EXTRUSION BLOW MOULDING

BLOW MOULDING

Blow moulding produces hollow three-dimensional articles from many of the thermoplastics materials which are available as granules or powders. The simplest tool consists of two female parts which contain a cavity when closed.

Granules or powder are softened in a plasticising cylinder and extruded into a vertical tube or "parison". The soft , warm parison is surrounded by the open mould which is then closed, thereby sealing the lower end of the parison. This is then inflated pneumatically (from the other end) to conform with the surface of the mould.

Clearly the outside dimensions of the article can be accurately determined, but the wall thickness, and its distribution, depends on the size of the parison and the geometry of the mould.

Die design and selection of parison size and its wall thickness is very important. Melt flow pattern, compression of melt, land length, back pressure are important factors in die design. Controls for varing parison wall thickness in number of steps are available. This feature allows to produce bottles with fairly uniform wall thickness.

It is not easy to mould-in lugs and bosses and holes can not be moulded-in.

Blow moulding machine provides the following functions:

FOLLOWING POINTS ARE FOR FURTHER STUDIES:

  • Melting ∓ pumping of plastics melt in to the Die by the EXTRUDER
  • Understanding of screw plasticising ∓ temperature profile is important.
  • Optimised screw design to handle specified polymer is required.
  • Forming of parison - hollow tube of plastic melt - by the DIE
  • It should supply melt with uniform pressure, viscosity and temperature at all around the exit of die. These should not vary with time.
  • Weld lines should be strongly merged by suuficient back pressure.
  • Balanced melt flow, compresion ratio, land length ∓ lback pressure are important factors in Die design.
  • Clamping two halves of moulds (cavities) around the parison and holding it clamped and cut off the parison - CLAMP UNIT

Clamp unit moves between PARISON STATION and; BLOW STATION. It close, clamp ∓ opens mould aroung parison at parison station. After parison is cut it moves the mould to Blow station. After the AIR BLOW into the mould , it opens and ejects the bottle. These movements are controlled by HYDRAULIC SYSTEM.

  • Expanding parison into the mould cavity with compressed air, thereby allowing parison to take up shape of the mould cavity,- BLOWING UNIT
  • At Blow station BLOW PIN enters the mould and blows air into the mould. Cooling of mould takes place.
  • Here HEAT EXCHANGE system in the mould requires good understanding. Cooling System should be well designed.
  • Exhausting the air from the moulded part and cool the plastics.

Ejection is carried out at Blow Station after the exhausting air from the mould.

ADVANTAGES:

  • Lower mould costs, mainly resulting from the lower pressures used, than for injection moulding.
  • Lower machinery costs than for injection moulding, per unit weight of material processed.
  • External threads can be moulded-in.
  • Large open ended parts can be made by splitting a closed symmetrical moulding.

DISADVANTAGES:

  • Close tolerances on the outer wall only. Wall thickness decreases with increase in the mould diameter: hence thinning occurs at corners and the thinnest wall occur at the largest diameter.
  • Holes cannot be moulded-in and therefore represent a post -moulding operation.
  • Cooling times are longer than for injection moulding because the moulding is cooled from the outside only. To accommodate a satisfactory wall thickness at the thinnest point, wall thicknesses elsewhere may be greater than they need to be,and this also means than cycle time will be longer and moulding heavier than thereinjection moulded counterparts.

Extrusion blow moulding is a process for directly producing hollow articles like bottles from plastics by using extruder, die, clamp unit (press) and mould. The bottle produces are expected to have following characteristics:

  • Barrier against gas and vapour,
  • Environmental resistance,
  • Impact resistance,
  • Clarity or colorful,
  • Printability.

BLOW MOULDING MACHINE

The machine provides the following functions:

  • Melting of plastic,
  • Forming the parison - hollow tube of molten plastic,
  • Clamping two halves of mould around the parison and holding it closed,
  • Expanding parison into the mould cavity with compressed air there by allowing parison to take up the shape of the mould cavity,
  • Exhausting the air from the moulded part and cool the plastic.

The automatic Blow moulding machine gives the following sequence of operations.

  • Mould close around an extruded parison and clamp,
  • Knife operates cutting the parison,
  • Mould shift to blow station - in case of top blow system, (with bottom blow system air blow and mould shift to ejector station simultaneously,)
  • Blow pin enters the mould and air blow takes place,
  • Mould opens,
  • Ejection,
  • Mould returns to parison head.

