Joining Sophisticated Medical Devices with Ultrasonics

Technology that delivers
The increasing functionality of medical devices places great demands on joining methods for thermoplastic parts. Requirements include the welding of dissimilar plastic types, absolute tightness and immaculate appearance. Ultrasonic technology achieves important quality stand-ards and offers high process speeds with increased productivity. Technical advancements such as the ability to program a variety of weld forces pro-vides solutions for difficult applications where hermetic sealing and optical quality are of utmost importance.

FIGURE 1: Dialyser filter housing and end caps are welded with ultrasonics.

This article discusses the use of ultrasonic technology in a critical application: dialyser components (Figure 1). A dialyser acts as an artificial kidney and replaces important functions of the natural organ. Because of the safety requirements for blood dialysis filters, the end caps are welded onto the housing using a special ultrasonic weld joint geometry: the double mash joint (Figure 2). Faulty parts could cost lives, thus an additional silicone sealing ring is placed inside the joint geometry. The welding process joins the parts and squeezes the sealing ring gently, but perm-anently to create a perfect and absolute hermetic seal. Another advantage of this particular weld geometry is the great bond strength of the assembly, which is an important feature in overcoming the resisting forces of the elastomeric sealing ring. Furthermore, the unique design of the mash joint accomplishes other critical tasks: it compensates for part tolerances and contains a flash trap to capture molten plastic material. The angled shape of the centring dome (Figure 3) ensures proper alignment and this is a feature that only ultrasonic technology can offer.

FIGURE 2: Ultrasonic double mash joint.

Dialysis filters are mostly disposable products for cost and safety reasons, yet quality is the essential criterion. In the production process membrane bundles are fed into a cylindrical casing and cut and sealed with polyurethane. Subsequently, the end caps and sealing rings are placed onto the housing and welded ultrasonically. Several inspection stations must be checked in a fully automated assembly process. All safety tests for membranes, surface properties, release properties and hermetic sealing must be passed as one hundred percent correct. Sophisti-cated quality controls integrated into the weld station offer traceability via special software, which provides timely and transparent protocols to ensure that defective parts can be immediately located and identified.

Hermetic seal with mash joint

FIGURE 3: Cross-section of a double mash joint with sealing ring in a dialyser application.

When using ultrasonics, high-frequency mechanical vibrations cause molecular and interfacial friction in the weld zone. The resulting heat plasti-cises the material. Specific component designs help to focus the mechanical ultrasonic sound waves into the joint area and the rapid and targeted melting is achieved by suitable welding surface contours such as spikes or sharp edges. These contours are called energy directors and prevent large-scale coupling and direct the ultrasonic energy into the joint. Certain thermoplastics pass from a solid to a fluid state in a narrow tempera-ture range when reaching their melting point, in particular, semi-crystalline polymers. With these materials, the use of conventional basic energy directors is pointless because of the relatively sudden change in compound stability that leads to rapid and uncontrollable melt flow. The solu-tion is a joint geometry that encases the melt and offers good hermetic qualities. The mash joint was developed several years ago through an extensive internal research programme. It has proven to be effective with its 30° angle, which acts as an energy director and creates a capillary effect pulling the melt upwards and downwards. Mash joint designs generally require larger generator power, because the joining process com-presses (literally “mashes”) the melt to greater density. The mash joint can be used with amorphous polymers, which are used in many medical device applications and have slower reaction to temperatures. Here, the mash joint can achieve good sealing results, while melt flow to the in-side or the outside of the component is prevented.

Sealing ring requires in-process weld force change

FIGURE 4: Microtome cutting double mash joint with sealing ring in a dialyser application.

The size of the dialyser components (an overall length of up to 30 cm and a weld diameter of 5 cm) and the addition of the sealing ring create difficult technical challenges. During the weld process the sonotrode meets the end cap and the mechanical friction creates heat to melt the dou-ble mash joints (Figure 4). After a few milliseconds the weld force experiences resistance, because the ultrasonic waves hit the sealing ring, which is made of a silicone elastomer, a thermoset with high temperature tolerance. A counter pressure develops that slows down the melting process and the actual joining velocity. However, to achieve maximum bond strength, the optimal joining velocity needs to proceed in a constant, linear manner.

To achieve this, in spite of the counter pressure, the weld process controller is programmed to switch to a second and higher weld force. The force change is controlled by a high-speed proportional valve that provides precise, closed-loop air pressure regulation with millisecond sam-pling rates to ensure consistent trigger and weld forces. Initial test welds determine the optimum welding forces so that the joining velocity pro-ceeds in a linear fashion over the entire welding process. The result is a hermetic weld with maximum bonding strength. Simultaneously, the sealing ring is compressed to the proper level without being damaged.

FIGURE 5: Screenshot of weld force change (red curve) when striking the sealing ring, which ensures a linear joining velocity (blue curve).

The touch screen of the machine control visualises each weld process, and provides a unique “fingerprint” of each weld (Figure 5). This graphi-cal illustration of the weld power, force and velocity allows precise interpretation of the quality of the joining process. Research has shown that the strength of the ultrasonic assembly depends on the melt rate: the more linear the joining velocity, the more stable the melting rate and there-fore, the more homogeneous the molecular structure. A linear melt profile results in excellent bonding strength. An ideal linear joining velocity (blue curve as seen in Figure 5) can be achieved by optimising the weld parameters. This also avoids unnecessary mechanical stress on the parts and ensures repeatability of the weld process, which is a major challenge for most other joining technologies.

The technical requirements for this accuracy involve determining the precise starting position for the ultrasonics power and the fast and accu-rate switch-off of the generator, which operates in the microsecond working range. To increase production output for the dialyser in the assem-bly process, both dialyser end caps are placed simultaneously onto the housing, and two horizontally positioned ultrasonic weld tools (the sono-trodes) travel onto the parts, one from the left and one from the right. To ensure that the ultrasonic waves from the two sonotrodes will not cancel each other out, the two generators work in a staggered manner with a delay of a few milliseconds. Precise calibration of the weld dis-tance means that the possible tolerances of the component can be reliably detected and compensated for. The melt volume remains constant at all times. A missing sealing ring can be easily detected on the ultrasonic power curve, which would then increase at a lesser degree. By using minimum and maximum limits automatic monitoring is established. No additional quality control step is necessary to check whether or not the ring is inserted.

A range of applications
As an economic plastic joining technology, ultrasonic welding offers solutions for a variety of medical applications including filters, valves, drug delivery components and many more. The technology is widely used by manufacturers, but it has not reached its full potential yet. The choice of material, the design of the joint area and the geometry of the sonotrode and fixture influence the weld results. Machine builders’ expertise plays an important role. By collaborating early in the assembly process, including on part design and joining technology, the best specifications can be configured. Then ultrasonic welding is a capable technology that allows for high process reliability at a consistent quality.

Bernd Frey
is a Mechanical Engineer and Area Sales Manager Southern Germany, and
Achim Rupp
is a Mechanical Engineer and covers sales in Europe, including the UK and Ireland, at
Herrmann Ultraschall, Descostrasse 3–9, D-76307 Karlsbad, Germany
tel. +49 7248 79–28
e-mail: achim.rupp@herrmannultraschall.com [5]
www.herrmannultraschall.com [6]