Ultrasonic Welds for Medical Devices

Ultrasonic welding is particularly suited for medical devices because it uses the part material itself to form a bond and does not introduce any additional materials such as glues or adhesives into the device. A fast, clean, efficient and repeatable process, ultrasonic welding is ideal for joining just about any set of plastic parts for many applications. To obtain these advantages, however, you need to select a readily weldable material, design the joint, design the fixturing and tooling appropriately, design and optimize the welding process and then control the process in production.

This Proven Process series on ultrasonic welding will focus on advanced techniques. Background for typical welding equipment, fixtures and general welding guidelines can be found through any of the major suppliers of ultrasonic welding equipment. (see “For More Information”)

Joint Design for Ultrasonic Welding

While the concept of ultrasonic welding is simple, the process of designing a robust joint and developing an effective weld process can be quite complex. Typically, it starts with a nominal joint design, nest and horn design and the energy coupler gain to obtain the nominal vibration amplitude at the welded surfaces. Then the process iterates between adapting the geometry of the joint and optimizing the weld process parameters (energy coupler gain, energy and energy envelope, pressure, and duration).

To create a good weld, the parts must make initial contact over a relatively small area compared to the finished weld area. This allows initiation of melting/softening soon after the ultrasonic vibration is applied.

Two common approaches in creating the small initial contact area:

• Energy director joint
• Shear joint

Figure 1.  A triangular energy director example

 

An energy director is a feature on the part that the horn contacts (the top part). See Figure 1. Triangular in section, with the point of the triangle providing the initial contact between the parts, the energy director concentrates the vibrational energy in a small contact area.

Figure 2. A shear joint example

 

A shear joint provides strength and may be used to create a hermetic seal. As shown, the parts are designed to interfere initially in a small area. As the ultrasonic vibration is applied and the horn presses the part down, the top part melts its way into the bottom part. Many different joint geometries are possible, but all effective approaches use some way of providing a small initial contact area for the weld.

Beyond the basics

Once you understand the fundamentals of joint design for ultrasonic welding, you can design joints to that have features and functions other than just the function of bonding the parts together.

An implantable, pressurized, combination device designed and developed at Proven Process has several ultrasonic welds applied during its manufacture. The final weld of the device assembly bonds the cover of the device housing to the base. These two large circular parts, approximately 70 mm in diameter, telescope together as they are welded. In addition to providing structural integrity, the same joint encompasses other functions and features as well.

Figure 3. A Proven Process pressurized device; a concentric, shear joint design; and a section of a successful weld.

• Joint features to control the weld and assembly geometry

Alignment is critical to the correct welding and the resulting quality of the joint. Various kinds of features can be molded into the parts to align and locate them with respect to each other. The ultrasonic tool nest and horn can also be designed with features to help with the proper alignment during welding.

Figure 4.  Examples of joints with alignment features

 

In Figure 4, the left sketch shows joint alignment by slip-fit adjacent surface. The right sketch shows an alignment feature chamfer that will push the upper piece inward as the weld progresses. The chamfer alignment feature is used to center the two concentric enclosure pieces of the pressurized device.

Figure 5.  In the device example, the two circular parts telescope together. Note the material slope or chamfer just below the initial shear contact area. For the two concentric pieces, this chamfer helps maintain center positioning between the parts as they are welded.

 

To maintain a consistent height of the assembly, you can mold in a stop that will keep the joint consistent. Note the shelf-stop below the weld.

Figure 6.   Examples of a joint with stop-weld features

• Joint features to contain flash and/or particles

 

Flash or loose particles generated by the ultrasonic welding process and landing in a particle sensitive area could cause failure in a medical device. Design the joint with features that contain any flash and/or particles, and make sure any loose particles can be removed by your cleaning process.

Figure 7.  This is a cross-section of the joint in the pressurized device. The interior of the device is to the left. The joint is designed so any flash created occurs on the exterior of the device where it can be removed by the normal cleaning process during manufacturing. The joint provides a "flash trap" to further contain the flash created when this shear joint is welded.

 

 

 

 

 

Figure 8.  Examples of joints with flash trap features

In the photo, a flash trap with a shear joint design is shown. Also shown is a captured standard round o-ring to enhance hermetic seal performance. In the right drawing, a second example of a flash trap with a shear joint is shown. The small rectangular open area provides a place for melting during welding and keeps flash and debris contained within the trap.

• Joint design features for additional functions

You can design the joint with features to retain functional or cosmetic components when you weld. In the device example, when the joint is welded, it holds and compresses a quad-sided O-ring to enhance a hermetic seal. Soft plastic suture feet around the periphery enable suturing the device in place.

Figure 9. Examples of joints with quad O-ring seal and suture feet features

 

Optimizing the design and the weld process

Developing an ultrasonically welded joint is not an exact science. Every part and every weld has its individual characteristics. Making the joint design and the welding process work together will require some iteration. Determine what the joint needs to do and how you will measure and evaluate the strength, features and functionality of the welds produced for a particular ultrasonically welded joint. You may want to perform pull and shear testing or pressure leakage testing in addition to visual inspection of the weld.

After you have developed an initial joint geometry, make a set of prototype parts at the nominal dimensions and prepare perhaps two more sets with different fits (tighter/looser, for example), while keeping the other dimensions the same. One approach for a disk-shaped assembly of two parts that telescope together might be to use the same ID on the outer part and vary the OD on the inner part for a tighter or looser fit. Adjust the design and run further tests until you have a joint that welds well.

When you have achieved a joint design and weld process that work together, you can look forward to consistently clean, fast welds in your medical device manufacturing process.

In the next papers in this series on ultrasonic welding for medical devices, Proven Process will discuss materials, horn and anvil design and further optimizing the process.