Beware the Boss

September 7, 2004
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The right thread design and driver settings can make a big difference in how well screws assemble plastic parts.

With so many ways to formulate plastic to get just the right combination of color, texture and durability, it's easy to forget how the parts will be assembled. However, if the parts will be assembled with screws, overlooking such parameters as thread style, driver speed and boss design could spell disaster on the assembly line.

"You have to match the thread and the torque to the application, rather than force one screw into every application," says John J. Souza, fastening consultant with Quality Screw and Nut Co. (Leominster, MA).

Many types of mechanical fasteners can be used to assemble plastic parts, including rivets, spring clips, and machine screws with mating nuts or inserts. But the most common mechanical fasteners for plastics are thread-forming and thread-cutting screws.

Thread-forming screws create threads by using the screw as a forming tool to push the walls of the plastic hole into the shape of a thread, says Bruno Marbacher, engineering manager at Bossard USA (Portsmouth, NH). The threads at the tip of the screw are smaller in diameter, so the pressure on the wall of the boss is applied gradually. Thread-forming screws can be safely used with plastics that have a flexural modulus of 200,000 psi or less. With a large enough hole, these screws can also be used in plastics with a flexural modulus as high as 400,000 psi.

Thread-cutting screws are required for harder, less ductile plastics. Thread-cutting screws remove material as they are installed, reducing stress on the boss. However, these screws cannot be installed and removed repeatably.

"With glass-filled plastics or hard plastics, such as Lexan, you want a less aggressive thread," says Souza. "When you're fastening very hard plastics, such as thermosets, you have to use a thread-cutting screw. A thread-forming screw will crack the material."

Self-threading screws for plastics are different from standard screws in several ways. One of the biggest differences is in the flank angle. Standard screws have a flank angle of 60 degrees. In contrast, screws for plastics have a flank angle of 30 to 48 degrees. The narrower flank angle exerts less outward force on the boss. This reduces the likelihood that the boss will crack and allows engineers to make the boss as much as 30 percent thinner.

The narrow flank angle also means the threads are longer, so more plastic is captured between each thread. The shear load can be distributed over a larger amount of plastic. And, the lower profile helps reduce installation torque.

Another difference is thread spacing, says Souza. Whereas a No. 8 sheet metal screw might have a thread count of 16 threads per inch, the same size screw for plastics might have just 10 threads per inch.

Self-threading screws for plastics are available in many unique designs. One example is the HI-LO screw, which has two thread forms of alternating height. The high thread has a 30-degree flank angle, while the low thread has a 60-degree angle. The low thread reduces wobble during installation.

Plastite screws take advantage of the natural resiliency of plastic. They have a trilobular thread that produces a smooth, burnishing action on the plastic, allowing the material to recover and fill-in behind the lobes. This maximizes resistance to vibrational loosening. The screws have deep, coarsely spaced threads, which provide a large shear area and deep thread engagement.

Some screws have an asymmetrical thread form, in which one side of the thread has a very shallow angle. When the shallow angle is on the pressure flank of the thread, it increases resistance to pull-out and puts less outward force on the walls of the boss. The greater angle on the entry side of the screw improves material displacement and flow.

The BosScrew has tiny indentations on the pressure flank of the thread. Plastic flows into the indentations once the screw is seated, increasing resistance to loosening.

For rapid installation in lightly loaded joints in ductile plastics, some fasteners have a thread configuration that allows the screws to be pressed into place. The threads are helical, not annular, permitting displaced air to escape during insertion. Mating threads are formed when the plastic relaxes around the shank. As a result, the fasteners can be removed and reinstalled for a limited number of times.

Regardless of what thread design is used, the head of the fastener should have a flat underside. Screws with conical heads, such as oval- and flat-head screws, should be avoided, because they put undesirable tensile stress on the plastic. In addition, screws with cone-shaped or pointed ends are more likely to cross-thread during reassembly, says Marbacher.



Design Tips

Because plastic is softer than metal, care must be taken to avoid overstressing the parts without compromising clamp load.

One design parameter that can make a big difference is the diameter of the boss, says Marbacher. The exact size of the boss will vary with the screw and the material, but as a rule of thumb, the outside diameter of the boss should be two to three times the diameter of the screw threads. In addition, the amount of material displaced from the side of the hole by the screw should not exceed 70 percent to 90 percent of the length of the threads. The softer the plastic, the more material that can be displaced.

