| orld has become very dependent upon plastic | | | | Like Injection molding, rotational molding had its |
| products. From household items to industry and | | | | roots in the 1940s. But it was not until the |
| aerospace, plastic in its many formulations has | | | | technology was more sophisticated and new |
| transformed modern manufacturing and created | | | | polymer and plastic formulations became available |
| conveniences and economies unimagined in the | | | | that the rotational process became a mainstream |
| early decades of the 20th century. | | | | manufacturing method. |
| Injection Molds | | | | Rotational Process |
| The injection molding industry took hold in 1946 | | | | The two processes are quite different. Let's |
| when James Hendry built a screw injection | | | | consider, for example, a 300 gallon water storage |
| molding machine. But, his technology was based | | | | tank made of polyethylene. Picture a master mold |
| on an earlier invention by John Wesley Hyatt who, | | | | made of aluminum or steel. The plastics |
| in 1868 injected hot celluloid into a mold to make | | | | manufacturer pours poly resin powder into the |
| billiard balls. Hyatt's method used a plunger to | | | | mold that is fitted inside an oven. Once sealed, the |
| force the material inside a mold. Hendry's | | | | mold is mechanically turned on at least three |
| improvement was revolutionary because it | | | | axes, moving much like a gyroscope. At the |
| eliminated the plunger and replaced it with an | | | | same time, the oven is raised to an appropriate |
| auger-type action that better distributed material | | | | temperature and the polymer - or other material |
| and facilitated the use of plastic inside molds. | | | | - tumbles inside and slowly coats the inner walls |
| Today's injection molds use much the same | | | | of the mold, melting as it rotates. |
| process and produce a wide variety of products | | | | Once the optimal temperature is reached, the |
| from car panels to outdoor furniture, small toys | | | | mold is cooled. As the temperature of the mold |
| and tools. Injection molding is ubiquitous in | | | | itself falls, the product on the inside shrinks away |
| manufacturing and uses many different materials | | | | from the inner walls and is easily removed. This is |
| from polymer plastics to aluminum, copper and | | | | not always the case with injection molds that are |
| other metals. The plastic bottles and kitchen | | | | often more difficult to successfully remove. The |
| implements people use in everyday life are | | | | shrinking action of rotational molding is particularly |
| products of the injection process. | | | | desirable when the product is very large and |
| Because the metal molds are generally expensive | | | | awkward to handle. |
| to produce, injection molding is most economically | | | | Rotational molding is also more economical for |
| used when thousands of pieces are being | | | | some products because less material is used. In |
| manufactured. Molds are made of hardened steel | | | | addition, the polymer that is left over from one |
| or, more recently, aluminum which is less | | | | mold can be used in another. The method itself is |
| expensive. | | | | more streamlined than injection molding, which |
| The Injection Process | | | | requires more interlocking parts. |
| Described very simply, molten plastic is injected | | | | Materials Improve and Expand |
| into the mold under high heat and pressure. The | | | | Most products made with the rotational molding |
| goal is to have the molten plastic material evenly | | | | method are from the polyethylene family. Other |
| flow to all parts of the mold, creating an exact, | | | | materials include nylons, polypropylene and PVC |
| consistent, solid plastic replica of the mold cavity. | | | | plastics. Some manufacturers have developed |
| After a brief cooling cycle, the mold or tooling | | | | formulas that integrate the use of natural |
| mechanically ejects the plastic part which then | | | | materials such as sand and chips of stone to |
| moves on through the manufacturing process. In | | | | make products. |
| the injection molding industry, this is a completely | | | | Plastic and resin products are now an integral part |
| automated process that's very fast and | | | | of everyday life and supply us with items as tiny |
| extremely efficient. | | | | as paper clips and as big as storage tanks. As the |
| Rotational Molding | | | | industry developed, so too has environmental |
| Rotational molding is yet another method of | | | | awareness about the safety and use of these |
| producing multiple products, most often made | | | | petrochemical-based products. Today, materials |
| with a variety of plastic powders. This process is | | | | can meet the specifications of FDA requirements, |
| usually used in making hollow products such as | | | | and other health and safety related regulations. |
| traffic cones, canoes, kayaks, bicycle helmets and | | | | Producers are also cooperating to create products |
| giant tanks used for water or chemical storage. | | | | that can be recycled. |