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issue: November 2003 APPLIANCE Magazine

Buyers' Guide to Plastics
Bulk Molding Compound: A Superior and Affordable Composite for the Appliance Design Engineer


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Appliance components requiring heat resistance with pleasing cosmetic appearance qualities have traditionally been manufactured using metal, coated substrates, or thermoplastics.

Components produced employing engineering thermoplastic have provided the appliance industry with specified color and heat-resistant properties while eliminating scrap generated by secondary painting operations. Measurable gains in manufacturing efficiency and profit have been achieved through the elimination of these secondary operations. Although thermoplastics have provided the properties necessary for use at elevated temperatures, they can often be expensive. These costs may nullify potential gains in profitability.


An example of a product manufactured using BMC.
Bulk Molding Compound (BMC) is a polyester-based thermosetting composite that is both injection and compression moldable. BMC has traditionally been molded into components requiring significant electrical, structural, corrosion-resistant, and/or heat-resistant properties. Successful applications have long been restricted to the non-visual or "working" areas of appliances such as brush holders, armature sleeves, and motor housings (rated UL VO-5V). Electrical (circuit breakers) and automotive (valve covers and headlamps) "work horse" applications represent typical BMC uses.

BMC is a moldable product, allowing for complex configurations, part consolidation, and greater design freedom. Through the use of traditional raw materials and new chemical additives, high-quality surface values are possible with BMC. Improved mold designs used in conjunction with superior molding equipment deliver the full aesthetic potential of these new materials to the end-use application. The inherent performance qualities and low price associated with these materials can be specified with deep gloss, rich color, and custom appearances as additional features.

This composite is currently specified by major appliance makers in components such as vent trims, range and refrigerator handles, control panels, and door skins. Core electric applications range from toaster oven end panels and door handles to iron sole plate covers, fryer bases, and lids. HVAC uses include humidifier housings and A-coil drip pans.

Surface Quality: Stain, Chemical, and Creep Resistance

Selecting a material for use at elevated temperatures requires careful consideration. Load bearing properties as a function of time and temperature are key criterion. Aesthetic requirements compound the complexity of material selection, as color consistency, color retention at heat, gloss factor, and stain resistance may also be critical.

Specific heat intensities will compromise the physical properties of all materials resulting in "creep," relaxation, and, finally, part failure. Potentially, the most outstanding performance characteristic offered by these materials is creep resistance.

Concerns regarding aesthetic durability in heated environments are not limited to the warp and relaxation attributed to creep. The retention of color is another critical concern for the heated appliance application. A bright white appliance must maintain its color and gloss when exposed to the various temperatures and stain potentials of the cooking milieu. Food, grease, condiments, and cleaning chemicals are all potential attackers to the aesthetic appearance of a kitchen appliance. BMC is one of the only "uncoated" plastics to date that is stain resistant, dimensionally stable, and chemically "clean" enough to be used in a food-contact environment. In addition, it maintains its color up to 500ûF (260ûC).

Technological Advancements - A Stainless Steel Appearance

Once reserved exclusively for restaurateurs and decorators with unusual means, major appliances clad in stainless steel have long represented the pinnacle of quality, durability, and price. Stainless steel appliances have traditionally been manufactured by companies focused more on industrial food service than retail sales venues. Over the past few years, however, there has been a growing retail niche for stainless steel that major appliance marketing teams have begun to cultivate successfully.

The extensive and highly successful history of BMC in automotive headlamps, in combination with BMC's growing penetration of the appliance market, led to an effective and low-cost alternative to stainless steel. Headlamps molded of these materials require several operations secondary to the injection molding process. The smooth, glossy substrate is transferred to a metallization station and coated with a thin layer of metal (most commonly aluminum). The thin metal surface obtains a mirror-like appearance with this coating and is then "top-coated" to protect the metallic deposition.

This same process is utilized to achieve the "Stainless" look. The mold is textured with a true stainless steel appearance. The parts from this textured mold are then vacuum metallized and finished with a highly durable clear coat. Gloss levels offered by the top coat can be customized to meet the specifications of the application. Additionally, a wide variety of metallic materials can be used, including bronze, brass, copper, and stainless.

Cost-comparative research between stainless steel and BMC composite and secondary processes showed a savings of approximately 40 percent, according to the technical paper, Stainless BMC: A Polymer Alternative to Stainless Steel. As the appearance of stainless steel becomes more popular in today's retail venues, major appliance OEMs will press the limits of the point-of-purchase debate: price versus value. The responsibility OEMs share is to deliver competitive quality while maintaining margin. BMC offers a viable alternative to the stainless clad market.

Technological Advancements - Weight Reduction Equals Cost Savings

Gas Assist Injection Molding (GAIM) has been labeled one of the most significant manufacturing advancements of the past 40 years. Through the manipulation of injected polymer and pressurized gas, innovators have improved plastic part quality while reducing processing and material costs.

GAIM has been practiced almost exclusively by the manufacturers of thermoplastic components. Recent technological breakthroughs have made a process similar to GAIM available with these composites. Gas Evacuation Technique (GET) allows the design engineer to reap the cost saving benefits of GAIM while enjoying the excellent properties and low costs associated with BMC.

GET with these composites offers molders and OEMs a reduction in cycle time and material. A 3-to-1 gain in part yield is realized, and a material savings of 40 percent, according to the technical paper, Thermosets Are in the GAIM: Gas Injection Molding with BMC.

Conclusion

BMC offers properties that are requisite for appliance applications. Recognized for its durability in harsh environments, BMC has a reputation for being able to "take the heat" and is no stranger to the appliance industry. The manufacture of cosmetic parts using these materials is gaining in popularity. Glossy, deeply pigmented components can be specified and productively manufactured by using modern formulas, mold designs, and process technologies. BMC offers an outstanding price-to-property ratio when compared to competitive thermoplastic and coated substrate alternatives.

This information is provided by Len Nunnery and Shelane Blevins of Bulk Molding Compounds, Inc. (West Chicago, IL, U.S.).

 

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