Rotational Molding:
New Resins, Integral Designs are Keys to Future Growth

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Rotational Molding

By Mark Shortt
Editorial Director, Design-2-Part Magazine

As the industry becomes more technically sophisticated, it struggles to reconcile its growth with a lack of public awareness and the need for a greater variety of moldable materials.

To many product manufacturers who are aware of its advantages, rotational molding is a dynamic and versatile method for producing one-piece plastic parts with complicated curves, smooth contours, and surface finishes ranging from matte to high gloss. Unusually shaped, custom parts-ranging in size from small ping-pong balls and ear syringes to 22,500-gallon liquid storage tanks-are routinely made by this relatively low-cost process, also known as rotomolding. But a disproportionately large number of product designers have never designed a part for rotational molding, and for them, actually seeing the technique for the first time can be an eye-opening experience.

"We love to get our first rotomolding job with a design group or OEM, because invariably they end up liking the process and using it for other parts," said Pat Long, president of Formed Plastics, Carle Place, N.Y., and president of the Association of Rotational Molders (ARM), Oak Brook, Illinois. "The struggle is to get somebody to try it for the first time. It just doesn't have the visibility of some of the other plastic processes."

In commercial use since the mid-1940s, rotational molding has long been recognized as a technique that is suited to producing relatively large, hollow, seamless parts-such as tanks-that can be either partially or totally enclosed. But the process has evolved to the point where numerous advanced molding techniques can be combined in one operation to produce more sophisticated parts, such as reinforced, multi-layer moldings of solid and foamed materials. By giving molders continuous feedback on the molding cycle, advanced diagnostics and computer controls now provide molders with unprecedented opportunities to influence the process. Today, rotational molding is being used increasingly to mold geometrically complex parts for nontraditional markets, such as unusually shaped ducts for aircraft heating and cooling systems.

Although the industry is growing at a rate faster than any other plastics processing method, its lack of visibility has inhibited the development of new materials for rotational molding. Without seeing a clearly demonstrated market need, many resin manufacturers have been reluctant to commit funds to research and development. "Our volume and usage of material is not as great as, say, for injection molding, vacuum forming, or extrusion," said Bill Burlingham, president of El Monte Plastics, South El Monte, California. "The dollars go to where the payoff is best."

Today, roughly 85% of the resins used by rotational molders consist of polyethylene. Polyvinyl chloride (PVC), polycarbonate (PC), and nylon constitute the bulk of the remaining 15%. However, specially compounded materials, such as fluorocarbons, cellulosics, styrenes, and polyurethanes, are also occasionally used by rotational molders.

Originally limited to vinyl plastisols, rotational molding began to penetrate new markets in the early 1960s as heat- and chemical-resistant polyethylenes became commercially available in a wider range of densities. These low- and high-density polyethylenes gave rotomolders entry to demanding agricultural and industrial applications-such as tanks and drums-that had previously used metal.

Markets for large fuel and chemical storage tanks became accessible to molders in the early 1970s with the introduction of cross-linkable polyethylenes, which provided improved chemical resistance and cold temperature impact properties. These properties were nearly matched by linear low density polyethylenes, which were introduced later that decade and popularized further in the 1980s. The '80s also witnessed the growing use of nylon, polypropylene, polycarbonate and other non-polyethylene resins by rotational molders.

The Rotomolding Process

The basic concept of rotating a mold to promote even distribution of a heated material over the inside surface of the cavity was first applied in the 19th century. And some of the machines built for the industry's early commercial moldings more than 50 years ago are still in use today. These facts have contributed to a common misconception of the process as a low-tech, easily understood technique.

Rotational molding consists of three stages: loading the resin into the mold, heating and fusion of the resin, and cooling and unloading the mold. The multi-arm turret machine, which can accommodate several molds at once, is the most commonly used processing equipment. It consists of an oven, a separate cooling chamber, and a turret for moving, or indexing, the arms and molds through the stages of the process.

The process begins with the placing of pre-measured liquid or powdered thermoplastic resin into a hollow mold. After the mold is closed, the molding machine moves the mold into an oven, where temperatures as high as 400C (750F) heat the mold and plastic to the required molding temperature.

As the mold is heated, it rotates continuously about its vertical and horizontal axes. The molding machine makes use of a series of gears and sprockets to achieve this biaxial rotation. Continuous rotation ensures that all surfaces of the mold make contact with the heated plastic material, which becomes evenly distributed over the inside surface of the mold.

