What is Rotational Molding: Process, Materials Used, and Industrial Applications?
Rotational molding is a relatively simple process used to shape thermoplastics in a high-heat, low-pressure environment. It is considered to be a comparatively low-cost process to run. The low pressure on the mold means that aluminum can be used for the tooling. Rotational molding is used in several industries for products such as road cones, outdoor furniture, store mannequins, and kayaks.
This article will discuss rotational molding, how it works, its purpose, applications, and benefits.
What Is Rotational Molding?
Rotational molding is a process for molding thermoplastics in a low-pressure, high-heat environment (c. 482-550 °F). Tooling used for rotational molding is commonly made from aluminum alloys. and the vast majority of resin used in the process is polyethylene, which does not break down chemically at the temperatures used for molding.
Rotational molding is used for parts requiring standard, uniform wall thickness, a high degree of structural stability, and a high-quality finish. As the final piece is created in one continuous process, parts do not require joining by welding therefore there are no weld lines or pinch-off seams, (as might be present in blow molding) upon completion. Rotational molding is ideally suited for open double-walled containers, or larger, single-pieced hollow parts. Examples of rotational molding include coolers, tanks, and kayaks).
How Has Rotational Molding Developed Over Time?
Rotational molding has changed over time from when it was first used to manufacturing the durable plastic products we use today. Historians believe rotational molding dates back to the Egyptians, who used the process to create their ceramic pottery. However, the official patent was first documented in 1855 to create hollow vessels and metal artillery shells. In the 19010s, the concept of rotational molding was applied to the food industry, where hollow chocolate eggs and bunnies were produced using this method! In the 1940s, toys were among the first objects to be mass-produced by rotational molding, and by the 1950s, many household items were being made this way.
The types of materials used in rotational molding have developed over time, initially thought to have started with ceramics and then evolved to metals, waxes, and plaster. Today, aluminum is used predominantly, with the distinct features of rotational-molded products being their durability, flexibility, and ability to withstand high heat without cracking.
The process of rotational molding continues to evolve. There has been a sharp rise in the importance of environmentally friendly molding materials. Environmentally friendly and bioplastic alternatives that will become the successors of today’s plastic materials are currently in the process of being researched, developed, and introduced.
What Are the Key Materials Used in Rotational Molding?
Several different materials can be used in the rotational molding process. However, they must be capable of being pulverized into a very fine powder, ranging from 74-2000 microns. Some key materials are as follows:
Polyethylene: It is available in a variety of classifications and grades. Polyethylene is a type of thermoplastic from the polyolefin family.
Polypropylene: It has a high heat distortion temperature, high resistance to environmental stress cracking, and excellent resistance to chemical stress cracks. It is typically used for manufacturing food industry products.
Polyvinyl Chloride: This material can be processed in both powder and liquid forms. It is commonly used in applications such as pet toys, dolls, and other children’s toys.
Nylon: Two grades of nylon can be used in rotational molding: nylon 12 and nylon 6. They are known for their high rigidity, tensile, and impact strength. Nylon has a very high heat resistance when compared to other rotational molding materials such as polyethylene.
Polycarbonate: This material retains its mechanical properties at higher temperatures than most thermoplastics, and is consequently a more expensive material than polypropylene, polyethylene, and even nylon. Rotational molding is usually used to fabricate furniture, shipping containers, and light fixtures from polycarbonate.
What Products Are Produced By Rotational Molding?
Many types of products are produced by rotational molding. Some examples include the following:
Kayaks
Tanks (for example storage tanks in agricultural, commercial, or industrial scenarios)
Store mannequins
Coolers
Bins and containers
Road cones
Road barriers
Automotive parts (including interior design elements, such as cup holders)
Sports equipment (for example dumbbells, helmets, golf carts)
How Does Rotational Molding Work?
Rotational molding works by creating a low-pressure environment (around 1.51 P, which is around 50% more than atmospheric pressure) with very high heat (approximately 482-550 °F). Plastic material is put into a hollow mold, where it is then rotated on two axes and placed inside the hot, low-pressurized oven. The rotational molding mechanism ensures the mold is slowly rotated upon two axes whilst being simultaneously heated to distribute and then fuse the resin inside the mold. Once the plastic has been completely melted and evenly distributed across the mold, it is then removed from the oven and cooled either with water spray or in the air. Once completely cooled, the rotation stops and the mold can be opened to reveal the finished piece inside.
What Are the Steps in the Rotational Molding Process?
The rotational molding process is relatively straightforward to understand. The steps in the rotational molding process are listed below:
A hollow mold is filled with powdered plastic resin (usually made from cast aluminum).
Close the hollow mold, and a pressure of 1.51 P begins to gently rotate bi-axially. Transfer the mold into a high-heat oven, the temperature of which averages 500 °F.
The mold continues the slow rotation as the plastic resin melts and completely covers the walls of the mold.
Remove the mold from the oven and cool until the resin has hardened into the desired design.
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