Fastening & Joining

Sept. 23, 1999
Innovative designs and better materials combine in fasteners that are strong, durable, and look good. The resulting systems provide a welcome balance between aesthetics and function

Selecting the right PSA
Determining the best possible pressure-sensitive adhesive (PSA) tape for an application requires analyzing the surfaces, environment, and manufacturing processes. With literally millions of combinations possible, choosing the right PSA tape can seem overwhelming. However, asking the right questions at the start of the design process can maximize performance and save time and money.

Substrate materials to be bonded are the first consideration. This may seem obvious but it is important to know the “actual” surfaces that will contact the PSA tape. An example is plastic. It’s a common substrate, but there are many different compositions available with various surface-energy levels. Surface energy relates to how well an adhesive “wets out” or flows uniformly over the substrate surface. High surface energy materials allow excellent “wet out” and provide the best adhesion. Some high surface energy plastics include Kapton, phenolic, nylon, polyester, ABS, polycarbonate, and rigid acrylics. Low surface energy materials don’t let the adhesive “wet out” and require more aggressive adhesives such as rubber-based and modified acrylic formulations. Some low surface energy materials may require special treatments to promote better adhesion.

Surface texture impacts the PSA bond. Textured materials don’t allow 100% surface contact and that results in lower adhesion. Using a heavier, softer, more-conformable adhesive or heat-assisting the lamination process to soften the adhesive and promote flow can improve a PSA’s performance on textured material.

Surface contamination also impedes surface contact and adhesion. Many types of contamination not visible to the eye can be detected analytically. If a material feels slippery, greasy, or slimy there is likely loose material on the surface that will impede a good bond. Some examples are antistatic agents, moisture, mold-release agents, silicone, oils, antiblocking powder, and plasticizers.

Surface contour influences PSA tape performance as well. Applications in which tape wraps around irregular angles or curves require a more-flexible construction. This means choosing either an unsupported transfer tape or a coated product that uses a conformable carrier such as tissue, cloth, or nonwoven material. It is, however, virtually impossible for an adhesive to overcome continuous stresses that result when a rigid material tries to return to its original form.

Environmental exposure is another consideration. For example, direct sunlight or other forms of ultraviolet radiation will prematurely age a PSA causing discoloration, brittleness, and a weakened bond. Temperatures above 120°F cause PSAs to become more viscous and this weakens bonds. Exposure to solvents such as gasoline or oil can also deteriorate a PSA bond. In these cases a firm acrylic PSA can typically withstand harsh environments. Low temperatures tend to solidify PSAs. At temperatures below 40°F, PSAs begin to lose their initial tack and become more difficult to apply. In such cases, experts often recommend rubber-based PSAs with lower crystalline points.

Performance requirements such as adhesion, tack, shear, and life expectancy are key to successful designs. Applications can require critical or noncritical bonds. A noncritical bond holds an item in place until it can be mechanically fastened. In such cases low-cost rubber-based PSAs will work as long as the surface is clean and uncontaminated. When the bond is critical, select a formulation that produces higher adhesion, tack, or shear levels.

Processing demands also come into play. Manufacturers coat PSA tapes in wide-web roll form and typically sell product in log or master rolls, 54 in. 3 250 yd (double-coated tapes) and 48 in. 3 180 yd (unsupported transfer tapes). For fabricated-foam products, two-sided PSAs are laminated to foams about the width of the tape’s log roll, and later fabricated into strips or die-cut parts. When logs of PSA tape are slit without a laminated-substrate support material, unsupported adhesive transfer tape constructions are typically not recommended.

Adhesive on unsupported PSA tapes without a carrier tend to flow outward toward a release liner’s edges. Long length rolls and higher pressures near the core tend to exacerbate the situation.

Another option is pattern-coating the adhesive onto the release liner during production; this permits unsupported transfer tapes to be slit into narrow rolls. Pattern-coating refers to the width and spacing of adhesive laid down across the PSA web stock during manufacturing. It is also called zone coating. Pattern coating the adhesive provides ungummed lanes when slitting rolls of tape from a master roll. The resulting ungummed slit edges, or fingerlifts, prevent edge pick and aid in liner removal.

Cost parameters must also be considered in any application. PSA tapes are typically priced by the “msi” (1,000 in.2) or the square yard (1,296 in.2) and can range from $1.00/msi to more than $25.00/msi. Expect to pay for high performance. However, in calculating the cost of PSA tape versus conventional fasteners be sure to look at the total applied cost. This includes costs associated with special processing equipment, production clean-up and change-over time, floor-space requirements, and complying with current or pending OSHA/EPA regulations, as well as end-product performance and potential design efficiencies.

The information was provided by Avery Dennison Corp., Specialty Tape Div., Painesville, Ohio.

Special considerations when plating screws
Fasteners can fail for many reasons, including overload, fatigue, and corrosion. Occasionally, fasteners fail as a direct result of improper plating. Poor plating procedures can cause poor thread fit, uneven plating thickness, or hydrogen embrittlement, a condition that can cause complete fastener failure when subjected to stress.

With hydrogen embrittlement, atomic hydrogen is absorbed and diffused throughout the metal during electroplating or in application. Atomic hydrogen has a tendency to collect in areas of high stress concentration. This can result in brittle failure, in most cases delayed, limiting reliable means of detection.

