What are Plastic Bags?
A plastic bag is a single-piece unwoven polyethene container used for storing, carrying, and packing a wide range of products and commodities. Plastic bags are durable, waterproof, and keep their shape even when very thin and easy to carry. Small Plastic bags are produced from several plastic polymers, the most popular of which are polyethene and polypropylene.
Plastic bag kinds and types are impacted by the materials used to make them andtheir shape, size, colour, closure types, and style. Polyethene plastic bags are classified as high, low, medium, or linear density, whereas polypropylene bags are crystal clear, have significant tensile strength, and are heat and chemical-resistant.
Polymers used in the production of grip seal bags are derived from fractured natural gas or refined crude oil that has been compressed and superheated to separate the pure polyethene chains that are polymerised to generate resin pellets. Variations in pressure and heat produce resin pellets of varying densities, which are then used to create the many kinds and types of plastic bags.
Because the features and attributes of various plastics vary, the types of resin pellets chosen for the extrusion process are critical to creating a wide range of plastic bags, including grey mailing bags and ziplock bags. Although each variety of resealable plastic bags appears to be the same, each has its strength, thickness, and durability.
Any packaging that allows the consumer or user to reseal or reclose the packaging is considered resealable. Packaging must frequently be resealed to retain product freshness or to prevent spillage. Reusable packaging has several applications which help to reduce waste. Grip seal bags are the perfect example of this; Mr Bags has an extensive range of colours, sizes and designs. Check out the full range here.
How are Plastic Bags Made?
Plastic bags begin as pure plastic resin pellets derived from crude oil or fractured natural gas. The first stop in manufacturing plastics is the cracking plant, which converts naphtha, a simple oil product, or ethane, a natural gas liquid, into ethylene, a hydrocarbon used to make various chemical goods. Ethane is the less expensive of the two hydrocarbons utilised in the cracking process to produce ethylene.
Ethylene from the cracking process is a gas that must be transformed into polyethene resin using pressure and catalysts, a process known as polymerisation. Plastics are made from ethylene by a chemical interaction between molecules that react together to produce polymer chains, which are the core component of all plastics. Plastics engineers manipulate and configure polymer chains to generate specified physical qualities in plastic resins.
Polyethene comprises a long chain of ethylene monomers and is the most often used polymer in producing plastic bags. It is the consequence of ethylene molecules reacting with a catalyst, which breaks the double bond of carbon molecules.
Polypropylene, which was initially presented in 1951 and perfected for commercial and industrial usage in 1957, is another type of plastic produced from crude oil. Polypropylene, like polyethene, is derived from crude oil, from which the monomer propylene is separated. The propylene monomer is converted into polymer polypropylene via chain-growth polymerisation.
Propylene monomers are linked together to produce a solid plastic substance in polypropylene manufacturing. To manufacture usable clear baggies, polypropylene is mixed with plasticisers, stabilisers, and fillers, which are blended with the molten polypro and cooled to form pellets or bricks.
Plastic colouring can be done in various methods, depending on the customer's needs and how the plastic will be utilised. Although there are several ways to colour plastic, the three most common are masterbatch, cube blends, and pre-coloured resin.
Masterbatches, also known as concentrates, are the most often used method for colouring polymers. The technique involves mixing concentrated pigments with polymer resin and heating them to guarantee optimal blending.
Blends of Cubes
Cube blends, often known as salt and pepper mixes, involve combining a masterbatch blend with polymers and allow users to precisely regulate the final output. The polymer preserves its original characteristics since it remains solid throughout the procedure.
The pigments are polymerised into the plastic resin during precoloring. Pre-coloured resins are simple to use and match the specified performance rate rapidly.
The extrusion technique is used to manufacture the plastic film that will be utilised to make self-seal plastic grip-seal bags. It starts by superheating plastic resin pellets to over 500o F to generate a uniform molten liquid. The heated resin must be flexible enough to pass through the circular die at the extruder's end.
Heating the resin to the proper melting temperature is essential for shaping and constructing plastic bags. The resin pellets are first inserted into the extruder through its hopper, placed at the beginning of the barrel above the auger. The initial hoppers were essential cone-shaped holes filled by hand with resin pellets.
Modern hoppers have evolved and come in various designs with technologically updated feeding mechanisms that eliminate the need for human hopper loading.
The extruder is the heating device used in the production of plastic bags. It is made up of a long tube called the barrel, which houses a helical screw called the drill. The auger steadily drives the resin pellets toward the die as they fall from the hopper into the barrel. The heating units on either side of the barrel progressively melt the resin pellets to generate the molten liquid passed through the die.
A round circular die at the barrel's end forces the molten plastic before it enters the cooling tower. A screening or breaker plate is placed before the die to remove any potential contaminants and ensure the cleanliness of the molten resin.
Pressure is applied to the molten plastic resin as it passes along the extruder barrel, forcing it past the screen or breaker plate and die. The breaker plate removes impurities and protects the die from the extremely high pressure caused by the extrusion process.
The die can be fed vertically or horizontally into the long cooling cylinder. The cooling cylinder is where the thickness and shape of the bags are decided. It is the part of the procedure that guarantees the bags are seamless and free of holes. The thickness of the bags is determined by the rate at which the molten material enters the cooling tower.