The size of the hole also determines how much friction is generated when the screw is installed. The tighter the screw fits the hole, the more friction that will be generated during installation. "If the hole is too tight, the prevailing torque could become higher than the final, target torque. If the friction is too high, you won't get any clamping in the joint," says Jayson D. Deman, a mechanical engineer at Weber Screwdriving Systems Inc. (Yorktown Heights, NY).

The length of the fastener also makes a difference. In general, the length of thread engagement should be two to three times the screw diameter.

For the most strength, bosses should be located next to walls or corners, advises Marbacher. "Pull-out force will be the lowest with a free-standing boss," he says. "A boss located at a 90-degree corner has the highest pull-out force."

Marbacher also recommends that bosses should have a small relief bore, equal to the outside diameter of the screw. "With a relief bore, the boss is less likely to crack," he says.

Because of the wide variation in grades of plastic, it's a good idea to test a number of different fasteners in the application, says Seshu Seshasai, executive vice president for technology at Textron Fastening Systems (Troy, MI). "Using the customer's CAD drawings and resin, we can mold just that portion of the parts that will be assembled," he explains. "That let's us test different fasteners in the application, determine the required torque, and see how many times the parts can be taken apart and put back together without any degradation in the threads.

"Often, engineers invest big bucks in hard tooling, and then they start looking at fasteners. By then, it's hard to modify the product."



Watch Your Speed

Just as the fastener can have a big impact on the reliability of plastic assemblies, so too can the screwdriver. For example, a common mistake when installing screws in plastic is to set the driver speed too high.

"You can't drive screws into plastic at speeds faster than 500 rpm-it just won't work," warns Souza. "The heat [from friction] will melt the plastic. You might get an instantaneous hit on your torque, but the joint will be weak, because you've damaged the molecular structure of the plastic."

The issue of driver speed often comes to the fore when assemblers automate a previously manual process, says Deman. A fastener that worked well with a pneumatic hand tool may not work as well at the higher speeds of automated screwdriving systems.

Ironically, assemblers aren't saving time by increasing driver speed. "With the threads on today's thread-cutting and thread-forming fasteners for plastics, fewer rotations are needed for installation, so the slower tool speed doesn't translate into slower fastening time," says Thomas C. Rougeux, technical sales manager for Visumatic Industrial Products (Lexington, KY). "You can get a half-inch screw to go in with six turns."

Torque is another issue. Many assemblers overtighten plastic assemblies, says Souza. "The standard torque formulas were developed for a machine screw going into a nut or a tapped hole in a metal part," he explains. "These formulas are based on the idea that the root of the screw will elongate like a spring. That doesn't happen in plastic; the screw never elongates."

When determining the torque required to fasten plastic parts, engineers must consider the tensile strength of the plastic, not that of the screw, says Souza. Instead of the diameter of the screw, engineers should take into account the circumference of the threads. And, because the fastener is creating its threads during installation, engineers should account for the high prevailing torque.

Once the torque specification has been set, torque control isn't as big an issue when driving screws into plastics as it is when driving into metal, says Rougeux. Torque control is an issue when assembling plastic parts that include a gasket, such as pump housings; when fastening hard, brittle glass-filled plastics; and when using metal inserts.

When evaluating screws for plastics assembly, engineers should compare strip-to-drive ratios. This is the ratio between the torque at which the screw will strip its threads and the torque necessary to drive it. For assembling plastic, this ratio should be at least 4-to-1. This ratio is influenced by such factors as the drive speed, screw finish, hole size and length of thread engagement.

From an automation standpoint, driving screws into plastic is often easier than driving screws into metal castings, says Rougeux. "The joints are fairly hard. The force needed to keep the bit engaged with the screw is usually low. And if the part design needs to be modified, it's usually easier to do than with a cast part," he says.

In most cases, screws for plastics feed just as well as machine screws. "Occasionally, you'll get screws with threads that are so sharp and so deep, that they can't help but mesh with one another," says Deman.

As with any screw, for optimal feeding, the overall length of the fastener should be at least 1.5 times the diameter of its head. And, as with any automation project, it's always a good idea to consult the equipment supplier before committing to molds. "Many manufacturers send their assemblies to us to check for potential problems with automation," says Rougeux. "If a boss needs to be thicker or if it needs to be moved to a slightly different location, they can usually accommodate that. Molds can be changed fairly quickly, and whatever money they spend doing that is more than made up in the automation process."



