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century using two sheets of cellulose nitrate material

clamped between two mold halves (2). Steam injected

between the sheets softened the material, sealed the

edges, and expanded it against the mold cavity (3).

Regrettably, cellulose nitrate is highly ﬂammable, thus

limiting practical use.

More useful cellulose acetate and polystyrene thermo-

plastics became available in the 1920s and 1930s (4). By

the mid-1930s Owens-Illinois Glass Co. and PLAX Corp., a

newly formed division of Hartford Empire Co., each began

to develop proprietary plastic bottle manufacturing pro-

cesses based in part on automated glass-blowing machin-

ery then in use (2). Unfortunately, commercialized

cellulose acetate and polystyrene bottles were signiﬁ-

cantly more expensive and, other than novelty, these

bottles offered no other marketable advantages over glass.

Low-density polyethylene provided the necessary ad-

vantage; in the mid-1940s, following World War II, PLAX

Corp. began blow molding the ﬁrst commercial plastic

container in high-volume, a squeeze bottle for Stopette

underarm deodorant (2).

While commercial development of the squeeze bottle

was a signiﬁcant event, blow molding of thermoplastic

materials in North America did not really begin until the

mid-1950s when high-density polyethylene became com-

mercially available (5); more important, commercial blow-

molding machinery becoming available in the late 1950s

(6). High-density polyethylene provided additional stiff-

ness while retaining some ‘‘squeezability’’ when required,

and commercial machinery provided an opportunity for

other companies to establish blow-molding manufacturing

operations. Until that time, a select few using proprietary

machinery were the only companies manufacturing plas-

tic containers. By 1960, over 55 builders were in the

business of manufacturing blow-molding equipment (7).

Many of these builders were unknown a year earlier and

most do not survive today.

BASIC PROCESS

Figure 2 illustrates the basic blow-molding process using

the extrusion molding method to create the parison.

In view Figure 2a, a blow-molding machine extrudes a

hot tube-shaped parison typically in a downward direction

from an extrusion head. Because the parison is tubular,

it has two substantially open ends. The machine places

the parison between two halves of a bottle blow-mold

cavity. In this view, the machine has a blow pin with an

end extending from the extrusion head and positioned

inside the parison. The blow pin is actually a metal tube

with a passage allowing a ﬂow of air to enter inside the

parison at the appropriate time. While this is a common

blow pin placement, machinery builders and designers

have developed several other arrangements with similar

function.

In Figure 2b, once the parison reaches a proper length,

the machine’s clamp system closes the two mold halves

against each other to close off and seal the parison open

ends. At one end, the mold pinches the parison ﬂat. At the

other end, adjacent to the extrusion head, the mold also

pinches the parison substantially ﬂat while sealing it

around the blow pin. The pinching of the parison creates

ﬂash material. Machinery, later in the manufacturing

Figure 1. An array of extrusion blow-molded bottles.

(Courtesy of Graham Engineering Corporation.)

138 BLOW MOLDING

process, trims and recycles this ﬂash. The ﬂash portion is

often 20–50% of ﬁnal bottle weight.

In Figure 2c, air ﬂowing through the blow pin at an

elevated pressure inﬂates, thereby pressing and conform-

ing the parison to the mold cavity surfaces. The machine

holds the air pressure for a period sufﬁcient to allow the

mold cavity to cool the conformed parison, thereby retain-

ing the molded shape of the container or bottle. Generally,

machinery circulates water through channels in the mold,

thereby maintaining a consistent mold temperature.

In Figure 2d, the machine ejects or removes the molded

bottle from the mold cavity, usually to one side or down-

ward. Typically, machinery trims away the ﬂash still

attached to the bottle in a secondary operation.

Although Figure 2 shows the extrusion method for

creating the parison or preform, using the injection

method does not alter process fundamentals—in particu-

lar, inﬂation of the parison or preform.

EXTRUSION BLOW MOLDING

Container manufacturers can make bottles, jars, or drums

of nearly any size using extrusion-based blow-molding

methods. Extrusion blow molding has two main process

categories, that is, container manufacture using continu-

ous extrusion machinery and intermittent extrusion ma-

chinery. Each category has machinery subcategories

tailored to speciﬁc application requirements. Compared

Extrusion

Head

Bottle

Cavity

Blow Mold

Parison Blow Pin

Molding

Machine

Mold-Close Direction

(a)

(b)

Extrusion

Head

Bottle

Cavity

Blow Mold

Parison

Blow Pin

Molding

Machine

Parison

Parison Pinched

Top & Bottom

(c)

Air Flow through

Blow Pin

Extrusion

Head

Blow-Molded

Bottle

Blow Mold

Parison

Blow Pin

Molding

Machine

Parison Pinched

Top & Bottom

(d)

Extrusion

Head

Bottle

Cavity

Blow Mold

Flash/Trim

Blow Pin

Molding

Machine

Mold-Open Direction

Flash

Blow-Molded

Bottle

Figure 2. Basic blow-molding process sequence: (a) Parison extrusion; (b) mold close and parison pinch; (c) parison inﬂation and bottle

cool; (d) mold open and bottle eject.

