Molding high pressure injection

Kubat, J. Manson, J.A. High Modulus/high strength polyethylene obtained by high pressure injection molding. Polym. Eng. Sci. 1983, 23 (16), 869. 

In 1985, a USA paint manufacturer introduced a high-pressure injection molding IMG process (HPIP). It was based on injection of the coating into a closed mold at 300-400 bars pressure, before the SMC was fiiUy polymerized. At this stage, the compound is still compressible, allowing the coating to flow between it and the mold. The technology overcomes the objections to conventional IMG and is used today worldwide (Table 5.27). 

Stribeck N, Bayer R, von Krosigk G and Gehrke R (2002) Nanostructure evolution of oriented high-pressure injection-molded polyethylene during heating. Polymer 43 3779-3784. 

The value of the pressure determines two injeetion techniques high pressure and low pressure injection molding. The high pressure injection molding process [GER 90], which is used most often, is directly inspired from the technology of injection... 

Figure 5.35. Principle of high pressure injection molding...

The rapidity with which isocyanate-polyol reactions take place has been used to advantage in reaction injection molding (RIM), In the typical process (Figure 14.28), two components are mixed by high-pressure injection into a special chamber and injected almost immediately into a closed, low-pressure mold. The finished parts can be cellular or solid depending on the amount of material injected... 

Injection Molding. In iajection mol ding a molten thermoplastic is iajected under high pressure iato a steel mold. After the plastic solidifies, the mold is opeaed and a part ia the shape of the mold cavity is removed. Geaeral descriptioas of the process and related equipment are given ia Refereaces 29—34. 

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. 

Polytetrafluoroethylene and fluorinated ethylene-propylene are the only resins composed wholly of fluorine and carbon. The polymer consists of fluorine atoms surrounding the carbon chain as a sheath, giving a chemically inert and relatively dense product from the strong carbon-fluorine bonds. Polytetrafluoroethylene must be molded at high pressure. Fluorinated ethylene-propylene c.m be injection molded and extruded as thin fdm. Both plastics have exceptional heat resistance... 

In injection molding, the polymer is fed from a hopper into a heated barrel where it softens and becomes a viscous melt. It is then forced under high pressure into a relatively cold mold cavity where the polymer has sufficient time to solidify. Then the mold is opened and the fabricated part is ejected. The cycle of operation is then repeated. 

Plastics are no different in this respect than other materials. If steel, aluminum, and ceramics were to be made into a different complex shapes and no prior history on their behavior for that processing shape existed, a period of trial and error would be required to ensure their meeting the required measurements. If relevant processing information or experience did exist, it would be possible for these metallic (or plastic) products to meet the requirements with the first product produced. Experience on new steel shapes always took trial and error time that included different shaped high pressure hydraulic steel cylinders that failed in service when used in a new injection molding hydraulically operating machine (author s experience). 

A knowledge of processing methods will be useful to the designer to help determine what tolerances can be obtained. With such high-pressure methods as injection and compression molding of 2,000 to 30,000 psi... 

The RIM process involves the high-pressure impingement mixing of two or more reactive liquid components and injection of the mixture into a closed mold at low pressures. Large and thick products can be molded using fast cycles with relatively low-cost materials. Its low energy requirements with relatively low investment costs make RIM attractive (9). 

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