What is a Rotational Molding Machine and How Does it Work?

What is a Rotational Molding Machine?

Rotational molding does not involve the use of a plasticizing unit. Rather than injecting or extruding molten material under pressure, the process uses materials such as powdered polyethylene that are cast within a hollow tool. Usually made of aluminum or steel, the tool is opened and closed manually to fill the powder and remove the finished part.

Excellent high-temperature performance and oxidation stability, prevent grease from deteriorating at high temperatures, and ensure long-term normal operation of lubricating parts at high temperatures, excellent adhesion performance, good mechanical stability, and colloidal stability, ensure that the lubricating parts will not be lost. Good lubricity, protect bearings and reduce wear. Excellent comprehensive performance ensures a long service life of the bearing.

Rotational molding is a labor-intensive and relatively slow process, so multiple tools are often cycled through the various stages. To achieve this, the machine can have up to 4 independent arms. Shuttles move the tool through the station in a linear fashion, so tend to have only one or two arms, but oscillating and carousel designs can be more complex. According to the modified performance characteristics of lubrication technology monograph to guide the use and maintenance of rotational molding machine equipment.

Rotational molding is usually used to produce hollow structural products. This molding process is suitable for thermoplastics and sometimes for some thermosetting plastics. This method is to measure the powdery or pasty resin, put it in the roto-molding mold, heat the mold, and roll and rotate it so that the resin in the mold is melted and plasticized to a flow state. And evenly filled with all parts of the mold cavity, after cooling and hardening, the finished product can be obtained after de-molding.

Rotational Molding Technology and Development:

There are many kinds of processing and molding processes for plastics and their composite materials. In addition to our common extrusion, injection, blow molding, and other processes, rotational molding is a processing method for plastic products. Since the roto-molding process came out in the 1930s, after more than 60 years of development, its equipment and technology have become more and more perfect. From small children's toys to common automotive plastic parts to huge engineering plastic products, especially super-large, and non-standard special-shaped hollow plastic products, due to the limitations of other plastic processing technologies, they can only rely on the rotational molding. Traditionally, rotational molding is applied to thermoplastic materials. In recent years, rotational molding of thermosetting materials such as cross-linkable polyethylene has also developed rapidly. Because roto molding does not require high injection pressure, high shear rate polymer metering, molds, and machines are less expensive and last longer. The rotational molding process has become an important branch of the plastic molding process.

Rotational molding is also known as a rotational casting, rotary molding, etc. The roto-molding process is to add plastic raw materials into the mold first, and then the mold is continuously rotated along two vertical axes and heated so that the plastic raw materials in the mold are gradually and evenly coated and melted, and adhered to the mold under the action of gravity and heat energy. The entire surface of the cavity is molded into the desired shape, and then the product is obtained after cooling, shaping, and de-molding.

Rotational molding relies on the centrifugal force of high-speed rotation of the mold so that the raw materials are distributed on the inner wall of the mold, which is normal pressure molding. Therefore, the residual stress of the product is low, and there will be no defects such as warping, and the mold can also simplify the process, thereby significantly reducing manufacturing costs, and manufacturing medium-sized, large-scale, or super-large fully enclosed and semi-enclosed hollow seamless containers.

The Production Process of Rotational Molding Technology:

The basic process of roto-molding is simple, powder or liquid polymer is placed in a mold, heated while rotating around two perpendicular axes, and then cooled. At the beginning of the heating stage, if powdered materials are used, a porous layer will be formed on the surface of the mold, and then gradually melted with the circulation process, and a homogeneous layer of uniform thickness will be formed. If liquid materials are used, then flows and coats mold surfaces, and stops flowing completely when the gel point is reached. The mold is then transferred to the cooling station, where it is cooled by forced air or sprayed with water, and then placed in the work area, where the mold is opened and the finished part is removed before the next cycle.

• The inside of the tool is sprayed with a release agent to allow for easier removal of the finished part.
• Put the required amount of powder into the tool and close it.
• The tool is attached to the arm of the roto-former, and the tool is then rotated by biaxial in the oven. This melts and spreads the material evenly over the inner surface of the tool.
• Move the machine's arm to the cooling station, where it continues to turn the mold through directed air or water jets.
• After solidification, the tool is opened and the part is de-molded.

• Feeding: Put the weighed plastic into the mold in the form of liquid or powder.
• Heating: The roto-molding machine rotates the mold in two directions and joins the heating furnace to heat the mold, and all the plastic adheres and sinters to the inner surface of the cavity.
• Cooling: With the continuous rotation of the mold, the mold is simultaneously transferred out of the heating furnace and enters the cooling chamber, where the rotomolded plastic is cooled and formed.
• De-molding: The rotational molding machine turns to the mold opening position, the mold stops rotating, and the mold is opened, and taken out.

Use and Application of Rotational Molding Machine:

It is suitable for the lubrication of various bearings, gears, chains, and other transmission mechanical parts under high-temperature environments. It is suitable for the lubrication of motors, bearings, and gears in metallurgy, mining, oil fields, machinery, transportation, and other industries under severe conditions such as high temperature, heavy load, and humidity. Especially suitable for high-temperature bearings, such as auxiliary boiler rotary burner bearings in large chemical plants, sleeve bearings for plastic extruders, high-temperature bearings for cracking furnace-induced draft fans, regeneration gas compressors, etc., stretching tenters, hot melt fans, lubrication of rolling bearings and sliding bearings of various machinery with high speed working at high temperatures such as heat setting drying room, a baking machine, and high-temperature dyeing vat.

Features of the Rotational Molding Process:

• The edge strength of the roto-molding product is good, and the roto-molding can realize the thickness of the edge of the product exceeding 5 mm, and completely solve the problem of the thin edge of the hollow product.
• Rotational molding can place various inserts.
• The shape of rotational molding products can be very complex, and the thickness exceeds 5 mm.
• Rotational molding can produce fully enclosed products.
• Roto-molding products can be filled with foam materials to achieve heat preservation.
• There is no need to adjust the mold, and the wall thickness of the rotationally molded product can be adjusted freely.
• The product is hollow and seamless, but the wall thickness can be adjusted to make the product have high strength.
• Products of various colors can be produced and can be used for anti-ultraviolet and anti-aging treatment.

• Because the material has to be ground and pulverized the cost increases.
• The processing cycle is long, so it is not suitable for mass production.
• There are fewer types of plastics available.
• Opening and closing the mold is relatively heavy physical labor.

Compared with other mold processes, rotational molding provides more design space. Under the correct design concept, several parts can be combined into a complete mold, allowing the container to be integrally formed, reducing assembly costs and leakage and seepage problems caused by welding.