Influence of injection molding parameters, melt flow rate, and reinforcing material on the weld‐line characteristics of polypropylene

Abstract

In this work, the relation of injection molding parameters to the mechanical properties of various polypropylene grades is investigated to find the optimal processing parameters to minimize weld‐line effects like reduced mechanical and/or optical performance. Injection molded test specimens of five polypropylene grades with different melt flow rates were characterized for their mechanical behavior and compared with equal‐shaped specimens with a colliding weld line in the middle of the specimen. For the production of these weld‐line specimens, a special mold was used and injection molding parameters were systematically varied, tensile (ISO 527‐2) and impact (ISO 179‐1/1eU) properties were measured, and statistical analyses were performed to gain insight on the correlation between melt flow rate and weld‐line behavior. It showed that mechanical properties of specimens with weld lines can be influenced by the processing parameters, but the effect is limited. Positive correlations were found between tool temperature and tensile modulus and strength. Polypropylene grades with low melt flow indexes seem to be more susceptible to weld‐line‐induced property reductions. In a second test series similar to the pure PP investigations, glass‐fiber‐ and talc‐filled PP were used to gain insight into the weld‐line behavior of PP composites. In addition to mechanical characterization, optical and scanning electron micrographs were taken of the weld‐line areas. It was found that the reinforcing particles align along the melt flow front in the weld‐line area. As a result, the mechanical performance of weld‐line specimens is poor. Injection molding parameters were found to have only a small effect.

Weld‐lines and its strength evaluation in injection molded parts: A review

“…The quality of the final product is depending on several process parameters such as melt temperature, mold temperature, injection pressure, packing pressure, cooling time, etc. 2,6,7 Primarily, plastics were viewed as a replacement for conventional materials such as metal and wood. However, they have now become as indispensable as the original sources themselves.…”

“…17 The general technical thumb rule says, when the angle between two mating flow fronts is below 135 , the mating line is called a weld-line, else it is a meld-line. 2,15 Based on the mating of the front flows, weld lines are classified into two different types as below-1. Stagnation weld-line: This is also called cold weld-line, these weld lines are formed when two melt front meets directly head-to-head, generally occurring in circular geometry or multiple gate locations.…”

“…Typically, in the case of stagnation weld-line, the center of the flow-fronts has a minimum mating angle, and it increases toward the surface however for the flowing weld-line mating angle is the minimum next to the obstacle and it increases when the distance from the obstacle increases. 1,2 The weld-line zone is a region where material characteristics vary from other part regions. The variation in the properties depends on many different parameters.…”

“…15,21,22 When fountain flow-fronts strike each other the mixing or interdiffusion takes place. 1,2 The characteristics of the interdiffusion area depend on viscosity, velocity, meld flow rate, temperature, and pressure of polymeric melt. At the time of the collision, the molecules and fibers are changing their orientation and move as per the fountain flow and bounce velocity vector.…”

“…At the time of the collision, the molecules and fibers are changing their orientation and move as per the fountain flow and bounce velocity vector. 2 Figure 3 shows the typical weld-line interdiffusion zone pictorial format and fiber orientation for reinforced materials.…”