In this installment of the Hunting for Insight series, the focus is on the Temperature at Flow Front result, an output that guides critical design decisions around part quality, weld line integrity, flow length capability, and processing window. This result provides insight into how the thermal condition of the melt evolves as it fills the cavity, and how that behavior influences both cosmetic and structural performance of the molded part.
What is Temperature at Flow Front, and Why Should Designers Care?
The Temperature at Flow Front result is available across all Autodesk Moldflow mesh types and is representative of the temperature at the mid-point at the wall. While the result representation is the same, the assumptions of each solver may vary. However, their exact nuances are not relevant for this article.
This result is important because the temperature of the melt at the flow front is an indication on how the viscosity of the material changes as the cavity continues to fill. Having an indication of the viscosity variation helps mold and part designers better understand if the cavity will consistently fill completely, weld line integrity, and ability to effectively pack out the part. If the flow front temperature drops too low, the material can begin to freeze off prematurely, leading to short shots, poor weld line strength, or inadequate packing. Conversely, higher flow front temperatures generally promote lower injection pressure, improved surface finish, and stronger molecular entanglement at weld lines. However, too much shear heating introduces the risk of property loss due to degradation, discoloration and could increase the shrinkage and cycle time.
For designers, this means the Temperature at Flow Front result is not just a processing metric, it is a design tool. It helps designers optimize their mold and part designs for processability by identifying regions at risk due to excessive cooling, long flow paths, or thin geometry, enabling early adjustments to wall thickness, gate location, gate quantity, or material selection prior to having a final tool design. This helps reduce the risk of fill and pack related defects and improves confidence in the processability of the part design.
The Temperature at Flow Front Result: What You’re Looking At
The Temperature at Flow Front plot is typically displayed as a shaded color plot, where the color scale represents the temperature of the melt at the advancing flow front. The result is a snap shot at single point in time and cannot be animated to show how the temperature changes, like in the temperature and bulk temperature results. As the material flows further from the gate, the temperature will generally decrease due to heat transfer into the cooler mold steel. Areas that retain higher temperatures are typically closer to the gate or located in thicker sections, while lower temperature regions are often found at the end-of-fill or in thinner geometry.
Figure 1: Shows the Temperature at Flow Front plot highlighting how the melt temperature decreases along the flow path from the gate to the end-of-fill.
Evaluating Processing Window: What is the perfect fill time?
The two competing thermal effects during filling are the shear heating between laminates and heat loss to the mold. As the material flows through the part and runner, it is generating heat from the adjacent layers sliding past one another. This heating lowers the viscosity and promotes easier flow conditions. At the same time, it is constantly losing heat to the colder mold surfaces, which increases viscosity and raises the risk of premature freeze-off. The constantly changing plastic temperature at the flow front is highly dependent on flow length, wall thickness, material properties, and processing conditions. Therefore, the Temperature at Flow Front result can be an indicator of the processing window.
The thermal response shear flow and contact with the cold mold is material-dependent. Semi-crystalline polymers typically show more pronounced shear-thinning behavior, so higher shear rates can significantly reduce viscosity and therefore reduce the required fill pressure. Amorphous polymers are generally more sensitive to shear heating and are more prone to thermal degradation if shear rates become excessive.
The “perfect” fill time would be achieved when you generate just enough shear heating to offset the heat loss and maintain a stable viscosity through the cavity, but not so much that you overheat the material and risk thermal degradation, or introduce excessive orientation at the gate.
This is why the Temperature at Flow Front result becomes such a powerful metric for evaluating the processing window. Because it directly reflects the balance between cooling and shear heating, the flow front temperature shows whether the material is staying within a stable rheological state as it fills the part. A consistent flow front temperature from gate to end-of-fill indicates that the process is well balanced, while large temperature drops or spikes reveal instability in that balance.
