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ROTATIONAL MOLDING Introduction Rotational molding (also referred to as rotomolding or rotational casting) is a unique molding process. The heating, shaping, and cooling of the plastic all takes place inside the mold, with no application of pressure. This method dates back to the early 1900s. Today the rotational molding industry is one of the fastest growing segments of the plastics industry, with an annual growth of 10%. This is largely due to the efforts of the Association of Rotational Molders (ARM), an international organization that includes molders, equipment suppliers, professional consultants, and design firms, and the Society of Plastics Engineers (SPE), who have recently formed a rotomolding division. Initially only vinyl plastisols were used for rotational molding; however, in the early 1960s, polyethylenes, the first polyolefin powders, were employed (1). This created the opportunity for molders to enter markets where vinyl products could not compete. In the early 1970s cross-linkable and modified polyethylenes opened up more new areas, especially in the large-tank market. In the mid-1970s, linear low density polyethylene, a major development in resins, was formulated. In the 1980s nylon, polypropylene, and polycarbonate were introduced for rotational molding applications. The 1990s saw rotolining resins, materials that bonded to metal, and one-step foam resin systems that provided for a finished part with a rotomolded outer skin and a foam-filled core. The year 2000 ushered in single-site catalyst resins, some known as metallocenes as well as numerous compounders providing blends of resins for very specific end use requirements. Most resin is used as a ground powder, ranging in particle size from 20 to 120 mesh (125–840 µm). Liquids and small-diameter micropellets can also be rotomolded. Rotational molding provides a more uniform wall thickness, for both single- and double-wall construction, than other methods. Thick corners impart strength. Inserts, ribs, and undercuts are easily included (2,3). The range of designs is limitless. The ARM organization has published a design manual for rotational molding (4). This was the first publication to address the design guidelines required in the rotational molding industry. Since that time, numerous books have been published on rotomolding (5). Molds and tooling costs are lower than those of other processing methods, since channels for cooling water and resistance to a clamping force are not required. Different articles and colors may be molded on the same machine and in the same cycle. Quick mold changes are possible when several short production runs are required. Large, hollow products are conveniently made by rotomolding. The largest article molded to date is an 85-m3 tank. Trimming can be eliminated since very little flash is produced. Rotomolded parts are comparatively stress-free. Corner sections are thicker than with other processes, which increases strength. Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Undercuts, intricate contours, molded-in inserts, and double-wall construction are routinely included; wall thicknesses are uniform.

Process In most rotomolding operations the molds or cavities are filled with a certain amount of powder or liquid (charging the molds); the mold halves are pressed together by bolts or clamps, placed in a heated oven, and rotated biaxially. During heating the resin melts, fuses, and densifies into the shape of the mold. The molds are moved into a cooling chamber, where they are slowly cooled by air and water. After removal from the cooling chamber, the molds are opened and the finished articles are released. Many rotomolded parts are produced in the color of the plastic. For colored parts, suppliers provide a compounded colored resin; however, many molders blend a dry color into the resin—for this operation, addition and dispersion are critical. New developments in liquid colors allow for use at both the compounder and rotomolder. Nylon and polycarbonate may require drying before molding. Although scrap or regrind is not produced in large quantities, only a small percentage of regrind should be used with the virgin resin. The particle size of the resin is extremely important; 500-µm (35-mesh) powder is the standard of the industry, although coarser or finer grinds may be employed. Wall distribution is determined by the rotation ratio. The fewer rpms in a certain ratio, the more uniform the wall thickness. Resin, molds, and final application must be taken into consideration to establish the most efficient cycle. Equipment. The equipment used in rotational molding is simple; many variations are available. The most common type is the so-called carousel type (Fig. 1). This machine consists of a heating station or oven, a cooling station (frequently an enclosed chamber), and a loading and unloading station. A carousel has three to six spindles or arms where the mold or molds are mounted. Most carousels have the freedom to revolve in a complete circle. The spindles are mounted on a central hub and driven by variable motor drives. New control systems allow each arm to operate independently in movement and control of oven temperature and time. Microprocessors are incorporated in the control system. Computer simulation software can be utilized in prototyping and manufacturing. The ovens are usually fired with natural gas and equipped with blowers to distribute the heat throughout the chamber. Some ovens can be heated by oil or propane gas, but natural gas is preferred. A new composite mold technology is also available to produce rotomolded parts. This is done without an oven as heating and cooling takes place inside the mold. A fan provides forced air to the cooling station for the initial cooling, whereas a water system cools the molds and products. A spray mist is used for even cooling. The cooling station may or may not be enclosed. In an open-flame machine a mold is rotated on a single axis over an open flame. After the article is formed, the excess resin is discarded and the flame is turned off to allow for air-cooling. This process requires a longer cycle time, but the equipment is less expensive.

