模内微装配成型预成型微型部件颈缩熔断的失效机理

Necking and Fusing Failure Mechanism of Preformed Micro Component in In-Mold Micro Assembly Molding Process

基于聚合物微型机械模内微装配成型加工面临的共性关键科学问题——在二次成型高温熔体充填流动环境下,如何避免预成型微型部件产生颈缩熔断失效问题,研究建立了高温熔体充填流动诱发颈缩熔断失效过程的机理模型。研究表明,预成型微型轴颈缩熔断损伤的直接驱动力是应变软化,一旦预成型微型轴出现应变软化,就必然会诱导颈缩熔断损伤。而应变软化现象的形成受控于其材料的初始屈服应力,初始屈服应力与二次成型注射温度呈负关联关系。当熔体注射温度由200℃增至240℃时,聚甲基丙烯酸甲酯(PMMA)微型轴的初始屈服应力由16.5 MPa降至9.89 MPa,降幅高达40.1%,而其颈缩断面的颈缩率由44.5%增至70%。二次熔体注射温度越高,微型轴越易诱发应变软化和颈缩熔断损伤。

How to avoid the necking and fusing failures of preformed micro component under the environment of high temperature melt filling flow is a critical scientific problem for the in-mold micro assembly process of polymer micro machine. In this paper, we established a mechanism model of necking and fusing failure for the preformed micro-component induced by high-temperature melt filling flow. The investigation results indicated that the direct driving force of necking and fusing failure for preformed micro-size shaft was derived from its strain softening. Once the strain softening was induced, the necking and fusing damages for preformed micro-size shaft might happen. The strain softening phenomena were controlled by the initial yield stress of preformed micro-size shaft and were negatively correlated with the secondary molding injection temperature. The initial yield stress of preformed PMMA micro-size shaft decreased by 40.1 % from 16.5 MPa to 9.89 MPa when the injection temperature increased from 200 ℃ to 240 ℃. The necking shrinkage of neck section increased from 44.5 % to 70 %. The higher the secondary molding melt injection temperature, the easier was the strain softening and necking fusing failure of preformed micro-size shaft.