Injection molding foamed high-pressure

In the high-pressure process, a resin melt containing a chemical blowing agent is injected into an expandable mold under high pressure. Foaming begins as the mold cavity expands. This produces structural foam products with very smooth surfaces, because the skin is formed before expansion takes place. 

Compressed gases—Most common gases used are nitrogen, air and carbon dioxide. These gases are dissolved under pressure in the resin and produce foam upon release of the pressure. The use of nitrogen in injection-molded foam products is typical. The nitrogen is injected under high pressure. When the pressure is relieved the gas becomes less soluble in the polymer and forms cells. 

Ultem PEI resins are amber and amorphous, with heat-distortion temperatures similar to polyethersulfone resins. Ultem resins exhibit high modulus and ate stiff yet ductile. Light transmission is low. In spite of the high use temperature, they are processible by injection mol ding, stmctural foam mol ding, or extmsion techniques at moderate pressures between 340 and 425°C. They are inherently flame retardant and generate Httie smoke dimensional stabiUties are excellent. Large flat parts such as circuit boards or hard disks for computers can be injection-molded to maintain critical dimensions. 

RIM is increasing in importance and emphasizes the production of thermoset PUs. Here, liquid monomers are mixed together under high pressure prior to injecting into the mold. Polymerization occurs within the mold. Most automotive dash panels are RIM-produced. Notice the tough surface and semiflexible underbelly of the dash. The amount of foam formation is controlled to give the finished product. 

Using the coinjection procedure, a solid melt is injected to form the solid, smooth skin against the cavity surface. Simultaneously a second short shot melt with blowing agent is injected to form the foamed core. With a full second shot, the mold can incorporate pins or a mold that opens similar to high-pressure foam molding. 

FIGURE 2.13 Structural foam injection molding, (a) During injection under high pressure there is very little foaming, (b) After injection, pressure drops and foaming occurs at hot core. 

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