Plastics Joining—Do You Know Your Options?
To meet joining needs for smaller, more delicate plastic parts, the Branson GSX ultrasonic welder blends precision and control. Its advanced “feather touch” electro-mechanical actuation system can begin a weld with very low trigger force. (Image courtesy of Emerson.)
Product designers and manufacturers face constant challenges in developing and cost effectively producing high-quality medical equipment and devices for everything from monitoring to drug-delivery, to daily care, wound care, surgical, and therapeutic uses.
Often, my work supports their manufacturing operations by helping them bring new product designs to market. It often begins with evaluating concepts and ideas—often in prototype form—to help manufacturers select and implement an assembly/joining process that meets product performance, quality, cleanliness, and cost goals.
Though some joining technologies are more popular and widely used than others, the approach I take is always “process neutral.” That means viewing the entire product assembly challenge with an open mind and considering the capabilities and limitations of all joining processes available.
One of the most popular forms of plastics joining is plastic welding, a family of joining methods that combine heat (or friction-generated heat) and pressure to create permanent bonds. Plastic welding methods are ideal for assembly when the plastic materials being used are compatible and the process (see the chart at the end of this article) and the application requires a permanent joint or seal between the components. Unlike mechanical and adhesive joining methods, plastic welds do not use consumables like fasteners or glue. Generally, the only costs for plastic welding involve the initial capital investment for purchasing a welder and creating part-specific tooling, plus the incremental cost of electricity to run it.
Plastic welding technologies have evolved to meet a range of assembly needs. In rough order of their popularity in the medical device field, these technologies include:
- Ultrasonic welding.
- Laser welding.
- Spin welding.
- Vibration and “clean” vibration welding
- Thermal processing
- Clean infrared technology
Let’s take a quick look at each of them.
Ultrasonic welding is a very reliable and cost-effective assembly technique. Through a sequence of components—a power supply, converter, booster, horn, and actuator—it delivers high-frequency, relatively low-amplitude mechanical vibration with downward force. This motion produces frictional heat at the interface of the parts that melts the plastic, while the downforce compresses the joint to create a strong bond. Ultrasonic welding describes a range of products that operate at frequencies from 15 – 40 kHz, with 20 kHZ being the most common. The length of the vibration, known as amplitude, is typically determined by an applications engineer based on the materials being welded.
When it can be used, ultrasonic welding offers the benefits of speed (most weld cycles are completed in less than a second), an ability to handle small or fragile parts, no consumables, no part set-up times, low capital equipment costs, and easy integration into automated production processes. The limits of the process center primarily on the relatively narrow range of materials that cannot be joined, the smaller size of parts, and the contours/geometry of parts. In the medical industry, ultrasonic welding is often used for applications such as syringes, catheters, and housings (e.g., glucose meters, urine meters for catheters).
For materials that are “easy” to ultrasonically weld, such as ABS, parts over 6 in. in diameter can be joined with a 15 kHz ultrasonic welder. (Note: larger parts = lower frequency). When parts are made of materials that are more difficult to ultrasonically weld, such as nylon, the size of the part to be welded decreases down to about 3.5 in. square (or diameter). Parts with deep contours may also prove difficult to weld, since these features can affect the range and performance of the ultrasonic process. (See the below table for a summary of advantages and limitations.)
Because it has a higher initial equipment cost, laser welding usually isn’t the first solution that manufacturers select. But those who need it rapidly learn that this cleanroom-capable technology is remarkably versatile and well suited to medical applications. It will join parts made of a vast array of materials in a wide variety of shapes and sizes, all while generating zero particulates and flash.
Laser welding uses heat provided by a laser light source in the 780-980 nm range. This light is concentrated through fiber optic bundles connected to the weld tooling, then distributed over the weld area of the parts according to the heating density required. Since it requires no vibration or relative movement between the parts, it joins fine part features and delicate components without risk of damage, while allowing for extremely precise alignment and part-to-part sealing. Thus, it’s ideal for assembling in vitro diagnostic (IVD) and micro-fluidic devices, but can be used for larger and less-delicate applications, too.
Relative to ultrasonic welding, laser welding joins a much wider array of materials. There are only two part design requirements: First, every assembly must have one part whose material is “transmissive” or “clear” to the laser wavelength used, while the mating part material is “absorbent” or “black” to that wavelength. Second, the part geometry and stack-up must allow passage of laser energy through the transmissive part to the weld zone, where the melt occurs in the upper section of the absorbent part. (See Figure 1 below.)
