Cooking Up Cash Savings

From your crisply ironed shirt and golden brown toast to your ice-cold beverage and juicy grilled burger, your day is full of good things brought to you by molded thermoset appliance parts.

Thanks to their unique properties, such as flexibility, durability, and low weight, thermoset materials are turning up in appliance applications previously deemed worthy of metals — and metals only. So the next time you butter your toast or bite into a burger take a closer look at your appliances. You’ll likely find thermosets carrying a hefty part of the load.

Driven by factors such as cost, aesthetics, and dimensional and thermal stability, design engineers are replacing stainless steel and aluminum appliance parts with parts made of bulk molding compounds (BMCs), phenolics, and other thermoset materials. The frigid temperatures of a Frigidaire or the extreme heat of a self-cleaning oven are no threat to the strength of these thermosets. Because they are crosslinked, thermosets create parts that are rigid in construction, dimensionally stable, and creep resistant, explains Gary Littell, applications development engineer at IDI Composites International.

Thermoset molding compounds, with resins such as polyester, vinyl ester, phenolic, or epoxy at their core, are crosslinked during the molding process. Richard Faulk, manager of new business development at Bulk Molding Compounds Inc. (BMCI), explains that the pressure and 280° to 350°F temperature of a die tool causes the compound to exothermically react, crosslinking the compound into an infusible solid state.

Although thermosets range in type from epoxies and melamine to sheet molding compounds (SMCs), those most commonly used in appliances are phenolics and polyester-based BMCs. BMCs are used in applications such as iron skirts, refrigerator handles, and range handles, while phenolics can be found in range strips, toaster end panels, coffee maker warmer rings, pot and pan handles, and blender motor housings, says Tom Wilczewski, manager of technical services at Plastics Engineering Co.

Fillers and glass reinforcements are used to increase the strength of thermoset molding compounds and allow the finished part to better withstand the impact, load, and other environmental conditions.

Anyway You Slice It
Harsh environmental conditions are exactly what a BMC is facing in Berkel Company’s X13A automatic food slicer. The material is exposed to sanitizers and cleaners as well as acids and proteins found in food. The single-molded base, deemed the “drip and drop zone” of the slicer, is constructed of IDI’s custom BMC.

As defined by IDI, “BMC is a thermoset plastic resin blend of various inert fillers, fiber reinforcement, catalysts, stabilizers, and pigments that form a viscous, ‘puttylike’ compound. BMC is highly filled and reinforced with short fibers. Glass reinforcement represents between 10 and 30 percent, with glass length typically between 1/32 and 1/2 in.”

Berkel traditionally relied on aluminum or stainless steel for the base of its slicers. However, when designing its XGA line, the company wanted to create a base that was sanitary and easier to clean. “We wanted to eliminate harborage points for germs,” says Todd Clemm, a Berkel product manager.

BMC has proven to be up to the task. Approved by the FDA and the NSF, BMC resists stains, scratches, chemicals, and sanitizers and is UL94-5 V flame resistant. Furthermore, BMC has allowed Berkel to incorporate a base that is simply one part, rather than a conglomeration of multiple parts welded together.
This reduction in parts as well as a reduction in weight means lower costs and simpler assembly for Berkel and improved quality for the company’s customers. “The part has basically cut cleaning time in half for our end users,” says Clemm.

Rose Polymer, the company charged with molding the slicer’s base, suggested BMC as a viable alternative to metal. Owner Bob Uhren sees increased opportunity for BMC in the appliance industry thanks to factors such as the rising cost of metals. “Aluminum and stainless steel are affected by energy costs,” Uhren notes. He also points to the design freedom and improved aesthetics offered by thermosets. BMCs can be molded to a unique net shape, giving design engineers the ability to mold in contours and virtually eliminate design constraints. “What BMCs give us is a test of our imaginations,” Uhren says.

Ready, Set, Mold
While the base for the Berkel food slicer is compression molded, appliance parts can also be transfer or injection molded. Shape, size, function, quantity, and application environment determine which of the three molding options is right for each application, says Uhren. However, he adds, there is typically a 50/50 split between compression and injection molding within the appliance industry.

“Compression molding consists of a heated matched die tool in which a BMC charge or charges are manually placed inside the tool and the tool is closed,” Faulk explains. “While the two halves of the tool are being compressed, the heat and pressure will allow the BMC to fill out the part and cure into the desired net shape.”

According to Littell, compression molding allows engineers to better manipulate the glass filler within the thermoset compound. The percentage of glass, Littell says, can be increased and glass lengths maintained — even at lengths from 1/8 to 2 in.

An unfortunate misconception within the appliance industry is that thermoset appliance parts do not lend themselves to injection molding. Faulk and others are quick to discredit the idea. “Sixty percent of all the materials we supply are injection molded,” Faulk says.

“Injection molding can usually provide for a more automated and efficient molding process,” he adds. Hydraulic pressure forces a predetermined volume of BMC to be delivered through runner and gate systems into a heated tool until the cavity or cavities are filled. “Again, heat and pressure cause the BMC to cure into the desired net shape.”

Wilczewski agrees that thermosets are suitable for contemporary molding processes, but notes that injection molding may not be the best fit for all appliance applications. “In some applications, a scar from a gate is unacceptable.” If the scar can’t be hidden in such a case, another molding option, such as compression molding, would be appropriate. “Advancements in compression molding, such as robotics,” he adds, “have made the process less labor intensive than it used to be.”

BMCI has developed new technology for injection molding appliance parts, such as oven and refrigerator handles. As defined by BMCI, Gas Evacuation Technology (GET) uses pressurized gas to force low-viscosity BMC back into the barrel of a molding machine. By creating this internal flow reversal, the material is reclaimed from the cored part and the polymer is positioned in front of the injection screw check ring to be used as a percentage of the next molded shot.

GET is said to reduce part weight by up to 50 percent and cycle times by up to 40 percent. “This unique GET process has provided for a substantial cost savings,” Faulk notes, “while maintaining the superior gloss appearance and inherent rigid quality feel associated with bulk molding compounds.”

Molding to net shape eliminates machining and other secondary operations associated with appliance parts. With phenolics, for example, threaded holes can be molded into the part. With BMCs, threaded inserts can be incorporated. “Thermosets basically get rid of all the ‘ings’ of secondary operations, including machining, drilling, and washing,” Wilczewski says. By eliminating these secondary operations, thermosets allow design engineers to cut costs as well as cycle time.