Shrinkage preform

Shrinkage of about 5-10% occurs at right angles to the direction of the preforming force. The amount of shrinkage is mainly dependent on the rate of cooling, but also to a minor extent, on the preforming pressure. 

Lowering of shrinkage by preforming some bonds in the liquid state. 

Sheets, rods, and lens preforms are cast from CR39 or CR39 prepolymer at 60-125°C. A relative high concentration (4-8 wt%) of peroxides or AIBN is needed to obtain complete polymerization. The shrinkage during polymerization is about 14%. 

Figure 12.10 shows a schematic of the two-step infiltration process. The first step consists of the preparation of a powder compact within the fiber preform by a routine slip-cast infiltration, the body being dried, and then strengthened by heating at a temperature close to the onset of shrinkage. The second step consists of the infiltration of the strengthened body by a liquid (polymeric) preousor that is first converted into a solid (a gel by reaction with water or diols if alkoxides are used ), or by the action of oxygen if polyvinylsilane (PVS) is used to... 

ASTM C 356-87 Standard Test Method for Linear Shrinkage of Preformed High-Temperature Thermal Insulation Subjected to Soaking Heat, 3 pp (Comm C-16)... 

In extrusion molding so-called slugs are produced from the paste as preforms. These are subsequently formed into their final shape. In the manufacture of insulators the piston is dried to a leathery hardness , which does not undergo any further shrinkage. These preforms are further processed with profiling tools. 

Figure 5.7 Effect of preform pressure on mold shrinkage. (Courtesy DuPont.)...

To minimize shrinkage and moisture absorption in hot-fill bottles, the first step is to heat-set the bottle. This is a process in which the bottle crystallinity, mainly in the sidewalls, is increased to about 30% under stress to maintain the bottle shape. The crystallization that is desired is strain-induced crystallization, which results in smaller crystallites that do not scatter light. The preform weight for hot-fill bottles is higher also, to help stabilize the finish area. Illustrations of the different preform weights commonly used in industry are in Table 12.1. 

There is a volumetric shrinkage of about 50% so the resultant structure formed in this process is a miniature of the precursor preform. The gases generated within the preformed structure must have time to diffuse out and not nucleate bubbles so one dimension of glassy carbon structures is limited to about seven millimeters. Hence materials or objects are limited to thin flat plates or tubes with thin walls. Massive equiaxed... 

When using a thermosetting resin, it is usual to take the carbon fiber reinforcement and prepare a prepreg by impregnating with the chosen resin. Alternatively, a dry preform of opf can be used for conversion to carbon fiber and subsequent initial treatment by the CVI process to provide a preform with some integrity [38]. Polymerization shrinkage will occur, which can be minimized by controlling the rate of temperature rise. 

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