Laser Welding Solutions for Transparent PET and Pure White PP Plastics

Laser Welding Solutions for Transparent PET and Pure White PP Plastics

Transparent and pure white thermoplastics rank among the most challenging materials for laser transmission welding. Leveraging the unique optical properties of these polymers—moderate transmittance coupled with high absorption rates in the 2μm wavelength band—this study employs a 2μm thulium-doped fiber laser as the energy source. The research successfully demonstrates welding feasibility for both transparent PET and pure white PP materials without requiring any absorbent additives.

PET Plastic (Left) vs. PP Plastic (Right)

Transparent PET Welding

Comparison of Transparent PET Welding Effects Using 0.808μm and 2μm Lasers

Figure (a) and (b) show the macroscopic morphology changes of transparent PET samples welded with 0.808μm and 2μm lasers under increasing input power, as captured by CCD imaging. Both sets of samples show no surface protrusions or scorch marks at the weld seams, indicating the PET surfaces remained undamaged. This successful outcome is achieved through the experimental setup where a special quartz glass with relatively high thermal conductivity (approximately 1.5 W/(m·K)) is pressed against both surfaces of the PET samples (with thermal conductivity no more than 0.3 W/(m·K)). The quartz glass not only provides appropriate clamping force for welding but also conducts heat away from the PET surface layers into the glass itself, thereby preventing thermal ablation damage that would otherwise occur from excessive surface temperatures.

As shown in the figures, the melt zones of both sample groups expand progressively with increasing input power. Notably, bubble formation is observed at the weld centerlines of some samples. These bubbles adversely affect the airtightness of welded components, as the ideal welding outcome should produce bubble-free, impurity-free seams identical to the base material.

Comparative analysis reveals that samples welded with carbon black additives, due to localized concentration of energy absorption at carbon particle agglomerations, already exhibit thermally decomposed bubbles at relatively low input power (2.2W). In contrast, samples welded without absorbers, lacking these high-absorption carbon black particles, only develop bubbles at significantly higher linear energy (4.4W). This clearly demonstrates that transparent PET samples welded with 2μm laser are less prone to pore defects under equivalent linear energy conditions.

Laser Welding of Pure White Polypropylene (PP)

Image: Comparative results of 0.808μm vs. 2μm laser welding on white PP:
(a1,b1) Macroscopic specimens; (a2,b2) Cross-sectional micrographs

Figures (a) and (b) demonstrate through-transmission laser welding outcomes using 0.808μm and 2μm wavelengths respectively on pristine white PP substrates. Both techniques successfully achieved defect-free surface bonding without thermal degradation.

Critical metallurgical distinctions emerge in cross-sectional analysis:
The conventional 0.808μm process (a2) - requiring carbon black absorbers - produces an opaque fusion zone that obscures interfacial bonding evaluation. In contrast, the advanced 2μm methodology (b2) leverages intrinsic polymer absorption, maintaining the material's original whiteness while delivering superior interfacial fusion visibility.

The experiments confirmed the advantages of 2μm laser welding for both transparent PET and pure white PP plastics. The tensile failure forces reached 57.8% and 93.3% of the base materials' strength respectively. Notably, this welding process required no additives, resulting in more aesthetically pleasing joints with enhanced bonding stability, while significantly improving the efficiency of laser transmission welding.

Songsheng Opto 2μm Coaxial Temp-Monitoring Laser Welding Head

Songsheng Opto developed this 2μm laser welding head with coaxial temperature monitoring for plastic welding. It uses real-time IR thermometry to adjust energy output, ensuring consistent welds and high yield. The built-in CCD camera tracks workpieces to prevent misalignment during welding.

Key Features:

• Closed-loop temperature control - Real-time monitoring and adjustment

• Ultra-fine weld seams - Precision joining capability

• Coaxial CCD monitoring interface - Integrated vision system

• Compact and rugged design - Space-saving robust construction

• Fully sealed internal structure - Protection against contaminants

• True coaxial integration - Combined laser, thermometry and imaging in single optical path

• Optimized for automation - Maintenance-free operation with no adjustment mechanisms, suitable for high-volume production lines

• Multi-spectral co-axial technology - Simultaneous laser welding, imaging, temperature measurement and guidance laser in unified optical system