Glossary of Polythene Related Terms

The following are a range of terms which you may encounter within the polythene industry.

Anti-bac:

Inhibits the growth of bacteria by acting as an antibacterial additive.

Anti-block:

A substance that is used to prevent the film from sticking together and becoming coagulated during extrusion. This additive is generally added to the polymer by the manufacturer.

Anti-static:

Anti-static film has the ability to discharge, and thus reduces the chance of static energy accumulating - this is typically taken into account when manufacturing packaging for electrical goods, or goods which you want to keep free of dust and dirt particles.

Barrier Film:

A film that blocks gases, odours, and flavours, and can be referred to as a high barrier material. Often used in food or medical packaging.

Biodegradable:

A material which is capable of being decomposed by bacteria or other living organisms - generally in the earth or in landfill sites. This process can be accelerated under certain conditions with a variety of additives

Block Headed:

The process whereby batches of bags are welded together, usually above a tear-off perforation, to allow them to be stored and removed easily one at a time.

Blocked Film:

When a film has been insufficiently cooled or overtreated it can make the extruded film impossible to open.

Blocking:

Where 2 layers of film stick to each other, making the bag difficult or impossible to open. When bags are in this state, they are considered ‘blocked’.

Blow Up Ratio (BUR):

This is the extent to which the polythene bubble is ‘blown.’ The size of the bubble is often represented in the form of a ratio (the diameter of the bubble: the diameter of the extruder dye). The greater the ratio, the stronger the film will be.

Co-extrusion:

When two or more polymers are extruded at the same time. These are then layered together, producing a film which is beneficial to each polymer layer's characteristics.

Compostable:

Those materials that will fully degrade in the presence of microbes or chemicals in a compost heap.

Corona Discharge Treatment:

The film is passed through a device which changes the surface resistivity of the film so that ink, adhesives etc can be applied easily.

Double Wound Sheeting (DWS):

This is film that is 2 layers thick and has been wound onto a reel making it extra durable. For this reason it’s often used for things like pallet covers as it is useful for protecting against dust & moisture.

Defence Standard 93-19/1:

The national standard set by the Ministry of Defence in 1976. All poly bags must be made in light of this standard.

Density:

This is the measurement used by the polythene industry to distinguish between various grades of polythene (such as HDPE & LDPE) Higher density polyethylene products have a tighter crystallisation structure.

Die Line:

A weaker seam in the film caused by a piece of contamination in the die. The film will tear easily along this seam under any amount of pressure.

Drawdown ratio:

Die gaps define the thickness of the heated polymer as it exits the die. Drawdown is the process of reducing the gauge to a thinner one.

Edge-trimmed:

The edges of a single wound or double wound sheet film are slit by fixed blades so that the width of the film is accurate to 2mm. Often referred to as “record edge”

Electro-conductive polymer:

A polymer with a high carbon content to allow the dissipation of static electricity.

Elongation:

The amount a film can stretch.

End (Bottom) Weld:

A bag with the seal at the base and separated from the end of the bag by a small skirt. Separate processes are involved in sealing and cutting. The bag length includes the skirt.

Ethyl Vinyl Acetate:

Derived from the random copolymerization of vinyl acetate and ethylene.

Euro pallet:

Smaller than the traditional UK pallet, measuring 1200mm x 800mm.

Euro slot:

A hole in a bag for hanging it on a hook - the shape is a punched hole with a horizontal slot on either side.

EVA:

An additive that lowers the freezing temperature of polythene and allows it to be stored in a freezer without cracking. It makes the bag rubbery and a little difficult to open. It is often added during the extrusion stage in quantities requested.

Film:

Sheets of less than .010 inch thickness.

Flexographic print:

Ink is coated onto a raised printing plate during the printing process. This plate is then rolled against the film to produce the printed image.

Flame Retardant:

A material that, when a certain additive is additive, reduces it’s flammability.

Flush:

To describe when they lay flat on a core is rolled right up to the edge so that there is no 'lip'.

Gloss:

The degree of the sheen of the surface.

Gravure print:

A type of printing process. During this, ink is painted onto an metal roller which has been engraved. This then proceeds to pass against the polythene film, resulting in the printed image. Often high-quality but expensive.

Gusset:

The bottom or side of the bags are folded. This ensures that the open width is larger than the closed width.

Haze:

A film is ‘hazy’ if it is cloudy. This cloudiness can vary in degrees.

High Slip:

An additive which makes the film more slippery.

HDP

High-density Polythene.

Impact Strength:

The ability to which a film can withstand processes such as shock loading, blunt puncturing, etc.

J-Fold:

A form of centre folded sheeting where the upper surface is a different width from the lower surface.

Laminated:

When two separate films are physically joined together by the use of an adhesive to combine the best physical properties of both.

Layflat Tubing:

The bubble of the tube wound up after blown film extrusion. A continuous tube.

