Solve Five Common Problems in Blown Film Coextrusion

A 360-degree look at resin conveying systems: types, operation, economics, design, installation, components and controls.

This Knowledge Center provides an overview of resin moisture and the drying process, including information on the best drying practices for your manufacturing facility. Monolayer Blown Film Plant

Solve Five Common Problems in Blown Film Coextrusion | Plastics Technology

Everything you need to know about plastics compounding technology—from feeding solutions to application profiles and expert advice.

Combat the skilled labor shortage using this comprehensive resource to train your own plastics processing experts.

Deep dive into the basics of blending versus dosing, controls, maintenance, process integration and more.

This Knowledge Center provides an overview of the considerations needed to understand the purchase, operation, and maintenance of a process cooling system.

Learn about sustainable scrap reprocessing—this resource offers a deep dive into everything from granulator types and options, to service tips, videos and technical articles.

Flat-to-downward trajectory for at least this month.

A mixed bag, though prices likely to be down if not flat for all this month.

Trajectory is generally flat-to-down for all commodity resins.

Flat-to-down trajectory underway for fourth quarter for commodity resins.

Generally, a bottoming-out appears to be the projected pricing trajectory.

PS prices to see significant drop, with some potential for a modest downward path for others.

Resin drying is a crucial, but often-misunderstood area. This collection includes details on why and what you need to dry, how to specify a dryer, and best practices.

Take a deep dive into all of the various aspects of part quoting to ensure you’ve got all the bases—as in costs—covered before preparing your customer’s quote for services.

In this collection of articles, two of the industry’s foremost authorities on screw design — Jim Frankand and Mark Spalding — offer their sage advice on screw design...what works, what doesn’t, and what to look for when things start going wrong.

In this collection, which is part one of a series representing some of John’s finest work, we present you with five articles that we think you will refer to time and again as you look to solve problems, cut cycle times and improve the quality of the parts you mold.

Gifted with extraordinary technical know how and an authoritative yet plain English writing style, in this collection of articles Fattori offers his insights on a variety of molding-related topics that are bound to make your days on the production floor go a little bit better.

In this three-part collection, veteran molder and moldmaker Jim Fattori brings to bear his 40+ years of on-the-job experience and provides molders his “from the trenches” perspective on on the why, where and how of venting injection molds. Take the trial-and-error out of the molding venting process.

Mike Sepe has authored more than 25 ANTEC papers and more than 250 articles illustrating the importance of this interdisciplanary approach. In this collection, we present some of his best work during the years he has been contributing for Plastics Technology Magazine.

In this collection of content, we provide expert advice on welding from some of the leading authorities in the field, with tips on such matters as controls, as well as insights on how to solve common problems in welding.

Mold maintenance is critical, and with this collection of content we’ve bundled some of the very best advice we’ve published on repairing, maintaining, evaluating and even hanging molds on injection molding machines.

Thousands of people visit our Supplier Guide every day to source equipment and materials. Get in front of them with a free company profile.

The global plastics industry has been navigating through what is arguably the most volatile period in decades. Unprecedented amounts of new production capacity are scheduled to start in North America, Europe, and China in the near term and compete for demand during a period of economic challenges. How will trade flows shift? Will this lead to regional cost disparities and rationalization? Energy transition and sustainability targets continue transforming the plastics market and increasing the competitive landscape. As the market evolves, what impact will new technology, policy, regulation, the growing role of chemicals versus fuel and other factors have on industry restructuring and business models? At GPS 2024, leading global experts will come together to discuss pivotal impacts and initiatives shaping the plastics industry. Join us and participants from across the globe to gain the latest insight and deep analysis as you connect with your peers and industry professionals. This year’s conference will explore the theme Disruptive Global Dynamics Reshaping Plastics and include a full day workshop focused on the Global Plastics Business and Plastics Transition to Circularity, 1.5 days of expert content and numerous networking functions.

Every three years, leaders from almost every major industry gather at NPE to advance their businesses through innovations in plastics. The largest plastics trade show in the Americas, NPE offers six technology zones, keynote speakers, workshops and opportunities to build partnerships.

