Yam, Kit L. (ed.). The Wiley encyclopedia of packaging technology

Подождите немного. Документ загружается.

excellent water-vapor barrier, good heat, and impact

resistance. Polypropylene is also a poor oxygen barrier,

making the material unsuitable for packaging oxygen-

sensitive foods requiring long shelf life. An oxygen mole-

cule can easy permeate through a wall made of polypro-

pylene. Ethylene vinyl alcohol (EVOH) copolymer, on the

other hand, is a high-cost thermoplastic material with

excellent oxygen gas barrier but sensitive to water, which

can signiﬁcantly deteriorate its barrier performance if

unprotected. In combination, a thin layer of EVOH pro-

tected by the polypropylene can greatly enhance bottle

performance, making it suitable for packaging products

previously not practical. Ethylene vinyl alcohol copolymer

does not generally adhere well to polypropylene, polyethy-

lene, or polyethylene terephthalate. Accordingly, multi-

layer structures often use a mutually compatible adhesive

between layers. Multilayer bottles not having the adhe-

sive can easily delaminate and separate its layers.

There are several methods with several variations

for establishing the multilayer bottle structure. These

multilayer methods generally fall into four broad groups:

(a) extrusion molding of a multilayer parison, (b) injection

molding of a multilayer preform, (c) coating the injection-

molded preform or blow-molded bottle interior or exterior,

and (d) bottle treatment.

Extrusion Multilayer

Blow molding machinery form the multilayer parison by

simultaneously extruding two or more thermoplastic ma-

terials in continuous layers within the parison’s wall (at

least for packaging applications, for example, automotive

Figure 12. Typical high-output preform injection-mold having 216 cavities for manufacturing a PET preform. (Courtesy of Husky

Injection Molding Systems.)

Figure 11. Typical high-output preform injection-

molding machine. (Courtesy of Husky Injection Mold-

ing Systems.)

148 BLOW MOLDING

part blow molding of an under hood duct might employ

intermittent and/or discontinuous layers). Creating the

multilayer structure is relatively straightforward because

melted thermoplastic materials ﬂow in a laminar fashion,

and the high-molecular-weight materials will not mix

while passing through the extrusion head. Figure 14

illustrates a cut-away schematic of a typical six-layer

extrusion head.

Most high-performance extrusion multilayer contain-

ers use polyethylene or polypropylene for its structural

layers and, as the gas barrier layer, ethylene vinyl alcohol

copolymer or a polyamide, typically poly-m-xylylene adi-

pamide or nylon-MXD6 developed by Mitsubishi Gas

Chemical. Product applications include: automotive and

small-engine fuel tanks, juice bottles, and ketchup bottles.

Figure 15a illustrates a typical extruded multilayer con-

tainer wall structure for food products. If we change the

polypropylene to high-density polyethylene, the structure

becomes typical of fuel tanks.

However, high-performance gas barrier applications

are not the only use for extruded multilayer structures.

Multilayer blow molding has become a signiﬁcant means

to recycle postconsumer high-density polyethylene scrap

where all of the layers are substantially the same mate-

rial. Containers for household chemicals and motor oils

are typical uses. A common structure is an inner product

contact layer of virgin high-density polyethylene, an outer

layer of pigmented virgin high-density polyethylene, and

middle layer of postconsumer commingled reclaim high-

density polyethylene (see Figure 15b). The commingled

HDPE is a mixture of homopolymer and copolymer HDPE

in a rainbow of colors that becomes an unpleasing yellow-

greenish-bluish gray color when molded. The commingled

material provides poor chemical stress-crack resistance,

a critical product requirement particularly for many

household and automotive chemicals. The virgin HDPE

copolymer inner product contact layer provides the re-

quired stress-crack resistance. A pigmented HDPE outer

layer provides a pleasing overall bottle appearance hiding

the middle layer.

Although technically not multilayer extrusion, some

extrusion heads for high-density polyethylene applica-

tions have a coextrusion feature for creating a ‘‘view

stripe’’ in an otherwise opaque pigmented container. As

the extrusion head establishes the tubular cross-sectional

form of the parison from a ﬂow of pigmented thermoplastic

material, before the parison exits the head, the pigmented

material ﬂow splits at one point with a secondary ﬂow

of unpigmented material from a separate plasticizer,

thus creating a pigmented parison with an unpigmented

translucent stripe. The stripe is in a position usually

corresponding with a side of the blow-molded bottle.

Graduation marks on the bottle in a position along the

view stripe allow consumers to easily judge content

volume.

Injection Multiplayer

As with extrusion multilayer, injection multilayer also

relies on laminar ﬂow of thermoplastic materials entering

the preform cavity to create individual layers. Although

there are subtle process variations between several tech-

niques, two or three individual plasticizers inject material

into the preform cavity through concentric injection

Figure 13. A horizontal rotary two-step

blow-molding machine with preform oven.

This is a high-output machine featuring 34

blowing stations capable of manufacturing

over 61,000, 0.7-L carbonated soft drink

PET bottles per hour. (Courtesy of Group

Sidel.)

Adhesive-Laye

Melt Channel

Parison Head Toolin

Manifold

Connector

Outer-Layer

Melt Channel

Reclaim-Layer

Melt Channel

Inner-Layer

Melt Channel

Barrier-Layer

Melt Channel

Figure 14. Cut-away schematic of a typical multilayered parison

extrusion head.

