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Medical Device Manufacturing: Components Bonding and Joining Techniques

Structural adhesives, in both liquid and tape formats, are used in a growing number of medical applications as replacements for mechanical fasteners.

Solvent Bonding

This method uses a solvent to fuse two plastics together. Simple in principle and simple in use. Solvent bonding is inexpensive and very fast. However this method demands the use of thermoplastic which will dissolve to fuse together. The application of a solvent at the bond lines induces sufficient mobility for the polymer chains to interdiffuse. Because the solvent must strongly plasticize the polymer surface, this joining technique is primarily applied to glassy amorphous thermoplastics, such as polycarbonate (PC), acrylic (AK) and Polystyrene (PS) resins.

These techniques have found wide use by virtue of their low cost and adaptability to high speed production. In addition adhesive and solvent bond provide a relatively uniform distribution of stresses over the assembled areas and high strength to  weight ratio.

Solvent Bonding is only for joining of amorphous thermoplastics, whereas adhesive bonding can be used with almost any plastic also Joints with large, induced gaps cannot be secured via solvent bonding.

  1. Welding (Fusion Bonding) :

Many medical devices can be joined without consumable adhesives or fasteners. Although snap and pressfit applications are the least expensive options, they are often not permanent or secure enough. When that is the case, welding is often the best solution.

In welding, heat is applied to melt the polymeric material at contacting surfaces enabling through intermolecular diffusion and polymeric chains entanglement process to form a joint. Welding processes for Medical Device assembly are often categorized and Identified by the heat method that is used. All processes can be divided into two major categories: External Heating and Internal Heating. Internal heating methods are further divided into two categories: internal mechanical heating and internal electromagnetic heating.

External Heating method relies on convection and/or conduction to heat to the weld surface. These processes include hot tool, hot gas, extrusion, implant induction and implant resistance welding. Internal mechanical heating methods rely on conversion of mechanical energy into heat through surface friction and intermolecular friction. These Processes include ultrasonic, vibration and spin welding. Internal Electromagnetic heating methods rely on absorption and conversion of electromagnetic radiation into heat. These processes include infrared, laser, radio frequency and microwave welding.

It is important to understand and compare different methods in order to determine which may be best for a given application. Some of the popular plastics welding methods for Medical Devices are presented here.

Ultrasonic Welding

The ultrasonic welding process is used quite commonly for medical devices because its strengths nicely complement the requirements of medical device assembly. Economics often favor ultrasonic welding because cycle times are typically very short. In ultrasonic welding, high-frequency vibrations generate heat at the interface of the parts to be joined, melting the plastic and creating a strong bond. Ultrasonic Welding is very popular technique for fusion and bonding of thermoplastics and thermoplastic components.

This technique is efficient, non-contaminating and requires no consumables. It produces a clean weld site with highly uniform joint quality, providing exceptional performance, consistency and reliability. Ultrasonic welding is often used in mass production because the welding times are relatively short (often less than 1 s). Ultrasonic welding is a flexible technique that can also be used for small lot size production - as long as the fixtures are designed to be flexible. It is applicable to both
amorphous and semi crystalline thermoplastics. However, Limitations includes; high capital & running cost of equipment and poor gap filling.

RF Welding

Radio Frequency (RF) welding, which is also often referred to as “dielectric welding” is a process that relies on internal heat generation by dielectric hysteresis losses in thermoplastics with polar side groups. It is most commonly used to weld PVC bladders, such as intravenous drip bags for the medical industry. RF welding has the advantages that it is a relatively fast process with typical cycle times ranging from less than 1s to 5s. It also does not require any special joint designs and produces
welds that are relatively appealing cosmetically. RF welding is almost exclusively used for welding thin sheets or films. Thickness usually ranges from 0.03 to 1.27 mm (0.001 to 0.050 in), depending on the material and application.

The limitation of RF welding is due to the fact that a strong electric field must be generated and this can only be achieved when the welding electrodes are brought together in close proximity (0.03 to 1.27 mm). Another limitation of the process is that the materials being joined must have the proper electrical properties.

Common applications include blister packs for packaging and medical applications such as IV bags and Urine Bags.

Laser Welding

When welding seams in the micrometer range are required, there is only one economical solution – laser plastic welding. Laser welding is a vibration-free, particulate-free, and, in most cases, flash-free process that is nearly silent. Other aspects make this joining technique additionally interesting for medical applications, such as: flexible production options, stress-free processing, very fine process control and hygienic high-precision processing chains – all assuring high quality and profitability. The process is nearly independent of part-size considerations.

Laser welding still has relatively high capital costs, faster cycle times come with higher capital costs also the process is sensitive to part fit-up.

Plasma Processing :

The medical device industry is highly regulated which demands reliable and reproducible components and manufacturing processing. Bonding mismatched plastics can be challenging for engineers and technicians in medical device manufacturing, even when the proper adhesives are utilized for maximum adhesion results.

Low temperature gas plasma can be a simple, safe, inexpensive, and powerful alternative to prepare materials for bonding for the manufacture of medical devices. Plasma processing is an excellent choice for increasing the surface energy of polymers enabling more wettable surfaces.

A Number Of Other Techniques Are Used For Joining & Bonding :

Hot gas welding, Speed tip welding, Extrusion welding, Contact welding, Hot plate welding, High-frequency welding, Injection welding, Ultrasonic welding, Friction welding, Spin welding, Laser welding, Solvent welding and plasma processing are some of the commonly used techniques. The applications, important features and the limitations for each technique is summarised in the attached table.

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