The part shown in Figure 2 has a narrow processing window due to a thinner nominal wall surrounded by thicker sidewall geometry that causes racetracking. Figure 3 details the fill pattern, highlighting the hesitation and racetracking due to the wall thickness variations. (click here to read more about the fill time plot)
Figure 2: Cross-section of the part geometry highlighting the thinner nominal wall relative to the thicker sidewalls.
Figure 3: As the material flow from right to left, the thinner center of the part hesitates through the nominal wall, as the flow racetracks through the thicker side walls.
Figures 4 A, B and C show how fill speed influences thermal stability and the processing window through the Temperature at Flow Front result. Small changes in fill speed cause large swings in temperature.
Figure 4A: The 0.60 second fill time is too fast. The material experiences minimal cooling and instead shear heats by approximately 37.3°F from the gate to the end-of-fill, indicating excessive shear heating.
Figure 4B: The 1.25 second fill time falls within the optimal processing window. A small amount of cooling (<1°F) is observed in the hesitation region, while the remainder of the flow front exhibits modest shear heating. The lower rate of shear heating suggests that enough energy is being generated to offset heat loss to the mold without excessively increasing temperature.
Figure 4C: The 3.35 second fill time is too slow. Most of the cavity is filled by a flow front that continuously cools as it advances, indicating insufficient shear heating to offset heat loss to the mold.
Understanding the Temperature Distribution: Are you thermally stable?
The Temperature at Flow Front plot provides a look at how heat is fluctuating throughout the filling process. In order for the part to cool and shrink uniformly, it is important that all regions of the part start at a similar temperature. This is why molders have traditionally favored faster fill times to reduce viscosity and maintain a more consistent melt state before packing. The Temperature at Flow Front result can tell you how your gating and filling strategy is impacting the thermal stability of your part, and whether or not you are setting yourself up uniform packing. It is preferred that the variation in temperature at the flow front remains less than 35 °F (19 °C), as shown in Figure 5.
Figure 5: Highlights a thermally stable filing progression with a temperature range of 4.2 °F at the flow front.
Flow Length Capability: Will the material make it?
As the melt travels further from the gate, it loses heat to the mold, reducing its ability to continue flowing. The Temperature at Flow Front plot helps evaluate whether the material can maintain sufficient temperature over the required flow length. Long flow paths combined with thin geometry will typically show significant temperature drop, indicating a higher risk of incomplete fill.
Short Shot Risk: Geometry and Wall Thickness Evaluation
Low temperature regions at the flow front are often associated with areas that are at risk of short shots. These regions can highlight thin walls, abrupt thickness transitions, or poorly placed gates that cause excessive cooling before the cavity is filled. Designers can use this result to identify these risks early in the design process and reassess the part or tooling strategy. This may include increasing wall thicknesses, repositioning or adding gates to reduce flow length, or modifying the fill pattern to better balance thermal loss across the cavity.
Surface Finish Quality
The temperature of the melt at the flow front has a direct impact on the surface quality of the molded part. Higher flow front temperatures generally promote better surface replication and gloss, while lower temperatures can lead to surface defects such as poor finish, hesitation marks, or visible flow lines. Variations in temperature across the part can also result in inconsistent surface appearance.
Figure 6 shows an example of hesitation and colder filling in a standing rib feature that leads to flow marks and a non-fill condition on a PA66 part.
Weld Line Integrity
When flow fronts converge, the temperature at the point of convergence plays a critical role in determining the strength of the weld line. Higher temperatures promote better molecular entanglement and improved weld line strength, while lower temperatures can result in weak interfaces and reduced mechanical performance. The Temperature at Flow Front plot can be overlaid with weld lines to help understand if excessive cooling could be affecting the integrity of the weld line.
Summary
Temperature at Flow Front connects part geometry, material behavior, and processing conditions to better understand how heat loss and shear heating interact at the flow front. When interpreted correctly designers can better anticipate filling consistency, weld line quality, surface finish, and overall process robustness. Ultimately, the result provides a practical way to evaluate whether a part design supports a stable, repeatable, and thermally balanced molding process.