BLOW MOLDING 149

nozzles, in generally either a simultaneous or a sequential

manner, to establish substantially three- or ﬁve-layer

structures (see Figure 15c). Usually based on polyethylene

terephthalate, a typical structure has a PET layer, a gas

barrier layer, and another PET layer. The barrier layer is

likely polyamide such as poly-m-xylylene adipamide or

nylon-MXD6. Ethylene vinyl alcohol copolymer is another

choice for barrier layer material, but the PET/EVOH is

more susceptible to layer delamination.

Creating an injection-based two- or four-layer structure

requires an insert or insert equivalent approach; that

is, an apparatus usually places the insert made of at least

one layer of material onto the injection cavity core, where

then the plasticizer of the injection molding machine

injects a second material over the insert to complete

the multilayer preform. Process technicians often refer

to this approach as ‘‘over-molding.’’ In general, the over-

molding approach is not as common or as efﬁcient, but it

does have an interesting advantage in that the insert can

be thermoformed, injection molded, or even extrusion

blow-molded.

Development of price competitive multilayer bottles

has allowed package designers to consider thermoplastic

materials, particularly PET, as a viable alternative to

glass and metal where high gas barrier is required. While

development of machinery processes is signiﬁcant, these

bottle opportunities would not be feasible without ongoing

development of high-performance barrier thermoplastic

materials both passive and active in nature.

Poly-m-xylylene adipamide or nylon-MXD6 is one of

many such materials. Nylon-MXD6 provides excellent

passive barrier to oxygen; however, eventually, enough

oxygen molecules can permeate through to change highly

oxygen-sensitive products. Interestingly, nylon-MXD6 can

become active by adding a ‘‘transition’’ metal salt such as

that based on iron, cobalt, or other ‘‘transition’’ metals.

The salt becomes a catalyst or oxidation promoter that

causes the nylon-MXD6 material to oxidize or attract

oxygen, thereby allowing the material to actively scavenge

oxygen molecules permeating through the bottle sidewall

before the oxygen molecule can react with the product

contained.

Sometimes an added ﬁller, such as a dispersion of

nanoclay material, creates a tortuous path for the per-

meating oxygen molecule forcing that molecule to travel

a greater distance and effectively improving barrier per-

formance. In addition, some barrier materials combine

oxidation promoter and tortuous path techniques in a

nanocomposite material.

Preform or Bottle Coating

While not a multilayer structure in the sense of extrusion

or injection-based approaches, applying a coating indeed

Inner PP Layer

(b)

(a)

(c)

Adhesive Layers

Inner Virgin

HDPE Layer

Reclaim Layer

Outer PP Layer

Barrier

EVOH

Layer

between

Adhesive

Commingled

Post-consumer

HDPE Layer

Outer Virgin HDPE

mented La

Inner PET Layer

Nylon Barrier Layers

Middle

PET Layer

Outer PET Layer

Figure 15. Common multilayer bottle wall

structures: (a) Typical PP barrier food packaging

application; (b) typical post-consumer recycled

HDPE application; (c) typical stretch PET bar-

rier food packaging application.

150 BLOW MOLDING

creates multilayered structures. All of these coating ap-

proaches seek to improve oxygen and/or carbon dioxide

gas barrier performance of polyester or PET bottles.

Makers of carbonated beverages, for example, are using

coatings to improve carbon dioxide retention in small

bottles, less than 500 mL (16.9 ﬂ oz) in capacity, where

bottle surface area relative to bottle capacity is too great.

Smaller bottles allow a greater rate of gas permeation

than larger bottles having less bottle surface area relative

to bottle capacity. For example, an uncoated 2000-mL

capacity PET bottle provides adequate 16-week shelf life

for a cola beverage, but shelf life for an uncoated 300-mL

bottle could be less than six weeks.

At least one company has developed a water-based ﬂow

coating material and process for covering preform exterior

surfaces for improved gas barrier in the blow-molded

bottle. Stand-alone apparatus applies and then cures the

coating on the preform before the blow-molding machine

reheats and then expands that preform into the bottle.

Given that coatings tend to be thin, most approaches

apply these coatings to the bottle instead of the preform.

Polyvinylidene chloride (PVdC)-based organic coatings

were an early barrier material. A more recent organic

coating development uses epoxyamine chemistry to create

a tough, glossy thermoset polymer ﬁlm on bottle exterior

surface. Stand-alone apparatus typically inline with the

blow-molding machine sprays or dips the bottle and then

cures the coating material.

Another coating approach uses a form of plasma-

assisted or -enhanced chemical vapor deposition process

to create an extremely thin inorganic ﬁlm on either the

exterior or interior bottle surfaces. Silica-based ﬁlm de-

positions create a silicon oxide (SiOx) barrier ﬁlm from gas

precursors containing, in part, silicon and oxygen. Car-

bon-based ﬁlm deposition create an amorphous carbon

barrier ﬁlm using an acetylene gas precursor. Fundamen-

tally, the deposition process involves placing the bottle in a

vacuum chamber, introducing appropriate quantities of

certain precursor gases into the chamber, exciting those

gases with energy and creating a plasma that causes a

compound (such as silicon oxide or amorphous carbon) to

form that normally would not form if not for the plasma,

and allowing the compound to condense on the bottle

surface. Specially built machinery for depositing these

coatings on bottles are often inline with blow-molding

machinery.

