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often inefﬁcient. Stickies in pulp cause paper-mill operat-

ing problems by reducing pulp drainage rates on paper

machines (20). They do this by forming deposits on paper-

machine wires, dryer fabrics, press rolls, and drying

cylinders. Reduced drainage rates force operators to run

paper machines at slower speeds or shut them down to

clean the fabrics. Both result in lower mill production

rates and reduced proﬁtability. Stickies also prevent good

ﬁber–ﬁber bonding, reducing paper strength, an impor-

tant consideration for corrugated containers. Both hot-

melt adhesives and pressure-sensitive adhesives can form

stickies.

Fractionation separates ﬁne particles and short, weak

cellulose ﬁbers from longer, stronger ﬁbers (21). Reﬁning

is used to develop the desired pulp drainage properties on

the paper machine and control paper bulk and density,

strength, surface smoothness, and porosity (22, 23).

Caustic soaking is also used to improve OCC ﬁber pro-

perties (24).

Extensive bleaching of OCC can produce a high-bright-

ness pulp suitable for use in manufacturing ofﬁce paper

(25). Economics compare favorably to those obtained for

manufacturing ofﬁce paper from de-inked recovered ofﬁce

paper (26).

A considerable fraction of corrugated containers are

coated with wax to increase water resistance. Waxes and

hot-melt adhesives can form a thin ﬁlm when making

linerboard from OCC and result in a slippery surface (27).

This can cause ‘‘telescoping’’ when the linerboard is wound

on a roll. When recycling OCC, removal of this wax is

difﬁcult. Flotation has been found to remove the wax

efﬁciently (27). However, high paper ﬁber losses make

this process uneconomical.

PLASTICS

Plastics recycling has several advantages (28):

. Conservation of nonrenewable fossil fuels: Plastic

production uses 8% of the world oil production, 4%

as feedstock and 4% for manufacture.

. Reduced consumption of energy.

. Reduced emissions of carbon dioxide, nitrogen oxide

and sulfur dioxide (SO

. Reduced amounts of solid waste going to landﬁlls.

The economic attractiveness of plastics recycling is

dependent upon the cost of the plastics feedstocks, crude

oil, and natural gas. For example, the price of recycled

polyethylene can vary from 85% to 105% of the virgin

polyethylene price, depending upon feedstock prices. If the

crude oil price doubles, as occurred in 2004–2005, and the

energy cost is 25% of the total cost of the virgin or the

recycled polymer, virgin polymer cost increases 75% while

recycled polymer cost increases 25%.

Primary recycling is the recycling of plastics that are

plant scrap and have not been sold for consumer use.

Secondary recycling is the physical cleaning and proces-

sing of postconsumer plastic products. Tertiary recycling

is the chemical treatment of polymers. This treatment is

usually depolymerization to produce monomers that are

puriﬁed and then polymerized to produce new polymer.

Using tertiary recycling, materials such as ﬁllers and

ﬁbers can be physically removed from the monomer. The

monomers can also be puriﬁed by distillation and other

processes prior to polymerization. The leading example of

tertiary recycling is poly(ethylene terephthalate) (PET).

Residential collection programs indicate high collection

rates for easily recognized types of containers (Table 2).

Sorted plastic packaging materials are shipped, usually

in bales, to processing plants to be converted to polymer

resins. The bales are broken and the bottles sorted to

ensure that only one type of polymer is further processed.

Processing consists of chopping and grinding the bottles

into ﬂakes. These ﬂakes are washed. Processing steps

such as ﬂotation are used to remove polymeric contami-

nants from the ﬂakes. The ﬂakes are melted and converted

into pellets. Process costs are summarized in Table 3.

For high-value food-packaging applications, minimal

migration of contaminants into food products is critical.

Currently the FDA requirement is a maximum 0.5 part

per billion (ppb) of noncarcinogenic compounds by dietary

exposure (29).

Poly(ethylene Terephthalate) (PET)

Originally, PET was used virtually exclusively to package

soft drinks. As a result the recyling stream was quite

homogeneous, consisting of unpigmented one- and two-

liter bottles with a small percentage of green bottles.

However, PET is now used to package a wide variety of

beverages besides soft drinks including water, fruit juice,

tea, and beer, plus other products such as peanut butter,

ketchup, and edible oils. These products are often pack-

aged in bottles with a wide range of pigments in many

sizes and shapes. In addition, various adhesives and

barrier resins are combined with PET (30).

