During injection molding the part geometry and obstacles to the melt flow may cause the melt to separate and rejoin. In that location a weldline (or meltline) will be formed, potentially impairing the mechanical properties of the final part. Understanding part design, mold design, processing conditions can help to avoid problems and improve the part esthetics.

This tool provides background information, tips and tricks on the prevention of weldline problems, as well as a rough indication of the strength at the weldline location (with 'head-on' melt flow as indicated in the image). The tool does not quantify the strenght of meldlines with (partially) parallel melt flow.

User input:

  • Grade selection
  • Relative humidity of application

Tool output:

  • Strength of original material without weldline at the selected relative humidity
  • Rough estimate of the weldline strength at the selected relative humidity for a 'head-on' flow front (worst-case) and otherwise perfect process conditions (best-case)
  • Injection molding recommendation for the selected grade
  • Design and processing solutions background information
Schematic weldline
  • All measurements are performed according to the ISO 527 1A standard for specimen (see image) with nominal specimen thickness of 4.0 mm (0.16 in), produced by injection molding.
  • In contrast to normal injection molding of tensile bars from one side, these bars were molded two-sided to induce a weldline in the center of the bar.
  • For grades containing fillers like glass fibers, it is known that the fibers have a preference to orient along the longitudinal axis of the tensile bar, which has a large effect on the stiffness and strength values of the stress-strain response. However, the weldline itself typically does not comprise any fibers.
  • Conditioned samples were conditioned according to the ISO 1110 standard ('accelerated conditioning') under N2 atmosphere and subsequently equilibrated at 50% relative humidity before testing.
  • Subsequently, the tensile bars were tested for their stress-strain response at both 23°C (73°F) under dry conditions and at 23°C (73°F) at 50% relative humidity. All these tensile measurements were conducted on a suitable floor-standing tensile machines of well-known manufacturers. Typically, a pre-load of 10N was applied, a test speed of 1 mm/min (0.04 in/min) for modulus determination up to 0.25% strain and subsequently a test speed of 5 mm/min (0.2 in/min) until rupture of the specimen.
Dimensions tensile bar according to the ISO 527 1A standard

Model creation

  • The model is based on 500+ physical experiments with injection molded tensile bars at room temperature, both dry and conditioned.


  • The weldline strength is difficult to quantify, because is does not only depend on the material properties, but also on geometry and processing conditions. For example, melt pressure, melt/mold temperature, injection speed, venting and potential contaminations all play a crucial role.
  • This tool only provides a rough indication of the weldline strength as we have measured this in our lab with our equipment. With different equipment or process settings, the result may vary (even more so for a different geometry other than a tensile bar).
  • On average, the standard deviation (1 sigma) we measured in our experiments is around 20%, but can be larger for certain grades. This 'engineering accuracy' is indicated in the table of this tool.

Please use the Hotjar feedback form on the bottom right of the Weldline tool page to submit your comments, questions, and feedback. Most frequent questions will be answered in this section.

Why can't I select all Envalior grades?

We're in the process of expanding this tool to more grades, but this depends on the availability of experimental data as well as on the demand for that particular grade. Let us know what grades you're looking for by filling in the feedback form on the bottom right of the page!

Why am I not allowed to export the data from the graph?

Everyone can use the download functionality once logged in; simply register (for free) and immediately start downloading.