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ROLE OF FIXURES IN ULTRASONIC PLASTIC WELDING-2

2021-01-19 15:35:20

 

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 customers needs, but the most common fixtures include aluminum, steel and poured urethane. Poured urethane fixtures are frequently used for supporting rigid amorphous plastic parts. Urethane fixtures are especially advantageous for providing good support with rigid parts and eliminating or reducing part marking that can occur during ultrasonic assembly. Urethane fixtures have also been used successfully as a part support for insertion and staking applications. Urethane fixtures have been used successfully for insertion applications with 8-32 or smaller inserts and staking applications with boss diameters less than 1/8 of an inch. 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 of the poured urethane would result in varying support at different locations around the part. The advent of new thin poured urethane fixtures provides good consistent support and a resistance to burning that was seen with the thicker poured urethane fixtures that were used in previous generations of ultrasonic welding. The use of the thinner poured urethane fixtures can be advantageous to welding, reduce part marking and 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 is aluminum. Aluminum provides good rigid support and can easily be reshaped if the plastic parts come in oversized. It is used for both crystalline and amorphous parts. Aluminum is almost 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 is generally 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 dont match, the vibratory energy can result in the rigid fixture marking the plastic part creating an unsightly scar. Steel is also used as a fixture choice for ultrasonic assembly. It is often chosen for its improved wear characteristics, particularly when welding highly abrasive materials. It is also 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 choice 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 probably 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 is frequently employed which causes the part to expand at the weld joint as the material melts. With this type of joint, you would want to 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. Due to the resiliency of urethane material, it would not be a good material choice for a shear joint welding type of application. 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.

 

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