Plastic joining--Do you know how to choose the welding technology that suits you?

Product designers and manufacturers face constant challenges in developing and cost-effectively producing high-quality medical devices and equipment for everything from monitoring to drug delivery, daily care, wound care, surgery, and therapeutic uses.

Typically, my work supports their manufacturing operations by helping them bring new product designs to market. It typically starts with evaluating concepts and ideas 鈥?often in prototype form 鈥?to help manufacturers select and implement an assembly/joining process that meets product performance, quality, cleanliness, and cost goals.

While some joining technologies are more popular and widely used than others, the approach I take is always 鈥減rocess neutral.鈥?This means looking at the entire product assembly challenge with an open mind and considering the capabilities and limitations of all available joining processes.

One of the most popular forms of plastic joining is plastic welding, a joining method that combines heat (or heat from friction) and pressure to form a permanent bond. Plastic welding methods are ideal for assembly when the plastic material and process (see chart at the end of this article) used are compatible, and the application requires a permanent connection or seal between components. Unlike mechanical and adhesive joining methods, plastic welding uses no consumables such as fasteners or glues. Typically, the only costs for plastic welding involve the initial capital investment of purchasing a welder and creating specific part tooling, and the incremental cost of electricity to run it.

Plastic welding technology has evolved to meet a range of assembly needs. In order of their popularity in the medical device space, these technologies include:

1. Ultrasonic welding;

2. Laser welding;

3. Spin welding;

4. Vibration and "clean" vibration welding;

5. Heat treating;

6. Clean infrared technology;

Let's take a closer look at what they are and understand:

Ultrasonic welding

Ultrasonic welding is a very reliable and cost-effective assembly technology. Through a series of components - power supply, converter, booster, horn and actuator - it provides high-frequency, relatively low-amplitude mechanical vibrations with downward force. This movement generates frictional heat at the interface of the parts of the molten plastic, while the downward force compresses the joint to form a strong bond. Ultrasonic welding describes a range of products that operate at frequencies between 15 鈥?40 kHz, with 20 kHz being the most common. The length of the vibrations (called the amplitude) is usually determined by the application engineer based on the materials being welded.

When it can be used, ultrasonic welding offers the advantages of speed (most weld cycles can be completed in less than a second), the ability to handle small or fragile parts, no consumables, no part setup time, low capital equipment costs, and easy integration into automated production processes. The limitations of the process center around the relatively narrow range of materials that cannot be joined, small part size, and the contour/geometry of the parts. In the medical industry, ultrasonic welding is commonly used for applications such as syringes, catheters, and housings (e.g., glucose meters, urine meters for catheters).

For materials that are 鈥渆asy鈥?to weld ultrasonically, such as ABS, parts over 6 inches in diameter can be welded using a 15 kHz ultrasonic welder. (Note: Larger parts = lower frequencies). When parts are made of materials that are more difficult to ultrasonically weld, such as nylon, the size of the parts to be welded decreases to approximately 3.5 square inches (or diameter). Parts with deep contours may also be difficult to weld because these features affect the range and performance of the ultrasonic process. (See the table below for a summary of advantages and limitations.)

Laser Welding

Due to the high initial equipment cost, laser welding is not usually the first solution chosen by manufacturers. But those who need it will soon learn that this cleanroom-capable technology is versatile and well suited for medical applications. It will join parts made of multiple materials in a variety of shapes and sizes while producing zero particulates and flash.

Laser welding uses heat provided by a laser light source in the 780-980 nm range. This light is focused by a fiber optic bundle connected to the welding tool and then distributed across the weld area of the part according to the desired heating density. Because it requires no vibration or relative motion between the parts, it can join fine part features and delicate components without damage while allowing extremely precise alignment and part-to-part sealing. As a result, it is well suited for assembling in vitro diagnostic (IVD) and microfluidic devices, but can also be used for larger and less complex applications.

Relative to ultrasonic welding, laser welding can join a wider range of materials. There are only two part design requirements: First, each assembly must have a part whose material is "transmissive" or "transparent" to the laser wavelength used and a mating part whose material is "absorptive" or "black" to that wavelength. Second, the geometry and stacking of the parts must allow the laser energy to pass through the transmissive part to the weld area, where melting occurs on top of the absorptive part. (See Figure 1 below.)

