Quality at Stake
Staking is an ideal process for assembling plastic parts. It's quick and inexpensive. It can join dissimilar materials. It eliminates the cost of fasteners and adhesives, and it allows engineers to loosen the tolerances for molded parts. Although a staked joint cannot be reopened, it can be cut or broken relatively easily to permit disassembly and recycling.
In staking, a part with one or more holes in it is slipped over corresponding posts on a base part held in a fixture. Then, a tool mounted on a press comes down, deforms the posts and traps the top part tightly against the base. In most cases, heat must be applied to the posts to melt the plastic. The process is used to assemble small products, such as printer cartridges, and large parts, such as automotive interior components.
The base part must be made of a formable thermoplastic, but the entrapped part can be made of any material: sheet metal, FR4, glass, rubber, thermoset plastic or another thermoplastic.
Almost all thermoplastics can be staked, but some work better than others. Acrylonitrile butadiene styrene (ABS), polyphenylene oxide, polypropylene and ABS-polycarbonate blends work particularly well.
"If we had our choice of materials, it would be ABS, across the board," says Eric Gregorich, applications and sales manager with Plastic Assembly Systems (Bethany, CT). "You'll get the cleanest look and a very strong boss. You rarely see any stringing or cracking with it."
Posts can be staked one at a time, or multiple posts can be staked simultaneously. Staking is usually done as a semiautomatic process, but the equipment also can be integrated with a variety of automated assembly equipment, including rotary indexing tables, pallet-transfer conveyors and even six-axis robots. Vision systems are sometimes used in conjunction with staking equipment to ensure that incoming parts have posts in the right spots.
The tool can be mounted on a manual press, a pneumatic press or a servoelectric press. The latter was introduced this year by Plastic Assembly Systems. With the servo press, engineers can control the position of the Z axis in 0.001-inch increments.
"The nicest thing about the machine is there's not one mechanical adjustment," says Gregorich. All process parameters, including ram speed, stroke length, forming height and forming time, are adjusted through a touch-screen display.
The servoelectric press has a maximum stroke of 20 inches, which is three to five times longer than the stroke lengths of most pneumatic presses used for staking. The ram speed can be set from 1 to 500 ipm. As a result, the press can accommodate large parts that need extra room to load into a fixture, without compromising cycle time. "When you actuate the machine, the head will come down at 500 ipm for the first 17 inches, but then for the last 1.5 inches of working distance, it can come down as slow as necessary," says Gregorich.
The chief drawback of the servoelectric press is its price. The press is approximately three times more expensive than its pneumatic counterpart. "I'm not saying it's going to be our biggest seller, but it's definitely a nice option to have," says Gregorich. "A lot of medical device manufacturers will use it, because they require very precise stopping distances."
There are several methods of forming a head on a plastic post. Which to choose depends on the material, the part design, cycle time requirements and cosmetic issues.
In hot-air staking, compressed air is heated and blown onto the post through a nozzle. The air can come from above the post or from the side. When the plastic softens, an air-cooled staking tool is pressed onto the post to form the head. The tool remains in place until the plastic solidifies. Cycle time is typically 8 to 12 seconds.
Joints made with hot-air staking have a good cosmetic appearance, says Mike Shirkey, president of Orbitform Inc. (Jackson, MI). With other methods, the plastic can stick to the tool, producing strings of material when the tool retracts. With hot-air staking, the plastic solidifies quickly on contact with the cold tool. "You end up with a really nice-looking form, and structurally, it's a little better," he says.
Hot-air staking can be used to form several posts at the same time, and the posts do not need to be located on the same plane. However, design engineers should provide enough room for each air nozzle to reach its post. Engineers should also make sure that there aren't any parts near the posts that could be adversely affected by the stream of hot air.
"The heat is very localized, so it's usually not a problem, but it's definitely a design consideration," says Shirkey.
Another staking option is to melt the post directly with a heated tool. The post melts as the hot tool presses down on it. After the post has been formed, a shot of compressed air is blown onto the tool for 1 to 2 seconds to cool it down slightly. This ensures a clean release from the part and prevents the plastic from springing back to its original shape. Total cycle time varies from 6 to 10 seconds, says Daniel Bolduc, northeast field sales engineer with Sonic and Thermal Technologies Inc. (Milford, CT).
Hot-tool staking keeps the heat localized to the post, and it's particularly good for plastics with a high glass content. In addition, a large number of posts can be formed simultaneously, and the posts do not have to be located in the same plane. "We've done up to 32 bosses at once. We've also done machines that can form multiple bosses and install inserts at the same time," says Gregorich.
Hot-tool staking is also the most flexible of the staking processes. Many suppliers offer quick-change tooling for their equipment.
