS/PC thermoplastic with 0.14-in.-thick walls and 0.08 in.-thick trim, requires a total of 17 lb of resin at an average cost of $1.25/lb. Although material costs will clearly be more expensive compared to the existing design, cost estimates for the four injection moldings in the proposed IP design are the same as the various parts they replace. Estimates include amortization of mold costs but not finishing costs, which are only critical for the outer surfaces of the IP and estimated to be the same for both designs.
However, the computer program showed that new tooling costs more. Tooling for the redesign is more complex because it consolidates many parts into a single structural component, and therefore cycle times also increase. Production also requires more molds, because parts must be molded on more than one machine. This increases tooling costs by $500,000. Combining assembly and manufacturing costs, the proposed design still results in a savings of $35.23/IP.
DFE analysis software also quantifies the difference between disassembling the current design and the proposed redesign during recycling. The MET scores are based on normalization of eight different quantifiable environment effects. The MET points are assigned to components assuming they are discarded at the end of life and replaced by new ones. However, when manufacturers disassemble and recycle parts, MET points are recovered and the score reduced. Eliminating the need to reproduce the same materials or manufacture more of the same parts lessens the negative environmental effects.
The DFE software uses the results of DFA analysis to establish a disassembly procedure. The program then analyzes the materials, processes, and intended end-of-life destination of parts. The software optimizes the disassembly sequence for the most rapid profit rates or for the quickest MET score recovery rate.
For instance, DFE analysis of the current IP design resulted in an estimated disassembly time of 1,560 sec for recycling and disposal. With a burdened labor rate of only $15/hr, the results indicated that further disassembly beyond 894 sec wouldn’t be economical. A maximum profit of $3.74 could be obtained per IP, mainly from recovery of polycarbonate and copper. If workers completely disassembled the IP, net profit would fall to $2.00. The highest net profit rate after 894 sec is $15.2/hr, at which point recycling cuts the baseline MET score of –33.7 — corresponding to simply discarding the IP — to –6.7 MET points, where a negative number represents adverse effects.
A DFE analysis of the proposed IP design showed that it would be easier to disassemble. The total disassembly time for the new design was estimated to be 260 sec. However, all of the material capable of being recycled could be recovered in 195 sec, at which point the bulk of the PC/ABS moldings are separated for recycling.
In the redesigned IP, the main structural moldings, parts of the cup-holder assembly, the glove compartment, the ashtray cover, and the air-deflector assemblies are all molded from PC/ABS resins and therefore can be removed as a single unit and recycled without disassembly. With more engineering thermoplastics in the design, the potential net profit from material recovery is $6.39/IP, almost twice that for the current design. This profit assumes a burdened labor for disassembly of $15/hr, and PC/ABS blend resins are estimated to have a recycling value of $0.18/lb, which is 15% of the cost of virgin resin for large-volume purchases.
The profit rate from disassembly of the proposed redesign is eight times higher than for the current design. With the new design, it is feasible to consider complete disassembly since it would only take 50 sec more after separating the main structural moldings. At this point, the MET score would decrease from –29.7 points if the IP was simply discarded, to only –2.4 points when the trim molding has been recovered.