Joint Design for Ultrasonic Welding
These tips will help you design plastic parts for optimal assembly with ultrasonics
When designing plastic parts to be assembled by ultrasonic welding, engineers have several options: a butt joint, a step joint, a tongue-and-groove joint, and a shear joint. Which to choose depends on many factors, including the materials; the size and rigidity of the parts; and performance requirements, such as joint strength, cosmetics and leak-tightness.
“It all depends on what the end-product needs to be,” says Stephen Potpan, product manager for Rinco Ultrasonics USA. “Does it need to be hermetically sealed? Or does it just need to be strong and not come apart?
“Part size also comes into play,” he continues. “For example, a shear joint is the recommended design for a hermetic seal, but the part geometry may not allow for it. The design must provide enough room to incorporate certain features.”
The butt joint with an energy director is one the most common joint designs for ultrasonic welding, and it’s the easiest to mold into a part. An energy director—a small, triangular ridge molded into one of the mating surfaces—is essential for this design. The ridge can be molded in the bottom part (pointing up) or the top part (pointing down).
The energy director limits initial contact to a small area and focuses the ultrasonic energy at the apex of the triangle. During the welding cycle, the concentrated ultrasonic energy causes the ridge to melt and the plastic to flow throughout the joint area, which helps bond the parts together.
“An energy director helps to initiate the melting process,” says Cristhian Mayorga, associate applications engineer with Emerson Industrial Automation.
Without an energy director, the ultrasonic energy will not be evenly distributed along the joint line, which means more time and energy will be required to produce the same result. That extra time and energy can translate into unacceptable flash or tooling marks on the surface of the part.
The point of the triangle can be molded at a 90-degree angle or a 60-degree angle. The former is best for amorphous resins, such as ABS or polystyrene. The latter is better for semicrystalline materials, such as nylon and polyethylene. As a rule of thumb, the width of the energy director should be no more than 20 percent to 25 percent of the wall thickness. The height of the energy director can range from 0.01 to 0.04 inch, depending on the material’s melt temperature.
A step joint with an energy director is relatively easy to implement in an injection-molding tool. This joint is usually much stronger than a butt joint, since material flows into the clearance necessary for a slip fit, establishing a seal that provides strength in shear as well as tension.
“My personal preference is a step joint with an energy director,” says Brian Gourley, welding group sales manager at Sonics and Materials Inc. “From a molding standpoint, this part design gives you a little more leeway in terms of tolerances for a dimensional fit. With a tongue-and-groove joint, the tolerances have to be a little bit tighter so the mating surfaces can fit together without having side-wall interference.”
“Another favorite is the shear joint,” adds Gourley, an engineer with 35 years of experience in plastics assembly. “One can be better than the other. It depends on the application. In some cases, if all you need is a mechanical weld, a step joint is probably acceptable. But, if the requirements for the application are stricter, the shear joint might be better than the step joint, especially if you need a hermetic seal or the assembly needs to hold pressure.”
The step joint supports self-centering of components, and it’s recommended when a good cosmetic appearance is required.
“A step joint will protect against flash, at least from one direction,” explains Jason Barton, director business development for the Americas at Dukane. “Most customers don’t want to see flash on the outside of the assembly, but they don’t mind if it goes into the inside. On the other hand, if it’s a vessel that’s involved with filtration, then the customer may not want flash inside or out. That’s where a tongue-and-groove joint comes in.”
Joint designs with energy directors tend to work best with amorphous materials—plastics with good stiffness and low to medium melt temperatures.
“Materials such as polyolefin, polyethylene and polypropylene don’t always work well with an energy director,” says Gourley. “Those materials are soft and compressible. They don’t transmit ultrasonic vibrations efficiently, so a step joint with an energy director is typically not our first choice. We’ll look at something that’s going to be a little bit more aggressive, such as a shear joint.”
The greatest strength is usually attained with a tongue-and-groove joint. Gap dimensions with very small clearances create a capillary effect that causes the melted plastic to penetrate through the entire joint area.
Like the step joint, this design facilitates self-location of the parts, and it hides flash both internally and externally. It also protects the weld line from ambient air flow, which can sometimes adversely affect the welding process.