To carry out the above mentions sequence of operations the Blow moulding machine incorporates the following:

  • Extruder : to plasticise the plastic, special screw for HOPE, RPVC, PP are offered.
  • Parison head: (cross head and die assembly) to create the desired size of parison,
  • Parison cutter: to cut off required length of parison,
  • Sealing unit : To seal the parison at the open end for blow moulding of jerry-can which does not have symmetrical shape around it's axis.
  • Mould clamp unit: to open, close and clamp the mould and to shift the mould to and fro between the center lines of extrusion and blow (pin) station. Another hydraulic cylinder is used for shifting mould clamp unit from parison head to blow/ejection station.
  • Blow and ejection stations: (also known as calibration station). This stations should be connected to source of compressed air to inflate the parison.
  • Deflating units: to remove flashes and pinches.
  • Hydraulic systems: to operate in proper sequence the mould clamp unit - mould open, close and clamp.
  • Pneumatic system: to operate in proper sequence

(a) Pre-blow-support air, air blow to inflated parison, movement of blow unit.

(b) Movement of blow pin or blow mandrel,

(c) Parison cutter,

(d) Wall thickness control, (can also be with hydraulic system. )

(e) Deflashing unit.

  • Electrical control panel:

(a) to control stepless speed variation of DC motor of extruder.

(b) temperature control of barrel heaters and die heater zones,

(c) to control the sequence of operation involving hydraulics and pneumatic.

  • Extruder: In present day machines the extruder is placed horizontally. It consists of screw, barrel with heaters, gear box, Dc drive for steeples variation of screw RPM. Screw of L/D ratio of 20 to 24 is used. Screw profile is designed to suit the polymer being used.

HDPE screw:(fig. 2-1) It is decompression type screw with two compression zones. Compression ratio are high i.e. of the order of 3.2 to 3.5. This is a proven international design to give high throughput and excellent mixing for HDPE.

RPVC screw: (fig. 2-2) It is specially designed keeping in mind the heat sensitive nature and high abrasion of PVC. It is therefore a three zone screw with a short feed zone and a longer compression zone with gradual taper and a lower compression ratio of 1.9 to 2.

PP screw: (fig 2-3) This screw has three zone with longer compression and metering zones.

In this process the parison is continuously extruded. Screw RPM of extruder is adjusted to give right length of parison during one cycle time.

EXTRUDER DRIVE

DC motor with thyrister control drive is used. The advantages are:

  • Infinite speed variation. A techo feedback system for continuous sensing and error correction to ensure actual speed equal to set speed.
  • Better speed / torque characteristics of a dc motor.
  • Better speed holding over a long period of time even under normal voltage fluctuations.

Ease of speed selection and variation compared to AC motor with a clutch system.

Laminated yoke motor with full wave full control drive eliminates the risk of sparking at commutator, thereby giving high machine up time.

The drive is specially designed for plastic processing machines. They are full wave, full control regenerative type with compact space saving design. It has extensive safety features such as

  • under voltage protection
  • Field failure protection,
  • Phase failure protection,
  • Oven load protection.

Thus the expensive motor is well protected from the electrical system faults.

Advantages of this drive

- very low ripple content

- Regeneration possible,

- DC out put voltage i.e. armature voltage can be reduced to zero of required.

Transmission system incorporates energy efficient helical gear box with integrated thrust bearing.

PARISON HEAD ASSEMBLY

It consist of mainly

  • Cross head
  • Torpedo spider mandrel
  • Die body,
  • Die ring,
  • Mandrel insert,
  • Die centering screw,
  • Heaters, Thermo-couples.

SIDE FED HEAD:

for HDPE (fig. 2-4)

This design is suitable for processing polyolifine. The melt coming from extruder and progresses towards the die ring, in the uniform tubular shape. The melt emerges from the die ring with balanced flow mandrel.

In order to control the temperature of melt in parison head, it is necessary to proves heaters and thermocouples on the e parison head. The entire surface of the head can be divided in to 2 or 3 heater zones with independent temperature controller.

Pre-blow air / support air connection is provided by a connecting hole in the die body and mandrel as seen in the figure. The parison is not allowed to deform it's tubular shape when the preblow air is provided during formation of parison. This stabilises the parison.

Mandrel is connected to an adjusting rod which can be moved up or down wards to vary the gap (wall thickness) between die-ring and mandrel. The plastic melt flow passage between die-ring and mandrel at die exit is divergent. Four adjusting screws are provided on the die body for centering the mandrel. Adjusting rod is moved by a pneumatic or hydraulic cylinder for parison thickness control. Right combination of inserts are to be selected for a given bottle.

TWO PARISON HEAD

for HDPE ( fig. 2-6 )

Two parison head is used for blow moulding small bottles of HDPE, LDPE AND PP. It melt flow is bifurcated in the adapter. For precise balancing the flow leading to both parison head, separate throttle valves are provided on both the side of the adapter.

