Rotational moulding, also sometimes known as rotomoulding, is a thermoplastic fabrication process by which plastic goods are formed and manufactured using a revolving mold and high heat. Rotomolded products are hollow and have single piece construction, with uniform wall thickness and high tensile strength. They are often inexpensive to manufacture and require little further fabrication work.
Rotational molders employ rotational molding techniques to manufacture plastic parts from stock molds, and rotomolding companies often offer custom rotational molding, which requires the fabrication of a unique rotomold. The molds are usually made out of either stainless steel or aluminum, which are welded together or die cast. The plastic rotational molding process is considered slower than other thermoplastic molding techniques, and parts are usually manufactured in smaller runs. Roto moulders are often able to mold different products simultaneously, as long as they are similar in size. Some are designed to fit many different smaller molds on their spindle arms, while other rotational molders are used to fabricate one large molded product at a time. Rotational moulders are used to fabricate plastic products for a wide range of industries and applications, including food and beverage processing, pharmaceutical, waste water management, agricultural, plumbing, sporting goods, packaging, material handling, chemical, fuel/oil, marine, road construction, automotive and toy manufacturing. Tanks, containers, bins and other hollow plastic products are just a few examples of possible rotationally molded plastics.
The process begins with a rotomoulding machine with arms called spindles. These spindles support the molds in which the plastic material is contained. The molding machine transports the molds from the loading area to a heating chamber and then to a cooling chamber. From start to finish, there are four different stages that are completed in anywhere from 15 to 90 minutes. In the first stage, which is referred to as the charging phase, the metal mold is loaded with raw plastic material and is sealed shut. From there, the mold is heated to high temperatures while slowly revolving on the vertical and horizontal axes of the spindles. This heats the plastic powder until molten, so it evenly coats the interior walls of the mold. Because roto moulders use low pressure, the plastic resin is in the form of fine powder to help it reach all surfaces of the mold cavity evenly. The plastic material can feature additives that contribute to corrosion and static resistance, and the finished molded product assumes these properties when it emerges from the molding process. Phase three is a cooling stage that cures the plastic within the mold, which is accomplished by using cold air and water for about 20 minutes. After the plastic has fully dried, the mold is reopened and the solid plastic part is manually removed. Because shrinking often occurs, removal can prove to be difficult. For applications that require additional strength, reinforcing ribs can be fabricated into the part.
Rotational molding services have several advantages over blow molding, thermoforming and injection molding. Molders are able to change colored plastics without extensive line cleanings, which saves on time and additional costs. Rotational molding does not require pressure, so the molds do not endure the same pressure as in injection molding or extrusion, which helps to limit equipment costs as well as energy-related costs. Rotational molding is often the method of choice in the fabrication of large plastic vessels and tanks, which are otherwise welded together with a laser, hot pressurized gas, ultrasonic vibration or a hot plate and pressure. While welding provides an air and water tight seal, bursting and cracking under high pressure is a concern. Rotomolded plastic tanks, however, are seamless and able to withstand higher amounts of pressure. Examples of other rotomolded products include outdoor furniture, kayaks and canoes, playground slides, safety cones, footballs, sports helmets, plastic rotomolded tanks, large trash containers and beverage bottles.
There are many other advantages to the rotational molding process. While being heated and rotated, the plastic resin adheres to the hot mold, coating it very evenly and seamlessly, producing strong outside corners with virtually no stresses. Rotomolding can produce one-piece constructions, and complex parts are formed without any part assembly required. The plastics used for rotomolds are often lightweight and may be high or low in density. For applications in the food and beverage and pharmaceutical industries, the thermoplastics used can be easily fabricated to be FDA approved, meaning they must not contain any recycled plastic or dies and cannot contaminate substances under any conditions, even high temperatures. The low cost of the materials and tooling makes rotational molding just as cost effective for producing prototypes as it is for large production runs. A variety of finishes, colors and textures are available. Companies are able to mold their logo, graphic or labels right into the parts. These imprints will not come off and are parts of the mold construction. Rotomolding is a versatile molding technique that provides durability to products, eliminates costly fabrication and assembly processes and reduces the need for replacement parts.
