Practical Research On Gas-assisted Injection Molding Of Car Bumper

A new molding technique, Gas-assisted Injection Molding(GAIM) which is capable of producing parts having both thick and thin sections without sacrificing surface quality, has received extensive attention in recent years. In this process, after molted resin is injected into a mold, pressurized nitrogen gas is injected into the core of the melted resin to assist the flow of resin. GAIM offers a number of advantages over conventional injection molding(CIM). (1) GAIM can substantially reduce molding cycle. The injection time and cooling time are decreased. (2) GAIM can reduce the clamp tonnage greatly. During the CIM, clamp tonnage must be large enough to meet the need of packing stage, especially to prevent sink marks. However, gas is utilized to reduce the clamp tonnage in GAIM. (3) Gas is injected to occupy part of mold, so a lot of materials is saved. (4) GAIM can eliminate the defects of parts greatly. Part quality can be improved by reducing residual stress and warpage. (5) High surface quality of injected parts is acquired by reducing sink marks and shrinkage that are normally encountered in CIM. The main results of the study are summarized as follows:1. External gas-assisted injection molding(EGAIM)Design cost of EGAIM is high. It can reduce or replace the packing process, and the strength of the mold is low and its molding machine is simple, which can reduce the molding cost of car bumper. In this paper, the principle, part design and mold design of external gas molding were introduced, research on common defects of molded parts was carried out, and the application of the technology was analyzed. The research results on EGAIM are limited, especially for large structural parts. This study demonstrated the feasibility of the technology in the production of car bumpers, gas injection components were designed, forming its own design of the external gas pressure, a PID algorithm was applied to the design of gas pressure control unit, which was beneficial to the practical research of the technology. 2. Common defects of car bumper molded by GAIMGas-assisted injection molding was used to improve the quality of car bumper, but warpage are still the main defects because of its thin wall and large surface. In this paper, Moldflow software was adopted to research the influence of gas channel and the main processing parameters on gas fingering in internal GAIM. Reasons and prevention methods of warpage are researched in the molding of car bumper, and the steps of warpage analysis were introduced in this paper. The results showed that there was the optimum scale of radius of gas channels. The gas fingering of parts with foursquare gas channels was serious. In the design of car bumper, gas channels were set up in the root of stiffening ribs and right-angle brim. The most important processing parameters are delay time, gas pressure, melt temperature and the volume of injected polymer melt. The four processing parameters influence gas fingering by changing the power of gas penetration. There are optimum value scales of the four parameters. In the research, we reduced warpage by choosing proper materials and optimizing the design of part and mold.3. Numerical simulation of car bumper moldingProcessing parameters of GAIM and CIM are compared in numerical simulation. In numerical simulation of CIM and GAIM, the clamp of GAIM was 47.1% less than it of CIM. There were less weld line, shrinkage ratio and warpage in GAIM than in CIM. In GAIM, it was easy to change the wall thickness of parts and a lot of materials was saved. According to the result of numerical simulation, the optimum number of injection locations was 6. Two main gas channels were set at the top and bottom of car bumper. Injection volume of melted resin is 92% of mold cavity and delay time is 1 s.4. Design of part, mold and molding machine of car bumperIn this study, we have adopted the combination of internal and external GAIM for the molding of car bumpers. Compared to CIM, convexity for external gas injection and gas channels were designed in parts molded by GAIM. In addition, we optimized strengthening ribs, wall thickness and radius of fillet radius. In the design of mold and machine, the unit of internal gas injection was optimized, and the unit of external gas injection was designed. The gas pressure control system was optimized.In the research, the mold of car bumper was designed, which was helpful to the practical research of external and internal GAIM applied to car bumper.