SPI Plastics Engineering Handbook - Chapter 12 - Blow Molding of Thermoplastics

8/17/2019 SPI Plastics Engineering Handbook - Chapter 12 - Blow Molding of Thermoplastics

1/8

12

Blow Molding of

Thermoplastics

Historically, the blow molding of thermoplas-

tic materials began during World War 11.

Polystyrene was the first material used with the

newly developed blow molding machines, and

low-density polyethylene was used

in

the first

large-volume commercial application, a

squeeze bottle for deodorant. In the beginning,

the plastic bottle business was dominated by

companies such as Ow ens-Illinois, Continental

Can, Am erican Can, P lax, Imco, and Wheaton

Industries, using proprietary technology and

equipment. The introduction of high-density

polyethylene and the commercial availability of

blow molding machines, mostly from such

German companies as Fischer, Bekum, and

Kautex, led to phenomenal industrial growth

and diversity in the

1960s

Basically, blow molding is intended for use

in

manufacturing hollow plastic products; a

principal advan tage is its ability to produce hol-

low shapes without having to join two o r more

separately molded parts. Although there are

considerable differences in the available pro-

cesses, as described below, all have

in

com-

mon production of a parison precursor), en-

closing of the parison in a closed female mold,

and inflation with air to expand the molten

plastic against the surface of the mold, where

it sets up into the finished product.

Difference s exist in the way that the parison

is made i. e. , by extrusion or by injection

Reviewed and updated by Samuel L. Belcher, Sabel Plas-

te ch Inc., Cincinnati, OH.

molding); in whether

it

is to be used hot as i t

comes from the extruder or injection molding

machine as

in

conventional blow m olding),

or

stored cold and then reheated as

in

cold pre-

form molding); and in the m anner in which the

parison is transferred to the blow mold or the

blow mold is moved to the parison.

The basic process steps remain the same,

however:

1.

Melt the material.

2 . Form the molten resin into a tube or par-

3 . Enclose the hollow parison in the blow

4. Inflate the parison inside the mold.

5. Cool the blow-molded part.

6 .

Remove the part from the mold.

7.

Trim flash, as needed.

ison.

mold.

In many cases, all these steps can be carried

out automatically, with the finished products

conveyed to dow nstream stations for secondary

operations and packaging.

Although there are many variations, the two

basic processes are extrusion blow molding and

injection blow molding. Extrusion processes

are by far the more widely used, but injection

blow molding and injection stretch blow mold-

ing have captured significant market segments.

While reviewing these methods, the reader is

urged to refer to Chapters 4 and 5 for additional

background material.

4


2/8


3/8

BLOW MOLDING OF THERMOPLASTICS

4

Fig.

2

n

3. Section through a typical extrusion die head.

Process

Variables*

Obviously, the process parameters to be con-

sidered in blow mo lding will be conditioned by

*This section courtesy of Soltex Polymer Corporation

the type of resin used e. g ., making an acetal

product would involve higher blow pressures

than would be required for polyethylene), the

type of blow molding unit used, and the prod-

uct being made.

The discussion below deals primarily with

the extrusion blow molding of high-density

polyethylene bottles-the techn ique, material,

and application

in

most comm on use today. The

process variables discussed cover the extruder

die for making th e parison) and the blowing

air.

Die. In a sense, the parison die has become

the key element in blow molding because it

controls material distribution in

the

finished

item and, in turn, the economics

of

the final

product. Therefore, increasing attention has

been devoted to making t he programming die

work to improve economics as well as proper-

ties. The main control factor in parison pro-

gramm ing is the core pin. T his pin can b e given

greater latitude by providing a taper at the die

face and providing for movem ent of the pin

so

the opening at the face of the die can be made

larger

or

sma ller as required to d eliver parisons

with thicker

or

thinner walls. Such a movable

Parisoncontrd

Support air

SUPP

air

Double ring

spider tOrp9do

Die

- -

Mandrel he ad w ith hear1 curve

Double

ring spider

torpebo)

head

auble

spider

head

Fig. 12-4.

Three basic panson extrusion die heads.