The 3D Printing Workshop @ NPE2024 – The Plastics Show, is an immersive, half-day workshop focused on the emerging possibilities for part production via 3D printing and additive manufacturing. Presented by Additive Manufacturing Media, Plastics Technology and MoldMaking Technology, the 3D Printing Workshop will build upon a successful model first introduced at IMTS 2014. Attendees will benefit from a program focused on practical applications of 3D technologies related to plastics processing. This event will conclude with a 3D Printing Industry Reception sponsored by Additive Manufacturing Media.

The Society Plastics Engineers (SPE) Extrusion Division and the SPE Eastern New England Section will co-host the Screw Design Conference-Topcon on June 19-20, 2024 @ UMass Lowell in Lowell, MA. This highly technical program will focus upon screw design principles for single and twin screw extruders with wide ranging topics relating to screw designs for feeding, melting, mixing, venting and pumping plastics products and parts. Areas of focus will include screw designs for melt temperature and gel management, gel minimization, bioplastics, recycled materials and foaming. In addition to the technical sessions, a tour of the UMass Lowel Plastics Processing Laboratories will be integrated into Day 2 of the event. This program is not just for screw designers, but to help anyone responsible for any type of extrusion operation to evaluate existing extrusion equipment; and also to prepare for future projects. Price to attend: Less than $1000! Registrations will be accepted in early 2024. Call for papers – To be considered to give a presentation, please submit a talk title and abstract on or before December 15 to: Technical Chair: Eldridge M. Mount III, e-mail emmount@msn.com Corporate sponsorships - A limited # of corporate sponsorships (15) are available on a 1st come basis. Included is a 6’ tabletop display (must fit on table), denotation in all promotional activities, and 1 no charge registration. To become a sponsor contact: Charlie Martin, Leistritz Extrusion, e-mail cmartin@leistritz-extrusion.com, cell 973-650 3137 General information: A reception on Day 1 and a tabletop display area will allow the attendees to meet and discuss state-of-the-art screw technologies with industry experts. The SPE Extrusion Division will issue a “Screw Design Certificate” to all participants who have attended the program. Students are encouraged to attend and will receive a discounted rate. For additional information contact: Program Chair: Karen Xiao, Macro Engineering, KXiao@macroeng.com

Debuting in 2010, the Parts Cleaning Conference is the leading and most trusted manufacturing and industrial parts cleaning forum focused solely on delivering quality technical information in the specialized field of machined parts cleansing. Providing guidance and training to understand the recognized sets of standards for industrial cleaning, every year the Conference showcases industry experts who present educational sessions on the latest and most pressing topics affecting manufacturing facilities today. Discover all that the 2022 Parts Cleaning Conference has to offer!

Presented by Additive Manufacturing Media, Plastics Technology and MoldMaking Technology, the 3D Printing Workshop at IMTS 2024 is a chance for job shops to learn the emerging possibilities for part production via 3D printing and additive manufacturing. First introduced at IMTS 2014, this workshop has helped hundreds of manufacturing professionals expand their additive capabilities.

In today's manufacturing environment, robust processes that meet strict industry and regulatory standards are essential. With the advent of servo-driven ultrasonic welding technology, enhancing product quality and maintaining consistency has become remarkably effortless. Discover the fundamentals of ultrasonic welding, delve into vital components within these systems, explore how servo-driven ultrasonic welding enhances weld quality via advanced control features and gain insights into optimizing your assemblies for welding in these high-performing machines. Join Dukane to unlock the potential of ultrasonic welding in modern manufacturing for plastic devices and components. Agenda: Fundamentals of ultrasonic welding Key components in an ultrasonic welding system Using servo-driven ultrasonic systems to control your welding process Designing your parts and components for servo-controlled ultrasonic welding

This webinar will help you make informed decisions to confirm the equipment access stairs in your facility are OSHA compliant and meet the highest standards of safety and ergonomics. Agenda: Identifying opportunities to increase safety in the work place Utilizing space saving stairways Ensuring code compliance for equipment access

