Blow Molding.ppt

8/10/2019 Blow Molding.ppt

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MFGT 142

Polymer Processing

Blow Molding

Professor Joe Greene

CSU, CHICO


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Blow Molding

Overview

Blown-Film Extrusion

Extrusion blow molding (continuous and intermittent)

Injection blow molding (hot and cold parisons) Molds and dies

Plant concepts (layout and capacity)

Product considerations (materials, shapes, designs)

Operation and control of the process


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Blow Film Extrusion

Blow molding of plastic film is a plastic forming process that

is well suited for the manufacture of film and bags. Process

Melting resin in extruder

Form molten resin into cylinder or tube.

Blow air inside the resin bubble.

Pull film into nip rollers through guide rolls.

Pull film through a series of rollers.

Wind-up film in take-up rolls


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Blown Film Equipment Equipment

Extruder and screen changer

2 to 8 diameters L/D ratios from 20:1 to 34:1

Screws are deep cut with melt separation flights.

Die block with oscillator, tubular die

Die diameters 6 to 36 (max. range from 2 to 100)

Air cooled ring: Single and dual-cooling orifice configurations

Tower structure with collapser and primary nip

Surface treater, secondary nip, and winder(s)


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Blow Film Extrusion

Process is similar to blowing up a balloon.

Tube is cooled by air from a cooling ring around die. The frost line is the zone where the temperature of the tube

has fallen below the softening point of the plastics.

Example, HDPE frost line actually appears frosty.


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Blow Film Extrusion

Size and finish of product is controlled by

extrusion speed takeoff speed

die or orifice opening

material temperature

air pressure inside tube.

Blow-up ratio is ratio of die diameter to the bubble diameter


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Blow Film Extrusion

Film producers may slit the tubing on one edge during

windup. If tube is blown to a diameter of 2m the flat film will have a width of

over 6 m.

Slot dies are not practical.Tubular films are desired as low-cost

packaging for some foods and garments. Only one heat seal is needed in the production of bags from blown

tubing.

Blown films are semi-oriented

Less orientation than highly oriented sheets from slot dies. Stretching from the tubing expanding under pressure results in less

orientation. This stretching provides balanced orientation.

Products are biaxially oriented. Machine and cross directions.

Improved physical properties result.


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Troubleshooting Blow Film

Table 11-3 (ITEC-041 Book) Pg. 179


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Blown Film

Auxiliaries

Output can be tubular, lay-flat, slitters, cutters Output can allow in-line production of slit-open or lay-flat

products, gusseted products, and variety of bags including T-shirt

bags with cut-outs.

Materials PE- LDPE, LLDPE, HMWHDPE, etc.

PA, ionomers, polyvinylidenechloride, PVOH, EVOH, EVA alone

or blended with PE

Products

Heavy duty films (0.1 to 0.2 mm) used for covers for agriculture

Packaging: wrap, can lining, garbage bags, T-shirt bags, garment

Multilayer: (3 to 11 layers) for barrier film


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Blown Film Operation and Costs Operation

Operating Pressure of 3,000 to 6,000 psi with max of 10,000 psi

Production rates with internal cooling range from 6 to 20 lbs per inch

of lay-flat width.

Typical Production rates of 700 lbs per hour of product & 300 ft/min

Typical production plant has 5 to 10 lines in operation.

Nip treater widths range from 24 to 144 (max. of 244)

Tension controlled nip treaters with typical tension levels of 0.125 and

1.0 lb/linear inch per mil of film thickness (1 mil =0.001)

Costs Every year 90 new blown film lines are built (60% to replace existing) Annual output of over 4 Billion lbs of resin over 2500 existing lines.

Production Costs (typical) are $200/hour per machine

Single-layer blown film line is $330K to $660K. Coextrusion line is $4 million


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Blown Film Costs Spreadsheet Operation

------------------------------------------------- --------------------- --------------------- ------------ --------------------- ------------------------------------------------- --------------------- --------------------- EXTRUSION TECHNICAL COST MODEL from IBIS Cost Model EXTRUSION TECHNICAL COST MODEL from IBIS Cost ModelRoplast uses rates 800 lbs/hr or 300 ft/min of LDPE, $200/hour billing machine------------------------------------------------- --------------------- --------------------- ------------ --------------------- ------------------------------------------------- --------------------- ---------------------

