Hot pressing models

Dynamic Mechanical Analysis and Stress Relaxation Behavior. Samples were compression molded into bars of the dimensions 38.xl2.5x0.78 0.007 mm and 65.x9.7xl.7 0.007 mm in a Carver laboratory hot press model C. A TA Instruments 983 DMA, which was operated in the fixed frequency mode, was used to characterize the storage and loss moduli as a function of temperature. Samples were scanned at fi-equencies from 0.05 to 10.0 Hz over a temperature range from -150 C to above the glass transition temperature. The displacement was 0.4 - 0.6 mm. Stress relaxation curves were determined for the same size samples at a constant strain. The sample was displaced for 10.0 minutes and then allowed to recover for 10.0 minutes. The stress data were taken in five degree increments. A microprocessor controlled Liquid Nitrogen Cooling Accessory (LNCA) was used for sub-ambient operations. 

In Coble s initial stage hot pressing model, the effective stress applied to the grain boundaries, p, is related to the externally applied stress, p, which is by Eq. (5.180). In that model, the total driving force (DF) is a linear combination of two effects (i) external applied stress and (ii) surface curvature, which determines the densification rate, and it is... 

As mentioned before, there is an important factor, stress intensification factor or stress multiplication factor. It is used to relate the mean stress applied to the grain boundary, pe, to the externally appfied stress, p. The significance of (p is such that while pa is the stress that is measured, pe is its counterpart that influences the rate of matter transport. The factor (p is geometrically dependent on the porosity and the shape of the pores of the ceramic compacts. 

In compensating for the presence of porosity in the powder system, we recall from our discussion of Coble s initial stage hot pressing model that the effective stress on the grain boundaries Pe is related to the externally applied stress Pa by Eq. (8.75). To account for the effects of the applied stress and the surface curvature on the densification rate. Coble argued that the total driving force DF) is a linear combination of the two effects that is,... 

In the hot pressing models, we found that the factor mean stress on the grain boundary... 

As discussed in Chapter 8, the hot pressing models predict that the densification rate can be written in the general form... 

Fig. 5.2 Comparison of creep behavior and time-dependent change in fiber and matrix stress predicted using a 1-D concentric cylinder model (ROM model) (solid lines) and a 2-D finite element analysis (dashed lines). In both approaches it was assumed that a unidirectional creep specimen was instantaneously loaded parallel to the fibers to a constant creep stress. The analyses, which assumed a creep temperature of 1200°C, were conducted assuming 40 vol.% SCS-6 SiC fibers in a hot-pressed SijN4 matrix. The constituents were assumed to undergo steady-state creep only, with perfect interfacial bonding. For the FEM analysis, Poisson s ratio was 0.17 for the fibers and 0.27 for the matrix, (a) Total composite strain (axial), (b) composite creep rate, and (c) transient redistribution in axial stress in the fibers and matrix (the initial loading transient has been ignored). Although the fibers and matrix were assumed to exhibit only steady-state creep behavior, the transient redistribution in stress gives rise to the transient creep response shown in parts (a) and (b). After Wu et al 1...

Huang L-H, Qin T-F, and Ohira Tatsuro (2004) Studies on preparations and analysis of essential oil from Chinese fir. Journal of Forestry Research, 75(1) 80-2 Hudson MS and Henrrksson ST (1956) The oscillation pressure method of wood impregnation. Forest Products Journal, 6(10) 381-6 Hughes C (1982) The natural durability of untreated timbers. What s New in Forest Research No. 112. New Zealand Ministry of Forestry, Forest Research Institute, Rotorua Humphrey PE and Bolton AJ (1989) The hot pressing of dry-formed wood-hased composites. Part 2 A simulation model for heat and moisture transfer, and typical results. Holzforschung, 43 T) 199-206... 

C-NMR spectra of water soluble cellulose model compounds indicate that formaldehyde is capable of reacting with wood cellulose functions under hot press conditions as well as at room temperature yielding hemiacetals. The formation of hemiacetals is reversible, and thus constitutes a reservoir for formaldehyde storage. Due to its affinity for water, formaldehyde released during the manufacture of UF-resin bonded products will be retained in the aqueous phase of wood. Wood contains about 9 wt% of moisture. Most of this is in the S-2 secondary cell walls that consist mainly of wood cel IuIose. 

In the hot embossing step, Zeonor 1020R plates (15 cm x 10cm, 2mm thick) were cleaned with methanol for 2 min in an ultrasonic bath and then cut into chips using a paper cutter. The chip was then pressed against the silicon master with heating (130°C, 667 N, 10 min, for a 3.3 cm x 3.3 cm chip) on a hydraulic press (model MTP-8 Tetrahedron Associates, San Diego, CA, USA). The cyclo-olefin... 

R. L. Coble. Diffusion Models for Hot Pressing with Surface Energy and Pressure... 

It is usually difficult to isolate and characterize a copolymer from a melt-processed polymer blend. Model studies of copolymer formation between immiscible polymers have been performed either in solution (where there is unlimited interfacial volume for reaction) or using hot-pressed films of the polymers (where the interfacial volume for reaction is strictly controlled at a fixed phase interface). Model smdies using low molecular weight analogs of the reactive polymers are useful but their applicability to high molecular weight reacting systems is limited. 

In the two-sphere model at the initial stage of sintering, is usually assumed to be equal to the area of the sphere projected onto the punch of the hot pressing die divided by the cross-sectional area of the neck, i.e., [Pg.351]

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