Didier Perret, medical business development manager, Branson Welding and Assembly at Emerson, considers medical device assembly scale-ups to meet market demands amid COVID-19.
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Whether for contact tracing, infection detection, controlling the spread, preventing or treating the disease, the ability to rapidly scale up manufacturing capabilities is vital in the global fight to control COVID-19. To do so, product-design and manufacturing teams must consider a variety of factors when deciding how to ensure proper scalability when choosing between adhesive or ultrasonic welding assembly methods — two popular methods for assembling thermoplastic medical devices and medical-grade textile materials into safe, reliable, finished products. These considerations generally fall into two broad categories: the speed and flexibility that are needed in the assembly operations; and the materials and shape or geometry of the part.
Safety syringes or auto disable (AD) syringes are commonly assembled using ultrasonic welding technologies or adhesives. Adhesives and ultrasonic welding technologies are also widely used to assemble in-vitro diagnostic (IVD) test kits, strips, collection swabs and biocontainment enclosures. Ultrasonic welding and adhesive tapes are quickly replacing traditional needle-and-thread assembly for nonwoven textiles used for personal protective equipment (PPE) such as face masks, gowns and surgical drapes.
Adhesives and ultrasonic welding are permanent joining methods. It is, therefore, important to also consider whether the assembled device will need to be disassembled to allow for maintenance, component repair or internal parts replacement. To allow for device disassembly, a product design likely needs to incorporate either mechanical fasteners or snap-fit components as opposed to permanent assembly methods such as adhesives or ultrasonic welding.
Flexibility
Of the two assembly methods considered here, adhesives often provide more flexibility in the assembly process. Adhesives can create bonds between plastic components that use a wider range of materials and shapes. They may be a good solution for assembly of products in small quantities, including prototype designs, product samples and high-product-mix production.
Adhesive processing methods do, however, come with many constraints. The first involves the optimisation and maintenance of the dispensing equipment. The applicator must be kept relatively clean, and the adhesive must be applied with consistency and care. When the number of adhesive dispensers increases (for scaling up production), ensuring process control becomes more challenging. Assembly managers must ensure that adhesives flow smoothly, and cure times are consistent and assure part strength. This often requires the periodic purging and cleaning of dispensing systems and applicators, which can lead to waste, production delays or costly equipment repairs. Data process management can also present challenges.
Another constraint on adhesive methods is that adhesives are consumables. Every adhesive bond represents an incremental production cost that rises in direct proportion to output. If product sales and production ramp up rapidly or new product variations are introduced, production costs will likely escalate accordingly.
Production volume change and scalability
If production volumes and expected sales increase rapidly, the benefits of ultrasonic welding become apparent. Ultrasonics offer an excellent opportunity to consider, or reconsider, adhesives versus ultrasonic assembly.
Using ultrasonic welding requires some upfront investment in welder capital equipment and product-specific tooling, which is needed to precisely hold the various plastic components in place for the welding process. This one-time investment can then be effectively and rapidly scaled up using existing equipment, processes and procedures. Whether a manufacturer is welding 1,000 or 1,000,000 devices, there are no incremental consumable or assembly costs.
Manufacturers with production volumes ranging from tens of thousands to millions per year can often realise a clear return on investment using ultrasonic welding processes. Manufacturers planning high-volume production from the outset often choose ultrasonic welding. With ultrasonic welding, once a product design is finalised, weld tooling can be completed and high-volume production can begin.
Cycle time
A key consideration for plastics assembly processes is the cycle time. Adhesive assembly requires curing time for each assembly. The most basic adhesive assembly process may consist of a relatively simple fixture, a handheld adhesive dispenser and a clamp or fixture to hold it steady during the curing process.
Higher-volume adhesive assembly methods often involve automation, but the need to hold the assembly during adhesive dispensing and curing remains. Robotic adhesive dispensing tends to be flexible, as robots can be reprogrammed to incorporate fixture changes, respond to changes to the adhesive chemistries, or adapt to the assembly of different parts with different geometries. All adhesive assembly methods, however, require dispensing and curing time.
By contrast, ultrasonic welding provides a permanent, welded bond in typically one second or less. Once the welded part is removed from the weld fixture, the weld cycle is complete and the next manufacturing step can begin as a new part is loaded immediately to start the next cycle. Ultrasonic processes can weld some products two at a time or in groups.
Materials
Material selection is an important variable in selecting an assembly process. In general, it is more difficult to bond dissimilar materials — rubber to plastics or plastics to metals — making mechanical fasteners, snap-fit or adhesives good considerations.
Adhesives generally offer greater options for bonding between dissimilar plastics. There are some exceptions, as some polymers may react chemically or degrade in the presence of certain solvent bond or adhesives.
For ultrasonic welding, similar or identical thermoplastic polymers tend to weld better than dissimilar polymers. Some dissimilar polymers, however, may also be welded if they have similar glass transmission temperatures (Tg) and melt flow indexes (MFI). Also, amorphous polymers tend to weld better than semi-crystalline polymers, as they have more gradual melt curves and more predictable melt flows between parts, which help to create more consistent bonds. Amorphous materials that weld particularly well include acrylonitrile butadiene styrene (ABS), polystyrene and polycarbonate.
Semi-crystalline polymers are more challenging to weld because these materials tend to melt and solidify more abruptly. These characteristics can make achieving a consistent melt and melt flow more difficult, making it correspondingly more difficult to get a consistent bond. Polyethylene, polypropylene and nylon are examples of semi-crystalline materials that are more challenging to weld.
Geometry
The use of adhesive joining methods allows for considerable variation in the geometry of parts.
Part geometry imposes a few more challenges when it comes to ultrasonic welding, as the structure of the part itself must adequately transmit the ultrasonic vibrations from the horn or sonotrode to the weld joint. Some part geometry or shapes will inherently do this better than others. An example of an easy-to-weld shape would be a cube with walls that are rigid enough to direct energy straight to the weld joint, such as those used in some lateral flow devices for rapid IVD tests. A more difficult shape to weld would be a sphere, as one half would tend to flex under load and, therefore, not transmit the energy as efficiently.
Easy-to-weld parts tend to have the following characteristics:
- Relatively flat surfaces (limited contours) so that good horn contact can be achieved
- Surface area on the top of the part over the weld joint area
- Side walls with enough rigidity to transmit energy to the weld joint
- A properly designed weld joint.
Every part is unique, so the most efficient first step in the design process is to speak with a knowledgeable professional who can evaluate the design, consider assembly needs and find the right solution.
Design for manufacturing: future considerations
Incorporating rapid product volume scale-up capabilities into manufacturing processes can greatly improve a company’s responsiveness to challenging market demands such as those related to COVID-19. A key consideration could be to make design choices that keep assembly options open to both adhesive and ultrasonic welding methods. Perhaps the easiest way to do this is to design a simple tongue-and-groove (Figure 1) joint into the mating surfaces of the assembly. This type of joint offers an inherent alignment feature — the groove — that’s ideal for capturing adhesive and aligning the tongue of the mating surface, as well as for producing a strong ultrasonic weld.
Figure 1: A simple tongue-and-groove joint.
Should production needs or volumes change, it is a straightforward process to convert a tongue-and-groove part from adhesive assembly to assembly using ultrasonic welding. All that is required is the addition of an ‘energy director’ — a small bead of sacrificial weld material — to the bottom of the existing tongue. Typically, this can be done with a modest ‘steel safe’ change to the mold. Then, during the weld process, the energy director on the tongue melts neatly into the groove, resulting in a very precise, high-strength weld joint that offers good sealing properties.