Welding plastics

Detailed knowledge of the capabilities of different welding techniques can bring significant savings. Roger Wise of TWI examines the current available technologies for welding plastics

Welding technology for plastics has been around for almost as long as the plastics themselves. In the 1930s, plastic compasses were being spin welded, and in the 1940s PVC was being hot gas welded for a range of military applications. Other processes, such as hot bar welding, ultrasonic welding and high frequency welding, were then developed as applications demanded improved fabrication and manufacturing technology.

Since then, the total number of commercially available welding techniques has risen to 20, each with its own unique features. A reasonable working knowledge of each technique is essential to exploit the full benefit of cost effective manufacture of thermoplastic products.

It is now generally accepted that welds are formed in polymeric components when appropriate values of temperature and pressure are applied for sufficient time to allow polymer chains from either component to interdiffuse. This situation is represented in Figure 1 where the individual polymer chains are shown crossing the weldline over a certain time period.

In order for this concept to work, polymer chains must be able to move. For most polymers, this only happens above certain critical temperatures called the glass transition temperature (for amorphous polymers) and the melting temperature (for semicrystalline polymers). The three basic welding parameters of pressure, temperature and time govern the weld strength.

Once the effect of these parameters has been determined, the next step is to work out which welding process to select to suit the material being welded, the geometry of the part and the required production rate.

Detailed knowledge of how welds are expected to fail is extremely important to anyone trying to design thermoplastic components which include welds.

With this knowledge, appropriate safety factors can be built into the design to ensure that components will perform satisfactorily in service for their required lifetime. Mechanical properties of welds, such as fracture, fatigue and stress corrosion cracking should be assessed if appropriate. Many of these properties will be time and temperature dependent due to the nature of thermoplastic materials.

Sometimes, a welded joint will be designed to consistently fail on the application of a predetermined load, for example in the packaging industry, to provide a means to access the item which has been packaged. A detailed analysis of how welds fail is outside the main scope of this article, although many of the advances made in welding technology result from a desire to improve the strength and consistency of welds.

Polymers may be divided into two groups: thermoplastic polymers like polyethylene and nylon, and thermosetting polymers like epoxy resin and cyanoacrylate. Thermoplastic polymers comprise linear chains which, above their glass transition temperature or melting temperature, are free to move around their neighbours at a rate governed by their length and their temperature.

Thermosetting polymers comprise a network of connecting chains which is preserved at all temperatures up until it is destroyed at high temperature. Welding demands movement of chains across the weldline and so only thermoplastic materials are weldable.

When two thermoplastic components are pressed together, and the linear chains are free to move (above glass transition or melting temperatures), the chains will only cross the weldline to form a weld if components are made of the same polymer type (in some cases dissimilar polymers will weld).

Temperature should also be maintained above the glass transition or melting temperature, and sufficient time must be allowed for chains to move across the weldline. These form the main rules for the weldability of the material. Although they can be rarely applied to fully explain any one process, they do provide a framework for understanding basic weldability issues.

There are believed to be approximately 20 distinct welding techniques if they are classified by method of heating. These 20 techniques have been divided into three broad groups, heating by friction, heating by conduction and heating by electromagnetism.

Heating by friction includes spin welding, vibration welding, ultrasonic welding, orbital welding* and friction stir welding*. Heating by conduction includes hot plate welding, hot bar welding, impulse welding, hot gas welding, extrusion welding, BCF welding, solvent welding, and forced mixed extrusion welding*. Heating by electromagnetism includes resistive implant welding, induction welding, EMA welding, dielectric welding, laser welding, microwave welding* and infra red welding*. Processes marked with an asterisk have been developed over the last 5 years.

It is believed that approximately five techniques have been developed during the last 5 years. This makes the job of selecting the optimum manufacturing processes for thermoplastic products particularly difficult.

Spin welding or direct rotational friction welding involves rotating one component relative to the other and then pressing them together under a known force or at a known rate. Heat is generated over the area of contact and the weld forms as the polymer melts and molten polymer is displaced as flash. Applications are limited to items with axisymmetry such as PVC pipe fittings to pipes and polyethylene water floats.

Vibration welding or linear friction welding involves components being brought into contact under pressure and moved such that a linear reciprocating motion is generated between them. This motion is usually approximately 1mm to 5mm and may be at the rate of 50-300Hz. Heat is generated by friction causing the thermoplastic to melt and flash to be produced. Applications include the manufacture of car bumpers which may be 1.5m long.

Ultrasonic welding involves the use of high frequency (>20kHz) sound energy to soften or melt the thermoplastic at the joint. Unlike ultrasonic welding of metallic materials where the motion is parallel to the joint and a rubbing action is produced, ultrasonic welds in plastics involve movement perpendicular to the joint resulting in a cyclic compression of the joint. The success or otherwise of this technique depends on the geometry of components at the joint line and provision for special features called `energy directors’ is usually required. Due to very low weld times, ultrasonic welding is widely used in industry, notable applications being in the automotive, electronics and biomedical industries.

Orbital welding involves the generation of heat between components to be welded by a relative motion which is described as orbital. This motion results in an even distribution of generated heat over the rubbing surfaces, until the plastic softens or melts. Currently available equipment is capable of welding medium sized components such as fluid reservoirs for automotive use.

