The Autoclave Process

Before presenting the general model formulation and some results, it will be helpful to briefly examine the autoclave process details and some of the evidence for voids. 

The transfer of autoclave pressure to the resin in the laminate does not occur hydrostatically because the resin is not enclosed in a constant-volume system. Flow can occur initially both vertically (thickness direction) and horizontally. Furthermore, the network of fibers can also eventually act as a network of springs to which the vacuum bag and bleeder assembly transfer the stress from the autoclave pressure. This stress can then be transferred 

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Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerisation process. The third, the soHd-phase polymerisation process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

The stability, growth, and transport of voids during composite processing is reviewed. As a framework for this model, the autoclave process was selected, but the concepts and equations may be applied equally effectively in a variety of processes, including resin transfer molding, compression molding, and filament winding. In addition, the problem of resin transport and its intimate connection with void suppression are analyzed. 

The real question in light of Figure 6.10 is what exactly is the resin pressure throughout the product part Is it equal to the gas pressure above the bag in the autoclave process, or the entry resin pressure in the RTM process If not, how can the actual resin pressure be reliably predicted ... 

Resin flow and pressure distributions are among the most challenging aspects of the autoclave process cycle to model. Accounting for tooling and bagging variations, such as inner bag... 

Heat transfer models of the autoclave process are the most accurate and well understood of all the process models. Much of this understanding is because the models are so easily verified through thermocouple measurements. Thermocouples are the most common part-sensing technique used in production. The challenging aspects are the incorporation of the affects of resin flow, resin kinetics, and autoclave position on heat transfer properties. The importance of incorporating resin kinetic models is to properly predict conditions that may lead to exotherms, especially for thick laminates [17]. 

Jumaa, M., and Muller, B. W. (1999), Physicochemical properties of chitosan-lipid emulsions and their stability during the autoclaving process, Int. J. Pharm., 183,175-184. 

Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high temperatures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7). 

The original process for high pressure polyethylene was based on use of a high pressure autoclave and used air to introduce free radicals sufficient to initiate polymerization of ethylene. Principal features of the autoclave process are summarized in Table 7.2. 

In order to reduce the volume stream and to optimize the efficiency of the following processing units the plastics have to be compacted mechanically before heat/pressure treatment starts. Bales of compacted wet plastics reach bulk densities of about 450 kg/ m3, they are put into moulds which will be transported into autoclaves. During the autoclave process plastics are heated and plastified under saturated steam conditions. Autoclave process is within a wide range independent from changing compositions of the plastics. The process temperature (about 170°C) does not provoke decomposition of plastics, used for household packaging materials. Hydrothermal autoclave conditions lead to further compaction and a stable product (bulk density up to 700 kg/m3) of the mixed different kinds of plastics. Fig. 1 shows a compacted plastic block after finishing the autoclave process. 

Fig. 1. Compacted block of mixed different kinds of plastic after finishing the autoclave process (laboratory autoclave in the background)...

CGTase from B. clausii strain El 6 was specific for P-CD formation displaying a P-CGTase action. The distributions of a-, P-, and y-CDs were 0, 89, and 11%, respectively, on soluble starch. The starches from different botanical sources influenced quantities and types of CD formed. It was also observed that root and tuber starches were more accessible to CGTase action. The process of starch homogenization can interfere whit the CGTase action and, consequently, on the CD formation. The gelatinization of starches by the autoclave process improves the CD production, mainly for cereal starches. 

An important medium for the growth of B. stearothermophilus is Spore Strip Broth. Strips containing spores of B, stearothermophilus are frequently placed in autoclaves to determine their efficiency. When removed after autoclaving, these strips are placed in the broth and incubated at 55 °G for 7 days. Growth indicates that the autoclaving process was deficient. 

For very large parts that cannot be accommodated in ordinary presses, the autoclave process can be used in conjunction with a vacuum bag [3]. In one example, prepreg is encased in a flexible plastic bag, which is situated in an autoclave (Fig. 2.2). A vacuum is applied to draw the bag against the mould surface and heat is applied. The process can be used to produce very large articles, but it can also be scaled down and has advantages for short... 

To date, the autoclaving process has been the one most frequently used to produce foam particles, in particular EPP (expanded polypropylene). In the autoclaving process, a suspension of compact polypropylene microgranulate and a liquid is placed under a pressurized atmosphere with inert gas and blowing agent while... 

HPMC is usually sterilized by autoclave, which is to its advantage, as opposed to other viscoelastics, because it remains unchanged by the autoclave process. HPMC does, nonetheless, have a tendency for microbial contamination, which is why special attention is given to filtering processes post-autoclave to ensure endo- and exotoxin removal from the substance. Quality control is of particular relevance here since HPMC products retain precipitates. More advantages of HPMC are storage at room temperature, availability and low HPMC production-costs. The necessary biotechnical purification-processes are, however, costly. Hydroxypropylmethylcellulose lasts 3 years, at room temperature. 

Autoclave molding, the process of curing thermoset resins at elevated temperature and pressure in an inert environment, has an important role in the fabrication of continuous fiber reinforced thermoplastics. While most companies view thermoplastics as an alternative to traditional autoclave long-cycle processing, they have come to accept the following reasons for the autoclave processing of thermoplastic matrices ... 

Thermoset based composite laminates are generally produced bj Autoclave/Vacuum Degassing Lamination Process (38, 39). The characteristics of this inocess are shown in Fig. IS. In this process, pr eg plies of desired shape are laid up in a prescribed orientation to form a laminate. The laminate is covered with successive layers of an absorbent material (glass bleeder fabric), a fluminated film to prevent sticking, and, finally, with a vacuum bag. The mitire system is placed upon a smooth metal tool surface into an autoclave, vacuum is a Ued to the bag and the temperature is increased at a constant rate in order to promote the resin flow and polymerization. The autoclave process will be used along this section as a case study to describe the influence of the matrix characteristics on the processing behavior of hi performance conqmsites. 

As discussed before the role of the matrix cm the inocessing behavior of hi performance composites fabricated in the autoclave process will be analyzed in detaS, as a case study, in this section. 

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