Bottle Treatment

Again, while not a multilayer structure in the sense of

extrusion- or injection-based approaches, bottle treatment

approaches seek to improve gas barrier of volatile hydro-

carbon chemicals in polyoleﬁn bottles by allowing polymer

molecules at or near the container’s surface to react with

certain gases, thereby creating a thin integral gas barrier

layer on the surface. There are two approaches: ﬂuorina-

tion and sulfonation.

The ﬂuorination approach is either downstream or part

of the blow-molding process. When part of the blow-

molding process, the molding machine uses pure nitrogen

gas to initially inﬂate the parison in the blow mold cavity,

quickly followed by a mixture of a small amount of ﬂuorine

gas with nitrogen gas to complete the blow-molding

process. The ﬂuorine gas easily and quickly reacts with

the polyoleﬁn material still hot from parison extrusion.

Chemically, the ﬂuorine replaces hydrogen atoms within

the polymer molecules that are on the interior surface.

To promote reaction, the downstream approach requires

reheating the bottle before ﬂuorine exposure. Fluorine is a

dangerous gas requiring special handling by trained

technicians.

Fluorination makes high-density polyethylene and

polypropylene bottles suitable for packaging nonpolar

chemicals, such as those in cleaners, mineral spirits,

turpentine, solvents such as carbon tetrachloride, and

gasoline additives, insecticides, herbicides, and the like.

Although treatment barrier is not quite as effective, the

sulfonation approach introduces sulfur trioxide to the

inside surface of the bottle or container downstream of

the blow molding machine. The sulfur trioxide reacts with

the HDPE, causing sulfonic groups to form on the contain-

er’s surfaces. The approach then introduces ammonium

hydroxide to neutralize any acidic byproducts followed by

a water rinse.

HEAT-RESISTANT PET BOTTLES

Conventional stretch or molecular biaxial-oriented blow-

molded PET thermoplastic bottles cannot withstand tem-

peratures much above 551C (1301F) without signiﬁcant

bottle distortion or shrinkage from release of molded-in

molecular strains and stresses created during the blow

molding process. Modiﬁed processes make heat-resistant

PET polyester bottles (molding technicians also call

these ‘‘heat-set’’ or ‘‘heat-stabilized’’ bottles), which in

part relieve stresses and ‘‘locks-in’’ remaining stresses

with an increased crystalline molecular structure that

prevents any signiﬁcant bottle shrinkage at desired pro-

duct ﬁll temperatures.

Product applications for heat-set bottles include high-

acid foods, such as fruit juice and isotonic drinks. Suppli-

ers using a modiﬁed ‘‘canning’’ process often ﬁll these

juices and drinks at an elevated temperature capable of

sterilizing the bottle, typically 681Cto961

C (1551Ft

2051F) and usually

at 851C (1851F). Patent literature is

full of bottle manufacturing process and equipment de-

scriptions further extending bottle heat-resistant perfor-

mance for low-acid foods requiring higher-temperature

pasteurization or retort sterilization processes (10).

Heat-set PET bottles are now routinely for packaging

products previously not feasible.

Molecularly, polyethylene, particularly high-density

polyethylene, is considerably crystalline in nature. On

the other hand, polyvinyl chloride is amorphous. Crystal-

line thermoplastic materials tend to be opaque. Amor-

phous thermoplastic materials tend to be clear or

transparent. Polyethylene terephthalate is a unique ther-

moplastic material in that, with relative ease, it can

assume a substantially amorphous molecular form or a

signiﬁcantly crystalline form depending on molding and

processing conditions. Moreover, PET having crystalline

BLOW MOLDING 151

structure can be either clear or opaque. Molding techni-

cians take advantage of this unique PET material cap-

ability to manufacture high-performance, transparent

heat-set bottles.

Polyethylene terephthalate crystals are generally

either spherulitic in shape or rod-like in shape. Spheru-

litic crystals are heat-induced in substantially amorphous

PET, and formations of these relatively large spherulitic

crystals cause clear PET material to become a white

opaque color. Fundamentally, the spherulitic crystal is

larger than light wavelength. Consequently, the crystals

reﬂect light attempting to pass through the PET material.

Rod-like crystals initially form in PET material from

molecular strain or biaxial orientation and continue to

grow with heat. Rod-like crystals are relatively small,

permitting light to pass without signiﬁcant reﬂection.

A conventionally molded PET bottle—for example, a

transparent carbonated soft drink bottle sidewall—is

about 20% crystalline. These crystals are substantially

rod-like in nature. Clear heat-set PET bottles for typical

hot-ﬁll applications have at least a 28% crystalline level.

Higher-performance heat-set bottles may have crystalline

levels around 30–32%. Theoretically, crystalline levels of

W50% percent are possible.

A crucial area of the heat-set bottle is the neck-ﬁnish.

The neck-ﬁnish cannot deform in normal use. Generally,

the PET thermoplastic material within the neck-ﬁnish is

amorphous, that is, noncrystalline. The amorphous struc-

ture is often not acceptable for higher performance bottle

applications. One means to gain additional heat-stability

is to deliberately crystallize the neck-ﬁnish with heat to

create spherulitic crystalline structure. These bottles are

easy to recognize in that its sidewall is clear and its neck-

ﬁnish is opaque white. An alternative approach, for typi-

cal applications requiring slightly less performance, is to

increase overall wall thickness of the neck-ﬁnish, permit-

ting it to retain its amorphous structure. While this

approach adds weight, it is often less expensive overall.

