3 litre double station double head auto deflashing machine

50 ml Blow moulding machine

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The machine automation can be done by a PLC control or by a wired relay logic (this become outdated because of the limitations and bulkiness of control panel).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.

Trouble Shooting of Blow Moulding Process

Pitted surface appearance

Possible Reasons:

• Air entrapment inside the mould
• Condensation
• Low Blow Rate
• Low Blow Pressure

Possible Solutions:

• Vent the mould to reduce the air entrapment.
• Check mould finishing, polished mould causes air entrapment. Very coarse mould surface results in rough part surface. Sand blasted mould surfaces are usually desirable.
• Check mould for condensation coolant temperature
• Increase the size of air orifice to allow faster blow rate
• Increase blow pressure.

Vertical groves and lines in the part

Possible Reasons:

• Dirty and / or damaged die

Possible Solutions:

• Clean die surface.
• Scratches in die or core, to be removed
• Check for contamination.
• Set the parison straight to eliminate hold up in die.

Irregular markings on the part

Possible Reasons:

• Water condensate or water leakage in the mould

Possible Solutions:

• Run faster cycle.
• Run warmer mould.
• Stop leakage in the mould.

Rough part surfaces and / or poor gloss.

Possible Reasons:

• Rough parison

Possible Solutions:

• Increase die temperature
• Increase mould temperature
• Increase melt temperature
• Increase extruder speed
• Use low molecular weight resin
• Clean die core

Rippling on parison / melt fracture

Possible Reasons:

• Unsuitable die shape
• Extrusion rate too high

Possible Solutions:

• Increase die land
• Increase die diameter
• Reduce extruder speed

Thin areas or holes on the part surface

Possible Reasons:

• Airflow in head is faster
• Parison is not straight
• Moisture in resin
• Contaminated resin in head or die

Possible Solutions:

• Reduce airflow through head
• Set parison
• Pre heat the resin
• Clean the head and die core

Poor wall thickness distribution – longitudinal

Possible Reasons:

• Parison sagging
• Parison control not working

Possible Solutions:

• Change extrusion rate
• Decrease melt temperature.
• Use low M.F.I. resin.
• Check parison control timings.
• Check parison control pressure.
• Check flow control valve setting.

Poor wall thickness distribution- circumferential

Possible Reasons:

• Non symmetrical parison
• Non symmetrical article

Possible Solutions:

• Set parison straight
• Use larger die
• Optimise article weight
• Use larger die

Excessive dimensional shrinkage of article

Possible Reasons:

• Low blow pressure
• Insufficient cooling
• Melt temperature is too high

Possible Solutions:

• Increase blow pressure
• Decrease mould temperature
• Increase cooling time
• Decrease melt temperature.

Warpage

Possible Reasons:

• Insufficient cooling
• Non uniform cooling

Possible Solutions:

• Reduce mould temperature
• Check for efficiency of water flow in mould channels
• Increase cooling time.
• Check efficiency of mould venting.
• Provide more cooling, especially in pinch of area of the mould.

Parison doughnut formation

Possible Reasons:

• Core temperature is low
• Core gone inside the die

Possible Solutions:

• Allow core to reach temperature equilibrium with rest of the system
• Lower the core slightly (to get the weight of the article, reduce the core diameter)

Thinning or stretching at parting line

Possible Reasons:

• Low blow pressure
• Pinch off is too sharp

Possible Solutions:

• Increase blow pressure
• Check efficiency of airline
• Widen pinch of land
• Reduce head and die temperature

Parison tail sticking to the article

Possible Reasons:

• Parison too long

Possible Solutions:

• Shorten Parison length.
• Increase pinch-of land to compression cool the tail

Black specks in parison

Possible Reasons:

• Degraded resin
• Hold ups in the die head

Possible Solutions:

• Thoroughly clean and purge extruder. Avoid prolonged shutdowns with resin held at high temperature
• Improve streamlining of the melt inside

Contamination

Possible Reasons:

• Poor house-keeping

Possible Solutions:

• Keep production area clean

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.