International Transportation Cooler, Coca-Cola Company, GA

Materials & Manufacturing Processes
RotoMold 
Spin Cast Mold
 

Rotational Molding  Rotational Molding or Roto Molding is a high temperature, low volume, low pressure thermoplastic forming process that applies heat to a closed clam shell mold representing the outside surface walls of the part, filled with a powder or liquid or pellet polymer resin and biaxial rotation (i.e. rotation on two axes) to produce hollow, one piece parts.   The rotational mold cycle allows for only one or two cycles or part an hour leading to relatively, higher piece price costs.  Advantages include a relatively low initial tooling cost usually machined aluminum or cast aluminum, combined with the capability of manufacturing large, hollow parts such as large oil or chemical tanks, kayaks, point of purchase displays, frames, cases, furniture, carts, dispensers, park equipment, among others.   Material waste is limited and excess material can often be re-used or recycled, making it an environmentally viable manufacturing process.

The rotational molding process cycle consists of four steps:

1.    Opening the mold, and loading a measured quantity of polymer (usually in powder form) into the mold.

2.    Heating & rotating the mold in an oven or self heated mold, (around 300 – 400 degrees), until all the polymer has melted and coating to the mold interior walls.   Molding time is critical. Too long or too hot and the polymer will degrade, reducing impact strength. Too low a temperature or heating time, the polymer melt may be incomplete and leave uncovered or thin areas or bubbles.

3.  Cooling the mold, usually by forced air or fan is the most time consuming part of the process.  May takes 5 – 15 minutes.  The part will shrink on cooling, away from the mold, allowing for easy removal of the part. Cooling rate is also important as rapid cooling, for example by a water spray, would result in cooling and shrinking at an uncontrolled rate, producing a warped part.

4.    Removal or ejection of the part.  Repeat Steps 1-4 (approximately 15 – 30 minutes)

Rotational molding offers many benefits over other types of molding such as injection molding, blow molding and thermo-forming:  Rotationally molded parts are only not size restricted by the size of the tool.  Because it is a low pressure, hollow or twin wall process there is no interior core to manufacture, it results in lower tooling investment, when compared to injection or blow molding.  Also with roto molding, offers a pretty consistent wall thickness with extra material build up in the corner for increased corner drop impact strength and one-part construction.    Production – tooling is 6-9 weeks compared with lead time injection molding of 3 6 months. 

Because of the relative simplicity of the tooling, minor alterations can be made to an existing mold to accommodate your changing production needs  do smaller runs on products as small as 10 pieces. Based on the advantages of plastics, rotationally molded products offer corrosion and chemical resistance including static, fire and weather-resistant.  The process allows for sharp corners including the use of internal molded in structure stiffeners and ultra-sonic welding together including other roto molded or metal parts to give limitless design opportunities.  Use on insert threaded fasteners and pad or screen printed graphics and sculpted or dimension graphics and logos is common. 

The Coca-Cola example above is 2 Part Rotomolded Design with a one piece, water based foam filled, rotationally molded product with a two piece, one part mold lid with molded in sheet metal panel graphics.  The interlocking lids allow easy access while meeting specified cooling requirements. The design is now produced in high volume and it is being used throughout Coca-Cola's international network