Often in medical part production, different materials have to be joined, like amorphous polystyrol (PS) and semi-crystalline polyethylene terephthalate (PET), such as in a dental applicator for treating caries at an early stage. Ultrasonic welding produces a controlled melt for a tight bond with minimal thermal load. The key to success is the variable programming of the weld force to uphold the desired joining velocity.
Because of the small dimensions, convex and concave shapes and thin film, the dental applicator was a difficult component to produce. Even the slightest dimensional variations in the injection-moulded parts could pose serious problems. The challenge was to design a weld joint that did not allow melt to escape laterally when welding the holder components, while simultaneously clamping the film. Also, the correct positioning of the delicate film during the production process had to be ensured. The already existing perforations in the film meant an additional challenge, since such “injuries” could have an effect on subsequent processing. They represent a disruption of surface, so-called notch effect points at which mechanical vibrations lead to stress peaks. Under certain circumstances, these can lead to undesirable plasticizing of the resin.
Herrmann Ultrasonics supported the dental company in designing the holder components in such a way that four locating pins and four small thin-walled joints ensured the best weld result. The thin-walled joint design (V-joint) is suited for small components with thin walls. The energy director has a clearly defined contact surface and facilitates self-locating.
Normally, only similar resins, or those with similar melt indices, can be welded. Amorphous and semi-crystalline resins have different melt indices and are more difficult to join. However, for the dental applicator, the fact that different resin grades are difficult to bond proved advantageous. While the amorphous holder warms readily and quickly as a result of the mechanical vibrations, the semi-crystalline film has a delayed response, which in turn protects it from degrading thermally. More precisely, it was necessary to find the ideal operating point at which the amorphous pin on the holder plasticizes successfully, while only the amorphous components respond adequately in the film and the crystalline components do not. If the energy input exceeds this value, the film melts too much and is destroyed.
When welding the applicator parts, the welder switches to a second weld force in the last third of the process, just before the end of the weld stroke of 0.22 mm. The melt generated with the first weld force is compressed when switching to a second, higher weld force. As a result, the velocity of the welding operation is retained until the end and allows the welding time to be shortened. Furthermore, the load on the film from the mechanical vibrations is also reduced. In addition, the cooling melt is pressed more strongly against the film during the holding period, thus increasing the strength of the weld.
From the machinery standpoint, the prerequisites for such accuracy are a precise determination of the starting position for the ultrasonics (trigger point), controlled melt build-up, and quick and accurate termination.
Trigger point: The approach motion of the sonotrode is observed and the ultrasonics are triggered only after the equipment comes to a stop upon contacting the part’s surface. Reference point zeroing ensures that dimensional tolerances for the part are equalized.
Melt build-up: The control permits programming the weld force in up to three steps to optimize a linear joining process. The result is controlled melt build-up, a condition for exact reproducibility of the operating point.
Termination: The generator operates in the µ-second range. This permits precise termination even with fast welding cycles.