Rotationally Molded Tank - Sherman Roto Tank
Rotationally Molded Tank - Sherman Roto Tank
Rotationally Molded - Roto Dynamics Inc.
Rotationally Molded Plastic - Roto Dynamics Inc.
Rotationally Molded Tank - Sherman Roto Tank
Rotationally Molded Plastic - Sherman Roto Tank
A very different process is involved in the fabricating rotationally molded plastic products such as fuel and water tanks. Unlike extrusion and injection molding methods where high temperature and pressure are involved, the rotational molding process does not require high pressure. However, it does need high temperature.
In rotational molding, or roto molding, low pressure and high temperature are involved, and a heated mold is rotated on two axes simultaneously to make hollow and one-piece parts, which do not need curing. Rotationally molded plastics are large containers with high capacities, and those cannot be manufactured easily by any other option. However, the process is lengthy and commonly takes a longer time to manufacture a product than injection methods.
The process has its own set of advantages such as the molds used in this process are cast or welded, so the mold tooling time is not a factor in the process, and the mold itself is substantially cheaper. Moreover, the wastage of material is low, as most of the excess material is recycled. These advantages make it an environmentally and economically viable process.
The rotational molding method can be divided into four steps:
The first step involves the loading of measured quantity of raw material into a mold; typically, a powdered polymer is used. Different types of machines have distinct loading mechanisms.
The second step is rotating the mold slowly and concurrently heating the mold in a heating station, which typically consists of an oven. The mold is rotated at two or more axes at varying speeds so that the plastic material does not accumulate. The mold remains in the heating station until the plastic material melts and spreads to the wall of the mold. In this step, the critical part is the time for which the mold remains in the heating station. If the mold stays for too long, the plastic material can degrade, which is directly linked to the finished product's ability to withstand impact. Conversely, if the mold remains for the adequate time, the plastic pallets would not melt and stick on the wall. The insufficient melting can lead to large bubbles in the finished product, rendering it useless.
The third step involves cooling the mold after rotation. For cooling, commonly a fan is employed. The mold is cooled until the plastic material is hardened and can be handled safely by a person. The mold usually sets in about ten minutes. As the material cools down and hardens, it starts to shrink, which facilitates the removal of the product from the mold. The cooling process is also critical, and the rate of cooling must be regulated carefully. If a product is cooled down rapidly, it can result in disfigurement of the finished product. There was a time when an operator would determine the length of the time for cooling down the mold; however, modern machines have sensors that monitor the temperature of the molds.
The last phase is the removal of the product from the mold by an operator.
Rotational molding is a low-pressure and high-temperature forming process for the production of plastic products. The method fundamentally differs from other molding processes, such as extrusion and injection, as it involves a cast or welded mold in the shape of a finished product. The process includes heating of raw polymer in a mold with the use of an oven and rotating the mold on different axes.
Many critics of the method characterize the technique as economically unviable since a couple of cycles are involved in the fabrication of a product. However, rotational molding has a set of advantages over other options.
Here, we are discussing those advantages as well as the disadvantages of roto molding.
The biggest advantage is the fabrication cost. This process can mold different parts with consistent thickness with stress-free corners, which cannot be achieved by the alternative methods. If a design demands reinforced joints, they can also be easily added.
In this method, to make a product, prefinished parts, such as internal pipes, structures, and metal threads, can be assembled into a single mold. Unlike alternative methods, which involve tooling of a die even for small changes, roto molding provides great flexibility. Due to this flexibility, the design time can be cut down significantly.
Rotational molding in some cases is an economically viable alternative to blow molding.
