Plastic molding is similar in many ways to metal molding. For most molding operations, plastics are heated to a liquid or a semifluid state and are formed in a mold under pressure. Some of the most common molding processes are discussed below.
1. Injection Molding:
The largest quantity of plastic parts is made by injection molding. Plastic compound is fed in powdered or granular form from a hopper through metering and melting stages and then injected into a mold. After a brief cooling period, the mold is opened and the solidified part is ejected.
2. Coinjection Molding:
Coinjection molding makes it possible to mold articles with a solid skin of one thermoplastic and a core of another thermoplastic. The skin material is usually solid while the core material contains blowing agents.
The basic process may be one-, two-, or three-channel technology. In one-channel technology, the two melts are injected into the mold, one after the other. The skin material cools and adheres to the colder surface; dense skin is formed under proper parameter settings. The thickness of the skin can be controlled by adjusting injection speed, stock temperature, mold temperature, and flow compatibility of the two melts.
In two- and three-channel techniques, both plastic melts may be introduced simultaneously. This allows for better control of the wall thickness of the skin, especially in gate areas on both sides of the part.
In rotational molding, the product is formed inside a closed mold rotated about two axes as heat is applied. Liquid or powdered thermoplastic or thermosetting plastic is poured into the mold, either manually or automatically.
4. Expandable-Bead Molding
The expandable-bead process consists of placing small polystyrene beads and a small amount of blowing agent in a tumbling container. The polystyrene beads soften under heat, which allows a blowing agent to expand them. When the beads reach a given size, depending on the density required, they are quickly cooled. This solidifies the polystyrene in its larger foamed size. The expanded beads are then placed in a mold until it is filled. The entrance
port is then closed and steam is injected, softening the beads and fusing them. After cooling, the finished, expanded part is removed from the mold.
Plastic extrusion is similar to metal extrusion in that a hot material (plastic melt) is forced through a die having an opening shaped to produce the desired cross-section. Depending on the material used, the barrel is heated anywhere from 250 to 600∘F (121 – 316∘C) to transform the thermoplastic from a solid to a melt. At the end of the extruder barrel is a screen pack for filtering and building backpressure. A breaker plate serves to hold the screen pack in place and straighten the helical flow as it comes off the screen.
6. Blow Molding
Blow molding is used extensively to make bottles and another lightweight, hollow plastic parts. Two methods are used: injection blow molding and extrusion blow molding Injection blow molding is used primarily for small containers. The parison (molten-plastic pipe) or tube is formed by the injection of plasticized material around a hollow mandrel. While the material is still molten and still on the mandrel, it is transferred into the blowing mold, where the air inflates it. Accurate threads may be formed at the neck.
In extrusion-type blow molding, the parison is inflated under relatively low pressure inside a split-metal mold. The die closes, pinching the end and closing the top around the mandrel. Air enters through the mandrel and inflates the tube until the plastic contacts the cold wall, solidifying. The mold opens, the bottle is ejected, and the tailpiece falls off.
Thermoforming refers to heating a sheet of plastic material until it becomes soft and pliable and then forming it either under vacuum, by air pressure, or between matching mold halves.
8. Reinforced-Plastic Molding
Reinforced plastics generally refer to polymers that have been reinforced with glass fibers. Other materials used are asbestos, sisal, synthetic fibers such as nylon and polyvinyl chloride, and cotton fibers. High-strength composites using graphite fibers are now commercially available with moduli of 50,000,000 psi (344,700,000 MPa) and tensile strengths of about 300,000 psi (2,068,000 MPa). They are as strong as or stronger than the best alloy steels and are lighter than aluminum.