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Dual pressure welding methods for ultrasonic assembly

The basic process

Ultrasonic plastics assembly joins thermoplastic parts through the use of heat generated by high-frequency mechanical impact motion. The welding tool, called a horn, contacts the plastic part under pressure. When the ultrasound is applied the impact energy travels through the part to the joint interface between the part contacted by the horn and the fixtured part. The resulting impact at the joint heats the plastic until it melts and flows between the part surfaces. This part of the cycle is known as the weld sequence. When the impact (vibrational) energy is turned off, the weld sequence is complete and the system begins the hold sequence. Pressure is maintained on the parts until the molten material solidifies, welding the two parts together. When the hold sequence ends, the horn retracts and the entire cycle is complete.

Welding methods

Three fundamental welding methods may be performed using Dukane's more advanced ultrasonic assembly systems. The Conventional method performs the weld sequence and the hold sequence at the same pressure. The Dual Pressure Hold method performs the weld sequence at one pressure (Pressure 1) and the hold sequence at another (typically higher) pressure (Pressure 2). With the Dual Pressure Welding method, parts are welded at two different pressures (Pressure 1 and Pressure 2) and held at the second, typically higher pressure (Pressure 2). Each method is described in detail below.

Conventional welding

Conventional welding consists of a weld sequence and a hold sequence, both performed at the same pressure.

Conventional Welding Sequence

After the weld sequence is initiated, the press descends until the horn contacts the part, and pressure begins to build. When the pressure reaches a specified level, the pressure switch (trigger switch) in the press head is activated and the ultrasound is turned on. Welding commences under Pressure 1 and continues until a transitional event occurs. Depending on the type of assembly system, a transitional event could be 1) a specified amount of time has elapsed, 2) the press head has traveled a specified distance, 3) a specified amount of energy has been put into the work, or 4) a specified peak power level has been reached.

Once the transitional event occurs, the ultrasound is turned off and the system enters the hold sequence at the same pressure. The hold sequence is usually complete after a specified amount of time elapses. When the hold sequence ends, the press head retracts, the weld cycle is complete, and the press is ready to begin another cycle.

Dual Pressure Hold method

The Dual Pressure Hold method has two sequences like the Conventional method, with one important difference. This method welds parts at one pressure (Pressure 1) and then holds the assembly at a second, generally higher pressure (Pressure 2). Many welds can be improved if the pressure during the hold sequence is increased over that required by the weld sequence.

Dual Pressure Hold

After the weld sequence is initiated, the press head travels until the horn contacts the part, and pressure builds. When the pressure reaches a specified level, the pressure switch is activated and welding begins under Pressure 1. Once the transitional event occurs, the ultrasonic energy is turned off, the second pressure valve is activated, and the hold sequence begins using Pressure 2. When the hold sequence ends, the press head retracts and the weld cycle is complete.

Dual Pressure Welding method

The Dual Pressure Welding method welds parts at two different pressures (Pressure 1 and Pressure 2) and then holds the assembly at the second pressure for the duration of the hold sequence.

Dual Pressure Welding

After the weld sequence is initiated, the press head descends until the horn contacts the part. Welding takes place under Pressure 1 until a transitional event activates the second pressure valve so that welding continues at another (generally higher) pressure (Pressure 2). A second transitional event causes the system to enter the hold sequence under Pressure 2.

Dual pressure welding advantages

The amount of force applied to a part prior to melt initiation can have a major effect on the weld. For many applications, there is an advantage to starting the weld under low pressure and then increasing the pressure to finish the weld. A good example is a brittle, thin-walled polyphenylene sulfide (e.g., Rytonª) assembly requiring a deep (0.090Ó) shear joint weld. A great amount of force is needed to weld the polyphenylene sulfide to such a depth. However, applying the maximum force immediately would fracture the part or damage the integrity of the joint. By initiating the weld at a low pressure, the material reaches its melt temperature before maximum required force is applied. A more complete, homogeneous weld is achieved without compromising the joint interface or part aesthetics.

Using dual pressure with weld by distance

Weld by distance uses precise measurements from a linear encoder to melt the same amount of plastic material in a joint on every weld, regardless of part height variations. This ability enhances the assembly feasibility of many applications. Combining the Dual Pressure Welding method with weld by distance assures consistent bonding of parts by controlling both distance and pressure variables. The following application illustrates.

Dual Pressure Welding with Weld By Distance

A Macroblend(TM) (Polycarbonate/ABS alloy) grille is to be welded to its housing using an angled shear joint. There is 0.060" of weld distance between the joint interface and the bottom of the housing. A felt barrier and a silicone pad must be captured between the grille and the housing. A large amount of clamping force is needed to achieve a 0.060" weld distance.

Initiating the weld sequence with a single, maximum pressure would either lock the parts together, resulting in a very weak bond, or deform the grille with frictional heat generated at the grille/horn interface before the plastic is able to melt at the joint. Using the Dual Pressure Welding method with weld by distance, the sequence is initiated at 40 psi (Pressure 1) for the first 0.015" of weld depth. At that point, the material has reached its melt temperature, and the pressure is increased to 90 psi (Pressure 2) for the remaining 0.045" depth. Once the total weld depth of 0.060" is reached, the ultrasound is turned off, and the assembly is held at 90 psi (Pressure 2) for the duration of the hold sequence. The combination of dual pressure welding with weld by distance ensures the weld is strong, consistent, and repeatable.