A basic experimental study of sandwich injection molding with sequential injection

An experimental study of sandwich injection molding is reported which involves sequential injection of polymer melts with differing melt viscosity into a mold. In isothermal injection molding the relative viscosity of the two melts is the primary variable determining the phase distribution in the mold. Generally the most uniform skin-core structure occurs when the second melt entering the mold has a slightly higher viscosity than the first melt injected. Large viscosity inequalities lead to nonuniform skin thicknesses. The influence of blowing agents and non-uniform temperature fields on the extent of encapsulation is described. Temperature fields are very important especially if the first polymer melt injected has a greater activation energy of viscous flow (or a greater temperature dependence of the viscosity function).     

微注塑成型过程塑料熔体流动的偏移行为

采用集成式热电偶传感器温度测量系统和可视化全息示踪技术, 对多型腔微注塑成型过程熔体流动前沿在型腔内的偏移现象进行观察和分析。结果表明, 当注射速度为140~220 mm·s^-1时, 主流道内的塑料熔体前沿呈“U型流”状态分布, 分流道内塑料熔体前沿向上侧偏移;当注射速度为10~70 mm·s^-1时, 主流道塑料熔体前沿呈“喷泉流”状态分布, 分流道熔体前沿向下侧偏移;当注射速度为80~120 mm·s^-1时, 主流道和分流道熔体前沿均没有明显的偏移。说明微注塑时注射速度不同, 产生的剪切热也不同, 熔体前沿偏移情况也不同。为此, 引入非平衡流动系数λ, 来判断熔体前沿的流动和偏移情况。     

电子血压计上壳注塑成型方案的计算机辅助分析

利用Moldflow Plastics Insight软件对电子血压计上壳塑件的不同注塑成型方案进行了分析比较,根据计算机辅助分析结果对浇注系统的设计方案进行了改进。实际注塑成型效果表明,改进后的浇注系统对注塑成型充填过程的平衡起到了明显的改善作用。     

Wall Slip of Linear Polymer Melts During Ultrasonic‐assisted Micro‐injection Molding

The wall slip of linear polymer melts under ultrasonic vibration is investigated by correcting the slip mechanism, and melt flow behaviors in ultrasonic‐assisted micro‐injection molding (UμIM) method are discussed. Based on the effect mechanism of ultrasonic vibration on the melt, theoretical models of the critical shear stresses for the onset of weak and strong wall slip during UμIM are established, and the change in rheological properties due to the onset of wall slip under ultrasonic vibration is experimental investigated by a built measurement system. The results show that the onset of weak and strong wall slip of the melt in micro cavity are promoted by ultrasonic vibration, which agree with the built theoretical models, and the melt filling capability in micro cavity is enhanced by reducing apparent viscosity and releasing shear stress of the polymer melt, which improves the molding quality of micro polymer parts via UμIM method. POLYM. ENG. SCI., 59:E7–E13, 2019. © 2018 Society of Plastics Engineers     

Rheology and product properties in injection moulding

The types of flow occurring in the runner, gate and cavity of a cylindrical bar mould have been investigated and the relation between the flow and local modulus of the moulding determined for crystal polystyrene. The results suggest that non-isothermal shear flow was occurring in the runner while the gate displayed the characteristics of shear flow when heated to the polymer melt temperature and a complex type of flow when cold. The cavity was found, under normal injection rates, to fill by a jetting mechanism which often produced surface irregularities in the product and progressive filling from the gate was only observed at low injection rates. Investigation of local moulding properties revealed that while considerable variations within a specimen existed, these were apparently not due to the mould filling mechanism. They have been attributed tentatively to the small opening of the mould which takes place during injection and to cooling strain.     

自然平衡流道结构对多型腔模具非平衡充填的影响

在验证国内外已有的关于几何对称型腔非平衡充填研究结论的基础上,设计了可更换流道的实验模具,研究了流道尺寸及浇口形式的变化对非平衡充填的影响,并考察了整个充模过程中塑料熔体在型腔中的流动情况.结果表明,改变流道尺寸及浇口形式有利于改善非平衡充填程度,并指出整个充模过程的非平衡充填是一个动态演化过程.     

