Types of Parison Die Heads

The sub assembly of an extrusion blow moulding machine, from where a tubular parison of molten resin comes out is called Die Head. There are different divisions of die heads commonly used. Based on the processing method the die head can be either a continuous parison die head or accumulator die head. Based on the number of parisons, it can be either a single parison or multiple parison die head. Then based on the number of layers – single layer, multi layer, view strip, multiple strip etc.
Continuous Parison Die Head
In this type of die head, the melt resin from extruder comes out continuously in the shape of a tubular Parison. This is similar to the extrusion heads for blown films. The continuous parison die heads can be either centre fed or side fed depending on the entry of melt resin from extruder to die head.

Accumulator Die Head
Here the melt resin from extruder is stored inside the accumulator head and ejected at the start of each molding cycle by external force. The volume of material can be stored is generally used as the size of head (like 5 litre, 20 litre etc). There are different types of accumulators.
The oldest one is external type where the accumulating chamber is external to the die head. In this type higher power is required to pushout the parison and the quality of parison is not good.
Then the built in vertical accumulator head is developed. The quality of parison is much better compared to the external type accumulator and power consumption to pushout parison is low. The initial models were LIFO (Last In First Out) type were the first entered melt resin stayed for a longer time leading to degradation of the resin. Then the FIFO (First In First Out) type developed leading to lesser degradation of resin. This gives the best quality of Parison.
Another type of accumulator head is screw type where the accumulator chamber is an extension of the extruder like injection moulding machines. These type have the lowest degradation of melt resin and suitable for sensitive materials like PC etc.

Multi Parison Die Heads
Here in a single cycle a multiple parisons are made resulting in higher productivity. Even though single parison type is most common, multiple parisons are common for smaller bottles and containers that require higher production capacity.

Multi layer Die Heads
In multi layer heads multiple extruders process different materials or same material of different colors and formed into a co-centric parison to enhance the property of end product.

The Process – EBM

The Extrusion Blow Moulding Process is discussed today. There are two main parts for EBM process – preparing the plastic resin by extrusion and blowing the melt resin inside the container mould.
Extruding:
The granules of the plastic resin is heated and melted into a viscous form. The heating required is supplied by electric band heaters around the extruder barrel. The screw inside extruder barrel is rotated by an electric/hydraulic actuator. This rotation of screw conveys the granules towards the die head of machine and during the travel the granules transformed into a molten viscous paste form. During the conveying the resin is normally compressed to a pre defined ratio thus removing the trapped air.
Forming Parison:
The melt resin is then formed into a Parison or tubular shape by the die head. The die head can be off two types – continuous or intermittent. In continuous type parison is formed continuously from the melt resin conveyed by the screw and barrel during the process. In intermittent type the melt is first accumulated into a accumulator chamber inside die head and during the start of each cycle pushed out to form Parison by applying external force.
Moulding:
The parison thus formed is hanged between the two mould halves and the mould halves closed by fluid/electrical force. Both the half moulds together will form the cavity inside as required and the parison is trapped inside. For complicated shapes of containers, the mould halves itself will have moving parts which will operate during the process.
Blowing & Exhausting Air:
The Parison inside mould will have an externally connected blowpin inside on one end. Compressed air will blow through the orifice of the blowpin to inside of the parison. The compressed air inside parison will press the melt plastic towards the wall of mould cavity. The mould will be cooled by chilled water and as the melt touches the inner wall of mould, will get cooled to a lower temperature. After giving sufficient time for the melt parison to cool to below the glass transmission temp of the resin, the compressed air inside is exhausted out.
Ejecting the blown container:
Once the compressed air inside mould is removed, mould halves are opened to free the blown container. Then the container can be removed from the mould either manually or by a robotic mechanism.
Trimming or deflashing the blown container:
The removed container is then trimmed to remove the excess plastic on the ends of container. This can be done either manually or by an automatic deflashing unit.
Quality Checks:
The deflashed container is then checked for quality like weight, wall thickness distribution, appearance; blow holes, other damages etc. Also periodically, drop test, stack test, leakage test etc are done to ensure the product confirmation.
Cycle time:
The EBM cycle time may be divided into three parts – blowing time, exhaust time and dry cycle time. The blowing time depends on the effectiveness of hest transfer thru the mould by cooling system. A chilled water cooling system for mould and uninterrupted compressed air supply will reduce the blowing time. The heat transfer rate of mould is also important. The exhaust time can be reduced by designing sufficient passage channel for exhaust air (through the blowpin and piping). The dry cycle time depends on the machine automation capability. The lower the dry cycle time of machine, higher the production of Machine.
Power consumption:
Other than the raw material cost, the main component of production cost for blow moulding is the power cost. Modern EBM machines are run by electricity. The enegy requirement may be divided into these sub sections:
Heating – required for heating the raw material to molten Parison.
Extruder Drive – generally an AC induction motor for conveying the material by rotating the extruder screw
Automation Power – generally hydraulic power system for the mould movements, parison pushout if required, wall thickness control etc
Compressed air: For blowing and other related automation process.
Ancillary Equipments like, Material feeder, Granulator etc.