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Load−unload station Cooling station

Molds

Fig. 1. Carousel-type machine.

The shuttle-type machine (Fig. 2) is used for large products, such as tanks. A frame for holding a mold is mounted on a movable bed. The drive motors for turning the mold biaxially are incorporated in the bed, which is on a track that allows the mold and bed to move into and out of the oven. After the heating cycle is completed, the mold is moved into the cooling station, which is not enclosed, Another bed with a mold is moved into the oven from the opposite end. (Some shuttle machines will use a movable oven.) The clamshell type of rotational molder utilizes an enclosed area that serves as both the oven and the cooling station (Fig. 3). This machine employs only one arm; heating, cooling, and loading–unloading stations are all in the same location. Rock and roll machines are commonly used for kayaks and canoes. These machines do not rotate biaxially but will rock back and forth while spinning on

Door

Oven

Door

Station 1

Station 2

Mold

Mold

Cooling, loading, and unloading

Cooling, loading, and unloading

Fig. 2. Shuttle-type machine.

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Ascending upper oven section

Mold

Drive

Mold

Drive

Fold down access door

Fig. 3. Clamshell-type machine.

the longest axis. This can be accomplished over an open flame or a variation, which uses a rocking oven. Machine maintenance is of extreme importance. A weekly inspection, as well as a good inventory of spare parts, is necessary in case of breakdowns. Filling devices can be automated but are usually manually operated. This requires an accurate weighing device and a container for each of the resins. Robotic systems for loading/unloading of powder and parts are now making their way into this industry. Some resins, such as nylons, require a nitrogen atmosphere. The nitrogen is introduced through a channel in the spindle and connected to the mold with rotary hoses. Molds. The molds used in rotational molding are among the easiest and least expensive to fabricate; two-piece molds are standard, but three-piece molds are sometimes required to remove the finished products. Molds can be as simple as a sphere, or complex, with undercuts, ribs, and tapers. Design considerations include heat transfer, mounting, parting lines, clamping mechanisms, venting, and material stability. Mold makers commonly use 2-D and 3-D electronic files to design new molds. Cast aluminum is the most widely used mold material. Small to medium articles are molded with a cast mold. Cast aluminum has good heat transfer, and is cost-effective when several moldings of the same article are required. Cast molds can be porous, however, and are easily damaged. Sheet-metal molds are normally used for larger parts. They are easy to fabricate; frequently the sections of the molds only have to be welded together. Sheet-metal molds are cost-effective for large articles. Other molds, such as electroformed nickel molds, give a product

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(c)

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(b)

(d)

Fig. 4. Parting lines (a) stepped, (b) pin and bushing, (c) fabricated, and (d) tongue and groove.

with very fine detail. Vapor-formed nickel molds also give good detail but are more expensive. Parting Lines. Each mold must have two or more sections, requiring good parting lines (Fig. 4) for a close fit and minimum flash. Mounting. The molds are mounted on the spindle or the arm of a rotational molding machine. Large sheet-metal molds are easily mounted by bolts or simple clamping systems. Several small to medium aluminum molds can be mounted on spindle or arm. A part commonly known as a spider is used to mount several cast aluminum molds. The spider consists of several arms or mounting legs where each mold is attached, normally by bolts. A central mounting location is attached to the spindle by bolts. The spider allows two or three dozen cast molds to be mounted on one central structure. The entire spider, or just one or two cast molds, can be removed or mounted efficiently. The spider or a single, large sheet-metal mold may be removed easily with a forklift or crane. This is important, since the rotomolding industry normally requires short production runs of a variety of parts. Figure 5 shows molds mounted on a straight arm and an offset arm. Clamping Systems. The most common clamping system for small to medium molds is that of spring-loaded clamps, which are welded onto the sections of a mold. As the molds get larger, nuts and threaded bolts are normally used; they are installed and removed with an air gun (impact wrench).

