Fasteners for Foam and Honeycomb
An innovative new fastening process combines elements of automatic screwdriving, plastic fastener anchors, and spin-welding technology for plastic parts assembly.
Celebrity auto critic Jeremy Clarkson once called the Audi TT “the Mac Daddy of sport coupes.”
The Audi may, indeed, be all that, but even the Mac Daddy can have assembly issues, and such was the case last year. Some components that were bonded beneath the rear parcel shelf with adhesive were becoming unstuck. That’s not good for any car, but it’s especially bad if the car’s base price exceeds $43,000. Audi engineers needed to solve the problem—fast.
The obvious solution was to replace the adhesive with a mechanical fastener. However, the shelf was made from expanded polypropylene foam (EPP). In addition to shelves, this strong, lightweight, recyclable material has myriad automotive applications, including seating, bumpers, stowage systems, door panels, pillars, floor levelers, head rests, tool kits, crash pads and sun visors. The low-density, closed-cell foam has excellent energy-absorbing and sound-dampening characteristics, but it’s not a great medium for threaded fasteners. They strip out too easily.
Audi called Weber Screwdriving Systems and fastener manufacturer EJOT for help. The automaker was already working with the two companies. It uses Weber’s automatic screwdriving equipment to install EJOT’s flow-drill screws in various Audi vehicles. Could the two companies solve Audi’s parcel shelf issue?
Working together, Weber and EJOT developed a unique friction-joining process that combines elements of automatic screwdriving, plastic fastener anchors, and spin-welding technology for plastic parts assembly. The system can be used to fasten foam and honeycomb core structures as well as carbon-fiber-reinforced plastic and glass-reinforced plastic.
How It Works
EJOT created two types of thermoplastic fasteners for the process.
Made of polypropylene, the EPPsys RSD is designed to be an anchor point for a threaded fastener—specifically, a Delta PT screw. The cylindrical fastener is 22 millimeters in diameter and 12.8 millimeters long. It is flat on one end and domed on the other.
Made of 30 percent glass-filled nylon PA 6, the EPPsys TSSD can be an anchor point for a Delta PT screw, or it can act as a direct fastening element. The fastener is a tapered cylinder with a blunt tip on the thin end and an undercut flange on the wider end.
The RSD and TSSD fasteners are “like a plastic anchor that you would install in drywall to hang a picture or shelf,” explains Jim Graham, president of Weber Screwdriving Systems. “You wouldn’t install a screw directly into the drywall—it would strip and pull out. Instead, you install an anchor in the drywall and drive the screw into that. The anchor gives you good retention force, and you can apply quite a bit of torque to the screw. The same thinking is behind the EJOT fasteners.”
The fastener is installed with the same type of equipment used to install flow-drill screws. A servo-driven spindle with a top speed of 5,000 rpm precisely controls the rotational speed of the fastener, while a servo-driven linear axis provides a maximum axial thrust of 800 newtons. Sensors and computer control ensure a consistent installation process.
In most cases, the fasteners do not require a pilot hole or other surface preparation. Once the driver moves the fastener over the target position, it spins the fastener and pushes it against the surface of the part. Friction generates heat to melt the surface materials and bore a hole into the parts. As the fastener spins, some of its surface material melts as well, mingling with the molten material and fibers of the base parts and creating a weld. When the fastener is flush with the base materials, the driver stops abruptly and holds the fastener in place. When the materials have cooled and solidified, the driver retracts.
“When the driver pulls away, you’re left with a very strong, very robust joint,” says Graham. “Our tests show a pullout force in excess of 750 newtons. In fact, in most cases, the plastic part fails before the joint fails.”
The installation process takes 2 to 4 seconds, depending on the density of the base material. An additional 3 seconds of cooling time is also necessary.
Fastener design and process control ensure that the installed fastener leaves a cosmetic surface. That’s important, since the fasteners are often installed in automotive interior components, like the Audi’s parcel shelf. (Slight flaring at the edge of the fastener can occur with foam densities above 100 grams per liter.)
Because precise control over axial force is a key variable, the fasteners must be installed with a Cartesian or six-axis robot. A blow-feed mechanism supplies fasteners to the driver.
“The key to this technology is the process control,” says Graham. “Attempts to install these fasteners manually with standard power tools or drills have been wholly unsuccessfully. But because we use our RSF technology [for flow-drill screws], with constant monitoring and control of depth relative to time, we’re able to…install this fastener and stop the system at the exact moment when the material has flowed consistently and hold pressure on the joint so it can stabilize and cool.”
The EPPsys RSD comes in two sizes. The RSD 50 has a 3.5 millimeter hole in the center for a Delta PT 50 screw. Maximum tightening torque is 2.5 newton-meters. The RSD 60 has a 4.5 millimeter hole for a Delta PT 60 screw. Maximum tightening torque is 4 newton-meters.
The center hole in the RSD fastener engages with a center pin in the driver bit. In addition, the bit engages the RSD fastener through five depressions around its perimeter.
A variant of the EPPsys RSD is designed to create a snap-fit. In this case, one RSD fastener has a post that mates with a snap in another RSD fastener.
The EPPsys TSSD is available in lengths of 12 or 17 millimeters and head diameter of 14 millimeters. Because it has a flange or head, like a screw, it can capture a thin sheet atop the foam or honeycomb sandwich. That joint is permanent. In addition, a Delta PT 40 screw can be driven into the fastener’s center hole to secure another part, such as a handle or bracket.
The EPPsys TSSD is driven with a male hex bit, which engages with a female hex indent in center of the fastener. (A variation on the EPPsys TSSD has a threaded metal insert to accept a screw. Instead of a hex, the bit engages with three round holes arranged in a triangle shape.)
Whether the RSD or TSSD fastener is used, the key variables in the process are torque, driver speed, axial force, insertion depth and installation time. In addition, the chemistry of the base parts and the fastener has to be considered. The plastics need to be compatible. Testing in a lab is critical for determining the optimal parameters, says Graham.
Both the RSD or TSSD fasteners can be molded in various colors to match an application.
Although Audi was the first to use the new process, other automakers are beginning to adopt it. In addition to automotive assemblies, the fasteners have myriad applications in heavy trucks, appliances, furniture, displays, LED and solar lighting, signage, packaging, recreational vehicles and aerospace interiors. In the latter case, the fasteners can be used for galleys, seats, dividing walls and overhead storage bins.
Honeycomb panels are commonly used in aerospace manufacturing. This three-layer composite structure consists of two load-bearing cover panels and a supporting core in honeycomb form. The support core can be made of cardboard, resin-impregnated paper, synthetic fiber or thin aluminum foils. The cover panels can be made of cardboard, plastic, fiber composites or metal sheet.
“Honeycomb is very lightweight and super strong, but it’s very difficult to join things to,” says Graham. “You have to drill it, install hardware, and secure it with epoxy. All that is expensive, and it takes time. Our solution is much more cost-effective.
“This technology is less than a year old, and we’re excited about it,” he continues. “It’s not in our normal wheelhouse—we’re used to driving screws—but it’s opening new markets for us.”
For more information on fasteners for metal, plastic, composites, foams and other materials, call EJOT Fastening Systems at 248-956-6350 or visit www.ejot-usa.com. For more information on automatic screwdriving systems, call Weber Screwdriving at 704-360-5820 or visit www.weberusa.com.
Editor’s note: See live demonstrations of this new technology at the Weber Screwdriving exhibit (booth 805) at The ASSEMBLY Show Oct. 25-27 at the Donald E. Stephens Convention Center in Rosemont, IL. For more information, visit www.theassemblyshow.com.