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TECHNICAL PAPERS

# Bridging Gaps in Laser Transmission Welding of Thermoplastics

[+] Author and Article Information
James D. Van de Ven

Mechanical Engineering Department, University of Minnesota, 111 Church Street Southeast, Minneapolis, MN 55455vandeven@me.umn.edu

Arthur G. Erdman

Mechanical Engineering Department, University of Minnesota, 111 Church Street Southeast, Minneapolis, MN 55455agerdman@me.umn.edu

K-mac Plastic, 3821 Clay Ave. SW, Wyoming, MI 49548.

J. Manuf. Sci. Eng 129(6), 1011-1018 (Jul 06, 2007) (8 pages) doi:10.1115/1.2769731 History: Received November 03, 2006; Revised July 06, 2007

## Abstract

Gaps at part interfaces pose a major challenge for laser transmission welding (LTW) of thermoplastics due to the reliance on contact conduction between the absorptive and transmissive parts. In industrial applications, gaps between parts can occur for a variety of tolerance and process control reasons. Previous experimental and modeling work in LTW has focused on gap-free joints, with little attention to bridging a gap with thermal expansion of the absorbing material. A two-dimensional comprehensive numerical model simulated bridging gaps in LTW. Using the model, operating parameters were selected for welding across a $12.7μm$ gap and a $25.4μm$ gap by creating sufficient thermal strain to bridge the gap and form a weld. Using these operating parameters, PVC samples were welded in a $T$-joint geometry with a designed gap. The quality of the welds was assessed visually, by destructive force testing and by measuring the weld size to quantify the weld strength. All the experimental samples, for the two gap sizes, bridged the gap and formed welds. The average weld strength of the $12.7μm$ gap samples was $16.1MPa$, while the $25.4μm$ gap samples had an average strength of $10.0MPa$. Gaps were successfully bridged with LTW by using a two-dimensional model to design the operating parameters. To achieve higher modeling accuracy, a three-dimensional model might better simulate the thermal diffusion in the direction of laser travel.

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## Figures

Figure 1

This diagram shows the heat transfer components of the two-dimensional numerical model. Material B is the transmissive part and Material C is the absorptive part. In this view, the laser beam is collinear with the positive z axis and is traveling into the paper in the direction of the positive y axis (not labeled).

Figure 7

Laser welding fixture mounted to the welding table. The arms pivot at the far right of the fixture. The absorbing part is placed in a slot approximately 50mm from the first pivot. The shims are placed on the absorptive part followed by the transmissive part, forming the T-joint geometry. The transmissive part is held in place by a rotating carriage, which is the second pivot on the pair of arms. On the far left side of the arms, a precision spring is set to a very low force to only maintain light contact between the parts and not to compress the shims.

Figure 2

The four output subplots of the model for the design solution bridging a 12.7μm gap. Subplot (a) shows the temperature of the transmissive part at the interface of the two materials as a function of time and width. Subplot (b) is a zoomed contour plot of the interface zone at the time step that the maximum temperature occurs. In this case, the interface is at a depth=3.2×10−3m. Subplot (c) shows the internal pressure as a function of time and width. Finally, subplot (d) shows the gap size along the width as a function of time. The width dimension is along the x axis (Fig. 1) with a mirror boundary condition at x=0. Note the large elapse time before the gap is closed and heating of the transmissive material begins.

Figure 3

Mesh and zoomed contour plots of the model bridging a 12.7μm gap. This frame coincides with the time the maximum weld width is reached. The interface between the two materials is at a depth=3.2×10−3m. Note the distinct temperature distribution difference between the absorptive and transmissive parts due to the large time elapse before heating in the transmissive part begins.

Figure 4

Model outputs from the model for bridging a 25.4μm gap. The operating conditions for this model include a laser power of 2.5W, a welding velocity of 0.001m∕s, and a laser beam diameter of 8mm. Note that the large time delay before the gap is closed and heating of the transmissive part begins.

Figure 5

A zoomed contour plot from the model bridging a 25.4μm gap. The operating conditions for this model include a laser power of 2.5W, a welding velocity of 0.001m∕s, and a laser beam diameter of 8mm. The time step of this frame coincides with a maximum width for a 485K temperature contour at the weld interface, which defines the weld width. The weld width predicted by this model is 3.00mm. Note that the interface between the two parts is at a depth of 3.2mm.

Figure 6

The T-joint geometry shown with the two shims on either side of the weld zone. These shims establish the desired gap size.

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