4.0, EUROMAP, OPC, OLE, QC, DSN, SQL, VNC, MES, ERP, FTP, CMS, SPI — are you confused by all buzzwords being tossed around in the plastics industry? Not convinced the data collection is necessary? Or are you unsure of how it could be implemented and improve your molding processes? Wittmann has been on the cutting edge of the data collection push for nearly 20 years. In this webinar, take a step back from the idea of the manufacturing facility of the future and discuss what you can do today to improve your process. Using readily-available technology, Wittmann can help reduce downtime, limit scrap and wasted material, and predict required maintenance. Let the experts at Wittmann help you understand: what data can be collected, what that data can be used for, what systems are used, and how to implement them. Agenda: Demystifying the terminology Tracking the material flow and lot information through the material handling system The data available from various auxiliary equipment, such as: dryers, blenders, mold temperature controls and robots Automating the process through changes in the data collected at the machines during production Adding visualization to increase productivity

Learn how targeted, modular, dosing and blending solutions — covering powders, granules, regrinds and liquids — provide plastics processors of all kinds with best-in-class accurate dosing while delivering significant raw material savings and ensuring highest quality. Agenda: Introduction to Movacolor Blending in plastics applications Movacolor feeding and dosing technology Hybrid blending to combine high material throughput and dosing accuracy

This presentation will explore the in-situ polyurethane (PU) overmolding of injection-molded and composite parts, allowing for direct out-of-mold class "A" surfaces. KraussMaffei will review the process and equipment required. It will also discuss tooling types currently available for PU systems for this process. KraussMaffei will compare the pros and cons of this technology over currently-available coating and painting systems. Agenda: Introduction and evolution of the ColorForm technology Overview of the ColorForm process Equipment required Tooling and PU systems Benefits of the system compared to typical spray-applied coatings Pros and cons of the technology

Consistent quality is paramount within the production of pipe and tubing applications. Additionally, significant material savings can be obtained by tightly controlling product dimensions with the correct process equipment. In this webinar, Conair will cover gravimetric control of an extruder and production line speed to ensure optimal quality and cost savings are achieved in your product run. A detailed discussion of the upstream material handling system includes: blending resins upstream of the feed throat, detection of the extruder rate at the throat, control of the extruder rpm and control of the product itself in feet per min — all accomplished with a simple recipe configuration which includes product weight per length desired and production line speed.

Medical-component specialist LightningCath has carved a niche meeting the needs of small to medium-sized entrepreneurs with complex catheter designs … quickly.

Fast Track service from BPM can repair every brand of rotor in two weeks or fewer.

Adds 52,000 square feet to Lebanon plant.

Nextpoint announced a $2 million investment, part of a $7.7 million raise for PlantSwitch, which uses agricultural waste to manufacture plastic.

Life cycle analysis of production at four plants in Mexico and Germany was conducted by C7-consult.

Tahara’s new eight-head, double-sided coex machine has many new mechanical and electronic features.

Topping five other entries in voting by fellow molders, the Ultradent team talks about their Hot Shots sweep.

Serendipitous Learning Opportunities at PTXPO Underscore the Value of Being Present.

Introduced by Zeiger and Spark Industries at the PTXPO, the nozzle is designed for maximum heat transfer and uniformity with a continuous taper for self cleaning.

Ultradent's entry of its Umbrella cheek retractor took home the awards for Technical Sophistication and Achievement in Economics and Efficiency at PTXPO.

technotrans says climate protection, energy efficiency and customization will be key discussion topics at PTXPO as it displays its protemp flow 6 ultrasonic eco and the teco cs 90t 9.1 TCUs.

Shibaura discusses the upcoming Plastics Technology Expo (PTXPO) March 28-30

Ahead of the first NPE since 2018, PLASTICS announced that its triennial show will stay in Orlando and early May for ’27, ’30 and ’33.

New features of NPE2024 aim to “bring the whole plastics ecosystem together to innovate, collaborate and share findings.”

Hundreds of tons of demonstration products will be created at NPE2024 next spring. Commercial Plastics Recycling strives to recycle all of it.

After what will be a 6-year hiatus caused by the COVID-19 pandemic, registration is open for the triennial show, which will take place May 6-10, 2023, in Orlando, Florida.