Table 2. Residential Recovery Rate by Package Type

Package Type Percent Recovery

Beverage bottles 65

Liquid detergent bottles 50

Other rigid containers 10

Packaging ﬁlm 5

Average of all plastics 30

Source: Reference 6.

Table 3. Approximate Polymer Recycling Costs

Costs

Process Step $ per Pound Percent

Collection 0.10 27

Sorting 0.12 32

(Subtotal cost) (0.22) (59)

Grinding and cleaning

0.15 41

Total 0.37 100

Source: Reference 6.

1078 RECYCLING

In 2006, 30.9% of plastic soft drink bottles were recycled.

Initial polyethylene terephthalate (PET) processing is si-

milar to that outlined in Table 3. Cleaning comprises

washing, rinsing, and drying. Poly(vinyl chloride) (PVC)

and PET have very similar density values; both will sink to

the bottom of the waterbath during rinsing. Therefore it is

difﬁcult to separate the two polymers after the bottles have

been ground into small particles (31). Melting PET contain-

ing PVC will produce black spots due to charring of the

PVC during processing to produce new bottles (32, 33).

For food applications, improved cleaning of PET pro-

duced by secondary recycling is needed. Supercritical ﬂuid

extraction using carbon dioxide (34) and solvents such as

propylene glycol (35) have been proposed. High tempera-

ture and the use of vacuum to remove volatile impurities

has also been suggested (35). Stripping of volatile compo-

nents at temperatures above 160 1C for 3 min has been

reported (36). Application of a multilayer approach, the

manufacture of a bottle with an inner layer of recycled

PET sandwiched between surface layers of virgin PET, is

used commercially for soft drink applications (37).

Two PET tertiary recycling technologies are used com-

mercially: methanolysis (38) and glycolysis (39). Both

cleave the ester linkages in the polymer to form mono-

mers. In methanolysis, methanol is used to cleave the

polymer ester linkages, producing stoichiometric amounts

of dimethyl terephthalate and ethylene glycol (38, 40). The

ethylene glycol is separated and puriﬁed by distillation.

The dimethyl terephthalate is puriﬁed by crystallization

and distillation. Both bis-hydroxyethylterephthalate and

oligomers are formed in glycolysis (37, 40). These are

recovered and puriﬁed by vacuum distillation and then

polymerized in the presence of ethylene glycol to form

PET. Glycolysis is claimed to be somewhat less costly than

methanolysis (38).

Hydrolysis yielding terephthalic acid and ethylene

glycol is a third process. High temperatures and pressures

are required for this currently noncommercial process.

The puriﬁcation of the terephthalic acid is costly and is the

reason the hydrolysis process is no longer commercial.

High-Density Polyethylene

Process steps in recycling high-density polyethylene

(HDPE) are similar to those outlined in Table 3. Cleaning

comprises washing, rinsing, and drying (32). Removing

labels is the worst problem in washing and drying stages.

Detergents are often used to improve the efﬁciency of label

removal. Metal foil labels can introduce metals into the

polymer. When metal foil labels are heat-sealed onto plas-

tic, the only way to remove them is using an extrusion melt

ﬁlter. This leads to plugging of the ﬁlter screens causing

more frequent changes and increasing production costs.

During the rinse cycle, polyethylene particles ﬂoat to

the surface of the waterbath. The higher-density PET and

PVC particles sink to the bottom of the bath and can be

separated from the polyethylene.

Blends of PET and HDPE have been suggested to

exploit the availability of these clean recycled polymers.

The blends could combine the inherent chemical resis-

tance of PET with the processing characteristics of HDPE.

Since the two polymers are mutually immiscible, about 5%

compatibilizer must be added to the molten mixture (38).

The properties of polymer blends containing 80–90% PET/

20–10% HDPE have been reported (41). Use of 5–15%

compatibilizer produces polymers more suitable for extru-

sion blow molding than pure PET.

Polypropylene

In 2005, polypropylene recycling was 10.1 million pounds

recovered. Polypropylene (PP) is used in packaging appli-

cations as ﬁlms and in rigid containers. Battery cases

could be considered another packaging application. Dead

batteries are often collected at the point of sale of new

batteries. In the United States, some states have laws

mandating this. Lead, acid, and plastics, particularly PP

from battery casings, are recovered and recycled (6). PP is

also recovered from bale wrap and other PP fabrics used

for wrapping in the textile industry and from other

containers (42).

