Lesson 3 Extrusion Blow Molding

8/10/2019 Lesson 3 Extrusion Blow Molding

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Extrusion Blow Molding

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Introduction

Extrusion Blow Molding is a process that can

create hollow parts. There are two different

industry segments that use Extrusion BlowMolding, packaging (bottles) and industrial (any

parts that are not bottles).


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To Read

Review Chapter 6.4.3 and 6.4.3.1 before beginning

the lesson.

To Do

Take Quiz 8afterthis lesson and before

scheduling Exam #3.


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Topics

What is Extrusion Blow Molding

Continuous and Intermittent Extrusion Blow

Molding

3D Blow Molding

Common Materials and Key Processing

Parameters Advantages and Disadvantages of Extrusion Blow

Molding

Competing Processes and Future Development


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Introduction

Extrusion blow molding is a process that can make

small to large hollow parts quickly.

The products range from less than 1 oz up to 1000

gal


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Some current products:

Containers

Gas Tanks

Toys

Seat Backs

Air Ducts

55 gal Drums


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The process starts with the formation of a

hollow tube of plastic, called aparison.

The tube is extruded downward until it extends

past the bottom of the mold.

The mold then closes on the parison..


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Air is introduced into the parison via a blow

pin at the top of the part or a needle in the

middle of the part, inflating the plastic and

pushing it against the mold sides.

The part cools in the mold.

The part is ejected from mold.


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Continuous Extrusion Blow Molding

The parison is continuously extruded

throughout the cycle.

It can only be used with materials that have

good melt strength.

It also can only be used for short cycle times.


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Intermittent Extrusion Blow Molding

The plastic material that is extruded is stored in

an accumulator.

When the mold opens, the accumulator

pushes the material out of the die to form a

parison.

It is used for parts with long cycles andmaterials with low melt strength.


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3D

It is used for parts with complicated tubular

geometry.

It is used with intermittent extrusion blow

molding.

Can be used with multiple materials (Co-

Extrusion). Usually an elastic material with a rigid material.


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There are two ways to get air into the parison.

Blow pins are used to bring the air in on the

parting line of the mold.

In bottles, it is inserted through the neck.

Blowing needles are used to create very small

holes in a part. Such pins can be located

anywhere in a mold.

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The plastic material expands through its

thickness as it exits the die. This is called die

swell.

It expands because of plastic memory. The plastic

material is squeezed as it goes though the land in

a blow molding die.

As it exits, it wants to go back to the shape it hadbefore it went through the land.

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During the formation of the Parison, it gets

longer and heavier. As it gets heavier, gravity

starts to extend the Parison. This is called

Parison Sag.

This creates an uneven wall thickness in the

Parison.

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Materials with High Melt Strength are needed

in Extrusion Blow Molding to reduce the

amount of Sag (and excess thinning) in the

Parison.

Lower barrel temperatures than Injection Molding

are used to increase viscosity.

Wide Spec/Higher Molecular Weight grades areused to give the parison more strength to hang

without tearing or excessive sagging.

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Common Materials

Olefins (Polyethylene, Polypropylene)

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Common Materials

Thermoplastic Elastomers (TPEs)

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Common Materials

Polycarbonate

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Common Materials

PVC

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Key Processing Parameters

Melt TemperatureChanges the viscosity of the

material.

Extruder SpeedDetermines the speed that the

parison is made.

Blow TimeAlthough blowing forms the product,

holding the pressure on the product keeps it incontact with the mold and increases the cooling.

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Key Processing Parameters

Blow PressureHelps to pick up surface detail in

the mold.

Parison Drop TimeHow long the parison hangs

and stretches before the mold closes.

Parison ProgrammingCreates local thickness

changes in the parison. If the parison is off-center, it will cause thickness

variation around the circumference of the part.

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Process Advantages

Molds are inexpensive vs. injection molding. Since

molding pressures are much lower, tooling can be

produced in aluminum.

Irregular geometry can be easily molded if the

mold can open without destroying the part.

There is the ability to combine severalcomponents into one part.

The inside volume of the part can be filled with

foam while the mold is still closed.

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Process Advantages - Co-extrusion

By using specialized polymers, better barrier

properties can be had.

Recycled materials can be layered in the middle.

Less costly materials can be used in a layer to

reduce the parts cost.

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Process Disadvantages

There are problems filling corners and deep

sections because the parison will be stretched

too thin.

In injection molding, the part is cooled and

constrained by two sides of the mold. With

extrusion blow molding, the part only touches

the mold on one side. This helps to create

wider tolerance than is seen with injection

molding.

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On long flat surfaces there is a tendency for the

flat surfaces to warp.

To ensure proper filling and a strong weldline, the

parison is extruded larger than the tool. Hence,

there is always some flash created as excess

material is squeezed between parting lines during

the blowing operation.

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Secondary operations are always needed to

remove flash. Holes and other features are put in

after the part is removed from the mold. There is wall thickness variation because of

parison sag and part shape. The parts tend to be

thinner at the top because of sag. When the parts

have a bigger local diameter, the parts will be

thinner in the thicker regions. The further the

parison has to travel; the thinner it gets.

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Disadvantages

Any surface defect that result from the extrusion

of the parison or sweating of the mold will cause

surface appearance problems on the part. If there is a change in parison thickness or if the

pinch-off is not designed correctly, there could be

a weak weld leading to part failure.

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Competing Processes

Injection Molding

Hollow parts can be made, but they must be molded in

halves and assembled. The parts will have constantwall thickness and tighter tolerances.

Rotational Molding

Parts will have more uniform wall thickness, but will

have a looser tolerance.

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Competing Processes

Twin Sheet Thermoforming

Parts will have an external seam/weld.

Injection Blow Molding Parts will not have a weld, have better surface finish,

and tighter tolerances, but cannot be produced as large

or as structural.

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Future Development

Extrusion Blow Molding is looking at converting

parts or products, that have two components that

are assembled together, to one Extrusion BlowMolded part. Because Extrusion Blow Molding is

a hollow part process, both sides of a product can

be molded together.

Lesson 3 Extrusion Blow Molding

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