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Plastic Welding Robotics
by Bennett Brumson, Contributing Editor
Robotic Industries Association Posted 11/20/2006
Take a look around and you will no doubt see plastic parts everywhere. Given the wide variety of plastic materials and processes used to produce these parts and weld them together, it should come as no surprise that manufacturers are increasingly incorporating highly flexible robots into their operations.
To Have and to Hold
Whether the robot manipulates the part or the welder depends on many factors, including degree of flexibility needed, the type of welding process being used, and integration considerations.
'We often use robots to manipulate the plastic part to a stationary machine that performs the actual welding as opposed to using the robot to manipulate a welding head around the part,' says David Williams, Senior Packaging Technology Leader, at Motoman Inc., West Carrollton, Ohio. 'For example, we have customers that swap out various types of welders, depending on the type of parts being run.'
'Sometimes the robotic system handles the welders. That has a lot of value in terms of positioning, accuracy, and repeatability because welding is often performed at a very critical point in manufacturing,' says Bashar Abdo. 'In most cases, the robot will hold the weld head, but there are times when the robot holds the part.' Abdo is the Cartesian Robotics Product Manager at Bosch Rexroth Corp., Linear Motion and Assembly Technologies Group, based in Charlotte, North Carolina.
How Do I Weld Thee? – Let Me Count the Ways
'Several types of welding processes are commonly used to join plastic parts. The oldest is probably ultrasonic or vibration welding, which has been around for decades,' Williams says. 'Ultrasonic sound waves are used to create a high-frequency vibration that changes the molecular structure of the plastic, joining the two parts,' Williams explains. This form of plastic welding uses an acoustic tool to transfer energy in the form of high frequency sound waves (20 kHz or greater) through the part to the joint area. This vibration energy is changed to heat through friction that melts the plastic. An advantage of ultrasonic plastic welding is that it can be used to weld plastics as well as plastic-based fabrics and films, providing for increased flexibility.
Friction welding is another type of plastic welding undertaken by robotics. 'Friction welding has two pieces pressed together and moved back and forth quickly. The pressure and motion create heat generated by the friction, that welds the parts together,' David Williams says. With friction welding, parts to be assembled are abraded together at a lower frequency than in ultrasonic welding, usually 100 to 300 Hz and higher amplitude, up to two millimeters. The friction caused by the vibration in conjunction with the pressure applied by the robot yields a strong weld. The robot keep parts remain clamped together until the weld joint cools and the plastic resolidifies. The friction movement can be linear or orbital depending on the geometry of the part.
Spin welding is also a type of robotic plastic welding. Motoman’s David Williams explains how this type of robotic plastic welding works. 'Spin welding has one part is located in a stationary fixture while the other one rotates. The combination of revolutions per minute and pressure generates heat that bonds the molecular structure of the monomers in the plastic, cross-linking them together.' This welding process is similar to friction welding where the robot supplies the pressure to keep the two parts together while the plastic material cools and begins to cure.
Laser welding is another useful means of connecting plastic parts together. 'Some plastic welding is done with a laser. Whether you hold the part or the head depends on the actual process,' Abdo says. 'If you are using a fiber-based plastic in a laser welding work cell, integrators have to keep in mind that the fiber needs to be rotated. In a lot of cases, you have to rotate the part and keep the welding head steady so you do not introduce fatigue into cables.'
Laser welding robotics have a laser light passing through one plastic component while the other piece absorbs the laser energy. The energy heating the parts in conjunction with pressure welds the parts together. The benefit of using a robot stems from the ability to heat both weld surfaces simultaneously as opposed to heating a single point as with spot welding.
'Robots can perform hot melt, laser, hot gas, and extrusion welding for plastics. All can be applied well in a continuous path application on a robot,' says Michael Sharpe. 'Robots can tend ultrasonic welding systems.' Sharpe is Engineering Manager for the Materials Joining Segment at FANUC Robotics America, Inc., Rochester Hills Michigan.
'In hot staking plastic welding, a Cartesian robot can be used to apply force as a complement to the weld head. The robot can provide some of the force in addition to the heat that is coming out of the welder to join the parts together,' Abdo says. Hot staking plastic welding has the robot applying pressure for a controlled amount of time.
