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.

Co-Extrusion Blow Moulding

MULTI – LAYER BLOW MOULDING
Multi layer blow moulding or co-extrusion blow moulding enhance the quality container than those produced from conventional single layered blow moulding process. The various combinations available are listed below:
No of Layers = 2 (Double layer)
Combination of Layers= Virgin - Recycled

Application :

• Economical due to usage of recycled material.
• Strong containers due to double layer structure. Ecco-friendly due to recycled material

No of Layers = 2 (Double layer)
Combination of Layers= Natural - Colored
Application:

• Cost reduction due to less usage of additives.
• Suitable for special packaging for sensitive materials where the color additives should not directly in contact with material to be packed.

No of Layers = 3 (Three Layer)
Combination of Layers = HDPE/PP – Recycled - HDPE/PP
Application :

• Economical due to usage of recycled material.
• Strong containers due to multi layer structure.
• Ecco-friendly due to recycled material
• Btter aesthetics due to recycled material is sandwiched between virgin layers.

No of Layers = 3 (Three Layer)
Combination of Layers = Virgin HDPE - Recycled with carbon black - Virgin HDPE
Application :

• Best suitable for light sensitive products like milk.
• Improves shelf life considerably.

No of Layers = 3 (Three Layer)
Combination of Layers = Nylon - Adhesive - HDPE/PP
Application :

• For pesticides and similar hazardous chemicals, solvents etc.
• Reduce the evaporation of product filled.

No of Layers = 4 (Four Layer)
Combination of Layers = Nylon - Adhesive - Regrind/recycled - HDPE/PP
Application :

• For pesticides and similar hazardous chemicals, solvents etc.
• Reduce the evaporation of product filled.
• Regrind from the machine is used for economy.
• Recycled material also possible to process.

No of Layers = 6 (Six Layer)
Combination of Layers = HDPE/PP - Adhesive - Barrier resin - Adhesive - Regrind - HDPE/PP

Application:

• For food packaging for long shelf life and aroma retention.
• For Plastic Fuel Tank in Automobiles.

Barrier Resins:
For enhancing packaging properties of blow molded containers, barrier resins are used. They have better barrier properties from gas, moisture etc. than normal polyolefin’s (HD and PP). The most common barrier resins are:

• Nylon
• EVA
• EVOH

Adhesive Resins:
When non-related materials with different characteristics are to be combined for better quality, an adhesive material is required, which can bond with both of the material. These are used to bond the barrier layer to holding layer of polyolefin’s. Barrier resins like Nylon or EVOH normally do not bond with HDPE or PP. Specially developed adhesives bond to both the barrier and carrying layer. The adhesive resin should be selected in accordance to the layers to be bonded. Some commercially available brands are:

• Bynel from DuPont
• Plexar from Equistar Chemicals
• Admer from Mitsui Chemicals
• Modic from Mitsubishi Chemical

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