BLOW MOLDING 139

to injection-based blow molding, mold tooling for the

extrusion blow process is relatively less expensive. In

addition, the extrusion blow process has relatively few

shape restrictions and is the only low-cost method for

manufacturing a bottle with an integral handle.

Continuous Extrusion

Machinery continuously forms the parison at a rate equal

to that required to blow, cool, and remove the molded

bottle. To avoid interference with parison formation, the

machine’s mold-clamping mechanism or system must

move quickly to capture the parison and return to a

blowing station, thereby creating space for the next par-

ison to form.

Builders have made machinery capable of molding

large containers, such as a 35-gal trash container, using

the continuous extrusion approach, but the approach is

better suited for smaller containers, typically 10 L (2.5 gal)

or smaller in capacity. In fact, most continuously extruded

blow-molded bottles are 4 L or less in capacity.

The continuous extrusion process is capable of molding

all commonly molded thermoplastic materials, but the

process is particularly ideal for molding heat-sensitive

PVC materials. Relatively low-pressure, slow uninter-

rupted ﬂow of melted material through an extruder or

plasticizer and parison extrusion head helps minimize

thermal degradation of heat-sensitive polymers.

Several machinery conﬁgurations exist, but two ap-

proaches generally predominate: shuttle continuous and

vertical rotary continuous. Less common approaches in-

clude rising-mold continuous and horizontal rotary con-

tinuous conﬁgurations.

Figure 3 illustrates the shuttle continuous extrusion

approach. With this method, the blowing station is located

on one or both sides of the parison extrusion head. As the

parison reaches proper length, the blow mold and clamp

(while open) quickly shuttle to a point under the extrusion

head, closes to capture and then cut the parison from the

head, and return to the blowing station where the blow pin

enters to seal and then inﬂate the parison against the

mold cavity surfaces. With dual-sided machine concept, as

shown in the illustration, two clamp systems shuttle on

an alternating basis. To increase production output,

machines use multiple extrusion heads with multiple

mold cavities—for example, a triple parison head with

two triple mold cavity sets. Generally, the mold and clamp

slides or swings on an incline. This provides extra clear-

ance from the ever-advancing parison as the mold and

clamp moves to the blowing station. An alternative means

for providing additional clearance is by ‘‘bobbing’’ the

extrusion head up during the return movement and then

down following clamp movement.

The shuttle machinery conﬁguration offers a number of

advantages. The conﬁguration allows critical dimensions

of the bottle neck-ﬁnish to be ‘‘pre-ﬁnished’’ or ‘‘calibrated’’

in the mold with a water-cooled blow pin. This eliminates

the need for post-mold secondary sizing or machining

of the neck-ﬁnish area. Machinery operators also value

the easy mold tooling and process setup. Job changes on

shuttle continuous equipment can be relatively quick,

making the equipment ideal for a short production run.

On the other hand, the process has a relatively slow

parison extrusion rate requiring the hot plastic material

to have exceptional ‘‘melt’’ strength. That is, the parison

must not appreciably change shape from material ﬂow

caused by its own weight as it extrudes from the head.

Shuttle continuous extrusion machines, particularly sin-

gle-sided machines, require the hot parison to hang for a

relatively long time.

A number of machinery builders supplying shuttle con-

tinuous blow-molding equipment have begun offering ‘‘long-

stroke’’ machines. These are either single-sided or dual-

sided alternating shuttle machines with relatively large

multicavity molds. These machines can have as many as 15

mold cavities in each mold set. Of course, the machine

Blow Mold

Extrusion

Head

Bottle

Cavity

Blow Pin

Mold-Shuttle Directions

Parison

Cutting

Knife

Blow Pin

Flash/Trim

Blow

Mold

Mold-open

& close is i

Direction of

View

Figure 3. Basic alternating shuttle

continuous-extrusion blow-molding

machine.

140 BLOW MOLDING

requires the extrusion head to extrude the same number of

parisons. In comparison, a more conventional shuttle ma-

chine typically has six of fewer cavities in each mold set.

