Effect of an obstacle during processing on the weld line of injection‐molded glassy polystyrene: Microhardness study

Abstract

ABSTRACT:The microhardness (H) technique is used to characterize the quality of the weld line in injection-molded glassy polystyrene by means of a cylindrical obstacle. In particular, the effect of the indentation location (closer or further from the obstacle edge parallel to the injection direction and across the weld line), both on the surface and in the bulk, was examined. Only for surface measurements close to the obstacle (up to 10 mm) a well-pronounced decrease in H (ϳ30%), followed by a sharp increase in a narrow distance (0.20 -0.25 mm), was observed. For the bulk measurements on the same location a slight decrease in H was detected. Additional H measurements made up to 60 mm from the obstacle for both cases showed that the weld line remains undetectable. The results obtained reveal that the presence of a cylindrical obstacle causes the formation of a weld line on and near the surface only at distances not exceeding the obstacle diameter. At larger distances, because of the effective mutual interdiffusion of polymer chains, the two parallel fronts coming from the two sides of the obstacle developed a homogeneous material without any weld line according to the microhardness test.

Visualization of the flow history contours at the cross‐section of a weld‐line in an injected molded part

“…This could suggest a correlation between material behavior at the core and the surface of the part. The correlation introduces an area which is highly affected by the obstacle pin (obstruction affected zone) broadened in a distance not exceeding the obstacle diameter; it is in agreement with the earlier studies 3, 4, 8…”

“…It is known that the poor bonding of the surface layers of a part, which includes V‐notch, is a result of insufficient time for polymer molecules to diffuse across the interface 7. This variation along the thickness direction was experimentally verified by a micro‐hardness study along the cross‐section of a weld‐line 8. Results showed that the core of the part is stronger than the surface layers.…”

“…7 This variation along the thickness direction was experimentally verified by a microhardness study along the cross-section of a weldline. 8 Results showed that the core of the part is stronger than the surface layers. In addition, a homogenous material with no presence of weld-line was found at the further distances from the obstacle pin, attributed to the effective interdiffusion.…”

Mechanical testing of micromolded plastic parts by nanoindentation

“…The latter has enabled to establish the relationships between the process variables and product characteristics [11,12]. At macroscale, some interesting findings, in this direction, have been reported, substantiating the validity of the microindentation technique for the assessment of the microhardness variation along the weld line of the macrosized PS and PC parts [13][14][15]. The same concept was applied to the assessment of welded joints in several polymers and metals [16][17][18].…”

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].…”