The alternative approach works because in a ﬁlling-line

environment, using a post-ﬁll cooling tunnel and coupled

with a generally poor thermal conductivity of the thermo-

plastic, the heaver neck-ﬁnish does not become hot en-

ough, long enough to allow the neck-ﬁnish to appreciably

deform.

While there are several variations, machinery builders

have developed two fundamental heat-set bottle-manufac-

turing approaches. First is a single-mold method where

the machine stretch blow-molds the heated preform into a

mold having a temperature of about 1201C to 1301C (2481F

to 2661F). Air pressure holds the molded bottle against the

mold cavity for about 3 s, which is sufﬁcient time to allow

stresses to relieve slightly and promote required addi-

tional crystalline growth. Second is a dual-mold method

where the machine stretch blow-molds the preform into a

ﬁrst mold having a ﬁrst shape slightly larger than ﬁnal

bottle form. The machine then reheats this ﬁrst shape in

an oven to relieve molecular stress but not alter biaxial

orientation. Heating the ﬁrst shape causes it to shrivel

into a second shape while increasing crystalline levels.

The second shape is then blow-molded in a second blow-

mold cavity to produce the ﬁnal bottle form.

Besides heat resistance, another consideration for

heat-set, hot-ﬁllable PET bottles is collapse from va-

cuum-related forces after ﬁll, seal, and cool to room

temperature. As the bottle and contents cool, it contracts

reducing volume that in turn creates a negative pressure

or vacuum inside. This contraction is different from

shrinkage that might occur from molecular stress relief

in a poorly heat-set bottle. Vacuum-related forces acting

against the bottle can often slightly distort bottle shape,

particularly in a lightweight bottle. To maintain a pleas-

ing overall appearance, clever hot-ﬁllable bottle designs

have ‘‘vacuum panels’’ or other surfaces that manage

these distortions in a uniform fashion.

ASEPTIC BLOW MOLDING

While any blow-molding process can directly or indirectly

link with a ﬁlling machine that sterilizes the bottle and

maintains an aseptic ﬁlling environment, aseptic blow-

molding usually involves the extrusion blow-molding

method where the bottle is blow-molded in a commercially

sterile environment with highly modiﬁed equipment. Of-

ten the product-ﬁlling process is an integral part of the

blow-molding machine. Generally, aseptic blow-molding

machines use one of two fundamental approaches.

Blow-and-Hold Approach

This is substantially a two-stage approach. The blow-

molding machine seals the bottle interior before removal

using moving plates in the mold to seal the blow-pin

opening after pin withdrawal. Following removal from

the machine, the bottle exterior is no longer in a sterile

environment, but the seal maintains sterile conditions

inside. In the second stage, a stand-alone aseptic ﬁlling

machine, with its own sterilized environment, sterilizes

the bottle exterior and then removes the seal, thereby

exposing the bottle interior. Next, the machine ﬁlls the

bottle with sterilized product and reseals before removal

and distribution.

Blow-Fill-Seal

Approach

This fundamental approach

has two variations. In one

variation, the blow-molding machine molds and then ﬁlls

the bottle within the blow-mold cavity. One or two seconds

after initial parison inﬂation, a measured quantity of

product ﬂows in to ﬁll the bottle. A few seconds later, the

blow pin and product ﬁller tube withdraw and another

small mold cavity, just above the main bottle mold cavity,

closes to seal the bottle and product inside. Bottles made

and ﬁlled, using this variation, often have a molded-in

twist-off seal that one can easily remove without a knife or

scissors. In another variation, the blow-molding machine

uses conventional means to manufacture the bottle and

then immediately transfers it to a ﬁlling station where the

machine ﬁlls and seals the bottle with a conventional

closure. Bottle manufacture, ﬁlling, and sealing all take

place in the same controlled commercially sterilized

environment.

152 BLOW MOLDING

Aseptic blow-molding machinery modiﬁcations gener-

ally include extensive use of stainless steel and plated

materials throughout. Special ‘‘clean-in-place’’ ﬁxtures

sterilize all internal surfaces, passageways, hoses, blow

pins, valves, and so forth. A cabinet encloses the molding

and ﬁlling area, and a ﬂow of sterilized, ﬁltered air having

laminar ﬂow characteristics maintains sterilized condi-

tions inside. Once molding process begins, nothing inside

the cabinet is touchable by human hands.

OTHER SECONDARY BLOW-MOLDING PROCESS

ADDITIONS

As well as those already discussed, three other additions

are commonly part of the blow-molding process.

In-mold labeling

In-mold labeling (IML) is a process addition where a robot-

like arm places a paper- or plastic-based preprinted label

onto the blow-mold cavity surface just before mold close

and parison inﬂation. A series of vacuum holes in the mold

cavity hold the label in place until the expanding hot

parison can contact and establish a 100% bond with the

label’s heat-activated adhesive.

Internal Cooling

Normally, cooling of the blow-molded bottle occurs from

the outside only—that is, from the blow-mold cavity itself.

