CLAMPING FORCE (TONNAGE)

Clamping Force is one of the main parameter in Blow Moulding. It restricts the size of container can be molded on a particular machine. The clamping force is required to balance the force created inside the container by blowing air. Blow molding machines don’t require very high tonnage like Injection molding machines.
For a standard 3-plate single cylinder direct mould clamping unit, the clamping force is calculated as below:
Clamping force (N) =cylinder Area (mm2) x Pressure (bar) /10
Clamping force (kgf) =cylinder Area (cm2) x Pressure (kg/cm2)
Clamping force (lbf) =cylinder Area (in2) x Pressure (pound/in2)
For example, if a unit has 100 mm dia and applying a pressure of 100 bar, then
Clamping force, N = 50 x 50 x 3.14 x 100 / 10 N
= 78500 N
= 78.5 kN

Clamping force, kgf = 5 x 5 x 3.14 x 101.9716 kgf (1bar=1.019716kg/cm2)
= 8004.77 kgf
= 8 Ton

Clamping force, lbf = (5/2.54) x( 5/2.54) x 3.14 x 1450 lbf (1bar=14.5psi)
= 17643 lbf

If the machine has more than one cylinder, multiply the result by the quantity of cylinders.
Now this helps us to find out the clamping force, but how you decide on what clamping force is required for a certain container? That depends on the blowing pressure and inside area of mold & pinch off length. The thumb rule is
Total Clamping force (kgf) = Surface Area (cm2) x Air Pressure (kg/cm2) /2.5
+ Total Pinch Off Length (cm) x 200

The 2.5 divider in first part is an assumption factor for round drums to find out the force acting in mold clamping direction. The 200 multiplier in second part is based on that 1 Ton clamping force is required for every 5 cm pinch off.

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.

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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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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NANO and Blow Moulding……

At last NANO is unveiled and booking will start soon. How the TATA’s managed to keep the price so low? Simple by innovative design and cost cutting. Plastics done a great role in cost cutting and blow moulding is one process which saved lot of money. Several metallic components are replaced by plastic thus reducing the weight of car and increasing fuel efficiency. Also blow moulding components like fuel tank, coolant tank, windshield water tank, air ducts etc reduces lot of manufacturing operations otherwise required on metallic components.
See several blow moulded auto components pictures below:

Types of Blow Moulding

There are two types of blow moulding – Extrusion & Injection blow moulding. In Extrusion Blow Moulding, plastic resin is heated up to melt and extruded into a tubular shape called Parison. This Parison is then trapped inside two halves of the required mould and blown by air to the shape of mould. In Injection Blow moulding, first the material is injected to form a preform and then it is blown to the shape. Extrusion Blow moulding is used to produce all types of drums, cans, bottles etc. and injection blow moulding is used to produce bottles.

Continuous Type Blow Moulding:
In continuous blow moulding the Parison – tubular molten raw material – is extruded continuously and the mould moves sidewards after receiving the parison. The blowing will be done in the blowing station and the blown containers ejected. The mould will again move towards the parison and next cycle starts.
Intermittent Type Blow Moulding:
In intermittent blow moulding the molten material is stored in a chamber called accumulator and Parison is ejected intermittently during the start of each cycle. Hence these machines are known as accumulator type blow moulding machines. In Accumulator machines Parison is ejected intermittently at the starting of each cycle. During the blowing time the molten material processed by extruder is stored in the accumulator chamber on the die head.
Injection Blow Moulding:
In normal injection Blow moulding process, the Preform made by injection moulding is directly blown to the required shape of mould. The injection process allows generating better neck formation to the container.
Injection Stretch Blow Moulding:
In normal injection stretch Blow moulding process, the Preform made by injection moulding is first stretched along the axis to orient the structure and then blown to the required shape of mould. This allows creating transparent containers with very low wall thickness and better strength.
Single Stage Injection Blow Moulding:
In single stage moulding, the preform is made and immediately blown to shape in the same machine. The normal injection blow moulding is done by this process. The stretch blow moulding also done in single stage process.
Two Stage Injection Blow Moulding:
In two stage process, first the preform made like any injection mould process. The ready preform then re-heated, stretched and blown to the required shape. Here the preform making and blow moulding are done in two different machines. This process is used normally for stretch blow moulding.

Start Up.......

This is the first time, I am writing this blog about Blow Moulding - a plastic processing technique for manufacturing different types of containers like bottles, jars, cans, carboys, drums etc. I hope to write regularly from now on this subject and my experience on it.
I am working on blow molding machine design and development from 1990 onwards based in Mumabi, India.