Understanding the extrusion process and the effect it has on your production chain of printing and converting film is an essential key to future success..
Plastic film has become so much a part of our daily life that we often don’t even notice it’s there – hidden in plain sight you might say! But its importance is undeniable, especially in the form of flexible packaging, and this is where its natural ability to repel liquids creates issues in the printing, laminating, and converting processes that are inherent in its production.
The fundamental issue is adhesion. Plastic poses a problem when it comes to the adherence of a liquid. Use a ballpoint pen to write on a plastic bag and you can smudge the ink by rubbing your thumb over it. The problem is a mismatch of surface energy between the two, and this is resolved by modifying the molecular structure of the plastic surface. This process is known as corona treatment and starts at the very beginning of plastic film manufacture, when it is extruded.
Controlled corona
By applying a carefully controlled electronic discharge at close range to the plastic film, the chemical make-up of its surface layer is changed by breaking down the long molecule chains which then allows the liquid to adhere. By increasing the surface energy of the plastic film (measured in dynes) it is possible to perform a range of different printing and converting processes that would otherwise prove impossible.
But, like most industrial processes, it’s not straightforward. Different plastic film formulations have different levels of surface energy and different processes demand different dyne levels to be successful. Basic materials like PP, PE-LD, PE-HD, and BOPP have a range of native dyne values from 29 to 32. However, the dyne level requirement, for example, of printing with solvent-based inks, or water-based inks, or for coating or laminating vary from 40 – 42 at the low end to 46 – 56 at the high end. To achieve these higher dyne levels, the corona system will require more power.
And it all starts with the extrusion process, which can raise dyne levels from 32 to 52 or more, whereas after extrusion, the polymer chains take 48 – 72 hours to post-crystallise, with additives like slip agents and those for anti-fogging migrating back to the surface and in turn affecting the adhesion level.
Securing correct treatment at extrusion is vital for two reasons:
- Dyne levels decline over time – typically 4 – 6 dynes over a period of 2 – 3 weeks before stabilising
- Because subsequent ‘boost’ or ‘bump’ treatment may be required later, depending on the intended process, and this may not be possible if corona treatment during extrusion was poor.
Increasing dyne levels at extrusion is easy, but after a period of crystallisation, additives like slip agents and those for anti-fogging migrate back to the surface and reduce the dyne level and lower adhesion. Correct treatment at extrusion is therefore vital for two reasons: first, because dyne levels decline over a period, and second, because subsequent ‘boost’ or ‘bump’ treatment may be required depending on the intended process, and this may not be possible if corona treatment during extrusion was poor.
Blown film extrusion
In this process the film is extruded vertically and wound into one flat tubular or two reels that are then converted by printing/coating/sealing into the final packaging product. Blown extrusion is typically for PE based materials, and the corona station is usually located at the top where the flattened tubular film is treated on the outside only. It is then split in two and can be either treated additionally, which is rare, or alternatively on the inside before being wound onto reels. On some extruders, the winder operates in both directions, so just one double-sided corona treater can be used.
The reels are then passed through a converting process where the welding takes place, and as corona treatment is notoriously the enemy of welding, because the oxidation generated by the corona process weakens the welding area, it is important to evaluate the likely effect in each case.
Blown film extrusion includes multi-layer polymers like LDPE, LLDPE, mLLDPE and MDPE, and applications include pallet hoods, collation shrink film, stretch hoods, lamination film, deep freeze film, heavy duty film, protective film, and food packaging film. These are single layer up to normally 3 layers but can be up to 11 layers in the case of food packaging film.
Cast film extrusion
In cast film extrusion, the substrate is produced from a flat die and wound as a single film after being side trimmed. In this type of installation, the corona treater is usually located before the winder but after the first edge trimming zone. Because the edges of the cast film are thicker than the rest of the web they need to be cut off before entering the corona station and winder to prevent any damage to the rubberised rollers.
Cast film extruders have a higher capacity and faster running speed than their blown film counterparts, so a corona treater capable of delivering a higher power is required to achieve the best result. Cast Polypropylene, normally called CPP and different to stretch film, is used for a wide range of packaging, and normally requires corona treatment. If an oxygen barrier is required, the outer LDPE or PP layer is normally corona treated for the next printing or laminating process. Diaper film is typically LDPE.
How to get it right
Extrusion is a composite process so there is a definite need for specialist knowledge to ensure the best results are obtained. As substrates become more complex to meet new packaging and safety criteria, the need for a detailed understanding of the surface treatment process and what it can offer becomes more important.
Often misunderstood, it is a process that if managed correctly pays dividends throughout downstream processing from the moment of extrusion.
If you don’t surface treat correctly at the extrusion stage, you are likely to limit your capability to further process the film – and that’s an expensive mistake to make!
