A-Z of Polythene terms

 

Anti-block An additive in the polymer to avoid the film sticking together and becoming blocked during extrusion. This Anti-block is usually added by the polymer manufacturer so that the film grade we buy is ready mixed.

Anti-static An additive added that reduces the likelihood of the film attracting static energy - useful when the film is for packaging electrical components. We buy Anti-Static masterbatch and add in a ratio of approximately 2% during extrusion on request. Customers often want a pink tint when they buy anti static film to denote it as such.

Barrier Film A term to describe film that is a good barrier to gases, aromas and flavours - typically, high barrier materials are going to be oxygen barrier materials, of which the most common are EVOH and nylon resins used in conjunction with polyethylene. To create the specific barrier required either coextrusion, lamination or coating is used.

Block Headed The process of welding together a batch of bags, usually above a tear off perforation so that bags can be stored in batches easily and taken one at a time.

Blocked Film When extruded film is impossible to open - this can be due to a number of factors such as not allowing the film to cool sufficiently, having the nip rollers too tight on the extruder, over treating of the film or using unsuitable polymer

Blow Ratio The degree to which the polythene bubble is blown - expressed as a ratio of the diameter of the bubble to the diameter of the extruder die. The greater the blow up ratio, the stronger the film will be as the polymer chains in the film become stretched and intertwined. A typical ratio will be 3:1.

Co-extrusion The extrusion of two or more polymers at the same time which are then layered together to produce a film to benefit from the specific characteristics of each polymer layer.

Defence Standard 93-19/1 Established by the MoD in 1976 as they felt there was no other acceptable national standard available at the time in relation to polythene film. Still used as a benchmark and MG Polymers tolerances are in accordance with this standard.

Die Line A line of weakness (seam) in the film caused by a piece of contamination caught in the die. Often difficult to spot, the film will tear easily along this seam under any pressure.

Electro-conductive polymer Polymer with a high carbon content to allow the dissipation of static electric. Due to the carbon it is a black polymer but can be extruded like any low density polythene film.

End (Bottom) Weld A bag with the seal at the base of the bag, separated from the end of the bag by a small skirt as the sealing and cutting are separate processes. Bag length is measured from top to bottom of the bag INCLUDING the skirt which is typically up to 1cm

Europallet A specific sized pallet somewhat smaller than the traditional pallet. A europallet measures 800mm x 1200mm and some customers insist on it for pallet deliveries. We have a stock of both euro and standard pallets.

Euroslot A hole in a bag for hanging it on a hook - the shape is a punched hole with a horizontal slot 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. We add EVA during extrusion on request normally at between 1 and 3%

Flexographic print A print process where ink is coated onto a raised printing plate (usually rubber or plastic) which is then rolled against the film to produce the printed image. The most common form of print for short run print. As opposed to 'Gravure' printing.

Flush To describe when the layflat on a core is rolled right up to the edge of the core so that there is no 'lip'

Gravure print A print process where ink is smeared onto an engraved metal roller which is then passed against the polythene film to produce the printed image. A high quality but relatively expensive print process. As opposed to 'Flexographic' printing.

Gusset A fold in the side or bottom of the bag so that the closed width is less than the open width. A side gusset is produced in extrusion using gusset boards that push in at the side of the bubble on the cooling tower.

J-Fold A form of centre folded sheeting where the upper surface is a different width to the lower surface

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

Layflat Tubing The bubble of tube wound up after blown film extrusion

Linear (LLDPE) A polymer similar to low density polymer (LDPE)

Light Proof The same as opaque - i.e. no light should pass through. The term light proof is often used to emphasise the importance e.g. the bag is going to be used in a film processing room

Low Slip Used to describe polymer that has not had slip additive added - often used for large sacks that are to be stacked and must not slide. We buy in ready formulated low slip LDPE.

Masterbatch A colour producing additive that is added in varying quantities depending on whether a tint or opaque colour is required - usually between 1% and 5%.