Steps in polypropylene recycling include size reduction

by grinding, washing, rinsing, and drying to remove

contaminants and produce PP ﬂakes (42). After extrusion,

molten polymer is ﬁltered through screen packs. The

polymer may be separated into different melt ﬂow ranges

to produce more uniform product grades.

Polystyrene

Polystyrene (PS) packaging applications include injection-

molded products such as beverage containers, dairy pro-

duct containers, and packaging for personal-care products

(43). Extruded solid-sheet PS packaging products include

salad boxes, dairy-product containers, baked-goods con-

tainers, and vending cups and lids. Extruded foam sheet

PS packaging products include poultry and meat trays,

produce trays, hinged-lid containers, egg cartons, and

foam cups. One formerly large-volume PS use, fast-food

clamshell containers, has greatly diminished because of

concerns over slow biodegradation of discarded polystyr-

ene containers. Blow- and foam-molded PS packaging

products include vitamin bottles, loose-ﬁll packaging,

and cushion packaging.

Polystyrene recycling processing steps include densiﬁ-

cation (for foams), granulation to reduce particle size,

washing, drying, extrusion, and pelletizing. High-density

baling is used to increase the bulk density of polystyrene,

often by a factor of 2. Contaminants are more easily

removed before this densiﬁcation step than after. A de-

monstration plant chemically decomposes polystyrene to

produce monomer (43).

Consumers readily recognize polystyrene foam pro-

ducts and turn these products in for recycling. However,

recovery rates for other PS packaging products is signiﬁ-

cantly less (43).

Commingled Plastic Wastes

relatively small amount

of PVC goes into packaging

applications and appears in municipal solid waste (33).

The greatest concern with PVC is as a contaminant in

other polymers being recycled, particularly PET.

RECYCLING 1079

The composition of nonmetal residues produced in

shredding automobiles is summarized in Table 4. Each

vehicle generates 500–800 lb of residue. The annual U.S.

total is about 3.5 million tons or about 1.3% of the

municipal solid waste generated annually (3). The mix-

ture is too complex to separate and recycle. Depending on

the amount of glass, water, metal, and dirt present, the

residue has a heating value of 4800–6800 Btu/lb (6, 44).

Incineration reduces residue weight by 50% and volume

by 80%.

Advanced waste recycling is the high-temperature/

high-pressure conversion of commingled plastic wastes

to form petrochemical process streams (45). Research is

in progress to determine the conditions that will favor

formation of certain types of chemical feedstocks, includ-

ing synthesis gas (hydrogen + carbon monoxide), hydro-

gen, crude pyrolysis oil (containing benzene, toluene, and

xylene), oleﬁns, and oxygenates (methanol, esters, methyl

formate, etc). This technology has also been evaluated for

producing fuels: (a) medium-Btu gas for boilers and (b)

liquid fuels such as diesel oil. None of these processes is

currently economic.

ECONOMICS AND STATISTICS

In the United States the number of beverage containers

sold increased approximately 25% between 1996 and 2006.

However, the number recycled remained essentially ﬂat at

approximately 75 billion weighing approximately 4 million

short tons (46). Between 1990 and 2006, the recycling rate

for all beverage containers declined from 49% to 39% after

reaching a high of 53% in 1992 (Anonymous, Container

Recycling Institute, http://www.container-recycling.org/

images/allrates/recrate-90-06.gif).

The materials used in beverage containers has changed

substantially between 1993 and 2004. The fraction of

aluminum containers has declined from 67% to 50% in

this period, while the fraction of class containers has

declined from 32% to 18% (47). In contrast, the fraction

of PET bottles has increased from 9% to 25%.

Overall, 2005 recycling for all packaging is over 25% in

a number of old EU Member States (Austria, Belgium,

Germany, Italy, the Netherlands, Norway, Sweden, and

Switzerland). In addition, new Member States like the

Czech Republic and Slovenia exceed 25% (48). In 2005,

35% of all PET bottles were recycled in the EI , equivalent

to 0.8 million metric tons.

Paper

Paper recovery now averages 346 pounds for each man,

woman, and child in the United States. Exports of paper to

be recycled have doubled since the late 1990s, according to

the American Forestry & Paper Association (49).

