Rotational molding (or Rotomolding) is a method that involves the slow tumbling, heating, and melting of a thermoplastic powder in a biaxially rotating mold to produce seamless, hollow plastic parts. This process is typically used to mold hollow parts, especially those with complex and varied shapes not easily obtainable by other hollow-art processes. It is a virtually shear-free and pressure-free process. The wall thickness uniformity and part weight can be easily maintained. There is very little waste of material due to scrap. Rotomolding molds are often less expensive than other types of molds. Raw Materials
Lower viscosity resins are typically used in this process as they are more readily sintered to ensure a good surface finish. The material must be capable of being ground into a 35-mesh powder that flows like a liquid. It must also be able to adhere to the hot surface of the cavity and fuse together without pressure. The material should be stabilized to resist oxidation, or a nitrogen purge must be used during processing. - Polyethylene (Low Density) LDPE, LLDPE
- Polypropylene PP
- Ethylene Vinyl Acetate EVA
- Polyvinyl Chloride (PVC)
Cost
Low tooling costs and high piece prices are typical of this process. Cycle times average around 40-45 minutes. Process Steps. The Rotational Molding process is essentially split into four operations: | | Step #1. Loading resin into the mold
The rotomolding process is begun by placing a pre-measured amount of plastic material (in either liquid or powder form) in a cavity. The mold is then closed and indexed into an oven where it and its contents are brought up to the molding temperature. As the mold is heated, it is rotated continuously about its vertical and horizontal axes. This biaxial rotation brings all surfaces of the mold in contact with the plastic material. Step 2. Heating and fusion of resin
The mold is rotated within the oven until all the plastic material has been picked up by the hot inside surfaces of the cavity and densifies into a uniform layer. While continuing the rotation, the machine moves the mold out of the oven and into the cooling chamber. Step 3. Cooling before unloading
Air, or a mixture of air and water, cools the mold and the layers of molten plastic material. This cooling process continues until the part has cooled sufficiently to retain its shape. The machine then indexes the mold to the loading and unloading station. Step 4. Unloading/Demoulding
When the polymer has cooled sufficiently to retain its shape and be easily handled, the mould is opened and the product removed. At this point powder can once again be placed in the mould and the cycle repeated. | | |
Advantages - Very large parts can be manufactured
- Chemical waste and storage tanks up to 2,500 gallons
- Minimum design constraints
- Very low tooling costs
- No polymer weld lines
- Stress-free products
Disadvantages - Slow cycle times mean high piece prices
- Narrow range of raw materials.
- Some geometrical features (such as ribs) are difficult to mold
Applications Rotational molding is incredibly versatile, able to handle a vast variety of shapes and sizes. Many parts cannot be readily produced by any other method. INDUSTRIAL & COMMERCIAL – Agriculture, Health & Science, & Point of Sale (POS): Specialty tanks and containers for fuel, water, and chemical processing - Livestock feeders
- Drainage systems
- Food service containers
- Instrument housings
- Vending machines
- Highway barriers and road markers
CONSUMER PRODUCTS – Recreational, Special Application, Toy, & Transportation: - Boats and kayaks
- Childcare seats
- Light globes
- Tool carts
- Planter pots
- Playing balls
- Playground equipment
- Headrests
- Truck/cart liners
- Air ducts
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Molding