Dave Williams said, 'Heat-staking is a related process used to join plastic part assemblies. An extrusion on one plastic part is inserted in the hole of another part. A heat source is applied to the far end of the extruded part, melting the end so that the parts cannot be separated.'
Hot plate welding robotics employs a temperature controlled heat source, which is positioned between the parts to be joined. The robot holds the parts during the welding cycle. The parts are melted for a specific amount of time to ensure that the proper amount of heat is applied to all parts.
If It’s Plastic, It Can Be Welded
The variety of plastics that can be welded robotically is very wide. Robots can weld acrylonitrile-butadiene-styrene, polyvinyl chloride, acrylic, polycarbonate, polyethylene, fiberglass and carbon reinforced plastics, along with fluorocarbons. There is robotic welding of parts for infrared ovens and heaters used by the thermoforming industry. In addition, robotics are deployed for the welding together of tubular pipes, flanges on the ends of pipes, and other plastic components that constitute solar energy systems.
David Williams of Motoman says 'A lot of automotive products, such as tail lights and marker lights, use various forms of plastic welding. The medical industry also uses plastic welding to molecularly bond parent resins together and to avoid introducing the use of any adhesives into the product.'
In the automotive sector, robotic plastic welding is used on instrument panels, bumpers, fuel tanks, manifolds, fuel filters, valves and sensors, anti-lock braking system components, fuel injectors, parts of electronic ignitions.
The consumer product area has a considerable amount of robotic plastic welding, including plastic watches, toys, toothpaste tubes, video and audio tape cassettes, ink-jet cartridges, computer floppy disks, dental devices, computer disk drives, and audio-video equipment.
The electronics industry uses robot-based plastic welding to form toggle switches, connectors, terminal blocks, switches, printed circuit boards, stencils and screens, and for other sensitive devices. Other uses for robotic plastic welding includes items in the medical, packaging, textile, and optical industries.
The role of vision systems in robotic plastic welding is a key to successful integration of these work cells. Bashar Abdo of Bosch Rexroth speaks of the part vision plays in plastic welding robotics. 'Vision is used to inspect the weld joints after the fact to ensure there is a good weld joint. A lot of integrators also use vision for set up. When applying heat inside of a part that is totally enclosed, it is often difficult for technicians to set the weld positioning using their eyes, so they use cameras where they cannot see.' He went on to say that vision allows operators to facilitate part orientation as well as inspection. 'Vision can be used in dual mode,' Abdo says.
'Vision systems may be used to ensure that the robot can accurately and repeatably present parts to the welder,' says Motoman’s Williams.
In any type of welding, the introduction of heat as part of the process can adversely effect the material being joined. If too much heat is applied, the plastic parts can be distorted. If not enough heat is introduced, there is a chance the parts will not be sufficiently melded together. Bosch’s Bashar Abdo addresses the issue of heat control:
'The weld head comes with a processor that continually monitors the heat at the tip and adjusts the power to compensate,' Abdo says. 'As heat is applied, the processor changes power settings to introduce more heat if necessary to the part should there be a loss of heat as it is applied.' Operators can read energy outputs to let them know if a good weld is being formed or not. Abdo continues by saying, 'The processor also monitors pressure, not just energy. By changing the pressure and energy at the same time, operators can get the proper application of heat to the part,' Abdo says. 'Time, pressure and energy all have to factored in the equation,' concludes Abdo.
For more information, you may contact any of the experts listed in this article or visit Robotics Online, Tips & Tech Papers.
Bashar Abdo, Cartesian Robotics Product Manager, Bosch Rexroth Corp. Linear Motion & Assembly Technologies Group, 704-714-8513, firstname.lastname@example.org
David Williams, Senior Packaging Technology Leader, Motoman Inc., 937-847-6200, email@example.com
Michael Sharpe, Engineering Manager for the Materials Joining Segment, FANUC Robotics America, Inc., 248-377-7298, Michael.Sharpe@fanucrobotics.com