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FABRICATION OF PLASTICBAGS
Catalina Amengual i Gar
Cristina Borrs Marqus
Bernat Darn Sellabona
Claudia Villar Egea
BACKGROUND
1965
Three employees of a Swedish company design and patent for "tubing for packaging purposes:
Plastic tube sealed at one side and allow the top side for the packaging of goods.
Also create handles for convenient carrying:
T-shirt plastic bag
T-SHIRT PLASTIC BAG
1975
The first plastic bags were introduced at retail stores
Plastic bags were better solution than paper bags:
Oil cheaper than wood
Less energy and water during production
Take up less space
More durable
Can hold more weight
Cost about 1/3 of a paper bags
NOWADAYS
About a trillion plastic bags are used every year.
Depends on the country environmental-friendly laws that regulate its use.
PRODUCT
PLASTIC BAGS
Day-to-day object
Used to transport and/or preserve small amount of goods
Most common form of packaging
Made of thin, plastic film
Lightweight and flexible
Thickness between 18 to 30 micrometers.
PLASTIC BAGS
Types according to their function:
Transporting goods T-shirt BagEconomicVery little amounts of materialUsually made of high density polyethylene which can carry up to 12 kilos
Keep foods protected against moisture Highly hygroscopic bagsHumidity absorbentsMade with polypropylene laminations
Medical purposesBeing manufactured under sterile conditionsNon-porous (isolating infectious body fluids)Less expensive than other re-usable options (such as glass)
PLASTIC BAGS, EXAMPLES
T-shirt Plastic Bag Retail Handle Bag Clear Colored Gift Bag
Market Bags With Block
Garbage Bag
Bag Roll
PLASTIC BAGS, EXAMPLES
Zipper Pouch BagStatic Shielding Bag
Cone BagMicro-Perforated Bread Bag Gusset Bags
Flap Seal Bag
THE MATERIAL
PROPERTIES
Flexible
Stretchable
Stiff
Low MFI High viscosity
Long and ramified molecular chains (prevents melt fracture, better mechanical properties)
COMMON POLYMERS
High Density Polyethylene (HDPE):
Easy to process, low cost
Good low temperature toughness
Great thermal resistance
Flexible
Weatherproof
Recyclable but not biodegradable.
COMMON POLYMERS
Low Density Polyethylene (LDPE) and Linear Low Density Polyethylene (LLDPE):
Very tough
Not good low temperature toughness
Good thermal resistance
Semi-rigid
Not biodegradable.
COMMON POLYMERS
Polypropylene (PP):
Difficult to process
Tough
Good dimensional stability at high temperatures
Rigid
Not biodegradable.
MANUFACTURING PROCESS
PLASTIC BAGS
The manufacturing process can vaty, but in general it includes severalmain steps:
Mixing stage
Blown film extrusion
Printing
Cutting and sealing
MIXING STAGE
The plastic polymer is transformed (adding additives) to improve their basic
mechanical, physical and chemical properties to obtain the requirement
commercial product.
Additives are combined to create the desire properties:
Blends of clarity
Strength
Stretchability
Sealability scruff and tear resistance
UV protection
Bacterial protection
Surface appearance
BLOWN-FILM EXTRUSION PROCESS
Blown film extrusion is the most important manufacturing process to
make plastic bags.
In this stage, tubular film is extruded vertically upwards. Air is
introduced inside of the tube, and as a result, the tube expands to a
bubble with a diameter large than the diameter of the die.
When the bubble is cooled sufficiently, it is flatten and pulled
through a set of nip rolls.
The layflat is guided over several idler pulleys (rodillos tensores) to
the winder (bobinadora) where it is rolled up over core.
BLOWN-FILM EXTRUSION PROCESS
Schematic of blown film process line.
https://www.youtube.com/watch?v=8CfL5xl2N1Q
BLOWN-FILM EXTRUSION PROCESS
The blown film extrusion is the most important manufacturing process to
make plastic bags, and hence it is important to analyze basic aspects
of the process, to know the effect that they will have on the final
product.
Important parameters:
Cooling rate
Bubble stability
Film speed (screw rotation, calendaring rolls velocity)
Temperature of the melted plastic (high viscosity polymers)
Air pressure (bubble expansion)
IMPORTANT PARAMETERS
Frost line height is the point where the film changes from the
melt to the solid state with semi crystalline polymers. Film
changes from a transparent amorphous state to a hazy or
translucent crystalline state.
Dd and DB are the die and the bubble diameters.
To and Tf are the initial and final film thickness. The film
thickness is dependent on the bubble size and the draw ratio.
Vo and Vf are the die exit and final film velocity. Higher draw
ratios result in relatively higher Vf values.