Unlike centrifugal casting, which uses centrifugal force to push the plastic against the surface of the cavity, the rotational molding process is characterized by very slow rotation speeds. According to The Society of the Plastics Industry's Plastics Engineering Handbook, rotation speeds range up to 40 rpm on the minor axes and 12 rpm on the major axes. For symmetrically shaped objects, a 4:1 rotation speed ratio is generally used to ensure uniform distribution of the melt. The molding of unusual configurations, however, requires a wide variety of ratios, the Handbook states.

While in the oven, the mold continues to rotate until the hot inside surfaces of the cavity have picked up all of the plastic material, which densifies into a uniform layer of melt. As the mold continues to rotate, the machine moves the mold out of the oven and into a cooling chamber, where either air or a mixture of air and water is applied to the mold and the plastic part. When the part has cooled sufficiently to retain its shape, the machine moves the mold to the loading and unloading station, where the mold is opened and the part removed. The part can then be placed in a cooling fixture to help control shrinkage. At this point, the process can be repeated.

Types of Molds Used

Cast aluminum molds are, by far, the most commonly used molds in the industry. Rapid heat transfer and low cost are among their advantages. However, they are porous and can be damaged easily. Molds made of machined metal, especially aluminum, are occasionally used for applications that require extreme precision. Machined aluminum molds are known to yield parts that are free of surface porosity or voids. They are, however, usually limited to special cases because of their high cost.

Sheet metal molds are most often used when only one cavity is required for the production of large parts. Many of these molds are fabricated in-house by custom molders, who simply weld together mold sections consisting of steel, aluminum, or stainless steel sheets. Their principal advantages include light weight and a cavity wall that is uniformly thin.

Although less common, electroformed nickel molds provide an advantage that would be difficult to achieve through other techniques-the replication of fine surface details of wood or leather graining. According to Mr. Beall, electroformed cavities are often used to make hollow undercut cavities for the molding of flexible materials, such as PVC.

Materials Requirements

The process requires materials that are available as a liquid, or capable of being efficiently ground into a fine, 35-mesh powder that flows as if it were a liquid. Resins with unfavorable flow characteristics are difficult to mold, and are rarely selected.

Because rotational molding is a low-pressure process, unlike injection molding, forces are not available inside the cavity to push the material together. Therefore, molding materials should be able to adhere to the hot surface of the cavity and fuse together without pressure, according to Glenn Beall, president, Glenn Beall Plastics Ltd., Libertyville, Illinois. A product designer and author of Rotational Molding: Design, Materials, and Processing, Mr. Beall chairs the Society of Plastics Engineers' Rotational Molding Division.

According to Mr. Beall, molding materials should also be able to retain their physical properties after being exposed for 20 to 60 minutes to oven temperatures ranging from 260C to 400C (500F to 750F).

Advantages of the Process

Rotational molding is often chosen to produce small and large parts of unusual shape that cannot be produced as one piece by other processes. Uniform wall thickness, even in unusually shaped parts, is considered a prime advantage of the process.

"It's the one process that will make a large, hollow object in one piece," said Bill Burlingham, of El Monte Plastics. "When people think of a large vessel, they think of injection molding or vacuum forming it and gluing it together," Mr. Burlingham explained. "Blow molding can do it, but a disadvantage of blow molding that rotational molding doesn't have is that the part gets thin at the corners and extremities. In rotational molding, the thickness is even all over, anywhere you measure."

Because it is a low-pressure process, rotational molding requires minimal mold strength. This favors the use of low-cost molds to produce large or complex parts. Inexpensive molds, in turn, make the process equally suitable for production of prototypes and both small and large quantities of production parts. They also permit frequent styling changes, which can be an important advantage in niche marketing.

Low processing pressures also tend to produce relatively stress-free parts, a necessity in large, load-bearing applications that require stress-crack resistance.

Rotational molding can be used to simultaneously process several different parts and colors. Other advantages of the process include its ability to produce double-walled parts for added rigidity; increased thickness on outside corners for added strength; production of minimal scrap; and the capacity for integrally molding in graphics that do not scratch off or peel away.

A Booming Industry Catches its Breath

The North American rotational molding industry reported total resin sales of $1.65 billion for the year 1997, an increase of 15% over the previous year, according to the lastest survey conducted by the Association of Rotational Molders (ARM). During the same year, rotational molders in North America used some 495 million pounds of resin, the survey indicates.