The Plastics Industry Association has hired from within, elevating Matt Seaholm to CEO and Glenn Anderson to COO.

Long-time leaders hailing from the U.S., Japan, Germany and Austria and across the entire supply chain, from machinery and materials to training and moldmaking, will be inducted.

Mixed in among thought leaders from leading suppliers to injection molders and mold makers at the 2023 Molding and MoldMaking conferences will be molders and toolmakers themselves.

After successfully introducing a combined conference for moldmakers and injection molders in 2022, Plastics Technology and MoldMaking Technology are once again joining forces for a tooling/molding two-for-one.

Multiple speakers at Molding 2023 will address the ways simulation can impact material substitution decisions, process profitability and simplification of mold design.

When, how, what and why to automate — leading robotics suppliers and forward-thinking moldmakers will share their insights on automating manufacturing at collocated event.

As self-imposed and government-issued sustainability mandates approach, injection molders reimagine their operations.

August 29-30 in Minneapolis all things injection molding and moldmaking will be happening at the Hyatt Regency — check out who’s speaking on what topics today.

Get your clicking finger in shape and sign up for all that we have in store for you in 2023.

Molding 2023 to take place Aug. 29-30 in Minnesota; Extrusion 2023 slated for Oct. 10-12 in Indiana.

Key technologies — such as multicolor molding, film molding and PUR overmolding for both exterior and interior applications — are at the forefront of this transformation. Join this webinar to explore the vast potential of eMobility in molding large components — including those with fiber reinforcements — thereby driving the need for large injection molding cells with a clamping force of up to 11,000 tons. You will also gain insight into Engel's innovative two-stage process, a solution for future recycling processes. This webinar will provide an in-depth overview of challenging applications, production concepts and best practices, including: BMW iX front panel production cell Smart rear panels concept based on IMD and 2C molding Sustainability concepts based on two-stage process Large tonnage equipment for battery moldings

Fast facts SPE International Polyolefins Conference 2024: The Preeminent Polymer Conference in the World dedicated to Polyolefins since 1975 The conference will in-person and virtual The powerful software platform for the conference will allow access to all papers on-demand, even several months after the conference, access to the program, access to virtual exhibitor/sponsor booths, and easy communication with speakers and other participants In-person participants can download a mobile or web app for the conference to access all the features of the software platform for the conference We anticipate over 900 people from around the globe to participate in-person or virtually The improved layout of the exhibit floor offers easy access and flow for the 60+ companies that we anticipate to exhibit Over 20 sponsors expected 150+ Technical papers, Sunday afternoon Tutorial Student Poster Competition 2 Networking Socials Meeting rooms available for rent to meet with customers and suppliers Exhibits from Monday through Wednesday until noon The Conference is organized by the SPE South Texas Section, the SPE Polymer Modifiers and Additives Division, the Thermoplastic Materials and Foams Division, the Engineering Properties and Structures Division, the Building and Infrastructure Division, and the Flexible Packaging Division.

The blown film industry has evolved from monolayer film structures to multilayer ones for a variety of food and non-food packaging applications. Here are typical problems with these structures and tips on how to solve them.

The blown film industry has evolved from monolayer film structures to multilayer ones for a variety of food and non-food packaging applications. Depending on the product requirements, a wide array of materials can be incorporated into the film structure to attain desired properties.

A process engineer working in this field must have a fundamental understanding of polymer rheology in order to properly specify equipment and select process conditions. Without sufficiently understanding polymer properties, unwanted layer interactions can occur during processing, which may lead to defects in the film.

Rheology is the study of the deformation and flow of a material. The extent of deformation and flow a material experiences is dictated by its rheological properties. For a polymer, these properties are dependent on physical characteristics such as molecular weight (MW), molecular-weight distribution (MWD), long-chain branching (LCB) and melting temperature. In blown film coextrusion, the most important rheological properties to be considered are shear viscosity, viscous dissipation, and elongational viscosity.

