FarragTech GmbH
 



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Internal Air Cooling Background

Blow moulding machines melt plastic resins in an extruder and push the melted plastic through the head {1} which forms the melt into preform {3}. The preform is then cut in a suitable length and transferred to a cavity {2} inside a mould where compressed air is introduced inside the preform through the blow pin {4} or blow needles. The pressure builds up inside the preform stretching it to the shape of the cavity.

The ambient air between the preform and the mould escapes through vents {5} designed in the mould. Chilled water runs continuously through cooling channels {6} around the cavity in the mould {7} cooling the mould down to a low temperature. The major difference between the temperature of the hot preform and the cold surface of the cavity allows for strong heat withdraw from the shaped plastic melt. The shaped product {8} solidifies due to the cooling and maintains the shape of the cavity. The mould is then opened {E} and the products are transferred to a trimming station where excessive parts are trimmed off.

The internal surface of the blow moulded (hollow) part remains at a much higher temperature during the mould cooling process.

The major difference between the outside and inside temperature causes material stress.

The wall thickness distribution is never equal across a blow moulded part. The mould cooling is not equal on the mould surface either. Heat transfer from heavy parts of a blow moulded product (such as the neck and the bottom corners of the bottle in the following illustration) through a limited mould surface is not equal to that of thin walled parts through large surfaces. This in fact causes more material stress and distortion in blow moulded products.

 

Material stress leads to a inferior quality and the product may fail leak, load or drop tests. Blow moulders are often forced to increase the wall thickness by up to 10% to pass the tests. Increasing the weight is combined with higher material cost and longer cycle time. The cooling time, which is the longest part of the total cycle time in the blow moulding process, is often extended to get the heat from the part all the way through the wall to the mould, but a difference in the temperature is always expected. Extending the cooling time slows the production and shrinks profits.

Lowering the chilled water temperature in the mould leads to a limited improvement. It is suggested to use pure chilled water at a temperature 6 to 8 ºC. The chilled water flow rates are to be as high as possible to create turbulent water flow in the mold cooling channels. This will help extracting more heat.

Adding antifreeze to the chilled water to apply lower temperatures has its disadvantages. Antifreeze agents normally have low thermal conductivity which lowers the heat withdrawal from the product in the mould and the majority of them have high viscosity which lowers the water pump performance and reduces the water flow rates.

Lowering the temperature under the ambient air dew point allows condensation to take place on the mould surfaces adding one more problem to the process (See Mold Area Protection).

 

Post cooling with internal air exchange adds one step in the process and requires more space and equipment. Some of the stress could have taken place during the mould cooling process or in the transition between the mould and the post cooling station.

A system injecting liquid Nitrogen or liquid Carbon Dioxide in a form of mist inside the product has proven to be very costly to apply and not ideal for internal cooling. It is difficult to guide the mist to the desired areas in the product and the accuracy of the injected amount of liquid is varies from cycle to cycle. The system is also hazardous and complicated. The dependence on liquid supply and the increasing liquid prices are also factors to be considered.

Rainer Farrag spent many years in Europe developing the ideal internal air cooling system. 

The most profitable blow moulding process is the one applying internal air cooling system with acceptable air flow, acceptable temperature (not higher than 5 ºC but not lower than to -40ºC) and good, turbulent air exchange. A compressed air chiller with integrated refrigeration circuits is the heart of the system.

Exchanging chilled air inside the product during the cooling time to extract heat from the internal surfaces dramatically reduces the material stress and the cooling time.

The proper air distribution inside the product is very important to achieve the desired improvement.

Blow pins and blow needles must be designed for individual products to guide the air to areas with thicker walls and areas which are not very well cooled by the mould. Creating turbulent air flow inside the product is also very important.

Blow valves must be designed to form the product with the highest air pressure available for the process and then drop the air pressure while chilled air is being exchanged inside the product. Sufficient pressure must be kept inside the product during the entire cooling time to keep contact between the product and the mould.

The following illustration shows the influence of the blow pin design and the blow valve blocks on air distribution inside blow moulded products.

Example (A) shows little or no air exchange. (B) and (C) show uneven distribution. The illustration (D) shows perfect air exchange and perfect air distribution.

The air exchange is started after the initial blow with stagnant air. The initial blow time is set to be just long enough to form the part and vent the mould cavities. 

The valves {B} and {C} are opened and the valves {A} and {D} are closed in the initial blow process. Compressed air {P} is flowing to the blow pins through the machine’s manifold {M} and the air distributor {L}. A simple blow pin in a coaxial configuration is illustrated in a large size showing the air flowing in both channels to the product. The pressure indicated on the gauge {G} shows maximum air pressure. The product is formed and the cavity is completely vented out.