Meeting these design requirements is not difficult. There are many "transparent" plastic materials, including colored materials, that are readily transmissive to laser light even though they appear opaque. The same applies to absorbing parts. In addition to carbon black, there are a range of colored pigments that absorb laser light. To ensure that your part color and pigment combination will work properly, consult your welding equipment supplier. (See the table below the figure for a summary of advantages and limitations.)

Spin Welding

The spin welding process, like ultrasonic welding, is a friction-based joining method. Spin welding is accomplished by rotating one half relative to a second, stationary half under a clamping load. The rotation generates the heat necessary to melt the material. Once the rotation stops, the actuator briefly continues to apply downward pressure to cure the bond and then releases the parts. Naturally, the joint between the two parts to be welded must be circular.

The process can join many thermoplastics, including parts formed in different molding processes (i.e., injection molding, extrusion, or blow molding), as long as the melt temperatures and flow indices of the mating materials are similar. Spin welding is also suitable for "remote field" welding鈥攚here the mating surfaces being welded are relatively far away (> 1/4 inch) from the contact surface of the horn鈥攁n advantage over ultrasonic welding.

Spin welding is typically used for relatively small, round parts such as syringe caps, cylindrical filter caps, and surgical cannulas, but large diameter parts can also be joined. (See the table below for a summary of advantages and limitations.)

Vibration (and 鈥淐lean Vibration Technology鈥? Welding

Vibration welding is a close cousin of ultrasonic welding, although it uses reciprocating linear motion, rather than vertical motion, combined with downward pressure to join two parts. The frequencies used in vibration welding are much lower than those in ultrasonic welding, ranging between 100 鈥?240 Hz, but the vibration amplitude is greater, ranging from 0.030 inches to 0.160 inches. As a result, the parts it joins are generally larger and more robust.

Vibration welding is highly versatile. It is able to join nearly 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.

Advances 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 vibrated together. Preheating reduces the amount of vibration required to achieve melting, limits the formation of flash and particulates, and is gentler on assemblies that may contain circuit boards or other sensitive electronics. Although CVT is fundamentally similar to vibration welding in terms of part loading and handling, the preheating process increases cycle time and increases energy consumption. While vibration welding has a cycle time of 3-5 seconds, CVT welding typically has a cycle time of 25 to 40 seconds.

Vibration or clean vibration technology is often used in medical manufacturing for larger two-part systems such as patient monitors, infusion pumps, or fluid collection containers. (See the table below for a summary of advantages and limitations.)

Heat Treatment

Heat treatment is another joining method often used in medical products that requires heat staking: placing a metal component into plastic. Heat staking is a process used to secure circuit boards, battery tabs or other electronic components into plastic components or housings. Basically, the metal component is heated to a certain temperature and then pressed into the plastic, which melts and then solidifies to secure the component. (See the table below for a summary of advantages and limitations.)

Hot-melting metal assemblies is critical for battery-powered products, such as portable or wearable meters or other devices. (A related hot working process, hot plate welding, uses a heated plate to heat the opposing edges of two parts before pressing them together. However, this process is not common in medical manufacturing.) 

Clean Infrared Welding Technology

Clean Infrared Technology can weld parts of any size, but it is most often used for larger parts and assemblies. The surface of the finished part is heated by a contoured, non-contact infrared emitter. Once the material softens, the emitter is removed and the parts are assembled together under pressure. The result is a clean, beautiful and virtually particle-free weld.

In addition to welding a wide variety of materials and part geometries, Clean Infrared Technology is gentle enough to join complex assemblies without damaging pre-assembled internal parts. However, infrared tools are generally more complex, more expensive to develop, and have a relatively long cycle. Therefore, clean infrared technology is selectively used in medical applications. An example is a blood filter. (For a summary of advantages and limitations, see the table below.)

So, which plastic joining technologyis better suited for your product?

