The objective of this project was to help Forintek members improve MDF manufacturing processes and maintain cost competitiveness by developing improved or new methods for mat preheating and/or humidification.
MDF mat heating and humidification with steam (the H & H system) has demonstrated its great ability to increase press speed and hence production capacity in Ranger Board and WestPine MDF. Higher mat temperature allows shorter press time but it tends to drive some moisture out of the mat. Moisture content can vary significantly across the thickness of the mat depending on the conditions of the H & H system and the configurations of the formers, which have a strong influence on panel properties. It was observed that board formaldehyde emissions can be significantly increased as a result of using H & H, although emission levels can be effectively controlled by increasing the use of formaldehyde scavenger, or via resin modification, or both. Achieving the balance of mat temperature and moisture content and hence obtaining the optimal H & H conditions is challenging. Trial and error has been the only approach available to the mills so far. The approach of computer modeling, however, should be an excellent enhancement to the empirical approach. Based on mill data from WestPine MDF and Ranger Board, a preliminary computer model for mat heating and humidification with steam has been developed. It can be used to develop a deeper understanding of the individual process parameters and their interactions within the H & H system, improving process efficiency and product quality as well as training mill operators.
In co-operation with Hydro Quebec, an experiment on microwave preheating of MDF mat was conducted. It was observed that moisture movement within the mat upon microwave heating was from the centre of the mat to both surfaces and towards the edges and corners. The moisture variation was much higher across the thickness of the mat than among different locations of a layer of the mat. The variations are thought to be related to the mat density profiles, electric field distribution and penetration depth. During hot pressing, PressMan profiles showed higher core temperatures for the microwave-preheated mat than the control for most of the press cycle.
Based on the work carried out for this project, the following conclusions and recommendations are made:
1. A preliminary computer model has been completed for the MDF mat heating and humidification system at Ranger Board. It is now ready to customize to mill conditions.
2. A preliminary computer model has been completed for the MDF mat heating and humidification system at WestPine MDF. It is now ready to customize to mill conditions.
3. Microwave preheating of MDF fibre mat increases the initial mat temperature and re-distributes and moves moisture towards the mat surfaces.
4. As a result of moisture re-distribution, the moisture gradient and temperature gradient are in the same direction at the early stage of the hot pressing. Due to this effect of positive coupling between moisture gradient and temperature gradient, heat can be transferred significantly faster from the surfaces to the core and heat flux is also increased.
5. With microwave preheating, the increase of core temperature in MDF fibre mat starts earlier and the rate of the temperature increase is greater.
6. As a result of microwave preheating, hot press cycle time may be substantially reduced, leading to increase of production efficiency.