Courresy

Barrenfeld-Ascher )


4/8

344 SPI PLASTICS ENGINEERING HANDBOOK

I

PUSH

ROD

Manual

adjurtmenll

PERED DIE

PIN

DIE BI~SHING

Replaceable)

Fig.

12-5. Manually variable

die. All

illustrations on

Processing Variables,

Courtesy Soltex Polymer

Corp.

core pin is schematically diagrammed in Fig.

12-5.

Die Dimension Calculations.

In sele cting the

die bushing and mandrel dimensions to be used

for the production of a blow-molded polyeth-

ylene pro duct, several features must be consid-

ered.

For bottles, the weight, minimum allowable

wall thickness, and minimum diameter are im-

portant considerations, as well as the need, if

any, to use a parison within the neck area and

whether there may be adjacent pinch-offs.

The type and melt index of the resin used are

factors because of swell and elasticity charac-

teristics.

Die land length and cross-sectional area must

be considered.

Some of the die dimensions will also depend

partly on the processing stock temperature and

the extrusion rate anticipated for produc tion.

Mathematical formulas have been de veloped

to permit the selection of die dimensions.

Al-

though these calculated dimensions are in-

tended as approximations or starting points in

die selection, they have been found to yield

products, in the majority of cases, within + 5

of the design w eight. In som e cases, only slight

changes in mandrel size or stock temperature

an d/o r extru sion rate ar e necessary to obtain the

desired weight.

Formulas f o r Calculating Die Dimensions.

The formulas presented here are for use with

long

land dies, those having a 20-30

:

1 ratio of

mandrel land length to clearance between man-

drel and bushing.

In their use, consideration must be given as

to the anticipated blow ratio, the ratio of m ax-

imum product outside diameter to the parison

diameter. Normally, ratios in the range of 2-

3 :

1 are recommended. The practical upper

limit is considered to be about 4 : 1

For large b ottles with small necks, this ratio

has been extended as high as

7 : 1 so

that the

parison fits within the neck. In such a case, a

heavier bottom and pinch-off results from the

thicker parison.

Also,

less material is distrib-

uted in the bottle walls

90

from the parting

line than in similar bottles with lower blow ra-

tios.

Whe n the neck size of a bottle or the smallest

diameter of the item is the controlling feature

as when the parison must be contained within

the smallest diameter), the following approxi-

mations may be used to calculate die dimen-

sions:

For a free fzlling parison:

d

0.5N,,

P d

D: 2Bdt 2 t 2

where:

Dd

= Diameter of die bushing, in.

Nd

=

Minimum neck diameter, in.

Pd = Mandrel diameter, in.

Bd

=

Bottle diameter, in.

t

= Bottle thickness a t

B , ,

in.

This relationship

is

useful with most poly-

ethylene blow m olding resins, and is employe d

when bottle dimensions are known, and a min-

imum wall thickness is specified. It is particu-

larly useful for round cross sections.

Th e 0. 5 figure presented fo r selecting the di-

ameter of the die bushing may cha nge slightly,

depending on processing conditions employed


5/8

BLOW MOLDING OF THERMOPLASTICS 45

stock temperature, extrusion rate, etc.), resin

melt index, and die cross-sectional areas avail-

able for flow. It may be slightly lower for a

very

th in

die opening small cross section) and

higher for large openings.

If

product weight is specified rather than wall

thickness for a process employing “inside-the-

neck” blowing, the following approximation

may be employed:

P d =

D:

2 W / T 2

M

where:

W = Weight of object, g

L = Length of object, in.

d

=

Density

of

the resin, g/cc

T

= Wall thickness, in.

This system is applicable to most shapes and

is of particular advantage for irregularly shaped

objects.

A controlled parison is one

in

which the di-

mensions are partially controlled through ten-

sion i.e. , the rotary wheel, the falling neck

ring, etc.).