MD and TD are the machine and transversal direction
orientation. (TD is also called the cross machine direction and
is designated as CD). This is an important parameter to
obtain the required mechanical properties of the bag,
especially for parameters that are closer to improve the
capacity of the bag to be loaded. The property ratios
depend on the bubble size and draw ratio. Larger bubble size
provides more transverse direction orientation and higher
draw ratios provide more machine direction orientation.
IMPORTANT PARAMETERS
Blown-up ratio (BUR) is defined as the bubble diameter divided by
the die diameter. It measures how large the bubble has expanded in
the transverse or cross-machine direction. The larger the blown up
ratio, the higher the transversal direction molecular orientation with
corresponding decreases in the draw down ratio.
The drawn down ratio (DDR) is the drawing taking place in the film
tower in the machine direction, and it is related to the blow-up. It is
also defined as the stretching ratio, as the relation between the linear
velocity of the stretching rolls (Vf ) and the velocity of the polymer at
the exit of the extruder die (V0)
ADVANTAGES OF THIS PROCESS
Advantages:
-Better property balance between the machine and transvers direction.
-Less scrap
-Equipment cost
-Capacity to make different widths with only one single die by
modifying the blown-up ratios.
EXTRUDING PLASTIC FILM
Printing stage
The rolls with the extruded material are introduced at the beginning of
the rotary flexographic machines, then the polymer film is forced to pass
through different rollers and cartridge holder until the film reached the
final line, when material is dried ink. Gravure-printing can also be used to
do this process stage, but is less common than the flexographic process.
Cutting and sealing
The next step in the manufacturing plastic bags is the cutting stage. The
first thing to be done is program the cutting machine with the required
parameters according to the type of bag which is produced. Product
width, high, geometry handles (is it is necessary) and pleats are fitted, in
order to obtain the final geometry.
Holes, vents, slits, perforations, handle punching are some of the different
options available in each production run.
Scheme of flexographic Printing process
CUTTING PROCESS
ENVIRONMENTAL IMPACT
LIFE CYCLE
CONTEXTUALIZATION
On average, plastic bags are used only for 25 minutes.
It takes between 100-500 years for a plastic bag to disintegrate (depending on the type of plastic)
1 million plastic bags are in use around the world every minute.
The average European uses about 500 plastic bags per year.
80% of marine litter is plastic
34 million tones of plastic carrier bags are produced in the UE each year.
THE THREE RS: REDUCE
It is a form of waste prevention: minimize the amount and toxicity of waste before it is generated.
Initiatives to reduce the consumption and use of plastic bags:
3rd of July is the Plastic Bag Free Day
Plastic bag free towns: the Spanish Government, the past March, approved a law that pretends gradually reduce the consumption of single use bags to suppress definitively in 2018.
THE THREE RS: REUSE
To reduce the consumption of plastic bags we can reuse the plastic bags that we have at home. This is a way to reduce the environmental impact.
Newer reusable plastic bags have been developed that both for minimize the amount of plastic being manufactured and maximize its use.
THE THREE RS: RECYCLE
Plastic bags are generally made of HDPE (high density polyethylene) and LDPE (low density polyethylene) and both are recyclable.
1. Workers hand-remove contaminants not meant to be recycled.
2. Bags are washed in flood tanks, removing all forms of non-metallic contaminants, and go through magnets, removing all the metallic impurities.
3. After wash the bags this are loaded into the recycling machine: https://www.youtube.com/watch?v=Q2IQWV8Q5JU
A RECYCLED MATERIAL: BIORESIN
The alternative to polyurethane-based plastic, bioresins are the result of the recycling process, and depending on the material of the original bag we can find:
Degradable resins: can continually be broken down into smaller and smaller pieces.
Compostable resins: can be used as mulch after being mixed with specific bio products at a composting plant.
There are a lot of companies that work producing bioresins, like the enterprise Bioresins.eu
DEGRADABLE PLASTIC BAGS
Plastic bags designed with changes in its chemical structure under specific environmental conditions. It can be broken down by chemical or biological processes.
Biodegradable polymers.
Compostable polymers.
Oxi-biodegradable polymers.
Photodegradable polymers.
Water-soluble polymers.
GREEN TRASH BAGS
Made from potato starch, a fully compostable polymer; without polyethylene and plasticizers.
Starch is a mixture of a linear (amylose) and a branched (amylopectin) polymer of glucose.
In a landfill environment, this organic material (leaves, grass etc.) releases methane gas a greenhouse gas contributing to global warming.
The procedure of manufacturing trash bags from potato starch is basically the same, but there are some differences: the complete process it is a little longer, because it is done with lower screw speed rotations, the blow up ratio is smaller and the film thickness is bigger.
THANKS.
CATALINA AMENGUAL I GARCRISTINA BORRS MARQUS
BERNAT DARN SELLABONACLAUDIA VILLAR EGEA