5.1 INTRODUCTION Blow molding is a process for producing hollow
objects, primarily from thermoplastic materials. Bottles and
packaging are the primary applications of blow molded parts.
Slide 3
5.1.1 Plastic Process To visualize the blow molding process,
think of an inflatable plastic tube that is closed on both ends
except at one point. If air enters at this point, the tube will
expand, causing the walls of the tube to become thinner. It is much
like blowing up a balloon. FIGURE 5.1(Page 305) Inflating an
expandable tube. FIGURE 5.1(Page 305) Inflating an expandable
tube.
Slide 4
The blow molding process begins with a hot, soft plastic tube
called a parison or preform. The parison is placed between two
halves of a mold that has one or more hollow cavities (Figure 5.2).
The mold halves are then clamped together. Air is blown into the
parison, expanding it against the inside walls of the mold to form
a hollow plastic part in the shape of the cavity. The mold is
cooled, usually with water, and the hot plastic cools by contact
with the mold. Once the part has cooled, the mold opens and the
part is ejected. FIGURE 5.2(Page 305) Basic blow molding process.
FIGURE 5.2(Page 305) Basic blow molding process.
Slide 5
5.1.2 Brief History of Blow Molding TABLE 5.1(Page 307) Key
Blow Molding Industry Events TABLE 5.1(Page 307) Key Blow Molding
Industry Events Bottles are the primary application of blow-molded
plastic parts; however, the blow molding industry also makes
products used in the aircraft, automotive, building and
construction, electronics, furniture, lawn and garden, medical, and
recreation fields. 5.1.3 Typical Blow Molding Markets and Product
Applications
Slide 6
FIGURE 5.3 (a) Assorted industrial and consumer blow-molded
parts; (b) intricate blowmolded case; (c) aerobic step; (d) dock
floates, foam filled for buoyancy. Applications include balls,
toys, bellows, and car bumpers (fenders). Many of the bottles and
jars are used in the food industry to package both solid and liquid
products, such as plastic containers for salt and sauces, and most
food-packaging containers are blow molded.
Slide 7
5.1.4 Blow Molding Materials 5.1.4.1 Materials Used
Thermoplastic materials are often suitable for many applications
without extensive material or component modification, but in other
cases they are not suitable. One reason is that many plastics are
permeable; that is, a liquid product or part of the product can
migrate through the walls of the container, or something from the
environment, such as oxygen, can enter the container through the
plastic walls. For a given plastic material, there are several ways
of overcoming this problem. They include (1)increasing the product
thickness; (2)coating the product with an impermeable material such
as glass, cross-linked PE, or more commonly, polyvinylidiene
chloride (PVDC); (3)incorporation of a platelike filler (e.g.,
glass flake); (4)and using a combination of plastic materials. Such
combinations, in which one parison is surrounded by another, are
usually produced by co-extrusion. Orientation (stretching the
parison) also yields improvements.
Slide 8
5.1.4.2 Chemical End Use and Physical and Processing
Requirements HDPE is impervious (unaffected by, or resistant) to
water, water vapor, inorganic chemicals, and inorganic materials in
aqueous (water-based) media, especially at the ambient temperatures
normally encountered during storage and transport. Its high
moisture resistance makes HDPE an ideal container material, keeping
moisture out as well as in. However, with organic chemicals and
fluids, HDPE must be used more selectively.
Slide 9
For hydrocarbon fluids, oils, and solvents such as those listed
below, absorption and permeation through HDPE become very important
factors. Most of these compounds are flammable and would not
normally be contained in HDPE containers without special
consideration. Their use as an ingredient in any product being
considered for packaging or storage in HDPE containers requires
proper testing to determine whether or not excessive permeation and
absorption will occur.
Slide 10
5.2 STAGES AND TYPES OF BLOW MOLDING
Slide 11
5.2.1 Stages of the Blow Molding Process 1. Plasticizing
(melting) the resin 2. Production of the parison (extrusion) or
preform (injection) 3. Inflation of the parison or preform followed
by cooling in the mold (This step takes the most time and controls
the machine cycle.) 4. Ejection of the part from the mold 5.