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Standard spindle (straight arm)

Offset (arm) spindle

Fig. 5. Mold mountings.

Venting System. Most rotational molds require a venting system to remove the gas that develops in the heating cycle. Depending on the mold size, vents range from 3 to 50 mm in diameter. A 12.5-mm-i.d. tube is used for an article of 0.76-m3 volume. Since vents leave a hole in the molded parts, correct placement is essential. They should be located in an area that may be cut out of the finished product or where a patch does not impair appearance. Vents must also be so located as to prevent water from entering the product while it is in the cooling cycle, because this may leave a water-track mark on the inside of the hot article. Improper venting can cause problems, such as blow holes in the parting line. Mold Release. Since most rotational molds are designed with little or no draft angle, they must be treated with a release agent. Molds are usually cleaned with a solvent and a lightly abrasive cloth to remove foreign particles from the surface. A light coating of release agent is applied and baked on to ensure adhesion. The amount of release agent required depends on the resins to

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be used. After the initial application, several hundred articles can be molded before the mold is stripped and another coating is applied; during this time touch-up may be required. Sandblasting is also used to remove resin buildup before reapplying the mold release. A more effective release agent is usually applied to the parting line to facilitate demolding. Teflon provides for a permanentrelease coating but care must be taken not to damage it, as it is normally very thin. Special Molds. Some molds may require a complicated design, inserts, and special machining. Alignment pins may be used to ensure a perfect fit. If a rotomolded article contains a large section where no resin buildup is desired, the mold can be shielded with an insulating material such as Teflon. Regular maintenance prolongs the useful life of the mold. For cleaning, nonabrasive tools should be used. Steel molds should be oiled to prevent rusting. Secondary Operations. Secondary operations may be required, such as automatic or manual drilling. Additional fittings can be added by spin welding. Sections may be joined by hot-bar, hot-gas, or ultrasonic welding. Flame treatment before painting provides a surface for adhesion. As in all plastic processing methods, secondary operations are time-consuming and should be kept to a minimum. Automatic computer-driven 3–5-axis routers are now commonplace for secondary operations. Decorating, labeling, and graphics have become an important area for parts that are rotomolded. Graphics, labels, and colors can be molded in or applied afterwards.

Resins In the early days of rotational molding only vinyl plastisols were used. The first polyethylene for rotomolding was produced in the 1960s. Today polyethylene is the principal material for the industry. The distribution of resin use in rotational molding is indicated below (6). 85% 10%

5%

Polyethylene Fluorocarbons Polycarbonate Nylon Poly(vinyl chloride) Polypropylenes ABS Acetals Acrylics Cellulosics Epoxies Phenolics Polybutylenes Polyesters Polystyrenes Polyurethanes Silicones

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A rotomolding resin requires good flow, which is measured by the melt index. The higher the melt index, the better the flow. Most rotomolding resins have melt indexes between 3 and 20. The melt index is also an approximate measure of molecular weight or chain length. A resin with a high index has shorter chains and a lower molecular weight; a low index indicates longer chains and a higher molecular weight. Molecular weight distribution is also important. The narrower the distribution, the more uniform the melt properties. Polyethylene. Linear low density polyethylenes (LLDPE) are the most widely used materials for rotational molding. Large pellets cannot be used and must be reduced to a smaller particle size to promote heat transfer from the mold to the powder. This also improves the flow of the particles during melting to prevent the deleterious effect of oxidation. This size reduction is done by the resin supplier, a custom grinder compounder, or the molder on mechanical grinding mills. Some resins, such as nylon, are supplied as small pellets and can be molded without grinding (see ETHYLENE POLYMERS, LLDPE). The molecular weight distribution of a polyethylene for rotational molding should be very narrow, allowing for uniform melting of the particles during the molding cycle. Polyethylenes are easily ground to 500 µm (35 mesh) at high rates, yielding acceptable powder at nominal upcharge. They are thermally stable, and with stabilization can be molded in high temperature, high speed rotationalmolding equipment without oxidation. Excellent low temperature physical properties, such as impact strength, allow use in a broad temperature range. Polyethylenes are inexpensive and available in a wide range of densities and melt indexes for a wide variety of applications. Ultraviolet stability or outdoor life is significantly improved by the addition of pigment or UV stabilizer. Most polyethylene resins meet FDA food additive regulations, except for some specialized cross-linkable polyethylenes and some resins with certain UV stabilizers. The high dielectric strength of polyethylenes makes them suitable for electrical applications. Density and melt indexes (2.0–20 g/10 min) are the main criteria for selecting a polyethylene for a particular application (Table 1). Polyethylene is classified by density. Type I has a density of up to 0.925 and is called low density (LDPE). Type II has a density range from 0.926 to 0.940 and is called medium density (LMDPE); most LLDPEs fall into this range. Type III has a range from 0.941 to 0.969 and is classified as high density copolymer (HDPE). Type IV is above 0.960 and is classified as high density homopolymer; it is not used in rotational molding. LDPE is flexible and tough, easy to process, and has excellent chemical resistance. LLDPE and LMDPE have better mechanical properties than LDPE, higher stiffness, excellent low temperature impact strength, and excellent environmental stress crack resistance. HDPE is the stiffest polyethylene, with excellent chemical resistance and good processability. XLPE contains a cross-linking agent that reacts with the resin during the molding cycle, similar to a thermoset. This reaction improves the toughness and stress-crack resistance of the rotomolded product. Ethylene vinyl acetate copolymer (EVA) is a low density polyethylene copolymer with excellent low temperature impact properties. The higher the level of vinyl acetate, the greater the flexibility of the material. Colorants, flame retardants, and foaming and antistatic agents may be added to polyethylene for specialty applications. Comparisons with other resins are given in Table 2.