Shear viscosity is a measure of the resistance a material experiences to shear flow and is impacted by both shear stress and shear rate. Shear stress is defined as the force per unit area applied to a material, whereas shear rate is defined as the rate of deformation of a material.

Polymers are non-Newtonian fluids, and therefore their viscosity does not remain constant with varying shear rates. Generally, the shear viscosity of a polymer will decrease with increasing shear rate, a behavior known as shear thinning. The rate at which a polymer shear thins determines how it is processed. For example, a highly branched polymer is more shear sensitive than a linear polymer, and will therefore experience a more rapid decrease in shear viscosity at higher shear rates. Processing highly viscous materials will result in increased melt temperatures, increased head pressures, and will require more applied torque from the extruder.

As a polymer is sheared, the chain entanglements within its molecular structure begin to unravel. This disentanglement process generates heat, a phenomenon known as viscous dissipation. Viscous dissipation is proportional to both the applied shear rate and polymer viscosity, meaning that an increase in polymer viscosity and/or shear rate will result in an increase in melt temperature. This phenomenon creates a temperature gradient between the extruder barrel and the melt. If not properly accounted for, this increase in melt temperature may result in the overheating and degradation of the polymer.

Blown film processors continue to add layers to structures to improve properties and performance. Shown here is a nine-layer line from Macro.

Elongational viscosity is defined as a material’s resistance to stretching. Similar to shear viscosity, the elongational viscosity of a polymer will depend on its molecular structure. A polymer with a high degree of LCB will have a high elongational viscosity because the long branches become entangled among themselves during extension. The maximum tension that can be applied to a melt without experiencing material fracture is known as melt strength. A material with a higher elongational viscosity will have a higher melt strength. The melt strength of a polymer will impact the resulting stability of the bubble being formed in blown film extrusion. A material with lower melt strength is more difficult to process because it will fracture more easily while being stretched during bubble formation.

Although producing multilayer films is advantageous and provides opportunity for improvements in film properties, the increased structural complexity creates new challenges for process engineers. There are four key factors to consider in order to successfully produce a multilayer film: 1) polymer selection, 2) process equipment design, 3) layer arrangement, and 4) process conditions. Problems in blown film coextrusion arise when insufficient consideration is given to one or more of these factors. Five common problems associated with blown film coextrusion, along with troubleshooting techniques for each, are detailed below.

1. Bubble Instability: The term bubble instability encompasses many different types of issues related to the stability of the extruded bubble. The main issues include bubble breaks, strain hardening, unstable frost line, and bubble fluttering.

Bubble Breaks: A bubble break occurs when molten material exiting the die is overstretched, resulting in a break in the bubble structure. This can occur when the melt strength of the extruded material is insufficient for the selected blow-up ratio (BUR). To avoid this problem, resin selection can be modified to incorporate a higher-melt-strength material into the film structure to improve the overall melt strength. An example of this is incorporating low-density polyethylene (LDPE) into a linear low-density polyethylene (LLDPE) film to increase its overall melt strength.

Strain Hardening: Strain hardening occurs when molten polymer is rapidly stretched in the machine direction (MD) and stiffens as a result. This causes fluctuations in internal bubble pressure and bubble width. This issue can be prevented by reducing the drawdown ratio. Another solution is to modify the resin selection to decrease the overall elongational viscosity of the film, allowing the film to stretch more in the MD without experiencing any strain hardening.

Unstable Frost Line: In a stable process, the frost line will remain at a constant height above the die and is controlled by the cooling rate, die throughput, and film-thickness uniformity. When a process becomes unstable, the position of the frost line will become unstable as well. One cause of this instability is a non-uniform temperature profile in the extrudate.

Variations in melt temperature may stem from improper screw design for the selected resin, a worn screw, or a failure in a heater or thermocouple. Melt temperatures should be taken for each stream prior to merging to determine which melt layer is the source of the temperature variation. Once located, the screw can be removed and inspected for polymer degradation and screw abrasion. All heaters and thermocouples should also be checked for functionality.

Another common cause of an unstable frost line is a blockage in the die. A non-uniform die throughput may be due to a buildup of degraded material in the die. To avoid this, the die should be inspected for material buildup periodically and cleaned if necessary.

