Film extrusion: Advancements and advice
Q: What are the key recent trends in extrusion, both in the market and in technology?
A: Recent trends in extrusion technology for films have been towards the use of twin-screw extruders with increasing outputs and technical complexity of packaging film production. This has been permitted as the design of high-torque, twin-screw extruders has been developed. For instance as outputs of oriented polypropylene lines began to exceed 3,500-4,000 kg/hr the continued use of tandem extrusion systems would have needed to add a second melting extruder (third extruder) to the successful tandem extrusion systems in use. At this point the use of a twin-screw extruder allowed a less expensive alternative.
In addition, the use of twin screw extruders has permitted the construction of oriented polyethylene terephthalate film lines without drying systems. Here the twin screw is operated with vents to remove moisture from the polyester polymer and maintain polymer molecular weight in film making without the drying systems.
Improvements in motor technology are also allowing the use of direct-drive extrusion systems eliminating the use of a gear-box reduction. It is expected that as motor costs continue to decrease, this technology will expand to smaller diameter extruders.
Advances in coextrusion technology continue to improve film-making systems and film properties. The introduction of microlayer or nanolayer coextrusion is showing dramatic improvements in stretch-film mechanical properties as well as gas barrier improvements with multilayer barrier resin combinations.
A: Sustainability is driving some end users to experiment with compostable films such as oriented and blown polylactic acid (PLA). Also the Walmart scorecard allows a multiprong approach to product designs for packaging. Any combination of improvements to life cycle assessment of the product can be used to lower the overall energy use and therefore the environmental impact and hence improve the score of the product.
This can drive packaging redesign, the use of thinner films, biodegradable materials, consolidated packaging to reduce weight and therefore transportation costs. This is due in large part to the relatively intelligent design of the scorecard recognizing the full impact of system improvements not just the use of biodegradability as a single measure of improvement.
Q: What is a common mistake related to extrusion?
A: Over-treatment is one of the most common errors made by film producers and converters. Treatment processes oxidize (burn) the surface of a film to make it more wettable, which in turn may improve adhesion when printed or laminated. However, as with any oxidation of a material, if the surface is over-oxidized, it can develop a loose layer of low molecular-weight burned material (or ash), which is poorly adhered to the film surface.
When the over-treated surface is printed or coated with adhesive, the ink or adhesive adheres to the ash, which is easily removed from the surface; the bonding of the ink or adhesive is low. The overtreatment can occur at any point in the film manufacturing and converting chain. When experiencing a sudden loss in adhesion you need to eliminate a formulation change in the ink or adhesive system and also to insure that the substrate is properly treated, not too little but also not too much.
Q: What other lessons learned can you share?
A: When coextruding a film, it is absolutely necessary to select the resins based on their relative rheological properties as well as their other desirable properties. In general, as you approach the surface of a film from the center, the viscosity of each subsequent outer layer should decrease in viscosity relative to the preceding inner layer. You need to have the viscosity as a function of temperature and shear rate to insure the proper engineering of the coextrusion system.
When establishing or evaluating a film extrusion system and in order to have the best film gauge and coextruded layer thickness control, it is imperative to have an extruder/screw combination that delivers a uniform polymer melt to the coextrusion or single-layer die. Melt pressure uniformity should be within 1% and the melt temperature should vary no more than 1-2 degrees over an extended time.
If you do not have these levels of melt pressure and temperature stability, the extrusion system/screw design should be questioned and the performance should be improved. Otherwise you will have poorer film flatness than necessary for the best slitting and converting yields.
Eldridge M Mount III is an independent consultant in extrusion, film manufacturing, metallization and converting and founded EMMOUNT Technologies, LLC in 2000. From 1978 to 2000 he worked at ICI Americas and ExxonMobil film divisions designing, developing, and manufacturing oriented films for flexible packaging.
He is a fellow and honored service member and a past vice-president and executive committee member of the Society of Plastics Engineers (SPE). Since 1981, he has served on SPE Extrusion Division Board of Directors. He can be reached at email@example.com; his company web site is www.emmount-technologies.com.