66183543 week 5 blow molding

7/29/2019 66183543 Week 5 Blow Molding

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Blow Molding


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Introduction

Plastic formation process-manufacture of

bottles and hollow-shaped parts

Competitive with other processes,injection molding

Advantages; cycle is very short (low cost),

lower mold cost than injection molding,

high production rates in producing hollow

parts with small or large opening


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Blow molding process; general

steps

Melting the resin- done in extruder

Form the molten resin into a cylinder ortube (this tube is called parison)

The parison is placed inside a mold, andinflated so that the plastic is pushedoutward against the cavity wall

The part is allowed to cool in the mold andis then ejected

The part is trimmed


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Image of Parison


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Blowing step in the blow molding

process


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Blow Molding Process

The parison can be formed by

Extrusion process

Injection molding process


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Extrusion blow molding

Parison is formed from by forcing the plastic

through an extrusion die.

Material enters the die, flow around the

mandrel so that extrudate would be cylindrical The die would have a hole at the center so

that air could be blown into the cylinder

In some blow molding operations, the air isintroduced from the bottom through an inlet


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This process can be; continuous extrusion blow molding

During the process, the extrusion runs continuously,

thus making a continuous parison. using multiple mold to match the mold cycle to the

extrusion speed

Intermittent extrusion blow molding

During the process, the extruder is stopped duringthe time that the molding occur

use either reciprocating screw or an accumulatorsystem

Extrusion blow molding


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Continuous extrusion blow molding

In this system, the output of the extruder ismatched by having multiple molds which sealand blow the parison and then move away fromextruder to cool and eject

In practical case, the mold cycle is longer than

time required to extrude a new parison If the mold cycle is twice than time needed for

creating a parison, a two mold system can beused

The method is sometimes called rising moldsystem - system of which two or more molds areused to mold parts from one extruder duringcontinuous process


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Two-mold system in continuous

extrusion blow molding

In this system, one mold captures the parison

(Mold A)

While a part is cooling in Mold B


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Mold A then withdraws the parison from the

extruder

While the part is ejected from in Mold B


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The part is cooling in Mold A (moved out from extruder)

Mold B is moving into position around the parison


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The cycle is completed by cooling of the part in Mold A

Capture a new parison in Mold B, blow the part into the

mold cavity


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Parison transfer system for continuous


System in which the parison is removed from the die andtransferred to a mold, thus give space for another parison

to be formed while the first part is cooling


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Multiple mold system for continuous


Use multiple molds to match the output ofthe extruder when forming the parison


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Disadvantages of rotating mold system-

the rotating wheel is much more

complicated mechanically, cost ofadditional molds is quite high

Advantage of rotating mold system mold

cycle that are much longer than extrusionrate can be accommodate

Continuous extrusion blow molding

methods are suitable for production ofsmall to medium size parts


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If the parts are large, the time to form the

parison by continuous extrusion is relatively long

Long extrusion time results in excessive coolingof the parison- cannot be effectively blown

Therefore method to forming parison that is

faster than extrusion speed must be used The rapid forming has been accomplished by

two methods; the reciprocating screw system

and accumulator system

In both systems, the parison is formed

intermittently rather than continuously, and used

only one mold


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Reciprocating Screw

Specific amount of resin is ejected to form a parison

A parison can be formed in one or two seconds

During the subsequent blowing and cooling, the extrusionscrew is retracting and accumulate another charge

No parison is formed during the part of the cycle so the

parison formation is intermittent If the blowing and cooling time are long, the screw may

stop when sufficient material has accumulate for the nextparison shot

The stoppage may cause some disruptions in theextruder, so another method which used accumulator andallow continuous running of the extruder is developed


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Accumulator system

The accumulator system also produce parisonintermittenly

In this system, the extrudate flow from the

extruder into an external chamber oraccumulator

At the appropriate moment in the cycle, a ram inthe chamber injects the resin through a die to

form parison The accumulator is heated to maintain theproper temperature of the resin


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Accumulator system in intermittent



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Injection Blow Molding

The parison is formed by the injection of moltenresin into a mold cavity and around a core pin

The parison is not a finished product, but it is

subjected to subsequent step to form the finalshape

Second step, blowing of the intermediate part ina second mold

Because of distinct separation of the two steps,the parison made by injection molding is called apreform


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Injection blow molding process

The mold is closed Resin is then injected to form a cylindrical part The mold is opened, and preform is ejected


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The preform is transferred to a blowing station while it is still hot or itcan be reheated

After the second mold is closed, air is injected into the heatedpreform (through a hole)

Blow the preform, against the inside walls, mold is open and the partis ejected


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Stretch blow molding

the plastic is first molded into a "preform" usingthe Injection Molded Process. These preformsare produced with the necks of the bottles,including threads (the "finish") on one end.These preforms are packaged, and fed later(after cooling) into an EBM blow moldingmachine. In the SBM process, the preforms areheated (typically using infrared heaters) above

their glass transition temperature, then blownusing high pressure air into bottles using metalblow molds. Usually the preform is stretchedwith a core rod as part of the process. Thestretching of some polymers, such as PET(PolyEthylene Terepthalate) results in strainhardening of the resin, allowing the bottles toresist deforming under the pressures formed by

carbonated beverages, which typicallyapproach 60 psi.
http://en.wikipedia.org/wiki/Bottleshttp://en.wikipedia.org/wiki/Bottleshttp://en.wikipedia.org/wiki/Image:Inside_the_mold.jpghttp://en.wikipedia.org/wiki/Bottleshttp://en.wikipedia.org/wiki/Bottles


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The preform can be stored until the

finished blow molded is needed

The flexibility of separating the two cycles

has proven useful in manufacture of soda

pop bottle



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Comparison of extrusion and

injection blow molding Extrusion blow molding

It is best suited for bottle over 200g in weight, shorterruns and quick tool changeover

Machine costs are comparable to injection blow

molding Tooling costs are 50% to 75% less than injectionmachine

It requires sprue and head trimming Total cycle is shorter than injection (since the parison

and blowing can be done using the same machine) Wider choice of resin Final part design flexibility


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Comparison of extrusion and

injection blow molding

Injection blow molding

Best suited for long runs and smaller bottles

No trim scrap

Higher accuracy in final part Uniform wall thickness

Better transparencies with injection blow molding,

because crystallization can be better controlled

Can lead to improve mechanical properties fromimproved parison design


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Common plastics for blow molding

HDPE (stiff bottle, toys, cases, drum)

LDPE (flexible bottle)

PP (higher temperature bottle) PVC (clear bottle, oil resistant containers)

PET (soda pop bottle)

Nylon (automotive coolant bottle, powersteering reservoir)


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Thickness of the part- wall thickness is varied

(bottom part is thicker, and thin toward the top)

Stiffness of the final part is dependent on the

wall thickness (good stiffness is required tostand straight & to withstand pressure of filling

and labeling)

Important parameter - Blow ratio= Mold

diameter/Parison Diameter

Blow ratio = 1.5 to 3 are common

66183543 week 5 blow molding

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