Because of this, the following relationships

are employed:

Dd 0.9Nd

Pd = J D; 3.6B,r + 3.6r2

Pd

=

d.D;

- 3 . 6 W / T 2 M

Derivation

of

Formulas

core pin blow sys-

tem). When a polymer is forced through a die,

the molecules tend to orient in the direction of

the flow. As the extrudate leaves the die, the

molecules tend to relax to their original random

order. Parison drawdown, the stress exerted by

the parison’s own weight, tends to prevent

complete relaxation. This results

in

longitudi-

nal shrinkage and some swelling

in

diameter

and wall thickness.

Through laboratory and field experience

it

has been found for most high-density polyeth-

ylene blow molding resins that:

D, O S N ,

where:

D,,

=

Die diameter

N,) = Minimum neck diameter

A,/

=

Cross-sectional area of the die

A,,

=

Cross-sectional area of the bottle

and that:

n

D =

D : P:

4

where:

P ,

= Mandrel diameter, in.

Bd

= Product diameter, in .

t

=

Product thickness, at Bd, in.

n

= 0 . 5

B :

B: 4 Bd t

4 t 2

r D ; P : )

= 0 5 - 4 t 2

4- 4B, f2 )

4 4

Dividing through by n/4 and rearranging

terms:

P : =

D: 2Bf / t t

or:

Pd

= JD

2Bdt 2 t2

Also:

= r i

where:

W =

Object weight, g

L =

Object length,

in.

A d 0 . 5 Ao

d

= Resin density, g/cc


6/8

346

SPI PLASTICS ENGINEERING HANDB OOK

: Since AD = 0.5AB:

R W

0 : P i )

4

= 0 5

M

4 w

0.5

i

P i

p d = JD: 2

wlRu

The same derivation is employed for con-

trolled parisons except that:

Other Considerations. As shown, the sizes

selected for the die bushing and mandrel de-

pend on wall thickness of the finished blow

molded part, the blow ratio, and certain resin

qualities included

in

the above formulas for

various polyethylene blow molding resins.

These qualities are parison swell increase

in

wall thickness as the parison exits the die) and

parison flare ballooning or puffing out of the

parison as it exits the die). Both depend on pro-

cessing conditions. It has been shown that cal-

culations can be made for the general die di-

mensions. The other dimensions of the die-

approach angles and lengths-vary widely with

machinery capabilities and manufacturer’s ex-

perience. Calculations for these dimensions

thus will not be given here. Instead, a few rules

of thumb suffice. For example, the land length

of the die see Fig. 12-6) generally is eight

times the gap distance between the pin and the

die. In simple tabular form, this works out to

be :

Gap

size in.)

Land length in.)

Above 0.100

Below 0.030

0.030-0.100

Notice that the land length is at least inch,

regardless of gap size. This land length is nec-

Fig.

12-6.

Die and

pin.

The die should be streamlined to avoid

abrupt changes in flow, which could cause

polymer melt fracture. When no further

changes are expected in die dimensio ns, the die

mandrel and bushing should be highly polished

and chrome-plated. This helps to keep the sur-

face clean and eliminates possible areas

of

resin

hangup. F inally, the edges of the pin mandrel)

and die should have slight radii to minimize

hangup w ithin or at the exit of the die area. The

face

of

the mandrel should extend

0.010

to

0.020 inch below the face of the die to avoid

having a doughnut at the parison exit.

Air Entrance. In blow molding, air is forced

into the parison, expanding

it

against the walls

of the mold with such pressure that the ex-

panded parison picks up the surface detail of

the mold. Air is a fluid, just as is molten

polyethylene, and as such it is limited in its

ability to flow through an orifice.

If

the air en-

trance channel is too small, the required blow

time will be excessively long, or the pressure

exerted on the parison will not be adequate to

reproduce the surface details of the mold. Gen-

eral rules of thumb to be used in determining

the optimum air entrance orifice size when

blowing via a needle are summarized below:

Orifice diameter in.)

Part size vo l . )

I.

16

I

4

I

U p to

one quart

quart-I gallon

1 callon-55 gallons

Normally, the gauge pressure of the air used

to inflate parisons is between

40

and 150 psig.