The air exchange follows the initial blow. The valves {B} and {D} are opened while {A} and {C} are closed. The air now flows to the outer channel of the blow pin through the distributor {L}, washes the bottle removing heat from the inner surfaces and flows back through the inner channel of the blow pin through the machines manifold {M} to the throttle valve {E} and then finally vented out. The gauge shows lower backpressure.

The illustrated throttle valve is manually set to govern the back pressure inside the product and the air flow rates during the cooling process.

 

 

 

The product is finally vented before opening the mould to remove the product. The valves {A} and {D} are opened and the valves {B} and {C} are closed. Now the total amount of air inside the product is vented out through both channels in the blow pin and both the machine manifold {M} and the distributor {L}. The gauge shows no backpressure and the mold is then opened to transfer the product to the trimming station. 

The simplest internal air cooling process with a simple coaxial blow pin and a simple blow block configuration is illustrated on previous page. 

The blow pin may contain multiple channels in other designs and the blow valve block may different in other applications.

Needle blow requires at least two needles in the product placed as far as possible from each other in the product. Both needles will be used to supply compressed air to the preform for the initial blow. During the cooling time both needles will be alternating; one needle blows air inside the product and the other needle vents hot air out of the product and the other way around. All needles will be venting the air out of the product before the mould is opened. It is obvious that a different blow valve block will be used when blowing through needles.

 FarragTech has also developed a single coaxial needle design. This design is operated as simple as a blow pin.

Increasing the air flow improves the results but the relation between air flow and cooling time is not linear. Exchanging the air inside a product 5 times might lead to a production increase of 10% but a 15% production increase might be the result when the air is exchanged 10 times during the cooling time.

Limiting factors such as limited size of the blow pin or the blow needles might not allow for a high rate of air exchanges. Compressed air cost must be taken in consideration as well. 

 

It is a fact that better cooling results are achieved with lower chilled air temperatures. However, the relation between air temperature and cooling time is not linear either. Lowering the temperature from 25 ºC to 5 ºC might lead to a production increase of 10% but a production increase of 15% might be the result when the air temperature is further lowered to -10 ºC. Air temperatures below -40 ºC are proven to be unnecessary.

Air chillers with integrated refrigeration circuits are safe and simple. Compressed air line components for up to 16 bar pressure and a temperature as low as -40 ºC including insulating material, flexible hoses and solenoid valves are standard components. 

 

 

A blow signal is required from the blow moulding machine to start the blowing process with the blow valves supplied with every internal air cooling system. This signal is always available in every blow moulding machine as it is necessary to operate the standard blow valves for the conventional blow process. An additional control signal is required from the blow molding machine to switch from initial blow (with maximum back pressure) to air exchange with low back pressure when the internal air cooling system is applied. The controls of many older blow moulding machines are not capable of delivering this signal to the internal air cooling system. In such cases an external control box is required to create the required signals.

Most blow moulding machine control systems are not designed to control blow valve blocks for needle blow applications with alternating blow. An external control box is also required in such cases.

 

The control box is the brain of the internal air cooling system. It is available in different configurations and is capable of controlling the blow process in two blow stations in addition to other functions.

Two inputs (20-280V, AC/DC) for blow signals from the blow moulding machine and 6 outputs  (24V DC) to control two blow valve blocks are available on  the box.

 

The internal air cooling system includes one of two air chillers. The Blow Moulding Booster (BMB) with air temperature at 1-5 ºC and the Blow Air Chiller (BAC) with a process air temperature as low as -35 ºC are available with complete sets of suitable blow valve blocks and individually designed blow pins or blow needles.

 

 

The Blow Moulding Booster (BMB) is a maintenance free compressed air chiller designed by Rainer Farrag for blow moulding applications. The air outlet temperature is designed to be above 0 ºC to avoid freezing the moisture of the compressed air inside the heat exchanger (evaporator) of the unit. 

When properly sized for the blow moulding application the chiller is capable of maintaining a compressed air supply temperature lower than 5 ºC for the blow moulding process.

 

The water cooled unit with constant temperature control is a very compact and is normally installed on top of the blow moulding machine, thus saving floor space. It is designed to handle any quality of compressed air. The air pressure is recommended between 6 and 12bar. Filtered cooling water at a maximum temperature of 20 ºC from the plant is required to cool the refrigeration circuit.

Production increase between 15% and 35% can be expected with BMB and the proper components of the internal air cooling system.

The sophisticated Blow Air Chiller (BAC) was designed by Rainer Farrag to chill the compressed air for internal air cooling systems to a temperature as low as -35 ºC. The compressed air is dried to a dew point lower than -40 ºC before it is chilled in the heat exchanger (evaporator) of the integrated chilling unit. The BAC units require a good quality of compressed air supply at a pressure dew point not higher than 8 ºC and oil content lower than 0.01 g/m³. This is standard air quality and it is available in most moulding plants.

