One more ultrasonic application involved bonding pressure-balancing membranes to pump housings. The housings are made of glass-fiber-reinforced polyphenylene sulfide (PPS). The membranes are made of polyester or polycarbonate. The customer wanted to replace expensive precut single membranes with less costly membrane tape.

This was a demanding application, because three consecutive ultrasonic process steps had to be performed:

• Punching out the membrane and welding it to a cap (35 kilohertz).

• Providing the cap with a protective cover (35 kilohertz).

• Welding the complete cap to the casing (20 kilohertz).

The newly developed ultrasonic tool system, MPW, does just that. It punches and seals at the same time using a membrane tape on a roll, saving money and time. This also eliminates the need for an additional optical scan to see if the membrane is correctly positioned.

The punch and weld module integrates many individual pre- and post-processing steps into the membrane welding operation. The MPW is available as stand-alone benchtop welder, or it can be integrated into automation.

Welding highly filled PPS is a challenge. The glass fibers make the PPS very strong, but also brittle and less able to react to the ultrasonic vibrations that generate heat at the joint line.

During ultrasonic welding, five operating modes are available: time, energy, power, absolute distance (end point of the weld distance), and RPN distance (referencing weld depth to a starting point). Through graphical monitoring of the various welding parameters on the ultrasonic welder, ultrasonic engineers were able to define a processing window. In this case, a uniformly increasing joining velocity was important for guaranteeing a consistent melt flow and good weld strength.

The joint geometry of the casing and the sonotrode design were chosen so that, in spite of the large amount of glass fiber, optimal material bonding occurred. ultrasonic engineers analyzed microtome section images of the joint to further optimize the process. The images showed a homogeneous binding of the polymer molecules, which was confirmed by leak testing at a burst strength of 10 bar.