Consequently, heat energy must travel through the entire

thickness of the bottle sidewall. Usually, this is not a

major problem for most applications; however, cooling of

heavy containers, such as a drum (or cooling heavy sec-

tions within the container), can take a signiﬁcant amount

of time. Assuming that the blow-molding machine plasti-

cizer is capable of delivering thermoplastic melt for the

next parison at a higher rate, removing some of the heat-

energy from inside molded container can improve molding

efﬁciency.

With varying cost effectiveness, molding technicians

have developed several internal-cooling approaches, in-

cluding injection of liqueﬁed gas, super-cold air with or

without injected water vapor, and air exchange. Effective-

ness highly depends on speciﬁc application conditions;

often apparatus and related gas or excess air delivery

expense is greater, offsetting any cost beneﬁt from higher

productivity.

Post-Mold Cooling

Post-mold cooling is another means that molding techni-

cians use to shorten molding time. Here the blow-molding

machine removes the partially cooled bottle from the mold

cavity, placing it into a typically water-cooled nest where

cooling continues. At least a portion of the nest mimics the

bottle contours. In addition, the nest may have a series of

ﬁne nozzles directing a ﬂow of high-velocity air toward

hotter areas. Often the nest is part of the trim tooling used

for removing bottle ﬂash.

BOTTLE DESIGN GUIDELINES

Figure 16 illustrates a number of bottle and neck-ﬁnish

terms. Good product design begins with a clear under-

standing of bottle manufacturing process and tooling

limitations. Consider injection blow molding, for example;

as a rule, available strength to minimize core rod deﬂec-

tion will generally limit its length to about 10 times its

diameter. In other words, height of the bottle should be

about 10 times or less its neck-ﬁnish diameter. Limits in

preform wall thickness and ability of tooling to properly

heat-condition the preform restricts preform blowup ratio;

that is, the bottle diameter or largest dimension perpen-

dicular to the core rod axis should be about three times or

less than the neck-ﬁnish diameter. For consistent wall

thickness, bottle cross-sectional shape also has limits.

Oval cross-sectional conﬁgurations require the oval’s ma-

jor diameter to be about two times (or less) its minor

diameter.

In comparison, extrusion blow molding is not as limited

as injection blow molding. For most thermoplastic materi-

als, molding technicians limit parison to bottle diameter

blowup ratio to about four times or less. This general rule

applies not only to overall bottle shape but also to isolated

Shoulder

Handle

Label

Panel

Handle Island

(trim piece)

Push-up

Bottom

Chime

Neck-finish

Figure 16. Common bottle and neck-ﬁnish terms. T, thread

diameter; E, external diameter; I, inside diameter; H, ﬁnish

height; S, thread start.

BLOW MOLDING 153

sections within the bottle. For example, bottle handle

designs that are deeper than wide across mold parting

face, particularly at the handle’s outer end, are difﬁcult to

mold while maintaining a reasonably consistent wall

thickness.

Generally, most blow-molded bottles perform better

when its sidewall has uniform or nearly uniform wall

thickness. Designers can help achieve this uniform thick-

ness by remembering to ‘‘radius, slant, and taper’’ all

surfaces. Bottles having square, ﬂat surfaces with sharp

corners will not likely perform well. Sharp corners will

usually become thin and weak. In contrast, ﬂat side panels

often are relatively heavy and thick and likely distorted.

Flat, square shoulder conﬁgurations offer little top-load

strength, an important consideration for bottle storage

and warehousing. Ribs do not always stiffen. Rib conﬁg-

urations create more surface area for the parison to cover,

which, in turn, thins bottle wall thickness, potentially

creating a bellows or accordion effect that ﬂexes easier.

Good designers think about the bottle as a whole structure

and attempt to determine how various forces acting on the

bottle impact performance. For example, what areas of the

bottle are likely to ﬂex when subjected to forces antici-

pated with double or triple stacked pallets of ﬁlled bottles

in a warehouse.

Blow-mold process conditions can inﬂuence bottle di-

mensions or, more important, bottle capacity. Consider

high-density polyethylene milk bottles, for example. By

tradition, U.S. dairies ﬁll milk bottles to within one half

inch (13 mm) of the top of the neck-ﬁnish. In other words,

the milk bottle has very little headspace above its ﬁll level.

Because the dairy ﬁlls to this level, it requires the bottle

capacity to be exact. To avoid overﬁlling or underﬁlling,

naturally, the dairy does not want its bottles too large or

too small.

Technicians at Dow Chemical determined that several

process conditions could signiﬁcantly change bottle capa-

city (11). High-density polyethylene bottles shrink over

time, with about 80% occurring within the 24 h after

manufacture, but small amounts of measurable shrinkage

still occurs after 10 days. Lightweight bottles are bigger

because of less thermoplastic material on the inside and

because the bottle bulges more from product weight. A 5-g

bottle weight reduction of a typical 1-gal (3.78-L) milk

bottle increases capacity by about 12 mL (5 mL for plastic,

7 mL for bulge). Faster manufacture cycle time, lower

parison inﬂation air pressure, and higher parison and/or

mold temperature also reduce bottle capacity. Lastly,

empty bottle storage temperature is an important consid-

eration. A signiﬁcant reduction of bottle capacity occurs

after 10 days of storage at 1401F (601C), a condition that

could easily occur in a semi-trailer on a hot summer day.

BIBLIOGRAPHY

1. R. Holzmann, ‘‘The Development of Blow Moulding from the

Beginnings to the Present Day,’’ Kunststoffe 69, 704–711

(1979).

