What is Double/Triple Bubble process, how it works, what kind

of machine is required

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ABOUT US EXTRU-TECH SOLUTIONS INC. is a consulting and service

providing company with head office in Toronto, Canada.

We provide consultancy services to the plastic packaging industry, research companies, as well as Extrusion machine manufacturers around the world.

We provide project management, extrusion troubleshooting, process improvement, Die (both Blown and Cast-any number of layers) and extruder maintenance services.

We also troubleshoot any downstream process problems specially the film quality (wrinkles, gels, instability etc.) and winding problems.

We also help in newer developments weather it is related to new equipment, new process or new film recipes (new products) etc.

Disclaimer:Most of the information shared in this presentation is gained through years of hands ON working experience on various Double and Triple-Bubble processes.The sole purpose of this presentation is to share the knowledge among interested professionals.

Single Bubble Process

Most of us are familiar with the Blown film process where we melt the plastic granules, extrude the homogenous melt through an annular die, blow the air through the die to make a bubble, cool it from outside using an air ring and also from inside using the internal bubble cooling (IBC) technology and the bubble is collapsed, treated (sometimes) and winded either in sheet or tubular form. This is a straight forward single bubble process called Blown film process.

Double-Bubble Process

As the name indicates, there supposed to be two bubbles… but how?

In this process the first bubble is formed in a similar manner as in the blown film using an annular die but the downstream cooling and forming process is different for different type of double-bubble process depending upon the polymer/recipe as well as the size requirements.

After pre-heating the primary Bubble/tube is re-blown to make a secondary bubble and this process is called as double-bubble.

The secondary bubble is collapsed and the film is winded either as tube or sheet depending upon the application.

Triple-bubble process Triple-bubble process is basically the

continuation of the double-bubble process. After collapsing the secondary bubble, this flat tube is re-blown third time between a set of circular IR heaters or hot air for annealing/thermoregulation and this process is called the Triple-bubble process.

Annealing/Thermoregulation is a very critical step in any shrink film process because with annealing we can control the percentage of shrinkage required in the final product as per application.

Typical shrinkage requirements: Sausage casings – 15%,High barrier shrink bags - 40-60% ,Lidding film – 0-<5%.

Why Double and Triple bubble

The main purpose of making the film using the Double or Triple-Bubble technology is to get a good controlled shrinkage (5-90%), improved mechanical properties out of thin film (down gauging).

Process Types: Double and Triple-Bubble

1. PVC Double-Bubble process: Shrink sleeves, Labels, safety seals etc.

2. Polyolefin (POF Shrink film) Double-Bubble process: common materials are Mettallocenes, Plastomers, Polypropylene. This process was initially started to produce the BOPP film but now many other POF films are made using this method.

3. PVdC Double-Bubble process: Usually Monolayer process using PVdC (Polyvinyledene Chloride) for sausage casings and Cling film.

4. Nylon casing Triple-Bubble process: Mono and Multilayer using PA (Nylon), PE and Tie resins.

5. Barrier Shrink film Double and Triple-Bubble process: Is typically a co-extrusion process using PE, PVdC, EVA, Tie resins, EVOH, PA, and PET etc.

1. PVC casing (Shrink Sleeve)

This is one of the oldest product made using the Double-Bubble technology.

Also this is the only Double-Bubble process where the first bubble is formed using the traditional blown film technique i.e. the bubble is formed using an air ring.

Typical Material: PVC

Applications: Shrink sleeves for bottles, Cap closure as safety seal, labels, bundling film etc.

Critical Components Extruder: That can provide the homogenous good

quality melt. Die: Short residence time, no restrictions and/or

Dead spots. Air ring: Single lip is commonly used to make the

First/primary bubble. Hot water tank: Hot water is used for pre-heating

the primary bubble. Re-blowing: Re-blowing is done after pre-heating

(Pre-heat temperature around 80 deg.C i.e. the Tgfor PVC) using compressed air or air blower. The water cooled sizer is used to stabilize the second bubble.

The sizing ring is similar to the one used in PPTQ lines or in modern days so called Aqua frost / Aqua quenched lines.

Winder: The finished film is collapsed and winded on a spindle/surface winder.

