Understanding strength formation during injection molding helps prevent weldline problems

As an engineer who designs application parts and is involved in simulation, you know that weldline strength plays a crucial role when designing new parts or evolving the design of failed parts that are injection molded. It is crucial to take into consideration the drop in strength resulting from weldline formation during the injection molding process and apply design and processing solutions to improve performance when designing an application.

Envalior’s Weldline Strength Tool is very useful to engineers designing applications with weldlines because it provides background information with tips and tricks on the prevention of weldline problems, as well as an indication of the strength at the weldline location. 

You can avoid weldline problems and improve esthetics 

It is best to use the Weldline Strength Tool as early as possible during the production development stage, so you can avoid potential problems later in the process. 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. You can avoid this problem by using our tool.  

As a user, you will need to input:

• Grade selection
• Relative humidity of application

The tool output includes:

• 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

Based on the information from the tool, you can decide if there are potential critical regions in the part design due to the formation of weldlines, for example, in regions that are highly stressed. Then you can consider methods to either reposition them or try and improve their performance. 

It is important to note that the tool does not quantify the strength of meldlines with partially parallel meld flow, opposed to head-on flow for weldlines.

Helping you accurately evaluate plastic parts designs that contain weldlines 

The main advantage of the Weldline Strength Tool compared to earlier work methods where only generic knock down factors were available is that grade specific data is now available, which can be applied to the strength value given on a standard datasheet. This combined with the links to processing and design advice makes the tool even more versatile and powerful in helping in the design and evaluation of plastic parts containing weldlines.

Weldline Strength Tool model and measurements 

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

The weldline strength can be difficult to quantify because it 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.

All measurements are performed according to the ISO 527 1A standard for specimen 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.

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.

At the moment, we are in the process of expanding the Weldline Strength Tool to more grades, depending on the availability of experimental data as well as on the demand for particular grades. Let us know what grades you're looking for by filling in the feedback form on the bottom right of this page.