Eighty-one percent of the paper recovered is recycled

by U.S. mills. Sixteen percent is exported to foreign

markets. The rest is used domestically to manufacture

products such as molded packaging, compost, and kity

litter. Mills buy recycled paper loose or in bales. Grades of

paper include mixed paper, mixed ofﬁce paper, sorted

white ledger, sorted colored ledger, computer printout,

newspaper, corrugated containers, and magazines (50).

More than 228 U.S. mills and industrial facilities

convert corrugated containers and other paper packaging

to new paper products and construction materials (51).

More than 60 of these produce linerboard and corrugated

medium with a recycle content of 100% (52). In North

America, minimills designed to process old corrugated

containers are becoming increasingly common. Located

in or near major cities, minimills are 25–50% the size of

conventional paper mills.

Metals

The U.S. aluminum industry recycled 51.9 billion alumi-

num cans in 2006, half a billion more than in 2005 (53). It

has the highest recycling rate of any beverage container at

51.6%. However, the recycling rate has declined from its

1997 high of over 65% (54).

An estimated ﬁve billion aluminium cans were used in

the UK in 2001, 42% of which were recycled (55).

In the United States, the recycling rate for steel

packaging was 63% in 1999 (56). In the United Kingdom,

the recycling rate for steel packaging, including transport

packaging, such as steel drums and bale wire, was 42% in

2002 (57). In 2005, over 2.3 million metric tons of steel

packaging were recycled in the 25 countries of the Eur-

opean Union (58). This represents an increase of 6%

compared to 2004 and a recycling rate of 63%.

Glass

In 2005, 25.3% of all glass containers were recycled (59).

The recycling rate of glass beverage containers declined

from 30% to 20% between 1996 and 2006 (60). Glass

makes up about 6% by weight of the material in municipal

waste streams recycled (59). Cullet from recycled glass

costs 80–95% of the virgin raw materials used in glass

production. In addition, the use of cullet saves 15% of the

energy costs associated with glass production from virgin

raw materials. This energy savings arises from the lower

melting temperature of cullet compared to virgin raw

materials and lower operating costs for pollution-abate-

ment equipment.

Seven billion glass containers were produced in the

United Kingdom in 2003 and the recycling rate has

remained relatively constant at approximately 33% since

2000 (61). This contrasts with much higher recycling rates

of 80–90% in European countries with more extensive

waste collection systems.

Table 4. Representative Composition of Automobile

Shredder Residue

Material Percent by Weight

Plastics 27

Rubber 7

Glass 16

Textiles 12

Fluids 17

Other 21

Source: Reference 6.

1080 RECYCLING

Plastics

The total pounds of plastic bottles recycled in 2005 in the

United States reached a new record high of 2.102 billion

pounds (62). PET and HDPE together comprise over 95%

of the plastic bottle market and 99.6% of the recycled

pounds (63).

In 2005, the recovery rate for plastics in the European

Union was 47% (64).

In 2005, 5.075 billion pounds of PET bottles and jars

were available for recycling in the United States (65). Of

this, 1.170 billion pounds was collected and sold. Of this,

58% were used by U.S. processors and 42% exported. The

favorable economics of PET recycling have been attributed

in part to forward integrated PET recyclers consuming

their own product to make bottle resin (32).

In 2005, the HDPE bottle recycling rate increased 1.2%

to 27.1%. Exports of collected HDPE bottles for recycling

overseas were 162.4 million pounds, nearly 18% of the

HDPE collected (63).

The largest used for recycled PET and HDPE resins is

fabrication of new bottles (63).

In 2005, 10.1 million pounds of polypropylene was

recycled in the United States (63). Most of this is derived

from the recovery of polypropylene bottle closures during

PET recycling. In addition, polypropylene bottles are com-

monly recycled with high-density polyethylene (HDPE). At

levels up to 5% by weight, polypropylene has a negligible

impact on the properties of recycled HDPE resins.

The costs of recycling commingled plastics has been

estimated at $1700 per ton (66). This is 10 times more

expensive than recycling easily separated homogeneous

products such as PET and HDPE.

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Europe,’’ www.endseuropedaily.com/docs/70514a.pdf (spring

2007).

65. ‘‘2005 Report on Post Consumer PET Container Recycling

Activity, Final Report,’’ National Association for PET Con-

tainer Resources, www.napcor.com).

66. J. R. Ellis, ‘‘Polymer Recycling: Economic Realities’’ in Ref. 5,

pp. 62–69.

