Role of Fixtures in Ultrasonic Plastic Welding
By Gary Clodfelter
Plastic Assembly Technologies, Inc.
The use of CAD/CAM technology has greatly improved the manufacturing of contoured fixtures to support the part during ultrasonic assembly. Exact mirror replicas of the part can be created to provide excellent support to the part. Due to complex contours seen on many parts, it is recommended that the horn and fixture be purchased as a set, so proper alignment is achieved between the two. The fixture used for ultrasonic assembly often is overlooked, but usually very critical to the success of the application.
The use of a fixture that is not properly designed usually will result in very poor welds or no weld at all. Typically, the fixture should support the half of the part to be welded that has the deepest draw. The horn will contact the other half to be assembled, so that the distance from the horn contact surface to the weld joint is minimized. The primary purpose of the fixture is to hold the part secure, in position and in alignment to the ultrasonic horn that will descend upon the assembly. It is important to support the bottom of the part to prevent it from deflecting under the pressure applied by the ultrasonic welder.
If the part does not fit correctly, alignment can be impacted and energy that is needed for the weld can be dissipated into locations not necessary for the weld. A sloppy fit actually can create a condition in which the parts vibrate in unison instead out of phase with each other. It is preferred that the part in the fixture be held sufficiently to allow the mating piece to vibrate against the fixtured part and create a melt flow. If the parts vibrate in unison during ultrasonic vibration, the frictional activity is decreased, which results in less melt flow.
Many different types of materials have been used to build fixtures for ultrasonic assembly, including plastics, stainless steel, tool steel, aluminum, cork, poured urethane and silicone rubber. All of these can play a role depending upon customer’s needs, but the most common fixtures include aluminum, steel and poured urethane.
Poured urethane fixtures are frequently used for supporting rigid amorphous plastic parts. These are especially advantageous for providing good support with rigid parts and eliminating or reducing part marking that can occur during ultrasonic assembly. Urethane fixtures also have been used successfully as a part support for insertion (applications with 8-32 or smaller inserts) and staking (applications with boss diameters less than 1/8").
Historically, urethane pours were not uniform and often used too much urethane in areas for supporting the part. Due to the resiliency of urethane material, this inconsistency would result in varying support at different locations around the part. The advent of thin-poured urethane fixtures provide good, consistent support and a resistance to burning that was seen with the thicker poured urethane fixtures. These fixtures can be advantageous to welding, can reduce part marking and can provide good support to the part, allowing for good transfer of ultrasonic energy to the part and not the fixture.
The most commonly used nest material, though, is aluminum. Aluminum provides good rigid support and easily can be reshaped if the plastic parts come in oversized. It is used for both crystalline and amorphous parts. Aluminum almost is always the material of choice for supporting parts made of crystalline type plastic materials. Aluminum fixtures may need to be hard coated if the plastic material includes abrasive fibers, but the material generally is clear anodized or chrome plated to prevent part marking. Part marking is a real concern with a rigid aluminum nest, particularly if the part does not fit the nest well. If the contours don’t match, the vibratory energy can result in the rigid fixture marking the plastic part, creating an unsightly scar.
Steel also is used as a fixture choice for ultrasonic assembly. It often is chosen for its improved wear characteristics, particularly when welding highly abrasive materials. It also is selected for use as anvil fixturing to provide knurling and various stitch patterns for textile weld and slitting applications. No coating is required if the material is stainless, but black oxide coating is frequently used for other tools steels.
The deciding factor for fixture materials usually comes from the type of support required and the type of weld joint used. The fixture usually supports the lower part – up to the parting line – to prevent part marking during the ultrasonic assembly process. If the part has a deep draw, it is advantageous to use aluminum material to improve loading of the part into the nest and unloading of the part out of the nest. On crystalline material applications, a joint called a shear joint frequently is employed, which causes the part to expand at the weld joint as the material melts. With this type of joint, make sure the fixture material is very rigid to suppress deflection and make sure the melt occurs along the vertical wall of the shear joint. Urethane material is not a good material choice for a shear joint welding application because of its resiliency. When a part is welded using an energy director joint, where there is no force against the outside wall, most of the support is needed on the part in the area under the energy director. With the energy director joint, the most common choices are aluminum, stainless steel and poured urethane.
The fixture material choice also will affect how the plastic material flows at the joint interface. A soft, resilient type of material will create a different flow rate than a rigid metal nest. Different fixture materials will create various frictional reactions at the weld joint. An experienced ultrasonic tool maker can assist with choosing the appropriate fixture material. Most of the time, they will be right based upon their application experience and exposure to the broad choice of plastic material choices used to manufacture plastic products. Fortunately, if they select wrong it rarely is a life altering event.
Most fixtures include a slotted aluminum subplate for mounting to a welder base or automation system. The slotted subplate allows for ease of alignment between the part and the ultrasonic horn. Many fixtures will incorporate leveling features on the aluminum subplate to adjust the nest if the fixture in not parallel to the machine and the ultrasonic horn.
Just about any type of attachment can be added to an ultrasonic fixture to improve alignment, enhance part loading and unloading, eject the part, clamp parts, provide slides to move the part or the fixture, etc. Servo and pneumatic drive motion control systems also have been used as fixture attachment devices to lift the part to a stationary vibrating ultrasonic horn. Fixture attachments include the use of clamps and slides. A clamp provides support to the part, but allows a split fixture to open wide for part loading. This clamp approach allows the fixture to provide excellent support at the joint, but still open up for part loading. This approach often is necessary due to limited stroke capacity of the ultrasonic weld machine. Slides also are incorporated into fixture designs to move the part into a position out from under the ultrasonic horn to allow for part loading. They also are used for welding at different locations on the part, to move a fixture component into position for improved support and to scan along a weld seam. Slides have been used to move a part along an X-Y axis to aid in installation of multiple inserts or to allow the horn to stake posts at multiple locations. With the use of slides, one finds other fixture attachments like stops and detents to control the motion position. The use of clamps and slides can be activated manually or automatically depending upon requirements for volume, ease of use and cost considerations.
Mechanical part ejection attachments often are added to fixtures to enhance part removal after the ultrasonic welding process. This eject feature is generally required if there is a deep draw to the part or the part imbeds itself into the nest during the assembly process. The mechanical eject attachment components can be as simple as a manual lever lift or a little more complex, like a lift driven by a pneumatic cylinder. Smart fixtures with plc controls have been used as a fixture attachment to provide sensors to ensure part placement before the weld cycle and to activate automatic part ejection after the weld cycle. It is feasible to envision that these smart fixtures also could incorporate part loading.
Customers that need rapid tool change are constantly requesting quick-change fixtures. For quick-change tooling, the use of dowel pins and bushings are commonly found fixture attachments. Coupled with quick tool change features for the ultrasonic transducer, booster and horn assembly, the end user has the capability to change jobs much faster, providing a substantial reduction in set-up time.