A Review of Joining Processes for Packaging with Plastics

I A Jones and F A Chipperfield

Pharmaceutical and medical packaging, Copenhagen, Denmark, 18-19 May 1999 and Addendum July 2001

Introduction

The medical industry, like many others, is driven by the need to supply cost-effective solutions in a rapid timescale, within an increasingly competitive marketplace. Benefit needs to be taken of the improvements and options available in manufacturing processes to assist in meeting these increasing demands. The requirements for packaging in the medical industry are very varied, calling for a range of different sealing solutions for the packages. This paper discusses the joining methods that are available and how they are used in medical applications.

Joining requirements for plastics in medical packages

Plastics' joining is used widely in medical applications for products of ever increasing diversity; ranging from tablet packs and sterile dressing packages to intravenous fluid bags and device packages. The wide range of applications; including joining of films, laminates, sheet and moulded plastics call for a variety of joining and welding methods. The joining processes currently used in medical packaging include:

• For flexible packaging
  • Hot bar or heat sealing
  • Impulse welding
  • Dielectric welding
  • Induction welding
  • Adhesive bonding
• For rigid packaging
  • Ultrasonic welding
  • Hot plate welding

The qualities demanded by the medical industry, such as high and consistent joint integrity, high precision and often high production rates increasingly call for improvements to the current methods of joining or the development of new joining processes. A number of new joining methods are therefore continually under development. These include laser and infrared lamp welding. These processes together with the others listed above are discussed in this paper.

Joining methods for flexible packages

Hot bar welding (sometimes called heat sealing)

Hot bar welding is a technique, which is mainly used to join thermoplastic films, i.e. materials having a thickness of less than 0.5mm. The technique is based on the principle that if two thermoplastic films are pressed against a heated metal bar, they will soften and a joint can be made between them. Since the technique relies on the conduction of heat through one of the films, this limits the thickness of material that can be welded. Sometimes two heated bars are employed, one either side of the films, and this has the effect of reducing the welding time.

The equipment comprises one or two metal bars, which are generally electrically heated. One of the bars is hinged to allow the placement and removal of the thermoplastic films, and the weld pressure is applied mechanically by the operator or via pneumatic cylinders. A coating of PTFE is often applied to the bars to prevent softened or molten thermoplastic from adhering to them.

Welding parameters important to this process are bar temperature, weld pressure and weld time.

Hot bar welding can be a rapid process with typical weld times, for thermoplastics of approximately 100µm thickness, in the order of 1-3 seconds. For this reason, hot bar welding has found application in a number of industrial sectors, but is most widely used in the packaging industry for sealing bags and films made from thermoplastics ^[1] . Many medical devices are heat-sealed in packages of thermoplastic and plastic coated paper. The heated region can be textured to reduce and control the joint strength. This offers the opportunity to supply a sterile component in an easy-to-open package. Outer packets for intravenous bags, laminated packs for powders, ostomy bags and some tablet blister packs are also heat-sealed.

Impulse welding

Impulse welding is a variation of hot bar welding where one or both bars are heated and then allowed to cool rapidly. In this way the parts being welded experience a well-controlled heating and cooling regime, while still being held under pressure.

Surgical gloves are sealed to maintain hygiene standards. They are usually made from polyethylene and are joined by impulse sealing (Fig. 1). This is a fast welding process, which creates minimal flash and can be used by an operator or can be automated.

Dielectric (high frequency) welding

Dielectric (high frequency or radio frequency) welding relies on certain properties of the material in the parts being welded to cause the generation of heat in a rapidly alternating electric field. This means that only certain materials can be dielectrically welded, mainly PVC. The process, which dates from the early 1940s, revolves around subjecting the parts to be joined to a high frequency electric field that is normally applied between two metal bars. These bars also act as pressure applicators during heating and cooling. The dynamic electric field causes the molecules in some thermoplastics to oscillate. Depending on their geometry and dipole moment, these molecules may translate some of this oscillatory motion into thermal energy and cause heating of the material. A measure of this interaction is the loss factor 'e' which is temperature and frequency dependent.

Fluid (e.g. blood and intravenous infusion bags) and ostomy bags are essential, disposable hospital products, which are joined using the dielectric welding process (Fig. 2). These products are used for collection, containment and storage. This process is selected as it is low cost, has a high production rate and has great design flexibility. Dielectric welding is also highly suited to cutting at a tool edge and for the production of tear/seal areas in film products allowing easy opening or user-adaptation of the product.