ROLL HANDLING

In web processes, such as slitting and rewinding, coating

and laminating, printing, as well as other applications,

rolls must be loaded from the rewind station after the

process. Because of weight and size, handling of rolls is

done by general-purpose equipment, such as cranes and

forklifts. For greater productivity, roll-handling equip-

ment is engineered to suit the characteristics of the

web-process equipment. An important fringe beneﬁt of

engineered handling systems is the reduction of safety

hazards associated with general-purpose equipment.

Slitter/Rewinder Overview. A slitter/rewinder (see Slit-

ting-rewinding machinery) converts master rolls into high-

quality, narrow-width, rewound rolls that may or may not

be the ﬁnal product. Variables in the web characteristics

and in the ﬁnal form of the product determine which of the

several types of slitting and rewinding techniques is used.

The slitter/rewinder machine is typically composed of un-

wind, slitting, and rewind stations. A turret used for

unwinding allows the loading of new master rolls while

the machine is slitting and rewinding. Continuous unwind-

ing can be attained by automatic splicing equipment.

1082 ROLL HANDLING

The slitting methods used depend on the material

characteristics and quality of the slit edge desired. Razor

slitting is used primarily on ﬁlms and score slitting on

many types of fabrics, nonwovens, and textiles. Shear

slitting is excellent for most materials but requires a

relatively high initial investment and operator expertise.

Locked-core rewinding is successful on good caliper webs

and pressure-sensitive tapes with relatively small roll

diameters. Differential rewinding is required for off-cali-

per materials and large-diameter rolls.

An effective method of rewinding is to alternate the slit

webs on two rewinding shafts. This method, termed

‘‘duplex rewinding’’, is particularly suited for narrow-

width slit material. Winding on a single shaft, called

simplex winding, is generally effective for slit and unslit

paper, nonwovens, and textiles.

Shaft Extraction/Insertion System. On duplex and sim-

plex slitter/rewinders, webs are slit and rewound on cores

supported by rewind mandrels. Rewind mandrels may be

extremely heavy, i.e., several thousand pounds (kilo-

grams), or cumbersome to handle manually. Each machine

cycle requires that a rewind mandrel be loaded up with

cores, put into the rewind station, removed from the

rewind station, and then extracted from the rewound

rolls. This process might take place many times a day,

and an operator might have to lift and maneuver many

tons of rewind mandrels per shift. Apart from the burden

on the operator, manual shaft handling hinders produc-

tion, because the cycle time for loading and unloading is

relatively long.

The use of a shaft extraction/insertion system mini-

mizes cycle time as well as operator fatigue. With this

concept, the ﬁnished rolls and rewind mandrels are trans-

ferred to a lift table where shaft extrators pull the rewind

mandrel from the rolls. The shaft is supported in the

extracted position until the rolls are removed from the

extractor table. Fixtures called core boxes are sometimes

engineered into the extractor table to locate cores in their

proper positions. The extractor inserts the shaft back into

the cores. The cored shafts then are brought back into the

rewind stations.

Shaft extractors are easily interfaced in close proximity

to turret winders, unwinders, and other simplex winders,

which often require the handling of steel shafts 6–10 in.

(15–25 cm) in diameter by several hundred inches (several

meters) long.

Handling Systems for Duplex Rewinders. A duplex slit-

ter/rewinder (see Figure 1), with the rewind stations

Figure 1. A duplex slitter/rewinder with roll-handling system.

Turret carriages are in full-roll downward position at slitter/

rewind station. New mandrels are in an upward position awaiting

turret index.

Figure 2. A slitter/rewinder roll-handling system

with a shaft-type unwind stand. This two-motor

drive system has an adjustable vacuum winding

capability and an overhead mandrel lift/transfer

mechanism.

ROLL HANDLING 1083

located above the main machine, interfaces well with an

overhead roll-handling system. After the rewind cycle, the

shafts with the rewound rolls are lifted out and the new

shafts with cores must be inserted into the machine.

A typical system (see Figure 2) may have an overhead-

powered carriage that supports dual vertically adjustable

shaft-clamp assemblies. With two clamps for each vertical

lifting mechanism, new shafts with cores can be loaded

into the machine immediately after the shafts with the

ﬁnished rolls are removed. In other words, a machine with

two rewind stations has four shafts circulating through

the system simultaneously. As the new rewind cycle

begins, the shafts with rolls are conveyed overhead to

the shaft extractors where the slit and rewound rolls

become free of the shafts and may be upended on pallets

for conveyor or forklift removal into storage.

