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.
-
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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