Relative to conventional shuttle machines, the long-

stroke machine, predominantly in the dual-sided conﬁg-

uration, is capable of high production output per unit of

factory ﬂoor space the machine occupies. Single-sided

long-stroke machines can simplify integration of in-mold

labeling equipment and downstream bottle handling

equipment relative to dual-sided machines having similar

production output. On the other hand, the large mold set

and clamping system must travel a longer distance be-

tween the blowing station and parison extrusion head.

Moreover, the mold set and clamping system has a greater

mass; therefore, it is more difﬁcult to accelerate and

decelerate. All things equal, an alternating shuttle ma-

chine with half as many parisons, mold tooling with half

as many cavities per side, and shuttle travel distance

substantially half as much will provide a faster per cavity

production rate than a single-sided long-stroke machine.

Figure 4 shows a typical alternating shuttle continuous-

extrusion blow-molding machine.

Recently, machinery builders have begun offering shut-

tle machines with tandem mold cavities. These machines

have a production output that approaches that of a rotary

machine while preserving the beneﬁt of the pre-ﬁnished or

calibrated neck-ﬁnish. Normally, at the blowing station

the blow pin enters the parison from the top, primarily

because the molded bottle leaves the machine in an up-

right position, greatly facilitating downstream handling.

However, the blow pin can also enter from a bottom

position, thereby blow molding the bottle upside down. A

shuttle machine having tandem cavities produce two

bottles from one parison, one in the upright position, the

other in the upside-down position. Incorporated trim

tooling later separates the tandem bottles.

Figure 5 illustrates the vertical rotary continuous

machinery approach. The mold cavities circumferentially

mount to a wheel having a horizontal axis of rotation like

that of a Ferris wheel. In this approach, one of the mold

sets on the rotating wheel captures the parison as it

rotates past the parison extrusion head, in this case

extruding the parison upward, while other mold sets

simultaneously shape and cool bottles from previously

captured parisons; another mold set opens, allowing bottle

removal.

Figure 4. An alternating shuttle continuous-extrusion blow-

molding machine featuring a three-parison extrusion head be-

tween two mold clamp mechanisms. (Courtesy of Bekum America

Corporation.)

Blow-Molded

Bottle

Extrusion

Head

Bottle

Cavity

Parison

Blow Mold

Mold

Rotation

Direction

Needle

Blow Pin

Flash/Trim

(a) Mold-close &

Parison Pinch

& Bottle Eject

Blow Mold

(b) Parison

Inflation &

Bottle Cool

Needle

Blow Pin

Mold-open &

Close is in

Direction of

View

Figure 5. Basic vertical rotary continuous-extrusion

blow-molding machine having a parison extruding

upward.

BLOW MOLDING 141

Generally, the machine extrudes a single parison and

each mold set has one cavity, but some machines have

multiple parison and cavity sets. Speciﬁc conﬁguration

depends on mold and clamp design details—for example,

two cavities inline, face-to-face made from a single par-

ison; or two cavities side-by-side, each capturing its own

parison extruded side-by-side and in parallel. Direction of

mold open and close motion is either parallel to wheel axis,

radial to wheel axis, or ‘‘book’’ where one mold half hinges

from an edge of the other mold half. Once the mold cavity

closes, a needle blow pin punctures the parison. Air

ﬂowing through the needle conforms the parison to the

cavity surfaces. Figure 6 shows a vertical rotary contin-

uous extrusion blow-molding machine having a single

parison extruding in an upward direction a moment before

the mold closes to capture. Note that the mold cavity in

Figure 6 has a shape to form a bottle having an integral

handle. In addition, Figure 6 shows a cylinder, (right-hand

corner) for actuating the needle.

The rotary approach has many advantages, primarily

for bottle applications requiring high production out-

put. The single parison extrusion head helps to maintain

bottle consistency between all mold cavities, particularly

for applications requiring a complicated multilayer or

coextruded polymer material structure. Another advan-

tage is that in-mold labeling systems, when used, are less

complicated because the same unit supplies labels for all

cavities. In addition, the vertical rotary approach has a

unique capability of holding the parison fast at the top and

bottom, allowing an opportunity to manipulate parison

material distribution in ways not as easily done with other

processes where the parison’s bottom end is free.

On the other hand, the vertical rotary approach, as

mentioned previously, typically requires the needle blow

pin to puncture the parison. For bottles, this puncture is

into a molded dome area above the bottle neck-ﬁnish. A

post-molding operation cuts off the molded dome with

other ﬂash trimmings. Establishing ﬁnal neck-ﬁnish di-

mensions further requires a sizing or machining opera-

tion. Machining the neck-ﬁnish area can allow plastic

chips to fall inside the bottle and potentially contaminate

any ﬁlled product. In addition, tooling job changes are

more complicated and typically take more time; however,

this is not necessarily a problem for long-term high

production applications.