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Table 1. Molding Properties of Polyethylenes Resin

Advantages

LDPE

Flexibility Excellent warp resistance Uniform shrinkage FDA accepted Flexibility Excellent cold temp. impact Uniform shrinkage FDA accepted Excellent ESCRa Good warp resistance More stiffness than LDPE FDA accepted Excellent stiffness FDA accepted Good impart strength Higher heat deflection Excellent impact Excellent ESCRa

BVA

LLDPE–LHDPE

HDPE

XLPE a Environmental

Disadvantages No stiffness No ESCRa

No stiffness No ESCRa

Less stiff than HDPE Lower heat deflection

Low ESCRa Warpage and shrinkage are not consistent Not accepted by FDA Longer molding cycle

stress-crack resistance.

Table 2. Resins Used for Rotomolding Resin

Advantages

Disadvantages

Polyethylene

Low cost, ease of moldability

Polycarbonate Nylon

Clarity, toughness Excellent impact strength High heat resistance Flexibility, paintability

Lower impact strength than other types Tendency to absorb moisture, harder to mold than PE High cost, harder to mold than PE Greater cost than PE, stiffness

Poly(vinyl chloride)

Poly(vinyl chloride). Poly(vinyl chloride) can be molded in liquid or powdered form. The liquid plastisols are fluid suspensions of fine particle-sized resins in a plasticizing liquid. PVC compounds are easily processed. They can be formulated to produce articles ranging from flexible to semirigid, with durometer hardness of 60 Shore A to 65 Shore D (see VINYL CHLORIDE POLYMERS). Molded plastisols possess outstanding chemical resistance. Plastisol systems for rotational molding are usually formulated to a Brookfield viscosity of 1000–4000 mPa·s (=cP) to provide high mold detail reproduction. Plastisols have excellent fluorescent pigment retention, which eliminates pigment bleeding. This can be an advantage in rotomolding when bright colors are required. Nylon. Nylon has excellent tensile strength, stiffness, impact strength, and high heat resistance, and properties are maintained at elevated temperatures; chemical resistance is excellent (see POLYAMIDES, PLASTICS).

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Table 3. Less Important Resins Used in Rotomolding Resin ABS Acetals Acrylics Cellulosics Epoxies Fluorocarbons Phenolics Polybutylenes Polypropylene Polystyrenes Polyurethanes Styrene acrylonitrile (SAN) Silicones

Advantages Good dimensional stability, low temp. impact strength Very rigid, good chemical resistance Good clarity Good chemical resistance, impact strength, and clarity Good electrical properties and chemical resistance Good thermal properties, excellent chemical resistance Good at high temp.

