Situation Transmission laser welding is used to join a wide variety of plastic parts. In these applications, the manner in which the two parts are connected can affect the quality of the weld. Butt joints can present challenges that are not posed by other types of configurations. The figure below depicts a simple butt-joint, where two flat pieces of plastic are flushed up against each other and laser welded.

For a Clearweld laser joint to form properly, the interface must allow the application of sufficient pressure, and the laser energy must reach it. A weld cannot be created if the two surfaces cannot be brought into contact.

For most weld joints, this would not be a significant problem. Pressure is normally applied to a holding fixture which forces the two parts together. The laser beam is then directed through the top substrate and converted to heat to melt the material around the joint. Since pressure is applied uniformly to both substrates, the material collapses and flows together to form the weld joint.

When welding a butt joint, the application of uniform pressure is more difficult to achieve than with other joint configurations. As a result, the laser beam should be directed perpendicularly or at an angle to the joint. The figure below illustrates the appropriate laser pathways for welding butt joints.

APPLICATION NOTE 2:LASER WELDING BUTT JOINTS

Laser

Laser

Laser

Laser

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Laser Welding Butt Joints For substrates with high NIR transmission, the laser beam can be directed from the side of the part at a 90° angle. For plastics with lower NIR transmission, the angle has to be reduced to between 34° and 72°. If the parts can act as a wave guide, the laser beam can be directed through the top of the top substrate and total internal reflection will deliver the energy to the joint.

Recommendations • The design of the holding fixture must allow the two parts to be pressed together so the melted areas can flow together. For flat substrates as illustrated above, two clamping cylinders are used during the welding process. The first applies a low pressure (60 to 90 N) to flush the two pieces together. Then higher pressure (350N) is applied to the faces of the parts while the laser beam is directed at the joint. • The condition of the surfaces to be welded also may affect the quality of the joint. Surface contamina-tion by a foreign material may interfere with the transmission of the laser energy, absorbing it and causing the plastic to char or discolor. • Surface texture also may affect the welding process. The greater the contact between the two surfaces, the stronger the weld will be. As a rule, smooth, molded surfaces are preferred. Rough or matte surfaces can be welded, however joint strengths may not be optimal. Air gaps between the substrates also should be avoided, as they can lead to charring of the weld joint for some substrates.

Weld Efficiencies Relative weld strength is often determined by subjecting the joint to tensile testing. However, for some joint geometries, such as overlap, the result is not a pure tensile value, but rather a combination of tensile and shear strengths. For butt joints, the result of a tensile test will provide pure tensile values that can be compared with the tensile strengths of the parent materials.

The table below provides weld efficiencies of Clearweld butt joints for various substrates (weld efficiency is calculated by dividing weld strength by the strength of the parent material). Also shown is percent transmission of the polymers studied. The welding tests were conducted using a 940 nm diode laser, and transmission values were measured at 930-950 nm.

®Clearweld is a registered trademark of TWI. The Clearweld logo is a registered trademark of Gentex Corporation.Patented. Crysta•Lyn Chemical Co. is the exclusive licensee of Clearweld products.

®®

Polymer*ABS, naturalAcetal copolymerAcrylic/PVCHDPE (high density polyethylene)LDPE (low density polyethylene)Nylon 6,6PC (polycarbonate)PEI (polyetherimide)PETG (polyethylene terephthalate)PMMA (acrylic)PP (polypropylene)HIPS (high impact polystyrene)PSU (polysulfone)PTFE (polytetrafluoroethylene)PVC (polyvinyl chloride)PVDF (polyvinylidine fluoride)UHMWPE (ultra high molecular weight polyethylene)

Weld Efficiency

0.830.79n/a1.00.860.450.760.310.960.821.0n/a0.73n/a0.960.780.47

Percent Transmission

0.6080.4310.0250.54646.51.5989.483.789.891.222.3

0.06383.7

0.43888.720.63.00

* Tests were preformed using Clearweld coatings. The Clearweld coating was applied via EFD741V-SS-CW needle tip dispensing valve.

Notes: 1. Acrylic/PVC and high impact polystyrene could not be welded because of low transmission values (0.025% and 0.063% respectively). 2. The PTFE did not weld even though it had a sufficient transmission value. The absorber degraded, but the plastic did not melt. 3. The weld efficiencies of nylon 6, 6, PEI and ultra high molecular weight polyethylene were less than 0.50. Nylon 6, 6 and PEI are high melting temperature plastics, and Nylon 6, 6 has a low transmission value. The high melt viscosity of UHMWPE impeded the welding process. 4. Weld efficiencies for ABS, acetal, LDPE, polycarbonate, PMMA, polysulfone, and PVDF were between 0.73 and 0.86. 5. Weld strengths equal to or approximating that of the parent material were obtained for HDPE, PETG, polypropylene and PVC. 6. The test samples were not ideal for obtaining the highest strengths possible using the Clearweld process. Weld surfaces were machined, resulting in some surface roughness and possible residual stresses. In addition, welding parameters were not optimized for each polymer.