Maxi grip bags A Self Seal bag with a double grip seal to make it more secure than a standard grip seal bag

Microperforation A process of perforating the film with a series of small holes by passing the film over a spiked hot or cold roller. This may be to allow moisture to escape as in bread bags. Cold microperforation is usually to create tiny pin pricks whilst hot microperforation is usually to create a series of tiny holes

Opaque Opaque is coloured film where you can not see sunlight through the film

Pallet Wrap Simply stretchwrap that is used to bind a pallet together - normally comes in large rolls

Printed Warning Notice (PWN) Normally in red or black ink, a simple printed warning about the danger of suffocation with polythene bags

Random Repeat Print Random repeat print is where a print is not kept in register and so the print will not be kept at the same position on each bag during conversion. The repeat length depends on the circumference of the print cylinder and the number of print plates in use.

Registered Repeat Print Registered repeat print is print which is kept in register to enable print to be kept in the same position on each bag. This is enabled by having a register mark produced when printing (the printed image itself can be used if it has a clear starting point) This register mark is then picked up by a 'magic eye' (photoelectric cell) on the bag maker. Registered print can therefore only be converted one-up.

Regran / Recycled Material Regran is short for regranulated and refers to the pellets that are produced from reprocessed scrap film. Regran film is of a lower quality than virgin and should not normally be run in less than 150gauge thickness.

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 (Also called gripper / grippa / minigrip) bags Sideweld bags with a ridge of polythene near the top that clicks together to seal the bag. This seal can be clicked open or shut. Self seal bags are primarily made abroad and imported. Please do not confuse with top tac bags.

Sheeting Layflat tubing that has been cut on either one side (for centre fold sheeting) or both sides (for double wound sheeting). If the sheeting is separated and collected on two separate take-offs, single wound sheeting can be produced.

Shielding bag A specific bag to shield the contents from electromagnetic radiation. A metallised bag is a form of shielding bag.

Shrinkfilm Film that shrinks on application of heat (e.g. a heat gun or tunnel). Useful for securely wrapping goods and to prevent tampering of goods. Whilst all film shrinks, the phrase shrinkfilm is used to describe film that shrinks in a known way. This depends on the film characteristics and the manner in which it is extruded. Low slip LDPE is one of a number of typical shrink films along with PVC and PP.

Side Weld A bag where the sides of the bag is created using a heated knife. It is weaker than an end weld bag but cheaper to produce and has no skirt. (You can produce a side skirt weld bag if required with conventional end weld seals but at the side)

Sleeve A sleeve is a bag that is open both ends rather than sealed at one. In order to produce sleeves the heat sealer is deactivated on the end weld bag maker leaving just the cutting mechanism

Slip Slip is defined as the inverse of the dynamic coefficient of friction. It is an additive added by the manufacturer to allow polythene to be converted easily (less friction with rollers) and for bags to open easily. Different formulations of LDPE have varying amounts of slip and so are termed either low or high slip. Normal slip is between 1.7 and 3.0

Standard pallet A standard pallet measures approximately 1000mm x 14000mm although there are some variations in size. Recently some companies have standardised racking to take europallets which are slightly smaller.

Stretchwrap A complex material, normally PVC or polyolefin based with plasticising agents added, designed to conform to whatever shape comes to hand

Tensile Strength Reflects the maximum load the film is capable of supporting

Tint Tint colour film is where only a very little masterbatch is added (usually about 1%) to give a tint of colour, the film remaining see through (translucent)

Top Tac bags (or mailing bags) A bag which has had an adhesive strip attatched to the top of the bag (usually on a lip). The strip consists of adhesive coated on a non stick silicone tape which is removed to allow the bag to seal. Depending on the adhesive properties, permanent or resealable bags can be produced.

Treating The process of bombarding film with a high voltage current so that the surface of the film becomes roughened up and enables print to adhere more easily to the film

Virgin Material Virgin material is used to describe polymer direct from the manufacturer rather than reprocessed material. As such, we can guarantee it is non-contaminated and can be used for food grade applications.

Water soluble bag A bag that dissolves after contact with water - useful in certain hygiene disposal applications. The polymer is formulated with alcohol to create the breakdown of the material on contact with the water. The bag can either be soluble in cold or hot water.

Wicketted Bags which have been bundled together by means of a small metal hanger that is put through 2 punched holes near the top of the bag - often used in retail applications or on-line packing machines for easy handling of the bags.