Preform Heating

During the preform-heating part of the cycle, the main problems to be considered are heat transfer and flow (or elastic deformation) of the compressed particulate matter. Referring to Fig. 13.45, the heat transfer problem can be described with the following form of the energy equation ... 

B. cereus can cause two distinct types of food-bome intoxicants (as opposed to infections) (1) an emetic (vomiting) illness with a short incubation time of a few hours and (2) a diarrheal illness with an incubation time of 8-6 h. The emetic form is caused by a preformed heat-stable enterotoxin of molecular weight less than 5000 Da. The long incubation form of the illness is mediated by a heat-labile enterotoxin of molecular weight of 50 000 Da, which activates intestinal enzymes and causes intestinal fluid secretion. 

B. cereus causes two different types of clinical syndromes. The first one is characterized by a short incubation period with vomiting, abdominal cramps, and to a lesser extent, diarrhea within 1 to 6 hours of ingestion of contaminated food. This syndrome is caused by a preformed heat-stable toxin. Similar to staphylococcal food poisoning, illnesses caused by B. cereus usually last less than 12 hours. The second syndrome has a longer incubation period (8-16 hours) and is caused by toxins produced in vivo after the ingestion of contaminated food. In this syndrome, patients experience diarrhea, abdominal cramps, and less frequently, vomiting. The heat-labile enterotoxin produced in this syndrome activates intestinal adenylate cyclase and causes intestinal fluid secretion. This illness usually resolves within 24 hours, but symptom durations of several days to weeks also have been observed. 

Samples were prepared using in-house molding facilities from preforms heated to 85°C prior to molding at 175°C with a process time of 90 seconds and a maximum pressure of 6.9 MPa. All samples were post mold cured at 175°C for 4- hours and were stored in a desiccator until testing. 

A soak time is used in preform heating for some stretch blow molding processes. Would you expect this to more likely be needed for thick or for thin preforms Explain. 

While processing with RFI technology, various aspects need to be considered, such as preform characteristics, resin flow through the preform, heating rates, resin characteristics, cure kinetics, etc. Pros and cons of RFI are listed in Table 14.14 and more information on RFI can be seen in Chapter 14. 

To improve homogeneity, the preformed article is heated to 370—390°C. The time required for heating and sintering depends on the mold dimensions cooling, which affects the crystallinity and product properties, should be slow. 

Applications. Preformed bags are opened by the packager, filled with food product, and closed by adhesive, heat-sealing, clipping, stitching,... 

For ordinary materials and higher production rates, P/M forging can be used (26,28). After parts are compacted and sintered to medium density, they are reheated, lubricated, and fed into a hot-forming or P/M-forging press. The part is formed by one stroke of the press in a closed precision die. A typical hot-forming press setup includes die sets, automatic die cooling and lubrication, transfer mechanism, an induction heating unit for preforms, and controls. 

Among the preformed polymers cured by minor additions of aHyl ester monomers and catalysts followed by heat or irradiation are PVC cured by diallyl fumarate (82), PVC cured by diallyl sebacate (83), fluoropolymers cured by triaHyl trimeUitate (84), and ABS copolymers cured by triaUyl trimeUitate (85). 

TriaUyl Gyanurate Gure of Preformed Polymers. TAC and TAIC are often used in smaU amounts with vinyl-type and condensation polymers for cured plastics, mbber and adhesive products of high strength, and heat and solvent resistance. In some cases, chemical stabUity is also... 

Al—Si, eutectic (I) BAlSi preforms, wine, rods. aluminum and car radiators, heat... 

Cu—X, sohd solution (II) BCu preforms, wine, rods. copper and copper heat exchangers. 

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