Blog 40: Introduction to Rotational molding

Introduction:

A minor portion of the plastics industry involves rotational molding, commonly referred to as Rotational Molding or roto casting. It started in the late 1950s with polyvinyl chloride (PVC) plastisol molding and advanced to polyethylene, where it mostly remained. This area of the plastics industry touches nearly every market. It has a wide range of products, from straightforward bulk storage containers to intricate automotive, medical, and aerospace applications, and it gives a great deal of design freedom and product scalability. No other process is as conducive to hollow, complicated structures as rotational molding. Small components like medical pipette bulbs can be produced in a similar way to how big boats are. As rotational molding is acknowledged by a larger number of designers and engineers, complex products like fuel tanks and pieces for aircraft ducting are becoming more prevalent.

Over the past 20 years, manufacturing technology has advanced substantially, and more and more businesses are using the versatile method known as rotational molding (or Rotational Molding). By pouring plastic powder to a shell-like mold, rotating, and heating it simultaneously, it is mostly used to create hollow pieces. The powder fuses during this phase creating a liquid layer devoid of bubbles that assumes the shape of the interior mold surface. The hollow portion is then taken out after cooling the bulk. As shown in the image below, this technology is frequently employed to make tanks, but it can also be used to create intricate toys, recreational vehicles, and much more.

This is an economical way to make big plastic pieces. A heated, gently rotating mold is filled with resin and rotated both vertically and horizontally. The resin on the interior surfaces of the mold is distributed and fused by simultaneous heating and rotating. A seamless product with uniform wall thickness and more material in the corners to absorb shocks and stresses where they occur most frequently is the end result.

How RIM works

The procedure starts by adding a powdered polymer to a metal mold (mostly polyethylene). The final shape can also include metal components that were inserted into the mold.

The mold is rotated on two axes inside an oven chamber. Depending on the material being utilized, the temperature is maintained between 500°F and 680°F (260°C and 370°C). The melted substance sticks to the interior surface of the mold during this procedure.

The produced object is removed after the molten material has cooled. The operation is then repeated as often as required by reloading the mold.

Trimming is used to remove the mold split (a line or ridge that emerges where the mold separates). Apertures, holes, screws, and slots are a few things that can be introduced at this stage.

  • Loading material inside the mold
  • Melting the material inside the mold
  • Cooling the mold
  • Post-processing

Materials that are used for Rotational Molding

The rotational molding industry first saw the first polyolefin powder, a low-density polyethylene, in a public demonstration in 1961. For Rotational Molding, polyethylenes continue to be one of the most often used materials due to their processability, variety of characteristics, and affordability. Major producers of thermoplastic raw materials, such as rigid polyvinyl chloride, polypropylene, nylons, and polycarbonate, have looked into specially designed powders for rotational molding. These powders may be foamed or fiberglass-reinforced. Many rotational molders have expanded the range of materials available for this process by using custom grinding services or having their own in-plant grinding facilities in addition to the raw material suppliers.

About 80% of the materials used in rotational molding belong to the polyethylene group, with the most popular one being:

  • Cross-linked polyethylene (PEX)
  • Low-density polyethylene (LDPE)
  • Linear low-density polyethylene (LLDPE)
  • High-density polyethylene (HDPE)

Other elements, such as PVC plastisols, nylons, and polypropylene, are also present in these materials. The essential specifications for these materials are:

  • High thermal stability
  • Sufficient resistance to oxidation
  • Ductility (easy flow through all the cavities of the mold)

What rotational molding is used for

Most industrial tanks and containers are made using Rotational Molding. Below is a complete list of all the additional applications:

  • Septic tanks 
  • Chemical storage tanks
  • Oil tanks 
  • Fuel tanks
  • Water treatment tanks 
  • Shipping tanks
  • Traffic signs
  • Containers
  • Toys and Leisure
  • Materials Handling
  • Marine Industry
  • Tanks for storing water and chemicals – up to 50,000 liters
  • Material handling products such as containers, crates, pallets, and insulated fish and cooler boxes.
  • Environmental products include litter bins, road cones, bollards, traffic dividers, and road signs.
  • Floats, buoys, and pontoons.
  • Automotive products such as truck mudguards, ducting, diesel fuel tanks, tool boxes, and tractor dashboards.
  • Kayaks, canoes, and boats
  • Products for the outdoors such as garden planters, water butts, and furniture.
  • Toys and playground equipment.

Advantages of Rotational Molding

Rotational Molding boasts a number of advantages over comparable molding methods:

  • Low-cost tooling: Since casting is the method used, there is no pressure. This indicates that molds are affordable and that a modest volume might be profitable. The minimal initial cost of Rotational Molding makes it particularly alluring if you have a brilliant idea for a new product but are unsure of how many you will sell or simply want to produce a small number of items. Low operating pressures enable the creation of Rotational Mold tooling from inexpensive metals like aluminum.
  • Consistent wall thickness: Rotational Molding produces walls that are consistently thick, with the corners typically being thicker. This improves 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. The walls are uniformly coated by the mold’s continuous rotation during both the heating and cooling phases.
  • Complex double-walled open containers can be made using double-wall construction without further processing.
  • High durability: pieces are molded as a single, solid piece, doing away with the need for weakening joining methods like welding and joint fabrication.
  • High durability: pieces are molded as a single, solid piece, doing away with the need for weakening joining methods like welding and joint fabrication.
  • High strength: Rotational Molding makes corners thicker, lowering the likelihood that they will fail at stress-concentration places.
  • Aesthetics: Surface finishes like fine-detail textures, logos, emblems, and letters are easily accommodated by the soft metal utilized for the Rotational Mold tooling.
  • Make Complex Forms: Making complex shapes is simple. Rotational Molding readily accommodates production complexities such as stiffening ribs, molded inserts and different surface textures.

Disadvantages of Rotational Molding

As with any plastic molding process, Rotational Molding has its disadvantages:

  • Long cycle times: Rotational Molding can take up to three hours to complete one part at eight rotations per minute.
  • Limited material choices: Poly-based resins are the only materials that can be used for Rotational Molding since they can easily be transformed from granules to a fine powder and have great thermal stability.
  • Expensive cost of raw materials: Prices of materials are high because of the need for high levels of thermal stability, the price of necessary additives, and the price of powdering the materials.
  • Lack of repeatability causes quality problems since the soft metal used in Rotational Mold tooling needs to be repaired or replaced after 3,000 cycles.
  • High labor costs: Rotational Molding requires more labor intensity than comparable manufacturing processes because mechanization and automation have not yet been attained for this process.

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