基于Moldflow的注塑模流道平衡设计与分析

以手机前盖及前饰板两个非等体件共模结构为分析实例,阐述在不同体积型腔的模具流道设计中,利用MPI软件对不同流道的设计进行对比分析。主要对比分析了三种典型的浇注方式对浇注时间、转换压力及锁模力三个要素的影响,再根据产品的需求选择合适的流道设计,最后利用分析出的两腔填充时间比值确定产品流道截面的尺寸。优化了分流道,使塑件的填充达到了流动平衡,尽可能减小了塑件的缺陷。     

基于Moldflow的复印机转印支架热流道注塑方案对比研究

针对某品牌复印机转印支架在生产中出现的缺料、缩水、翘曲等不良现象,将原模具的两点进料热流道方案改为单点进料热流道方案,并应用Moldflow软件对两种方案的熔体充填过程、模腔压力分布及塑料件的熔接痕质量、流动前峰温度、凝固情况和顶出时的体积收缩率等进行了比较和分析,从理论上预测了新方案的可行性。经生产企业实际应用,取得较好效果。     

Some relationships between injection molding conditions and the characteristics of vitrified molded parts

The various mold filling phenomena influencing the characteristics of fabricated parts are surveyed. The phenomena leading to jetting in injection mold filling are considered. These are associated with the magnitude of swell by the melt as it exits the gate into the mold. Special attention is given to the influence of non-isothermal runner flow. A theory of extrudate swell of polymer melts with temperature profiles is developed using Tanner's unconstrained recovery theory. In the. absence of jetting, mold filling by a simple advancing front takes place. The hydrodynamics of the advancing front and the stress fields in the flowing melt are determined. Analysis and modeling are presented based on the use of hydrodynamic lubrication theory involving a solid layer along the mold wall and a hot isothermal melt core. This theory is compared with experimental measurements of pressure losses in mold filling. The development of birefringence in injection molding processes is analyzed. Birefringence distributions are due to frozen-in flow birefringence. A new experimental study is presented and its results compared with theoretical predictions. The problem of thermal stresses in injection molded parts is considered.     

Automated Design for the Runner System of Injection Molds Based On Packing Simulation

Multiple injection cavities are automatically balanced by adjusting runner and gate sizes based on an iterative redesign methodology integrated with computer-aided engineering (CAE) packing simulation. For runner balancing, each cavity must be filled simultaneously at uniform pressure. In addition, the time-pressure history of the polymer melt over the entire molding cycle should be considered. Based on the proposed methodology, a multicavity mold with identical cavities is balanced to minimize entrance pressure differences among various cavities at discrete time steps of the molding cycle. The results have shown more than a 95% reduction of the entrance pressure differences over other related studies, and also have demonstrated increased searching performance over other optimization techniques. A family mold with different cavity volumes and geometries is also balanced to minimize pressure differences at the end of the melt flow path in each cavity on a basis of discrete time steps of the molding cycle. The methodology has shown uniform pressure distributions among the cavities during the entire molding cycle.     

Boundary conditions in the modeling of injection mold-filling of thin cavities

A governing equation for injection mold-filling of thin cavities with a power-law fluid is derived. The interaction between upstream delivery channel flow and cavity flow results in a continuously changing gate condition as the total viscous dissipation of the delivery channel-cavity assembly is minimized. Depending upon the relative magnitude of pressure drops or viscous dissipation across the channel and the cavity, the boundary conditions which determine the cavity filling process will lie between the following two limiting cases: a Cauchy type gate condition such that the location of the melt front is completely determined by the upstream flow; a Cauchy type melt front condition in which the gate condition is controlled by the downstream flow. For most injection molding cases this may be manifested as equilibration of dissipation density on the melt front. Experimentally observed melt front locations from isothermal, Newtonian filling of a constant gap rectangular cavity and of a bi-gap rectangular cavity are reported and the validity of the limiting cases are tested.     

Flow visualization for injection molding of polyethylene and polystyrene melts

Flow patterns have been observed during the injection molding of rheologically characterized low-density polyethylene and polystyrene melts under various molding conditions. Some studies of high-density polyethylene were also carried out. Various mold designs were included in the study and the flow patterns investigated under both isothermal and cold cavity wall conditions. In addition to investigating injection molding of single polymer melts, flow patterns in the sandwich molding of polyethylene and polystyrene were studied.     

Experimental and theoretical study on filling imbalance in geometrically balanced injection molds

Experimental and theoretical studies were performed on filling imbalance in geometrically balanced injection molds. Balancing the melt flow between cavities was investigated using several different runner systems at various operating conditions. Experiments indicate that injection rate, mold, and melt temperatures substantially affect the filling imbalance. It is strongly dependent on runners layouts geometry, and it has never been eliminated completely. It is most difficult to remove for high injection rates and low melt temperatures. Standard element geometry and circled element geometry cause positive imbalance which means that inner cavities fills faster, and it is opposite for one/two overturn element geometries which induce negative imbalance. A special modeling procedure is required to simulate properly the imbalance. This includes inertia effects, geometrical modeling of the nozzle where the imbalance starts, 3D tetrahedron meshing with minimum 12 layers. Simulations were consistent with experiment, however, when the imbalance increased, the discrepancies between simulation and experiment also increased. It can be stated that filling imbalance problem is still unsolved. There are serious thermo‐rheological aspects to explain for better understanding this phenomenon. Trends in modeling of injection molding are presented and new concepts solving the problem are discussed including simulation/optimization approach and a novel concept of global modeling of injection molding process. POLYM. ENG. SCI., 59:233–245, 2019. © 2018 Society of Plastics Engineers     

