C4. expanded polystyrene foam processes - Expandable polystyrene (EPS) is supplied by manufacturers in bead form. The beads contain a blowing agent (usually pentane) that expands them from 2 to 50 times upon exposure to heat. By controlling time and temperature, the amount of expansion can be controlled. Expansion of the beads during molding produces a component with excellent insulating and flotation properties. Processing usually involves a pre-expansion of the beads as a first operation.

C4a. pre-expansion of EPS beads - can be accomplished with any of several batch methods involving ovens, steam chambers, or hot water baths, and several continuous methods involving hot air, radiant heat, or steam heat. All these methods expand the beads to approximately the level of expansion specified for the eventual molded part. The continuous steam-heating approach is by far the most common. It involves a continuous feed of beads to a steam chamber where the beads are mechanically agitated and mixed with steam. As they are heated by the steam, they expand from the vaporization of the blowing agent. Additional expansion comes from the absorption of steam. Agitation prevents the beads from fusing together. The amount of expansion is controllable and depends on the temperature of the steam, the feed rate of the beads and the amount of air introduced to the expansion chamber. As the beads expand, they tend to rise to the top of the chamber and overflow it and are conveyed by air flow to an open storage bin. In the storage bin, the beads cool and their expansion subsides somewhat and gradually stabilizes. The bins are open and subjected to additional air flow to dry the beads. After several hours (typically from about 3 to 12), the beads are dry and ready for molding.

C4b. shape molding of EPS beads - Fig. 4C4b illustrates this process. Pre-expanded beads are conveyed by air to the mold where a measured amount is introduced to the mold. The mold closes and steam is introduced through small holes, heating the beads and causing them to soften, fuse together, fully expand, and fill the entire moldcavity. The expanded beads block the small steam holes, preventing further introduction of steam. The mold is then cooled with water until the part stabilizes. The mold is opened and the part is ejected. Drinking cups for hot beverages, picnic coolers, fastfood containers, fitted support blocks in shipping cartons, and display figures, are typical products molded from EPS with this method. Drinking cups are produced from small beads with typical densities of 2 to 5 lb/ft 3 (32 to 80 Kg/M 3 ). Sometimes, a vacuum is drawn on the moldcavity to provide room for the steam and to facilitate the expansion of the beads. Other heating methods - hot air or conduction through the mold - may be used to heat the beads.

Fig. 4C4b Shape molding with EPS beads.

C4c. block molding of EPS - to produce sheets or slabs of foam material proceeds quite similarly to shape molding. In this method, the moldcavity is rectangular and can be rather large – as large as 4 × 16 × 3 feet. Larger beads are also used for such components. Vacuum assistance may be employed to remove air from the mold. After molding, the blocks produced are sliced into thin slabs or other shapes by hot wire or band saw. Building insulation panels and flotation blocks are products made by this approach. Typical densities of insulating board are 1 to 2 lb/ft 3 (16 to 32 Kg/M 3 ).

C4d. expanded polyolefin foam process - Recently, polyolefins have been produced in a fashion similar but not identical to EPS. Special polypropylenes (EPP) have been manufactured to meet this market. Unlike EPS beads, EPP beads do not retain a blowing agent very well, so, EPP beads are usually pre-expanded by the resin manufacturer. The converter fills aluminummolds with the pre-expanded beads and applies high-temperature steam, heated air, and other gases to heat the beads until the surfaces are tacky. The mold is then cooled under pressure until the beads are fused together. EPP foams are very soft and ductile. They are used in packaging for shock mitigation and in vehicle bumpers. EPP foams tend to be porous and so are not used in liquid containers.

C5. two-color injection molding - This method is used for computer keyboard keys, two-color automobile tail lights and similar parts with inlaid color effects. The part is molded in two operations using two different mold cavities. The part molded in the first operation is used as an insert when the final part is molded in the second operation using a different color plastic. Fig. 4C5 illustrates the process sequence.

Fig. 4C5 Two-color injection molding: a) The mold for the outer shell. b) Plastic is injected in this mold to make the outer shell. c) The molded outer shell ejected from the mold. d) The second color is injected into a second mold that contains the outer shell as an insert. e) The second mold opens and the two-color part is ejected.

C6. insert molding - simply involves injection (or compression) molding in which other components are placed in the mold before the mold closes and the plastic material is introduced. Plastic material flows around the inserted components and holds them in place. This is the oldest method for combining metal and plastic parts and is extensively used. It provides excellent holding power for the insert. However, placement of inserts slows the molding cycle, adds the risk of damage to the mold from a fallen insert and has the possibility of allowing plastic to enter an unwanted area of the inserted part, necessitating rework. Metal inserts are sometimes used when screw threads, studs, or other elements, having strength or wear resistance greater than that of the plastic material, are required. Another application is the production of electrical plugs, sockets, and switches, when the metal electrically conductive elements are encapsulated in an insulating plastic. One common molding method uses duplicate mold halves on a turntable so that the inserts can be loaded in one moldcavity on the turntable while molding takes place at another turntable location. Inserts are most commonly metal, but inserts of other plastics, paper (e.g., labels), ceramics, or other materials can also be used. (Other methods for combining plastics and other materials in a single component are discussed in section N1 below.)