Intermittent Extrusion

An intermittent extrusion blow-molding machine quickly

extrudes the parison after bottle removal from the mold

cavity. The mold-clamping system does not transfer to the

blowing station. Bottle molding, cooling, and removal all

take place under the parison extrusion head. This ar-

rangement allows the clamping system to be more simple

and rugged. Manufacturers typically use the intermittent

extrusion approach to make containers from 100 mL (3.4 ﬂ

oz) to 210 L (55 gal).

Two machinery conﬁgurations predominate: recipro-

cating screw and accumulator head. A less common

conﬁguration is a side-ram accumulator or shot-pot

approach. Polyoleﬁn thermoplastic materials, parti-

cularly HDPE, are ideally suited for the intermittent

extrusion; unfortunately, its start–stop–start character

and extremely rapid parison extrusion rate is not best

for heat-sensitive materials.

Manufacturers typically consider reciprocating screw

machinery for containers less than 10 L (2.5 gal) in capacity.

The plasticizer of the reciprocating screw blow-molding

machine has an extruder screw that moves back as it helps

to melt the thermoplastic material. That melted material,

for the next parison, accumulates downstream of the

screw’s tip. During parison extrusion, the screw quickly

moves forward to push the accumulated material through

the extrusion head. Next, the mold clamping system closes

on the parison and blow pin. The parison then inﬂates

against the mold cavity while the reciprocating screw

begins to accumulate material for the next parison. Once

the molded container has cooled, the clamp opens allowing

container removal. The process then repeats.

For many bottle design conﬁgurations, the reciprocat-

ing screw machine allows ‘‘pre-ﬁnishing’’ of critical bottle

neck-ﬁnish dimensions during the molding process. Posi-

tioning the blow pin inside the parison before mold-close

allows a choice of two pre-ﬁnishing approaches. The

ﬁrst, a so-called ‘‘ram-down’’ or ‘‘calibrated’’ pre-ﬁnishing

Figure 6. Mold cavity and upward extruding parison of a rotary

continuous-extrusion blow-molding machine. Previous mold (now

closed) holds the parison in position. Note, the needle blow-pin

actuating cylinder near upper left-hand corner and mold cavity

for a handleware bottle. (Courtesy of Graham Engineering

Corporation.)

142 BLOW MOLDING

approach provides a result substantially identical to the

approach used with shuttle continuous extrusion. The

second, a so-called ‘‘pull-up’’ pre-ﬁnishing approach, is

unique to intermittent extrusion machinery. The blow

pin for the pull-up approach moves upward a spit second

before the mold opens. During this upward movement, the

blow pin shears plastic material in a critical neck-ﬁnish

inside diameter area to create a precise, round, and

smooth inside surface for a closure to seal.

The pull-up pre-ﬁnishing approach is an innovation that

perhaps is singly most responsible for the success of HDPE

milk bottles. Substantially all of the HDPE milk bottles

made in the United States feature a neck-ﬁnish made using

the pull-up approach. The pull-up approach permits ex-

tremely lightweight containers that in turn reduces man-

ufacturing cost from less material expense and faster

(shorter) molding process from a reduced need for cooling.

Bottles featuring the pull-up made neck-ﬁnish use a low-

cost closure not requiring any secondary sealing material.

Instead, the closure features a standing circumferential

wedge that, when applied to the bottle, tightly engages the

smooth precise inside surface of the neck-ﬁnish to establish

a simple and highly effective seal. Figure 7 shows a

reciprocating screw blow-molding machine.

The accumulator-head intermittent-extrusion process

is ideal for heavyweight containers, such as 210-L (55-gal)

drums, and noncontainer items, such as parts for toys,

automotive ductwork, and the like. Generally, manufac-

turers will consider accumulator head machinery for

containers larger than 10 L in capacity. The accumulator

head features a tubular melt reservoir as part of the

extrusion head itself. The machine’s plasticizer feeds

melted thermoplastic material into the reservoir on a

ﬁrst-to-enter, ﬁrst-to-leave basis. Once a sufﬁcient quan-

tity of melted material accumulates, a tubular plunger

within the head quickly extrudes the parison through a

head tooling annulus.