Characteristics of hydrogen embrittlement include:
• The higher the applied stress in the fastener, the shorter the time to failure.
• There is a threshold value of hydrogen concentration for a given steel at a certain heat-treat level, above which embrittling is critical.
• The greater the amount of hydrogen in the steel, the more prone the fastener is to failure, the shorter the time to failure, and the lower the stress level at which the fastener will fail.
• The higher the tensile or hardness of the steel, the greater the susceptibility to embrittlement.
• Different types, grades, and heat treatments of steel will experience varying degrees of embrittlement from the same amount of hydrogen.
• Greater embrittlement relief results from longer baking times and higher temperatures.
Steels with the highest tensile strengths fail at lower stresses and in shorter times. Also, heat treatment does not have a strong influence on screw failures due to hydrogen embrittlement, but control of the heat-treating process does.

The very design of high-strength fasteners — with their many stress risers and notches and their typical exposure to dynamic stresses — make them susceptible to hydrogen embrittlement failure. A design that incorporates thread root, radius root runout, and compound fillets, such as found in Unbrako screws, reduces this potential. Nevertheless, every precaution should be taken to prevent potentially disastrous failures.

One precaution is to control heat treating. Steels with higher hardness and carbon contents are prone to hydrogen embrittlement, making it beneficial to use the most modern furnace equipment. Atmospheres must be carefully controlled. This ensures that tensile strengths are within required values with no trace of carburization. Surface carburization should be avoided in electroplated, high-strength fasteners with a critical combination of high hardness, high carbon, and high surface stresses.

Proper grinding helps safeguard fasteners from hydrogen embrittlement. For many larger screws and bolts, grinding heat-treated blanks removes surface imperfections, establishes close dimensions, provides a good surface finish, and assists in decarburization. However, improper grinding operations can be another source of trouble. Improper grinding causes high surface tensile stresses and overheating, often severe enough to produce hard spots or cracking. The combination of surface tensile stresses and high surface hardness which accompanies the hard spots makes such parts more prone to failure by hydrogen embrittlement.

Cleaning is an essential step to help prevent hydrogen embrittlement. Before plating, remove all heat-treat scale and foreign materials from high-strength fasteners. Avoid heavy acid cleaning; it adds large quantities of hydrogen to the heat-treated parts. Cleaning methods depend on the type of material and fasteners.

Actual plating procedures are the most important factors in producing trouble-free plated fasteners. Closely control conditions at every stage to ensure freedom from hydrogen embrittlement. Check the plating baths regularly to ensure chemical analyses are held within recommended limits. Check every lot of plated parts for plating thickness, coverage, and adherence, and periodically subject samples to salt-spray tests. Select and test samples for hydrogen embrittlement by sustained loading.

Finally, start the baking process promptly after the plating. Times and temperatures depend on a number of factors, including the type of plating material, fastener size, and individual plating specifications.

Information for this article was provided by John Grey of SPS Technologies, Industrial Fasteners Group, Cleveland.

Captive screws ease service
The manufacturer of a new airline-ticket desktop printer recently faced the challenge of attaching internal components while avoiding the danger of loose screws falling onto the power supply. Datasouth Computer, Charlotte, also had to ensure quick and easy access to components for service.

As a solution, the company specified PEM Type PF11 self-clinching panel fasteners from Penn Engineering & Manufacturing, Danboro, Pa. The components install as complete assemblies and feature a captive-screw design that eliminates the risk of loose hardware. The fasteners meet UL 508 operator-access-area requirements and permit quick disassembly by hand or screwdriver. They also improve MTTR (mean time to repair) by eliminating a service technician’s need to search for and fumble with lost screws.

Each Datasouth Journey Desktop ATB2 Printer uses seven panel fasteners: two on the floppy drive bracket, one on the power supply shield, and four on the PCB bracket. The 0.060-in.-thick floppy and board brackets and 0.036-in.-thick power supply shield are preplate galvannealed CRS-1008-1018.

Fasteners install permanently with a standard machine press. The fasteners are assembled by inserting into prepunched or drilled holes (0.312 in. for #8-32 thread size) and applying parallel squeeze forces. The installation force causes the sheet metal to cold flow into an undercut beneath the fastener head, so the fastener becomes an integral part of the sheet. A serrated clinching ring prevents the fastener from rotating once installed. This type of panel fastener features a distinct shoulder, which provides a positive stop during installation.

Type PF11 fasteners have a combination slot/Phillips drive, and a large knurled knob promotes easy hand actuation, which is a key advantage when components need to be replaced. The thumbscrew portion of the fasteners allows quick screw-in using one hand.

The Journey Desktop ATB2 airline-ticket printer, which was initially developed to fulfill a special customer requirement, is part of the company’s full line of heavy-duty thermal and dot-matrix printers for transportation, distribution, manufacturing, and health-care markets.

The information was provided by Penn Engineering & Manufacturing Corp., Danboro, Pa.

Ultrasonic welding strengthens turbine-wheel bond
Ultrasonic welding is ideal for bonding large plastic parts and hard-to-weld materials. The technology transmits vibratory energy to the bonding area, causing the plastic material to soften in a fraction of a second. When the material resolidifies, it creates a strong molecular bond.

Polaris Pool Systems, San Marcos, Calif., used ultrasonic welding from Sonobond Ultrasonics for the turbine wheel assembly of its automatic pool cleaners. Loose-fitting bearings that were press-fit into the turbine wheel caused gear jam-ups. Ultrasonic welding of the bearing into the turbine wheel produced a perfectly aligned, solid bond with no bearing movement, eliminating the jamming problem.

Sonobond designed a custom welding horn and fixtures into its Sureweld 20 ultrasonic plastic welder for the Polaris application.The Sureweld equipment incorporates a highly stable bench press with a rectangular support column that minimizes deflection when pressure is applied against the part for welding. Average weld time is 0.3 to 0.4 sec. An integrated 1,000-W power supply decreases space requirements, and the unit sets up easily for quick operation.

Information for this article was provided by Sonobond Ultrasonics, West Chester, Pa.

© 2010 Penton Media, Inc.

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