基于气体反压的注射成型及其关键力学问题研究

Injection molding is one of the most versatile and important manufacturing processes capable of mass-producing complicated plastic parts in net shape with excellent dimensional tolerance. Defects such as weld lines, sink marks, and warpage are caused by melt fronts collision, un-balanced flow, uneven cooling, non-uniform internal stress and inhomogeneous nucleation and growth of crystals as the part solidifies. Nowadays, some new molding techniques have surfaced that make use of specific pressure (and shear induced) profiles prior to or during molding, to control the flow pattern and/or the internal structure and morphology of the plastic as it is being shaped，as part quality and yield requirements become more stringent. The ultimate challenge is to predict why the properties of a molded part depend on the thermal-mechanical history of the polymer during processing, and how it can be modulated by multi-field control means. If the model is capable of integrating all the effects of viscosity and morphology, at macroscopic and microscopic levels, it will be possible to determine the optimum melt processing conditions without recourse to an excessive number of experiments. In this project, a method and apparatus for effectively restricting the fountain flow during injection molding by gas counter pressure (GCP) will be studied. This system consists of four main units: (1) conventional injection system; (2) a mold system with gas injection/release valves; (3) a signal measuring system; and (4) a GCP system. Then, the relevant influence of control mechanism and processing conditions on the skin thickness, crystallization morphology, orientation and mechanical properties will be investigated. In addition, the mechanical properties of parts molded from different pressure control in various conditions are also compared. During the filling-packing-cooling cycle of injection molding with GCP, the polymer properties, mold design, and molding conditions interact to produce the thermomechanical history experienced by the polymer melt which in turn determines the physical properties of the molded article. Understanding the links between process conditions and product properties is crucial for successful operation, and a predictive model enables process optimization for given product specifications. Mathematical models and numerical simulation of injection molding with GCP control will be implemented based on experimental results, conservation equations, a theory of molecular orientation and crystallization. The effect of crystallinity on viscosity of the polymer melt and temperature due to latent heat of fusion is considered. To investigate the effect of processing parameters on the crystallinity, the injection molding process with GCP of semicrystalline plastics will be simulated and the molded parts micro morphology will be studied under different processing conditions.

注射成型制品约占塑料制品种类的 1/3 以上，由于其优良的加工和使用性能，广泛应用于国民经济及人民日常生活的各个领域。复合注塑成型能大幅提升注塑产品的竞争力与附加值，是塑料加工领域重要的发展趋势，但复合成型相关的模拟技术有待完善。反压（即施加额外压力场）注射成型工艺，在国家重大项目、军工等领域有重要应用，相较于外加热场、力场的复合注塑成型，研究较少且不系统。 本项目拟结合实验与数值方法，针对反压注塑成型工艺开展下面的工作：1）反压注塑成型工艺对透明料、结晶料、纤维增强塑料制品微观形貌与性能的影响，明确反压注塑成型工艺的适用范围与推广条件；2）反压注塑成型模拟理论与算法的研究，包括假设与简化条件、数学模型的建立、边界条件的确定及数值模拟技术的实现；3）反压注塑过程中微结构形态及演变机理的试验与数值研究。研究结果可用于探索新的聚合物成型加工方法，在低成本制备特殊功能塑料制品的方法上实现突破。

复合注塑成型能大幅提升注塑产品的竞争力与附加值，是塑料加工领域重要的发展趋势，但复合成型相关的模拟技术有待完善。反压（即施加额外压力场）注射成型工艺，在国家重大项目、军工等领域有重要应用，相较于外加热场、力场的复合注塑成型，研究较少且不系统。.本项目拟结合实验与数值方法，针对反压注塑成型工艺完成如下工作：1）完成反压注塑成型实验，研究工艺对透明料、结晶料、纤维增强塑料制品微观形貌与性能的影响。通过实验发现反压工艺可以提高PC微孔注射的表面质量，在给定工艺条件下，微孔注射制品的平均表面粗糙度最多可提高83.7％,与传统注射成型的表面质量在一个量级；反压工艺可提高PP微孔注塑制品的结晶度（细化晶粒）9.6%，拉伸强度提高18.2%;对于纤维增强塑料，纤维取向度降低10％，有效改善制品性能。2）完成反压注塑成型模拟理论与算法的研究，包括假设与简化条件、数学模型的建立、边界条件的确定及数值模拟技术的实现。结合已有成果和工程应用，提出了“类反压工程模拟”和“反压模拟”两类方法，前者可应用商品化的软件（如Ｍoldflow和ANSYS）完成，量化反压工艺对熔体流动形态、熔体前沿、速度、温度、取向等的影响；后者需要考虑本构方程、边界条件等变化，自主完成部分编程，以适合新工艺、新产品研制；3）反压注塑过程中微结构形态及演变机理的试验与数值研究。根据现有的条件和研究手段，选取“纤维取向”作为实验与数值模拟验证参数，在此基础上，进而分析预测反压工艺对射流(jetting)缺陷、黏度的影响，但对结晶与性能预测待进一步完善。相关的研究结果，在气体辅助注塑成型、珠光免喷涂新材料成型方面尝试应用，可进一步用于探索新的聚合物材料成型加工方法，对低成本制备特殊功能、性能的塑料制品有指导作用。