Understanding Plastics, A Teachers Guide
(29.5 minutes – see lower portion of this web page)
Reproduced with permission of University of Wisconsin – Stout, School of Industry and Technology
The purpose of these videotapes is to provide the viewer with a clear understanding of the basic process used to convert plastic molding materials into finished products. In addition to the processing technique, the video also considers the history and growth of the plastics industry, problems related to the disposal of discarded plastics, and some of the advantages of using plastics in the manufacture of many consumer products.
Plastics are either thermoplastic or thermosetting materials. Thermoplastics can be heated and molded into a specific shape, and upon cooling will retain that shape, the material will again soften and the plastic can be molded into a different shape. When thermosetting resins are subjected to heat and pressure, they cure and harden and cannot be softened and reshaped.
Plastic materials can be formed into consumer products by any one of the several molding techniques. Seven of the more common molding processes are highlighted in this video.
- Extrusion – produces thermoplastic parts of uniform cross section and continuous length. Co-extrusion is included.
- Blow molding – a process used to produced hollow, closed products with small openings. Both extrusion and injections blow molding are covered.
- Compression molding – a thermoset material is placed directly into an open, heated mold cavity. The mold is closed causing the material to flow into the recesses of the cavity where it solidifies.
- Transfer molding – a thermoset plastic is placed in a hot transfer chamber, a plunger forces the material the sprue and runner system into the hot mold cavity where it is held under pressure until it solidifies.
- Rotational molding – a thermoplastic material is placed in a mold which is heated and rotated on two axes simultaneously to produce a hollow part which is completely closed.
- Thermoforming – thermoplastic sheet stock is heated and formed into the desired shape by mechanical pressure and/or a differential in air pressure.
- Injection molding – plastic material is softened in the barrel of the machine and is forced into a closed mold to form the part. Both thermoplastics and thermosets can be injection molded.
Suggestions For Effective Use
- A brief, discussion of the history and development of the plastic industry.
- Explaining that the many different plastics in use today’ all can be classified into two categories, thermoplastics and thermosets.
- Point out that thousands of plastic articles are commonly made by one of the seven processes show in the videotape.
- Exhibit several common plastic items which were made by the processes shown in the videotape.
Blow Film Extrusion
Injection Molding 2