Pneumatic support cells used in the care of pressure sores are also dielectric welded from PVC sheet. Three other types of packaging are produced by this technique:

• Sachets: Extruded flexible vinyl 'lay-flat' (a tube which is folded flat along its length) is clamped, filled and sealed in section, each with the required capacity - to give discrete sachets of powders, dressings or fluids.

• 'Blister' packs: Some blister packs are produced by sealing rigid transparent or translucent film to a substrate material.

• Transparent boxes: Boxes can be made in translucent or transparent acetate, or rigid vinyl film with creasing, by application of heat and welds at the final tear-seal.

Induction welding

Induction welding is a technique where heating is generated by an induced electric current in a conductor built into the product. The two most commonly encountered mechanisms by which heat can be generated by an induction field, are eddy current heating and heating due to hysteresis losses.

Induction welding by eddy current generation is similar to resistive implant welding in that an implant is generally required. Since most unfilled thermoplastics can be described as good insulators, an electrically conducting implant must be present at the joint line. A work coil which is connected to a high frequency power supply, is then placed in close proximity to the joint. As electric current at high frequency passes through the work coil, a dynamic magnetic field is generated whose flux links the implant. Electric currents are induced in the implant and when these have sufficiently heated the conducting material, the surrounding thermoplastic parts melt and soften. If some pressure is applied to the joint, this aids wetting of the molten thermoplastics and a weld forms as the joint cools.

This technique is used as it enables flexible design features, high production rates and can be either a fully automatic, or occasionally a manual, joining process.

Glass and plastic bottles, which are used in the medical and consumer-health industry, have their plastic coated aluminium tops sealed by the induction welding process. A metallic-coated plastic tab (e.g. polyethylene coated aluminium foil) is attached to the top by an induction field. The process is rapid, facilitates seal removal and provides tamper evidence, while retaining product quality. Most tablet blister packs are also sealed in this way. A thermoformed tray is filled with the tablets and then they are sealed into individual compartments by induction heating of a plastic coated aluminium foil with an appropriate clamp.

Other consumer-health products manufactured using this technique include toothpaste tubes, which are filled with dental cream prior to sealing the end. Common materials used are HDPE/aluminium/LLDPE.

Adhesive bonding

Adhesives are widely used in the medical industry with the largest day-to-day usage probably being the wound dressings everyone is familiar with. These are typically pressure sensitive adhesives based on elastomers (e.g. natural or synthetic rubbers) or acrylics. However, there is a broad range of adhesive types, classified often by chemical type, physical form or end use. These are also applied in packaging and medical device manufacture.

By chemical function, the main adhesives of interest to the medical industry are acrylics and pressure sensitive adhesives. Practically every other adhesive type (e.g. epoxies, silicones and polyurethanes) also get used in one application or another, mostly related to medical products used outside the body, or internal but often protected from direct exposure to the internal bodily fluids/organs. As well as bonding various medical devices, acrylics have seen particular interest as surgical tissue adhesives to dress exposed or internal wounds without the need for needles, stitches or other clips. In these cases, the adhesives must be benign and many adhesives grades are now classified as biocompatible and even in some cases, biodegradable. These types of adhesives are often based on a cyanoacrylate, which can be cured by moisture on the skin or tissue, or from the atmosphere.

Joining methods for rigid packages

Ultrasonic welding

Ultrasonic welding involves the use of high frequency mechanical sound energy to soften or melt the thermoplastic at the joint line. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20 or 40kHz. The actual mechanism responsible for the generation of heat at the joint line is not well understood and the heating effect of the ultrasound varies with the degree of crystallinity of the material being welded. The ability to weld a component successfully is governed by the design of the equipment, the mechanical properties of the material to be welded and the design of the components. Ultrasonic welding is a fast process (weld times are typically less than one second) and can easily be automated. It is therefore ideally suited to welding components in mass production.

Incontinence pads are required to provide maximum patient comfort with minimal disruption to lifestyle. The material used is generally polyethylene. Both continuous automatic ultrasonic seam welding and adhesive bonding techniques are used on these products. Ostomy bag flanges can also be welded ultrasonically. Short joining times can be achieved.

Another medical device that uses this technique is a blood filter (Fig. 3), which recycles the patient's own blood (autologous transfusion). The filter is usually made from ABS (acrylonitrile butadiene styrene). The ultrasonic welding process is used as short weld times and an hermetic seal is required and the process can be easily automated. Collection flasks requiring hermetic seals and anaesthesia filters are also ultrasonically welded.