Bubble Fluttering: Bubble fluttering initiates below the frost line and appears as linear marks on the bubble surface in the transverse direction (TD). This fluttering is caused by high air velocity coming from the air ring. Decreasing blower speeds or adjusting air-ring components to reduce the air flow along the bubble surface will prevent bubble fluttering. However, reducing air flow will subsequently decrease the cooling efficiency of the air ring, which may then cause the frost line to drift away from the die, leading to new problems. To avoid this situation, resin selection can be optimized to reduce the overall melt viscosity of the extrudate and decrease the overall melt temperature.

2. Gauge Variation: In blown film coextrusion, it is inevitable to have some degree of gauge variation in the film, typically in the TD. An example of a gauge profile for a film with a low degree of gauge variation can be seen in Fig. 1a. However, if unexpected issues begin to arise during processing, the degree of gauge variation can increase. There are a few different issues that may lead to increased gauge variation, and determining the cause of the issue can be done by examining the shape of the gauge profile for the bubble.

Misaligned Die: A common source of gauge variation is a misaligned die gap. A misaligned die will result in a non-uniform flow distribution of material leaving the die. A common indicator of a misaligned die is an offset gauge profile, like the one shown in Fig. 1b. To correct this, the die gap should be checked for uniformity along the circumference. If misaligned, the die can be recentered using the die-adjustment bolts.

FIG 1 Examples of gauge plots for film samples experiencing: A) minimal gauge variation; and B) offset gauge variation.

Non-Uniform Air-Ring Cooling: Poor or non-uniform air flow coming from the air ring will result in non-uniform film cooling, which will impact the drawdown ratio of the film. This can result in portions of film being stretched more than others, which may lead to gauge variation. To avoid this issue, the air channels in the air ring should be inspected and cleaned periodically to remove any impurities that may cause air-flow disturbances. The air ring should also be checked to confirm it is properly centered on the die.

Non-Uniform Melt Temperature: Non-uniform melt temperature will result in variable bubble cooling rates and die throughputs. A telling sign of a non-uniform melt temperature is a sinusoidal gauge profile. The formation of this sinusoidal gauge variation is related to the flow of a material having a non-uniform melt temperature through a die, a phenomenon known as port lines. As previously described, variations in melt temperature are likely caused by an improperly designed or worn screw.

3. Interfacial Instability: The term interfacial instability refers to instabilities occurring at the interface between two layers. The stability of the interface will depend on factors such as material properties, process conditions, and equipment design characteristics. Three known types of interfacial instability that may arise during blown film coextrusion.

Zig-Zag Interfacial Instability: This form of instability occurs when an interface experiences excessive shear stress, resulting in the formation of “zig-zag” distortions along the film surface in the TD direction. An image of this instability is shown in Fig. 2.

FIG 2 Image of a polymer interface experiencing high shear and exhibiting zig-zag interfacial instability. (Source: Zatloukal, M., Kopytko, W., Vlcek, J., and Saha, P., SPE ANTEC 2005)

There are a few known causes of the zig-zag instability. The first is differences in shear viscosity between the materials creating the interface. If the materials have distinctly different shear viscosities, the individual layers will experience differing shear rates at an applied shear stress, causing the zig-zag deformation to appear. This can be mitigated by selecting materials with similar shear viscosities (if possible) to create the interface.

The second cause is an improper layer ratio. If the layer ratio chosen is such that the interface is too close to the die wall, the excessive shear stress will trigger the deformation. To prevent this, the outer layer thickness can be increased to shift the interface away from the wall and reduce the shear stress applied onto it.

The third cause is an improper die design. When designing a die, it is important to properly optimize fluid channels to create a uniform velocity profile throughout the die. Improper die design may lead to the formation of high-shear points and a non-uniform velocity profile, resulting in this instability.

