EXTRUSION 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.
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:
BLOW MOULDING MACHINE
The machine provides the following functions:
The automatic Blow moulding machine gives the following sequence of operations.
To carry out the above mentions sequence of operations the Blow moulding machine incorporates the following:
(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.
(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.
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.
DC motor with thyrister control drive is used. The advantages are:
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
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
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.
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.
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.
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
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.
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.
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 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.
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.
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.
Where in can enter parison thickness for ten points of the parison length and cycle time.
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 MACHINE
PARISON HEAD ASSEMBLY
SIDE FED HEAD:
TWO PARISON HEAD
TOP FED HEAD
BLOWING AND EJECTOR STATION
MICROPROCESSOR CONTROLS - KWIC MICRO
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