Attachment anxiety

May 23, 2002
High-tech components used in appliances put more demands on adhesives and sealants.

By Christine Salerni, Sean McGrath
Loctite Corp.
Rocky Hill, Conn.

Edited by Sherri Koucky

In clothes dryers, light-cure acrylics are used in motor assemblies for flange bonding and to coat and seal the laminate.

New washing machines use toughened anaerobic threadlockers to withstand the increased temperatures and vibrations from high-performance motors and pumps. Also, cyanoacrylates bond switches on the control panel and affix identification tags to motor housings.

On refrigerators, cyanoacrylates bond door handles, shelves, and touch-up switches, and attach identification tags on compressor motors. Cyanoacrylates also find use as instant adhesives in glassto-glass bonding of large shelf assemblies inside the refrigerator.

Advances in appliance technology are bringing new roles for adhesives. Traditional uses still abound. For example, adhesives find widespread use in appliance-manufacturing applications for structural bonding, cylindrical assembly, threadlocking, flange and thread sealing, thermal management, wire bonding and harnessing, gasketing, and sealing. Adhesives can bond dissimilar materials, reduce component inventory, and decrease total manufacturing costs, making possible many appliance designs that would be impossible or cost-prohibitive using mechanical-fastening methods.

New uses are coming about because appliances are incorporating more high-performance power components, such as motors and more electronics. Also, pressures to cut production time are encouraging manufacturers to deploy adhesives more frequently.

New washing machines use small, high-performance motors and pumps that run at high speeds, producing elevated temperatures and increasing vibrational shock. While traditional threadlockers can withstand some level of vibration, the combination of heat and continuous vibration requires using toughened anaerobic threadlockers that retain commutators and bearings to the motor shaft, secure bolts and screws, and seal the metal gear pin to the transmission housing. These "toughened" threadlockers resist shock and vibration by maintaining a certain level of flexibility.

Anaerobic adhesives cure at room temperature when deprived of oxygen in the presence of metal ions. Offering high-shear strength, good solvent and temperature resistance, rapid cure, and easy dispensing, these adhesives are commonly used as threadlockers and thread sealants to minimize loosening of threaded-metal fastener assemblies, as well as retaining compounds and flange sealants.

Cyanoacrylates are used for general-purpose bonding of switches located on the control panel, to attach rubber pads to the washer's base legs and bumpers to the lid, to bond brake pads to brake shoes, and to affix identification tags to motor housings. In the tumbler drum, cyanoacrylates bond felt to the air seals, and hot melts bond the liner to the drum.

Cyanoacrylates or instant adhesives cure rapidly at room temperature when pressed into a thin film between two surfaces. Available in a variety of specialty formulations and viscosities, these adhesives are solvent-free, easy to dispense using automated systems, and adhere to most substrates, even after thousands of hours of exposure to temperatures as high as 250?F. Cyanoacrylates are commonly used for bonding identification tags and labels on large appliances and for applications involving plastic-to-plastic and plastic-to-metal bonding, for example, when attaching door handles, drawer pulls, and similar devices.

Silicones are primarily used in transmissions for gasketing and flange-sealing applications. Epoxies retain the motor's seat to the shaft, and acrylics bond motor magnets to the housing.

Silicones are flexible, rubberlike materials that cure at room temperature and are resistant to heat and moisture. These materials bond to a wide variety of substrates, including glass. Offering exceptional chemical/electrical resistance and good gap-filling capabilities, they are naturally suited to gasketing, sealing, and caulking applications, and are frequently used to seal stove tops and dryer doors, and act as liquid-gasketing materials on motors in appliances.

