Flexible, heat-resistant, one-part RTV silicones have traditionally been used to bond smooth glass or porcelain stovetops to metal stove frames. As a stove heats and cools, its components expand and contract at different rates, causing stress on adhesive joints.  Silicones inherently exhibit very high elongation, which allows them to absorb this stress without failing cohesively or cracking the glass.

Because silicones are based on the inorganic bond of silicon-to-oxygen, they can withstand much higher temperatures than epoxy or urethane adhesives that are based on organic carbon-to-carbon bonds. Throughout the life of a stove, adhesives must survive thousands of hours at elevated temperatures without degrading, a demand easily achieved by silicones.

Although one-part RTV silicone adhesives offer superior flexibility and heat resistance, their curing mechanism has long constrained manufacturers of smoothtop or glass/ceramic cooking ranges, negatively affecting both the cost and efficiency of the manufacturing process. RTV silicones require ambient moisture and significant cure time to develop the strength required to manipulate stovetops to apply the burners to the assembly.

Enlarge this picture Fig. 1. Cure time vs. strength for one-part RTV and new two-part silicones.

During the curing process, stovetops are typically racked from four to 24 hours in high humidity environments, a requirement that results in large quantities of work in process (WIP). This build up of WIP to allow sufficient curing extends the manufacturing cycle and prevents the next assembly process from beginning. To meet the moisture requirements of RTV silicones, manufacturers invest in costly humidity chambers and devote substantial floor space for stove top racking. Lower ambient humidity due to seasonal variations can compound manufacturing complexity further by extending required racking times.

There are several other drawbacks to RTV silicones. These adhesives emit strong acetic acid (vinegar) vapors that corrode electronics and can be offensive to line workers. The maximum UL temperature rating of these materials is 140 DegC with a maximum cure depth of 3/8-in., factors that limit their usefulness in a variety of appliance assembly applications.

Two-part silicones

Loctite 5600 Silicone Sealant is applied using Baron’s robotic/automated assembly equipment onto a glass stovetop just prior to assembly of the frame.

Recently, new, highly engineered two-part silicones have been developed specifically for the appliance market to overcome the problems associated with RTV silicones. Fig. 1 illustrates a typical strength-vs.-cure-time curve for an RTV silicone and the two-part technology. The graph highlights the superior strength/cure time performance of two-part silicones.

The two-part silicones offer a number of benefits over RTV silicones. These products skin over in 5 minutes or less, develop sufficient bond strength to allow burner assembly in 3 to 5 minutes, and are ready for packaging in 5 to 10 minutes. Racking is not required, and parts can be continuously processed without costly WIP. As the initial curing process does not require moisture, the expense and floor space requirements related to humidity chambers are eliminated. In addition, no offensive or corrosive odors are emitted.

New two-part silicones provide high adhesion and excellent bond strength on glass, metals, and other materials, and can be used for bonding, gasketing, potting and sealing. These adhesives carry a UL temperature rating of 180 DegC, an improvement of 40 DegC over the temperature ratings of RTV silicones. The higher temperature rating allows two-part silicones to be used in range/oven applications that historically required welding   or mechanical fasteners. The shelf life of the two-part silicones exceeds one year, and bond strength is greatly improved as a result of added adhesion promoters.  Table 1 highlights the major benefits of a two-part silicone vs. an RTV silicone.

Cost savings

Enlarge this picture Table 1. Comparison of properties between RTV and new two-part silicones.

Although every appliance manufacturer is unique, and costs vary from plant to plant and region to region, significant cost savings can be achieved by switching from RTV silicone to the fast-curing two-part silicones. Table 2 illustrates typical cost savings achieved with the two-part silicone technology.

The savings can be attributed to a number of areas:

• Labor: An RTV silicone line typically requires two workers to transport assembled stovetops onto racks, while two-part silicones require no labor as parts are processed on a slow conveyor for 5 to 10 minutes before proceeding directly to burner assembly. Based on typical, fully burdened labor rates of $30,000 per year, immediate cost savings of $60,000 can be realized using the two-part silicones.
   
• Production time/work-in-process: Cost-per-minute of finished goods varies with the number and type of units manufactured, the average cost of all units produced, and overhead. In a high-volume facility that produces 750,000 smoothtop ranges annually, the WIP value is estimated at $22,500. A facility producing 250,000 units annually has an estimated WIP value of $9,000.
  
• Floor space: Parts racking can consume as much as 2,000 square feet of floor space for RTV silicones.  Using a floor space cost of $10/ft2 per year, converting from an RTV silicone to new two-part silicones can deliver $20,000 per year in savings.
   
• Capital equipment: Starting from scratch, both silicone technologies require a one-time capital equipment investment. The RTV silicone will require at least one 55-gallon drum un-loader, header pipe and multiple dispense guns. Average cost would be approximately $15,000, depending upon complexity. The two-part silicone system will need two drum un-loaders and metering equipment that costs approximately $50,000. Assuming the existing drum un-loader can be utilized, the switch from an RTV manufacturing process to the two-part silicone requires a capital investment of $35,000. The payback period for the equipment required would be less than 5 months, as shown in Table 2.

By using the two-part silicones, appliance manufacturers also eliminate the energy and equipment maintenance costs associated with the humidity chambers needed to cure one-part RTV silicones. This benefit further supports the economics of a switching to the new adhesive technology.

Other applications

Enlarge this picture Table 2.  Cost savings with new two-part silicone in high-volume and medium-volume stovetop production.

New, two-part silicone adhesives are also being used in a variety of other high-temperature, appliance-assembly applications. Two-part silicones are replacing heat-cure silicones in bonding mounting brackets onto glass front dishwashers. In this application, manufactures have eliminated the energy costs associated with cure ovens and the work-in-process involved with post-cure cooling.

This new class of silicone can also be used to replace the welds that mount rails on the underside of stainless steel gas ranges. Welding equipment is expensive to maintain, and the process can cause surface cosmetic damage that is costly and labor intensive to remove from the range top. By replacing welds with two-part silicones, manufacturers can realize significant cost savings.

The extremely fast fixture speed and high temperature limits of new two-part silicones make them an excellent alternative to double-sided tape in oven-door assembly applications. These new silicones are also being used on slide-in/drop-in ranges to bond smooth-top glass to aluminized rails and enameled burner boxes, significantly decreasing WIP.

Case history
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New two-part silicones from Henkel Corp. have become an integral production component at Baron Systems & Solutions, a division of Schott North America. Baron is a leading contract manufacturer of appliance subassemblies such as glass range cooktops, oven doors, and control panels. Baron, whose appliance industry experience spans 15 years, was looking to replace the slow, labor-intensive RTV silicone technology it had been using with materials that offered better process speeds and lower costs.

Representatives at Baron evaluated numerous materials and eventually settled on Loctite“ 5600™ Silicone Sealant for bonding glass/ceramic stovetops to stove frames.  Combined with Baron’s existing automated dispensing systems, fast-curing Loctite 5600 silicone sealant produced one completed stovetop every 8 seconds, with little or no waste. In addition, the assemblies could be handled within minutes. Switching to the new two-part silicone allowed for continuous processing of stovetops and eliminated costly WIP.

For more information, email: john.lafond@us.henkel.com