What is plastic?
What are the different types of plastic?
How are plastics processed into products?
The term 'plastic' derives from the Greek 'plastikos', meaning fit for molding, and 'plastos', meaning molded. In line with this root etymology, and in the broadest sense, a plastic is a material that at some stage in its manufacture is able to be shaped by flow such that it can be extruded, molded, cast, spun, or applied as a coating.
Although there are many plastics found in nature, the term “plastics” more commonly refers to synthetic materials created by joining and repeating molecules into very long lines in a process known as polymerizing. Polymers alone rarely have the physical qualities to be of practical value, so most plastics contain various chemical additives to facilitate the manufacturing process or produce a particular desirable property, such as flexibility or toughness.
There are hundreds of different types and blends of plastics today, however there are only a few that are most commonly used. In an attempt to bring order to plastics classification for recycling purposes, the Society of the Plastics Industry (SPI) devised some voluntary codes in 1988 for these commonly used plastics.
They are commonly called SPI codes, and the numbers within the recycling arrows refer to different types of plastic resins.
Each of these plastics has unique properties and are used for varying applications. An example of common uses are:
- #1 PET (polyethylene terephthalate): plastic soft drink bottles, water bottles, beer bottles, mouthwash bottles and many more
- #2 HDPE (high density polyethylene): milk bottles, detergent bottles, oil bottles, toys, plastic bags
- #3 PVC (polyvinyl chloride): food wrap, vegetable oil bottles, blister packaging
- #4 LDPE (low density polyethylene): bread bags, frozen food bags, squeezable bottles, fiber, tote bags, bottles, clothing, furniture, carpet, shrink-wrap, garment bags
- #5 PP (polypropylene): margarine and yogurt containers, caps for containers, wrapping to replace cellophane
- #6 PS (polystyrene): egg cartons, fast food trays, disposable plastic silverware
- #7 Other: This code indicates that the item is made with a resin other than the six listed above, or a combination of different resins.
There are many ways to process plastics, the main purpose being to convert plastic pellets into a useable product, i.e. bottles, film, fibers, toys and all other plastic products you use daily. The most common manufacturing processes are; extrusion and injection molding.
Extrusion molding: A heated plastic compound is forced continuously through a forming die made in the desired shape (like squeezing toothpaste from a tube, it produces a long, usually narrow, continuous product). The formed plastic cools under blown air or in a water bath and hardens on a moving belt. Rods, tubes, pipes, sheet and thin film (such as food wraps) are extruded then coiled or cut to desired lengths.
Plastic fibers also are made by an extrusion process. Liquid resin is squeezed through thousands of tiny holes called spinnerets to produce the fine threads from which plastic fabrics are woven.
Injection molding: This is the second most widely used process to form plastics. The plastic compound, heated to a semi-fluid state, is squirted into a mold under great pressure and hardens quickly. The mold then opens and the part is released. This process can be repeated as many times as necessary and is particularly suited to mass production methods. Injection molding is used for a wide variety of plastic products, from small cups and toys to large objects weighing 30 pounds or more.
Blow molding: This is a secondary processing as it takes the injection molded part and pressure is used to form hollow objects, such as the soda pop bottle or two-gallon milk bottle, in a direct or indirect method. In the direct blow-molding method, a partially shaped, heated plastic form is inserted into a mold. Air is blown into the form, forcing it to expand to the shape of the mold. In the indirect method, a plastic sheet or special shape is heated then clamped between a die and a cover. Air is forced between the plastic and the cover and presses the material into the shape of the die.