Investigation on Core-back with Secondary Injection Foam Injection Molding Technology
Supervisor:zhao guo qun
Core-back foam injection molding process is a polymer foam injection molding process with variable cavity volume.Its technical feature is to move the dynamic side or core of the mold along the direction of mold opening at a certain speed after high holding pressure,thus providing more space for polymer foaming.In core-back foam injection molding,the mold opening action in the core-back stage can provide sufficient space for the nucleation and growth of cells,which will significantly improve the foaming ratio of polymer foams,and increase the uniformity of cell size and distribution in the plastic parts.Therefore,core-back foam injection molding process can effectively improve the weight loss,heat insulation,sound insulation,energy absorption and cushioning properties of the plastic parts,increase the specific surface area of the plastic parts,and effectively improve the adsorption capacity of the foamed plastic parts.Due to the high weight-reducing effect of core-back foam injection molding parts,the curb weight can be effectively reduced,and the goal of higher lightweight can be achieved,so it has gained wide attention in the field of transportation.At the same time,due to the good thermal insulation,sound insulation,energy absorption and cushioning properties and specific surface area of core-back foam injection molding parts,it has also received extensive attention in the fields of building materials,sports goods,adsorption and catalysis.At present,scholars have carried out a lot of research work around core-back foam injection molding process,such as the core-back foam injection molding mold,the core-back foam injection molding material,the design of the core-back foam injection molding process parameters and the most common weld mark defects of the polymer injection molding.However,the conventional core-back foam injection molding mold has simple structure,its parting surface has been opened after the core-back action,and it can’t be operated by secondary injection,which limits the expansion and application of core-back foam injection molding process.Supercritical fluid is commonly used as blowing agent in core-back foam injection molding,the cost of supercritical fluid generating and aerating equipment is high,which increases the cost of equipment.In order to improve the performance of weld lines of plastic parts,most of the research is based on the traditional structure of weld lines,but the research results have limited effect on the performance of weld lines.In view of the above problems,core-back foam injection molding is systematically studied from the following aspects:core-back foam injection molding process principle,forming mechanism,process optimization and innovation,mold design and manufacture,structural analysis of plastic parts,structural analysis of weld lines,forming principle of special structure of plastic parts and weld lines,influence of process parameters on plastic forming process and performance.The main research work and results are as follows:(1)The principle of core-back foam injection molding was studied.A new method called core-back with secondary injection foam injection molding was proposed.A cavity depth control system was designed.With this cavity depth control system,a set of core control system which can precisely control the distance of core-back rate was manufactured.After the first injection,a special skin structure different from the conventional foam injection specimen was obtained by core retraction,and the polymer melt was filled into the closed skin structure by holding pressure in a second time.Finally,a foam injection part with a special structure was obtained.It is found that this special skin structure effectively inhibits gas spillover from polymer melt during foaming stage,and significantly increases the power of cell nucleation and growth.The gas enclosed in the skin can naturally provide an internal gas counter pressure for the flow front of the newly injected polymer melt,thus inhibiting the springing effect during the filling of the polymer melt,and significantly improving the internal cell structure of the specimen.This special dense structure of skin plays an important role in improving the mechanical properties of foamed plastic parts.(2)The influence of process parameters on the foam injection molding parts of core-back foam injection molding was studied.The relationship between process parameters and the mechanical properties and weight reduction effect of the parts was analyzed.It was found that the optimum dosage of chemical blowing agent AC was 7%.When the dosage of chemical blowing agent was low,the foaming effect was not good.Excessive dosage of foaming agent would destroy the formed cell structure because of excessive decomposition and heat release.Injection rate and mold temperature affect the surface quality of plastic parts;the main function of injection quantity is to control the strength of the special skin structure of parts.The first stage holding pressure determines the filling rate of polymer melt into the skin structure cavity in the secondary injection stage,which is unique to core-back foam injection molding method.The first stage holding time determines the degree of filling the skin structure cavity by polymer melt in the secondary injection stage.The second stage holding pressure is to fix the size and quantity of the final cells and to compensate the plastic parts.The function of core-back rate is to control the appearance and shape of the skin.The lowest weight loss rate was 16%and the highest weight loss rate was 46%by core-back with secondary injection foam injection molding.(3)The effect of core-back with secondary injection foam injection molding method on the weld lines of plastic parts was studied.A kind of plastic parts with special weld lines was obtained and a new weld line structure was found.It was found that there was a special reticular structure in the weld line of the plastic parts obtained by core-back with secondary injection foam injection molding.The reticular structure could induce the cross flow and intertwining of newly injected polymer melts,thus enhancing the mechanical properties of the weld line.The special reticular structure itself plays a similar role in strengthening the fibers in the weld line area,which further enhances the mechanical properties of the weld mark.The tensile strength,specific tensile strength and elongation at break of core-back with secondary injection foam injection molding specimens with special weld lines are 30.7%,53.3%and 550%higher than those with conventional weld lines,respectively.This weld line structure with special reticular structure plays an important role in improving the mechanical properties of weld lines.(4)The influence of technological parameters on the formation and properties of fusion structure of plastic parts was studied.The relationship between technological parameters and mechanical properties and weight loss of plastic parts was analyzed.The results show that the injection volume affects the thickness of the reticular structure and the whole thickness of the reticular structure in the special reticular structure.The reticular structure becomes thicker with the increase of the injection volume,and the whole thickness of the reticular structure also increases.However,the excessive injection volume will cause the network structure to be too thick,resulting in new defects in the weld line of the specimens.When the injection rate is constant,the strength of the special reticular structure is affected by the injection rate.With the increase of the injection rate,the strength of the reticular structure increases,but when the injection rate exceeds a certain value,the strength of the reticular structure decreases.The weight loss and mechanical properties of the plastic parts with special structure weld lines obtained by core-back with secondary injection foam injection molding are better than those obtained by conventional foam injection molding with common structure weld lines.(5)The foaming injection method of core-back with secondary injection foam injection molding was applied in production practice.A kind of B-pillar guard plate of a brand automobile with high specific strength and weight loss was formed by core-back with secondary injection foam injection molding.The structure and mechanical properties of the B-pillar guard plate obtained by conventional foam injection molding method were compared and analyzed.It was revealed that the foam injection method of core-back with secondary injection foam injection molding was improved.The influence mechanism of uniformity of cell distribution,optimization of cell structure,improvement of mechanical properties and weight reduction effect in automobile B-pillar guard plate is discussed.The results show that the foam structure of automotive B-pillar guard plate obtained by core-back with secondary injection foam injection molding is excellent,and can reduce weight by 24.4%on the premise of guaranteeing mechanical properties.Compared with the B-pillar guard plate obtained by conventional foam injection molding method,its weight loss rate,tensile strength,flexural strength and impact strength with notches have been improved.After comprehensive consideration of weight reduction,the specific tensile strength,specific bending strength and specific impact strength of B-pillar guard plate obtained by core-back with secondary injection foam injection molding are much higher than those obtained by conventional foam injection molding method.