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The Process of Polythene Manufacturing in the UK

March 20, 2018 Comments Off on The Process of Polythene Manufacturing in the UK

Types of Polythene

• Low Density Polyethylene – LDPE
• High Density Polythene – HDPE
• Linear Low Density – LLDPE
• Metallocene Linear – MLDPE

Properties

LDPE – Natural in colour with good clarity. Flexible and pliable with good tensile strength HDPE – Natural in colour with cloudy appearance. Stiffer more rigid film with high strength properties LLDPE – Is a polymer which when added increases performance particularly in it strength characteristics MLDPE – This is linear polythene, with good strength characteristics, again natural in col-our Film Substrates Virgin Reprocessed Mixed Density Co-extruded

• Virgin: Extruded from its natural Material for more critical and consistent Applications
• Reprocessed: Extruded from recycled polythene for non critical applications such as waste bags
• Mixed Density: Mixing polymers together to form different characteristics.
• Co-extruded: Multi layered polythene containing different polymer grades. Polythene Additives

Masterbatches

Polythene is natural in colour and masterbatches are added to give its colour. Different % of masterbatches is added to give the polythene its opacity of colour. For a hint of colour about 1-2 % is added to give it a tint. For a deep colour where the polythene needs to be opaque between 6 and 12% addition Only certain colours have good opacity properties e.g. White cannot be opaque so co-ex (multi layered) needs to be made with a coloured inner layer.

Bio-Degradable

This is an additive that we add to the film to make it degrade, there are a number of different additives on the market which we use.

The Polythene Manufacturing Process

The Blown Film Process

The blown film process is the method by which the vast majority of all polythene products are manufactured. Polythene granules are fed into an extruder. As the granules are transported down the extruder screw, they are continuously heated and eventually become molten.

The melt passes through a narrow slit forming a tube which is extruded vertically until it can be clamped at the top nip rollers, which continuously pull the tube from the die. Once clamped the tube is inflated to create the desired width, the speed of the nip rolls determines the film thickness. The film is air cooled to set the film dimensions.

The tube is then collapsed prior to the nips to form a lay flat tube to a set width and thickness. The flat film is passed through path rollers to a floor mounted winder. Further processing can take place during this section for example: Slitting, Trimming, Perforation or treatment for print.

The cast film process

In the cast film process the material is forced through a slit die to form a sheet. The sheet is (as with blown film) drawn off via nip rolls again the wall thickness is determined by the speed at which the material is drawn off.

Cast film is cooled as it exits the die via large water cooled chill rolls which also leave a micro embossed pattern on the film. Winding styles and terminology Lay Flat Tube (LFT) – The most basic blown film product – formed by collapsing and flattening the bubble / tube. All other blown film products are derived from the lay flat. Centre Fold Sheet (CFS) – This is a lay flat tube with one edge fold removed by either a trim or an edge slit. Single Wound Sheet (SWS) – Again both edges of the edge fold are removed. In this instance the two sheets of film are separated and are wound up simultaneously. As with double wound sheet (DWS), the roll can now be slit down into multiple rolls by inserting slitting Blades. Because cast film is extruded through a ‘slit die’ the final product is exclusively Single Wound Sheet.

Double Wound Sheet

(DWS) – Both edge folds of the lay flat are removed leaving two single sheets of film wound onto one core. The roll may now be slit down into multiple rolls by the addition of slitting blades. Gusseted Lay Flat (GLFT) – Gusseted film is formed by using a pair of wooden (or metal triangles) to alter the shape of the bubble as it passes through the collapsing frame. J Fold Sheet (JFS) – This is a lay flat tube with one trim removed from a section from the edge fold to a specified width on the upper or lower web.

The Printing Process Flexography is the major process used to print packaging materials. Flexography is used to print corrugated containers, folding cartons, multiwall sacks, paper sacks, plastic bags, milk and beverage cartons, disposable cups and containers, labels, adhesive tapes, envelopes, newspapers, and wrappers. Flexographic presses are capable of producing good quality impressions on many different substrates and are the least expensive and simplest of the printing processes used for decorating and packaging printing. The use of flexographic printing presses is on the rise. There are two primary reasons for this: 1) it is a relatively simple operation; and 2) it is easily adapted to the use of water-based inks. The widespread use of water-based inks in flexographic printing means a large reduction in VOC emission compared to thick headset web or gravure printing processes.

Publication flexography is used mainly in the production of newspaper, comics, directories, newspaper inserts, and catalogues. Packaging flexography is used for the production of folding cartons, labels, and packaging materials. Large quantities of inks are used during normal runs on flexographic presses; however, some printers are able to recycle a majority of their spent inks and wash waters. Major chemicals used in flexography include plate making solution, water and solvent based inks, and blanket/roller cleaning solvents. Flexography is a form of rotary web letterpress, combining features of both letterpress and rotogravure printing, using relief plates comprised of flexible rubber or photopolymer plates and fast drying, low viscosity solvent, water-based or UV curable inks fed from an “anilox” or two roller inking system. The flexible (rubber or photopolymer) plates are mounted onto the printing cylinder with double-faced adhesive. Plates are sometimes backed with thin metal sheets and attached to the cylinder with fastening straps for close register or ink alignment. This adds additional cost to the plate and requires more make-ready time, but when quality printing is critical this type of plate can make the difference.

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