The fastest of the forming methods is ultrasonic staking. With this method, the heat to melt the plastic doesn't come from the tool itself, which stays cool throughout the process. Instead, the heat comes from friction between the plastic and the tool face, which is vibrating at ultrasonic frequencies. Cycle time can be less than 1 second.
Other staking processes need a certain amount of recharge time, because heat has to transfer out of the tool. "With hot-air staking, the tool has to cool down between cycles. With hot-tool staking, the tool has to heat up between cycles," explains Tom Kirkland, business unit manager for Dukane Intelligent Assembly Solutions (St. Charles, IL). "Ultrasonic staking [melts the plastic] faster than either of those processes, and in terms of overall cycle time, it doesn't need any recharge time."
Ultrasonic staking is usually done with low pressing force and high-amplitude vibrations. The speed of the ram must be carefully controlled, so that it does not exceed the melting rate of the plastic. In addition, the horn must start vibrating before it contacts the post. "You want the post to stand still and the horn to vibrate, so the heat concentrates at the tip of the post," says Kirkland. "If you don't get the ultrasonics going before the tool reaches the post, there's a very high probability that...you'll fracture the post at the root."
When to stop the vibration is just as critical. "It's important to shut off the ultrasonics before you contact the part you're trapping," advises Kirkland. "You don't want to hit a circuit board and make it vibrate, for example. You want to finish forming the head with pressure alone."
Because the tool has to vibrate, there are limitations in the number and position of posts that can be formed simultaneously with ultrasonic staking. "If you're staking one to four posts, and they're relatively close together, the acoustic design of the tool isn't too complicated," says Kirkland. "But if the posts are spread out...it's difficult."
In the InfraStake process from Extol Inc. (Zeeland, MI), infrared light is used to heat the post. A reflector inside the forming tool concentrates infrared energy on a narrow spot, uniformly heating the post and limiting the amount of heat transferred to surrounding parts. The focused nature of the heat source is particularly advantageous when staking parts such as circuit boards, which have joints that can't be exposed to heat or vibration.
Like ultrasonic staking equipment, the infrared staking tool does not need time to warm up or cool down. "The light energy is directed at the plastic, which keeps the tool cool and allows you to get on and off the part quickly," says Chris Keizer, sales manager of Extol Inc.
The process works with most opaque plastics, but does not work well with clear and translucent plastics, because the energy passes through them, says Keizer.
In some cases, heat may not even be necessary to form a head on a post. Radial and orbital forming machines, or a simple arbor press, can be used to cold-form plastic posts. Any thermoplastic with good impact resistance, such as acetal or nylon, can be cold-formed.
"You can cold-form some plastics, as long as they're glass-filled. Otherwise, they have too much memory," says Daniel P. Baumann, president of Schmidt Technology Corp. (Cranberry Township, PA).
To prevent buckling the post, its length should not be more than twice its diameter. To prevent fracturing the post, the forming force must be applied gradually. A cold-formed plastic post will naturally want to return to its original shape, but holding the post under pressure for a period of time after forming will limit how much "spring-back" occurs. Nevertheless, cold-forming should not be used to assemble parts exposed to heat. Cold-formed plastic posts are also more susceptible to crazing and chemical attack.
The post in the base part is usually round and solid, with a flat top or a point, but it can also be hollow. As a rule of thumb, the diameter of a solid post should be 50 percent of the nominal wall thickness of the part. This will help prevent voids and sink marks on the flat side of the part at the base of the post, says Kirkland. If the post must be larger, a hollow post may work better. A radius or a recess around the base of the post will help it withstand the stress of staking.
The cavity on the face of the forming tool should be designed to melt just enough plastic to form a head on the post. Too large a cavity will produce an irregularly shaped head. Too small a cavity will produce excessive flash. The same is true of the length and width of the post itself. Too large a post will produce a misshapen head; too small a post will produce flash.
"You don't want a stake with a high aspect ratio," warns Kirkland. "A post that's very tall relative to its girth...is very difficult to stake well. It's too spindly.
"Ideally, you want a post that's a little bit squat. However, there is such a thing as too much of a good thing, and a post that's too squat doesn't give you enough material to work with to produce a head with sufficient strength to hold the part."
The post can be formed in many shapes, including a dome head, flat head, pan head and rosette. The post can also be formed flush with the mating part.
The rosette head is a good design for posts 1.5 to 4 millimeters in diameter. It works well with most thermoplastics, except for those with abrasive fillers, which can dull the point on the forming tool. The dome head is good for posts that are less than 3.18 millimeters in diameter, for multiple-stake applications, and for plastics with abrasive fillers. A hollow post is preferred for posts with an outside diameter of 4 millimeters and an inside diameter of 1.5 millimeters.