“A tongue-and-groove design is best for a hermetic seal, but if the part thickness can’t accommodate that, then we’ll use a shear joint,” says Mayorga. “A shear joint can be done with a thinner wall thickness, but it will require more amplitude to weld the parts.”
The tongue-and-groove joint design requires relatively thick walls. Whereas the minimum wall thickness for a step joint is 0.062 inch, the minimum wall thickness for a tongue-and-groove joint is 0.09 inch. The tongue will feature an energy director.
The shear joint has proved to be successful for welding semicrystalline plastics. With large joining distances, this joint design typically produces air-tight and high-strength welds.
“If a major criteria is leak-tightness, the shear joint is a good choice, depending on the material and the geometry,” says Barton. “If you’re trying to weld a vessel made out of nylon, you want a shear joint.”
Shear joint welds are achieved by first melting the contacting edges, then continuing the melt along the vertical walls as the parts telescope together. Telescoping prevents exposing the weld region to air, which could cool it too rapidly, causing brittleness. Weld strength is determined by how far the top part telescopes into the bottom part. A weld depth of 1.25 to 1.5 times the wall thickness of the part will create a weld that is nearly as strong as the surrounding wall.
The parts are guided together by a lead-in, and the weld is controlled by the amount of interference between the two parts. Rigid side-wall support is important with shear joint welding to prevent part deflection during welding.
“A shear joint will require more work of the ultrasonic welder to overcome the interference fit between the two parts,” notes Gourley. “As a result, the parts will be exposed to the ultrasonic vibrations longer than they would be with a step joint, so there is a possibility of tooling blemishes on the parts.”
Where flash is objectionable, flash containment traps should be included in the design.
“Flash traps are tricky, though,” cautions Gourley. “That plastic material flowing outside of the weld joint can actually aid in weld strength. The flash that escapes from the weld joint will travel between the two parts. But, if there’s a flash trap, that material flows into the trap. You’ll get a cosmetic joint, but you may not get the cross-sectional strength from the additional material that is bonding the two pieces together. It’s a fine line.”
Big Picture Design
Outside of the joint itself, there are some general considerations engineers should think about if they intend to assemble a product via ultrasonic welding.
One is wall thickness. “Thin walls may not have enough stiffness to transmit vibrations to the joint area to create an effective weld,” Gourley points out. “If the wall is too thin, maybe 0.06 inch or less, it’s going to act like a spring. It will move in a lateral direction instead of a vertical direction, and it will rob energy from the joint area.”
Another consideration is the overall size of the assembly. “There are limitations in terms of a single horn contacting a single part,” says Barton. “For ultrasonic welding, the part typically has to fit inside a 10-inch by 10-inch box or maybe a 12-inch by 12-inch box. But that depends on the materials, too. You’re not going to weld a 10 by 10 nylon box, but you might be able to weld a 10 by 10 polystyrene box.”
And, because the horn must have intimate contact with the part, engineers should avoid features that could prevent the horn from reaching its target. Machining excessive pockets into the face of the horn could impair its ability to vibrate.
Designs with straight lines or curves can be welded, but rounded corners are preferred over sharp ones. “Ultrasonic energy tends to migrate to sharp areas and cause cracks,” notes Potpan.
Near-field welds—in which the horn is 0.25 inch or less from the joint—are preferred over far-field welds, where the horn is more than 0.25 inch from the joint. The latter will require higher amplitudes and longer weld times than the former.
The Early Bird
While ultrasonics professionals might have their personal preferences for joint designs, there’s one piece of advice they all agree on: Get resin and welder suppliers involved early in the design stage. Many suppliers offer design guidelines and worksheets to help engineers get started when designing a part for ultrasonic welding.
“The biggest issue we see is that the parts have already been designed and are off to the mold maker,” says Potpan. “Then, when we get involved and mold changes are needed, it becomes very expensive.
“We always try to get with the engineers when they are doing the design work in CAD. That way, when we recommend changes to the joint design, we are just changing a computer model not a steel mold.
Gourley agrees. “Get us involved early in the design stage,” he advises. “If we can review a CAD file and have a conversation about design, we can help engineers save time, money and anguish. It’s a lot easier to make changes on a CAD file than it is to change a mold.”