The Cooling Tower
The molten plastic resin is driven past the die and brake plate into the bottom of the cooling tower cylinder as it exits the extruder. The bag is filled with compressed air to transport the molten material into the cooling tower, which shapes and thickens the bag. Inflating the molten plastic resin is remarkably comparable to inflating a massive balloon.
The plastic tube slowly fills with air and moves into the cooling tower. The plastic substance cools and takes on the proportions of the finished plastic bags during the process. The pace of movement determines the thickness of the plastic film. Plastic bags with thin walls that expand quickly will be incredibly light. The final bags will have thicker walls and be more durable if the molten plastic grows slowly and gently.
As it climbs toward the nip rollers that tightly press the sides of the plastic bag together, the tube of newly created plastic film cools. The plastic movie extends axially and radially, forming a line many times the size of the die ring. The nip rollers remove any air bubbles, blisters, or other flaws.
The tube's diameter is determined by the type of plastic being processed. The height of the cooling tower, which can range from 25 to 35 feet, is another factor in determining the cooling process's speed.
Gussets are folds, creases, indentations, or expansions in the side of a plastic bag that increase its capacity while allowing the bag to lay flat when stored. Before the bag material flattens against the cooling tower walls, boards in the walls indent the fully inflated plastic as it advances near the nip rollers.
The boards generate a crease in the shape of two overlapping rectangles, which are forced together by the nip rollers when the bag material exits the cooling tower. This gusset is a side gusset and is relatively frequent in plastic bags, notably tougher and stronger ones.
Nip rollers are situated at the top of the cooling tower and provide traction, tension, and winding for depositing the completed bag material onto rollers. As the inflated plastic film cools, it crystallises at the frost line, cool enough to be flattened and changed into lie flat or collapsed tubing. The nip rollers offer the force to draw the cooled plastic film upward such that it can be shaped into rolls for further processing.
Nip rollers aim to give pressure to make a solid and secure plastic bag winding. They manage the hardness of the roll by eliminating air from the web of plastic from the cooling tower. In the design of nip rollers, the difficulty is to make the winding sufficient for the nip to remove surplus air and wind a hard rigid roll but not harm the plastic film web.
Plastic Bag Material
Polyethene, the raw material for plastic bags, is derived from ethylene produced from natural gas or oil. It is specifically processed to transform it into a polymer. Polyethene is modified to suit how it will be used and the many sorts of bags that will be made. Polythene Mailing bags are a great example of clear polythene bags; they are widely used in the eCommerce and shipping industries as they offer a cheap and secure way to transport your goods. Check out our range of grey mailing bags here; we offer various sizes and package quantities.
Plastic is derived from polymers, which are simple to synthesise and are composed of monomers. Polyethene comprises a chain of carbon atoms combined and linked to hydrogen atoms. Plastic's structure allows it to be easily transformed into different densities and shapes.
High-Density Polyethylene (HDPE) is a robust material with high tensile strength. It is resistant to many solvents and can endure temperatures of 120° C or 248° F. The linear form of the molecular chain results in a robust polymer that can resist repeated use.
Low-Density Polyethylene (LDPE) Plastics are thick, flexible, puncture, and tear-resistant. Mils, or one-millionth of an inch, are used to quantify their thickness. A greater mil count indicates a thicker bag. Mil counts for kitchen bags range from 0.9 mils to an extra-thick 6.0 mil for contractor bags.
LLDPE (Linear Low-Density Polyethylene) bags are identical to LDPE bags but have superior strength and puncture resistance. They are used for gusseted bags with a lighter gauge. LLDPE bags are less expensive than LDPE bags and are better for the environment. Copolymerisation of ethylene with butene, hexene, and octene yields them.
Medium Density Polyethylene (MDPE) MDPE is more challenging and harder yet still flexible enough for easy processing, despite being substantially thicker and suitable for handle and loop bags. MDPE's density makes it resistant to shock, drops and stress cracks.
Metallocene (mPE) (mPE) Metallocene is not a monomer in the same way that ethylene, propylene, or vinyl acetate is. It is composed of charged metal ion particles and carbon atoms. It follows the same density guidelines as polyethene, with lower densities being softer and higher densities more robust. It is combined with LDPE and LLDPE throughout the production process to create an improved poly film that seals faster and more robustly. The disadvantage of metallocene is its high cost, which is caused by its mixing with polyethene.
Vinyl EDC, or ethylene dichloride, is produced by mixing chlorine and ethylene. Vinyl EDC is created by polymerising vinyl chloride monomer (VCM), then polymerised and transformed into a PVC resin powder. The use of modifiers and additives changes the physical properties of the material. Vinyl is pliable, waterproof, long lasting, flame retardant, and resistant to wear and tear, UV radiation, and stress. Its advantageous characteristics make it perfect for keeping and conserving valuable goods.
Polypropylene comprises long chains of propylene molecules that provide flexibility while maintaining strength. It is fatigue resistant and does not suffer damage from prolonged use. Polypropylene has a low density, making it lightweight while maintaining a high strength-to-weight ratio. Polypropylene bags are used in the same way as conventional plastic bags. Its most popular application is for totes and shopping bags.