Floor Loading and Unloading. Floor loading and unload-

ing features are available as an integral part of many

unwinds and simplex rewinds. These units use hydrauli-

cally pivoting arms to raise and lower rolls to the ﬂoor. The

need for auxiliary equipment is eliminated. Lift tables,

shaft extractors, and upenders are designed to work with

simplex (single-position) rewinders that produce large-

diameter rolls of paper, laminates, and nonwovens (see

Figure 3). Driven ‘‘V’’-trough conveyors can then move the

rolls from the rewinders to weighing stations, stretch

wrapping, and storage conveyors.

Robotic Roll Handling. An example of robotics in spe-

cial-purpose roll handling is shown in Figure 4. A simplex

slitter/rewinder with an automatic roll pusher pushes the

slit and wound rolls onto the robot shaft. The robot shaft

then expands to lock the cores to the shaft. The robot is

programmed to shift the axes of the rolls 901 and to

automatically position them over a pallet. A rotary table

supports the pallet and will index every 901 to allow

unloading of ﬁnished stacks of rolls in the four quadrants

of the pallet.

Total Automation. The ultimate in roll handling is

shown in Figure 5. This slitter/rewinder for duct tape

automatically cuts cores, loads cores at their proper loca-

tion on dual rewind shafts, feeds the loaded shafts into

rewind stations, winds the slit tapes, unloads the rewound

Figure 3. A surface center slitter/rewinder

with hydraulically pivoting rewind arms,

preset constant surfacing pressing in the

center surface rewinding mode, and an auto-

matic minimum gap for center winding.

Figure 4. A roll unloading and stacking robot with hydraulically

controlled cantilever mounted boom with 901 rotational capability

on vertical and horizontal axes for pallet ofﬂoad.

Figure 5. A fully automatic, pressure-sensitive tape turret slit-

ter/rewinder with automatic core cutter and loader.

1084 ROLL HANDLING

rolls from the machine, and feeds the empty shafts back in

for core loading. This completely ‘‘hands-off’’ machine is

microprocessor-controlled. Cycle times are better than 30 s.

REFERENCES

‘‘Roll Handling’’ in R. E. Mastriani, ed. Wiley Encyclopedia of

Packaging Technology, 1st edition, Arrow Converting Equip-

ment, Inc., pp 571–573.

ROTATIONAL MOLDING

PHIL DODGE

Quantum Chemical Company

Cincinnati, Ohio

Reusable, large industrial containers can be made by

rotational molding. Rotational molding generally is not

thought of as a technique for making packaging. However,

some manufacturers use this process, which can make

complex, hollow, and seamless products of all sizes and

shapes to produce large bulk containers for shipment of

powders and liquids, 5–55–gal drums, trash and recycling

containers, insulated food-product containers, and double-

wall shipping trays for equipment components.

ADVANTAGES

Compared with blow molding or thermoforming, rota-

tional molding offers signiﬁcant advantages:

. Mold costs are low and comparable with those for

blow molding and thermoforming.

. The process is economical for short production runs.

. Secondary tooling is minimal or not needed.

. There can be little or no scrap.

Numerous design features are possible; for example,

double-walled parts, good surface detail, intricate con-

tours and undercuts, stress-free parts, minimal cross-

sectional deformation and warping, and molded-in inserts

can be used.

POLYETHYLENES MOST WIDELY USED

Generally, powdered resins are used in rotational molding.

However, a few liquid resins are available and some high-

ﬂow resins, such as nylons, have been used in small pellet

form.

Although many thermoplastics can be rotationally

molded, only a few actually are used to make commercial

products. Polyethylenes—low-density (LDPE), linear low-

density (LLDPE), high-density (HDPE), and copolymers—

are the most widely used materials (see also Thermoset-

ting polymers).

LLDPE rotational molding grades typically have den-

sities ranging from 0.935 to 0.940 g/mL and a melt index

(MI) of 3–10 g/10 min. LLDPE rotational molding resins

offer good stiffness and low-temperature impact strength,

excellent environmental stress-crack resistance (ESCR),

and warp resistance.

LDPE rotational molding grades generally have den-

sities of 0.915–0.920 g/mL and an MI of 10–25 g/10 min.

LDPE rotational molding resins offer good impact

strength, low shrink-age, good warp resistance, and

ﬂexibility.

