Chapter 2: Rotational Molding Process

The steps involved in a rotational molding operation are the following:

Pulverized Resin: Milling or pulverizing are used to change pellets or coarse powders into fine or extra-fine powder. The particle sizes of individual machines differ, which requires that the plastic material be passed through several pulverizers to achieve the proper consistency. The different pulverization methods include batch pulverization, dry milling or grinding, or wet pulverization. The selection of the pulverization method depends on the type of molding process to which the pulverized plastic will be applied.

Raw materials for rotomolding vary in accordance with their physical properties and intended applications. Additives and colors are added to achieve the required characteristics and properties. The various types of polyethylene are mainly used for the rotomolding process and are thermoplastics that can easily be reshaped by heating. The five types of polyethylene are linear low-density polyethylene (LLDPE), medium density polyethylene (MDPE), high-density polyethylene (HDPE), low-density polyethylene (LDPE), and cross-link polyethylene (XLPE).

Loading: A measured quantity of the polymer, which is in powdered resin form, is placed in a hollow mold and secured tightly. The powdered resin must be in fine sizes, homogeneous, and dried to achieve a good flow and prevent bubble formation. The amount of resin loaded is one of the factors which determine the wall thickness of the part.

The hollow mold is made from cast aluminum or fabricated steel sheet and gives the molded part its shape.

A mold release agent is a coating present on the inner walls of the mold. It is used for effective removal of the molded part after cooling, as it prevents sticking from the mold surface. The types of mold release agents are as follows:

Sacrificial coating: This type of mold release agent, usually silicone, comes off with the molded part when it is released from the tool. Hence, it is applied at the start of every loading process.

Semi-permanent coating: Semi-permanent mold release agents are commonly used in most industries. It lasts after several cycles of heating and cooling of the polymer. It is re-applied or topped up before being used up.

Permanent coating: This type eliminates the need for the re-application of a mold release agent, as it is permanently fixed on the mold surface. However, the permanent mold release agent layer can wear off due to scratching and mishandling. The most common permanent mold release agent coating is polytetrafluoroethylene (PTFE).

Heating: The powdered resin is heated inside the hollow mold while being rotated slowly until all the resin is melted. As the resin melts, it coats the entire inner wall of the mold. The simultaneous action of heating and rotating ensures the uniform distribution of the resin inside the mold. The mold rotates biaxially and is usually slow (less than 15 rpm).

To achieve a good wall thickness distribution, the proper rotation ratio must be determined. This value is the number of rotations per minute (RPM) on the horizontal axis over RPM on the vertical axis. Spheres or cubes can be molded at a rotation ratio of 4:1. For irregular solids, the ratio must be at 1:8 or 8:1, depending on how the manufacturer optimized this factor.

The heating time of the polymer is critical and is one of the parameters which determines the quality of the finished part. Excessive heating time will result in thermal degradation of the polymer and will reduce the end mechanical properties, such as less resistance to wear and impact. On the contrary, insufficient heating time will result in incomplete melting of the polymer. Unmolten grains will not coalesce with the molten resin, which results in bubble formation. This variation has adverse effects on the end-mechanical properties of the product.

Cooling: At this stage, the molten polymer inside the mold hardens and solidifies into its desired shape. The outside of the rotational mold is cooled by natural or forced convection, usually using air. Cooling air is sometimes supplied to the mold internals to maintain dimensional stability during cooling. Water sprays may be used to reduce the cooling time, but this can affect the mechanical properties and dimensions of the part.

The cooling time of the polymer is as crucial as the heating time. Thus, the proper cooling rate must be determined. Cooling rapidly results in uncontrollable warpage and shrinkage of the part. Slow cooling, on the other hand, causes the flow of the molten resin resulting in inconsistent wall thickness.

Demolding or unloading: The cooled part is carefully removed by the operator from the hollow mold tool. An air ejection system can help in lifting the part out of the tool. Once the parts are removed, it proceeds to the next processes, such as inspection and packaging.

Secondary Processes: This can include painting, coating, assembly, welding, the addition of inserts, and so forth. Each type of secondary process depends on the application of the finished product.

Chapter 3: Materials Used in Rotational Molding

The commonly used polymers in rotational molding are presented below. Most polymers used for this process are thermoplastics.

Polyethylene: Polyethylene accounts for more than 80% of the polymers used in industries that use rotational molding. This is due to its low cost and ease of molding. It is readily available in powdered form, unlike non-polyethylene polymers, which are difficult to grind. It also has good chemical resistance and low water absorption.

Polyethylene grades that can be used in rotational are High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Medium-Density Polyethylene (MDPE), and Linear Low-Density Polyethylene (LLDPE).

Polypropylene: Polypropylene is the second-most processed plastic and is one of the most versatile polymers available. It has characteristics between LDPE and HDPE. Its valued properties are good chemical, heat, and fatigue resistance.

Polyvinyl Chloride: Polyvinyl chloride is the polymer form of vinyl chloride monomer. It is a strong and rigid plastic and is compatible with many additives to modify its mechanical properties.