Similar to the accumulator-head, the side-ram or shot-

pot intermittent-extrusion process uses a single melt

accumulator that receives melted thermoplastic from the

plasticizer. The melt accumulator is not a component of

the extrusion head; instead, it features a piston or plunger

along side the plasticizer that pushes the material into a

manifold and then through multiple extrusion heads. It is

generally used for heavy container applications requiring

an excessive amount of accumulated melt that is greater

than the amount the reciprocating-screw process is cap-

able of accumulating. Recently, machinery builders are

offering the ‘‘shot-pot’’ equipment for classic reciprocating-

screw applications, particularly in situations where a ﬁne-

mesh screen ﬁlters the melt to remove contaminates likely

in post-consumer recycled material.

Head Tooling, Parison Programming, and Head-Tooling

Ovalization

By deﬁnition, the extrusion process requires the parison to

be a hollow tube having a diameter and wall thickness

adequate to properly form the container. Head tooling

consists of two components: a generally circular die sub-

stantially establishing, in cross-section, the parison’s

outer diameter and a generally circular mandrel substan-

tially establishing the parison’s inner diameter. In combi-

nation, these two components create an annulus through

which the parison extrudes. The diametrical difference

forms a gap between the die and mandrel that establishes

parison wall thickness. For simple less demanding con-

tainer applications, the gap remains uniform.

Parison inﬂation in the blow-mold cavity is not even.

Some areas will expand more, thus tending to thin more.

Parison programming is a technique to compensate parti-

cularly for more demanding container applications. Pro-

gramming permits the molding technician to place extra

material in the container where needed and remove

Figure 7. Typical reciprocating-screw intermittent-

extrusion blow-molding machine. (Courtesy of Graham

Engineering Corporation.)

BLOW MOLDING 143

material where it is not needed, ultimately allowing

improved container performance while making container

wall-thickness more consistent and likely reducing over-

all container weight and cost. Programming dynamically

changes annulus gap size by moving either the die or the

mandrel relative to the other to a speciﬁc proﬁle as the

parison extrudes to create axial circumferential zones

or rings of thicker or thinner material in the parison

corresponding to areas within the blow-molded container

requiring more or less material.

Head-tooling ovalization is also a technique to compen-

sate areas of the parison that tend to thin more. By

changing the shape of either the die or mandrel from

round to slightly oval, the corresponding gap of the

annulus created by the die and mandrel circumferentially

varies. During parison extrusion, ovalized tooling creates

‘‘stripes’’ of thicker or thinner material parallel to the

parison’s axis that correspond with areas of the container

requiring more or less material. In other words, the

parison in circular cross section has areas thicker than

other areas within that cross section.

Generally, ovalizing head tooling is a ﬁxed approach,

not dynamic, as is the case with parison programming.

While ovalized head tooling and parison programming

separately or together can signiﬁcantly improve material

thickness distribution in the blow-molded container, some

areas of the blow-molded container will always remain

slightly thicker or thinner than other areas. The molding

technician selects a degree of ovalization in combination

with parison programming that is a compromise of wall

thickness requirements in various container areas. For

example, the container’s sidewall may be slightly thicker

than necessary to have adequate wall thickness in the

container’s chime area.

In general, it is not practical or cost effective for most

container applications to have dynamic ovalization pro-

grammed to a speciﬁc proﬁle as with conventional parison

programming. However, in large somewhat heavy contain-

ers, such as the 210-L (55-gal) drum, dynamic ovalization

is justiﬁable. Even with parison programming and ﬁxed

ovalization, a drum’s chime area can still have signiﬁcant

differences in material thickness. Dynamic ovalization

features a die having ﬂexible lips. Applying a force to

the lips during parison extrusion causes the normally

circular die to slightly distort into various oval shapes as

determined by a programmed proﬁle suitable to speciﬁc

container requirement, thereby reducing an otherwise

necessary material thickness compromise in other parts

of the container.

INJECTION BLOW MOLDING

In the classic injection blow-molding process, the plastici-

zer injects melted plastic into a closed mold having a cavity

and a core rod to create an injected-molded ‘‘test tube’’-

shaped preform having an integral neck-ﬁnish. While still

hot, the injection-molding machine transfers the preform

on the core rod to a bottle blow-mold cavity. Pressurized air

passing through a valve in the core rod inﬂates the pre-

form, conforming it to the blow-mold cavity surface. Once

cooled, the mold cavity opens and the machine removes the

blow-molded bottle from the core rod.

Early injection blow-molding techniques were substan-

tially two-position tooling adaptations of conventional

injection molding machines (8). A shortcoming of these

adaptations was that the injection and blow-mold stations

stood idle during bottle removal. The blow-molding in-

dustry generally credits Angelo Gussoni with inventing a

three-station injection blow-molder (9). A third station

improved efﬁciency by allowing bottle removal while the

other two stations molded either the next preform or the

next bottle.