Updated: 3/2/99 per hour per year PRODUCT SPECIFICATIONS VARIABLE COST ELEMENTS --------------------- ---------------------

Part Name Sheet NAME Material Cost $12.68 $634,128Width 100 cm WDTH Direct Labor Cost $0.41 $20,658

Maximum Wall Thickness 1 mm THKM Utility Cost $0.01 $503Average Wall Thickness 1 mm THKA

External Surface Area 100 sq cm SAREA FIXED COST ELEMENTS --------------------- ---------------------Projected Area 100 sq cm PAREA Equipment Cost $37.82 $1,890,863

Tooling Cost $15.60 $780,000Number of Cavities 1 CAV Building Cost $0.40 $20,116

Number of Actions in Tool 1 ACT Maintenance Cost $16.31 $815,461Surface Finish [3=best] 1 [1,2 or 3] FIN Overhead Labor Cost $6.62 $331,060

Cost of Capital $30.40 $1,520,144Annual Production Volume 50 (000/yr) NUM =========== ===========

Length of Production Run 1 yrs PLIFE TOTAL OPERATION COST $120.26 $6,012,933

MATERIAL SPECIFICATIONS ------------------------------------------------- --------------------- ---------------------Material Type HDPE MAT

Material Price $0.88 $/kg PRICE INTERMEDIATE CALCULATIONSScrap Credit Value $0.00 $/kg SCPRI Part Name Sheet

Density 0.94 g/cm^3 DENS Material Designation HDPEThermal Conductivity 0.24 W/mK TCOND Product Weight 100 g

Heat Capacity 1675 J/kgK HTCAP Raw Material Price $0.88 /kgMelt Temp 220 C MTEMP Material Scrap Price $0.00 /kgTool Temp 45 C TTEMP Material Density 0.94 g/cm^3Eject Temp 80 C ETEMP

Adjusted Material Scrap 0.005PROCESS RELATED FACTORS Cumulative Rejection Rate 0.001

Dedicated Investment 0 [1=Y 0=N] DED Effective Production Volume 50050 /yr Operation Rejection Rate 0.1% REJ Tool Complexity Factor 19542

Material Scrap Rate 0.5% SCR Energy Adjustment Factor 2.6Average Equipment Downtime 20.0% DOWN Clamping Force 424 kN

Direct Laborers Per Station 0.5 NLAB Cooling Time 0.9 sec

OPTIONAL INPUTS CALCULATED Cycle Time 9144.0 secProduction Rate 9144 cm/minute OCYCLE 9144.0

Equipment Cost per Station $330 (000) OEQUIP $330,000 Runtime for One Station 2546.6%Tool Cost per Set $30 (000) OTOOL $30,000 Number of Parallel Stations 25.47

Productive Tool Life 1 yrsEXOGENOUS COST FACTORS EXOG Tool Sets/Station 1

Direct Wages 12 /hr WAGEIndirect Salary $50,000 /yr SALARY Equipment Investment/Station $330,000 /station

Indirect:Direct Labor Ratio 0.4 ILAB Tooling Investment/Set $30,000 /tool setBenefits on Wage and Salary 30.0% BENI

Working Days per Year 260 DAYS Power Consumption/Station 0.1 kWWorking Hours per Day 24 HRS Building Space/Station 26.3 sq mCapital Recovery Rate 15.0% CRR

Equipment Recovery Life 8 yrs ELIFE Equipment Annuity $3,257,501 /yr Building Recovery Life 20 yrs BLIFE Tooling Annuity $844,818 /yr

Working Capital Period 3 months WCP Building Annuity $63,572 /yr Price of Electricity $0.051 /kWh ELEC Working Annuity $1,847,042 /yr

Price of Natural Gas $6.50 /MBTU GASPrice of Building Space $600 /sq m PBLD ############################# ############ ############

Auxiliary Equipment Cost 20.0% AUXEquipment Installation Cost 50.0% INST

Investment Maintenance Cost 5.0% MNT


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Blow Molding Blow molding is a plastic forming process that is well suited

for the manufacture of bottles or other hollow parts. Process


Form molten resin into cylinder or tube (parison) into mold

Close mold and inject air.

Part is cooled.

Part is ejected.

Part is trimmed.