Disadvantages Hard to mold Hard to mold, high cost Low impact strength Hard to mold Hard to mold Hard to mold Hard to mold, low impact strength Hard to mold

Good abrasion properties, long-term water resistance Good chemical resistance Good electrical properties and clarity Good insulating properties and abrasinn resistance Good chemical resistance and clarity

Low impact strength Low impact strength Hard to mold

Good moisture resistance, excellent, chemical resistance

Hard to mold, high cost

Hard to mold

Polycarbonate. Polycarbonates (qv) provide resins with excellent mechanical properties, including stiffness, tensile strength, creep resistance, and the highest heat resistance of all rigid plastics. Other Resins. Many other resins have been processed by rotational molding. In most cases their disadvantages outweigh their advantages. Disadvantages include high cost, poor impact strength, and low flowability (Table 3). Applications Almost any type of part or product can be produced by rotational molding. Rotomolding has fewer limitations than any other plastic processing method. It is used for tanks, ranging in size from 19 L to 88 m3 . Rotomolded tanks are utilized in the agricultural, chemical, and recreational vehicle industries. Rotomolded containers are used for packaging and material handling. Rotational molding is used for portable toilets, battery cases, light globes, vacuum cleaner and scrubber housings, toys, and garbage containers. Rotomolded kayaks, canoes, boats, and playground equipment are commonplace.

Economic Aspects Compared with blow molding, rotational molding offers lower equipment and mold costs and broad design parameters. Blow molding, however, provides a quicker

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Table 4. Plastics Processinga

Equipment cost Mold cost Cycle time Design limits Materials available a1

Rotomolding

Blow Molding

Thermoforming

2 1 3 1 1

3 3 1 3 3

1 2 2 2 2

= lowest costs; 3 = highest costs.

cycle and can be used with more resins. Compared with thermoforming, rotational molding offers cheaper mold cost and more design freedom. Thermoforming equipment is cheaper, cycles are shorter, and the resin selection is wider. Table 4 compares these processes. Rotomolding resins are priced slightly higher than most other resins because of higher stabilizer requirements. Grinding costs are 11–15 cents/kg. Rotomolding machine prices range from $80,000 for a small machine up to $800,000. Grinding systems are $50,000 to $100,000.

Safety Rotational molding is a reasonably safe industry. Most resins used are nonhazardous. Some resins may cause mild irritation; suppliers provide safety information on their handling and use. Most resins used for rotational molding are ground to 500 µm (35 mesh), and appropriate procedures should be followed to prevent a dust explosion and fire. A rotomolder is a relatively safe piece of processing equipment; fans are enclosed with guards; spindles and arms operate at a low speed. The oven is equipped with a temperature control.

BIBLIOGRAPHY “Rotational Molding” in EPSE 2nd ed., Vol. 14, pp. 659–670, by Philip T. Dodge, Quantum Chemical Company, USI Division. 1. USI Chemicals, Petrothene Polyolefins, A Processing Guide, National Distillers, Cincinnoti, Ohio, 1971. 2. P. B. Brains, Basic Principles of Rotational Molding, New York, 1971. 3. Engineering Design Handbook, Rotational Molding of Plastic Powders, U.S. Army Material Command, Alexandria, Va., 1975. 4. G. Beall, The Engineers’ Guide to Designing Rotationally Molded Plastic Parts, Association of Rotational Molders, Chicago, Ill., 1982. 5. R. Crawford, Rotational Moulding of Plastics, John Wiley & Sons, Inc., New York, 1992; P. Mooney, Analysis of the North American Rotational Molding Business, 1995; R. Crawford, Rotational Moulding of Plastics, 2nd ed., John Wiley & Sons, Inc., New York, 1997; P. Mooney, The Recent Pace and Growth in North American Rotational Molding, 1997; G. Beall, Rotational Molding, Hanser/Gardner, 1998; P. Mooney, The New Economics of Rotational Molding, 1999; P. Nugent, Rotational Molding: A Practical Guide, 2001; P. Mooney, New Market Dynamics in Rotomolding, 2003.

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6. P. T. Dodge, Materials for Rotational Molding, Plastics Design Forum, Denever, Colo. 1984;

GENERAL REFERENCES Association of Rotational Molders Library, www.Rotomolding.org. Society of Plastics Engineers, Rotomolding Division. www.rotomolding.net.

PHILLIP T. DODGE STEVE ANDRZEJEWSKI DUANE MAHAN Equistar Chemicals, LP, Lyondell Chemical Co.