Trimming or finishing the part (the trim step is frequently
performed while the other four steps are cycling) It should be
noted that when several pieces are made at one time in multiple
molds on one machine, the first four steps may overlap.
Slide 12
5.2.2 Types of Blow Molding 1)Extrusion 2)Injection 3)Stretch
Figure 5.5-page 314 describes these types and subtypes. Figure
5.5-page 314 describes these types and subtypes.
Slide 13
Blow Molding Extrusion 1)Continuous 2)Intermittent
(Accumulator) Co-extrusion Three- Dimensional Extrusion 1)Suction
2)Vertical manipulation 3)Horizontal manipulation Injection
1)injection station 2)blow station 3)ejection station Stretch
1)Heating a molded and cooled preform 2)Closing blowing mold
3)Stretching
Slide 14
5.2.3 Extrusion Blow Molding There are two basic types of
extrusion blow molding: 1) Continuous Blow Molding 2) Intermittent
(Accumulator) Method
Slide 15
5.2.3.1 Continuous Blow Molding In the continuous method, the
parison is extruded continuously from a head or die unit. The
extruder produces an endless parison. In one variation of the
continuous method, a shuttle press carries one or more molds. As
soon as the mold closes, it moves away and an open mold moves into
place as the parison or parisons continue to be extruded. When the
mold closes, air is blown into the parison. Once the parison has
expanded, air pressure holds it against the mold as the part(s)
cool.
Slide 16
5.2.3.2 Intermittent (Accumulator) Method In this method, the
extruder runs continuously. A chamber, called an accumulator,
accumulates a substantial volume of well- plasticized melt that is
delivered by the extruder. Once a plunger or ram forces the melt
out through the die head to form the parison (Figure 5.6).
Slide 17
A benefit of the intermittent or accumulator method is that it
allows the delivery rate of hot plastic from the die head to be
independent of the delivery rate of the extruder. Accumulator
capacity determines the maximum size of large blown parts. A large
accumulator may hold enough plastic melt to make a parison for a
150-lb (68-kg) part. The advantages of an accumulator are: 1)It
holds a large volume of melt for large items requiring very long
molding and cooling cycles. 2) It permits high production rates.
3)It permits the fast extrusion of large parisons and consequently
a short suspension time for the parison, allowing comparatively
little sag and better control of wall thickness. 4)It provides more
uniform shot size (weight of plastic), which minimizes waste. 5)It
decreases idle mold time to a minimum.
Slide 18
5.2.3.3 Co-extrusion Blow Molding Co-extrusion refers to the
technology used to make products that contain multiple layers in
their wall structures. Such products are said to be co-extruded.
The layers may be made of the same or different materials, colored
or uncolored material, or recycled and virgin materials. The main
difference between multiple-layer and single-material extrusion
blow molding is in the extrusion system. In co-extrusion, each
material is extruded from its own extruder. Examples of products
made from this process are ketchup bottles and automotive gas
tanks.
Slide 19
Arrangement of Extruders for Co-extrusion An arrangement of
extruders to produce co-extruded, multilayer structures is
illustrated in Figure 5.8. FIGURE 5.8-Page 316: Co-extrusion blow
molding. FIGURE 5.8-Page 316: Co-extrusion blow molding.
Slide 20
Multilayered Structures A co-extruded, multilayered structure
(Figure 5.9a) may be created to provide one or more characteristics
that cannot be provided by a single-layer product. for example, a
better heat barrier or increased resistance to permeation. or that
a costly color be used in only one layer of the structure instead
of throughout the entire wall thickness.
Slide 21
Co-extrusion Systems The extrusion system in co-extrusion blow
molding must supply several streams of melted material to the die
simultaneously. Some streams are smaller, by design, in volume than
others, in order to produce thinner layers. The processing
conditions may also differ from one material to another. See Figure
5.9b which shows a typical co-extrusion die head. See Figure 5.9c,
which illustrates the multiple layers in a packaging
container.