LDPE:

Low-density polyethylene .

LLPDE:

Linear low-density polythene. Straighter chain of molecules than LDP and therefore stronger.

Light Proof:

If something is ‘light proof,’ then light cannot pass through. Another term for this would be opaque.

Low Slip:

Used to describe polymer that has not had slip additive added - often used for large sacks that are to be stacked and not slide.

Machine Direction:

The direction film travels during extrusion.

Masterbatch:

A colour-producing additive that is added in varying quantities depending on whether a tint or opaque colour is required.

Maxi grip bags:

A self-seal bag with a double grip to make it more secure.

Metallocenes:

Usually these are difficult to process & extract, however they are really strong and have excellent clarity.

M-Fold:

Centerfold sheeting that's folded inward on the crease side.

Micron:

Measurement of thickness.

Micro perforation:

A process of perforating the film with a series of small holes by passing the film over a spiked roller. This may be to allow moisture to escape. The spiked rollers can be hot or cold depending on the size of holes you want to create.

Mono:

When only a single layer of film is extruded. This type of film is commonly used in the manufacturing of clear films/polybags and some dark-coloured mailing bags.

Oxygen Transmission Rate:

The rate at which oxygen permeates a film at a certain temperature.

Pallets:

UK pallets typically measure at 1000mm x 1200mm. It is worth noting, this can vary slightly.

Pallet Wrap:

A pallet is bound together by stretch wrap, also known as pallet wrap.

Photodegradable:

When exposed to sunlight, these materials can degrade.

Printed Warning Notice (PWN):

Normally in red or black ink, a simple printed warning about the danger of suffocation polythene bags.

Random Repeat Print:

Random repeat print is where a print is not kept in the register and so the print will not be kept at the same position on each bag during conversion.

Recycled:

Plastic, once it has been used, can be reworked to create a new product.

Registered Repeat Print:

Print is kept in a consistent position on each bag through the process of repeat print. Ink will be kept in the register.

Regran/Recycled Material:

Regran is short for re-granulated and refers to the pellets that are produced from reprocessed scrap film. Usually of lower quality than the original.

Reprocessed:

A processed plastic which has not been used and subsequently reworked.

Sack grade:

Sack grade is a term to describe a type of polymer with a different melt flow index, suitable for sack making.

Self-seal/side-weld bags:

Sideweld bags have a ridge of polythene near the top that clicks together, closing the bag. A side-weld bag is where the sides of the bag are created using a heated knife. Whilst it’s weaker than an end weld bag, it is cheaper to produce.

Single Wound Sheeting (SWS):

A single layer of film wound onto a reel.

Sheeting

Layflat tubing can be cut on just one side for centre-fold sheeting. It can also be cut on both sides for double-wound sheeting.

Shielding bag:

This type of bag shields any contents from electromagnetic radiation.

Shrink-film:

Film that shrinks on the application of heat. Useful for securely wrapping goods and preventing tampering.

Sleeve:

A sleeve is a bag that is open at both ends.

Slip:

This can be applied to polythene film in order to make the surface more lubricated and have less friction.

Tensile Strength:

The ‘pulling strength’ required to break films.

Tensile Yield:

The ‘pulling strength’ required to deform films.

Thermoplastic

This material can be repeatedly softened by heat and hardened by cooling.

Tint:

Tint colour film is where only a very little masterbatch is added to give a tint of colour.

Top-tac bags:

These bags have adhesive strips. The strip consists of an adhesive coated on non-stick silicone tape which is removed to allow the bag to seal. Both permanent and resealable bags can be produced.

Translucent:

A film that permits the transmission of light, but does not provide a clear view through the material (similar to a frosted glass for example)

Transparent:

A film that permits the transmission of light, and provides a clear view through the material.

Transverse Direction:

The cross direction of film during extrusion (90 degrees from the direction of travel).

Treating:

Film is blasted with a high voltage current. This process roughens the film's surface and allows print to adhere easier.

UV absorber:

A compound designed to only absorb selected wavelengths of UV rays.

UV blocker:

A compound designed to block selected wavelengths of UV rays.

UV Stabiliser:

An additive which reduces the tendency of plastic materials to photo-degrade as a result of ultraviolet rays such as sunlight.

U-Fold:

A double layer of film, folded on both sides, with a (centre) section cut out on one side.

Vapour Corrosion Inhibitor:

A compound added to film for the prevention of oxidation or corrosion of packaged parts.

Water soluble bag:

These bags are useful for hygiene disposal purposes as they dissolve when they come into contact with water.

Water Vapour Transmission Rate:

The rate at which water vapour permeates through film at a certain temperature.

Wicketted bags:

Bags have been bundled together by a small hanger. This hanger is put through 2 punched holes near the top of the bag. You’ll often find them used in retail applications or line packing machines as they make handling them easier.

Yield:

How much film is yielded per tonne.