Rotational Molding (also called Rotomolding)

To see pictures of rotationally molded parts, click here.

To watch a movie about rotational molding, click here.

Process Overview

Rotational molding is generally used to produce large, hollow parts, parts that are too large to make with other processes. The parts usually have walls around 0.3" to 1.2", which is much thicker than what is usually found in other processes.

There are four basic steps in a rotomolding process (refer to Figure 1 throughout this discussion). In the first step at the loading / unloading station, a powdered resin is loaded into a cold mold and the mold is assembled, usually manually. Next, the molding machine moves the mold into a heated chamber. The mold is allowed to heat soak while being rotated around two axes. The powder fuses together and coats the mold walls (it does not melt). Next, the mold is moved into a cooling chamber where cold air or water is sprayed on it. When the mold contents are cold, it is finally rotated back to the loading / unloading station, where the mold is disassembled, the part is removed, and more powered resin is loaded into the mold.

Materials

There are many factors that affect the feasibility of using a resin in rotomolding. Polyethylene is by far the most common (probably used in 90% of applications). Other materials are much more difficult to mold. Some other materials are PVC, ABS, nylon, polycarbonate, high impact polystyrene, acetal, and crosslinked polyethylene. Some material properties that affect ability to be rotomolded are:

Thermal stability: The resin will see a high temperature for a long time (much longer than in other processes). Many materials are not thermally stable, especially resins like PVC and ABS. These resins can be rotomolded if extreme care and process control is practiced.

Excessively high fusion temperature: Some engineering resins have fusion temperatures that exceed the capability of commercial rotomolding ovens.

Grindability: Materials must be ground into a fine powder. Plastics with low melt temperatures can be difficult to grind because they melt in the high-speed impact mills.

Pourability: A material must have a high pourability rating to tumble well in the mold. This property is measured by ASTM Test 1895-69 by timing the flow of a given weight of plastic powder through a standard funnel. An acceptable rotomolding powder flows at 185 grams/minute.

Fusability: The powder must easily fuse together during the heating cycle. Molecular weight plays a large role in fusability. If molecular weight is too high, higher temperatures will be needed, which could degrade the plastic. If molecular weight is too low, the powder may melt, forming puddles in the mold and producing non-uniform walls.

Chemical Crosslinking: Some manufactures use chemical crosslinking when an easy flowing resin is needed for processing, but a high molecular weight resin is needed for mechanical properties. In the crosslinking process, individual polymer molecules are joined together by a molecular bond. This increases molecular weight, increasing properties such as toughness, creep resistance, and stress crack resistance.

Summary

There are several advantages to rotational molding over other processes.

· Because there is no pressure, the mold does not have to be very strong, and is relatively inexpensive

· Compared to injection and blow molding, the machine cost is small

· Very large parts are able to be made with a relatively small capital investment

· Hollow parts can be formed (difficult or impossible with injection molding)

· The plastic is stress free because there is no pressure and very slow cooling

Some disadvantages are listed below:

· Molding cycles are very long compared to other processes

· The mold is usually assembled / disassembled manually, so it is a labor intensive process

· Not very many types of resins can be used (polyethylene is used probably 90% of the time)

· The process is not very energy efficient

Some characteristics of rotomolded parts are:

· Parts are generally large and hollow, like storage tanks and large children's toys

· Complex geometry cannot be molded