First results from the ongoing research project GEO-FaserMap
Introduction
Weld lines occur when two flow fronts meet during mold filling. Due to insufficient diffusion of the polymer chains and altered fiber orientation, these areas can be significantly weaker than the unaffected material. The aim of the thesis [1] was to investigate the extent to which the geometry of flow obstacles influences the fiber orientation and thus the mechanical properties of weld lines. The central question was: Does a change in the obstacle geometry lead to a measurable change in weld line strength? The findings of this thesis can be compared with the contents of the article "Estimating the Weld Line Strength of Plastic Components Easily" by PART Engineering, which deals with methods for predicting the strength of weld lines.
Experimental set-up and methodology
To analyze the influence of the geometry, test plates were made of various short-fiber-reinforced plastics. The materials included PA6, PA6-GF15, PA6-GF40, PBT-GF30 and PP-LGF40, as these are widely used in technical applications. The test plates contained specifically placed flow obstacles (Fig. 1) of four different geometries (cylinder, rectangle, 90° and 120° obstacle) and two plate thicknesses (2 mm and 4 mm).
Mechanical tests were carried out to determine the weld line strength. In addition, computed tomography (CT) analyses were used to visualize the fiber orientation in the weld lines. Parallel to the experimental investigations, simulations were carried out to determine the flow processes as well as integrative structural simulations with MatScape, Converse and Simcenter Nastran. The aim was to compare the experimental results with the simulation.
Results
The results show that the geometry of the flow obstacles has no significant influence on the fiber orientation or the weld line strength. Much more decisive are the base material, fiber content and length, as well as the plate thickness (Fig. 2).
While the simulations predict certain geometric influences on the fiber orientation, the experimental investigations show that this influence is negligible.
Adjusting the local stiffness and strength in the weld line does not seem to improve the result. Rather, a suitable factor for reducing the weld line strength is sufficient for the assessment of simulation results according to these test results.
Such a weld line factor, as described here, can be calculated in MatScape and used in S-Life Plastics for strength assessments. However, this factor cannot be directly compared with all the factors determined here, as the weld lines examined here were not exclusively stagnant weld lines.
In the case of stagnant weld lines, the diffusion of the polymer chains is limited, which leads to lower mechanical strength (e.g. tensile specimen injected on both sides). With flowing weld lines, the material continues to flow, as in the plates used here.
The CT analyses show that the fibers in the area of the stagnant weld line reorient themselves and in some cases align themselves transversely to the direction of flow. This reduces the mechanical properties. However, a comparison between weld lines caused by different flow obstructions showed no significant differences in fiber orientation to each other (Figure 4). Although it was found that the angle of the flow fronts varies depending on the geometry of the obstacle, this difference had no measurable effect on the fiber orientation in the weld line itself.
It turns out that the fiber content and the fiber length have a more significant influence on the mechanical properties of the weld lines. As the fiber content increases, the loss of strength in the weld line area increases, as the fibers there are often not optimally aligned and the strength is therefore more dependent on the matrix. Especially with highly filled materials such as PA6-GF40 and PBT-GF30, a significantly reduced relative weld line strength can be observed.
Fiber length also plays an important role: While long fibers generally offer higher mechanical reinforcement, they can be unfavorably aligned at the weld line, which increases the relative weakening. This was found with the PP LGF40. Short fibers are easier to process and result in a more uniform reinforcement but offer lower maximum strength overall than long fibers.
It was also shown that the thickness of the plate or sample makes a greater difference, but only with higher fiber contents (Fig. 5).
Result
In this first investigation on panel test specimens, it was shown that the geometry of the flow obstacles has no decisive influence on the fiber orientation and thus the strength of weld lines. Instead, the choice of material, the fiber content/length and the board thickness play a greater role. Anisotropic simulations and the reduction of the permitted strengths in weld line areas provide improved results compared to conventional isotropic material models without taking the weld line into account. Accordingly, it has been shown that a simple estimating methodology, such as in the article "Estimating the Weld Line Strength of Plastic Components Easily", is sufficiently precise.
Autor:
M.Sc. Daniel Lattek, Technical University of Braunschweig, Braunschweig, Germany
Co-authors:
Steffen Jacob, Development Engineer, Kunststoff-Zentrum in Leipzig gGmbH, Leipzig, Germany
Sascha Pazour, Simulation and Sales Engineer, PART Engineering GmbH, Bergisch Gladbach, Germany
Literature:
[1] Daniel Lattek, Experimental investigation of the geometry dependency of fiber orientation on weld lines of injection-molded test plates, Technical University of Braunschweig, 06.02.2025