Initially, Gussoni also adapted this three-station concept

to available conventional injection-molding machines. La-

ter, purpose-built three-station injection blow-molders

evolved that today have become an industry standard.

Figure 8 illustrates the basic three-station injection

blow-molding process. Note that the process requires three

core rods for each preform-mold and blow-mold set.

The injection blow-molding process has a number of

advantages. The process is ﬂash- or scrap-free. Molded

bottles do not require secondary trimming. Injection mold-

ing the preform accurately maintains critical neck-ﬁnish

dimensions, an important consideration for complex child-

resistant and snap-on closures common on pharmaceuti-

cal bottles. Bottle weight control is extremely precise,

accurate within a range of 0.2 g or less. Bottle surface

gloss and texture is pristine and consistent. For typical

applications having a capacity less than 250 mL, the

injection blow-molding process is generally less expensive

than extrusion blow-molding processes. For vials—for

example, 1, 5, or 10 mL in capacity—and other very small

bottles, bottle manufacturers have only one cost-effective

process choice, injection blow molding.

On the other hand, machinery conﬁgurations generally

limit bottle size to 4 L (1 gal) or less capacity. Because of

the relative high cost of mold tooling, the injection blow-

molding process is usually not economically justiﬁable for

typical bottle applications having a capacity greater than

500 mL. Finally, the process limits bottle shape and pro-

portion. Although not a problem for most bottle applica-

tions, the process is not ideal for extremely ﬂat, overly tall

and thin, offset neck-ﬁnish, or handleware bottle shapes.

Table 1 contrasts important advantages of injection blow

molding versus extrusion blow molding.

Pharmaceutical and cosmetic product applications

often require small bottles with precise neck-ﬁnishes.

Furthermore, these products are often expensive, de-

manding an equally high-value, quality package that

injection blow provides and not always available from

the extrusion-blow alternative.

Many thermoplastic materials are injection-blow mold-

able, which could be difﬁcult or impossible using extru-

sion-blow

processes. General-purpose

polystyrene is a

good example of a thermoplastic that injection-blow-mold-

ing machines routinely process. Because the core rod helps

to support the melted material during transfer to the

blowing station, injection-blow process does not require

the same degree of melt-strength that extrusion blow,

particularly continuous extrusion blow, requires. In addi-

tion, the injection-blow process adds a degree of molecular

144 BLOW MOLDING

orientation to the general-purpose polystyrene bottle that

enhances its impact resistance.

Other materials commonly molded with classic injec-

tion blow are high- and low-density polyethylene,

polypropylene, polyvinyl chloride, and polyethylene ter-

ephthalate. While the injection-blow process approach

provides beneﬁcial molecular orientation to polystyrene,

other thermoplastics such as polypropylene or polyethy-

lene terephthalate, unfortunately, do not beneﬁt. The core

rod of the classic injection-blow process remains inside the

preform; additionally, the preform length is approximately

the same as the container is tall, conditions that inhibit

beneﬁcial molecular orientations of many other materials,

particularly in an axial direction.

EXTRUSION—INJECTION-MOLDED NECK PROCESS

This unique hybrid process combines advantages of injec-

tion blow with extrusion blow. Originally developed by

Owens-Illinois in the 1950s, the process is not much in use

today for manufacturing bottles, but a version is in use for

manufacturing constant velocity joint (CVJ) boots and

parts for other automotive bellows applications. The hy-

brid process features an injection-molded neck with an

extrusion-blow-molded body that provides a dimension-

ally accurate neck-ﬁnish without some of the bottle shape

and proportion limitations of injection blow. The hybrid

process can mold a ﬂat oval bottle having an offset neck-

ﬁnish and integral blown-handle conﬁguration typical of

extrusion blow with a high-quality neck-ﬁnish typical of

injection blow. While machinery is not high output, it is

ideally suited for integrating an in-mold labeling feature

without production penalty.

STRETCH BLOW MOLDING

Thermoplastic materials are polymers of relatively high

molecular weight—that is, of long polymer chains made of

hundreds to thousands of linked individual monomer

molecules. Normally, these long-polymer-chain molecules

entangle with other in random complex coils. Careful,

proper processing of some materials, such as polyethylene

terephthalate (PET) and polypropylene (PP), will estab-

lish and retain partial molecular orientation of the poly-

mer chains. In other words, the polymer chains become

somewhat aligned with each other in uniaxial and biaxial

directions.

