Research Of Local Control Process On The Weld Line Strength Of Injection Molded Parts

In recent years, with the increase in demand for plastic products and the higher quality requirements, injection molding technology has been putting forward higher and higher demands. However, injection molding has the disadvantage that it can’t be overcame, that is, when products due to the multiple gates that causes the melt apart and converge in injection molding process, it will inevitably form the weld lines. Weld lines as a common injection molding flaw, it exists tend to decrease the local strength of products and then affect the whole allowable strength of structure parts, which greatly limits the extensive application of plastic parts.In this paper, the effect on the weld line mechanical properties of injection molded parts was explored through the moldflow-structure coupling finite element analysis and experimental research. And on the basis of studying the effect on weld line tensile strength of the injection molded parts under conventional injection molding process parameters, local control process was proposed to use in the region of the weld line to improve the weld line strength of injection molded parts. And through the experiment and simulation analysis, more researches were studied to the molding process on the improvement of strength of weld line.The specific research contents include the following:(1)According to the forming mechanism of weld line, the PP samples either in single gate without weld lines or in double gate with weld lines were processed to the moldflow-structure coupling simulate analysis by using the finite element analysis software Moldflow and Algor, which was in order to study the influence on weld line mechanical properties of injection molded parts. Results show that:under the same load condition, the stress and strain in no weld line sample are consistent in the whole area, while the sample with weld line produces the maximum stress and strain in the middle of the length direction of weld line position. This shows that the weld line effects on injection molded parts mainly causes stress concentration and stress change on the weld line position when the products under the condition of stress, and then reduces the mechanical properties of plastic parts.(2) In virtue of orthogonal test, combining with signal-to-noise ratio analysis and variance analysis, the influence on weld line strength of PP sample was studied and analyzed under different conventional injection molding process parameters, which was in order to provide a feasible basis for exploring the new technology. Results show that: for PP spline tensile sample, injection pressure and mold temperature are the key factors that affect the weld line strength. So under 40MPa to 55MPa of injection pressure and 30℃ to 75℃ of mold temperature, it can get the higher weld line tensile strength during the higher injection pressure and higher mold temperature. Therefore, it can provide the theoretical basis to the local control process through the experimental analysis.(3) Based on the effect of mold temperature on weld line tensile strength, combined with the actual production, the local heating molding process was proposed to use to improve the weld line tensile strength of injection molded parts. To do this, the influence on weld line mechanical property of PP sample was studied combine with moldflow-structure coupling finite element analysis and experimental research. Results show that:local heating molding process can reduce the weld line stress and strain in a certain extent, and improve the weld line strength. Under the mold temperature of 30℃ to 75℃, with the increase of heating temperature, the weld line tensile strength of sample is first raised and then decreased, and got the maximum value under the heating temperature of 60℃. Meanwhile, by comparing with the experiment of overall heating molding process, it shows that local heating molding process has certain advantages on improving production efficient and saving energy at the same time of improving the weld line tensile strength of injection molded parts(4) Based on the effect of the injection pressure on weld line tensile strength, the local compression molding process was proposed to use to improve the weld line tensile properties of injection molded parts based on experiments. Results show that:1) The weld line tensile properties of modified PP sample by local compression molding process is higher than the one of conventional injection molding process as a whole, and the improvement on the weld line tensile properties of sample is different under different combinations of process parameters.2) The effect of mold temperature and pressure time on weld line tensile properties of local compression molding sample is:under 35℃ to 65℃ of mold temperature and 5s to 15s of pressure time, with the increase of mold temperature, the tensile strength of sample weld line improves while the elongation at break decreases; And the response of tensile strength and elongation at break to pressure time has an optimum value under 10s.