Blow Molding - Polymer Processing - Lecture Slides, Slides of Software Engineering

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MFGT 142

Polymer Processing

Blow Molding

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

Blown Film Equipment

• Equipment

– Extruder and screen changer

• 2” to 8” diameters

• L/D ratios from 20:1 to 34:

• 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)

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.

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.

Troubleshooting Blow Film

• Table 11-3 (ITEC-041 Book) Pg. 179

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 Model Roplast uses rates 800 lbs/hr or 300 ft/min of LDPE, $200/hour billing machine

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Updated: 3/2/99 per hour per year PRODUCT SPECIFICATIONS VARIABLE COST ELEMENTS --------------------- --------------------- Part Name Sheet NAME Material Cost $12.68 $634, Width 100 cm WDTH Direct Labor Cost $0.41 $20, Maximum Wall Thickness 1 mm THKM Utility Cost $0.01 $ Average 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, Tooling Cost $15.60 $780, Number of Cavities 1 CAV Building Cost $0.40 $20, Number of Actions in Tool 1 ACT Maintenance Cost $16.31 $815, Surface Finish [3=best] 1 [1,2 or 3] FIN Overhead Labor Cost $6.62 $331, Cost of Capital $30.40 $1,520, Annual Production Volume 50 (000/yr) NUM =========== =========== Length of Production Run 1 yrs PLIFE TOTAL OPERATION COST $120.26 $6,012,

MATERIAL SPECIFICATIONS ------------------------------------------------- --------------------- --------------------- Material Type HDPE MAT Material Price $0.88 $/kg PRICE INTERMEDIATE CALCULATIONS Scrap Credit Value $0.00 $/kg SCPRI Part Name Sheet Density 0.94 g/cm^3 DENS Material Designation HDPE Thermal Conductivity 0.24 W/mK TCOND Product Weight 100 g Heat Capacity 1675 J/kgK HTCAP Raw Material Price $0.88 /kg Melt Temp 220 C MTEMP Material Scrap Price $0.00 /kg Tool Temp 45 C TTEMP Material Density 0.94 g/cm^ Eject Temp 80 C ETEMP Adjusted Material Scrap 0. PROCESS RELATED FACTORS Cumulative Rejection Rate 0. 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. Average 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 sec Production Rate 9144 cm/minute OCYCLE 9144. 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. Productive Tool Life 1 yrs EXOGENOUS COST FACTORS EXOG Tool Sets/Station 1 Direct Wages 12 /hr WAGE Indirect Salary $50,000 /yr SALARY Equipment Investment/Station $330,000 /station Indirect:Direct Labor Ratio 0.4 ILAB Tooling Investment/Set $30,000 /tool set Benefits on Wage and Salary 30.0% BENI Working Days per Year 260 DAYS Power Consumption/Station 0.1 kW Working Hours per Day 24 HRS Building Space/Station 26.3 sq m Capital 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 GAS Price of Building Space $600 /sq m PBLD ############################# ############ ############

Auxiliary Equipment Cost 20.0% AUX Equipment Installation Cost 50.0% INST Investment Maintenance Cost 5.0% MNT

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)

– Melting resin in extruder

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

– Close mold and inject air.

– Part is cooled.

– Part is ejected.

– Part is trimmed.

Blow Molding

• Principle-

– Inflate a soft end thermoplastic hollow preform (or parison) against a cooled

surface of a closed mold (extrusion blow molding).

– OR inject 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).

Extrusion Blow Molding

• Extrusion Blow Molding the parison is formed from an

extrusion 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 molded

individually.

• 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.

Injection Blow Molding

• Injection blow molding forms a parison by injecting a

molten resin into a mold cavity and around a core pin.

• The parison is formed by injection called preform and then

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

• Traditional injection molding machine is used to create

preform.

– 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.

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

improved properties to pass 6 foot drop test.

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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.