Often, too high a blow pressure will “blow

out” the parison. Too little, on the other hand,

will yield end products lacking adequate sur-

essary to get the desired parison flare.

face detail. As high a blowing air pressure as


7/8

BLOW MOLDING OF THERMOPLASTICS 347

possible is desirable to give both minimum

blow time resulting in higher production rates)

and finished parts that faithfully reproduce the

mold surface. The optimum blowing pressure

generally is found by experimentation on the

machinery with the part being produced. The

blow pin should not be so long that the air is

blown against the hot plastic. Air blowing

against the hot plastic can result

in

freeze-off

and stresses in the bottle at that point.

Moisture

in

the blowing air can cause pock

marks

on

the inside product surface. This de-

fective appearance is particularly objectionable

in

thin-wdled items such as milk bottles. Use

of a dryer is recommended to prevent this prob-

lem.

Parison V ariations. T o obtain even wall dis-

tribution in blow-molded products, the parison

can be modified from its normal concentric tu-

bular shape. Die bushings can be “notched”

or “ovalized” to provide a nonuniform cross

section to accommodate nonround product de-

signs see Fig. 12-7). Parison thickness can be

varied in the lengthwise direction as well, by

using a process called parison programming

see Figs. 12-8 and

12-9).

Credit for develop-

ing the first parison programmer is given to

Denes Hunkar of Cincinnati, Ohio . His system

moved a tapered die mandrel in relation to a

fixed die bushing during extrusion to increase

or decrease the wall thickness. Others operate

in one

of

the following ways:

1. By varying the extrusion rate.

2. By varying the extrusion pressure.

Thick

. He a vy

Thick

Design

normal

Die

Gap

Die

Gap

Notched

Oval

Fig. 12-7. Notched and ovalized die bushings for making

non-round products.

Fig. 12-8. Effect of moving core on thickness of parison

wall.

3 . By moving a tapered die bushing

in

re-

4.

By varying the take-off rate in a contin-

lation to a fixed mandrel.

uous parison operation.

Early programmers had the capability to set

eight points along the parison length; today,

parison programmers are available that can

change the thickness up to 128 times. The ad-

ditional control over wall thickness allowed the

blow molding industry to expand rapidly into

Fig.

12-9.

Bottle wall distribution effect of parison programming


8/8

348 SPI PLASTICS ENGINEERING HANDBOOK

markets other than bottles, such as automotive

air ducts, fuel tanks, furniture, and

so

on.

Types

of

Extrusion

Blow

Molding

Continuous Extrusion.

One of the basic

forms of extrusion blow molding is based on

producing a molten tubular parison without in-

terruption. The many variations on continuous

extrusion blow molding come about because of

the need to move the blow molds

in

and out of

the die area to capture the needed length of tub-

ing

for

each part and remove it for blowing and

cooling. Methods for introducing the blowing

air also vary. Normally, the size and design of

the product handle

or no

handle, center

or off-

set finish, etc.) and the number to be produced

will

govern the choice of process.

Shuttle mold systems remove the parison to

a position below

or

to one

or

both sides of the

extrusion die for blowing. When the tube

reaches the proper length, the blow mold is

moved under the die head, where

it

closes

around the parison, pinching one end closed;

the tube is severed by a knife or a hot wire, and

the mold moves to the blow station to clear the

way for the next parison. For higher productiv-

ity, more than one parison can be extruded from

the die head at a time see Fig.

12-10). n

the

common rising mold type of machine, the blow

mold rises from below to close around the tube;

the blow pin enters from the bottom see Fig.

12-11). Other adaptations of the shuttle mold

process move the blow mold on an incline or

use alternating molds moving in from left and

right. In these cases, the blow pin normally en-

ters the precut parison from the top see Figs.

12-12 and 12-13). Effects of moving heavy

molds at high speeds limit the shuttle mold pro-

cess to products of about 2 gallons

(8

liters) in

capacity

Fig.

12 10.

Twin parison, dual

shuttle

blow molding machine. Courtesy

Johnson

Contro ls, Inc. )

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SPI Plastics Engineering Handbook - Chapter 12 - Blow Molding of Thermoplastics