The influence of level of interfacial healing on the weld-line strengths of injection molded parts

Abstract

Weld lines are commonly present within injection-molded plastic components and generally reduce strength. In designing a plastic part, the designer can often vary the wall thicknesses, or move the gate locations to position the weld line, so that its structural loading is minimized. By altering such variables as melt temperature, mold temperature, and packing pressure, slight increases in weld-line integrity can be achieved. These approaches often do not increase the part strength or performance to an acceptable level, resulting in the exclusion of numerous applications for plastic conversion. This study investigated a hot runner system, sequential valve gates, and heat degree, to achieve economical methods for predicting weld-line properties. By calculating melt front temperatures with MoldFlow analysis, we can predict the strength of the weld line according to the definition of " heat degree " , which is the effective cross-sectional area of the melt front multiplied by the temperature gradient. An effective crosssectional area of the melt front is defined as the area of the temperature above the melting point. In contrast to the test result, when the " sum of heat degree " is lower than 1150 mm 2 ° C for PA6 (Mitsubishi 1030J) and 500 mm 2 ° C for polypropylene (PP) (FCFC K1012), the molecules cannot bond adequately at the interface of the melt fronts. Therefore, the mechanical property declines significantly.

Evaluation of healing models to predict the weld line strength of the amorphous thermoplastic polystyrene by injection molding simulation

“…Correlations of the viscosity or temperature profile in the weld line with the weld line strength were found. [18,27,28] However, these only allow a qualitative prediction of the weld line strength, which can only be used to a very limited extent as a basis for part design.…”

“…Explanation approaches are the relaxation of the strongly oriented molecules or air inclusions in the weld line. [1,[16][17][18][19]…”

Untersuchung der prozess- und geometrieabhängigen Zug- und Schubfestigkeit von Bindenähten in Polystyrol

“…Die Bildung einer V-förmigen Nut, welche aufgrund ihrer Kerbwirkung unter Belastung zu einer Spannungskonzentration in der Bindenaht führt, wird in der Literatur mehrmals beschrieben [10,11]. Die Ursache für die Entstehung dieser V-Nut ist derzeit noch nicht vollständig geklärt [12][13][14]. Mögliche Ursachen sind Lufteinschlüsse [2,14,15] oder eine lokal höhere Volumenschwindung in Richtung der molekularen Orientierungen [3,16].…”

Effect of weld-line on tensile properties for thermoplastic glass filled material

“…Katsuyuki et al 2 have reported research on the characterization of the microstructure using polarized laser Raman spectroscopy, micro-cutting analysis, and fracture testing and found significant reduction by weld-line attributed to increasing molecular weight or broadening molecular weight distribution. Chung-Ching Huang et al 9 developed a hot runner system, sequential valve gates aimed to achieve cost effective methods for the prediction of weld-line properties. The authors have predicted the strength of the weld-line using the temperature of the melt front as suggested by mold flow simulation software.…”

“…stress at break or strain at break will not be the right approach. 1,5,8,9 This may lead to the wrong design and subsequent failure of the part. To avoid this, it becomes necessary while designing a plastic injection molded part to identify the weld-line areas and strength of part near weld-line areas.…”