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Blow Molding Blow molding is a plastic forming process that is well suited

for the manufacture of bottles or other hollow parts. Process (Figure 12.1)


Form molten resin into cylinder or tube (parison) into mold

Close mold and inject air.

Part is cooled.

Part is ejected.

Part is trimmed.


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Blow Molding

Principle-

Inflate a soft end thermoplastic hollow preform (or parison) against a cooledsurface of a closed mold (extrusion blow molding).

ORinject a thermoplastic material into an injection mold featuring a neck ring

and core pin. Air is injected to blow the material against a cooled surface

(injection blow molding).


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Blow Molding

Materials

Good stretchability and a high MW are preferred

HMWPE is the most widely used for high volume packaging

PP used in processes that promote orientation

PVC is used for bottles in Europe (homopolymer can be crystal clear)

PET is primarily used for injection blow molding. Preforms

injected into cold mold to an amorphous state and then reheated to

100C for blowing

Nitrile, SAN, PVDC, PPO, PC, and PA

Products

Packaging, bottles for drinks, containers for cosmetics and

toiletries, automotive containers and bumpers.

Coextrusion products for chemical resistance and structural


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Extrusion Blow Molding

Extrusion Blow Molding the parison is formed from anextrusion die that is similar to one from blown film.

Blown film is continuous. The film is made continuously.

Extrusion blow molding is discrete. Each part is moldedindividually.

Cycle time reduction

Two mold shuttle system (Figure 12.2)

Parison transfer system (Figure 12.3)

Rotating mold or carousel system (Figure 12.4)

Accumulator system for intermittent extrusion blow molding (Fig.

12.5) Very large parts (up to 120 gal) that are several times the

injection volume.


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Injection Blow Molding Injection blow molding forms aparisonby injecting a

molten resin into a mold cavity and around a core pin. Theparisonis formed by injection calledpreformand then

blown with air to form final shape. (Figure 12.6)

Traditional injection molding machine is used to createpreform.

Mold is closed with core pin in place.

Resin injected to form a cylindrical part around the core pin.

Threads, if any, are also formed at this stage.

Mold is opened, core pin removed, and parison ejected.

Parison is transferred to a blowing station either still hot or cooled.

Second mold is closed and air is injected to form part.

Mold opens and part is ejected.


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Injection Blow Molding The injecting and blowing cycles of injection blow

molding need not be done at the same time or even at thesame location.

Parisons can be made by injection and then either stored

until the finished blow molded parts are needed or shipped

to a satellite location where they can be blown.

Parison must be reheated if not blown right away.

Example, Soda pop bottles

Parisons are made in central location on a large, multi-cavity injectionmolding machine which give economies of scale and close engineering

control over injection mold step (critical step).

Parisons are moved to blow molding site, reheated, and blown into bottles.

Advantages

The blow molder doesnt need expensive injection machine andinjection molds, but just and oven and a blowing station.


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Injection Blow Molding Injection blow molding allows formation of a parison that can

have a non-constant cross-section resulting in better wall

thickness uniformity than from extrusion blow molding.

Stretch blow molding (Figure 12.7)

Mechanical assistance stretches the part in the longitudinal

direction at the same time blowing the part causing a stretch in the

part along the hoop or radial direction.

Results in biaxial orientation and increased properties.

Process results from a telescoping mandrel or core pin that extends

to push on the bottom of the preform at the same time that the air is

being injected to push against the walls to stretch the material

radially.

Advantage is improvement in mechanical properties.

Higher burst strength and higher impact strength.

PET soda pop bottles use this technique to cause some crystallization and

im roved ro erties to ass 6 foot dro test.

C i


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Comparison

Extrusion and Injection Blow Molding

Extrusion blow molding is characterized by: Best suited for bottles over 1/2 pound (200 g) and shorter runs.

Machine costs re comparable to injection blow molding.

Tooling costs are 50% to 75% less than injection blow molding.

Generates 20% to 30% scrap due to sprue and head trimming. Requires additional equipment to grind scrap and reuse.

Total cycle time is shorter due to less parison transfer time.

Wider choice of resins possible due to resins with higher

viscosities can be used.

Final part design flexibility can be greater with the use of

asymmetrical openings.

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Comparison

Extrusion and Injection Blow Molding

Injection blow molding is characterized by: Scrap free.

Better suited for long runs and smaller bottles.

Higher accuracy in the final part.