Slide 22
FIGURE 5.9 (a) Multilayer structures; (b) multilayer extrusion
die and manifold delivery system; (c) multiple layers used in
packaging applications.
Slide 23
5.2.4.1 Three-Dimensional Extrusion Processes Three types of
three-dimensional technology methods are available: (1) the suction
blow module, (2) vertical clamp parison manipulation, and (3)
horizontal segmented mold parison manipulation. FIGURE 5.11(Page
319) Suction blow molding. Suction Blow Molding: FIGURE 5.11(Page
319) Suction blow molding. (1) The suction blow molding
Slide 24
Parison Manipulation In the vertical clamp method the mold
opens vertically, the lower half slides out, the parison is placed
in the cavity, and the mold slides back and blows (Figure 5.12).
This method is ideal for multilayer applications: for example, fuel
filler pipes. Conventual mold halves provide a lower mold cost. The
mold is also very accessible, and double-clamp stations can be used
for higher volumes (Figure 5.13- Page 320). FIGURE 5.12(Page 319)
Vertical clamp method. FIGURE 5.12(Page 319) Vertical clamp method.
(2) Vertical clamp parison manipulation
Slide 25
FIGURE 5.13 Integrated mounting. (Courtesy of SIG Kautex, Inc.,
North Branch, NJ.)
Slide 26
(3) Horizontal segmented mold parison manipulation In the
segmented mold/horizontal clamp process (figure 5.14) the parison
is extruded, and as the mold begins to close, the parison is
inflated. Then the parison is manipulated to the cavity
configuration and the mold completes closure, cools, and opens with
part ejection. The parison manipulation is accomplished with A
sixaxis robot. Complex shapes can be produced by using segmented
mold technology, with the incorporation of value-added design
features through the use of A clamp function and pinch-off for twin
tubes, brackets, and so on. The part quality is high due to the
minimum contact between parts and mold. FIGURE 5.14(Page 320)
Horizontal clamp. FIGURE 5.14(Page 320) Horizontal clamp.
Slide 27
5.2.4.2 Double-Walled Parts and Containers A typical part is
shown in Figure 5.15a.
Slide 28
see page 321 for figures 5-15 The parison is usually produced
on a continuous extrusion machine with a transfer parison into a
mold or a shuttle press (Figure 5.15b). In Figure 5.15c the parison
has been prepinched on the bottom. It has been preblown with low
pressure to form a pillow. As the mold closes, the parison bulges
and forms over the male form. The side and top of the parison begin
to be trapped around the edges, essential for the successful
double-walled part (Figure 5.15d). The mold closes further, with
air preventing the walls from collapsing (Figure 5.15e). Cooling
and high-pressure air are used to conform the mold core and cavity
(Figure 5.15f ). The high-pressure air is usually blown through a
hollow needle.(see page 321 for figures 5-15)
Slide 29
5.2.5 Injection Blow Molding Injection blow molding is used to
produce a molded parison called a preform. Injection blow molding
is usually preferred over extrusion blow molding for making small
parts that require high production volumes and closer control of
dimensions. (Figure 5.16-Page322). The injection blow molding
process occurs in two steps: (1) injection molding a preform onto a
support pin or core, which provides neck threads that are already
formed to their required dimensions; and (2) blowing the preform,
still on the support(core) pin, to its final shape in a separate
mold(Figure 5.16-Page322).
Slide 30
5.2.5.1 Injection Blow Molding Machine In phase 1(injection
station) of this process, the preforms are molded by injecting the
plastic material into a matched metal mold, consisting of top and
bottom split cavities, over a core that forms the inside of the
tubular preform. The preforms are allowed to cool only long enough
to hold their shape. In phase 2(blow station) the mold opens and
the hot, semiviscous preforms are indexed to the next station
(Figure 5.18), where split cavities in the shape of the part are
closed over the preforms. Here, the hot preforms are blown to the
shape of the cavities, then cooled. In phase 3(ejection station)
the blow molds open and the parts are indexed to the next station
for ejection. In this three-phase process, all three phases take
place at the same time (Figure 5.19).
