Advances in Joining Methods

Norbert Sparrow

Identifying an appropriate welding process or adhesive for a specific application can be critical to maintaining product integrity and containing production costs.

Many types of components for medical devices must be bonded together in some manner. Selecting the optimal method, however, is not always a simple task. Part of the difficulty stems from the availability of alternative joining technologies, each of which has inherent advantages and limitations.

Several welding methods are suited for the bonding of plastic components, from laser systems to ultrasonic or vibration technology. Because the choice of a process can directly affect product integrity and manufacturing costs, careful consideration of the appropriate technique is critical. Criteria that should be considered include part geometry, joint design, the types of materials that are being joined, cost, processing time, and operating conditions.

Bielomatik Leuze GmbH + Co. (Neuffen, Germany) has developed an Nd:YAG laser system that, according to the company, produces a high weld-joint strength and achieves short cycle times. Because this low-heat, vibration-free laser welder produces flash-free joints and does not leave behind residue that might contaminate components, it is suited for bonding sensitive parts. While most thermoplastics can be laser-welded using this system, a successful bond is dependent on one of the bonded parts being transparent and the other containing a heat-absorbent additive.

An Nd:YAG laser manufactured by Electrox (Letchworth, Herts, UK) is suited for processing a wide range of metals such as stainless steel, titanium, and a variety of alloys used in medical applications. Increased throughput and superior weld quality are among the advantages of this process compared with alternative methods, according to sales manager Ian Bell. "The heat input is low, so material stress is reduced and distortions are negligible," says Bell, who adds that "because laser welding is a noncontact operation, tooling wear is minimized."

Ultrasonic joining methods have been refined by machine manufacturers such as Telsonic AG (Bronschhofen, Switzerland). The company has developed a CNC ultrasonic welding machine with a servomotor drive and programmable force/time profile. The use of brushless three-phase motors in lieu of a conventional pneumatic drive system enables these welders to achieve short reaction times even in the presence of high welding forces. The optimized welding-force profile permits joining very precise parts while suppressing material blistering typically caused by polymeric vapourization.

New Adhesives See Light

According to George W. Ritter, research engineer at the Edison Welding Institute (Columbus, OH, USA), welding may be the preferred joining method if a device incorporates compatible metals or thermoplastics. If, however, the bonded materials are characterized by a mismatch in the coefficient of thermal expansion, cooling rate, or modulus, or if there is poor molecular weight distribution, adhesives may be your best option, says Ritter.

But which adhesive? To determine whether an acrylic, epoxy, urethane, silicone, or UV-curing formulation is the most appropriate material, manufacturers should consider such factors as the substrate, bond strength, and resistance to impact, temperature, humidity, and chemicals, says Ritter. Engineers should also make it a point to stay current with developments in the medical-grade adhesives field, where evolving technologies frequently result in enhanced material properties and processing characteristics. A case in point is a new family of light-curing cyanoacrylates from Loctite Engineering Group (München, Germany).

USP Class VI– and ISO 10993–compliant cyanoacrylates are popular with device manufacturers because of their rapid curing capabilities, bond strength, and compatibility with a variety of substrates. These materials do have their limitations, however. Fillet cures can be slow unless a solvent-borne accelerator is used, notes Loctite application engineering chemist Christine M. Salerni. Cyanoacrylates are also prone to blooming, a white residue that results when the cyanoacrylate monomer volatilizes and settles on the part. Light-curing acrylic adhesives, on the other hand, will not cure in areas that light cannot reach. The company's light-curing cyanoacrylates combine the desirable characteristics of these adhesives while eliminating their process limitations. Not only do these new materials cure rapidly, but they will cure anionically in shadowed areas. In addition, cyanoacrylate's propensity for blooming and stress cracking has been minimized.

Cure Accelerators May Hinder Performance

While the device industry's need for fast, repeatable, and traceable assembly methods has led many manufacturers to the use of UV- and visible-light cure adhesives, industrial sales manager Ruben Burga of EFOS Inc. (Mississauga, ON, Canada) cautions against the ill-considered use of photoinitiators. These additives may well accelerate the curing rate, but at a cost of diminished adhesive performance. To achieve an appropriate cure, manufacturers should use an adhesive tailored to the application, says Burga, and ensure that the material is cured at the appropriate intensity for the correct length of time. Makers of light-source equipment, such as EFOS, he adds, can help device manufacturers to establish curing parameters that will achieve repeatable, traceable, and documented manufacturing processes.