Although still registering healthy gains, the industry slowed somewhat in growth during 1998 and the first part of 1999. Its growth during this period has been in the range of approximately 8 1/2-10%, according to unofficial estimates. One reason for the slower growth rate, Mr. Beall said, is that some of the larger mass merchandisers have dropped large rotationally molded items, such as playground equipment, from their product lines. Once a major market for rotational molders, these companies believed that the cost per square foot of floor space occupied by such huge merchandise was too high in relation to the income it generated.

"The other thing is, this industry had to stop somewhere and catch its breath," Mr. Beall said. It had been growing too fast, at a rate of 10-15%. There weren't enough technically qualified people to keep up with the growth."

"But let's go back to the 10-15% growth years. What they did was they spawned enough business to get people excited about doing research and development to develop new technology.

"The things that happened were very important improvements in machinery, including microprocessor controllers, which were slow to come to rotational molding. But now they're here, and they're being adopted the molders."

Major Technological Advances

In recent years, the industry has benefited from numerous advances that have lent greater sophistication to the process. A major technological breakthrough was made in 1990, when Roy Crawford, working with Paul Nugent at Queen's University, Belfast, Northern Ireland, first introduced the temperature-sensing transmitting unit known as the Rotolog.

The Rotolog, which mounts directly on the mold and rotates right along with it, transmits electronic signals that communicate the temperature of the material inside the mold to a receiver outside of the oven. This information is processed and printed out by a computer in real time. Although not suited for ongoing use in day-to-day manufacturing, the Rotolog has become a valuable diagnostic tool. It has also shown promise as a means for controlling the process in the future, Mr. Beall said.

Until the introduction of the Rotolog, it had been difficult for molders to obtain an accurate reading of the mold temperature. "Think of that mold flopping around out there in mid-air, going through all those gyrations," Mr. Beall explained. "You couldn't put a wire to it to know what temperature that mold actually was, like you can with injection molding. Molders controlled the process by controlling the temperature of the oven. But we knew that wasn't the same as the temperature of the material."

But if molders had found it difficult to accurately record the mold temperature, they were at a greater loss when it came to actually looking inside a rotating mold. That all changed in the early '90s, when rotomolders welcomed the development of a technique for mounting an insulated video camera inside the mold.

Randy Syler of State Industries, Inc., Ashland City, Tenn., developed the technique, which calls for mounting the video camera and a light source in an insulated box, which is then mounted on the inside of the mold. The camera focuses on different locations in the mold and lets molders observe the slow melting and fusion of the powder. Primarily used as a diagnostic tool, the camera is yet another opportunity for analyzing and optimizing the process.

Another significant advance is the development of an infrared temperature-sensing device that communicates directly with machine control to permit closed-loop molding. Mounted in the oven and in the cooling chamber, the infrared unit picks up the temperature of the mold as the mold rotates. It is said to be capable of operating in the heat of the oven for an indefinite period of time, and is especially useful with unconventionally shaped molds (such as for canoes), where temperatures vary in different areas of the mold. A computer program processes the various temperature readings, analyzes the data, and reports it back to the molder as one temperature. Dr. Paul Nugent, Remcon Plastics, Inc., Reading, Pennsylvania, developed the infrared temperature-sensing device, which becomes available from Ferry Industries this year.

"You now have continuous feedback. You can actually adjust your cycle in mid-cycle," Mr. Beall said.

Today, rotational molders also have the benefit of microprocessor controllers that are capable of controlling all phases of the process-heating, cooling, and speed and ratio of rotation. Similar to those used by injection molders, the controllers remove much of the human error from the process. They also allow molders to stop the mold from rotating in one direction, and reverse the direction of rotation.

One of the more popular applications of advanced technology has been the use of in-mold graphics, a process that integrates printing with rotational molding to permanently embed graphics and lettering into the wall of a rotomolded part. Mold-in graphics, a patented process of Mold In Graphic Systems, Clarkdale, Arizona, grew out of a rotational molder's unsuccessful attempts to apply graphics onto naturally resistant, oily polyethylene products. The process is widely used by rotomolders for applications where graphics need to withstand tough, everyday abuse from exposure to UV rays, extreme heat or cold, water, chemicals, or abrasion. These include refuse containers, water and chemical storage tanks, real estate signs, kayaks, and surfboards. Because they do not scratch or wear off, molded-in graphics are being used increasingly for corporate logos, instructional messages, warnings, and identification purposes.

