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February 1, 2000

8 Min Read
Effective plastics design for durable goods

Editor’s note: Robert J. Cleereman, global director of the Materials Engineering Center at Dow Chemical, recently addressed an audience of designers at the Appliance Show in Nashville, TN. His message? How to create customer-preferred, innovative, and cost-effective appliance designs based on the intelligent use of plastics. Portions of his discussion, excerpted here, challenge the status quo among designers of durable goods.

Today, the use of plastics as engineering materials in durable goods markets remains minimal. In fact, less than 3 percent of all durable goods are plastics-based. Despite their potential to create superior solutions that meet market needs, these materials still face several obstacles to achieving widespread acceptance. Several pioneering projects, however, have shown how plastics can be used alone and in combination with traditional materials to bring costs down and add product function. The key for designers is to understand their options from a more complete perspective so that they can develop alternative solutions based on plastic’s strengths.

Reasons for the perception about plastics in the appliance industry are numerous, but the most compelling rationale involves the structural performance of plastics. Specifically, most appliance designs (and many others as well) are based on modulus of elasticity, or stiffness.

While plastics compete well with metals on a tensile strength basis, they are not as attractive from a stiffness point of view. For example, 30 percent glass-filled PP gives 1000 kpsi modulus per dollar, while cold-rolled 4340 steel offers 7000 kpsi for the same dollar. Add to this the fact that designers are more familiar with steel, and you see why the motivation for switching to plastic dwindles.

On the other hand, plastic-based durable goods designs can support significant loads, be more economical, and demonstrate inherently higher quality than traditional-material-based products. Plastics can do this because they can be fabricated in complex net-shape or near-net-shape geometries that eliminate or reduce secondary operations. This quality, dubbed fabrication functional performance, refers to the free-form nature of plastics, one that is foreign to steel, aluminum, and wood.

Classic System
Durable goods are generally complex because they exist to provide features that accomplish some functional purpose. When the product is based on traditional materials achieving this purpose normally requires elaborate assemblies. That’s because these materials are available only in flat or constant shape geometries. Therefore, constructing complex products from these very simple forms dictates a multitude of pieces and parts.

Picture nearly any durable goods product (Figure 1). Basically, it consists of a base upon which everything that makes it function is mounted. Achieving the proper spatial arrangement of these parts requires complex use of brackets and fasteners. Once the assembly is complete, a cover or enclosure is needed to hide all the ugly brackets and fasteners.

The end result of this type of design is a large number of pieces, parts, and finishing costs. Figure 2 depicts the manufacturing cost breakdown for typical, high-production durable goods. Obviously, the raw material and reform (stamping) steps are not very expensive, but costs escalate when it’s time to put the pieces together and make them look good and perform properly.

Technology has come a long way with respect to mass production, but all the efforts and investment since Eli Whitney invented the process have been based on improving this last step—namely, putting pieces together. Raw material cost is not an issue when you can still get a pound of 30,000-kpsi modulus steel for 25 to 35 cents.

When designers try to substitute plastics in this system, using it as a pseudo-traditional material, they are bound to fail. In other words, substituting plastics directly into traditional-material-based designs actually raises the manufacturing cost (Figure 3). This is due to the higher cost per stiffness issue discussed early. Note that this is part-for-part substitution, not new system solution thinking.

Application Solutions
It is clear that plastics cannot compete unless their intrinsic advantages are used to offset the stiffness disadvantage. To do this, designers need to examine the durable good product’s function and create a design solution that addresses these functional requirements without constraints on form. This change in thinking allows fabrication functional performance to exert its leverage, which can then create lower-cost products with superior performance (Figure 4).

Reducing manufacturing cost by using materials that cost more on both a per pound and per stiffness basis is the result of creating designs as application solutions based on the advantages of plastics. The reality of this statement has been demonstrated many times over in the past 10 years.

For example, the John Deere Stealth rear engine riding mower used engineering polymers that cost more than $2/lb. Yet the end cost of making the mower was lower than the steel version, which had been assembled at a state-of-the-art manufacturing facility. Why? Because the plastic redesign consolidated 153 pieces down to three and eliminated secondary operations.

