Lasers excel in high speed plastics welding

TWI Bulletin, November/December 1994

Ian Jones graduated from Jesus College, Cambridge with a degree in Natural Sciences, specialising in Materials Science. He joined The Laser Centre at TWI in 1989, and since then has worked on a wide range of laser applications in the Laser Processing Section. Specifically these have included development of high power laser welding of steel, nickel-based, titanium and aluminium alloys, as well as plastics processing. Ian is also involved in EUREKA initiatives for the advancement of laser processing in European industry.

Nicki Taylor gained a degree in Metallurgy and Materials Science from the University of Birmingham in 1985. Following this she joined the Plastics Joining Group at TWI, where she is currently the Technology Manager. Her work has included the development of welding techniques for continuous fibre-reinforced composite materials, and a wide range of application studies involving welding of thermoplastic and composite materials.

Laser technology may be used as an alternative high speed, non-contact technique for welding plastics. Ian Jones and Nicki Taylor review the processes currently available for joining plastic sheet and compare these with recent studies using lasers.

Plastic packaging is widely used in most industries, notably the food and medical industries. These two areas increase the requirements from sealed units. The sealing operation is critical to quality assurance of the product in manufacture of medical and food goods.

As well as the packaging industry, thin plastic sheet and film are used for a number of other applications, for example automotive interiors. Often, for these applications, a cut/seal joint is what is required, to minimise processing.

Polymeric fabrics are used for items such as disposable dressings in the medical industry or as a filtering medium. Again, they require cut/seal joints to be made, often through a laminate of fibrous materials.

This article reviews the welding techniques currently used in plastics packaging and then describes work carried out using laser technology to provide an alternative flexible, non-contact, high speed technique for a number of application areas.

Current welding techniques

Ultrasonic sealing involves generating heat by microfrictional effects induced by ultrasonic excitation. One shot sealing of plastic packages can be achieved by this process, with typical sealing times of below one second. Its limitation, however, is the size of the package. Ultrasonic sealing is also used in the cut/seal operation for fabrics. Here a good joint can be obtained, but the maximum speed expected would be in the region of 30 m/min.

Hot air is used in a variety of ways for processing plastics. Packages, such as food trays, can be sealed by using a precise arrangement of tiny directed jets of hot air around the tray rim. The major drawbacks of this technique are that equipment must be custom-made for each product, and the cycle times for the process are quite long. Hot wire welding is widely used in simple shrink-wrapping equipment. The hot wire is often allowed to burn through the film, performing a cut/seal operation. This process is cheap and simple to perform, but not very fast.

Induction sealing is best known for use in sealing diaphragms on to jars or bottles. It is also incorporated into a number of other systems where aluminium foil is a constituent of laminate materials. The obvious disadvantage with induction sealing is that it is only applicable where there is a metallic element to the joint.

Dielectric welding is used for sealing and cut/sealing of a large number of items, such as book covers, inflatable products, interior car door panels and seat covers. In the medical industry, glass bottles have been replaced by PVC welded bags and automatic machines produce intravenous blood, urine and colostomy bags at rates of up to 1000/hr. The major disadvantage of the process, however, is that because it relies on the presence of a polar molecule within the plastic, it is only applicable for use on materials such as PVC and some polyurethanes.

The disadvantages listed for the above processes have led to research being performed on use of lasers for processing thin plastic sheet, film and fabrics where there may be advantages for high speed processing.

New techniques

High speed welding of plastics using lasers

Industrially, two laser types are predominantly used; the CO ₂ laser and Nd:YAG laser. The CO ₂ laser uses a gas mixture to produce laser energy with a wavelength of 10.6µm and lasers are now available in the power range from 30W to 40kW. The lasers normally operate in a continuous wave mode. For this wavelength, the beam can be transmitted through air and is reflected and focused using mirrors and special lenses.

The Nd:YAG laser uses a solid crystal rod excited by flashlamps to produce laser energy with a wavelength of 1.06µm and lasers are now available in the power range from 30W to 2kW. The lasers can operate in a pulsed or continuous wave mode. For this wavelength, the beam can be transmitted through a fibre optic beam delivery system which is advantageous for remote access applications and complex robotic manipulation.

Applying lasers to joining plastics, CO ₂ laser energy generally couples very well and Nd:YAG laser energy is transmitted by clear plastic materials. The high coupling efficiency of the CO ₂ laser has enabled lasers to be used for cutting a range of plastic materials, mainly in sheet, where high cutting speeds and precise details can be achieved. However, the tendency for vaporisation of the plastics, because of the high power densities created by the CO ₂ laser energy, prevents their use in joining if similar parameters to those used in cutting are adopted. Various approaches have been attempted to enable plastics to be joined by lasers, including laser cutting of polyethylene (PE) and polypropylene (PP) plates (6mm thick) followed by pressure welding, ^[1] volume heating of PP and PE (15mm thick) using a 100W CO ₂ , laser, ^[2] and following laser heating by local application of pressure. ^[3] In this work, little attention has been paid to joining thin sheet plastics (<1mm) or application of Nd:YAG lasers to joining plastics.

