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MFGT 142Polymer 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 201 to 341
• 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

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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
• 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.

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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)
• 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.

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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).

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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
  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.

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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.

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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 the same 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 injection molding 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 and injection 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 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.

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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 and from 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 of die/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.

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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 toward the 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
  not need 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 of thermal 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 to parison 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 pinch point 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

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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.

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Trouble-shooting Guide for Blow MoldingTable 12.1
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Case Study 12.1 Making Soda Pop Bottles

• Performance requirement (after 120 days)
• less than 15 loss of CO2
• no off-taste, no change of shape (swelling), no
  fall in liquid level,
• drop test of 6 feet with no cracks or leakage,
  burst test for CO2
• PET had excellent barrier properties versus PVC
  (2x), HDPE (52x), PP (57x), and LDPE (114x).
• Stretch blowing development improved properties
  of PET.
• PET is injected at 480F-540F and then quenched.
  (resin is dried)
• Preforms can be stored or shipped for final
  forming.
• PET preform is heated to 200F (60F higher than
  that Tg)
• Hot PET is stretched and blown to form crystals
  which are small and do not reflect much light.