The Curve Ball

Sometimes, even the best combination of driver settings, fastener style and joint design can't prevent unforeseen problems. "Plastics are...interesting," says Weber's Deman, with a hint of sarcasm.

Deman remembers helping a toy manufacturer that was experiencing a mysterious problem while installing screws in a plastic part. Assemblers were getting high-torque rejects in the morning, but not in the afternoon.

"The manufacturer was both molding and assembling the parts in the same facility," he recalls. "The parts that were being assembled in the afternoon were molded that morning, so they were still somewhat soft. The screws would go right into them, with little or no prevailing torque. The parts that were being assembled in the morning were molded the previous afternoon, so they had time to fully cure and harden. When screws were installed in those parts, the prevailing torque was much higher, causing the rejects."

The bottom line, says Deman, is that manufacturers must work closely with their resin, screw and tool suppliers to develop the best solution to their assembly needs.



Case Study: ‘Driver' Fits Golf Car Maker to a Tee

Every golfer knows that the driver is one of the most important clubs in the bag.

The engineers who designed the new Precedent golf car from Club Car Inc. (Augusta, GA) also know the value of a good driver-a screwdriver, that is. Automated screwdriving machines and self-drilling fasteners are keeping assembly costs for the golf car "under par."

Gerald Skelton, a manufacturing engineer with Club Car, says the car has many innovative features that improve performance for golfers and facilitate manufacturing. "One of our objectives was to get rid of as many parts...and operations as possible," he says. "We wanted the car to be leaner and less labor-intensive, while at the same time, improving quality through the process."

That goal is reflected in the car's chassis, which consists of an automotive ladder-style aluminum frame and two sections of fiber-reinforced composite that have been bonded together under high pressure to create a single structure. This design has twice the torsional rigidity of the old frame and provides built-in protection for the car's wiring, brake and pedal systems.

To limit the amount of welding used to assemble the car, fasteners were to be used to attach various components to the chassis, explains Skelton. However, the company didn't want to tap holes in the parts during fabrication, and then worry about the holes matching up later on the assembly line. Nor did the company want to perform separate drilling operations on the assembly line.

TEKS self-drilling fasteners from ITW Shakeproof (Broadview, IL) solved the problem. This screw drills its own hole in thin sheet metal, heavier gauge metals and nonmetallic materials. Once the hole is drilled, the fastener taps the hole in the same way as thread-forming or thread-cutting screws do in a predrilled hole.

The next step was finding equipment that could install the fasteners at the right speed and pressure. "Operators cannot install these fasteners by hand," says Skelton. "It takes so much pressure and so much time, it would wear them out. We tried it for a while when we were building prototypes, and it was murderous."

Weber Screwdriving Systems Inc. (Yorktown Heights, NY) provided five semiautomatic screwdriving systems to handle the job. Some stations have four spindles fixtured together, so four fasteners are installed simultaneously. Other stations have a single spindle mounted to an X-Y-Z positioning system, which installs a fastener at multiple locations.

After locating the parts in a fixture, the operator activates the machine. The spindles come down, install the fasteners, and retract. If the right number of fasteners were installed, and if all were installed at the correct torque, the controller allows the operator to release the assembly to the next station.

The application was challenging, because the speed and pressure requirements at each station were slightly different, says Skelton. Some fasteners were going through steel into aluminum. Others were driving through composite into aluminum, and others were drilling through composite into steel.

"The screwdrivers have been working great," says Skelton. "In fact, they are critical to our process. If any of them goes down, we don't build."

Besides automated screwdrivers, the assembly line for the Precedent features many other manufacturing innovations. Customer order information and instructions are sent electronically to the assembler at each station to ensure each car is built to specification. The car's body panels are made of Surlyn Reflections, a strong, flexible plastic from DuPont (Wilmington, DE). Molded in color, the material has a high-gloss, car-like finish, and it's resistant to nicks, dings, collisions and ultraviolet radiation.

The data collected from the assembly process is used to track quality trends for future improvements. Through electronic data collection, statistical programs are developed to address potential issues before they become a problem. And, like the major automakers, Club Car has a system for assessing the quality of the parts and assemblies provided by its suppliers.

For more information on automated screwdriving, call Weber Screwdriving at 914-962-5775 or visit www.weberusa.com.

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