Introduction
The use of high-power near-infrared diodelasers for joining plastics is growing.More development work is being performedin institutions and in R&D labs,and applications in industry are slowlyincreasing. Several different approachesare being developed for laser welding ofplastics. The main principle now used tolaser-weld plastics is known as “transmissionwelding.” Transmission welding hasdemonstrated that precise, controllableheating and melting of low melting pointthermoplastics can be produced at the interfacebetween a transmissive and anabsorptive plastic. The principles of transmissionwelding were explained in a previousApplication Note in this series.
Welding trials
Although it is often required to produce awelded seam by relative motion betweena high-power diode laser beam and thetarget, there are some situations in whicha single spot-weld can suffice. In addition,taking the relative motion out of thewelding process can lead to a simpleranalysis of that process and can help incomparing the weld performance of differentmaterials. Clear and colored acrylicsheets (often referred to as Perspex1) arewidely used for signage and other applicationswhere their optical clarity is required.It is also readily available in a widerange of colors. Because of their readyavailability and high infra-red transmission,acrylic sheets have been widely usedfor laser welding experiments. Similarly,polycarbonate (PC), tradename Lexan2,has also been used. As this material hasbetter mechanical properties, it is used inmore demanding applications, where, forexample, toughness is required. Therefore,these materials were used for the first stageof these trials.A series of experiments was designed toidentify the laser parameters required toproduce high-strength spot-welds betweenthese two widely used materials.A large diameter laser spot was used toreduce power density to an appropriatelevel for laser-welding of plastics. Thisspot was produced using a CoherentFAP™ System, an 800 m diameter fiberand a 1:1 Optical Imaging Accessory to Figure 4. Acrylic to acrylic weld100908070605020 40 60 80 100 120Spot Area (mm2)Pulse Energy (J)10 W30 W40 WEffect of Energy Input, Acrylic to AcrylicMax Spot Size
Figure 1. Spot welding data for acrylic1401201008060402000 2 4 6 8 10 12Spot Area (mm2)Weld Time (sec)10 W20 W30 W40 WSpot Area vs. Weld Time, Acrylic to AcrylicWeld Damage ThresholdFigure 3. Comparing spot welding of pc and acrylic materials1401201008060402000 1 2 3 4 5 6 7 8 9 10 11Spot Area (mm2)Weld Time (sec)pc/pc, 10 Wac/ac, 10 Wpc/pc, 40 Wac/ac, 40 WSpot Area vs. Weld TimeWeld Damage ThresholdFigure 2. Spot welding data for polycarbonate1401201008060402000 2 4 6 8 10 12Spot Area (mm2) Weld Time (sec)10 W20 W30 W40 WSpot Area vs. Weld Time, PC to PCWeld Damage Thresholdprovide a collimated beam of approximately12 mm diameter. These parametersgive a power density ranging from9–35 W/cm2. Results are given in Figures1 and 2.The weld damage threshold identifies thepoint at which thermal damage was firstnoted in the melt spot. This was usually inthe form of bubbles generated in the meltpool. These were noted after the end ofthe laser pulse, and were not typical ofshrinkage porosity. It was concluded thatthese defects were probably water vaporgenerated within the material by the laserheating process.Further important information can be extractedfrom Figure 3 that compares thepolycarbonate and acrylic materials. Toachieve a particular weld diameter with theacrylic material requires less energy thanto achieve the same weld diameter withthe polycarbonate material. This is mostlikely due to the higher thermal capabilitiesof the polycarbonate material.An alternative plot of this data for onematerial combination, acrylic to acrylic, isgiven in Figure 4. This plot emphasizesthe role of energy input and shows thatwelding at higher average power andhigher average power density is more efficient– less total energy is required toachieve maximum weld-spot size. Itshould be noted that average power, measuredin watts, is simply the rate of inputof laser energy, measured in joules. Hence,these results are readily explained by lowerconduction losses at shorter welding times.Keeping the pulse duration to a minimum,therefore, reduces heat loss through conductionto the component, which is alwaysa prime objective for a precision weldingprocess such as this.To expand the scope of these trials, the laserweldability of a completely differenttype of polymer, polypropylene (PP), wasexamined. Polypropylene is very widelyused in industry because of its very lowsurface energy. This makes it a very difficultmaterial to bond, either to itself or toother substrates. Polypropylene is also attractivebecause of its extremely low costand its recyclability.

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