Exact and clean joining of plastic parts
Laser welding allows joining of plastic parts precisely and cleanly without the risk of particles or impairment of materials near the weld seam. This showcase demonstrates how this was solved for a medical filter assembly without affecting the delicate filter membrane negatively.
Customer problem and requirements
In medical devices, thin membranes are used for many different filtration applications. The filter membranes usually have to be fixed on a carrier or mounted in housing without impairing the functionality of the membrane. The weld seam must be leak-tight all around so that all liquid has to pass the filter membrane. The welding process must never impair or damage the filter membrane so that its functionality is no more guaranteed.
A consortium of medical technology companies, research institutes, and a hospital as end-user was facing this task within the framework of an EU-project.
Adhesive bonding of the membrane was not desired because of the materials involved, concerns that adhesive could flow into the membrane and the possible contact of the adhesive to the liquid to be filtered.
With ultrasonic welding, the danger of damaging the membrane with the mechanical vibrations was too large. In addition, a rib must be melted down during ultrasonic welding, which can lead to contamination by particles.
With laser welding, a solution could be found to weld all 4 plastic parts of the assembly leak-tight together and fix the filter membrane without any damage.
In the first step, the filter membrane is inserted between the filter holder plates. The structures in the filter holder plates allow the liquid to be evenly distributed. The filter membrane is mechanically clamped between the filter holder plates, which are then sealed during the welding process.
In a second step, the previously joint filter plates are welded leak-tight into the cover lid with the liquid inlet, so that no liquid can flow around the filter membrane.
In the third and final step, the cover lid with filter plates is welded onto the housing, whereby again a leak-tight and firm connection must be achieved.
Laser-absorbing and laser-transparent
For first functional samples, the plastic parts have not been specifically colored for the laser, but a laser-absorbing dye (ClearweldTM) has been applied manually to the filter holders and the housing.
For the prototypes and series, the filter holder plate 1 and the housing are produced from a laser-absorbing plastic. Although the laser is absorbed well by the colorant, it looks for the human eye only slightly blueish and is acceptable for the visible appearance of the product. Furthermore, the colorant is compatible with medical requirements as e.g. FDA approval.
Equipment plastic welding