Tg : Glass Transition TemperaturePPTQ: Polypropylene Tubular Quenched

Applications

2. Polyolefin (POF Shrink film) Double-Bubble Process

In this process the size of the primary tube is dictated by the size of the water cooled mandrel being used. The primary bubble can be formed using a internal water cooled mandrel or an external mandrel similar to the one used in PPTQ/Aqua frost/Aqua quench process. With internal cooling mandrel, outside cooling is done using a water ring.

The diameter of the secondary bubble is dependent on the diameter of the primary tube and film recipe with minor variations due to film thickness. Usually these lines are dedicated to a fixed lay flat. Typical B/R is 5:1.

Primary tube thickness range: 350-750 micron, Final film thickness range: 12-30 micron (Approx.)

The size of the water cooled mandrel is relative to the die size. (Approx. B/R :1/1).

Typical Structures: 3 and 5-layers PP/PE/PP PP/PP/PE/PP/PP PP/PE/PE/PE/PP

Critical components Water quenching:

1. The primary bubble is formed either like PPTQ process means the melt is slightly cool down using an air ring and then passed through a water quenched mandrel.

2. The second method is by using a combination of water cooled mandrel and a water ring for outside cooling. The bubble is supported by a continuous supply of compressed air. Sometimes this compressed air can also be used to maintain the size of secondary bubble.

Scrap winder: If for any reason the second bubble is lost, this scrap winder comes in handy and takes care of the primary tube.

Primary nip: Usually one of the rubber roll has two ‘V’ grooves at the location of both the edges of primary tube to prevent the edge deformation (cracks) in the thick primary tube.

Critical Components Pre-heating IR chamber: This IR heater chamber is just slightly

larger than the primary bubble diameter for better heat transfer. The length is usually in the range of 1-1.5 mtrs.

Film guides: The bubble guide rings play a crucial role in keeping the tube in center of the IR chamber for uniform heat transfer and also to prevent the tube from coming in contact with the IR heaters that can lead to fire.

Second IR Heater tunnel: This heater tunnel provide the desired temperature to re-blow the second bubble. The tunnel is slightly bigger than the possible diameter of the secondary bubble.

Third IR Heater chamber: Sometimes this is used to control the thickness variation of the secondary bubble. The segmented heaters work in the similar fashion as the die lip heaters in a regular blown film line.

Cooling air rings: There are two or three air rings for cooling and stabilizing the second bubble. As the amorphous primary tube inflates it starts crystallising and relieves lots of heat in this process. The secondary bubble will blow up to a certain diameter after that the increase in tensile strength will not allow it to blow further. The air helps in removing this excess heat.

Annealing: Annealing is done using a combination of hot and cold rolls just before the winder.

Applications: Scratch resistant film, bundling film, Safety seal etc.

Applications

3. PVdC Double-Bubble Process

PVdC processing is very complicated and require special equipment as well as special skills.

PVdC comes in powder form and is very corrosive when melted. Usually the Screw, barrel and die are made out of Dura nickel alloy or good quality Stainless steel.

The screw design is crucial because there is a very narrow process window means we need to melt and mix the material within this temperature range, also the residence time should be kept to a minimum otherwise PVdC will degrade.

Using the special design screw and die the molten PVdC is extruded through an annular die in to a cold water tank.

After cooling, this tube passes through a hot water bath . After re-heating the water is wiped out and the secondary bubble is blown

using either a powerful air blower or compressed air. This secondary bubble (5-6 times larger diameter) is collapsed. The film can be separated as sheet or can be winded as double layered film.

Critical ComponentsOil Sizing: the primary bubble is formed by filling the oil inside through the center of the die and by trapping this oil with the help of nip rolls, this bubble is formed in a water tank. The oil level is maintained using level sensor and automatic refill arrangement.Hot water Tank: The cold flattened tube is passed through warm water (about 35 deg. C) to re-heat the tube for re-blowing.Randomization: During the re-blowing a neck is formed which is tightly hold using a set of frictional rollers assembled in a ring formation. The ring and so as the bubble is rotated in order to randomize the gauge. Collapsing frame and Winding: The secondary bubble is collapsed using a special sturdy collapsing frame with aluminum/steel rollers and can be winded in single (sheet) or double (tube).

