Ultrasonic Welded Joint Design
These tips will help you design plastic parts for optimal assembly using ultrasound.
When designing plastic parts assembled by ultrasonic welding, engineers have a variety of options: butt joints, step joints, tongue and groove joints and shear joints. Which one to choose depends on many factors, including the material; the size and stiffness of the part; and performance requirements such as joint strength, cosmetic and sealability.
Butt Joint
Butt joints with energy directors are one of the most common joint designs for ultrasonic welding and are the easiest to shape. The energy director - a small triangular ridge forming a mating surface - is essential for this design. The ridges can be molded at the bottom (upward) or at the top (downward).
The energy controller limits the 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 ridges to melt and the plastic to flow throughout the joint area, which helps to bond the components together.
Step Joint
Step joints with energy directors are relatively easy to implement in injection molding tools. Such joints are generally much stronger than butt joints because the material flows into the gap required for the slip fit, creating a seal that provides shear and tensile strength.
Tongue and groove
The maximum strength is usually achieved by the tongue and groove joint. The very small gap size of the gap creates a capillary effect that allows the molten plastic to penetrate the entire joint area.
Like the step connector, this design facilitates self-positioning of the part and hides the flash inside and outside. It also protects the weld line from ambient air flow, which can sometimes adversely affect the welding process.
Shear joint
Shear joints have proven successful in welding semi-crystalline plastics. This joint design has a large connection distance and typically produces airtight and high strength welds.
Shear joint welding is achieved by first melting the contact edges and then continuing to melt along the vertical walls as the components retract together. Stretching prevents the weld area from being exposed to the air, which can cause it to cool too quickly, resulting in brittleness. The weld strength depends on the distance the top is telescopic to the bottom. The weld depth is 1.25 to 1.5 times the wall thickness of the part and will result in a weld that is almost as strong as the surrounding wall.
These components are guided together by the lead-in and the welding is controlled by the amount of interference between the two components. Rigid sidewall supports are important for shear joint welding to prevent part offset during welding.
Large picture design
Outside the joint itself, if the engineer intends to assemble the product by ultrasonic welding, the engineer should consider some general factors. One is the wall thickness and the other consideration is the overall size of the component.