HDPE rotational molding grades have densities of

0.942–0.950 g/mL and an MI of 2–8 g/10 min. HDPE rota-

tional molding resins offer high stiffness, high impact

strength, and excellent chemical resistance.

Ethylene-vinyl acetate (EVA) copolymers for rotational

molding have densities of 0.925–0.945 and an MI of 10–25

g/10 min. EVAs offer good low-temperature impact strength

and ﬂexibility.

Postconsumer polyethylene resins (PCR) are made

from HDPE blow-molded containers. HDPE PCRs have

much lower melt indices than the MI of typical rotational

molding resins. Therefore, for rotational molding, a HDPE

PCR is blended with a virgin resin. HDPE PCR content in

blends can range between 10–25%. The blends can be

made by either melt compounding or dry blending PCR/

virgin blends.

How the PCR/virgin resin blend is prepared affects the

inner surfaces of rotationally molded parts. Parts rota-

tionally molded from dry blends can have very rough inner

surfaces. Parts made from HDPE PCR/virgin resin extru-

sion blends exhibit smooth inner surfaces.

The color of parts rotationally molded from HDPE PCR/

virgin resin blends will be inﬂuenced by the type of HDPE

PCR in the blend. Because some HDPE PCR are green

(e.g., PCR based on HDPE copolymers), rotational molding

resins made with these resins have a green tint. Other

HDPE PCR are gray and have black specks. Thus, parts

rotationally molded from blends made with these HDPE

PCR have a dirty white color.

Compared to the virgin LLDPE resin, the toughness of

HDPE PCR/virgin resin blends is much less, and it

decreases as HDPE PCR content increases.

The environmental stress-crack resistance (ESCR) of

HDPE PCR/virgin resin blends is considerably lower than

that of the virgin polyoleﬁn resins.

Crosslinked polyethylene rotational moldings can be

made by using materials that contain a chemical cross-

linking agent. Crosslinking improves physical properties,

in particular impact strength, dimensional stability, creep

resistance, and environmental stress crack resistance

(ESCR).

Polyethylene rotational moldings also can be crosslinked

using electron-beam treatment. Electron-beam processing

allows the degree of crosslinking to be controlled; therefore,

the level of property improvements can be controlled. Also,

parts can be recycled before they are subjected to the EB

treatment. Thus, scrap losses are reduced.

Polypropylene rotational molding resins provide higher

heat resistance (containers can be autoclaved) and have

higher stiffness than polyethylenes. Reactor-produced poly

ROTATIONAL MOLDING 1085

propylene copolymers (TPOs), i.e., copolymers produced in

a reactor, have not been subjected to the mechanical and

thermal degradation associated with melt blending of

rubber compounds into a polypropylene base. Other ad-

vantages of the reactor-produced TPOs include their in-

herent temperature and chemical resistance, low speciﬁc

gravity, and high ﬂexibility. Also, unlike rubber blends—

which can degrade or crosslink—the reactor-produced

TPOs are readily rotationally molded.

Nylon rotational molding resins, both liquid and pellet,

offer good heat resistance, toughness, good wear and

abrasion resistance, high strength and stiffness, and

good chemical resistance.

Polycarbonate rotational molding resins offer high heat

resistance, good impact strength, and clarity.

Vinyl rotational molding resins, both powder and li-

quid, are available. Vinyl plastisols are liquid rotational

molding resins that offer a wide range of stiffness, from

very soft (low-durometer) to very rigid (high durometer).

A SIMPLE, BUT VERSATILE PROCESS

Six basic steps exist to the rotational molding process and

are as follows:

1. A predetermined amount of resin is placed in the

bottom half of a two-piece mold.

2. The mold halves are closed.

3. The mold is placed in a heated oven and rotated

biaxially.

4. The resin melts against the inside surface of the

mold, fuses, and then densiﬁes into the shape of the

mold cavity.

5. The mold is removed from the oven and placed in a

cooling chamber. A combination of air and water is

used to slowly cool the mold.

6. The mold is transferred from the cooling chamber

and opened, and then the ﬁnished part is removed.

Many variations exist of the basic rotational molding

process.

Generally, the walls of rotational moldings are made of

one material. However, multilayer rotational moldings

can be made. For example, there can be a foam layer, a

solid layer, a layer of recycled material, a crosslinked

layer, and different-colored layers.

