Fluorinated ethylene propylene or FEP is a copolymer of hexafluoropropylene and tetrafluoroethylene. It differs from the PTFE (polytetrafluoroethylene) resins in that it is melt-processable using conventional injection molding and screw extrusion techniques. Fluorinated ethylene propylene was invented by DuPont and is sold under the brand name Teflon FEP. Other brandnames are Neoflon FEP from Daikin or Dyneon FEP from Dyneon/3M,or Ruiflon from Ruide.
FEP is very similar in composition to the fluoropolymers PTFE (polytetrafluoroethylene) and PFA (perfluoroalkoxy polymer resin). FEP and PFA both share PTFE's useful properties of low friction and non-reactivity, but are more easily formable. FEP is softer than PTFE and melts at 260 °C; it is highly transparent and resistant to sunlight.
Two methods of physical foaming are established in the polymer industry: continuous foaming (extrusion) and discontinuous foaming (injection moulding). Used for the purpose of weight reduction, the foaming process also reduces the cost of plastic parts. It can also be used to compensate for shrinkage or for maintaining the holding pressure as is also the case in gas-assist technology.
The fluids (nitrogen or carbon dioxide) are inserted into the melt. This occurs in the place of plasticization, i.e. the gas is either applied directly into the plasticizing cylinder (Mucell method) or fed into the plasticizing cylinder through a special nozzle (SULZER method). The gas dissolves in the melt at a certain pressure and expands as soon as the melt is injected into the cavity. During volumetric filling, gas expansion assumes the role of holding pressure, which prevents voids and warpage. The gas streaks that may occur during melt injection, are prevented by means of gas back pressure or selection of special material or additives.
The produced part is comprised of a compact outer skin and a so-called microcellular foam or foam with integral density distribution (integral foam). The number and size of pores depends on the nucleation (number of crystallization seeds) and other physical parameters such as pressure, temperature and mould filling level.
Chemical foaming involves the introduction of blowing agents (powder or pellets) to the melt. During the plasticization process and with rising temperature, the chemical additive decomposes and gives off gas (mostly carbon dioxide). This process also involves dissolution of the gas in the melt and maintenance of a minimum level of pressure. The next stages are identical to those described for physical foaming. This method has the advantage of not requiring any special equipment.
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