Light-cure adhesives come in acrylic, silicone, and cyanoacrylate formulas, and cure upon exposure to the proper wavelength of UV or visible light. These adhesives provide unlimited time to align and adjust parts, and adapt easily into automated production lines. Available with a wide range of physical properties, light-cure acrylics offer good environmental resistance, excellent gap-filling properties, clear bondlines for improved aesthetics, and good thermal and environmental resistance. However, acrylic formulations do not cure in shadowed areas or through opaque materials. Light-cure cyanoacrylates are dual-cure adhesives that fixture instantly when exposed to light, yet cure in shadowed areas at room temperature due to the secondary moisture cure mechanism. Similar to cyanoacrylates, light-cure silicones use light as the primary cure mechanism and moisture as the secondary cure for shadowed areas. Ideal for highvolume bonding applications, light-cure adhesives are commonly used for bonding LED, clock, and timer displays where rapid fixturing and cure is required.

Epoxies are commonly used as structural adhesives and offer high-bond strength, good adhesion to substrates, excellent gap-filling capabilities, superior electrical properties, and good temperature and solvent resistance. Curing by heat exposure or chemical reaction, these adhesives are commonly used in small-appliance applications, for example, bonding flex hoses on to vacuum cleaners or attaching plastic or paper filters to metal components in air purifiers and air conditioners.

Most clothes-dryer adhesive applications are similar to those of washing machines, with a few exceptions. Light-cure acrylics are used in the motor assembly for flange bonding and to coat and seal the laminate.

Adhesives range from simple, natural compounds to complex chemical formulations. The most common adhesives used in appliance manufacturing are hot melts, cyanoacrylates or instant adhesives, anaerobics, lightcure adhesives, silicones, and epoxies.

Both traditional and new high-performance hot-melt adhesives are solid at room temperature but turn to liquid when exposed to heat. Low cost, clean, and versatile, these adhesives can be easily integrated into high-speed automated manufacturing processes. Hot melts are excellent gap fillers and bond to a wide variety of porous and nonporous materials, including difficult-to-bond plastics. These hot-melt adhesives are not traditional EVA chemistry, but rather polyolifin and polyamide chemistry which make them high performance. Solventfree and environmentally friendly, high-performance hot-melt formulations withstand severe operating environments including elevated temperature and high-flexibility applications. Hot melts are commonly used to affix insulation in refrigeration systems, tack wire harnesses, fasten labels and tags, and attach plastic hoses to inlets and felt to dryer drums, as well as for general-purpose bonding and potting.

Refrigerator manufacturers rely upon anaerobic adhesives primarily in motor assemblies, where the adhesives function as threadlockers to secure bolts and screws, thread sealants that protect against harsh chemical refrigerants, and retaining compounds that bond commutators. Threadlockers are also used on threaded assemblies located throughout the appliance.

Silicones are used to seal the door gaskets and for a variety of gasketing applications. Hot melts seal the bottom edges of the door assembly. Acrylic adhesives are used for motormagnet bonding. Light-cure adhesives bond plastic and metal trim to glass shelves.

The wide variety of available adhesives makes it difficult to choose the right one for an application. Beyond the nature of the application itself, important selection considerations include the end-use environment, substrates involved, and stresses encountered during manufacturing and equipment operation (e.g., exposure to heat, humidity, solvents, and ultraviolet light). For appliance manufacturers, automated production is critical, and adhesives can also be selected on how easily they integrate into the manufacturing process.

Adhesives have significant benefits over mechanical and thermal-fastening methods. Rather than concentrating stress at a single point, adhesives distribute stress load over a broad area, resulting in more even distribution. A joint bonded with adhesive better resists flex and vibration stresses than, for example, a riveted joint. Adhesives form a seal as well as a bond, eliminating corrosion that often occurs in a mechanically fastened joint. They join irregularly shaped surfaces more easily than mechanical and thermal fastening, increase the weight of an assembly negligibly, create virtually no change in part dimensions or geometry, and quickly and easily bond dissimilar substrates and heat-sensitive materials.

Limitations include setting and curing time (the amount of time it takes for the adhesive to fixture and strengthen fully), surface-preparation requirements, and the potential need for joint disassembly.

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