The standard technique for making multilayer rota-

tional moldings is to use a ‘‘dump box.’’ After the ﬁrst layer

is molded, the dump box—which is attached to the mold—

is opened to allow the material for the second layer to

enter. The dump-box cover is closed, and the second layer

is molded.

Foam ﬁlling, which uses rigid polyurethane foam,

injected into the hollow space between the walls of rota-

tional moldings adds stiffness and structural strength.

Integral skin foam moldings also are possible with

resins that contain foaming agents. These one-step foam-

ing resins expand inside the mold, and the material

directly against the mold surface forms a dense skin.

This approach to multilayer foam molding is faster than

other techniques, and tooling costs are less.

Molded-in decoration also is possible with rotational

molding. By embedding a graphic into the surface of a

rotational molding, the graphic becomes permanent and

will not peel and cannot be scratched or rubbed off.

Molded-in graphics eliminate surface pretreating required

with other decorating methods. Also, no paints or painting

equipment are required.

Precolored polyethylene powders save rotational

molders from the hassle of dry-blending pigments with

natural-colored resin powder.

Rotolining is a process that chemically bonds a reactive

polymer to the inner surface of hollow metal products. The

metal product is mounted on the rotational molding arm

as opposed to a mold.

SEVERAL TYPES OF EQUIPMENT

Many different types of rotational molding equipment

exist.

Open-Flame and Slush Molding Machines. These ma-

chines (Figure 1), which are the oldest type of equipment,

usually rotate on only one axis. This equipment is used

mostly to produce open containers.

Carousel Machines. These machines (Figure 2) are the

most common type. Carousels consist of a heating station

or oven, a cooling station or enclosed chamber, and a

loading and unloading station. The carousel can have

three to six spindles or arms for mounting molds. Most

carousels have the freedom to rotate in a complete circle.

Figure 1. Rotational-molding equipment.

1086 ROTATIONAL MOLDING

Clamshell Machines. These machines (Figure 3) have

only one arm, and the heating, cooling, and loading/

unloading stations are all in the same location.

Rock-and-Roll Machines. These machines (Figure 4),

which rotate on one axis and tilt on another, are used for

long items.

Shuttle Machines. These machines (Figure 5) generally

are used to make large parts. A frame for holding one mold

is mounted on a movable bed. The bed is on a track that

allows the mold and the bed to move into and out of the

oven. After the heating cycle is complete, the mold is

moved into an open cooling station. A duplicate bed with

a mold is then sent into the oven from the opposite end.

RECENT TRENDS

Considerable progress has been made in recent years to

improve processing controls. Microprocessors, which in-

dicate processing set points and conditions, greatly im-

prove precision. Mold-cycle data for the oven and cooling

chambers can be stored for future production of parts.

Cycle time, oven temperature, major and minor axis

speeds, as well as fan and water-spray times are other

functions under computer control. New temperature-mon-

itoring systems also are available.

A key beneﬁt of rotational molding is the low relative

cost of its tooling. Cast aluminum molds are most fre-

quently used, especially for small-to-medium-sized parts.

Cast aluminum has good heat transfer and is cost-

effective when several molds for the same part are

required.

Sheet-metal molds are normally used for larger parts.

These molds are easy to make because sections can be

welded together. Other molds, such as electroformed and

vapor-formed nickel molds, yield an end product with ﬁne

detail. Cost for these is typically higher.

To assist in part removal, most rotational molds require

a mold-release agent, which is either baked or wiped on.

Many molds now use a ﬂuoropolymer coating to eliminate

the use of mold release. Also, waterborne release agents,

Figure 2. Carousel/turret machines have a center pivot with

three, four, or ﬁve arms that hold molds. The arms index

simultaneously from station to station, i.e., load/unload station,

to oven and to cooler.

Figure 3. Single-station machines, also called clamshell ma-

chines, have an oven that has a hinged cover and a hinged front

panel. The mold rotation arm can swing into and out of the oven.

The cover and front panel are closed during heating and are

opened for part cooling, removal, and reloading of the mold.

Figure 4. Rotational moldings with long length:diameter (L/D)

ratios can be made on equipment called ‘‘rock-and-roll machines.’’

The mold is rocked back and forth on a stationary, horizontal axis

while it is rotated about a moving axis that is perpendicular to the

rotating axis.

Figure 5. Shuttle machines either move the mold, along an oval

or straight track, from the load/unload station, to the oven, and to

the cooling station.

ROTATIONAL MOLDING 1087