Hot air
In a convection oven, hot air flows continuously around the profile and carries the curing heat in from the outside – a proven, robust process that works for almost any compound. Depending on the profile, the air is injected from the front, the back, or both; simple nozzles are often enough, and for demanding cross-sections a 180°-deflection speed nozzle gives a more even flow. Heating runs on gas or electricity, depending on the system.
How it works
Hot air transfers its heat through forced convection: air that's significantly hotter than the profile flows continuously around its cross-section, giving up energy to the surface as it passes. How much heat actually arrives, though, isn't just a matter of air temperature – flow velocity and how turbulent the air is at the profile surface matter just as much.
Influencing factors in detail
Flow velocity
The faster air flows past the profile, the thinner the boundary layer at its surface gets – and the more heat arrives every second. That can't be pushed indefinitely, though: past a certain point, moving the air faster costs more energy than the improved heat transfer actually gains you.
Degree of turbulence
Turbulent flow mixes the air at the profile surface far more effectively than calm, laminar flow, which noticeably improves how well heat gets through. That's why nozzle shape and arrangement aren't an afterthought: the flow-optimised 180° arrangement in the GM 500, for instance, is deliberately built to create high, even turbulence across the whole profile.
The "kink" in the heating curve
Measure the profile surface during hot-air vulcanization and the same thing happens every time: within a few seconds it takes on almost the temperature of the surrounding air – engineers call this the "imposed temperature." From that point on, the surface barely absorbs any more energy; whatever still makes its way into the profile travels there purely by conduction. RWTH Aachen's IKV measured this effect directly, and it's what determines, very practically, how long a heating zone needs to be.
Throughput vs. temperature
To reach the same cure state, running faster at a higher temperature is usually more energy-efficient than running slow and cool. That might seem counterintuitive, since higher temperatures cost more energy – but the shorter time the profile spends in the line more than makes up for it.
Scientific basis
- GEO
- Research project on flow data and heat transfer in hot-air vulcanization, together with the University of Paderborn's KTP – published as "Holistic energy concept for hot-air vulcanization using innovative air guidance and heat transfer" (GAK trade publication, 2016).
- IKV HeatStraD
- Gerlach Maschinenbau and RWTH Aachen's IKV developed the "HeatStraD" measurement device in a publicly funded ZIM project (2015–2017, funded by Germany's Federal Ministry for Economic Affairs and Energy via AiF, grant reference ZF 4019901JA5) that for the first time made the temperature profile inside a profile cross-section visible during hot-air, infrared, and microwave vulcanization. Project lead Dr.-Ing. Florian Lemke went on to deepen the findings in his dissertation "Vulcanization Strategies for Sulfur-Curing Rubber Profiles" (RWTH Aachen, 2022).