Slide 31
FIGURE 5.18 Uniloy Milacron transfer turret, indexing
injection-molded performs on cores to blowing station. FIGURE 5.18
Uniloy Milacron transfer turret, indexing injection-molded performs
on cores to blowing station.
The majority of machines are of the three-station type
described, with 120 indexing and matching injection and blow molds.
(Figure 5.20-page 324). Special four-station machines are indexed
on 90 turns, with the fourth station being used for special
conditioning of the parison core rod after stripping (pick-off) and
before blowing (Figure 5.20-page 324). Four-station machines are
often used for two-color or multicolor bottles.
Slide 34
5.2.5.2 Stretch Blow Molding Stretch blow molding [referred to
earlier as injection stretch blow molding (ISBM)] involves:
1)conditioning (heating) a molded and cooled preform to a specific
temperature, 2)closing it in the blowing mold, 3)then very quickly
stretching it in two directions, length and diameter. Often, a rod
is used to stretch the hot preform in the axial direction, with air
pressure then used to stretch it in the radial direction.
Apllication of this type of Molding: improve impact strength,
transparency, surface gloss, gas barrier, and stiffness
properties.
Slide 35
FIGURE 5.21: with the most common application for stretch blow
molding being the soda bottles made of clear or tinted polyethylene
terephthalate (PET)
5.2.6 Control System FIGURE 5.23 Uniloy Milacron Carmac 486
process controller for extrusion blow molding machines.
Slide 38
5.2.6.1 Reprogramming 5.2.6.2 Universality 5.2.6.3 Safety
5.2.6.4 Troubleshooting 5.2.6.5 Ancillary Control 5.2.6.6 Open- and
Closed-Loop Control 5.2.6.7 Statistics 5.2.6.8 Automatic Quality
Control
Slide 39
5.2.7 Advantages of Extrusion and Injection Blow Molding
Figures 5.24 to 5.27 The blow molding process is a natural
production process for containers and hollow parts. It is the
preferred process for high-volume containers for packaging
applications such as food, personal care items, and household
products, as well as for industrial high-strength applications such
as automotive and agricultural tanks, pressure vessels, and air
ducts Figures 5.24 to 5.27 show several examples of blow- molded
parts. 5.2.7.1 Extrusion Blow Molding
Slide 40
FIGURE 5.24 Cannondale bike bellows.
Slide 41
FIGURE 5.25 Panels.
Slide 42
FIGURE 5.26 Assorted bottles and containers.
Slide 43
FIGURE 5.27 Medical prescription containers.
Slide 44
5.2.7.2 Injection Blow Molding The main advantages of injection
blow molding are: There is no scrap or flash to trim and reclaim.
The neck finish and details are very accurate and of high quality.
There is no process weight variation. Injection blow molding
typically offers the lowest part cost for high-volume bottles
weighing 37 g or less.
Slide 45
5.2.8 Disadvantages of Extrusion and Injection Blow Molding A
disadvantage of blow molding is uneven wall thickness. Although
this may be minimized with programming, the wall is usually thicker
at pinch-off areas and thinner in corners. The tendency of thicker
walls to shrink more in the center of the part has to be
compensated for in the product design. For example, rectangular
containers usually have outwardlycurved surfaces. Another
disadvantage is that close dimensional tolerances are difficult to
achieve, the exception being bottle threads produced by injection
blow molding. The accuracy of surface finishing details on
extrusion blow molded products is also relatively low. 5.2.8.1
Extrusion Blow Molding
Slide 46
5.2.8.2 Injection Blow Molding The disadvantages of the
injection molding process are: The cost of tooling is higher than
for extrusion blow molding. Bottle sizes and shapes are limited to
an ovality ratio of 2 : 1 and a blow-up ratio no greater than 3 :
1. Slightly offset necks are possible with this process, but
handles are not.