A common error manufacturers make in the adhesives selection process, says Ritter, is to "ask it to do something it was not meant to do." He cites the example of stress management in relation to joint design. "The objective is to transfer loads, not to bear them," he says, noting that ignorance of this principle can lead to serious design flaws.

In fact, Ritter's sage advice is equally applicable to the selection of any bonding method. To determine which type of joining technology is best suited to your product--and to pinpoint precisely what it can and cannot do to ensure that your device is safe and cost-effective—contact the companies profiled in this section. They all have acquired substantial expertise in joining technologies for the medical device industry.

Epoxy Technology

Featuring more than 100 standard epoxies in its product range, Epoxy Technology (Billerica, MA, USA) supplies several USP Class VI materials suited for bonding applications in the medical device sector. The 353ND epoxy, for example, is used to bond and coat endoscopes, catheters, diamond scalpels, dental tools, pacemakers, and related devices that require failure-free performance. "Welch Allyn uses our optical epoxies for bundling the fibre-optic assemblies in their laryngoscopes," says vice president of technical operations Dick Estes. "We also have a product called 301, which is virtually inert in terms of mutagenicity, that is used to mould the housings of titanium injection ports that are sewn under the patient's skin," adds Estes.

The company is preparing to launch OG602, which was undergoing USP Class VI testing at the time of writing. This UV-curable single-component adhesive "is totally transparent through the spectral range and cures within three seconds," explains Estes. Because of its chemistry, he adds, the company is very confident that testing will reveal it to be a nontoxic inert material. It is suited for multiple uses, including temporary tacking or applications where very precise alignments are required, says Estes.

The company is certified to ISO 9001, and its research and manufacturing facilities are certified by the US Defense Electronic Supply Center for lab suitability.

EFD

Dispensing systems designed by EFD (Dunstable, Beds, UK) incorporate valves and controllers to precisely deposit adhesives and other fluids. The company builds bespoke systems to meet customer requirements based on five variants of dispensers and six basic types of valves, says UK sales manager Gary Cordier. "Typically, we will take a standard valve and change certain parts to meet an application," says Cordier.

One recent project involved depositing precise, repeatable amounts of UV adhesive to bond a cannula to a polypropylene hub. Sales of the finished product—a disposable syringe with a needle that automatically retracts after injection to prevent accidental needle sticks—exceeded projections. To keep up with demand, the manufacturer, which had been using a benchtop system designed by EFD, asked the company to develop a semiautomated dispenser that would increase production volumes without affecting overhead, says Cordier.

"We recommended a setup incorporating 10 adjustable needle valves suited for making microdeposits of low- to medium-viscosity fluids," says Cordier. After placing a work holder in position, the operator simply lowers a bar to cycle the valves and make 10 identical deposits. In addition to increasing output tenfold compared to the benchtop dispenser, the system was designed to maintain a high degree of process control. "We fitted each valve with an individual microprocessor-based controller," says Cordier. "If there are any variations in the dispense cycle, the operator can easily adjust that parameter and document any changes that were made."

Ease of adjustment was also built into the dispenser. Each valve's dispense-time setting can be adjusted in increments down to 0.001 second, and once the optimal settings have been determined for the first system, the remaining nine programmes can be easily set to produce the same results.

EFD also produces a cyanoacrylate dispenser that automatically deposits identical dots or uniform beads of adhesive by means of a timed air-pulse system. The system is designed to replace squeeze bottles, toothpicks, and other manual methods of adhesive application.

Dymax Europe GmbH

A company that offers UV- and light-curing adhesives as well as related equipment has announced the availability of a family of adhesives that comply with ISO 10993, the international standard for assessing medical device biocompatibility.

"ISO 10993 was written primarily for finished products," says Harald Schulz of Dymax Europe GmbH (Frankfurt, Germany). "The standard was not articulated with components in mind, so it proved to be rather difficult to apply to adhesives." Nevertheless, Schulz feels that doing so was worth the effort, in the interests of both Dymax customers and European industry at large. "It's very important for us to have an international standard," he says. "We were using USP Class VI because nothing else was available, but ISO 10993 is, in fact, more stringent than the US standard," says Schulz. "And, it's international!"