TOP FED HEAD

for RPVC (fig. 2-5 )

It is also known as spider / torpedo head. This design is suitable for materials like PVC, PC etc. as they make high demands on the precision of the temperature control and flow behaviour. This requires flow path of low resistance eliminating dead zones to avoid stagnation of material.

With this head design, the melt flows axially on to the cone shaped torpedo tip and two web spider. Then it passes through the die ring and core in tubular form. The length of the flow between spider web and die ring exit should be enough for rejoining the melt at the operating back pressure on the melt. Pre-blow air \ support air connection is provided by a connecting hole in the web of spider and torpedo as shown in the figure.

Centering screws are provided on die body to make the die gap uniform at die exit.

ACCUMULATOR HEAD

For large blow moulded containers, where continuously extruded parison may sag excessively due to higher weight. Accumulator allows the molten plastic to accumulate inside the head and a hydraulic piston forces the plastic through the tubing die whenever the machine gets ready for blowing. This feature is available in KBM30.

PARISON CONTROL

(fig. 2-7)

It is required to facilitate dimensional precision of the parison to ensure uniform wall thickness of the moulded bottle. Uniform wall thickness is necessary for a minimum value of weight /impact strength ratio of a blow moulded bottle. With constant wall thickness of parison the bottle of intricate shape produced may not have uniform wall thickness in the bottle. This is because the parison expands during blowing operation and parison stretches and it's wall thickness lowers. The extent of change in wall thickness depends on the amount stretching it undergoes. To overcome the problem of wall thickness variation in the finished bottle it is necessary to provide thicker wall where melt is expected to stretch and expand more and similarly provide thinner wall thickness where expansion or stretching is not much. Such wall thickness variations can be programmed on the parison control so that parison of suitable wall profile can produce bottles of intricate shape with uniform wall thickness in finished bottle.

Parison control is for the control of:

- longitudinal length and

- cross sectional wall thickness.

The longitudinal wall thickness are controlled by extruding the parison through a variable orifice die. The inner profile of die ring and shape of mandrel insert provide variable annular gap at the parison exit by moving the mandrel up/down by means of either pneumatic / hydraulic cylinder mounted on top of the head. Stroke of the movement is controlled by timer and speed of it's movement is controlled by flow control valve.

Starting from the established die gap, the timer programmed wall thickness control offers facility of influencing the parison wall thickness by increasing or decreasing it and then automatically returning to original condition. For parison control we have three parameters to control, namely time, stroke and speed.

The preselected time determines, at which point the die is reset in relation to the parison length. The stroke determines, by how much the die is moved to change the die gap. It is controlled by limit switch. Here one size of thick and one size of thin parison can be possible. In other words only two sizes of wall thickness can be set. Speed determines, how fast the transition from thick to thin or thin to thick is to take place and it is regulated by a flow control valve. The parison control cylinder can be pneumatically or hydraulically operated.

PARISON CUTTERS

(fig. 2-8)

Cold parison shear type cutter.

This type of cutter is most commonly used and is popular.

The knife blade is operated by a pneumatic cylinder through a rack and pinion arrangement. The position of knife blade with respect to die ring is adjustable. The knife moves left to right in one cycle and then moves right to left in the next cycle.

BLOWING AND EJECTOR STATION

(FIG. 2-9)

The machine can have Top blowing (mounted above the mould) or BOTTOM blowing (mounted below the mould) arrangement. Movement of the blow pin in to the mould and its withdrawal are operated by pneumatically.

In top blowing arrangement the blow pin / blow mandrel is introduced in to the still plastic parison gripped by the mould on reaching the blowing station. The shaping of neck also takes place (compression moulded neck), while blowing compressed air inflates the parison t take the shape of the mould.

In bottom blowing, it is possible to arrange the parison to fall on the blow pin directly before mould closing takes place. It is possible to have pre-blowing (earlier than inflating air blowing or blow mould is closed.) in order to prevent the crumpling of narrow parison. In this method blowing (inflating)and shifting of the mould to ejection station can be done simultaneously thereby saving in cycle time. The movement of blow pin is by pneumatic or hydraulic cylinder.

SEALING UNIT:

(FIG. 2-10)

This is required for blow moulding of jerry can. The parison is required to be shaped - some what oval shape - for moulding flatter or rectangular cross section of the can perfectly. This is achieved by sealing the open end of tubular parison and slightly inflating by support air. This unit is mounted on the single parison HDPE die head for jerry-can moulding.