While all blow-molding processes involve ‘‘stretching’’

the parison or preform in some fashion, ‘‘stretch blow

molding’’ or biaxial-orientation blow molding is a modiﬁed

process to achieve and retain polymer chain alignment in

biaxial directions within the blow-molded bottle, in parti-

cular within the bottle sidewall. Biaxial orientation can

signiﬁcantly improve bottle impact strength, transpar-

ency, surface gloss, stiffness, and gas-barrier performance,

particularly in bottles greater than 250 mL in capacity.

Filled soft drink PET bottles, highly pressurized from

carbon dioxide gas, are an everyday reality because of

the stretch-blow process.

Fundamentally, the stretch-blow process requires pre-

cise conditioning of the preform to a temperature warm

enough to allow rapid inﬂation and molecular alignment

but cool enough to retard re-randomization of its molecu-

lar structure once aligned. Preform manufacture is by

either extrusion molding or injection molding; however,

dimensional precision provided by injection molding has

allowed that approach to dominate in bottle manufactur-

ing today.

Most common polymers biaxial-oriented are polyethy-

lene terephthalate (PET) and polypropylene (PP). Other

polymers include polyvinyl chloride, acrylonitrile-based

copolymers, and a high-performance polyester, polyethy-

lene naphthalate.

Figure 9 illustrates typical preform and bottle propor-

tions. Note that the preform is signiﬁcantly shorter than

the bottle. In comparison, conventional extrusion-blow

parison and injection-blow preform are substantially the

Air Flow

through Valve

in Core Rod

(b) Bottle Blow-

Molding Stage

Transfer

Direction

Preform Mold

Injection

Nozzle

Core Rod

Neck-Ring

Blow Mold

Neck-Ring

Blow-Molded

Bottle

Bottle Ejector

Core Rod

Transfer Plate

Molded

Preform

(a) Preform Injection

Molding Stage

Bottle

Figure 8. Basic injection blow-molding machine

process sequence.

BLOW MOLDING 145

same length as the bottle is tall. The ﬂow of air at elevated

pressure stretches the preform in a ‘‘hoop’’ direction—that

is, a direction substantially perpendicular to the preform’s

axis. At the same time, the ﬂow of air, often in combination

with a stretch rod, stretches the preform in an axial

direction—that is, a direction parallel to the preform’s

axis. Needed air pressure is extreme; for example, PET

applications often require 600 PSI (4100 kPa) to properly

inﬂate the preform. In comparison, most extrusion blow-

molding applications require about 90 PSI (600 kPa).

There are two basic stretch-blow methods: one-step and

two-step. While at least one machinery builder has pro-

duced a one-step machine capable of producing a 20-L

container, most stretch-blow containers are less than 4 L

in capacity.

One-Step Method

Production stages of preform injection molding, heat con-

ditioning, and blow molding take place in the same

machine. Molding technicians generally consider it better

for manufacture of wide-mouth jars and better for bottles

having extreme oval or other unusual cross-sectional

shapes. The one-step method minimizes blemishes to

provide a pristine bottle-sidewall surface appearance. In

addition, the preform does not require a handling ring, as

the two-step method requires. Eliminating the handing

ring can improve the bottle’s overall aesthetic appearance.

Tooling and machinery setup is relatively easy, making

the method ideal for short-run production applications. In

addition, the method potentially saves energy in that the

preform does not require reheating from room tempera-

ture (see Figure 10).

On the other hand, one-step machinery typically has an

equal number of preform cavities and blow-mold cavities.

Dictated by speciﬁc preform and bottle shape, weight, and

performance requirements, rarely is an optimum process

or ideal production rate for preform molding stage in

perfect balance with that needed for bottle blow molding

stage. Accordingly, one process stage, usually preform

injection molding, will govern overall machine productiv-

ity while retarding productivity of the other process

stages. Sometimes process technicians are tempted to

make compromises, usually in preform conﬁguration,

that reduce overall bottle performance in exchange for

greater overall production output.

There are three general variations of one-step machin-

ery. One variation features a four-station rotary table.