2. J. H. Dubois, Plastics History USA, Cahners Publishing Co.,

Boston, 1972, pp. 14–16.

3. Early patent describing process approach: W. B. Carpenter,

Process of, and Apparatus for, Molding Hollow Forms of

Celluloid or like Plastic Material, U.S. Patent No. 237,168,

February 1, 1881.

4. J. H. Dubois and F. W. John, Plastics, Reinhold., New York,

1967, pp. 37–43.

5. R. V. Jones, ‘‘High Density PE—Polymer Success Story,’’

Plastics World April, 32–39 (1969).

6. G. S. Brown, ‘‘The Mechanical Processes of Blowmolding,’’

Plastics World May, 16, 17, 20, 21 (1959).

7. W. O. Bracken, Blow Molding Developments, Technical Papers,

Vol. VII, 17th ANTEC, Society of Plastics Engineers, 1961.

8. U.S. Patents to Moreland and Gussoni are typical examples of

early two-station tooling adaptations: S. T. Moreland et al.,

Machine for Molding Thermoplastics, U.S. Patent No.

2,298,716, October 13, 1942; and A. Gussoni, Method for

Producing Hollow Blown Articles from Thermoplastic Mate-

rials, U.S. Patent No. 3,011,216, December 5, 1961.

9. A. Gussoni, Machine for the Manufacture of Bottles from Plastic

Material, U.S. Patent No. 2,853,736, September 30, 1958.

10. Examples include: T. J. Boyd et al., Blow Molding Method for

Producing Pasteurizable Containers, U.S. Patent No.

6,485,670, November 26, 2002; H. Sugiura et al., Double-

Blown PET Bottle Shaped Container Having Essentially No

Residual Stress and Superior Heat Resistance, U.S. Patent

No. 5,928,742, July 27, 1999; G. Denis et al., Equipment for

the Manufacture of Polyethylene Terephthalate Container,

U.S. Patent No. 5,334,007, Aug. 2, 1994.

11. Operators Guide—Controlling Shrinkage of HDPE Bottles,

Dow Chemical Co., Midland, MI, 1979.

General References

C. Irwin, ‘‘Blow Molding’’ in A. L. Brody and K. S. Marsh, eds.,

Encyclopedia of Packaging Technology, 2nd edition, John

Wiley & Sons, New York, 1997, pp. 83–93.

D. V. Rosato, A. V. Rosato, and D. P. DiMattia, Blow Molding

Handbook, 2nd edition, Hanser Gardner Publications, Cincin-

nati, OH, 2003.

BOTTLE DESIGN, PLASTIC

ANDREA S. MANDEL

Andrea S. Mandel Associates,

Packaging Consulting Services,

West Windsor, New Jersey

Like many of today’s popular packaging techniques, blow

molding of plastic bottles became popularized soon after

World War II (see Blow molding). The original applications

took advantage of the ﬂexibility of plastic material

to create squeeze bottles for dispensing of deodorants

or medicines. The availability of reasonably priced

higher-density polyethylene for rigid containers in the

1950s led to the widespread use of plastic bottles for

detergents (1).

As both molding and plastic material technology devel-

oped, conversion to plastic bottles expanded beyond

154 BOTTLE DESIGN, PLASTIC

household products into health and beauty aids, foods and

beverages, and general goods. By the last decade of the

twentieth century, most soft drink bottles were converted

from glass to plastic, and oil and other automotive product

containers were changed from metal to plastic. The 1990s

and early twenty-ﬁrst century brought technical break-

throughs in materials and manufacturing that allowed

the economical production of high barrier, multilayer and

high temperature resistant plastic bottles. As a result,

foods and drugs requiring hot ﬁll and good ﬂavor, odor,

oxygen, and vapor barriers (such as sauces and baby

food) began to be packaged in plastic bottles and jars in

increasing quantities.

The advantages of plastic over glass bottles are abun-

dant. They are safer, lighter, easier to handle for consumer

and manufacturer, and easier to manufacture, and they

offer improved versatility in design. However, many of the

earlier conversions suffered because of the marketer’s

attempt to imitate the original glass bottle as closely as

possible. It was feared that the consumer (especially in

beauty aids) would perceive the change to plastic as

strictly a cost-savings measure resulting in reduced qual-

ity. Today, the majority of plastic bottles are designed to

take advantage of the material’s unique properties, and

the consumer has come to expect all the inherent advan-

tages in the items they purchase.

In the 1970s and 1980s, plastic bottle designers and

manufacturers were faced with widely ﬂuctuating plastic

material and additive costs and availability. The latter

1980s and 1990s added the additional challenge of envir-

onmental concerns. The early twenty-ﬁrst century has

brought us dual concerns: high energy prices with in-

creased costs and the potential for shortages, along with a

renewed interest in environmental considerations. The

package must not only minimize material usage to reduce

costs and environmental impact, it must also take into

account its eventual disposal.

BASIC STEPS IN THE PLASTIC BOTTLE DESIGN AND

DEVELOPMENT

There are several basic steps for the proper design and

development of plastic bottles. They can be summarized as

follows:

1. Deﬁne bottle requirements—product to be con-

tained, use, distribution, aesthetics, and environ-

mental issues.

