How to Choose a Blow Moulding Machine

Buying a blow moulding machine can be a nerve racking experience for the fresh entrepreneurs. There countless specification, capability and price factors to be considered. This blog sets out to help anyone who wants to buy a blow moulding machine but does not know where to start.
Mould Size:
The most important spec is the mould size of the machine. The range of product you want to produce should be matching the model. For this refer the maximum mould size mentioned in the machine specification chart.
Production Capacity:
Estimate your production capacity required like containers per month and cross check with the machine supplier whether this can be met. It is advisable to keep a 25% extra capacity. This relates to the extruder size, plasticizing capacity and possible cycle time of the machine.
Quality of the machine:
Quality of the machine is a relative subject. Normally the cost of the machine will vary proportionally with the quality of the machine. Quality itself is in two sections – machine built quality & product (container) quality. The product quality depends on your end user. The machine built quality affects the maintenance cost and breakdown cost of the plant.
Other Machine Specifications:
The Die-Core (Cup-pin) size of the machine should be enough for the proposed containers intended to manufacture. The mould clamping force also critical with respect to your products. Then for the accumulator head type machines. The accumulator shot capacity is important.
Automation:
The machine automation can be done by a PLC control or by a wired relay logic (this become outdated because of the limitations and bulkiness of control panel). The PLC control has very wide range of options available. This can be from ordinary custom built micro processor controller to industrial grade established control systems like MOOG, B&R etc.
Additional Options:
Automatic Deflashing:
Here the product is automatically deflashed or trimmed in the attachment of the machine. Fully ready container will come out of the machine. If deflashing option is not present, every product (container, bottle etc) have to be trimmed by an operator manually.
Ancillaries:
Selection of ancillaries like air compressor, scrap grinder, cooling system are also required to be selected in accordance with the machine manufacturers recommendations.
Parison control:
A parison control system helps to improve the product quality and reduce the flash waste. This will be useful for large containers.

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.

Parison Thickness Control

Parison control means dynamically controlling thickness of Parison to get a uniform thickness container on an Extrusion Blow Moulding machine. It helps to maintain the required thickness axially and radially on the product made out of Extrusion Blow Moulding (EBM).
What is Parison?
Now what is a Parison? It is the molten tubular form made on an Extrusion Blow Molding machine by extruding thermo plastic resins. This viscous melt resin called Parison is put into two halves of Blow Molds and shaped to the form of cavity of mould by blowing compressed air.
Why control required?
Feedback SensorWhen the Parison comes out of the die head of the EBM machine it will be in the molten state. Due to the self-weight the Parison will elongate resulting in a thinner top portion compared to the bottom portion. This will cause uneven thickness in the article made. Also the complex geometries of the product to be molded necessitate varying the parison thickness to get more uniform product wall thickness.

The advantages:
· Uniform article wall thickness and hence improved quality due to lesser thermal stresses during processing.
· Even distribution of thickness and hence possibility for reduction in weight.
· Reduction in top and bottom wastage and hence higher production.
· Reduced cooling time due to uniformity of thickness and hence higher production.
Different types of Parison Control
Relay logic control (timer based)
In single stage parison control the thickness is adjusted once in a parison. This can be either thick-thin or thin-thick type. In three stage control the thickness is adjusted three times in a parison (thin-thick-thicker or thick-thin-thicker type). This control is achieved by a simple On-Off hydraulic valve operated by a set of timers and relays.
Closed loop control (Multi-point Parison Programmer)
Closed loop electro-hydraulic control system can be fitted on the machine to control the parison thickness at several points (30 times or 100 times or more). Since this is a closed loop system the accuracy is much better than ordinary system and the resulting container will have most uniform thickness.
Axial Wall thickness Control
All the above described systems are for thickness control in axial direction of Parison. This is the most common Parison controls.
Radial Wall thickness Control (PWDS)
Other than the axial wall thickness control, for some specific and complex products, thickness control in radial direction is required. This is achieved by transforming the uniform annular space through which parison comes out to elliptical shape.
MOOG
MOOG is the pioneer in Parison programming or Parison control.
Other Providers
Other than moog several other parison control systems are available. They include:
· B & R, Austria
· Gefran, Italy
· Beckoff, Germany
· Yuken, Japan
· Barber & Colman, USA
· Seitlure Systems, India
Time based and position based Control
In the continuous extrusion blow moulding time based systems are used. That is the parison thickness is controlled on a definite interval of time. Here the total time for one parison cycle is divided by the number of points to calculate the time interval.
In accumulator type extrusion blow moulding either a time based or a position-based system can be used. In position based system a potentiometer senses the position of the push out (accumulator) cylinder stroke and programs the profile points with respect to the position. In time based system the profile points are programmed with respect to the time taken for parison push out. To overcome the variations in time the controller automatically updates the time in each cycle and uses it for programming the next cycle.
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