Meeting these design requirements is not difficult. There are many “clear” plastic materials, including colorized materials, which readily transmit laser light even though they seem opaque. The same holds true for absorbent parts. Beyond carbon black, there are a range of color pigments that absorb laser light. To be sure your combination of part colors and pigments work properly, consult with your weld equipment supplier. (See the table below the figure for a summary of advantages and limitations.)
Above: Figure 1: A laser heat source connected to the weld tooling is directed through a transmissive part layer into an absorptive part layer, where the melt occurs. Downward force from the tooling completes the laser weld.
The spin welding process is, like ultrasonic welding, a friction-based joining method. Spin welds are achieved by rotating one part half against the second, stationary half while under a clamp load. The spin creates the heat needed for the materials to melt. Once the rotation stops, the actuator briefly continues downward pressure to solidify the bond, then releases the part. Naturally, the joint between the two components to be welded must be circular.
The process joins many thermoplastics, including parts formed in different molding processes (i.e., injection-molded, extruded, or blow-molded) so long as the melt temperatures and flow indices of the mating materials are similar. Spin welding also accommodates “far-field” welds—welds where the mating surfaces of the welds are relatively far (> ¼ inch) from the horn-contact surface—which is an advantage relative to ultrasonic welding.
Spin welding is commonly used for relatively small round parts such as syringe caps, caps for cylindrical filters, and surgical trocars, though large-diameter parts can be joined as well. (See the below table for a summary of advantages and limitations.)
Vibration (and “Clean Vibration Technology”) Welding
Vibration welding is a close cousin to ultrasonic welding, though it uses reciprocating linear motion, rather than vertical motion, plus downward pressure to join two parts. The frequencies used in vibration welding are considerably lower than in ultrasonic welding, ranging between 100 – 240 Hz, but the amplitude of the vibrations is larger, ranging from 0.030 in. – 0.160 in. Thus, the parts it joins are typically larger and more robust.
Vibration welding is quite versatile. It is able to join virtually all types of plastics and handle complex shapes and large sizes. The process and its tooling are scalable, so multiple parts can be welded in a single cycle.
Advancements in vibration welding have led to a recent innovation called clean vibration technology (CVT). CVT utilizes an infrared heat source to precisely preheat the weld surfaces before they are vibration-welded together. Pre-heating reduces the amount of vibration required to achieve the melt, limits the formation of flash and particulates, and is much gentler to assemblies that may contain circuit boards or other sensitive electronics. Though CVT is essentially similar to vibration welding in terms of part loading and handling, the preheating process adds cycle time and increases energy consumption. While a vibration weld cycle is 3-5 seconds, 25- to 40-second cycle times are common for CVT welds.
Vibration or clean vibration technology is typically used in medical manufacturing for larger two-part systems such as patient monitors, infusion pumps, or fluid collection vessels. (See the below table for a summary of advantages and limitations.)
Thermal processing is another joining method that is often used in medical products that require heat staking: the placement of metal elements into plastic. Heat staking is the process used to fasten circuit boards, battery tabs, or other electrical components into plastic components or housings. Basically, the metal component is heated to a temperature and then pressed into the plastic, which melts and then solidifies to secure the component. (See the below table for a summary of advantages and limitations.)
Heat-staked metal components are essential for battery-powered products, such as portable or wearable meters or other devices. (A related thermal process, hot-plate welding, uses a heated platen to heat the facing edges of two parts before they are pressed together. However, this process is not common to medical manufacturing.)
Clean Infrared Technology
Clean infrared technology can weld parts of any size, though it is most often used for larger parts and assemblies. Facing-part surfaces are heated by contoured, non-contact infrared emitters. Once material has softened, the emitters are removed, and the parts are brought together under pressure. The result is a clean, aesthetically pleasing weld that is virtually particle-free.
In addition to welding a wide range of materials and part geometries, clean infrared technology is so gentle that it can join complex assemblies without damaging pre-assembled inner parts. However, infrared tooling is typically more complex and more costly to develop, and the cycles are relatively long. So, clean infrared technology is selectively used in medical applications. One example is in blood filters. (See the below table for a summary of advantages and limitations.)
So, What Process Is Right for Your Medical Product?