New approach to develop a 3D non-isothermal computational framework for injection molding process based on level set method

The simulation of three-dimensional (3D) non-isothermal, non-Newtonian fluid filling process is an extremely difficult task and remains a challenging problem, which includes polymer melt flow with free surface coupled with transient heat transfer. This paper presents a full 3D non-isothermal two-phase flow model to predict the complex flowinmelt filling process,where the Cross-WLFmodel is applied to characterize the rheological behavior of polymer melt. The governing equations are solved using finite volume method with SIMPLEC algorithm on collocated grids and the melt front is accurately captured by a high resolution level set method. A domain extension technique is adopted to dealwith the complex cavities, which greatly reduces the computational burden. To verify the validity of the developed 3D approach, the melts filling processes in two thin rectangular cavities (one of them with a cylindrical insert) are simulated. The predicted melt front interfaces are in good agreement with the experiment and commercial software prediction. For a case with a rather complex cavity, the dynamic filling process in a hemispherical shell is successfully simulated. All of the numerical results show that the developed numerical procedure can provide a reasonable prediction for injection molding process.     

The injection molding of thermoplastics part I: Theoretical model

A mathematical model is proposed for the quantitative treatment of the injection molding of thermoplastics as it relates to the behavior of polymer in the cavity. The model is based on setting up the equations of continuity, motion, and energy for the system during each of the stages of the injection molding cycle (filling, packing and cooling) and the coupling of these equations with practical boundary conditions. The treatment takes into consideration the non-Newtonian behavior of the melt, the effect of temperature on density and viscosity, the latent heat of solidification, and the differences in thermal properties between the solid and the melt. In employing the model, it is necessary to know the pressure-time variation at the cavity entrance. Numerical solutions have been obtained for the case of spreading radial flow in a semi-circular cavity. The numerical results yield significant data on the progression of the melt front, the flow rate, and the velocity profiles at different times and positions in the cavity. They also yield temperature and pressure profiles throughout the packing and cooling stages.     

充模过程的Level Set两相流模拟

采用Level Set两相流方法模拟了熔体充模过程,避免了处理复杂的边界以及用Ghost方法将熔体内的速度值外推到熔体外的情况。分别对型腔水平中面与垂直中面的充模过程进行了模拟。讨论了不同注射速度、不同注射口数量以及不同Reynolds数对充模过程的影响,得出了不同时刻各种情况下熔体界面的位置与充模过程刚结束时型腔内的压力分布,分析了熔体在型腔内运动的不同阶段的特点及形成不同阶段的原因。结果表明,在注射口宽度与型腔宽度相差不大的情况下,如果采用中低速充模,则整个充模运动过程以比较平稳的扩展性运动为主,充模较完全,熔体不发生破裂,制件效果较好。充模速度越大,熔体达到平稳流动的时间越短,充模过程越短。数值模拟结果与实验结果一致,同时表明Level Set两相流方法在求解拓扑性质发生较大变化问题时具有很大的优势。     

注射成型中模腔内振动剪切流动的理论模型

采用Ｌｅｏｎｏｖ本构模型,首次研究了在注射成型中模腔内聚合物熔体的振动剪切流动时所产生的振动剪切应力。结果表明,振动剪切应力的振幅随着聚合物熔体的粘度,振动频率和应变振幅的增加而增加,随着熔体温度的增加而减小。     

塑料注射成型的流动仿真

注塑件大多为薄壁制件,在进行熔体的流动仿真时,可采用合理而必要的假设,建立塑料熔体在流道和型腔中流动的数学模型;采用有限元和有限差分混合方法求解;编制相应的模拟程序,并将程序的预测结果与实验值进行比较。     

Effects of surface roughness on microinjection molding

Injection molding is one of the most common processes for cost‐effective mass production of microplastic parts. When the dimensions of the part, and thus the cavity of the mold, are small, microscale factors which are normally neglected in the analysis of conventional injection molding may play an important role. This investigation addresses the effects of mold surface roughness on the injection of polymer melt, which is a non‐Newtonian fluid, during the filling stage of microinjection molding. The surface roughness effect on the volume of the mold cavity is discussed. A simple, but effective model, to describe the conductivity and the specific heat of the surface roughness is proposed. Subsequently, by employing the finite volume method and the level set method, a numerical procedure incorporating the proposed surface roughness model to describe the flow behavior of the polymer melt in the cavity is implemented. Finally, simulation on the melt flow injected into a microdisk cavity is performed using the proposed model and the results are found to be in good agreement with experiment. POLYM. ENG. SCI., 47:2012–2019, 2007. © 2007 Society of Plastics Engineers