Wave Interfacial Instability: This form of instability is caused by non-uniform elongational deformation occurring at an interface between two materials, resulting in wave-like patterns forming along the bubble surface as it is extruded from the die. An image of this instability can be seen in Fig. 3. The elongational deformation causing this instability has been found to occur from two known sources. The first is associated with an improper layer ratio. If one of the combining layers of the structure is too thin, it will experience more acceleration at the merge point within the die, resulting in a higher rate of elongational deformation in that layer. Layer ratios should be adjusted to allow for a more uniform velocity profile to form downstream of the merge point.

FIG 3 Image of a polymer interface exhibiting wave interfacial instability. (Source: Tzoganakis, C., and Perdikoulias, J., Polymer Engineering & Science, Vol. 40, p. 1056, 2000)

The second source is a difference in the elongational viscosities of the materials creating the interface. Two materials with differing elongational viscosities experiencing the same elongational force will deform at different rates. If possible, materials having similar elongational properties should be chosen to create the interface.

Reactive Interfacial Instability: A reactive interface may form during the coextrusion of a multilayer structure consisting of a polar barrier resin and a tie layer. Barrier resins are typically coextruded together with layers of polyolefin resin. These structures typically require a tie layer between them to improve their adhesion to one another. Tie layers are polymers consisting of both polar and nonpolar end groups. Occasionally, unwanted chemical reactions may occur between the polar end group of the tie layer and the polar barrier resin, creating an unstable interface. The result of these reactions is a reduction in optical clarity and a film with a more grainy appearance.

An example of this instability is an interface between EVOH and a tie layer with a maleic anhydride (MA) functional group. Unwanted reactions may occur between the MA of the tie layer and hydroxyl groups along the EVOH chain. A visual example of this instability is shown in Fig. 4, which compares a PP/EVOH multilayer film with no tie layer (Fig. 4a) and a MA tie layer (Fig. 4b) coextruded at the same conditions. The addition of the MA tie layer resulted in a drastic deterioration of optical film clarity, caused by unwanted interactions occurring at the interface between the EVOH and the MA tie layer. To prevent this from occurring, the tie layer can be replaced with one consisting of a different functional end group.

FIGURE 4 Comparison of optical clarity for: A) a nonreactive PP/PP/EVOH/PP/PP film;and B) a reactive PP/PP-MA/EVOH/PP-MA/PP film.(Source: Bondon, A., Lamnawar, K., and Abderrahim, M., Polymer Engineering & Science, Vol. 55, p. 2542, 2015)

4. Gels: The term gel is commonly used to refer to any form of small defect that creates an optical distortion in the final film. Gels are problematic because not only do they create an optical distortion, they can also reduce the mechanical properties of the film. Gels can be divided into three main categories: unmelted resin, degraded materials, and foreign contaminants.

Unmelted Resin: Unmelted resin is a common type of gel that is typically observed at high extruder throughputs. They are a result of non-uniform melting caused by low-shear regions located within the extruder. To determine if a gel is unmelted resin, heat the gel above its melting temperature, and then let it cool back down. If the gel does not re-appear after cooling, it was unmelted resin. If unmelted resin is present in the film, the extruder screw should be examined and redesigned to minimize the presence of low-shear zones.

Degraded Materials: Material degradation can occur during extrusion if a polymer is exposed to high levels of energy for an extended period of time. These conditions lead to the formation of highly oxidized or crosslinked materials, which appear as gels in the film. Typically these gels are not present immediately after startup. They appear on the film over time as the degraded polymer accumulates within the process.

Highly oxidized gels appear as brittle black specks and can be identified by their fluorescence under UV light. Figure 5 shows an image of a highly oxidized gel taken under polarized light, and an image of the gel fluorescence under UV light. Crosslinked gels have a dark brown appearance and consist of oxidized material, however the degree of oxidation is too low to cause fluorescence under UV light.

FIG 5 Images of a highly oxidized gel in a multilayer film under A) polarized light and B) UV light. (Source: Campbell, G., Spalding, G., Analyzing and Troubleshooting Single-Screw Extruders, Hanser, Vol. 1, p. 486, 2013)

Crosslinked gels are sometimes mistaken for highly entangled resin. To distinguish between the two, heat the gel up above its melting temperature, apply a stress on it to break it up, and allow for it to cool. If the gel shape reappears after cooling, the material is crosslinked and not highly entangled. To determine the source of the degraded material, the extruder screw should be removed after operation without purging. Once the source of the degradation is identified, the screw design can be optimized in that region to minimize the stagnation and low shear forces present.

