Molded part

Fabrication. Acetal resins are most commonly fabricated by injection mol ding. A homogeneous melt is essential for optimum appearance and for performance of injection molded parts. A screw compression ratio of no less than 3 1 is advised and the size of the injection molded shot should be 50 to 75% of the rated capacity (based on polystyrene) of the barrel. 

The beater additive process starts with a very dilute aqueous slurry of fibrous nitrocellulose, kraft process woodpulp, and a stabilizer such as diphenylamine in a felting tank. A solution of resin such as poly(vinyl acetate) is added to the slurry of these components. The next step, felting, involves use of a fine metal screen in the shape of the inner dimensions of the final molded part. The screen is lowered into the slurry. A vacuum is appHed which causes the fibrous materials to be deposited on the form. The form is pulled out after a required thickness of felt is deposited, and the wet, low density felt removed from the form. The felt is then molded in a matched metal mold by the appHcation of heat and pressure which serves to remove moisture, set the resin, and press the fibers into near final shape (180—182). 

Sufficient time must be allowed to cool the molded part under pressure to 90°C to prevent vacuum voids and distortion. 

By the 1950s P/M parts were used in postage meters and home appHances. In the 1970s P/M superaHoys for aerospace appHcations were used, followed by steel P/M forgings. The 1980s opened the way for rapid soHdification processing, P/M tool steels, and metal injection molded parts. 

The injection mol ding process eliminates the restriction of straight-sided components required when parts are ejected from a die, and offers opportunities for external undercuts and threads. A wide variety of alloys can be processed, including alloy steels and stainless steels. Material properties of injection molded parts are available (32). 

Process details may be summarized powder sizes are fine (usually < 20 Ilm) low (generally < 69 MPa (10,000 psi)) injection pressure low (ca I49°C) mol ding temperature shrinkage (molded part to finished size) typically 20% final part densities are usually 95—98% + of maximum pore-free density and ductility is exceptionally high, elongation values are > 30%. 

Materials Standardsfor Metal Injection Molded Parts, Metal Powder Industries Federation, Princeton, N.J. 

Early phenoHc resins consisted of self-curing, resole-type products made with excess formaldehyde, and novolaks, which are thermoplastic in nature and require a hardener. The early products produced by General BakeHte were used in molded parts, insulating varnishes, laminated sheets, and industrial coatings. These areas stiH remain important appHcations, but have been joined by numerous others such as wood bonding, fiber bonding, and plywood adhesives. The number of producers in the 1990s is approximately 20 in the United States and over 60 worldwide. 

Recycling. Thermosets are inherently more difficult to recycle than thermoplastics and thermosetting phenoHcs are no exception. However, research in this area has been reported, and molded parts have been pulverized and incorporated at 10—15% in new mol ding powders. Both German and Japanese groups had instituted this type of practice in 1992 (71,72) (see Recycling). 

Converters who manufacture EPS parts and components for packaging are called shape molders. Others who mold large billets, eg, measuring 0.6 X 1.2 X 2.4, are called block molders. The package user can obtain EPS parts without the benefits of a mold parts can be fabricated from billets by... 

The largest segment of the CASE family of polyurethanes are elastomers. Cast polyurethane elastomers reached a new dimension when high pressure impingement mixing led to reaction injection molding (RIM). This technology is used widely in the automotive industry, and reinforced versions (RRIM) and stmctural molded parts (SRIM) have been added in more recent years. 

Commercial engineering thermoplastic nylons are mainly crystalline resins. Nylon-6,6 [32131 -17-2] is the largest volume resin, followed by nylon-6 (48). Other commercially available but much lower volume crystalline nylons are -6,9, -6,10, -6,12, -11, and -12. The crystallinity of the molded part decreases with chain size (49). A few truly amorphous commercial nylon resins contain both aromatic and ahphatic monomer constituents (50). For example, Trogamid T resin is made from a mixture of 2,2,4- and 2,4,4-trimethylhexamethylenediamines and terephthahc acid (51). 

Both PET and PBT resin serve some of the same markets as other plastics. Nylon, unsaturated polyester fiber glass, phenoHc, PC, and polyarylate resins sometimes compete for the same molded part. 

Cured phenolics are universally brittle in nature. This is true of both resoles and novolacs and does not depend much on the source of methylene used to promote cure. Consequently, the fillers used in molded articles are highly important to the design of the manufactured product. With resoles, the fiber or filler are usually the primary component of the final composite, with the resole acting as a binder or impregnating agent. With novolacs the resin may be the major component in the molded part. Poly-silanes and other organic polymers are also added in some applications to promote impact resistance and toughness [192]. 

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