Rotational molding, also known as rotomolding, is a thermoplastic molding technique most suitable for large, single-piece hollow objects and double-walled open containers, such as coolers, kayaks, and tanks. It is perfect for entrepreneurs, start-ups, and micro businesses, as it is a cost-effective option for production numbers of fewer than 3,000 yearly.
Rotomolding is often used for components that need high stability, homogeneous wall thicknesses, and high-quality finishing. The rotomold may be easily customized with inserts and spin weld attachments, and foaming can be utilized to add rigidity and thermal insulation. With no pinch-off seams or weld lines, rotomolding creates a completed product without needing supplementary procedures, unlike blow molding and thermoforming.
How Does It Work?
A rotational molding procedure involves the following steps:
Pellets or coarse powders are milled or pulverized to produce a fine or extra-fine powder. Since the particle sizes of various machines vary, the plastic material must be processed through numerous pulverizers to attain the desired uniformity.
Batch pulverization, dry milling or grinding, and wet pulverization are all types of pulverization. The pulverization method is determined by the molding process to which the pulverized plastic will be subjected.
The polymer, a powdered resin, is metered and tightly pressed into a hollow mold. For a proper flow and to avoid bubble formation, the powdered resin must be dry, homogenous, and in fine particle sizes. Therefore, one of the elements affecting the part’s wall thickness is the amount of resin loaded.
In this stage, the powdered resin is heated while gently rotating inside the hollow mold until all the resin is melted. The interior wall of the mold is also completely covered with resin as it melts. Moreover, the mold’s resin is distributed uniformly because of the simultaneous heating and rotating actions, and the biaxial mold rotates at a modest speed of approximately less than 15 rpm.
The polymer heating time is crucial, and one of the criteria is influencing the quality of the completed product. For instance, the polymer will thermally degrade if it is heated for a long time, lowering its final mechanical attributes and making it less resistant to wear and impact.
On the other hand, if the heating process is rushed, the polymer will only partially melt. As a result, unmolten grains won’t combine with molten resin, which causes bubbles to develop. This fluctuation negatively impacts the product’s end-mechanical qualities.
At this point, the liquid polymer inside the mold hardens and takes on the intended shape. Next, the outside of the rotating mold is cooled by using air, either forced or natural convection. To preserve dimensional stability during cooling, cooling air is occasionally delivered to the interior of the mold. Using water sprays to speed up cooling might impact the part’s mechanical characteristics and size.
The polymer’s cooling time is just as important as its heating time. As a result, the correct cooling rate must be established. Rapid cooling causes the component to deform and shrink out of control. Meanwhile, slow cooling results in the flow of the molten resin, which results in uneven wall thickness.
5. Unloading or Demolding
The operator carefully removes the cooled portion from the hollow mold tool. Then, the part can be lifted out of the tool with an air ejection mechanism. Following the removal of the pieces, other procedures — including inspection and packing — are carried out.
6. Secondary Processes
The application of the produced product determines which kind of secondary procedure is used. This can involve assembling, welding, coating, painting, adding inserts, etc.
Rotomolding can be more beneficial than conventional molding techniques. Here are some of its advantages:
There is no pressure applied in rotomolding because the process involves casting. As a result, molds are affordable and low-volume production may be profitable.
Because of its inexpensive initial expenditure, rotomolding is particularly appealing if you have a brilliant concept for a new product but are unsure how many you will sell or need a low-volume production run.
In rotomolding, complex forms are simple to create. Production complications — including stiffening ribs, molded inserts, and various surface textures — are easily accommodated by this technique.
Consistency and Uniformity
Rotomolding produces uniformly thick walls, with often thicker corners that improve the integrity and strength of the product. Other techniques, like blow molding, strain the molten material at corners or sharp edges, potentially causing weak points and thin places.
Rotomolding allows a single machine to simultaneously mold many pieces. Tool management is simple, with some rotational molding equipment that includes independent arms; one mold may be scheduled for maintenance while the others are in operation.
Lerner Molded Plastics for High-Quality Rotational Molding Services
Lerner Molded Plastics is an ISO 9000-2000 certified plastic injection mold and tooling maker with over 25 years of expertise! We also provide rotational molding services to customers in various industries.
Plastic injection molding may be a complex procedure with many variables to consider. We can walk you through these factors and assist you in deciding on the optimal injection molding tools and component design for your project.