Materials and joint configurations

For this work, PE and PP in the thickness range 0.1-2mm have been evaluated. Trials have been carried out to produce welds in lap joint, cut/seal and butt joint configurations.

Laser types and equipment

CO ₂ laser. Continuous wave 120W and 1500W CO ₂ lasers have been used in the laser welding trials. The laser beam was focused with a ZnSe or KCl lens and argon was used as gas shielding.

Nd:YAG laser. Continuous wave powers of up to 500W from Nd:YAG lasers with fibre optic beam delivery systems have been used in the laser welding trials. An argon shroud was used as gas shielding.

Workpiece handling. Trials were performed using a high speed table capable of traverse speeds of up to 100 m/min. Simple clamping of the plastic sheets was used in all cases.

CO ₂ laser

Trials were carried out on PE and PP sheets of 0.1-0.2mm thickness respectively, in lap joint and cut/seal configurations.

Lap joint configuration. The welding speeds and tensile strengths of welds (compared with parent material) for the 0.1mm thick PE and 0.2mm thick PP are summarised in the Table. A typical joint in 0.1mm PE is shown in Fig 1. The results showed that welding speeds of up to at least 100 m/min could be achieved for both 0.1mm thick PE and 0.2mm thick PP using laser powers of 900W and 800W respectively. A 100W laser could also be used to weld the 0.1mm PE at 16 m/min and the 0.2mm PP at 51 m/min. In tensile tests, the parent material failed rather than the joint.

Table: Summary of welding conditions and tensile properties

For thicknesses greater than 0.2mm in a lap joint configuration, vaporisation of the material caused the top sheet thickness to be reduced due to loss of material, see Fig.2, and this produced reduced tensile strengths.

Cut/seal joint configuration. The welding speeds and tensile strengths of welds (compared with parent material) for 0.1mm thick PE are summarised in the Table and a typical joint is shown in Fig.3. The results showed that welding speeds of up to 10 m/min could be achieved for 0.lmm thick PE at 100W. In tensile tests, the joint failed at 94% of the parent material value. At a higher power of 400W, the cut/seal could be achieved at 50 m/min, but at some loss of strength (only 20% of the parent strength).

Nd:YAG

Trials were carried out on 0.5mm thick PE and 0.2 and 2.0mm thick PP sheets in lap and butt joint configurations.

Lap joint configuration. The welding speeds and tensile strengths of welds (compared with parent material) for PE and PP are summarised in the Table. Welding speeds of 0.1 and 0.2 m/min were obtained for 0.5mm thick PE and 0.2mm thick PP at power of 80W. In tensile tests, the joints failed at approximately 70% of the parent material value. The speed could be increased to 0.5 m/min when a 400W laser power was used. The nugget-type weld obtained is shown in Fig.4.

Butt joint configuration. The welding speeds and tensile strengths of welds (compared with parent material) for 2.0mm thick PP are summarised in the Table. Welding speeds of 0.1 m/min were obtained at 100W power. In tensile tests, the joints failed at approximately 30% of the parent material value.

Discussion

This work has demonstrated the potential of laser technology for joining plastics. In particular, low power CO ₂ lasers have the capability of producing joints in lap and cut/seal configurations at high speeds (at least 100 m/min) which could have applications for high speed packaging where a non-contact, flexible, computer-controlled system would have advantages over ultrasonic, dielectric and heat sealing techniques. For the Nd:YAG laser, the lower absorption capabilities of the shorter wavelength, produced the possibility of welding sheet material up to 2mm thick, with the associated benefits of low distortion and low general heat input which has enabled laser technology to be applied in mass production sheet metal welding industries such as automotive fabrication.

However, a number of differences in the melting of the plastic materials and the effect of pigmentation have been noted particularly in the Nd:YAG laser work. Further work is required to study the effect of additives and pigmentations on the coupling efficiency of different laser wavelengths and the relation to joint properties, as well as further materials studies on other thermoplastic materials (for example, nylon, high temperature plastics).

With regard to potential applications for this technology, joining of thin sheets of PE and PP and other plastic films should be relevant to the packaging industry. The technology could also be suitable for other applications using woven and foam-based materials (such as sanitary ware, dust protectors) and medical applications (like catheter sheathings).

Conclusions

CO ₂ lasers (up to 1000W) can be applied to joining thin (<0.2mm) plastic sheets in PE and PP at high speeds (up to 100 m/min) in lap joint and cut/seal configurations.

Nd:YAG lasers have potential to be applied to joining plastics (PE, PP) in the thickness range 0.2-2.0mm in lap and butt joint configurations. Further work is required to examine other materials and the effect of pigmentations/additives.

Applications for laser technology should be found in high speed flexible packaging for medical, chemical and food industries.

References