Effect of packing pressure on stagnation weld-line strength for thermoplastic semicrystalline glass fiber reinforced material

Abstract

Weld-Line in an injection molded part develops when two or more melt fronts are converged together. The weld-line is unavoidable when the design is complex where two melt flow fronts are mating each other head-to-head, this mating area is mechanically weak compared to other portions of the modeled part. The decrease in strength in the weld-line area can be attributed to several factors like molecular diffusion, fiber orientation effect, unoptimized process conditions, surface tension effect, internal residual stresses, etc. The strength of the weld-line area can be increased by optimizing injection molding process parameters like melt temperature, injection speed, packing pressure, changing gate type and location, etc. The present work focuses on studying the effect of packing pressure on strength of stagnation weld-line. This has been examined with experimental testing and scanning electron micrographs of the fractured surface on the weld-line and without weld-line specimens. The special injection mold is designed and fabricated to produce plaques having stagnation weld-line. The plaques are prepared out of 30% glass-filled Polyamide 6 material. The four sets of plaques are produced by changing the magnitude of packing pressure equal to 60% of filling pressure, with increment of 20% up to 120% of filling pressure. The tensile test specimens are machined on these plaques for two different angular orientations and testing is conducted as per ISO 527-2. The results demonstrated that, without weld-line specimens, the tensile modulus, stress at break marginally increases with an increase in packing pressure, and strain at break decreases with an increase in packing pressure. However, for specimens with stagnation weld-lines, both the tensile modulus and stress at break are observed to be 42% of without weld-lines samples for minimum packing pressure. The results obtained are evident from stress-strain graphs and scanning electron micrographs.

Comparative study of weld‐line strength for unfilled and glass‐filled thermoplastic polyamide‐6 materials

“…For 90 weld-line only normal stresses act along the weld-line plane however for 45 weld-line angle specimens the normal stresses and shear stresses are distributed equally and therefore both stress at break and strain at break values are more for 45 weld-line compared with 90 weld-line specimens though the tensile modulus is lower. [8,24] Figure 9 shows strain at break for no weld-line(B), with 90 and 45 weld-line angle specimens for unfilled(0GF), 30% glass-filled (30GF), and 50% glass-filled (50GF) materials. As strain at break values for glass-filled material are very small compared to unfilled material, the detailed view on the bar chart is shown.…”

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

“…13,27,41 The algorithms of these tools are robust and provide a good correlation with actual reality. The weld-line location prediction is well captured in these tools, however, to evaluate the strength of the weld-line for the plastic material non-linear algorithms are required and these flow simulation tools are mainly based on a linear algorithm, therefore prediction of weld-line strength still under development 15,42 and there are no standard results output available to get weld-line strength. The researchers worked on weld-line strength prediction topics by developing customized algorithms or using multi-scale simulation methods.…”

“…The size and shape of the V notch is depending on process parameters and properties of processed plastic material. 5,10,15 Furthermore, the flow fronts no longer melt together uniformly and homogeneously, which might result in an optical and mechanical weak point. 5,12 Figure 1 shows the melt flow front before mating and after mating each other.…”

“…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.…”

“…Which involves a degree of molecular interdiffusion, anisotropy, material type, type of reinforcement, process parameters, design of the part, etc. 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.…”