When it comes to joining plastic parts, it's hard to beat staking. Quick and inexpensive, the process can join dissimilar materials, and eliminates the cost of fasteners and adhesives. It also allows engineers to loosen the tolerances for molded parts.
To create a staked joint, an operator first slips a part with one or more holes in it over corresponding posts on a base part held in a fixture. A tool mounted on a press then comes down and deforms the posts, trapping the top part tightly against the base.
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. Posts can be staked one at a time, or multiple posts can be staked simultaneously. Staking is usually done as a semiautomatic process. However, staking equipment also can be integrated with a variety of automated assembly equipment, including rotary indexing tables, pallet-transfer conveyors and even six-axis robots.
In hot-air staking, compressed air is heated and blown onto the post through a nozzle. 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.
Another staking option is to melt the post directly with a heated tool. With this hot-tool approach, the post melts as the 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.
Because hot-tool staking keeps the heat localized to the post, it's good for sensitive plastics with high glass content. In addition, the technique makes it possible to stake a large number of posts simultaneously. Hot-tool staking is also the most flexible of the staking processes. Many suppliers offer quick-change tooling for their equipment. Total cycle time varies from 6 to 10 seconds.
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. Other staking processes need a certain amount of recharge time, because heat has to transfer out of the tool. But, with ultrasonic staking, recharge time is not a factor. Therefore, cycle times can be less than 1 second.
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.
Finally, 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.
When using the approach, the length of the post should not be more than twice its diameter. Otherwise, there may be buckling. 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.