Using the same systems solution approach, Dodge redesigned a passive restraint instrument panel for its 1997 Dakota truck. The plastic-based design eliminated dozens of parts, reduced costs, and gave a higher structural performance in terms of femur loading during crash. In addition, the plastic IP is torsionally stiffer than its steel-based predecessor, quieter on a noise, vibration, and handling basis, and is perceived as higher quality by passengers.

Appliance of the Future
What could the next appliance solution look like? Let’s examine the question from the perspective of what we would like to do today that we can’t because it costs too much. For a refrigerator, that would be a large-capacity, highly energy-efficient model that takes up the same floor space as today’s lower capacity and efficiency versions. How can plastics help achieve this solution?

High capacity equates to a small volume of insulation. Conversely, high efficiency equates to extensive insulation. To avoid the trade-off, one answer would be vacuum panels, thin but highly efficient insulators. The problem is that the cost would be high if these panels are used in today’s complicated refrigerator liner designs. If we could move to a simple liner, however, we could make use of the vacuum panels more cost-effectively. Using plastics’ advantages, we can change the liner design.

In a typical cost breakdown for a $1000 side-by-side refrigerator, roughly 30 pieces and parts account for nearly half the cost of manufacture. Replacing these with a comprehensive molded-plastic subassembly would eliminate the need for a complex liner and give us the solution we need.

Using plastics’ part-consolidation advantage, we can mold one module to replace many of these parts, and then use the vacuum panels in the simplified liner. Product identification would then occur at the end of the production process to allow for mixed model manufacture.

Challenges Ahead
A few obstacles to this vision of the future remain. One of the primary challenges is that the design cycle time for plastics-based product designs appears significantly longer than traditional-material-based designs. One answer: change the design/development process to fit the material. Replace design-build-test with do-it-right-the-first-time. Using an application solutions approach, designers can evaluate all options before selecting the best one.

In terms of design cycles, durable goods designers typically follow a design-build-test sequence that includes many prototyping test iterations. It works well with traditional materials because prototyping is quick and low cost, with mock-ups produced in the shop with a band saw, welder, and drill press. These prototypes can be tested for functionality, kinematics, and stress-strain issues, and problems are addressed by sawing the offending design apart and rebuilding the prototype.

Even with traditional materials, this process is less efficient on a speed-to-market basis than many realize. But the situation is exacerbated when plastics are simply substituted, because prototyping a plastic part is not quick or cheap. Prototype tooling, for example, can take 12 weeks or more to make and can cost from 30 to 80 percent as much as production tools. Moreover, when product design and engineering efforts are curtailed to fit a build schedule, insufficient testing leads to plastic parts that perform poorly. More time is wasted in corrective action, and the cost of the after-the-fact fix-up leaves everyone with a poor taste for plastics. Worse yet, the lack of engineering time results in field failures. Remember, however, that this scenario refers to simple plastic substitutions.

How can designers execute the design-build-test process if they cannot build cheap, timely plastic prototypes? One answer: change the design/development process to fit the material. Replace design-build-test with do-it-right-the-first-time. Using an application solutions approach, designers can evaluate all options before selecting the optimum one. It may sound obvious, but it rarely happens, because one part of the solution is locked in before considering other factors. For example, if the fabrication method is assumed, that decision locks out many options.

Keeping solutions open means evaluating concepts using sound engineering to identify fabrication and materials combinations early and often. Feasibility studies on several option combinations can often quickly target the one or two best concepts, which can then be engineered in more detail. Once this phase is complete, rapid prototyping can be used to verify functionality, kinematics, ergonomics, and customer issues. Mock-ups can suffice for focus studies and marketing efforts.

Other engineering testing, such as stress-strain, dynamic impact, and vibration, can be prototyped virtually in the computer. Designers need adequate data on product performance specs, design, fabrication techniques, and raw material properties. While virtual testing doesn’t eliminate the need for final product testing, it does give greater confidence in the engineering phase.

Contact information
Dow Chemical
Robert Cleereman
Midland, MI
Phone: (517) 636-5425
Fax: (517) 636-3926
Web: www.dow.com
E-mail: [email protected]

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