Friction stir welding was first developed in 1991 as a means to weld aluminium alloys but has been successfully applied to thermoplastics. The technique involves placing components to be welded together, clamping them securely in place and then driving a rotating metal tool along the joint line. The frictional contact of the rotating tool with the thermoplastic causes softening or melting of the material. Friction stir welds have also been produced where the relative motion between metal tool and joint was in a linear reciprocating fashion (Figure 2).

Hot plate welding or heated tool welding involves pressing components to be welded against a metallic plate heated to a temperature above the glass transition or melting temperature of the thermoplastic. When the surfaces of the components have melted and flash produced, the hot metallic plate is removed and the still molten surfaces pressed together until the thermoplastic solidifies.

Hot bar and impulse welding are both techniques which are widely used for joining thermoplastic films and each involves the generation of heat in a metallic component. With hot bar welding, films are pressed against the bar and allowed to fuse together before being removed. With impulse welding, films are pressed against a heated element and remain there while the element cools down after the weld has formed.

Hot gas welding involves softening or melting abutted thermoplastic components using a stream of gas which has been heated by being passed over hot electric elements. A consumable rod of the same type of thermoplastic is also melted by the hot gas and a weld is formed as the consumable and components are melted together. Applications for hot gas welding are many and varied, and include the repair of car bumpers and the welding of geomembranes.

Extrusion welding involves the use of a stream of hot gas to soften or melt the thermoplastic components to be joined, and the addition of a consumable thermoplastic which is molten. The consumable is added to the extrusion welding machine in rod or granule form and is made molten by being passed through a small, electrically powered extruder. This material emerges as a rod of molten thermoplastic which is introduced to the softened thermoplastic components by a shoe shaped to produce a smooth, even weld bead.

Bead and Crevice Free (BCF) welding was developed as a means of joining thermoplastic pipes without any weld bead or flash of any kind. The technique involves closely butting the pipes together and clamping them, and then heating the joint with an electrically heated metal collar.

At the same time, an inflatable bladder is inserted into the pipe and inflated so that the joint is constrained in every direction. As the thermoplastic under the collar melts, polymer chains from either side of the joint line begin to mix, but weld flash cannot form.

Solvent welding involves the application of thermoplastic in solution or gel form to the components before pushing them together. The solvent softens the surface of the components allowing thermoplastic in solution and from the components to mix together. When the solvent has evaporated a weld is formed. Applications include welding fittings to pipes.

Forced mixed extrusion welding was developed in the early 1990s as a means to weld thick section (>10mm) thermoplastic components with parallel sided joint preparation in order to minimise weld distortion (Figure 3). The technique involves positioning components to be joined a fixed distance apart and then heating and welding the components using a moving head. The heating is achieved using a hot gas stream followed by a heated metal plate which contacts the opposing faces of the components. A rotating metallic rod then removes material from both faces and mixes this material with extruded thermoplastic introduced from the centre of the rotating rod. In this way, a parallel sided joint is achieved using extruded material and material from the thermoplastic components.

Resistive implant welding involves clamping an electrically conducting material between thermoplastic components, connecting it to a power supply, and then passing sufficient electric current through the conductor to cause heating and melting of the surrounding thermoplastic material.

With induction welding, an electrically conducting material is clamped between thermoplastic components and then sufficient electric current is induced through the conductor to cause heating and melting of the surrounding thermoplastic. Electric current is induced by a dynamic magnetic field usually generated by a workcoil which is connected to a high frequency power supply. The conductor must make a continuous loop to allow the induced current to flow. Applications for induction welding include the sealing of tops to plastic bottles where the tops are discs of metal foil coated with thermoplastic.

EMA welding involves clamping a strip containing small particles of ferritic material between thermoplastic components and then applying a dynamic magnetic field which is driven by a power supply at 3-10MHz.

Dielectric welding (also called radio frequency welding) utilises a property of some thermoplastics in being able to align themselves with an applied electric field. If this electric field oscillates in the MHz region, a loss mechanism within the polymer produces heat which can cause melting of the polymer. Polymer films are clamped between metallic plates across which a dynamic electric field is set up. The plates apply weld pressure and long welded joints can be produced by overlapping the ends of shorter lengths.

With laser welding, a laser beam is directed at a joint in a thermoplastic component and the energy is used to melt the thermoplastic. The energy density of the laser must not be so high as to cause degradation of the polymer. When welding overlapping films of thermoplastic material, weld speeds of over 500m/min have been achieved.

Microwave welding involves clamping a strip of microwave susceptible material between thermoplastic components and then exposing the assembly to microwave energy (Figure 4). The microwave susceptible material could heat by several possible mechanisms including eddy current, hysteresis or dielectric loss. This technique potentially allows welds of any geometry to be made but will only work provided that the components themselves are not susceptible to the microwave energy.

Welding with infra red requires heating the surfaces of thermoplastic components to above their glass transition or melting temperature using infra red energy, before pressing them together, to allow molecular mixing to occur. The application of the infra red energy is usually via a tungsten filament lamp (Figure 5) or using electrically heated ceramic material. Weld times are generally in seconds although the infra red absorption characteristics of thermoplastics can vary considerably.

SUMMARY

Joining of thermoplastic materials, or thermoplastics to other materials is a field generally comprising three broad options, mechanical fasteners (eg bolts and rivets), adhesives (eg epoxy resins, cyanoacrylates and so on) and welding. The selection of the most appropriate option will depend on many interdependent factors, including in service performance, type of material to be joined, required joining rate and cost. Consideration should be given to as many of these factors prior to design.

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{{TABLE 1: Factors affecting welding techniques