split illustration comparing vertical oven-style airflow that wrinkles an ODF wet film with parallel hot-air drying that keeps the film flat and uniform in a drying tunnel.

Parallel Hot-Air vs Vertical Oven Drying for ODF Lines

For companies planning an oral dissolvable film (ODF) line, “which drying concept should we choose?” is often one of the hardest questions to get a clear answer to. Many equipment suppliers focus on heating power, temperature range and nominal capacity, but rarely explain how airflow organisation affects ODF quality in practice.

In reality, the core challenge in ODF drying is not simply removing solvent, but keeping the film surface stable while the wet film has not yet set. The way air moves around and across the web directly influences thickness uniformity and process reproducibility.

This article compares two common concepts: vertical oven-style airflow and parallel hot-air drying, and provides a clearer selection logic for ODF projects.

1. The Essential Difference: How Airflow Acts on the Wet Film

At first glance, the two approaches may look similar – a heated tunnel with air movement and exhaust. The crucial difference is how the airflow vector interacts with the wet film:

  • Vertical oven-style airflow: air is directed towards the film surface with a vertical or strongly angled component, creating an impact on the wet layer.
  • Parallel hot-air drying: the main airflow vector is aligned with the web direction, reducing vertical disturbance to the wet film.

This may sound like a simple “direction” issue, but for ODF it is equivalent to asking: are we gently removing solvent, or are we applying extra mechanical force to a fragile wet film?

For more background on wet-film behaviour and drying, see HUANGHAI’s article on why drying determines thickness uniformity and quality .

2. Impact on ODF Thickness Uniformity

Ultimately, dose uniformity per unit area depends strongly on film thickness uniformity. Drying plays a bigger role here than many teams expect:

  • Under vertical airflow, the wet film can be disturbed, lifted or redistributed: local areas may be blown thin or allowed to pool thicker, making thickness variation more likely.
  • Under parallel hot-air drying, the wet film can stay flatter and more stable as it sets, making thickness variation easier to control.

In many trial phases where “no setting seems to be stable”, the root cause is not that parameters have not been found yet, but that the source of variation is built into the airflow concept. The system is creating noise faster than the engineers can tune it away.

3. Impact on Quality Control and Batch-to-Batch Consistency

When thickness variation is high, quality control becomes a game of “finding problems without being able to remove the root cause”:

  • results appear as QC variability,
  • but the true cause is process instability in the drying stage.

With a drying concept that reduces wet-film disturbance:

  • batch-to-batch consistency improves naturally,
  • process windows are easier to define and maintain,
  • and quality metrics can be held within tighter limits over time.

The difference is not the sophistication of the measurement system, but whether the process itself is inherently stable enough for measurement to be meaningful.

4. Impact on Commissioning and Scale-Up: How Much Manual Intervention Is Required?

A large part of the hidden cost in ODF line projects appears during commissioning and scale-up:

  • With vertical airflow, wet-film disturbance often leads to thickness issues that require intensive manual tuning just to keep the line stable.
  • With parallel hot-air drying, it is easier to form a stable process window, so less time is spent “fighting the system” and more time is spent optimising efficiency.

This distinction matters more than theoretical drying speed. For companies that want to move quickly into validation and routine production, a concept that is stable but slightly less aggressive is often far more valuable than a concept that is fast on paper but unstable in reality.

For a deeper look at why some ODF lines commission in days while others struggle for weeks, see this related article on ODF line start-up time .

Conclusion: Choose Drying Based on Wet-Film Stability, Not Just Power

Choosing a drying concept for an ODF line should not be based only on heating capacity or catalogue numbers. The key question is: how does the airflow interact with the wet film during its most fragile stage?

Parallel hot-air drying reduces the risk of breaking up the wet film, helping achieve thickness uniformity, stable quality control and reproducible process windows. Vertical oven-style airflow, by contrast, can introduce structural variation at the wet-film stage, leading to thickness non-uniformity, longer trials and higher commissioning costs.

For teams that want to move from “we can make films” to “we can reliably manufacture ODF at scale”, drying concept selection is a critical, strategic decision – not a minor detail.

Contact HUANGHAI to discuss drying options and complete ODF line configurations that are designed around wet-film stability and industrial robustness.

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