Joining choices: welds, fasteners or adhesives?
No matter which method is used, the goal is low production costs and a quality end product.
Design engineers consider a number of factors when deciding which joining method is best suited to a particular application. Each has pros and cons, and this article will look at some of the reasons behind the choices for one over the other.
- Welding, done correctly, produces a strong permanent joint between substrates. There are methods for welding metals, using heat, for welding plastics and for using solvents.
- Fasteners offer post-production versatility that meets maintenance and repair needs.
- Adhesives bring an interesting set of solutions that can replace or supplement welding and fasteners within a given end item.
No matter the join method, there are common aspects to all production lines that are taken into account when making design plans.
Reducing Production Costs
Production costs are unseen to the consumer but very visible to the manufacturer. Decreasing costs helps to maintain or increase profits while allowing consumer pricing to remain attractive to buyers. Reducing production costs on a product line calls for a good understanding of each step within the production process and a good bit of creativity.
Specialized skilled labor can be costly, and involves extensive training. Welding requires a specialized, skilled labor force. Specialized skilled labor can be costly, and involves extensive training. A much shorter learning curve is needed to become proficient in applying adhesives or fasteners.
Line speed is another significant factor in production costs. Adhesive application often is less time consuming than fastening or welding, and the resulting increase in line speed can help to lower the per part costs.
For example, adhesives replace the need to pre-drill holes for fasteners. Eliminating fasteners allows for much more flexibility in aligning parts during assembly.
Welding operations introduce heat into the process that can slow down production while the parts cool enough for handling and setting up for the next step in the line. An inline adhesive process allows engineers to maximize the use of efficient movements throughout the production line.
Joining with adhesives, fasteners or welding all offer the versatility of manual, semi-automated or fully automated robotic systems. Converting from one system to another can often yield significant cost savings, even after factoring in new equipment costs.
Welds and fasteners add weight to a finished item. Adhesive bonding achieves weight reduction because it allows for thinner sheets of metal, plastics or composites to be used in product design. Similar to the large panels on trucks, panels on appliances are selected for their ability to withstand welding or fasteners without distortion, or loss of structural integrity. Bonding distributes stress across the entire joint, and reduces substrate fatigue at fastener locations.
Weight reduction can have additional benefits— and not only in the appliance industry— including lower shipping costs, functionality improvements (such as lighter hand tools reducing operator fatigue or the increased fuel economy in lighter vehicles and aircraft), less wear and tear on production equipment and tools, and production line efficiencies that can be attributed to moving lighter components.
Structural adhesives are designed to increase the strength of the part. When fasteners are used to join two thin sheets, you still have two thin sheets. Bonding the two sheets with adhesives unitizes them, creating a single, stiffer and stronger panel. The overall strength will be higher even if the bond is only on a portion of that panel.
The strength of joined parts is directly related to stress distribution. Stress can be caused by a load on any joint, or because differences in thermal expansion between two different substrates create movement as temperature changes. For example, a metal to plastic part that is assembled with fasteners has stress points at every fastener location that can cause the component parts to split, crack or craze. Adhesives are better than fasteners when it comes to distributing these stresses; resilient bonds are formed when the adhesive cures. The stress is absorbed, greatly increasing the robustness of the part.
Rivets, fasteners and weld seams can each mar the appearance of the assembly. Fasteners can complement or interfere with design goals. Adhesives allow tight, nearly invisible joints and seams, giving designers greater creative opportunities.
Noise reduction is a key target in many design plans, but is especially desirable for appliances. Noisy operations of consumer products lead to complaints and a potential loss of revenue. Customer satisfaction and product longevity are both affected by noise. Many appliances have decibel ratings on their government-mandated stickers! Welded seams eliminate rattle noises. Fasteners, when used without a threadlocker, can work loose from vibration and produce rattling noises. Properly applied and cured adhesive bonds don’t rattle. Bonded parts are unitized so that they move as one piece – by design.
Adhesives can be formulated and used to create seals that keep liquids and contaminants in their proper area, whether sealing a lubricant in or sealing liquids or dust out! Welding is also an effective sealing method that is complemented/enhanced with adhesives that are used to fill porosities that can occur during the weld process.
Fasteners are not typically used alone to create seals, however there may be specialty products for unique applications that are beyond the scope of this article.
We’ve also taken a look at the reasons some engineers are hesitant about replacing welding or fasteners with adhesives. Overwhelmingly, the primary reasons for not switching are concern over choosing the right adhesive or implementing a solid process to ensure consistent results.
Appliance manufacturers are ahead of the curve when it comes to confidence with switching to adhesives. This may be because adhesives and sealants are commonly used throughout the manufacture of the motors and motion assemblies within appliances, making these engineers more accustomed to adhesive processes. Following are a few typical adhesive applications within the appliance industry.
Motor Magnet Bonding
Brittle rare earth magnets used in motor manufacturing originally were designed to be clipped into place. Those clips created stress points that caused the magnets to fracture. Adhesives bond across the entire magnet and distribute the stress across the part, increasing the life of the motor.
Several types of adhesives are available to accommodate the differences in process and strength requirements in motor magnet bonding.
Single component heat cured epoxy adhesives are available in a flowable formulation or as a non sag product. When the adhesive is required to flow into the joint, a bead of flowable epoxy is dispensed onto the top of the magnet up against the can. During the heat curing process, the epoxy flows into the joint before curing. In other applications a non-sag epoxy is applied within the joint and maintains its position during curing.
Toughened Acrylic formulas are suited to applications requiring rapid fixture and strength development time. These offer a fixture time that is generally less than 30 seconds and develop strength rapidly, reducing the clamping time. They are available for application in a two-step process, or as an external mix using precisely positioned nozzles in an automated or semi-automated line.
An increasing trend in the industry is to decrease the size of motors. For many small motors high temperature resistant cyanoacrylates can be used.
Anaerobic adhesive threadlockers are used throughout appliances to prevent loosening or noise from vibration, or thermal expansion/contraction. They also seal, prevent corrosion, and provide a controlled off torque. Threadlocker strengths range from permanent to removable, to meet the application requirements.
Anaerobic retaining compounds provide permanent bonding of co-axial metal joints. Mechanical joining techniques provide about 20 percent surface to surface contact; retaining compounds provide 100 percent surface to surface contact. This allows more than 5 times the load carrying capacity. Retaining compounds unitize the joint by filling the gap between metals, and as a result, machine tolerances can be relaxed, lowering production costs as well as increasing product quality.
Anaerobic thread sealants generally seal to the burst pressure of the pipe. Grades are available for potable water, gas, air, and hydraulic systems. They are used throughout water pumps, water heaters, soda fountains, fire sprinkler systems, and wherever a seal is required on threaded metal pipe.
Form in Place Gasketing
Anaerobic form-in-place gaskets allows manufacturers to eliminate their inventory of different sizes and shapes of gaskets and replace them with one item. Form in place gaskets are non-shimming and provide uniform stress distribution with no relaxation or shrinkage.
In conclusion, in appliance manufacturing as elsewhere, there are a variety of methods for joining materials and a myriad of reasons engineers are switching joining processes from welding or fastening to adhesive bonding. Design engineers have different goals for each joining application, and one size does not fit all. Engineers and production managers can depend upon well established, reputable companies in the fastener and adhesive industries to offer reliable support and advice to help them make the best choices every time.
In the end, no matter which method, or combination of methods is used, the goal is low production costs and a quality end product. Understanding all of the options available supports the best decision for each application.