The company supplies flexible and rigid adhesives in a range of viscosities for bonding cannulae, catheters, face masks, and related devices. UV-cure adhesives contain fluorescent particles to facilitate 100% in-line inspection. A complimentary application evaluation service is also offered: customers are invited to send samples to the company's laboratory, where Dymax personnel will determine the most appropriate bonding process for a specific device.

Dymax also produces a line of curing lamps, including the Medi-Cure UV Lamp, which is housed in a stainless-steel enclosure and includes a built-in filter for use in clean environments.

Bielomatik Leuze GmbH + Co.

Laser welding is suited for joining rigid medical parts, housings that contain sensitive electronic components, and other assemblies susceptible to heat or vibration. The Laser-Tec welding system developed by Bielomatik Leuze GmbH + Co. (Neuffen, Germany) yields flash-free joints and does not leave behind residue that could compromise the integrity of a component. In addition, it enables manufacturers to achieve high weld-joint strength and short cycle times.

While most thermoplastics are suited for laser welding, one of the welded parts must be transparent to permit penetration of the laser beam, and the other part must contain a heat-absorbent additive. Weld-joint width can be adjusted from 0.1 mm to several millimetres. Tests conducted using materials such as polycarbonate, polypropylene, POM, and PA6 have shown that the weld-joint strength produced by this process is equal to that of the materials being bonded.

Telsonic AG

Unlike conventional longitudinal welding equipment, ultrasonic torsional welding machines produce twisting vibrations, causing plastification to occur as a result of frictional heat. Because a minimal load is transferred to the welded part, this technology is suited for the assembly of delicate medical devices, according to Telsonic AG (Bronschhofen, Switzerland), which manufactures both types of machines.

The oscillating movement causes the welding action to occur horizontally, thereby subjecting the welded part to minimal vibration. The method is suited for welding parts onto thin membranes as well as joining components with sensitive electronic assemblies. Films and foils can be easily welded, and flanging can be performed without the need to clamp the part.

Part sizes should not exceed 100 mm diam, however. In addition, only round or semiround forms are suitable for ultrasonic torsional welding because of the linear reduction in amplitude toward the centre of the system's sonotrode.

The company also offers conventional ultrasonic welding machines, including CNC models with a servomotor drive and programmable digital force/ time profiles. The latter enable short reaction times, even with high welding forces, and are suited for the welding of precise medical components.

Permabond

A manufacturer of engineering adhesives, Permabond (Eastley, Essex, UK) recently launched a division dedicated to the development and marketing of medical-grade adhesives and sealants. The company also announced that it will introduce new lines of photocurable adhesives, cyanoacrylates, epoxies, silicones, and urethane adhesives suited for medical device assembly applications. Industry can expect to begin seeing these product launches as early as March, according to general manager for Europe Nab Kalsi. "These hybrid technologies will represent a step change for industry," Kalsi predicts. This flurry of development work was made possible, in part, because of the expertise available to Permabond through its parent company, National Starch & Chemical Co. (NSC; Bridgewater, NJ, USA).

"We can draw on years of adhesive technology at NSC," says Kalsi, "and we are not limited by any single existing product range." The company has no incentive to favour one type of adhesive over another, he explains, because it supplies such a broad range of products. Customers can rest assured that Permabond will recommend the most appropriate bonding solution for any given application. "Bespoke solutions that involve reformulating the chemicals can be offered...we have the resources to create our own polymers, if that is required to meet a customer's needs," says Kalsi.

A photocurable adhesive that bonds various plastics within a fraction of a second, and a cyanoacrylate that produces quick, strong bonds in a variety of metal and polymer substrates are among the products currently available.

Permabond 4L35 is suited for bonding polycarbonate, polystyrene, acrylics, and other plastics. Because of its wetting capabilities, the material is particularly well-suited for joining hard-to-bond plastics such as highly plasticized polyvinyl chloride. It withstands gamma, EtO, and E-beam sterilization methods.

The cyanoacrylate Permabond 4C10 bonds such substrates as steel, brass, plaster, stone, aluminium, ABS, polyester, PVC, and many other plastics within 5 to 20 seconds. Suited for the high-speed production of medical devices, the colourless, transparent liquid has a viscosity of 40 cP at 77°F.

These and other products suited for device applications are described in a brochure titled Permabond Adhesives for the Medical Device Industry that is available free from the company.

EFOS Inc.

