Elastic Reservoir Molding (23:35 - 21.04.2011)
The starting charge in elastic reservoir molding (ERM) is a sandwich consisting of a center of polymer foam between two dry fiber layers. The foam core is commonly open-cell polyurethane, impregnated with liquid resin such as epoxy or polyester, and the dry fiber layers can be cloth, woven roving, or other starting fibrous form. As depicted in figure, the sandwich is placed in the lower mold section and pressed at moderate pressure – around 0,7 MPa (100 lb/in2). As the core is compressedi it releases the resin to wet the dry surface layers. Curing produces a lightweight part consisting of a low-density core and thin FRP skins.
(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p. 328)
Reinforced Reaction Injection Molding (RRIM) (00:31 - 22.04.2011)
The principle of the RRIM process centers on bringing together two fast reacting liquids into a mixing chamber and then into a mold cavity. Relatively low clamping force pressure is used, and external heat is not normally used on the surface of the mold. The rate of polymerization reaction between two liquids gives a cycle time of typically less than 60 s in the mold. The chemical reaction starts immediately when the two liquids are combined through a mixing chamber and progresses as material flows into the mold cavity.
In general, RRIM has been developed using polyurethane formulations, with one reactant stream being a polypl and the other benig a di-isocyanate. The glass reinforcement normally used in the RRIM process is milled fiber added to the polyol stream. An alternative processing method involves placing a glass fiber preform into the mold cavity and then injecting the two liquids into the mold cavity. While polyurethanes are the dominant resin matrix used in this process, other matrix materials such as phenolics are used to achieve specific property requirements.
(Fiberglass and Glass Technology: Energy-Friendly Compositions and Applications, Frederick T. Wallenberger,Paul A. Bingham, p.144)
(Figure, taken from: Structural Composite Materials, F. C. Campbell)
Austenitic Stainless Steel (01:29 - 23.04.2011)
Austenitic stainless steels contain 16 to 26 percent chromium (Cr), 10 to 24 percent nickel (Ni) and manganese (Mn), up to 0.40 percent carbon (C), and small amounts of Mo, Ti, Nb, and tantalum (Ta). The balance between Cr and Ni + Mn is normally adjusted to provide a microstructure of 90 to 100 percent austenite. These alloys have good strength and high toughness over a wide temperature range, and they resist oxidation to over l000°F. This group includes types 302, 304, 310, 316, 321, and 347. Filler metals for these alloys should generally match the base metal, but for most alloys should also provide a microstructure with some ferrite to avoid hot cracking.
Austenitic stainless steels are commonly welded. Two problems are associated with welding austenitic stainless steels: sensitization of the weld-heat-affected zone and hot cracking of weld metal. Sensitization is caused by chromium carbide precipitation at the austenitic grain boundaries in the heat-affected zone, when the base metal is heated to 800 to 1600°F. Chromium carbide precipitates remove chromium from solution in the vicinity of the grain boundaries, and this condition leads to intergranular corrosion. The problem can be alleviated by using low-carbon stainless steel base metal (types 302L, 316L, etc.) and low carbon filler metal. Alternately, there are stabilized stainlesssteel base metals and filler metals available which contain alloying elements that react preferentially with carbon, thereby not depleting the chromium content in solid solution and keeping it available for corrosion resistance. Type 321 contains Ti and type 347 contains Nb and Ta, all of which are stronger carbide formers than Cr.
Hot cracking is caused by low-melting-point metallic compounds of sulfur and phosphorus which penetrate grain boundaries. When present in the weld metal or heat-affected zone, these compounds will penetrate grain boundaries and cause cracks to appear as the weld cools and shrinkage stresses develop. Hot cracking can be prevented by adjusting the composition of the base metal and filler metal to obtain a microstructure with a small amount of ferrite in the austenite matrix. The ferrite provides ferrite-austenite boundaries which control the sulfur and phosphorus compounds and thereby prevent hot cracking.
(Manufacturing Engineering Handbook, H. Geng, p. 21.22-21.23)
Martensitic Stainless Steel (01:36 - 23.04.2011)
Martensitic stainless steels contain 11.4 to 18 percent chromium (Cr), up to 1.2 percent carbon (C), and small amounts of manganese (Mn) and nickel (Ni). They will transform to austenite on heating and, therefore, can be hardened by formation of martensite on cooling. This group includes types 403, 410, 414, 416, 420, 422, 431, and 440. Welding on these stainless steels is difficult. Weld cracks may appear on cooled welds as a result of martensite formation. The Cr and C content of the filler metal should generally match these elements in the base metal. Preheating and interpass temperature in the 400 to 600°F range is recommended for welding most martensitic stainless steels. Steels with over 0.20 percent C often require a postweld heat treatment to avoid weld cracking.
(Manufacturing Engineering Handbook, H. Geng, p. 21.22)
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