Once a cosmetic baseline is in place, the next step is figuring out how the material behaves as it moves through the mold. The Rheology Curve Study focuses on how injection speed affects pressure, fill time, and shear.
This is where flow behavior becomes measurable. Instead of relying on visual confirmation alone, you start defining the range where the material fills consistently and responds predictably.
This study establishes the relationship between injection velocity and fill pressure, then identifies the speed range where the process remains stable.
Within that range, the cavity fills consistently, the flow front stays balanced, and cosmetic defects are minimized. You also get a predictable pressure profile that carries into later studies.
That range becomes the working zone for everything that follows, including cavity balance, pressure drop, and process window development.
Plastics don’t behave like simple fluids. Their viscosity changes with shear rate, meaning they flow more easily as injection speed increases, but only up to a point.
At low speeds, the material resists flow, which can lead to short shots or visible weld lines. At very high speeds, shear heating builds up, increasing the risk of burns, splay, or flash.
Between those extremes is a stable region where flow is controlled and repeatable. That’s the zone this study is designed to find.
The process starts with the cosmetic baseline, typically running at 95 to 98 percent fill to maintain consistency.
From there, a series of shots are run at increasing injection speeds, usually across five to seven steps. For each shot, fill time, peak injection pressure, and part weight are recorded.
Once collected, the data is plotted with injection speed against peak pressure. This reveals how the material responds to shear and highlights the region where the relationship becomes linear.
The midpoint of that linear region is selected as the target injection speed, giving a stable and repeatable operating point.
Each run tracks injection velocity, fill time, peak pressure, and part weight, while keeping melt temperature constant. Visual observations are also recorded to connect numerical data with part quality.
As speeds increase, fill time typically decreases, while pressure follows a trend that helps define the material’s shear behavior.
The curve usually breaks into three distinct regions.
At low velocity, the material flows sluggishly, and pressure tends to be high and unstable. This often leads to incomplete fills or visible defects.
In the middle, the curve becomes linear. Flow stabilizes, and pressure changes predictably with speed. This is where the process performs consistently.
At high velocity, the curve starts to break down. Pressure may drop unpredictably due to shear heating, and defects like burns or flash become more likely.
Selecting a speed from the center of the linear region gives you a process that stays stable even as conditions shift slightly.
Numbers define the curve, but the parts confirm it.
Each sample is compared to the cosmetic baseline. Slower speeds often show flow lines or blush, while excessive speed can introduce burns or splay.
At the optimal speed, parts match the baseline visually while running at lower and more consistent pressure. That alignment between data and appearance is what locks in confidence.
The selected injection speed directly affects how evenly cavities fill. Running outside the stable region often creates imbalance between cavities.
By locking in a consistent shear condition here, the next phase can focus on verifying uniform fill across the tool.
This study produces a rheology curve plot showing injection speed versus peak pressure, with the stable region clearly defined. It also establishes a recommended fill speed and includes visual documentation for each test condition.
These results are stored alongside the rest of the process development data, forming a direct link between flow behavior and validated process parameters.
When the curve doesn’t stabilize, it often points to machine limits or material viscosity being too high. Irregular pressure spikes can indicate gate restrictions or venting issues.
A sudden drop in pressure usually signals shear overheating, while a flat curve may mean speeds are too low to reveal meaningful differences.
Each of these patterns gives direction for both process adjustments and tooling improvements.
Flow behavior isn’t guesswork. When measured correctly, it reveals how the material wants to move through the mold.
Working within that natural range changes how the entire process performs, leading to more consistent filling, fewer defects, and a foundation that holds up through every downstream study.