Hot plate welding

Hot plate welding, sometimes called heated tool welding, is probably the simplest welding technique for thermoplastics. The parts to be welded are held in fixtures, which press them against a heated tool. The heating takes place in two stages; the heated tool melts the surfaces and material is displaced so that a smooth surface is obtained; mechanical stops in the equipment or a reduction in pressure prevent further displacement, but the parts continue to be heated by the tool until they are softened some distance away from it. The fixtures then open, the heated tool is withdrawn and the fixtures then force the parts together.

Infrared lamps can be used in place of the hot plate to provide a non-contact heat source and reduce part contamination.

Vibration welding

The principle behind vibration welding is that the parts to be joined are brought into contact, under pressure, before being moved so that the joint area rubs together with a linear reciprocating motion. Heat is generated by friction, and once the material at the joint line has melted the vibration stops, the parts are aligned and the joint cools under pressure.

The main processing parameters are weld time, weld pressure, vibration amplitude and frequency of vibration.

Vibration welding is a commercially practised technology and equipment is readily available. The technique can be applied to almost any thermoplastic material, but industrial applications tend to be based around linear joints which are sufficiently long, such that ultrasonic welding cannot easily be accomplished (approximately 200mm), and hot plate welding would typically take many minutes to perform.

New and alternative joining techniques

Laser welding

Lasers can be used to join either flexible or rigid products and packages.

The laser welding technique involves the generation of an intense beam of radiation, usually in the infra red or visible areas of the electromagnetic spectrum, for melting thermoplastic in a joint. Lasers can be used to weld films of thermoplastic in lap joint configuration very effectively. Weld speeds can be many hundreds of metres per minute making the process ideal for high volume production, in areas such as the packaging industry. By careful control of the laser beam profile, it is also possible to make a weld and cut at the same time in an operation called a cut seal.

Most recent developments have been in transmission laser welding of plastics sheet and mouldings. This technique requires one part to be transmissive to laser light and the other to be strongly absorbing. The light is absorbed at the opaque surface to create a melt and a weld if the two pieces are in contact. An opaque surface coating may also be used to weld two transmissive materials. The process has been demonstrated with Nd:YAG and diode lasers and may also be carried out using an infrared lamp. The welding is rapid with low distortion and does not mark the surface of the component ^[2] .

Mechanical fastening

Snap-in devices are used for 'locking-in' edges. An example of this is medicine bottle tops, which incorporate childproof devices, so that they are not easily opened by small fingers.

Summary

The joining technology for a medical package is integral to its success. The selection of joining method is dependent firstly on the materials and component design to be joined (e.g. are the materials compatible for welding or is a coating or alternative method required?), and then on the performance, quality control, speed and economics of the procedures. The main joining processes for medical packages are listed in Table 1, together with some indication of the typical weld times achievable and the application areas. 

Table 1. Summary of processes for joining plastic packages.

Process	Typical weld time/speed	Characteristics	Applications
Flexible packages
Heat sealing/ Hot bar welding	1-3secs	Suitable for film welding only	Bag sealing, ostomy bags, device packets, powder sachets
Impulse welding	1-3secs	Suitable for film welding only	Bag sealing, blister packs
Dielectric welding	<1min	For high loss plastics only (PVC, PU)	IV bags, tear/seal joints, air cells, boxes
Induction welding	<1min	Needs electrically conducting implant	Tamper proof bottle tops, blister packs, toothpaste tubes
Adhesive bonding	depends on curing requirements	Tape, pressure or reactive types	Plasters, skin, some packages
Rigid Packages
Ultrasonic welding	<1 sec	Strong material dependency, joint design critical	Blood filter unit, collection flasks
Hot plate welding	10secs-1hour	Not suitable for nylons	Pipes
Vibration welding	1-10secs	Vibration in the joint plane	Large devices
Laser welding	up to 750m/min	Film or rigid part welding by controlling absorption position of the light/heat	In development

References

N°	Author	Title
1	Mistry K,	'Looking into the future: materials and joining technology', Medical Device Technology, March 1998, pp36-40.
2	Jones I,	'Laser sealing of plastics for medical devices', Medical Plastics conf. 8-10 Sep 1998.

Addendum (July 2001)

Further developments have now been carried out which enable the laser welding of clear to clear plastics (or a wide range of coloured plastics) using the transmission laser welding technique. Named Clearweld®, the process utilises a consumable absorber for the laser beam, placed at the joint interface. The resulting welds are clear and almost colourless and of high integrity. Melt occurs only at the joint interface and not throughout the component and the process is applicable to sheet and moulded parts, films and fabrics in a wide range of plastic types.

Additional information can be found on the Clearweld® website - www.clearweld.com.