High pressure foam molding

In the high pressure process, a resia melt containing a chemical blowiag ageat is iajected iato an expandable mold under high pressure. Foaming begias as the mold cavity expands. This process produces stmctural foam products with very smooth surfaces siace the skin is formed before expansion takes place. 

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. 

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

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. 

Two streams of PUR chemicals collide with each other violently and under high pressure generally at 1,500 to 3000 psi (10.3 to 24.1 MPa) inside the mixer. When these impinging streams collide, the flow is very turbulent and the reaction begins. The stream exits the mixhead and is directed into the mold. After the pour a piston inside the mixhead scrapes the walls of the chambers completely clean so that no reacted foam is left inside the mixhead. 

Classification. Phenolic foam is divided broadly into two categories. One is the novolac type and the other is the resol type (see Figure 55). In the novolac type, phenol and formaldehyde react in the presence of an acid catalyst and link the linear condensation product by a methylene bond. The basic curable crosslinking agent, such as hexamethylene tetramine and blowing agent are added to the linear condensation product and the resultant product is molded at elevated temperatures and high pressure. 

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. 

Water-Base Mold Releases. Water-base release agents are being used more widely. Solvent-base agents evaporate too quickly in warm areas. One example is a hydro-alcoholic solution of saponified fatty acids claimed to produce a dry-fihn coating. Axel Plastics Research Laboratories in Woodside, NY has a new product. Mold Wiz H40-3U, for flexible polyurethane foam. This material is heated to evaporate the water and is then applied with high-pressure spray equipment (44). 

There are a number of different systems and machines available for producing structural foams. The most important categories are low-pressure molding and high-pressure molding. Low-pressure molding... 

Foam-Kon. [LNP ICI Advanced Materials] Thermoplastic structural foam cone. for high pressure foam systems, inj. molding compds. 

---