 

BAC units are water cooled compressed air chillers and they require small amounts of filtered cooling water at a temperature not higher than 15 ºC. There is virtually no maintenance required when good air and water quality (industry standard) is provided for the unit. Air filters are supplied with every unit for additional safety only.

 

BAC is equipped with Farrag Intelligent Terminal (FIT), a microprocessor with graphic display for accurate control and data display.

The compact BAC units are normally floor mounted units but they can be installed on the extruder platforms of larger industrial blow moulding machines.

A production increase of 25% to 50% with the application of BAC as a part of an internal air cooling system can be expected when compared with a conventional stagnant air process. Some cases in industrial blow moulding have shown a production increase higher than 100%.

 

Foam insulation on all the chilled air lines is very important to keep the blowing air at a low temperature all the way from the unit to the blow tools.

Experiments in blow moulding with internal air cooling systems have proven that the temperature distribution across the wall of a container is more even and the temperature all over the product is lower when the internal air cooling system is efficiently applied. The product quality improves and the production line output increases. 

 

 

 

 

 

 

 

The following illustration shows the difference in a practical example.

A small bottle is produced in a shuttle blow moulding machine. The chilled water temperature used to cool the mould was measured at 10 ºC. The cycle time in a conventional stagnant air blow process was 11 seconds with a cooling time of 8 seconds (left side of the illustration).

A specific point was chosen to measure the difference and the temperature profile was measured across the wall of the product at this point. A dramatic difference in the temperature between the inner and the outer surfaces was detected.

The blow system was then changed into internal air cooling system with chilled air supplied at a temperature of 3 ºC. The chilled water temperature remained unchanged with the same cycle time of 11 seconds and a cooling time of 8 seconds (middle of the illustration).

The overall temperature was much lower than that measured in the conventional blow process and both the inner and the outer surfaces showed a much lower temperature with a peak in the centre of the wall.

The third test (right side of the illustration) was performed with the same chilled water temperature in the mould and the same compressed air temperature but the cooling time was cut down to 5.2 seconds. The total cycle time dropped down accordingly to 8.2 seconds. The overall temperature level was just a little under the values measured in the conventional process and the temperature of both the inner and the outer surfaces was identical and just a little higher than the second test but still lower than the conventional blow in the first test. The temperature distribution across the wall showed a peak in the middle of the wall.

 

The production in the first test with stagnant air was at a rate of 327 bottles per hour but in the last test with internal air cooling the production rate increased by 33% to 440 bottles per hour. Samples were collected in all tests and the bottle dimensions were set in a comparison which clearly showed that the best dimension stability was achieved during the second test followed by the samples of the third test. The samples collected during the first test with stagnant air were far behind. Load tests and drop tests were performed and the results were similar to those of the dimension tests.

    

The product shown (7.4 litres / 590 grams) is a part of a car assembly. Due to its location in the car the part (oil cooler) has a complex form. As a consequence the Polypropylene product has different stretch areas, varying wall thicknesses (1.35 to 3.80 mm) and a wide range of temperature distribution, making the part rather difficult to blow.

Because of safety reasons in the automotive industry this oil cooler has to be strong and stress free, which was very difficult to achieve with a conventional stagnant blow process. A comparison between the customary process and the internal air cooling process was made to justify the investment and to get an approval from the car manufacturer for applying the internal air cooling system. 

Again, samples were collected from the normal production with stagnant air blow and after the change over to the internal air cooling system.

 

DATA FOR THE COMPARISON:

 

Standard
Blow Process

Internal Air Cooling

Preform temperature [°C]

195

199

Chilled water inlet temperature [°C]

11,9

11,4

Chilled water outlet temperature [°C]

12,7

12,5

Compressed air inlet temperature [°C]

20

-29

Exhaust air temperature [°C]

 

56

Compressed air pressure [bar]

8,5

7,5

Average air consumption [l/s]

 

18

Air consumption [l/h]

 

38000

Cooling time [s]

44

24

Cycle time [s]

61

41

Hourly production rate

59

87,8

Production increase [%]

 

48,8

THE FINANCIAL CALCULATION:

 

Standard
Blow Process

Internal Air Cooling

Hourly machine rate

117,8

117,8

Operator cost per hour

51,8

51,8

Daily operation hours

24

24

Product weight [g]

590

540

Resin cost per kg

1,85

1,85

Resin cost per product

1,09

1,00

Additional energy for BAC [kW]

 

7

Additional energy cost for BAC per h

 

0,18

Compressed air consumption [Nm³/h]

 

64,91

Compressed air cost per h

 

1,95

Production cost per / h

234,02

260,75

Daily production cost

5617,00

6258,00

Daily number of products

1416

2107

Manufacturing cost per product

3,97

2,97

Daily profit related to Internal Cooling

 

2107,00

Investment in the Internal Air Cooling System

 

70000,00

Amortization in days

 

33,22

AMORTISATION – ONLY 33 DAYS TO PAY BACK!