2. Deﬁne manufacturing and ﬁlling requirements—

types of molding available and ﬁlling and packa-

ging systems.

3. Select materials.

4. Rough drawings.

5. Part drawing.

6. Model and/or rapid prototyping.

7. Mold drawing.

8. Unit cavity.

9. Unit cavity sampling and testing.

10. Finalize drawings.

11. Production mold.

12. Production mold sampling and testing.

13. Production startup.

This is a basic list of steps for bottle production. Some

items may be done in a different order, and some can be

performed concurrently to save time. Many items will be

performed more than once as the design is reﬁned. In

addition, mating ﬁtments and secondary packaging are

frequently being developed in the same timeframe and

become part of the critical path.

DEFINING BOTTLE REQUIREMENTS

What Will the Bottle Hold?

Information about the product to be contained is of

primary importance for bottle design, particularly in the

selection of material and neck ﬁnish. Will the product be

liquid, powder, or solid? What is its viscosity? Is it homo-

geneous, or does it tend to separate out? Will the product

have to be shaken to be used? What plastic materials and

additives is it compatible with? How sensitive is it to

moisture or oxygen gain or loss? Does the product have

components (such as those found in detergents) that could

make the bottle prone to stress cracking? Will the product

outgas into the headspace and cause a pressure buildup?

Will the product absorb oxygen from the headspace and

tend to collapse the bottle? Will the product be ﬁlled into

the bottle hot or cold? How much product should the

container hold (in weight or volume)? Is the product

considered hazardous or in some way regulated?

WHAT ARE THE SPECIAL REQUIREMENTS FOR BOTTLE

PERFORMANCE?

Product End-Use. How will the product be used by the

ultimate end-user for dispensing and storage? Examples

of this are squirt bottles, roll-on bottles, trigger-spray

bottles, drainback closure/measuring cup systems, bottles

with integral funnels, and bottles that are stored on their

caps. What are the storage conditions (temperature, hu-

midity, pressure) of opened and unopened containers?

Secondary Packaging. Will the primary package be sold

in an intermediate secondary package? Examples of this

are carton, blister, and tray.

Distribution Requirements. Will the unitized product be

sold in shipping case or partial-height tray? What type of

inner partitions or shrink wrap can be used? Do inter-

mediate customers require smaller unitized packs and if

so, of what type? Will the bottle itself contribute to

stacking strength? Does the ﬁnal outer package have to

ﬁt on a standard GMA 40-in 48-in. pallet (consider load

optimization and stack heights)? Will cases be shipped in

mixed loads at any time? Will shipment generally be by

full

truckload, partial truckload

(LTL), or individual ship-

ment by ground or air (such as United Parcel Service or

Federal Express)? Does the product travel in a rack

BOTTLE DESIGN, PLASTIC 155

system or bulk package for any part of its distribution?

Are there shelf size requirements for the retail market or

for storage? Will the distribution system require the

product to be shipped over high altitudes or in unusually

hot temperatures or low and/or high humidity environ-

ments? Will international shipment lead to special condi-

tions or requirements?

Aesthetic Requirements. The plastic bottle is frequently

required to act as a salesperson at point of purchase and

after the sale. Common concerns include:

1. Shelf facing size—maximization of width and/or

height vs depth while maintaining package stability

2. Clarity, opacity, and color

3. Label area and label qualities

4. Requirement for recycled and/or recyclable material

WHAT ARE THE SPECIAL REQUIREMENTS FOR BOTTLE

MANUFACTURING AND FILLING?

Options for Bottle Manufacture

Ideally, the designer should be able to select the best

manufacturing technique for the particular bottle. How-

ever, because of cost, available capital equipment, and

other factors, it may be necessary to compromise if possi-

ble. The major choices are:

1. Molding process—injection blow, extrusion blow,

stretch blow (with variations)

2. New or modiﬁed molds (or parts of molds and mold

bases)

The various advantages and disadvantages of the differ-

ent plastic bottle manufacturing methods and molding

processes are summarized as follows:

1. Process: extrusion blow molding

a. Description: A tube (parison) is extruded through

an annular die. Two halves of a bottle mold are

clamped over the parison, sealing the top and

bottom except for a hole for air injection. Air is

injected, expanding the parison to match the

mold. Clamped material (excess material at the

top and bottom commonly called necks and tails)

is removed and generally put back into the

system as regrind.

b. Advantages

i. Relatively inexpensive mold, mold modiﬁca-

tion and equipment costs for a basic system.

ii. Multilayer bottles and extruded side stripes

possible.

iii. Larger size containers are economical.

iv. In-mold labeling available.

v. Good for handleware or other designs requir-

ing a molded-in ‘‘hole.’’

c. Disadvantages

i. High built-in waste due to necks and tails.

ii. Requires in-mold or postmold trimming of

necks and tails.

iii. Finish (neck area that typically receives a

cap or ﬁtment) dimensions and quality are

not as consistent.

d. Variations

i. Extrusion dies can be ovalized to improve

material distribution in oval or rectangular

bottles.

ii. Extrusion dies can be programmed to open

and close while extruding, providing a top to

bottom variation in wall thickness.

iii. For high-volume large-container production,

continuous wheel machines can be used in-

stead of molds that shuttle in and out.

iv. Instead of direct blow into a bottle, a dome

may be created that must be trimmed off the

ﬁnish. A calibrated neck, on the other hand,

uses the blow pin to help mold the ﬁnish. The

top of the ﬁnish may be further improved by

posttrim reaming.

v. High blow ratio (ﬁnish to maximum width) or

off-center neck bottles can sometimes be im-

proved by blowing ‘‘outside’’ the neck; a wide

parison is used and waste is trimmed off the

sides of the ﬁnish on the parting line.

