Words of Wisdom: Rotational molding, then and now

In 1868 John Wesley Hyatt created the first manmade plastic. That thermoplastic material was sold under the trade name Celluloid. This discovery was a landmark event that led directly to the evolution of the giant plastics material manufacturing industry.This was an important material, but in its original form it was only useful as a coating. Celluloid could, however, be cast into sheets and that led to the development of blowmolding, thermoforming, and compression molding. These molding machines were produced in conjunction with the Charles Burroughs Company. Burroughs is now considered to be the father of the large plastics processing machinery business. The availability of these processing machines opened markets for celluloid, leading to financial success and worldwide recognition for Hyatt.

In the majority of instances a new material precedes the development of the equipment required to convert it into a commercially acceptable form. Rotational molding is an exception. The rotational molding process uses biaxial rotation and heat to produce hollow one-piece plastic parts. This molding process existed long before the introduction of plastic materials.

The first recorded use of a process that resembled rotational molding was a British patent issued to R. Peters in 1855. The Peters? patent described the use of heat and biaxial rotation to mold hollow metal artillery shells. It would be 91 years before the plastics industry adopted this molding process. Over the years the molding equipment evolved and found use in the production of hollow metal, wax, plaster of Paris, chocolate, and rubber products. At the dawn of the 20th century, the use of rubber dwarfed the fledgling plastics industry. The rubber molders refined rotational molding and kept it alive awaiting the arrival of plastic materials.

That long-awaited material appeared in 1946 when Union Carbide introduced a moldable grade of liquid polyvinyl chloride (PVC). This was the material the industry had been waiting for. The plastics industry adopted the rotational molding of PVC and quickly replaced many hollow rubber parts. PVC is a versatile material. There is, however, a limit to what can be achieved with only one material.

A rotationally moldable polyethylene (PE) was marketed in 1961 by U. S. Industrial Chemicals (now Equistar Chemicals). Pulverized into a fine powder, PE flowed like a liquid. It sintered onto the cavity in uniform thicknesses. It had the chemical resistance to be used for storage, shipping, and processing tanks. PE was easy to mold. Best of all, it was relatively low in cost.

Processors quickly put PE to use in applications that could not be satisfied with PVC. The molding industry prospered and other material manufacturers entered the market with nylon, polycarbonate, polypropylene, and the fluorocarbons. The rotational molding industry grew steadily through the 1960s and 1970s with explosive growth in the 1980s and early 1990s.

Process Description

Wheeled, roll-out refuse containers are a significant market. To rotationally mold such a container, a mold defining that shape is mounted on the arm of a machine capable of biaxially rotating and moving the mold through the four phases of the process (Fig. 1).

Plastic material, in a liquid or a powder form, is placed in the mold?s cavity (Fig. 1A). The machine simultaneously rotates the mold in two directions and moves it into the oven (Fig. 1B). The mold becomes hot and all the material adheres to or sinters onto the inside surface of the cavity. While it continues to rotate, the machine moves the mold out of the oven and into the cooling chamber, where the plastic is cooled to the point that the formed part will retain its shape (Fig. 1C). The machine then moves the mold to the open station. The mold can then be opened and the molded part removed (Fig. 1D).

The Rotational Molding Business

A major development in the business of rotational molding took place in 1976 with the creation of the Assn. of Rotational Molders Int?l. This full service trade association has 353 company members in 55 countries. The combined financial support of its members has allowed the Association to promote the advantages of the process. As a result, the industry grew two to three times faster than the GDP in the 1980s and early 1990s. The most recent analysis of the overall processing industry was published in 2000 by Plastics Custom Research Service (PCRS). The future growth of rotational molding was projected at 5% per year. The only processes with greater potential were blowmolding and structural foam at 8% per year. Injection molding received a rating of only 3.5%.

A 2003 analysis of rotational molding by PCRS indicated that?on the basis of the amount of plastic material consumed?the industry?s growth was 9.7% per year from 1991 through 1996. This impressive growth rate dropped to an average of 3.2% during the next five years. The reasons for this decline were the worldwide economic slow-down and the decline of the toy business. In 1995 toys accounted for 42% of rotational molding?s market. By 2002 that number had declined to 20%. As an aside, it is now estimated that 80% of all of the toys being sold in the U.S. are imported.

Rotational molding has weathered the current economic slow-down better than many other molding industries. There has been some consolidation, but it has not suffered the number of bankruptcies and plant closings that have decimated the injection-molding and tool-making industries.

Today the rotational molding industry is going through major changes. The industry is experiencing the same increased customer expectations as other industries. Lower cost, higher quality, and quicker delivery are now standard. Some molders are adopting the latest technologies and others are not. Many are now ISO certified. Some are embracing lean manufacturing. Most are providing additional free services such as product design, warehousing, and assembly. The progressive molders, who are adopting the new ways of doing business, will undoubtedly be the survivors when the global economy settles into whatever the new normal turns out to be.

The entire manufacturing community was hurt by the last recession and outsourcing. Rotational molding fared better than the other plastics processing industries, due in part to the entrepreneurial spirit of this relatively new industry. While some were waiting for the good times to return, true entrepreneurs were out hustling for new projects. Thomas Edison once offered the following words of wisdom that are apropos to the current state of the rotational molding industry: "Everything comes to him who hustles while he waits."