Almost every product assembly has key features or performance requirements that lead to initial consideration of one or two specific joining methods. In addition, engineers may prefer a specific joining process based on past experience. But no matter how the evaluation and selection process begins, the process must cover a range of factors:

Materials. Part material is a major factor, as it must conform to the joining process requirements. Whenever a product combines small plastic components, ultrasonic welding is almost always considered. However, the effectiveness of ultrasonic welding may be limited when the parts are made of olefin materials (such as polypropylene or polyethylene), highly modified materials, glass-filled materials, or composite materials. For such parts, manufacturers may consider alternative materials that can be ultrasonically welded. Or, they must choose among other processes that can more effectively join the part materials.

Part Size.  While vibration welding and CVT accept large parts, ultrasonic welding does not because of the limitations of its acoustically tuned tools. Laser welding is certainly capable of joining larger parts and producing clean and aesthetically pleasing assemblies, but manufacturers typically use it for smaller parts due to its relatively high cost.
For small devices that must be produced in large quantities, ultrasonic welding is often the technology of choice. Typically, manufacturers use higher frequencies/lower amplitudes (and lighter downforces) to assemble small, fragile parts. For example, many device manufacturers might use a 40 kHz ultrasonic welder with very light downforce to successfully assemble devices without bending, deflecting, or even cracking fragile components. The latest generation of ultrasonic technology can adjust downforce (the force required to initiate a weld) with greater sensitivity and predictability than ever before. And, with joins made in fractions of a second, cycles are very fast and energy efficient.
As parts become larger and more robust (thicker walls, longer surfaces, etc.), the frequency decreases, but the amplitude and downforce increase, along with the downforce used to hold the part in place. Therefore, many medium-sized parts might be ultrasonically welded in the 30 kHz to 20 kHz range, then down to 15 kHz until the size limits of the process are reached. Then, for larger and more robust parts, vibration welding or CVT, using lower frequencies and higher amplitudes, are the logical answer.

Part shape or geometry.  Any joining process that generates heat through friction (ultrasonic, vibration, or spin welding) must have parts with relatively straight or flat joining surfaces so that the tool can contact and the vibratory motion can be transferred through the parts. Spin welding requires a round part with a profile or notch that can be used to grip the part and apply a rotational force.
Processes that rely on direct heat, such as CVT or infrared welding, are more versatile because 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 demands for cleanliness and purity. Many are manufactured and packaged in cleanroom environments, with part details and flow paths that have little tolerance for impurities such as flash and particulates.
When cleanliness is critical, laser welding is often the answer, especially for medical devices that require particle-free quality. However, if part mating surfaces can be designed with features that contain melt flash and particulates, ultrasonic welding, vibration welding, or CVT can provide a more cost-effective answer.

Above: This chart illustrates the probability of obtaining good connection results by process based on experience with different component and material properties. Anomalies do occur. Materials and parts with "limited" connection probability usually depend on application or material specific factors.

Internal components.  The market for in vitro diagnostic and implantable medical sensors, analyzers and drug delivery devices is exploding. Applications like this, where assemblies contain electronics, require a gentle joining method, so high-frequency ultrasonic welding (40 kHz) or vibration-free laser welding may be candidates. Laser welding provides an aesthetically pleasing join without causing deformation of intricate features or small parts. It also creates a hermetic seal between small parts without minimal flash or particulates, a quality that is critical for cleanroom-quality assembly and packaging, and for products that can be trusted to deliver precise insulin, hormone or drug delivery therapies. And, because it does not introduce vibration or mechanical movement between parts, laser welding enables extremely precise weld alignment and part-to-part sealing. Welds are fast, perfectly clean, with zero minimal particles and zero flash.

Production Speed.  With welding cycles measured in fractions of a second, there is no faster joining process than ultrasonic welding, making it ideal for mass production of medical products and devices that meet dimensional and material requirements. Its cousins 鈥?spin welding and vibration welding 鈥?also join parts quickly, with typical cycle times of one to several seconds. Of the joining methods that directly or indirectly heat parts, laser welding is the fastest, typically followed by CVT and clean infrared technologies.

Capital Cost.  Once you have determined a high-quality product design and the best joining method, the actual cost of the joining equipment should be your final consideration.

The best option for selecting the most advantageous technology for your application is to keep an open mind and remain 鈥減rocess neutral鈥?in the decision-making process. Understand the advantages and limitations of each available process and work closely with the equipment/solution provider to develop a solution that best suits your manufacturing and application requirements.