Re-blowing Process & Applications

The re-blowing process is quite simple. The primary tube is re-heated using warm water, 35 deg.C (Tg for amorphous PVdC) at that the amorphous polymer is soft enough to be re-blown using a low pressure high volume blower. The primary bubble start re-blowing and at the same time started crystallizing. The bubble can not blow after a certain diameter because of the increase in tensile strength due to crystallization.

The bubble is collapsed and passed over a set of hot and cold rolls (annealing process) to control the shrinkage in the final film.

The MD shrinkage is achieved by controlling the stretching in MD. The TD shrinkage has many factors, like resin type, additives used, quenching

temperature, hot water temperature, distance between die and quench water, etc.

Film thickness: 10-25 micron for single sheets, 20-50 micron for double wounded.

Applications: Sausage casings, Cling film, cheese, poultry etc.

Applications

4. Nylon Casing (Triple-Bubble Process)

Nylon casings are made on dedicated lines for various sizes (Tube diameter/Calibre).

These lines can be in horizontal or vertical configuration.

Typical Structures: 3,5,7, and 9 layersPA/Tie/PAPA/Tie/PA/Tie/PAPA/Tie/PE/Tie/PAPA/EVOH/PA/Tie/PAPA/EVOH/PA/Tie/PE/Tie/PA

Note: The size (diameter) consistency is very crucial for the sausage filling process.

Critical Components Vacuum Sizer: This is one of the critical

components in the Nylon casing process. As we all know the melt strength for Nylon is very poor and in order to form a tube we need to apply the vacuum. In order for the vacuum to work we must have an opening through the center of the die.

Elevating Mechanism: The vacuum sizer must be assembled on an elevating mechanism with at least 8-12” traveling distance from the die lip.

Additional Cooling chamber: For high output lines it is important to have an additional cooling chamber to properly cool the primary bubble/tube in order to keep it in amorphous state.

Critical Components Moisture removal: It is important to wipe

out all the moisture from the surface of primary tube before it enters in to the pre-heating IR heaters otherwise this moisture will leave water marks on the surface of final product. The moisture can be removed using air knives and squeeze rolls.

Pre-heating : Preheating can be done using Hot water or IR heaters.

Cooling: An air ring is used to stabilise the secondary bubble. As the amorphous primary tube inflates it starts crystallising and during this process eliminates lots of heat that needs to be removed to stabilize the size of secondary bubble.

Size control: For automation we need to measure the secondary bubble diameter as well as third bubble diameter. On basis of the feedback from this device we can increase or reduce the length of these two bubbles that will eventually change the diameter of these bubbles.

Critical Components Annealing: Annealing is a heat stabilization process

where the polymer stresses (developed during bi-ax orientation) are relieved and also the shrinkage can be controlled for the final product . In PA casing process this can be done either using IR heaters, Hot air or Steam.

Randomisation: It is one of the crucial step and is a must for this product. Because Nylon film is very stiff it is important to have a uniform roll geometry otherwise the gauge bands will remain as memory and the film can not be used on automatic filling machines.

Winding: Winding is done on spindle winder.

Applications: Sausage casings

Applications

5. High Barrier Double/Triple Bubble process

This is typically a co-extrusion process and the film is produced as 3,5,7,9 and 11 layers with various barrier resins like PA, EVOH and PVdC along with PE, EVA and tie resins. The secondary bubble can be made using IR heaters similar to the POF process but the best shrinkage properties are achieved using a combination of hot water and hot air/IR heaters. Here we will discuss about this second method because the first one is already covered in POF process.

The PVdC based barrier film needs to be cross linked (By Irradiation) in order to improve the mechanical properties.

Common Structures:

PE/Tie/PVdC/Tie/PE

PE/Tie/EVOH/Tie/PE

PA/Tie/EVOH/Tie/PE

PE/PE/Tie/PVdC/Tie/PE/PE

PA/Tie/PA/EVOH/PA/Tie/PE

PET/Tie/PE/Tie/PA/EVOH/PA/Tie/PE

Specifications: Primary tube thickness: 300-1000 micron, Final film thickness: 40-60 Micron,

Typical B/R: 2.5-5.0, MD Stretching: 3-5 times.

Applications: Fresh meat packaging, Cheese Packaging, other high barrier packaging like coffee, Lidding film, Balloon film etc.

