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Study on the Laser Transmission Welding of Transparent Thermoplastic of PMMA

Author: LiXiaoYu
Tutor: ChenJiMin
School: Beijing University of Technology
Course: Optical Engineering
Keywords: laser transmission welding PMMA plate experimental technique finite element modelling (FEM) temperature field stress field
CLC: TN249
Type: Master's thesis
Year: 2011
Downloads: 123
Quote: 0
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Abstract

Plastics as a substitute materials for the metal and nonmetallic materials in the products become more and more widespread use. For a lot of large and complex parts, it is hard to form the parts as a whole. Therefore different joining methods are demanded to connect the different parts to be a unit. In the plastics joining field, laser welding plastics has many unique advantages which make it have a rapid development in recent years. But there are rarely researches on the field of laser welding plastics in domestic joining fields. Compared to the international study, the gap becomes more and more greater. In the field of laser welding plastics, the welding of two transparent materials is a relatively difficult sub-field. In this paper, we focused on the two transparent materials (PMMA plate) welding and developed a new welding method which used the Zn film as the energy absorption layer to melt the PMMA plate to accomplish the welding. At last, we analyzed the influence factors which affect the welding strength.In this paper, first we found the suitable metal powder Zn as the absorb material and was used an innovation patent: the laser induce deposition method to form the thin film on the PMMA. The orthogonal test method was adopted in the research to find suitable process range. Via the method of macro topography of welding seam, extension test, morphology and infrared spectroscopy how the influence factor affect the welding result was explain. We found that the thickness of Zn film was a major influence factor which determined the welding experiment successful or failed. When the process parameters was P1=8W, V1=1000mm/s, the scanning interval was 0.05mm, we achieved the maximum shear strength attain to 34.5MPa. Compared to the shear strength of 14.6MPa used in Clearweld absorber, the shear strength had a significant rise.In the portion of analyzing what influence the welding strength, there are two kinds of situation inducing the welded parts being splited. In the first case, the top and bottom parts were splited totally. In this situation, we considered that the main reason caused the phenomenon was that the Zn film didn’t absorb enough energy. It made the upper and bottom plats not joined together well so as to cause the strength fall down. Another case was the welded parts were fracture around the welding seam. In this situation, we thought that the tensile nonuniform, micro-flaw and residual stress were the main factors inducing the welding strength fall down.Because of the thermal effect lasting in the entire welding process, we used the finit element software-ANSYS to analyse the property of the material development in the different phases under the thermal effect. In the software, we analysed the feature of temperature distribution and compared it with the test data. The model completly tallied with the test data. In the portion of thermal stress analyse, it revealed that around the heat affected zone, the material suffered the reiterative change of tensile stress to pressure stress which made the material fatigue and caused the material strength fall down. The thermal stress analyse aslo verified the reason that caused the material fracture under the material yield strength.

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