Unlike print-on graphics, which can fade away, molded-in graphics are embedded into the resin during molding. According to the company, the process works on virtually all types of molds, ranging from cast aluminum to fabricated steel or aluminum molds.

Another advance has been the introduction of single-shot foams. Previously, foamed parts had been produced by molding a layer of solid material, then depositing the foam material into the mold to produce double-walled parts. Now, molding a part with a solid polyethylene skin on the outside and foam on the inside can be done several ways. In the most common, Mr. Beall said, the two materials that go into the making of the outside skin and the inside foam are mixed together. The material with no foaming agent, which will become the outside surface of the part, has smaller powder size. These smaller particles melt quicker, stick to the mold first, and become the outer skin. As the mold continues to heat, the thicker pellets become hot enough to melt, a process that activates the foam that will become the inside layer.

According to Mr. Beall, researchers are working to develop a one-piece chair cushion that has foamed material on the inside and a soft, pliable, elastomer-type material for the outer skin. "It's fascinating when you stop and think about it. Now think about all the shape capabilities that rotational molding has, and think about the contoured, highly styled seats in the transportation industry, and you begin to see some wonderful opportunities."

Limitations

Advances in automation, reduced cycle times, and a greater variety of moldable resins are at the top of the list of industry needs.

"The big limitation is the number of materials that can be run," said Pat Long. "We've gotten as far as we can with the materials available for rotational molding," he stated. A flame retardant ABS, he said, would open additional markets to rotational molders because of its favorable stiffness and heat distortion properties. The Association of Rotational Molders, he said, has been trying to encourage the development of new materials through research and development activities at universities such as Brigham Young University and the University of Akron.

Development of methods that would reduce cycle times by permitting quicker opening and closing of molds would also help. "What's holding that back is that it's a real tough environment, because you're constantly taking these molds and heating them up, cooling and heating them up, and cooling and heating them up," said Mr. Long.

A Need for Integral Designs

Despite the recent slowing of the industry's growth rate, Mr. Beall disagrees with those who interpret it as a sign of long-term decline.

"I don't believe it is practical to expect this industry to continue to grow at 10-15% per year. What I do know is that the use of plastic itself, over the last four years, has grown at the average of approximately 8.5% per year. That's more than double the gross national product."

Mr. Beall anticipates that the plastics industry will continue to grow at a rate faster than the GNP as long as consumer purchasing power-already at an all-time high-continues to rise. He expects rotational molding to grow right along with the plastics industry, but at a faster rate than the older, more established processes of injection molding, thermoforming, and extrusion. The reason, he said, is that whole market segments have not yet discovered rotational molding. When asked to name these market segments, he declined to give specifics, but offered a roundabout answer.

"Many people think of rotational molding as the ideal process for producing large tanks," he explained. "And it is, but we're not limited to tanks. The trick today is to use it to make a tank, but at the same time, combine all the other features around the product with the tank."

For an example, he described a housing for a large industrial drill that is currently being produced. The hollow housing doubles as the fuel tank, which, in conventional drills, is molded onto the side of the tool. It also incorporates the drill handle and mounting features for the gasoline engine.

"That's what will be driving us in the future, as people combine these things together, instead of just thinking of it as a simple tank," Mr. Beall said.

In a recent edition of Rotational Molding News, a newsletter of the Society of Plastics Engineers' Rotational Molding Division, Mr. Beall discussed possible automotive uses of rotationally molded parts. One of the more interesting possibilities is a two-seat electric car with an unpainted, rotationally molded polyethylene body, shown at last fall's K '98 Exposition in Dusseldorf, Germany. Although not yet commercialized, the car is rumored to be currently undergoing tests in the U.S. market.

"Just let your imagination wander for a moment about what could be done with a rotationally molded car body. Those same body components could incorporate the fuel, radiator overflow, and windshield washer tanks. Heating and cooling ducts, fuel and electrical conduits, arm rests, glove compartments, and consoles could all be molded in. The weight would be reduced, and that is critical in an electric-powered car."

Deciding Whether to Outsource

For product manufacturers who seek to reduce the number of parts and assembly operations required by their products, rotational molding offers the advantage of producing one-piece parts. It can also appeal to engineers who are finding it difficult to justify the higher tooling costs of other plastics processing operations. For many companies, the question of whether to purchase rotational molding equipment for in-house operations or contract the work to a job shop is easily decided.

