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February, 2012 Consistent QC for Laser Plastic Welding By Josh Brown, LPKF Laser & Electronics, Tualatin, OR
breakthrough applications. From micro-fluidic devices, created with extraordinary accuracy, to cleanroom class V applications, impossible with other welding methods. Laser plastic welding is leaving a noticeable mark in the assembly world. This welding process makes use of a finely-focused laser beam. The part to be welded is com-
L
prised of two pieces with dissimilar characteristics: an upper, laser transmissive layer and a lower, laser-absorbing layer. The laser energy projects through the upper part and is absorbed by the lower piece. The absorbed heat melts the plastic at the interface between the two parts and creates a weld. The benefits of laser welding vs.
other plastic joining methods are many: precise heat placement, a clean welding process, unmatched precision and the ability to weld com- plex and three-dimensional parts.
Whether quality controls are company-imposed or controlled by government regulation, they are a prerequisite to competing in the manufacturing world.
But, likely, the most important
capability of laser plastic welding is quality control from its robust process monitoring systems. The simple fact that laser plastic welding
Typical laser welding system.
high tolerance requirements; there- fore, the quality assurance and vali- dation must be equally as precise, able to measure the tiniest devia- tions from the tolerance abilities boasted by laser welding. In itself, the process of laser
plastic welding is extremely reliable and repeatable. However, the process can be hindered easily by small devi- ations in the parts to be welded. The two causes for concern are geometric
Melt-collapse Monitoring. The most robust and often used process monitoring method is collapse moni- toring. This technique makes use of the natural convergence of the join- ing parts as they move together under clamping force. Typically parts are designed
with a collapse rib. This rib once melted and introduced to clamping pressure will collapse. The measure- ment of this collapse can be used to
aser plastic welding, a roughly decade-old assembly technolo- gy, includes its own resume of
is capable of producing highly-com- plex parts requires that it also have the ability to monitor the complexi- ties of its own process at a very accu- rate level.
Process Monitoring Methods Laser plastic welding often is
selected as the joining method of choice for precision and intricacy. Many of the parts in question have
and optical deviations. There exist five different types
of process monitoring techniques for laser plastic welding, ensuring that any part, regardless of its nuances or the variations in the part, are able to be monitored effectively. These five techniques are: Melt-collapse Monitoring, Pyrometer, Reflection Diagnosis, Burn Detection, and Camera-assisted Vision Systems.
determine weld quality. The laser welding process itself is highly reli- able, but even the smallest devia- tions in part dimensions can result in poor welds. If the two joining parts are even slightly warped, this can leave gaps. Gaps of more than 0.05mm are known to degrade weld quality significantly. Introducing a collapse rib that is greater in height than the part tolerances can ensure that the distance of collapse will overcome the tolerances. Once an adequate melt-collapse
is determined in testing, parameters are entered into the system. If a part fails to fall within the proper collapse parameters during production, it will be marked for rejection and all data will be stored for later evaluation. The device that measures the col- lapse is known as a linear voltage distance transducer. It is accurate to less than 0.01mm, which is overkill because even the most precise injec- tion molding processes are incapable of producing dimensional tolerances of less than 0.02 mm. Pyrometer. Pyrometers measure the temperature within the welding zone. Temperature inconsistencies are directly correlated to inconsisten- cies in the weld. Predefined upper and lower
temperature limits are developed in testing. If anomalies from burned contaminates or inconsistent part dimensions cause the radiation to fall outside of the defined “tempera- ture envelope,” the part will be flagged and the data stored. Pyrometers typically are used
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