How plastic welding technologies help in joining single-use medical devices

Extending effective treatment to those who need it often requires bringing care closer to the patient or making clinic-based testing and treatments simpler and faster to administer. For medical manufacturers, these demands require solutions such as microfluidic “tests on a chip,” or improved at-home in-vitro tests, oxygen therapies and peritoneal dialysis equipment.

Design and manufacturing challenges arise as diagnostic tests and therapies like these evolve for use by people without professional medical training.. At-home test results must be reliable compared to those obtained in a laboratory, oxygen and fluid flows must move evenly and reliably, and patients must be protected from adverse outcomes, such as periodontal infection resulting from nonsterile fluid connections.

As a result, single-use components are critical. They must perform properly, be lightweight, easy to ship, and readily affordable. To reliably and affordably manage these complicated requirements, medical manufacturers develop and mass-produce a wide range of engineered plastic flow management devices. However, the complexity of these devices generally requires that they be assembled from multiple injection-molded parts, and final assembly methods typically use some form of plastic welding.

Ultrasonic plastic welding
Many single-use assemblies, such as automated peritoneal dialysis (APD) cassettes, are assembled using ultrasonic plastic welding, a widely used technique that employs vibratory motion, heat and gentle compressive force to create solid-state welds between plastic components.

Components are held in tooling, then subject to high-frequency (10-70 KHz), low-amplitude (1-250 µm) mechanical vibration that generates intramolecular friction, melting the mating surfaces of the components and creating a strong molecular bond in the finished part. The process suits efficient, high-volume production because weld cycles are so fast — typically less than one second — and the welding equipment integrates easily with automated production lines. The latest ultrasonic welding innovations allow exacting control over downforce and weld depth for added accuracy.

Like other forms of plastic welding, ultrasonic welds join plastic assemblies without the need or expense of consumable items like adhesives or mechanical fasteners. There is no need for clamping glued assemblies together so that they set properly. However, parts designed for ultrasonic welding must meet certain geometric and material requirements.

Hot-plate welding
Other flow management products may be welded using hot-plate welding, another proven plastic welding technology. In hot-plate welding, the two opposing components to be joined are pressed against a heated metal plate until the edges soften. The hot plate is then withdrawn, and the parts are brought together under pressure, allowing the two sections to bond as the plastic cools. This process offers a high degree of control over heating and weld quality, which is why it is still used today.

While the fluid paths used in APD and blood separator cassettes are relatively large, medical flow management devices that require the use of much smaller microfluidic channels — including single-use disks, cartridges or cards used for in-vitro diagnostic tests, clinical chemistry and immunoassays — may require a more precise plastic welding process, such as laser welding.

Laser welding
There are two types of laser welding: simultaneous through-transmission infrared (STTIr) and quasi-simultaneous. Both transmit laser light through a transmissive (transparent) part to the surface of an absorptive (dark) part. The laser energy generates heat at the interface between the two parts that melts the plastic. Then, with the help of controlled compressive force, the softened part surfaces are brought together and the weld is completed.

Both types of laser welding apply heat to plastic parts with exceptional precision, but do so in different ways.

Simultaneous laser welding delivers laser energy to the weld zone using specially designed waveguides, fiber-optic bundles that are custom-shaped to heat the entire weld joint of the part simultaneously. The simultaneous method allows for use of a lower power density and also offers advantages for mass production, since cycle times are shorter

Quasi-simultaneous welding utilizes a laser and a set of movable mirrors to trace a beam of laser energy along the contours of weld joint in a continuous, closed loop, so weld cycles take longer to complete and require a higher power density.

Both of these laser welding methods can control the direction and depth of energy application and weld zone heating with high accuracy so they are well suited for welding parts with fine details, narrow flow paths or heat-sensitive internal components. When needed, customized part masking can also be used to block heat-sensitive areas. Masking may also be used with a wide beam when parts require surface welding. Weld depths are typically controlled to tenths of millimeters, and laser welded joints are hermetically sealed. In addition, because laser welded joints do not require vibratory motion, they are always free of particulate and therefore ensure unimpeded flow paths. Medical Design & Outsourcing