Preform injection molding in station one. Unlike injection

blow molding, the machine removes the preform core rod

before transfer to the next station. The preform is heat

Table 1. Injection Blow Molding Versus Extrusion Blow Molding

Injection Blow Molding Extrusion Blow Molding

Used for small, heavier bottles, typically less than 500 mL Used for larger bottles, typically larger than 250 mL

Best process for polystyrene; ideal for most other

thermoplastic materials

Best process for polyvinyl chloride; with adequate melt

strength, ideal for many other thermoplastic materials

Scrap-free: no ﬂash/trim to recycle; no pinch-off scar on bottle;

no post-mold trimming required

Fewer bottle shape limitations permitting extreme

dimensional ratios: long and narrow, ﬂat and wide, doubled-

walled, offset neck-ﬁnish, molded-in handle, and other odd

shapes

Injection-molded neck-ﬁnish provides more accurate neck-

ﬁnish dimensions that better address special shapes

required for complicated safety and tamper-evident closures

Precise and repeatable bottle weight control from injection-

molded perform

Low-cost tooling often made of aluminum; ideal for short- or

long-run manufacturing applications

Excellent and consistent bottle surface texture Adjustable weight control ideal for prototyping applications

Stretch Rod

Heated

Preform

Blow Mold

Bottle Cavity

(a)

Blow-Molded

Bottle

Action of

Stretch Rod

& High

Pressure Air

Flow

(b)

Figure 9. Basic stretch blow-molding process sequence: (a) Mold

close on preform; (b) stretch blow molding. Note that for many

PET applications, preform length is typically about one half the

bottle height.

146 BLOW MOLDING

conditioning in station two by heating and/or cooling

speciﬁc areas within the preform. Preform blow molding

into the bottle occurs in station three and bottle removal

occurs in station four. The second variation features a

three-station rotary table. Some machines combine pre-

form injection molding with heat conditioning in stage

one. Other machines combine bottle blow molding and

bottle removal in station three. The third variation com-

bines in one machine a substantially conventional preform

injection-mold tooling and apparatus with a substantially

conventional bottle blow-mold tooling and apparatus con-

nected by a preform transfer mechanism that includes a

short preform reheat oven. Preform injection-mold cavity

count is higher than bottle blow-mold cavity count, allow-

ing a more balanced operation.

Two-Step Method

Production stage of preform injection molding is separate,

taking place in a separate machine from heat conditioning

and bottle blow molding stage. Molding technicians often

refer to the two-step method as ‘‘reheat and blow.’’ The

method allows technicians to optimize preform and bottle

design and overall production efﬁciency. Equipment with

extremely high output is available. Figure 11 shows a high-

output PET preform injection-molding machine; although

not evident in the ﬁgure, these machines have a robotic

takeout that includes a post-mold cooling system. Figure 12

shows a typical high-output PET preform injection mold.

Figure 13 shows a high-output horizontal rotary bottle

stretch blow-molding machine with a preform oven for heat

conditioning. Bottle manufacturers often link a group of

injection molding machines producing the preform with a

group of blow-molding machines producing the bottle with

conveyors that reduce part handling and ‘‘work-in-process’’

inventory. Nonetheless, manufacture of the preform and

the bottle can be separate and in different facilities.

Molding technicians tend to use the two-step method

for containers having a substantially round cross-sec-

tional conﬁguration; however, with improved reheat ovens

capable of heating regions within the preform to a differ-

ent temperature than other regions and precise preform

positioning in the blow-mold cavity, two-step machinery is

often suitable for containers having a substantially oval or

rectangular cross-sectional conﬁguration.

MULTILAYER

All materials—whether metal, glass, paper, or plastic—

have certain strengths and weaknesses. Layering two or

more materials can economically overcome individual

material shortcomings. Multilayer blow molding is a way

to combine the strengths of two or more thermoplastic

materials to economically manufacture bottles that can

package a product far better than any of the materials

could individually. In recent years, multilayer structures

and related barrier polymers and machinery has

been an active development area within the blow-molding

industry, particularly for PET bottle applications. Ther-

moplastic materials in bottles now routinely protect sen-

sitive products previously not feasible.

Ideal characteristics of many bottles include: low cost,

strength, clarity, product compatibility, and gas barrier.

Polypropylene, for example, is a relatively low-cost mate-

rial suitable for food contact and having acceptable clarity,

Time

Temperature

Two-Step

Method

PET Material

Melting in Injection

Molding Machine

Break in Time

Melting Point

Crystalline PET

∼

490°F (255°C)

Temperature

Maximum PET

Crystal Growth

∼

350°F (177°C)

PET Glass

Transition

Temp. ∼

∼

180°F

(82°C)

Room

Temperature

PET Material Drying

in Heated Hopper

One-Step

Method

Preform Injection Molding

∼

535°F (280°C)

Preform Reheat

Preform

Conditioning

Rapid Preform Cooling

Bottle

Cool

Bottle

Stretch Blo

Molding

Figure 10. Comparison of basic one-step and two-step

stretch blow-molding processes of PET.

BLOW MOLDING 147