Uniform wall thickness.No seam lines or pinch marks

Transparencies are best because crystallization can be better

controlled and the blowing can be more stress free.

Improved mechanical properties from improved parison design andfrom stretch blowing.


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Blow Molding Molds Molds and Dies

Made from tool steel and is similar to extruded pipe and blown film dies.

Programmed Parison Formation (Figure 12.8)

Improves part uniformity of extrusion blow molded parts.

Method employs an extrusion die that has a mandrel with a conical

shaped end.

The slope of the sides of the cone on the mandrel is not quite as steep

as the slope of the sides of the opening.

When the mandrel and outer die move relative to each other, the gap

between them will open or close, depending on the direction of

movement.

Feature is used to make prison that is thicker at the bottom than at the

top, thus compensating for the natural thickness variation in blow

molded due stretching.

Variation in thickness is accomplished by timing the movement ofdie/mandrel with the extrusion of the parison.


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Blow Molding Molds General Mold Considerations

Does not require high pressure like injection molding.

Aluminum (cast or machined) is the most common material.

Disadvantage to Al is the excessive wear.

Steel inserts can be used for wear areas, e.g., pinch-off.

Cast steel can be used for long production runs. Epoxy molds with Al filler can be used for prototype.

Molds are two halves with cooling lines.

Proper venting is important to prevent air traps.

Surface of the mold is not polished or chromed. Engraving of logos is common.

Blowing point is through a hole in the top or bottom of mold.

Ejection can be through gravity or mechanical assist.

l ldi ld


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Blow Molding Molds Sliding/Compression Blow Molds

Molding a recessed ring or lip onto blow molded part by applying

compression to certain areas of part, e.g., flower pot, undercuts, etc.

Process (Two flower pots are made)

Mold closes around parison which is blown in normal method.

The parison flows around a recess in the side of the mold (top of flower pot).

Before the parison is cool, the upper and lower sections of the mold slide towardthe center portion, which is fixed. The sliding motion compresses the material

that is in the gaps between the sliding parts of the mold and the fixed parts. The

reinforcement ring is formed.


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Plant Concepts

Blow molding equipment are self contained and do notneed extensive cooling or part removal as in extrusion.

Removal and handling of scrap.

Scrap removal station automates pinch-off removal.

Scrap is chopped and blended with virgin material.

Common practice is 2% regrind. Max is 50%. Properties drop after 20%.

On-line filling and labeling is done with automation.


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Product Considerations Materials

HDPE (stiff bottles, toys, cases), LDPE (flexible bottles), PP

(temperature resistant bottles), PVC (clear bottles, oil resistant),

PET (soda pop), PC (housings), nylon (fluid containers), and FEP.

Polyolefins (HDPE, LDPE, PP) are easiest to process but are

sensitive to oils and can have stress cracking problems.

PVC is very temperature sensitive and rarely used because ofthermal degradation and safety concern for HCl.

Post consumer regrind can be used with virgin material.

Shapes

Hollow parts that are usually cubical or cylindrical.

Molded parts can be cut in two to yield two parts.

Wall thickness is limited to 1 cm (0.5 in) or less.

Wall thickness variation is a problem with thicker at bottom due toparison sag.


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Product Considerations Shapes (continued)

Corners and edges should have generous radius to reduce

stretching effect around corners. The shape of the bottom should be concave rather than flat to

ensure a thicker bottom and provide more stability (non-rocking).

The opening at top of the bottle can have the threads molded in,

which will have variations in dimensions. Volume of container can be adjusted with the use of inserts that

reduce to volume of cavity in the mold and can be seen as a round

indentation as in milk jugs.

Handles can be molded in part by blowing the part past a pinchpoint that is opened so that further blowing can fill the handle.

Handles and inserts can be attached to the outside of part when the

insert is added to cavity before blowing.

Important parameter is blow ratio (1.5 to 3 common) Ability to expand

DiameterParison

DiameterMold

RatioBlow_

_

_


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Operation and Control

Important process control parameters stretch (sag) of parison,

temperature of the parison and temperature of work space,

melt flow characteristics of resin

speed of parison formation

crystalline nature of the polymer,

cooling capacity of the mold.

T bl h i G id f Bl M ldi


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Trouble-shooting Guide for Blow MoldingTable 12.1


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Documents

Blow Molding.ppt