Foreign Contaminants: Occasionally, foreign contaminants may gain entry into the process. Foreign contaminants typically enter the extruder with the main feedstock through the hopper, and can range from clothing fibers to foreign polymer resin. If the gel is found to have a distinctly different melting temperature than the feedstock resin, or a very irregular shape (such as a fibril), it is likely a foreign contaminant. To avoid these gels, the hopper should always be properly cleaned, and the virgin feedstock container should be enclosed to prevent any foreign objects from being mixed in.

5. Film Curling: Film curling is a defect observed with multilayer structures consisting of materials having varying degrees of crystallinity. This defect forms during the cooling of the molten film and causes the film to roll up on itself. Film curling is related to two issues: differences in material properties (i.e. melting point, degree of crystallinity) and improper layer arrangement.

A common example of film curling is shown in Fig. 6, which illustrates a three-layer film consisting of polyethylene (PE), nylon (PA) and a tie layer. As the multilayer film exits the die, all layers are molten and the film begins to cool. As it cools, the nylon layer crystallizes and becomes rigid while the PE layer shrinks, as it is still molten. Once the temperature decreases below the melting temperature of PE, it starts to become rigid as well. However, at this point the nylon is already crystallized and cannot shrink further, causing the PE to curl away from it during solidification.

FIG 6 Film curling example of a three-layer blown film coextrusion line consisting of PE, PA, and a tie layer.

One method to prevent film curling is rapidly cooling the film to prevent crystallization. As opposed to using air to cool the film, the film can be extruded directly into a water bath for rapid cooling. This allows for materials with differing crystallization rates to be coextruded without the film exhibiting any curling. Another possible solution is to optimize the structure of the multilayer film.

For example, modifying the structure of a PE/tie/PA film, which is likely to curl, to PE/tie/PA/tie/PE/tie/PA. The additional layers in the new film increase the overall mass of the film which will mitigate the extent of curling that the film experiences.

About the Authors: Tomislav Ivancic is a process-engineering technologist at Macro Engineering and Technology Inc., currently involved in R&D and rheological process modeling. He received both his MASc. and B.Eng in chemical engineering from McMaster University, with a specialization in polymer materials and processing. Contact: 905-507-9000; tivancic@macroeng.com; macroeng.com.

Dr. Karen Xiao is vp of technology at Macro Engineering. She has almost 25 years of experience in the industry in both equipment and film manufacturing. Prior to joining Macro in 2020, she was the extrusion technology leader for Celgard LLC, an Asahi Kasei company, responsible for battery-separator film development for nine years. Xiao is also on the Board of Directors of the SPE Extrusion Div., having served as the Division Chair and Technical Program Chair. She was elected Fellow of SPE in 2016. She has over 50 publications and six pending and granted patents in film extrusion. Contact: kxiao@macroeng.com.

Not all film webs are created equal. This creates challenges for both winder and operator. Here’s how to handle them.

You rightfully worry about melt temperature, but don’t overlook head pressure, because the two are closely linked and will influence line performance.

Just like selecting the extruder size and drive combination, the L/D should be carefully evaluated.

A troubleshooting timeline is essential to help you quickly identify problems and their causes. Here we'll describe such a timeline and how to use it to solve one common problem—melt fracture in tube and profile extrusion.

The biggest cause of wrinkles in the blown film process is instability of the bubble between the air ring and where the bubble reaches full diameter.

The overall performance of a blown film line depends on a properly tuned and maintained IBC control system. Follow these preventive-maintenance tips to keep your system running smoothly.

Three real-world case studies show how processors solved common problems in blown film via the mapping technique.

Multilayer Extrusion Plastics Technology covers technical and business Information for Plastics Processors in Injection Molding, Extrusion, Blow Molding, Plastic Additives, Compounding, Plastic Materials, and Resin Pricing. Learn More

Solve Five Common Problems in Blown Film Coextrusion | Plastics Technology