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issue: October 2009 APPLIANCE Magazine

APPLIANCE Engineer - Technology Report
Engineering a Cost-Saving Washer Tub

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Paul Roggema, Europe correspondent

Shifting structural loads and changing to a different material grade result in a washer tub that is less expensive to produce.

The tub was tested for a 50-L, 1400-rpm unit, and tested for 4700 cycles.

Does a $2.50 saving on a washer tub really make a difference?

Saving money was one of the goals when engineers from Italian engineering and moldmaking company Meccanica Generale (MG) began a €1.8 million, two-year project to develop a new generation of washer tub.

Already a strong player in horizontal-drum washer tub engineering, the company began with the notion that the current design leaves little room for improvement in material use and production cycle time. MG came up with the idea to separate the water-containing and the structural functions of the tub, thus allowing for more accurate dimensioning.

Three-Section Design

Instead of the main back section and the front section, the new design is composed of three parts. In addition to the front and back tub is a bottom plate, which carries the bearing support and the structure. By switching the weight of the drum to the bottom plate, the weight of the back and front drum can be reduced—which gives designers a great deal of freedom to change the design.

There is also a new system for attaching the counterweights that is engineered to be safer and more cost-effective.

To some, the concept might be counterintuitive: saving resources by designing-in an extra part. But it begins to make sense when explained by Stefano Mancini, MG’s director of R&D and inventor of the new concept.

“We took an unusual step. One of the changes is to use a glass fiber–filled material for the full range, instead of calcium carbonate–filled PP (polypropylene),” Mancini says. “Glass fiber might be more expensive, but measured in strength units, it is more cost-effective. Second, the total tub weight is approximately 40% lower.”

Creating a box section between the tub and the back plate, Mancini says, results in a design that resembles the iron cross construction used in stainless-steel tubs. “The advantages are significant: on a typical 1200-rpm tub you normally need 6 kg of PP with calcium carbonate, but our design only uses 3.5 kg of the 30% glass fiber PP.”

The standard plastic is usually calcium carbonate PP that costs about €0.5 per kilo less than glass fiber PP. But measured in strength units, glass fiber costs only 67% of the cost of calcium carbonate PP.

The tubs also save 40% in material processing and 15% in molding time. Looking at the savings on a mass-production scale, there are notable environmental advantages: The energy saved to process 500,000 parts is equivalent to 1261 barrels of crude oil; the reduction in material weight for the same quantity of parts saves 39,000 tons of water and reduces CO2 emissions by 1400 tons.

A bottom plate handles bearing support and allows more design flexibility for other tub sections.

Joining Techniques Provide Speed and Savings

“The structures are attached using vibration welding,” Mancini tells APPLIANCE magazine. “Welding only takes 20 seconds. We also designed a new method for attaching the counter weights, using special springs, achieving a 50% reduction in fixing costs. Applied together, our ideas can shave €2 to €3 off of the typical tub costs of around €10.”

It only takes a little math to see how significant such savings can be. “If you make 3 million to 5 million washers a year, and your net profit is 100 million, the cost reduction is very interesting,” Mancini says. Particularly, he adds, when one considers that the payback time is about one year.

Advantages can extend to the performance of the appliances itself. “A lighter tub also affects the energy use of the actual wash cycle,” Mancini says. “There is less material to be heated; the end user saves up to 8%.”

The tub was tested for a 50-L, 1400-rpm unit, and tested for 4700 cycles. Future development may include integrating components such as pump, recirculation, or air trap.

Mr. Montevecchio, MG’s director of business development, says the design project was completely self-initiated. No specific customer was a partner in the development of the new tub. However, the company already works with most European appliance OEMs and several are evaluating the new tub.



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