Critical Components- PVdCbased shrink film

Water Quenching /Vacuum Sizer:

Both of these methods are used to make the first bubble called Primary tube in the range of 0.3-1mm thickness and the lay flat from 40mm-120mm.

In the water free fall quenching method the primary tube diameter is maintained using compressed air inside the tube.

In case of vacuum sizing the primary tube diameter is dictated by the inner diameter of the sizer.

Water quenching is important to keep the polymer in amorphous state that helps in re-blowing the second bubble.

Starch Powder: There is an arrangement to spray the starch inside the primary tube that helps to prevent the blocking of tube during pre-heat process. This starch also helps in easy opening of the finished bags during packaging process.

Critical Components Hot Water: For some recipes it is possible to re-blow

the second bubble just by pre-heating the primary tube in hot water. The best benefit of using Hot water is the uniform heat for the entire circumference of the primary tube.

Hot air /IR Heater chamber: Stiff polymers need to be re-heated close to their melting temperature and require a secondary source of heating that can be either Hot air or IR heaters. Sometimes, a combination of both hot water and either hot air or IR heaters are used for pre-heating. It is important to maintain the uniform heat around the entire primary tube circumference (for gauge uniformity) and the heat sources must be carefully selected.

Cooling Air ring: This is probably the most crucial component of this entire process. After the pre-heating section the primary tube immediately enters in to the cold air ring. The design of the air ring also dictates the size of the secondary bubble in co-relation with the size of the Primary bubble/tube.

Critical components Re-blowing process: Once the desired pre-heating temperature

(depends upon the recipe) is achieved the secondary bubble can be easily blown using compressed air .

Typical orientation ratios are in the range of 2.5-5 X both in MD and TD depends upon the polymers/recipe being used.

Collapsing frame: This process is forced re-blowing and as the amorphous primary tube starts inflating it crystallises and the tensile strength increases. In order to collapse this bubble very strong collapsing frame is required (usually stainless steel rollers are used).

Annealing Section /Triple Bubble:

-One option is to use hot and cold rolls before the winder.

-Second option and probably the best is to make a third bubble that will pass through IR tunnel but is more complicated and expensive option.

Both methods do their job of thermally set the bi-ax orientation.

Annealing helps in controlling the final lay flat as well as the shrinkage percentage during application.

Irradiation: The finished rolls are transferred to the Unwind-rewind station with Irradiation equipment. This is done to cross link the skin layer of the film that improves the mechanical properties and provides the much desired increment in puncture resistance necessary for packaging meat having sharp bones.

Film properties

Excellent barrier against O2 and moisture,High or controlled shrinkage depending upon

the recipe (raw materials) being used,Excellent optical properties (high gloss and

transparency),Excellent puncture resistance (keep in mind

PVdC based films need to be cross linked),Low temperature, high strength sealability,

thanks to the requirement of low melting temperature skin resins in case of PVdC as a barrier layer.

Applications

Common Elements of Double/Triple-Bubble

There are following critical steps that are common in all the double/Triple-bubble processes discussed above:

Typical resins: PVC, PVdC, Nylon are the resins soft enough at their amorphous Tg so that they can be easily re-blown in to secondary bubble. Some other resins like EVOH, PE, Ionomers etc. required to be heated almost up to the melting temperature.

Sizing: in all the above processes we need to have some way of sizing the primary bubble either vacuum sizing, water ring, cooling mandrel, compressed air with free water fall ring, oil sizing in case of PVdC etc.

Pre-heating requirement: all the Double/Triple- Bubble formation require some kind of pre-heating for the primary bubble/tube. Some resins can be re-blown at low temperatures (below 100 deg.C that means hot water can be used), the others will need higher temperatures (close to their melting temperature) means other heat sources will be required , that can be hot air, hot oil, IR heaters etc.

Annealing: All double/Triple-Bubble processes require some form of annealing in order to relieve the stresses developed during the bi-axial orientation process. This procedure is also called as thermoregulation by which the final shrinkage levels can be achieved.

More Questions???

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Contact: Ajay BeniwalExtru-tech Solutions Inc.11 Seascape Crescent, Brampton, ON, Canada, L6P3C4www.extru-techsolutions.com