Nylon: Nylon comes from the another company. Aside from film and fiber production, this polymer can be used as a molding compound. It is generally tough, with good thermal and chemical resistance.

There are requirements in selecting the polymer to be used for rotational molding, considering the nature of the process steps. This limits the thermoplastics in the following ways:

The molten plastic will be exposed to oxygen at high temperatures, which may result in oxidation and loss of the desired mechanical properties of the polymer. Therefore, the molecule of the polymer material must have groups with antioxidant properties.

The polymer must have high thermal stability for the material to resist permanent changes brought by high temperatures.

The molten material must easily flow within the walls of the mold since flow is dependent on rotational movement only, and there is no pressure involved. The flow characteristics of the chosen polymer at high temperatures must be considered during the optimization phase.

Primary additives improve the mechanical properties of the part and aid in the molding process. Flow modifiers aid in the flow of polymer resin in the molten state to achieve good thickness distribution. Heat stabilizers prevent thermal degradation induced by high temperatures. Fillers increase the stiffness and impact modifiers increase impact strength; however, its amount must be controlled since it causes rough surface and reduced flow. Secondary additives are also utilized to give the finished product special characteristics, such as colorants, flame retardants, and anti-static agents.

Chapter 4: Advantages and Disadvantages of Rotational Molding

The concept of rotational molding is a simple one, but in fact, it is challenging for some manufacturers to achieve a good product out of the process. Rotational molding is valued for its advantages compared to other molding methods. With proper design and settings, the manufacturer and end-user can benefit from the following:

Uniform wall thickness: A consistent wall thickness, on all sides, edges and corners, increase the part's durability. With proper rotational speed and cooling cycles, a uniform wall thickness may be achieved, even on producing thick-walled parts. The corners and edges produced are thicker with rotational molding when compared to blow molding, which stretches the molten material in those areas.

Ease of producing double-walled parts: Double-walled parts are easily made without the need for secondary processing such as welding and joint fabrication. The parts produced have seamless edges, which eliminates the stress points, resulting in increased durability.

Inexpensive tooling: Since the mold does not have to withstand high pressures, it can be manufactured using low-cost materials such as aluminum. Less investment is required for the tooling when only short production runs are required.

Flexibility of production: Different parts can be molded in a single machine at the same time. With some rotational molding equipment that has independent arms, it makes tool management easy; one mold may be scheduled for maintenance activities while the other molds are in use.

Larger parts can be produced: Rotational molding makes the production of large hollow parts possible. The only limitation is the size of the heating and cooling chambers.

Less downstream processes required and minimal waste in production: The part manufactured in rotational molding is only a single part. Hence, it is not required to undergo trimming or stripping steps. Rotational molding also generates less wastage of polymer resin in the form of runners, sprues, and cut-offs.

Ease of decoration: A designer can easily incorporate details such as textures and symbols through the addition of such details on the surface of the tooling.

As rotational molding offers many advantages over other types of molding processes, it does not mean that it is the best for all manufacturers. Here are some disadvantages of this process:

High cycle times and costs: Rotational molding may not be suitable for high-volume manufacturing. The slow rotation during heating to the molten state and gradual cooling of the part and entire tool to room temperature after mold consumes a lot of time during the molding cycle. Cooling water or air systems are available, but it requires additional cost.

Lastly, there are still manual steps involved in the process (e.g. demolding) due to the limited availability of automation features. This also adds to the overall cost of the operation.

Limited material options: Few polymers qualify as the raw material for this process since they require being converted into powdered form to be processed successfully. Polymers other than polyethylene are costly and difficult to grind. Also, this process requires the polymer to have high thermal stability, which limits poly-based resins to be selected.

Shorter service life of the tool: Since it is only made from thin and soft metal, the tool must be replaced after several mold cycles to ensure the quality of the parts being produced due to a lack of repeatability.

Some details and designs are difficult to mold: Uniform thickness on a large flat surface is difficult to mold due to the flow of the resin. Also, rotational molding machines are not capable of molding high-tolerance parts and sharp edges; high-pressure molding may be considered.

Chapter 5: Applications of Rotational Molding

Rotational molding has many applications such as industrial and automotive parts, furniture, materials handling equipment, medical devices, toys, and much more. Some of the notable products made from rotational molding are the following:

Material handling equipment such as durable crates, stackable pallets, containers, and insulating boxes, which are only manufactured by rotational molding

Plastic storage tanks, gallon drums, and carboys for containing small to very large volumes of water and chemicals

Laboratory and medical supplies such as syringes, oxygen masks, and squeeze bulbs

Gardening and agricultural tools used for planting, such as pots, troughs, composting bins, and gardening carts

Sanitary products such as containers for refuse, trash cans, and septic tanks

Marine vehicles and equipment for transportation and water sports such as rowing boats, canoes, buoys, and kayaks produced in a rock and roll molding machine

Safety barricades, traffic cones, and other similar items found on roads and highways

Toys and sporting equipment such as doll parts, footballs, playground slides, gym equipment parts, and floatable objects for swimming pools

Small shelters and housing (i.e. tornado shelters, portable toilets, testing facilities)

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