The most expensive step of injection and extrusion molding is the tooling of the die. In rotational molding, the die can be welded and cast quickly for different applications. The cost of the development of the mold for rotational molding is a fraction of what it takes to tool a new die for the other options.
When short runs and rush deliveries are involved, rotational molding saves the day.
The same mold can be used without purging in the rotational molding when different colored products are needed to be made. In other methods, purging of the die is an essential step for swapping colors.
Thin panels of consistent wall thickness can be made with the method without much effort.
Only a little amount of material is wasted in this method, as the polymer can be recycled easily.
There are some disadvantages too in this method.
The biggest limitation is the time it takes to fabricate a product; unlike injection molding, which takes seconds to make a product, rotational molding can take up to half an hour to manufacture a fuel tank.
As the process is a low-pressure method, it is not possible to make sharp threads like injection molding.
The other drawback is the longer time it takes to cool down the mold. In alternative methods, only the product is cooled down; however, for making rotational molded plastics, the whole mold is cooled down.
The space a rotational molding machine takes in a manufacturing facility is also another concern; the footprint of the machines is larger than the alternative molding methods.
Regardless of the limitations, the range of rotational molded plastic products is expanding with the advent of new technology.
Rotationally molded products are typically hollow with large capacities, such as fuel tanks, with consistent thickness and density. Common molding methods, such as blow molding, extrusion, or injection molding, cannot mold such products; thus, a unique method has been developed, involving a large rotating mold, which is the size of the finished product. For making products, a mold is preheated by a heating system, and then the plastic pellets are introduced into the mold; the mold rotates on different axes, and the softened pellets start to take the shape of the mold. The mechanism ends with the cooling process, where the mold is cooled down.
The process is called as rotational molding, or roto molding, and the machines are called rotomolders.
Rotomolders commonly have four parts: oven, molds, mold spindles, and cooling chamber. Molds give the shape to plastic and are manufactured by two methods, welding or casting. The metal typically is either steel or aluminum. The oven is used for heating the molds, whereas the mold spindles help in giving uniform coating; they are mounted on a rotating axis. The cooling chambers assist in hardening the material by exposing the mold to low temperatures.
Rotomolders come in a variety of designs --carousel, clamshell, rock and roll, vertical, and shuttle machines.
This is the most common bi-axial machine that, based on an application, can have four and six stations. They are designed in such a way that if one arm remains in the heating station, the other would be the cooling station, and the third arm would be the loading or unloading area.
In this type of machines, a mechanical arm is used for rotating a mold, so that it can be rotated at different axes easily. The advantage of this machine is that the cooling and heating processes are done in the same chamber. This makes the machine compact, so it takes a lot less space than other options. Moreover, the design is less expensive.
Rock and Roll
This machine has a specific application; it fabricates long and narrow parts. The machine can have one or more arms based on the need; machines that have two arms are called shuttle type machines. The arm rotates a mold at 360 degrees at one axis, and concurrently, rocks the mold at 45 degrees along a different axis. The machine is used for making high-volume products and is considered cost-effective due to the use of smaller heating chambers.
These machines are also known as over rotational machines. They have different cooling and heating areas and can be loaded and unloaded from the front. Based on the requirements, these machines can be of varying sizes. They are energy efficient because they have small and compact cooling and heating chambers. When it comes to capability, they are the best, still taking a lot less space than multi-armed machines.
As the name implies, these machines shuttle the mold between the heating and cooling chambers and can rotate it bi-axially. Shuttle type machines are considered low-cost, as they can produce large-size products. For different needs, the footprint of a machine can be modified.
Other than standard extrusion and injection molding, there is another molding method in which plastic is molded, rotational molding. The process is entirely different from conventional molding and involves a rotating mold. For making rotationally molded plastics, a hollow mold is heated to a predetermined temperature and raw material is put into the mold. Then, the filled mold is rotated along two perpendicular axes slowly, and as the centrifugal force starts to act on the softened raw material, the material moves towards the wall and sticks. Under this action, plastic is molded various applications, including plastic tanks, plastic containers, and roto molded tanks.