It consists of a two pneumatic cylinders, sealing rods, toggle linkage etc. The long levers of the toggles pivoted at the center, support the sealing rods at one end and engage short toggle levers at the other end. The short levers in turn are pivoted around a pin, which is connected to the piston rod of the pneumatic cylinder. The long levers with sealing rods open when piston rod goes in. Similarly long levers with sealing rods close when piston rod comes out. With the closing of sealing rods the sealing action takes place as parison is pinched by the parallels sealing rods - parallel to mould parting line.

There is a provision for adjusting the position of sealing unit. It should be ensured that knife movement does not foul with sealing unit.

DEFLASHING UNIT

The blow moulded bottles are deposited on the transfer device(guide rails) in proper position. The scrap removal is carried out by punching or cutting, operated pneumatically. Logically arranged guide baffles lead this scrap to a centralised recycling unit for reuse.

MICROPROCESSOR CONTROLS KWIC MICRO

It consists of Monitor, key board, Input/output status display etc.

Key board consist of numeric key for data entry and manual operation keys for Reset, Carriage in, Mould close, support air, Blow pin out, Sealing, Cutter left, Cutter right, Carriage out, Blow pin in, Blow air, Mould open, Parison thick, Parison thin, Special program key, Mode selection keys for Setting, Manual, Semi-auto, Auto.

Main menu offers following pages:

  • Data entry menu,
  • Mode selection menu,
  • Temperature profile menu,
  • Parison profile,
  • File maintenance.

Data entry is by percentage. The maximum value is show with in bracket.

Under data entry menu following pages are available:

Mould open/close parameters:

Wherein you can enter mould close delay, mould closing speed, mould safety speed, mould safety time, mould safety pressure, lock force, two mould open speed.

Carriage parameters:

Where in you enter carriage in delay, two carriage speeds, carriage movement pressure, carriage out delay, three carriage out speeds.

Blow pin parameters:

Where in you enter blow pin in period, blow pin speed, blow pin pressure, blow air delay, blow air period , exhaust air period, blow pin release period, blow pin release speed, blow pin release speed, blow pin release pressure, blow pin out speed, blow pin out pressure.

General parameters:

Where in you can enter support air selection on or off, support air delay, support air period, sealing delay, sealing period, cutter selection on or off, cutter delay, air ejection selection on or off, batch counter selection on or off, batch counter number.


Parison profile parameters:

Where in can enter parison thickness for ten points of the parison length and cycle time.

Temperature profile:

Where in you can enter temperatures for four zones of barrel three zone of die, two zone of die ring, hot wire temperature, selection of heating on or off . Actual temperatures are entered for barrel zones and die zones. Percentage is entered for die ring zones.

This microprocessor control offers excellent repeatablility and consistency in quality and productivity.

EXTRUSION BLOW MOULDING

BLOW MOULDING

BLOW MOULDING MACHINE

EXTRUDER DRIVE

PARISON HEAD ASSEMBLY

SIDE FED HEAD:

TWO PARISON HEAD

TOP FED HEAD

ACCUMULATOR HEAD

PARISON CONTROL

PARISON CUTTERS

BLOWING AND EJECTOR STATION

SEALING UNIT:

DEFLASHING UNIT

MICROPROCESSOR CONTROLS - KWIC MICRO

HOME Contact-to buy this book Things to know- Process Things to know- Polymers Things to know- Part Design Things to know- Mould
Things to know- Machine Injection Moulding Process Screw Plasticating Moulding Key factors influencing proces Shear strain curve
pvt diagram

Thermal stability Balancing melt Flow Balancing Heat Freezing Quality Problems
Three Balances

Moulded-in Stress Shrinkage Inconsistent Dimension Sink-mark Weld-lines
Moulding Problems

CYCLE TIME Plastics Part Design Flow Analysis .. ..
Engineering Basics in Machine Glossary of technical terms- Physics Glossary of technical terms- Polymer process Myth Understanding Mould Cooling Channels Ideal Moulding Shop


HOME-book

Foreword Preface Contents Introduction Summary
Author: Prabodh C. Bolur Archives of articles PLASTINDIA-2000 Photos MoldFlow Users Meet Seminar on FRP IPI's Lecture Meeting at Mumbai
PETNOLOGY seminar

IPI-AITEC Seminar Training At Nilkamal Plastics Windsor Functions-old Answers to YOUR questions ...


Barrel Temperature

Typical Barrel temp. Barrier Screw Energy Efficiency Failure Analysis Metering Stroke
Mouldability

Plastics Part Design Polymers Applications Polymers Applications Polymer Basics Properties of Polymers
Properties of Polymers Process Guide Process Limitation Quality in Moulded Parts Residence Time Speed Setting in IMM
Tolerance in molded parts
Volume Change -- -- -- --