Press cycle

During the press operation, which is actually a form of compression mol ding, the resin-treated laminate pHes are heated under pressure and the resins cured. The initial heating phases cause the resin to melt and flow into voids in the reinforcing ply and bond the individual pHes together. The appHed heat simultaneously causes the resin to polymerize and eventually to cross-link or gel. Therefore, resin viscosity reaches a minimum during the press cycle. This is the point at which the curing process becomes dominant over the melt flow process. Dynamic mechanical and dielectric analyses (11) are excellent tools for study of this behavior. 

With a sufficiently long press cycle, a state of complete cure is reached. At this point, the laminate is cooled in the press, under pressure, and removed for finishing operations. If the press is opened at a temperature above the boiling point of trapped volatiles, vaporization occurs causing interlaminar blistering which mins the laminate. 

Fig. 2. Pressing cycle for a single level part (a) cycle start (b) charging (filling) die with powder (c) compaction begins (d) compaction completed (e)...

The higher the moisture content of the particles, the easier can the face layer be densified at the start of the press cycle this leads to a lower density in the core layer. 

Influence parameters are especially the press temperature and the moisture content in the mat. Additional parameters are the wood density, the porosity, the swelling and shrinking behavior of the wood, the structure at the surface and the wetting behavior. During the press cycle several mechanisms are in operation ... 

Simulation models describe the various conditions occurring during a press cycle (gradients of the temperature, the moisture content, the steam pressure and the formed bond strengths) which lead both to microbuckling of the wood cell walls by their moisture and temperature-induced densification (Fig. 6) [215-218]. 

Wet/Wet Pressing, Representative press cycles, conceptual schematics of mass transfer in the sheet, and density profiles through the thickness of the sheet are portrayed in Figure 7, Pressing of wet mats starts with a steady pressure rise to 400 psi platen pressure so as to compress the mat to minimum void volume and express water retained from cold pressing. Platen steam pressures up to 400 psig heat the mat and reduce water viscosity and raise its vapor pressure. This high-presstire inversion cycle is followed by a period of low platen pressure intended to dry the sheet to anhydrous condition. 

Dry/Dry Pressing. Dry/dry pressing also enables short press cycles because dewatering is avoided but, dry-formed mats have no cohesive properties and must be compressed slowly to allow air to escape without mat rupture. Slow platen closure plus high plate temperatures at platen steam pressures up to 550 psig cause premature heating of the mat surfaces before the inversion pressure peak is reached. This leads to precure, so to speak, of the surfaces with low surface density and properties. 

Most particleboard plants have multiple-opening hot presses which produce one panel per opening per press cycle. The press loader also has storage area for the number of mats equal to the openings of the hot press. When the press loader is filled and the press opens, all mats in the press loader are simultaneously... 

The yield of bark extracts from four West Coast coniferous barks varied from 320 to 370 lbs of extract solids per ton of oven-dry bark. When a small amount of paraformaldehyde is added to wood particles which have been sprayed with bark extract and processed into board, formaldehyde released during the hot-press cycle reacts in situ with polyphenolic compounds present in the extract and forms a boil-proof bonding agent. 

The pressing can be done by hydraulic presses in either a batch or a continuous mode. A multiple number of melting/crystal-lization/pressing cycles is utilized for obtaining fractions of increased purity ... 

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