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.