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Advanced Technology for Plastics Staking and Swaging

Advanced Technology for Plastics Staking and Swaging
A new “pulse staking” process could enable medical device designers to use more diverse and complex designs and a wider range of plastics than existing heated-tip and ultrasonic technology allow.

Plastic swaging and staking technology is favored for many types of plastic-based assemblies because by forming a plastic post or flap, it is possible to securely connect or “capture” components made of a diverse range of materials—not just plastics, but metals, fabrics, filter media, and even PCBs, switches, and electronics. Permanent and strong thermoplastic stakes or swages can be made quickly and at relatively low cost, without the need for processes involving labor-intensive mechanical fastening or expensive adhesive-fastening processes. Typical processes include:

  • Heat staking: dome, hollow, knurled, or flush
  • Thermal insertion
  • Rim swaging
  • Bonding panels or other materials
  • Embedding bolts, pins, screws, or holes in plastics
  • Polymer-to-polymer bonding
  • Polymer-to-mesh bonding
  • Attaching membranes or mesh to plastic parts

For years, global manufacturers have leveraged two popular staking technologies—traditional thermal staking and ultrasonic staking—to efficiently assemble a range of products for medical, automotive, appliance, consumer electronics, and other applications. Today, there is a new “pulse staking” platform developed in Europe called PulseStaker, which is now becoming available worldwide. Developed by HTE Engineering Services Ltd. (Dunboyne, County Meath, Ireland), which was acquired in October 2018 by Emerson, this newer approach accomplishes all of the same staking and swaging tasks as existing heated-tip and ultrasonic technology, but it allows for more diverse and complex product designs, is gentle to electronics and circuitry, and bonds a much wider range of plastics than ever before.

Three Types of Staking Technology

Conventional thermal staking or swaging uses a continuously heated tip that is moved into contact with plastic, where it melts the plastic and forms it according to the shape of the tip. The entire forming process, from tip insertion to removal, takes place at one temperature, so it can be challenging to balance the heat input needed for good melting and forming with the cooler temperatures needed for good strength and aesthetics of the finished shape. Target applications include simple, vertical shapes that allow direct vertical access. Because thermal tips radiate heat, posts and features must be adequately spaced for proper processing, and components being staked should not be sensitive to heat exposure.

Ultrasonic swaging or staking employs vibratory energy, also applied through metal tooling, to create frictional heat that is used to melt and form plastic stakes or swages. Similar to thermal staking, target applications for ultrasonic staking include plastic parts with simple shapes and flat, 2-D surfaces. Unlike thermal, ultrasonic works very well with heat-sensitive fabrics and components because melt heat is highly localized.

The PulseStaking process opens up new application possibilities because of the innovative design of tips (Figure 1) that can provide localized, changeable temperatures during the plastic-forming process. Each tip combines an electrical heating element with a compressed-air cooling system. This design instantaneously heats or cools the tip by applying “pulses” of heating or cooling that precisely manage the temperature of the plastic. Compared with thermal tooling, this process can operate more closely to the melt temperature of a given plastic, which reduces stress in the finished part.

Figure 1: PulseStaking Tip

The PulseStaking Cycle

The technology uses either open- or closed-loop controls to complete a four-part staking or swaging cycle (Figure 2) in a matter of seconds. The four parts are described as follows:

Figure 2: The PulseStaking cycle.

  • Main heat: When the PulseStaking tip touches the plastic point to be staked, an electrical current is passed through the tip, which rapidly heats up and causes the plastic to soften. The tip is pushed onto the plastic, where it displaces the melted plastic and forms it into the contours of the PulseStaking tip.
  • Dwell or Weld: Brief periods when the current that heats the tip is switched off enable the rapid build-up of heat from the tip sufficient time to be conducted into the surrounding plastic. Non-powered periods (dwell, weld, cooling) promote more even heating of the target area while preventing overheating or burning of the forming plastic.
  • Cooling: To cool the melted plastic and enable it to set quickly, a compressed air blast is directed to the inside of the tip.
  • Reheat: When the tip and plastic cool, the plastic can sometimes stick to the tip and will not allow the tip to be pulled away without pulling some of the plastic away with it. To overcome this problem, the current is passed through the tip and it is heated again for a very short time prior to extraction. This softens the plastic in contact with the tip and allows the tip to be pulled away cleanly without pulling away any of the plastic, resulting in a superior cosmetic result. As the cycle ends, the tip cools, preparing for a new cycle.

Above: Unlike thermal staking tips that radiate heat at all times, PulseStaking tips are independently and instantaneously heated and cooled. Their targeted, localized heating effect enables them to form plastic stakes and swages with consistent strength and a high-quality finish, even when these must be made in close proximity to other plastics, like this impeller disk. Image courtesy of Emerson.

Process Benefits

The unique characteristics of PulseStaking technology enable it to perform all of the same types of stake or swage processes as can previous technology, often with a higher degree of aesthetic consistency and quality. However, the greatest strength of the technology is its ability to perform in very challenging, high-value staking or swaging applications that were either very difficult or impossible to complete with other approaches. These extremely challenging staking or swaging applications involve:

  • Complex 3-D part designs with varied surface contours and multiple, closely aligned post or flap features.
  • Parts made with any of a growing number of advanced, blended, glass-reinforced, or chromed/metallicized plastics.
  • Parts that must capture fragile or heat-sensitive components (weaves, filter elements, fabrics, ceramics, metals).
  • Parts that must capture and hold delicate, heat- or vibration-sensitive electronic components such as printed circuit boards, soldered components, or sensors.

PulseStaking technology offers advantages for working with complex, contoured, and closely aligned part features because of the unique tip design. Unlike traditional tips, which radiate heat at all times, pulse tips are independently and instantaneously heated and cooled and localized in their heating effect. Therefore, pulsing tips can be positioned much more closely and in more complex configurations than can traditional heated tips. And, because pulsing tips heat up only during their short operating cycle, there is no risk of unintended radiant heating, even if tooling or tips pass very close to nontarget surfaces.

The ability to vary tip and plastic temperatures within a staking or swaging cycle enables the PulseStaker platform to deliver superior, particle-free, and cosmetic results for a wide range of materials, including materials with levels of glass fill exceeding 30%. For example, glass fibers tend to stick to traditional thermal tips and pull away from a finished stake when the tip is removed. However, a pulsing tip can first melt, form, or cool a finished stake, then pulse a different, lower temperature to release the tip without sticking.

Above: Many PulseStaking tips can be built onto a single piece of tooling, enabling multiple stakes or swages to be performed in a single operation on simple or geometrically complex parts, both large and small. Image courtesy of Emerson.

Compared with other forming technologies, PulseStaking technology can also join a wider variety of different materials to plastic components or housings, including metal shims; plastic keys or buttons; filters, fibrous cloth, or insulating materials; printed circuit boards, electronics, or sensors; and fragile glass or ceramic elements.

And, beyond the realm of current applications, the flexibility and versatility of PulseStaking technology open up a variety of new part design and production options. First, heating tips are available in many standard and custom shapes (e.g., domed, rectangular, lozenge) and can be operated singly or, if production and cycle times require it, densely grouped into larger tools capable of performing many forming operations simultaneously on a single part. Second, the localized heating characteristics of each tip enable swaging or staking operations on angled or geometrically complex part surfaces and access to deep cavities or other difficult-to-reach areas.

Review of Staking/Swaging Technology

With the newer PulseStaking technology in mind, below is a quick, “process-neutral” comparison of plastic staking and swaging technology, highlighting key differences and helping to identify when the various technologies can be employed to maximum benefit and value.

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