I A Jones
Published in Medical Device Technology, 1, March 2005.
Using the correct welding process can deliver function and appearance at the right price and rapidly
Plastics increasingly provide a solution to balancing the need for speed and cost of production against requirements for function and appearance. In some cases, plastics enable a product to be fabricated that once would have been inconceivable. This can be seen, for example, with microfluidic devices. Plastics have enabled these devices to be made using rapid mass production procedures at a fraction of the cost compared with using glass- and metal-based components.
A factor that will influence the choice of welding technique for a plastic is whether or not the joint will need to be flexible, as in a blood-warming bag, or rigid, as in the hard enclosure of a medical instrument such as aglucometer. Speed, number of welds and production quality are also relevant criteria. The scale of the choice available becomes apparent by reviewing some of the welding options.
Heat sealing. If a product is made from thermoplastic film, that is, a material thinner than 0.5mm, then pressing a hot bar against two layers will join them together; the transmitted heat fuses the material along the line of the bar. For a 100-um thick film, a weld can be formed in 1-3 s. Sealing sterile packages is an obvious application; many devices, instruments and products can be heat sealed behind plastic-coated paper.
Impulse welding. This method uses the same basic principle as a hot bar mentioned above; heat and pressure are used to produce a weld between two sheets of thermoplastic. However, impulse welding introduces much greater control with the same rapid heating experienced with the hot-bar process (1-3 s process time) and cooling that is managed according to an automatable and precise regime. This is a rapid and clean method that produces minimal waste.
Dielectric welding. This process works on the principle that causing a molecule to oscillate generates heat. High-frequency radio energy is used to excite dielectric molecules that are typically poly(vinylchloride)(PVC). The energy is applied between two metal bars that act as pressure applicators during heating and cooling. This is a versatile and low-cost process that is suitable for cutting at the tool edge and for the production of easy opening customised products; material blends can be used to provide controlled weld strength and controlled interaction with the dielectric heating source. Blood, intravenous-infusion and ostomy bags are good examples of products that are joined in this way. The process is versatile enough to be used in the volume manufacture of barrier products such as medical gloves, condoms and inflatable cuffs on balloon catheters.
Induction welding. This process delivers a weld by inducing an electrical current to flow in a wire that is embedded in the product itself. Heat is produced by eddy currents, that is, hysteresis losses. Electromagnetic hysteresis losses generate heating by virtue of the changes in the ferromagnetic domains or regions in the metal implant lagging behind the alternating induction field. The heating that is produced is dependent on the frequency of the inducing field. The current is induced in the embedded wire within the product by a work coil that, when placed close to the product, radiates a high-frequency alternating electromagnetic field. The resultant heating causes the surrounding material to melt and when some pressure is applied, wetting occurs and a welded joint forms on cooling. It is an efficient process that enables high rates of automated production. Applications are many and varied and include the sealing of plastic-coated foil caps on medicine containers and tablet blister packs.
Infrared welding. This process uses an infrared source, which is a tungsten filament line heater or a ceramic plate. The components to be joined must be brought close enough to the heat source for the correct amount of time for melting to occur. When this has happened, the heat is withdrawn and the parts are pushed together to form a weld. The advantages of this process include speed; it is a rapid way of sealing mass-produced flexible components. It is also a non-contact process, meaning that there is nothing to contaminate the joint and brittle components can be welded without damage because no shear force is applied to the parts during heating. This method is used, for example, to weld PVC containers for storing biological samples in liquid nitrogen; each container has 17 welded seams. These are welded by pulsed application of infrared radiation and heating instruments combined with forced cooling of the heating element and the seams. The result is reduced weld volume and reduced overall heating compared with RF welding.
Ultrasonic welding. This process uses ultrasonic energy at a frequency of 20-40KHz. The heating effect is dependent on the crystalline structure of the material being welded. Amorphous plastics readily transmit the ultrasonic energy due to their higher modulus of elasticity compared with typical semi-crystalline plastics. This means that a weld can be made through a greater thickness of amorphous plastic, and using less energy than is required with typical semi-crystalline materials. However, the process is applicable to most types of plastic; procedures and equipment exist to suit the different materials and design of components. It is suitable for flexible and rigid joints and is an efficient way of assembling components in a mass production line. Welds often take less than a second to be formed and the process can be readily automated. Incontinence pads, often made from polyethylene, can be continuously seam welded using this technique. Products such as the flanges on ostomy bags and filters for blood and anaesthetic filtration, often made from acrylonitrile butadiene styrene plastic, can also be welded this way.
Hotplate welding. This is the most basic way of forming a weld between two plastic objects. The faces to be joined are heated by direct contact with a hot metal surface. This causes melting and smoothing of the joint faces, which, when the heating tool is removed, are pushed together and a weld is formed. This process differs from heat sealing in that the parts for heat sealing are put together first and heat is applied to the joint though the plastics of one or both of the components. Clearly this limits heat sealing to welding of thin films whereas hotplate welding is typically used for much thicker components.
Vibration welding. This method is a frictional process in which the surfaces to be joined are pushed together and then vibrated to generate frictional heat. When the heat has caused sufficient melting, the vibration is stopped and the components are aligned and allowed to cool. This technique is particularly useful for joining linear objects that are too large for ultrasonic welding or where hotplate welding would take too long.
Laser welding. Laser welding is growing in its use and range of applications. Lap joints between plastic components can be easily welded using lasers, providing the upper layer allows the laser beam to pass through to an absorbing lower layer. The laser wavelength used passes through most plastics in their natural state. Certain additives such as titanium dioxide and particularly carbon fillers will reduce the laser transmission. It is necessary to check that the plastic formulation used for the component is compatible with the welding process. Lasers can be high-power diode or Nd:YAG sources emitting energy in the infrared range of 808-980 nm. The energy can be delivered to the weld site via a fibreoptic to allow easy and flexible operation with gantry or robot manipulation.
If neither of the components to be joined is fabricated from a material that absorbs light, then a laser absorber must be added by printing or as a film coat at the joint line. Until recently, a carbon-based black absorber had to be applied at the joint interface, which alters the appearance. However, clear, non-absorbent materials can now be laser welded using virtually colourless infrared absorbing inks or other material forms (Gentex Corp., www.clearweld.com).The infrared absorbing medium is printed/painted onto one surface of the joint, integrated into the bulk plastic, or produced in the form of a film that can be inserted into the joint mechanically or potentially in the moulding process. It absorbs infrared laser light and enables an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. There is a range of equipment types available for applying the laser welding process and, as research continues, the options for welding a greater range of materials increases at a pace.
Joining in the future
As medical device manufacturers become increasingly aware of cost and end-users more appearance-conscious, the pressure to use new materials in new ways will increase. Welding will be an important facilitator in making design aspirations manufacturing realities. For those with the courage and imagination, welding is there to help.
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