Nearly every product assembly has key characteristics or performance requirements that lead to initial consideration of one or two specific joining methods. In addition, an engineer may prefer a particular joining process based on past experience. But regardless of how the assessment and selection process begins, the process is going to have to cover a range of factors:
- Materials. Part material is a primary factor because it must be compatible with joining process requirements. Whenever a product combines small plastic parts, ultrasonic welding is almost always considered. However, the effectiveness of ultrasonic welding may be limited when parts are made with olefin materials (e.g., polypropylene or polyethylene), highly modified materials, glass-filled materials, or composites. For parts like these, manufacturers may consider alternate materials that can be ultrasonically welded. Or, they must select among other processes that will join the part materials more effectively.
- Part size. While vibration welding and CVT accept large parts, ultrasonic welding does not, given the limitations of its acoustically tuned tooling. Laser welding is certainly capable of joining larger parts and producing clean and aesthetically pleasing assemblies, though manufacturers often limit its use to smaller parts due to its relatively high cost.
For small devices that must be produced in quantity, ultrasonic welding is often the technology of choice. Generally, manufacturers use higher frequencies/lower amplitudes (and lighter down-forces) to assemble small, fragile parts. For example, many device makers may utilize a 40 kHz ultrasonic welder with a very light downforce to successfully assemble devices without bending, deflecting, or even cracking fragile parts. The latest generation of ultrasonic technology can regulate downforce (the force needed to initiate a weld) with far greater sensitivity and prevision than ever before. And, with only a fraction of a second required for joining, cycles are very fast and very power-efficient.
As parts get somewhat larger and more robust (thicker walls, longer surfaces, etc.) the frequency is reduced but the amplitude and downforce are increased, along with the downforce used to hold the part in place. So, many mid-sized parts may be ultrasonically welded in the 30 kHz to 20 kHz range, moving down to 15 kHz until the size limit of this process is reached. Then, for larger and more robust parts, vibration welding or CVT, which use a much lower frequency and higher amplitude, are a logical answer.
- Part shape or geometry. Any joining process that generates heat through friction—ultrasonic, vibration, or spin welding—must have parts with comparatively straight or flat joining surfaces, so tooling can make contact and vibratory motion can be transmitted through the part. Spin welding requires a circular part with a contour or notch that can be used to grip the part and apply rotational force.
Processes that rely on direct heating, such as CVT or infrared welding, are more versatile, since their tooling and heat-transfer surfaces can be contoured to accommodate nearly any part size or geometry.
- Part cleanliness/aesthetics. Obviously, medical products and devices often must meet high requirements for cleanliness and purity. Many are manufactured and packaged in clean-room environments, with part details and flow paths that have virtually no tolerance for impurities, such as flash and particulates.
When cleanliness is at a premium, laser welding is often the answer, particularly for medical devices that demand particulate-free quality. However, if part mating surfaces can be designed with features that contain melt flash and particulate matter, ultrasonic welding, vibration welding, or CVT could provide an even more cost-effective answer.
Above: This chart illustrates the probability for a good joining outcome by process, based on Emerson’s experience with varied part and material characteristics. Exceptions do occur. Materials and parts with “Limited” joining probabilities often depend for success on specific factors of the application or material. Consult a plastics joining expert to learn more.
- Internal components. The market for in vitro diagnostics and implantable medical sensors, analyzers, and drug delivery devices is exploding. Applications like these, where assemblies contain electronic components, require gentle joining methods, so either high-frequency ultrasonic welding (40 kHz) or vibration-free laser welding are likely candidates. Laser welding provides a cosmetically attractive connection without causing deformation of intricate features or small parts. It also creates hermetic seals between small parts without generating minimal flash or particulates, a quality essential to products that require cleanroom-quality assembly and packaging and that are trusted to deliver precise insulin, hormone, or medication drug-delivery therapies. And, because it introduces no vibration or mechanical motion between parts, laser welding enables exceptionally precise weld alignment and part-to-part sealing. Welds are fast, perfectly clean, with zero minimal particulate and zero flash.
- Production speed. With weld cycles measured in fractions of a second, no joining process is faster than ultrasonic welding, so it is ideal for mass production of medical products and devices that meet size and material requirements. Its cousins—spin and vibratory welding—are also capable of joining parts quickly, with typical cycles of one to several few seconds. Of the joining methods that apply direct or indirect heat to parts, laser welding is the fastest, typically followed by CVT and clean infrared technology.
- Capital cost. Once you’ve settled on a high-quality product design and an optimal joining method, the actual cost of joining equipment should be your final consideration.
Your best option in selecting the most advantageous technology for your application is to have an open mind in the decision making process and be “process neutral.” Understand the advantages and limitations of each process available and work closely with equipment/solution providers to develop a solution that works best for your manufacturing and application requirements.