Visible-light, UV, and spot-curing systems with up to 20,000 mW/cm² of curing power are designed and manufactured by EFOS Inc. (Mississauga, ON, Canada). The manual and automatic equipment is fitted with changeable filters that enable the user to tailor the output for wavelength-specific applications and to protect sensitive substrates. According to industrial sales manager Ruben Burga, establishing appropriate curing parameters at the outset is crucial to ensuring optimal adhesive performance.

"Although UV- and visible-light-curing adhesives can be heavily loaded with photoinitiators to enable extremely rapid cures, this will often adversely affect the final performance of the bond line," says Burga. Performance may be inhibited by such factors as insufficient wetting time, uneven curing stresses within the adhesive, excessive shrinking, excessive outgassing, and partially terminated polymeric chains in the adhesive. To ensure that curing is carried out at the appropriate intensity for the correct length of time, Burga recommends consulting with a light-source supplier such as EFOS. These parameters can be programmed into a system such as the Novacure, he adds. "This will ensure consistent manufacturing with repeatability, traceability, and reporting capabilities that will satisfy the rigorous manufacturing protocols of medical device manufacturers," says Burga.

Electrox

Because of their short wavelength, Nd:YAG lasers can use conventional glass optical components, enabling the incorporation of fibre-optic delivery systems. Consequently, the laser beam is readily absorbed by a range of metals. A pulsed Nd:YAG laser manufactured by Electrox (Letchworth, Herts, UK), the Scorpion is suited for these types of applications, particularly those requiring highly accurate welds.

Rated at 0–400 W average power, the Scorpion attains up to 7 kW peak power and delivers 0–70 J of pulse energy. The unit's 225–1000-µm focal-spot diameter produces peak-power intensities that enhance throughput and weld quality. The laser beam's pulse can be shaped with respect to time, thus enabling the welding of a variety of materials, but the device is most effective with highly reflective and coated materials. Spot welding requires access from a single side of the workpiece.

"Laser welding is a noncontact operation, so tooling wear is minimized," says sales manager Ian Bell. "Since electrodes are not used, you can achieve very clean welds, with no weld nuggets, as such." Bell adds that the low heat input reduces material stress. "That, and the hermetic nature of the weld, are among the primary advantages of laser welding over alternative methods," says Bell.

The company, which celebrates 20 years of designing and manufacturing industrial lasers this year, is currently developing diode-pumped lasers for welding applications. Using the diode as the pumping mechanism increases the lifetime of the device. "This technology is available in marking lasers," says Bell, "and we have been able to develop diode-pumped welding lasers, but the cost is a limiting factor at this point."

SEBRA

Thermoplastics such as polyethylene, fluoropolymers, polyvinyl chloride, polypropylene, and polyurethane can be welded into finished catheters by means of a system developed and manufactured by SEBRA (Tucson, AZ, USA). The CE-marked PIRF system is designed to produce high-quality, maximum production yields while complying with rigorous international standards related to safety and emissions.

About the size of a briefcase, the catheter manufacturing device provides fast, precise, and repeatable control of heat time, cool time, and insertion pressure. A proprietary process ensures accurate mould-temperature control during heating.

Loctite

Cyanoacrylates are popular adhesives with device manufacturers because they cure rapidly, exhibit high bond strength on a variety of substrates, and are USP Class VI and ISO 10993 compliant. The material does have some disadvantages, however, including a propensity for blooming under certain conditions. This is particularly problematic in medical applications, because device manufacturers typically package their product shortly after manufacture, which can cause vapour traps that lead to blooming. Other drawbacks include a susceptibility to stress cracking on some plastics, limited gap cure, and slow fillet cures if accelerators are not used. A family of light-curing cyanoacrylates recently introduced by Loctite Engineering Group (München, Germany) reportedly overcomes these limitations while retaining the material's desirable characteristics.

"With our light-curing cyanoacrylates, exposed fillets cured rapidly even when low-intensity sources were used, thereby preventing blooming," says application engineering chemist Christine M. Salerni, based at Loctite's North American Engineering Center. "We were able to achieve cure times as low as three seconds," she adds.

Loctite's material cures in depths greater than 0.25 in. within 25 seconds, Salerni adds, which prohibits the uncured liquid adhesive from coming into contact with the plastic and creating an environment conducive to stress cracking. Other benefits include rapid cure speeds that are obtained without oxygen inhibition, and curing in shadowed areas by means of anionic reactions.

In addition to light-curing cyanoacrylates, Loctite offers more than 30 medical-grade adhesives, including other light-cure adhesives, instant adhesives, and activators, primers, and specialty sealants.