This method may look new; however, it has its roots in the 19th century, when it was used for making artillery shells. Let us explore the history of rotational molding to know how it has evolved over time.
Like many things, rotational molding also originated in Europe; it was the idea of a British man, who in 1855, used rotation and heat to make metal shells for artillery. He modified his techniques to make other hollow materials, too. With this method, it was observed that the wall thickness and density of hollow objects could be made consistent. However, the method did not become popular and remained in the shadows of alternative molding methods.
The next development in this method came from an American in 1905, when the method was used for manufacturing hollow wax objects, and soon, the method was applied for making hollow chocolate-it was the beginning of chocolate eggs. The method for a long time was applied in the confectionery industry and was not used in large-scale industrial manufacturing.
The precursor of the present day roto-molding was the plaster of Paris molding, which started in the second decade of the 20th century. Gradually, in the 1940s, rotational molding of plastic found traction, and toy makers started to make hollow toys using the process. The machines used for manufacturing was quite crude which had parts from automobile, like the rear axle of a vehicle. The machine was driven by an electric motor, and gas burners were used for heating. For making molds, nickel and copper were used, and polyvinyl chloride was the raw material. Cold-water baths were used to cool down the molds.
Since the method produced toys with consistent thickness and density, it got further traction, and industries started to employ the technique to produce other items such traffic safety equipment, armrests, and buoys. As a number of manufacturers embraced the method, more efficient and bigger designs started to become available on the market, which had an indirect heating system.
The next stimulus to the method came in Europe, where a new raw material, polyethylene, and a new cooling system were used for making large hollow containers. The cooling system had burners that could be switched off during the process, so the hardening process could start while the mold was still in motion.
Soon, fabricators began using other raw materials, such as nylon and polycarbonate, which were molded into plastic fuel tanks, water tanks, and industrial molds.
- A common process used to form hollow, thermoplastic shapes, such
as bottles and containers. Two general types of blow molding are extrusion
and injection blow molding.
- The process in which individual polymer particles are joined together by a molecular bond.
- The ability of a material to withstand various chemical agents. Plastic is capable of enduring many of these chemicals, making it very useful for holding tanks and other containers.
- The oxidation and deformation of a material, such as the rusting of iron, due to a chemical reaction.
- The time it takes from the first stage of rotational molding, in which the plastic is loaded, until the plastic completes its final stage and is removed from the mold.
- The removal of unwanted material from a molded product, especially from the areas where parting lines of the mold may have caused excess material to be formed.
- A significant parameter for polyethylene (PE) that affects the properties, such as stiffness and impact strength, of the end product.
- The ability of a material to return to its original form after stress has been applied and quickly removed.
- The temperature at which the plastic resin melts and begins to adhere to the surface of the mold.
- The ability of a plastic to be ground into a fine powder. Most plastics are ground prior to being loaded into the mold.
- The measure of the ability of a plastic to withstand fracture by shock.
- A generic name for semi-synthetic or synthetic materials that can be extruded or molded into films or objects. Plastic materials are also used for coatings and adhesives.
- A chain of molecules that is formed by the bonding of individual repeating units (monomers).
- The measure of the speed of a plastic powder when poured through a funnel. For a plastic powder to be acceptable for rotomolding, it must be capable of flowing at least 185 grams/minute.
- Any of various semisolid or solid natural organic materials that are usually translucent or transparent. Resins are mostly utilized in plastics, adhesives and coatings.
- The "arms" of the rotational molding machine that are responsible for moving the molds between different stages. Spindles also spin on the horizontal and vertical axes to create a uniform filling within the mold.
- The ability of a resin to withstand specified temperatures. Less thermally stable materials can be rotomolded only under controlled conditions with extreme care.
- A material that will soften when repeatedly heated and cooled. Examples of thermoplastics include polyethylenes, polypropylene (PP), vinyl and nylons.