2. Process: injection blow molding

a. Description: The neck ﬁnish and ‘‘cigar’’-shaped

body are injection molded in the ﬁrst phase. This

parison mold preform is then moved to a blow

station containing a mold shaped like the bottle

body. Air is injected through the ﬁnish, and the

completed bottle is removed from the pin. The

injection molded performs can either be premade

and stored for later transport to be reheated

for the blow molding process, or can be molded

and then blow molded all in one continuous

process.

b. Advantages

i. Good quality and control of ﬁnish dimensions

due

to injection process

ii. Capability of control and design inside the

ﬁnish area (as long as there are no

undercuts).

iii. Economical for large volumes of smaller

containers.

iv. Waste not built into molding process (injec-

tion mold is hot runner).

v. Can use PET as material.

vi. Finished container does not have a ‘‘pinchoff’’

area at the tail (the line where the extruded

parison is actually cut off the container dur-

ing the manufacturing process) that can pro-

vide a weak point for drop test or stress-crack

failure.

c. Disadvantages

i. Additional need for injection molds increases

cost and lead times (although existing pre-

form molds can sometimes be used to save

costs and time).

156 BOTTLE DESIGN, PLASTIC

ii. In-mold labeling and handles (with ‘‘holes’’)

not generally available.

iii. Not good for larger containers or high blow

ratios.

3. Process: stretch blow molding

a. Description: This is a variation on injection blow

molding wherein the hot parison mold, or pre-

form, is stretched in length by a push rod placed

in the bottom prior to blow molding. The preforms

can be premade and stored for later transport to

be reheated for the blow-molding process, or it

can be molded and then blow molded all in one

continuous process.

b. Advantages

i. Improved bottle impact and cold strength.

ii. Improved transparency, surface gloss, stiff-

ness, and gas barrier.

iii. Good for pressure containers such as soda

bottles.

iv. Opportunity to save plastic material by opti-

mizing strength through stretch process and

good control.

c. Disadvantages

i. Commonly used PET requires special hand-

ling and drying process.

ii. Extra step required in process.

iii. High capital equipment costs.

iv. Additional need for injection molds increases

cost and lead times (although existing pre-

form molds can sometimes be used to save

costs and time).

v. In-mold labeling and handles (with ‘‘holes’’)

not generally available.

Bottle Filling and Packing

Bottle design needs to take into account the ﬁlling and

packing operations for the bottle. Important criteria

include:

1. Current equipment and change parts. There are

frequently size limitations (height, width, and depth

on equipment). Likewise, change parts and change-

over times can be minimized by determining com-

mon critical dimensions or easier adjustments

(frequently height) with other bottles that might

run frequently on the same line.

2. Forces and conditions imparted by the ﬁlling and

packing operation on the bottle. Common conditions

include downward compression due to insertion of

items in the neck (such as plugs and balls) or on the

ﬁnish (such as snap-on closures); bottle body torsion

and neck distortion due to high-torque cap applica-

tion; side-to-side compression due to labeling and

starwheel pinch points; multidirectional compres-

sion and abrasion due to bulk handling such as in a

bottle unscrambler; and bottom drop impact due

to automatic drop packing into cases or trays

(Figure 1). In addition, aesthetic damage can be

done at various points to a labeled or unlabeled

bottle due to abrasion at transfer points and hand-

ling throughout the line. In some cases, machinery

can be selected or modiﬁed to reduce these condi-

tions or insure that they occur at protected or strong

points on the container. If this is cost prohibitive or

not possible, the bottle needs to be designed with

these requirements in mind (see Bottle Design and

Speciﬁcation).

BOTTLE DESIGN AND SPECIFICATION

Once the special requirements for the bottle have been

determined, sketches and models of various bottle designs

meeting these criteria can be produced. As the process

continues, the ﬁnalists need to become more detailed

(dimensions, angles, ﬁnish) and begin to incorporate

good plastic bottle design principles. Computer modeling

and rapid prototyping can be used as part of this process.

GENERAL DESIGN CONSIDERATIONS

1. Sharp Edges and Changes in Direction Should Be

Avoided. Surfaces should be blended and generous

radii used as much as possible at bends and corners

(2). Sharp edges can lead to thin and high-stress

areas, resulting in stress cracking or failure on drop

testing.

2. The bottle must have minimal or no undercuts.

Thought must be given to how the bottle is to be

removed from the mold. The inside of the ﬁnish must

be stripped from the core rod or blow pin, and the

front and back halves of the mold need to open up

from the outside of the bottle. Any plastic that would

hook onto the tooling or molds can cause the bottle to

distort or even break. Very small and rounded

undercuts, such as small retaining rings on the

inside of the neck, can sometimes be successfully

molded; trial and error may be necessary for a

particular design and process to see how much is

possible.

3. W

all Thickness

Minimum and Distribution Should

Be Considered. Thin spots can lead to stress

Figure 1. Common stresses on a bottle on a ﬁlling and packing

line.

BOTTLE DESIGN, PLASTIC 157