Surface finish foaming

Extrusion Uniform melt flow, enhanced physical properties Higher takeoff speeds Foam density Higher takeoff speeds Reduced melt fracture Surface finish Foam cell structure ... 

The surface finish of injection-moulded foamed articles are not ideal because of external defects, burst bubbles, etc. This can be avoided by sandwich moulding. This is accomplished by initially injecting a slug of polymer melt into the mould followed immediately by a polymer containing a blowing agent, which chases the initial polymer into all the interstices. 

Carbon dioxide gas was used as a physical blowing agent to produce medium density polypropylene foam sheets using a single screw extruder. The mechanical properties of the foam were similar in the machine direction and in the transverse direction. Abetter surface finish and a lower density was produced by using a commercial wrapping film as a cap layer. The process conditions and the die design data are presented and an attempt made to relate them to the product characteristics. 4 refs. 

Lost-foam sand casting Sand (expendable but recyclable) pattern is consumed Ferrous and nonferrous As above plus no need for parting line better surface finish than conventional sand casting increased dimensional accuracy Increased cost, added process steps As above... 

The mold can be modified to meet certain different shapes. The molding can be made either by withdrawing cores or by special press motions that partially open the mold halves (such as the compression molds used in coining to provide 2-D action 3-D mold actions are also used). The degree of foam density, wall thickness, and surface finish depends on the foam mixture (constituents and amounts). The machine controls the time cycle and the mold action required. 

The GMP s process eliminates the use of sheet metal for the skin of the refrigerator door. In this application, the thermoplastic film forms a durable, protective outer skin with a wide choice of color options that are applied directly to the film. In addition more innovations exist apart from the film and thermoplastic interior liner, the doors consist entirely of polyurethane. GMP backs the thermoplastic film with an approximately 4 mm thick layer of the Baydur 110 structural foam polyurethane RIM system from Bayer AG that creates a rigid, dimensionally stable outer shell with no need for sheet metal. Then, GMP fills the space between this shell and the inner liner with insulating polyurethane foam, a rigid, low-density foam. The result is a self-supporting door that satisfies all stability, thermal insulation, and surface finish requirements. 

Process aids are added primarily to PVC-U (including foam) to improve fusion characteristics and melt flow during the processing stage. Addition levels are typically from 1-6 phr with resultant improvements in output and surface finish. Transparency can also be improved in clear formulations. Chemically, these products are high molecular weight methyl methacrylate and alkyl acrylate copolymers. 

After a short stabihsing period, the completed pattern is strengthened by dipping in a refractory material which coats the foam pattern leaving a thin heat-resistant layer which is then air dried. This ceramic coating also provides a good surface finish for the finished casting. 

PS Up to 40% by weight of this material can be mixed in. Impact grades add toughness to the mix. Non-impact PS (crystal) tends to cause surface finish problems. Expanded PS (EPS) should be avoided as a foam because of its low bulk density. Testing shows considerable strength improvements at 10 to 40% levels of densified EPS. 

A typical blade of large wind turbines consists of outer skins supported by a main spar and stiffeners. The blades are generally constructed using fibre/ polymeric matrix composites and may have a sandwich construction with low density polymer foam or balsa wood cores. The epoxy-based eomposites are of greatest interest to wind turbine manufacturers beeause they dehver a combination of environmental, production and cost advantages over other resin systems. Epoxies also improve wind turbine composite blade manufacture by allowing a shorter cure cycle, increased durability and improved surface finish. The utilisation of epoxy infusion or prepreg manufacture (see Sections... 

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