Research In Rigidity Of Thin-wall Plastic Parts Based On Modal Experiment

Thin wall molding technology has become a new research focus of the plastic molding industry. Plastic consumption industries take thin wall as their target because of its obvious advantages. Research in the rigidity of thin-wall plastic products is necessary and important due to the lower rigidity resulting from the thinner wall.The paper, taking the display back-shell, a large thin-wall plastic product, as example, analyzes the current status of domestic and international research in thin-wall plastic product rigidity. The key problems and features of thin wall molding technology are discussed. The author explores the reasons of thin-wall plastic product deformation and analyzes the influence of structure of the product on deformation. Through the analysis of warpage in the plastication, mould filling, cooling and demoulding stages and discussion of the structure and viscous flow of the testing piece, the paper studies the rigidity of plastic product from the approach of experimental modal analysis. Based on the multi-degree-of-freedom system, real modal and complex modal theories, the author suggests confirming modal parameters by frequency domain method.Such modal parameters as intrinsic frequency, damping ratio and vibration are obtained after identifying the structure's input and output signals collected in the experiment. Then the features of the main modes of different phases in certain frequency range we are interested in are identified. Deformation of plastic products can be clearly observed in vibration animation. Thus, its structure can be corrected increase the rigidity. The experiment conducts thin wall molding on the thin-wall plastic display back-shell by frequency domain method. Multi-input Single-output is adopted for the 299 test points on the thin-wall plastic part. Excitation is given by hammering and the responses are measured. The signals, after sampling by A/D converter, are put into computer and transfer function is generated. The modal parameters of the first ten phases are acquired after the entire fitting of transfer function.After studying the vibration animation, the author finds curve on the load-bearing bottom side and warpage on the tube end of the display back-shell, which means insufficient rigidity. The result provides significant reference for structure design modification. Thin wall molding in rigidity study of plastic product, with no hypothesis in theory, has approximation in boundary conditions. Such an approach is drawing more and more attention in the plastic product research and has guidance meaning for the rigidity problem in the production of large-scale thin-wall plastic parts.