Sometimes, however, the decision rests on whether the product manufacturer has people on staff with the skills needed to mold specialty materials, such as nylon. Although most OEMs are likely to be familiar with the process of molding polyethylene, many lack the necessary knowledge and capabilities for molding the other materials used in the process.

"If you don't have prior experience with nylon, rather than try and learn how to do it yourself, it's easier just to subcontract it," said Mr. Beall. Similarly, rotomolding jobs that involve materials such as polycarbonate (PC), polyvinyl chloride (PVC), fluorocarbons, cellulosics, styrenes, and polyurethanes are often subcontracted because only a few molders work with each of the materials, Mr. Beall said.

Another factor that has encouraged outsourcing is the small number of product designers who have the knowledge and experience to design a rotationally moldable part. Aware of the shortage of experienced designers, many custom molders have responded by providing a design service for their customers. Many of these molders also offer secondary operations: machining holes or threads that are not molded in, as well as finishing and decorating.

Some OEMs, such as Boeing Commercial Airplane Group, Seattle, Washington, do in-house molding and outsourcing. In Boeing's case, most of the rotational molding is done in-house. At its manufacturing plant in Seattle, the firm devotes some 30,000 square feet to rotational molding of nontraditional parts, such as air conditioning ducts and connectors that are used in various environmental control systems. According to John Moser, technical operations engineer, injection and blow molding are not conducive to the smaller production quantities that are typical of aircraft parts. The cost of blow molding components, he said, tends to be high because of tooling costs and tool changes.

Boeing uses machined aluminum molds almost exclusively because of the tight tolerances required of its parts. "We have really super-tight tolerances on our parts compared to traditional rotomolded products. Typically, we hold a tolerance of 0.030 on a piece four feet long."

However, the firm also works with a limited number of qualified partners and suppliers (including Formed Plastics, Carle Place, N.Y.) who meet its strict process specifications. Certification of its suppliers involves many complicated issues, and on the average, most rotational molders are "not anywhere near being qualified" to provide the type of work that Boeing seeks, according to Mr. Moser.

What Contract Molders are Doing

The Niland Co., El Paso, Texas, provides rotational molding of contract architectural products for the lighting industry. The firm manufactures rotationally molded polyethylene globes for streetlights, as well as air ducts, dashboards, and cab storage compartments for the heavy truck industry. It also molds polycarbonate globes for high-heat applications. Niland has developed a 108-inch-long air duct, molded in two pieces and put together for Greyhound Bus. The duct reduces the number of parts that would required if it were molded in metal, and is said to be stronger, lighter, and quieter than a comparable metal part.

Tom Niland, president of the company and a past president of the Association of Rotational Molders, says that his company has improved shrink rates on complex parts by applying small amounts of vacuum or pressure to molds during the molding cycle. Mr. Niland said that molders can deter shrinkage, one of the main impediments to maintaining precision tolerances in rotationally molded parts, by applying vacuum at the initial stage of a molding cycle, or a small amount of pressure during the final stage. According to Mr. Niland, jobs that were once considered impossible are now "fairly routine" as a result of the vacuum/pressure method, which has also made possible reductions in cycle time and part cost, he said.

El Monte Plastics, a custom rotational molder since 1968, produces items such as plastic holding tanks for recreational vehicles, eight-foot-wide bumpers for mobile homes, and automated trash barrels for municipalities. One of the firm's interesting moldings is a cylinder-shaped, polyethylene pool filter housing, with 1/4-in. wall thickness, that is designed to withstand high water pressure.

At its 40,000-square-foot facility in Porterville, California, Quadel Industries, Inc., has capabilities for molding tanks with capacities up to 3500 gallons. The firm also devotes some 12,000 square feet of floor space to rotational molding at its plant in Coos Bay, Oregon, where it molds tanks up to 1300 gallons. Quadel is a full-service molder that builds its own ovens and offers a design service in addition to its rotomolding operations. Projects include air plenums, electrical boxes, and trays for Monaco Coach. One of its largest items is the Bear Den, an underground tornado shelter.

Information Resources

The Association of Rotational Molders (ARM) has created an on-line, searchable database that connects users to its 460 member companies throughout the world. In addition to listing products and services, the database has more than 75 links to member companies' web sites or e-mail. ARM also offers a comprehensive range of educational literature, including a troubleshooting manual, a listing of rotational molding equipment, and a glossary of terms.

The Rotational Molding Division of the Society of Plastics Engineers provides a forum for the exchange of technical information on rotomolding.

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