Numerical And Experimental Research On Surface Quality Of Spray-Free Injection Molded Products

The appearance of plastic products is constantly improving with the development of the plastic industry and the improvement of people’s living standards.In order to achieve the aesthetic effects of products such as even high gloss,dazzling color,and pearlescent sparkle,traditional plastic products are mostly processed by spraying,electroplating and other processes,which are often accompanied by environmental pollution and energy waste.Spray-free plastic,a kind of plastic composite materials usually containing fillers of pearl powder and metal powder,is only through injection molding without the subsequent processing of polishing,spraying and so on to obtain the finished products.Compared with traditional multi-process products,spray-free plastic injection molding has the advantages of strong personalization,high efficiency,low cost,good appearance,and environmental protection.In the past ten years,it has been widely used in automotive interior parts,home appliance exterior parts,daily packaging,building materials and other fields.However,spray-free plastic pigment fillers usually have poor compatibility with polymer matrix materials.Undergoing the complex thermal and mechanical fields of injection molding,the orientation and distribution of pigment fillers affect the microscopic appearance and performance,especially when the product contains ribs,holes and other structures.The surface defects such as weld(melt)lines,black line flow marks,and uneven optical flicker have to avoid.Nowadays the spray-free injection molded products are with small sizes and relatively simple structures because of the different orientation and distribution of pigment fillers,which are not benefit to engineering application of spray-free plastic.The thesis combines numerical simulation and experimental methods to study the cause and mechanism of optical flicker of aesthetic injection molded products,and provide references for the design and production of spray-free injection molded products.Acrylonitrile-butadiene-styrene terpolymer(ABS)and polycarbonate(PC)are selected as the polymer matrix,and metal aluminum powder is the filler.Through injection molding experiments and numerical simulations,the influence of different runner and gate system,and cavity(product)structure on the surface quality of the injection molded product is studied,the formation mechanism of black line flow marks is analyzed,and a numerical prediction method for black line flow marks of spray-free injection molded products is proposed.The main research contents and conclusions are as follows:(1)Research on the properties of ABS/Al composite materials to obtain a suitable injection molding process.Through research,we found:1)The addition amount of aluminum powder filler in the spray-free plastic is relatively small,and the impact on the injection molding process conditions of the matrix material is limited.After aluminum powder is added,the glass transition temperature(T_g)of the material is reduced by 2.5℃,which has little effect on the rheological properties of the material.2)Compared with pure ABS products,the tensile strength of ABS/Al composite products is reduced by 12.3%,and the notched impact strength is reduced by 30.5%.3)The addition of aluminum powder filler reduces the surface gloss of the product.4)The optical performance can be improved by adjusting the injection molding process parameters.For example,the surface gloss of the product increases with melt temperature or injection rate increase.(2)The influence of runner and gate on the optical appearance performance of spray-free injection molded products,and establish runner and gate design rule and numerical prediction methods.The first-level runner(two-plate mold structure)and the second-level runner(three-plate mold structure)were used to complete the preparation of ABS/Al and PC/Al composite rectangular flat injection molded products.The influence of melt flow pattern,temperature and shear rate on the surface quality of injection molded products is focused on.Through numerical simulations and experiments,it is found that the"ear-shaped"flow imbalance of the melt front caused by the three-plate mold will form black line flow marks on the surface of the product,and the cause of the black line flow marks is mainly caused by the unbalanced melt flow and the uneven orientation of the aluminum powder filler.The design rule for spray-free plastic runner and gate are given.The study found that:1)The uneven orientation of aluminum powder,especially in the thickness direction is the main reason of surface defects.2)The spray-free plastic injection molding avoids small-sized gate structures as much as possible.The"ear-shaped"flow imbalance at the front of the melt caused by the shear heat generation of the point gate during the filling stage.The change of process parameters,such as an increase in the injection rate or a decrease in the melt temperature,will aggravate the imbalance of the melt flow.3)The numerical result of melt flow front and filler orientation are used to prediction.The"V"profile shape of the melt front seems have better prediction of the black line flow mark.(3)The influence of the cavity(product)structure on the appearance quality of ABS/Al composite injection molded products,and the rib and hole structure design rules are built.And we try to use the"textured"surface to weaken the adverse effects of black line flow marks.Design flat products with ribs and holes,and study the influence of weld(melt)lines and black line flow marks produced by the injection molding process on the surface quality of the products.The study found that:1)The effect of rib structure on the black line flow mark of the product is related to the cross section(the part connected with the main structure of the product)and the filling direction.When the large size direction of the rib structure cross section was arranged along the melt filling direction,there are obvious black line flow marks on the surface.If the ribs structure array were nearly perpendicular to the melt filling direction,there are no effects on the appearance of the product.2)Spray-free plastic injection molding,the influence of the hole structure on the surface quality of the product includes two parts:the weld(melt)line and the black line flow mark;it is different from the weld(melt)line formed by the hole structure in the conventional plastic injection molding process and weld(melt)line affected area is equivalent to the aperture.3)Numerical simulation method is used to analyze the formation mechanism of the weld(melt)line and the black line flow mark on the surface of the product and the difference between the two.4)The numerical simulation result is reliable for the prediction of the weld(melt)line,and the prediction function of the black line flow mark needs to be improved.The proposed"V"profile shape of melt front can predict the weld(melt)line and black line flow marks caused by the hole structure.5)In engineering practice,the textured treatment on the mold cavity surface can cover and modify the weld(melt)line and black line flow mark defects caused by the product structure.