Introduction
The fast development of the wood-based panel industry during the last decades resulted in a substantially increased production capacity. Production speed of continuous press lines up to 120 m/min combined with an increasing demand on quality and cost efficiency as well as the large variety of panel types require an advanced in-line control. The aim of this study is to give an overview about the state of the art of nowadays inline control technology, ongoing research and developments as well as a preview on future trends within this field of expertise.
The most important measuring parameters for further process steps and the quality of wood-based panels are moisture content, weight per area unit, board density, cross-and lengthwise density distribution and density profile, thickness, weight of the board. Moreover, other factors such as surface quality, particle geometry, internal defects (e.g. blisters and foreign bodies) as well as trending and forecast of the board’s quality are well-established and taken into consideration by inline measuring systems today. All these parameters influence strongly the final mechanical properties of the product, e.g. internal bond (IB) and modulus of rupture (MoR), modulus of elasticity (MoE), surface soundness (SS) but also physical characteristics like thickness swelling (TS) after 24h, free formaldehyde emission, surface color, coating capability etc. (Hasener & Barbu, 2009; Hasener, 2009).
The standardized quality control in the wood-based panel industry is usually based upon results obtained from time consuming destructive tests in the laboratory. For a comprehensive and continuous process- and board quality inspection, modern inline-techniques and process control systems were developed and fitted to the wood processing and available since two decades.
The wood-based panel producers have to face some continuous changing aspects like:
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The wider diversification of wood-based panel types with properties which meet specific customer requirements.
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The increased flexibility for producing dozens of different panel types per day with customer tailored product quality.
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The high productivity kept on the one hand by reducing the incidental stops and rejects amounts, further reducing of production safety limits etc.
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The permanent changes in the raw-material supply – various types of processing residues and recycled wood and increasing costs.
A quick reaction of equipment based on the inline process control systems allows to reduce the variations and to maintain a high panel quality (Kruse, 1997).