BLOWN FILM PROCESS SUZANA SHAFEI MOHD HAIROL MIZZAM HARIS
BLOWN FILM PROCESS All right reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording, or in any information storage or retrieval system, without permission in writing from the publishers. eISBN: Published by: Politeknik Sultan Abdul Halim Mu’adzam Shah, Bandar Darulaman, 06000, Jitra, Kedah. Tel: +(60) 049146100 Fax: +(60) 049174232 / 049146323 Website: http://www.polimas.edu.my Copyright © 2022 by Politeknik Sultan Abdul Halim Mu’adzam Shah, Kedah, Politeknik Malaysia www.polimas.edu.my
THE AUTHORS SUZANA BINTI SHAFEI obtained a Bachelor of Mechanical Engineering in 2005. In August 2007, she was assigned as a lecturer at Politeknik Sultan Azlan Shah (PSAS) for 5 years and Politeknik Sultan Zainal Abidin (PSMZA) for 4 years for the Diploma in Mechatronic programmes. Moved to Politeknik Sultan Abdul Halim Mu’adzam Shah (POLIMAS) in 2015 until now and is involved in teaching for Diploma in Mechanical Engineering (Plastic) programmes under the Mechanical Engineering Department. MOHD HAIROL MIZZAM BIN HARIS obtained a Bachelor of Mechanical Engineering in 2007. In August 2008, he was assigned as a lecturer at Politeknik Kota Kinabalu (PKK) for 4 years for the Diploma in Mechanical Engineering programmes. Moved to Politeknik Sultan Abdul Halim Mu’adzam Shah (POLIMAS) in 2012 until now and is involved in teaching for Diploma in Mechanical Engineering (Plastic) programmes under the Mechanical Engineering Department. i
PREFACE First at all, we grateful to Almighty Allah S.W.T for established us to complete this book. This book was successfully prepared with effort of some lecturer of Malaysia Polytechnic. This book was written based on the blown film process for plastic processing process that are used in manufacturing for plastic industries . This eBook accompanied with notes, mind map, questions for the final examination and some simple activities that can be carried out during the lecture. There are also some useful tips and hints to help student understand better about the course. Lastly, we would like to thank for choosing this module as a medium of learning and hopefully it could benefit you all. ii
TABLE OF CONTENT CHAPTER PAGE _______________________________________________________________ AUTHORS I PREFACE II INTRODUCTION 1 BLOWN FILM COMPONENT AND FUNCTION 4 BLOWN FILM PROCESS 7 PRODUCT OF BLOWN FILM 9 DEFECT OF BLOWN FILM 11 WASTE MANAGEMENT IN BLOWN FILM 13 ADVANTAGE & DISADVANTAGES 16 CREATE MY OWN MIND MAP 18 EXERCISES 19 REFERENCES 21
1.0 INTRODUCTION Blown films extrusion is one of many polymer manufacturing processes. This method is used to produce commodity and specialized polymer films that are typically used in packaging such as shrink, stretch, barrier films (used to protect deli meat), frozen food packaging, and shopping bags. There are many types of polymers that can be used in blown film extrusion with the most common being polypropylene and polyethylene (LDPE, HDPE and LLDPE). This method can manufacture monolayer films as well as more complex multi-layer films that involve co-extrusion to combine more than one plastic into a single film. . 1
There are a few different types of dies used in blown film extrusion with the most common being annular, which is a simple circle die. The molten polymer enters the die head and air is injected via a hole in the die center to radially inflate the polymer into a thin tube that is many times its original, extruded diameter. It is this step of the process that can be adjusted to achieve the desired film thickness and width. 2 Blown film extrusion can be complex so we will now explain each step of the process in detail. The first step is to melt the polymer in an extruder. Polymer resin, often in the form of beads, is loaded into a hopper and fed into a heated barrel with a screw. The screw is used to transport the polymer down the barrel. The beads are gradually heated to melt the polymer. The heat profile is an important part of extrusion as the polymer may thermally degrade if overheated. When the molten material reaches the end of the barrel, it is extruded through a die. It is at this point that blown film extrusion differs from other extrusion processes
The hot tube film is then cooled, typically with high speed air, and pulled upward by equipment called nip rollers. On most medium to large size film lines, this vertical tube may extend several stories into the air. As the film cools it crystallizes at what is called the frost line. As the tube reaches the nip rollers, at the top of the line, the film is now cool enough to flatten and is then referred to as lay-flat or collapsed tubing. The film is then transported downstream by rollers for further processing (e.g. slit, printed, vented, converted into bags) and is eventually wound into rolls. 3
2.0 BLOWN FILM COMPONENT AND FUNCTION EXTRUDER The extruder consists of a resin feeding hopper, a heated barrel, a rotating screw, a screen changer, a die adapter and a base. DIE The film extrusion die is attached to the adapter. A good die design ensures smooth and complete melt flow, thus preventing resin degradation from overheating. The functions of the die are : 1. Force the melt into a form approaching its final shape. 2. Maintain the melt at a constant 3.Meter the melt at a constant pressure and rate to the die land for uniform film gauge, with allowance for gauge reduction. 4
AIR RING External air rings were the original method used to cool the blown film bubble COLAPPSING FRAME At its upper end, the cylindrical bubble is gradually flattened and guided to a nip roll assembly by a series of guide rolls or a forming tent. Collapsing angles should be kept to less than 11°. Collapsing frames generally are made of wood slats, metal rolls or coated hollow metal beams. The frame can be raised or lowered to meet specific film processing requirements. Air actuated collapsing frames provide a layer of air between the frame and the bubble to reduce distortion and wrinkles in the film. 5
GUIDE ROLL From the haul-off unit on top of the tower, the film web is fed down through a series of guide rolls. These rolls can have non-slip coverings to improve web tracking and improve release of tacky film surfaces. WIND UP ROLL Film is tightly wound generally onto a cardboard or metal tube called a core. The core is turned by a winder. Once a film roll is full, windup is switched to an empty core via a system of multiple stations. There re numerous types and designs of film a winders, characterized by: 1. Take-up roll drives, including surface, center or center/surface assist. 2. Film roll changing, including manual, semi-automatic or fully automatic. 3. Roll stand configuration, including stacked, face-to-face or back-to-back. 6
3.0 BLOWN FILM PROCESS Figure 3.0 Blown Film Process Plastic melt is extruded through an annular slit die, usually vertically, to form a thin walled tube. Air is introduced via a hole in the centre of the die to blow up the tube like a balloon. Mounted on top of the die, a high-speed air ring blows onto the hot film to cool it. The tube of film then continues upwards, continually cooling, until it passes through nip rolls where the tube is flattened to create what is known as a ' layflat' tube of film. This lay-flat or collapsed tube is then taken back down the extrusion ' tower' via more rollers. On higher output lines, the air inside the bubble is also exchanged. This is known as IBS (Internal Bubble Cooling). 7
Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film which is drawn down along the extrusion direction only. 8 The lay-flat film is then either kept as such or the edges of the lay-flat are slit off to produce two flat film sheets and wound up onto reels. If kept as lay-flat, the tube of film is made into bags by sealing across the width of film and cutting or perforating to make each bag. This is done either in line with the blown film process or at a later stage.
4.0 PRODUCT Blown films extrusion is one of many polymer manufacturing processes. This method is used to produce commodity and specialized polymer films that are typically used in packaging such as shrink, stretch, barrier films (used to protect deli meat), frozen food packaging, and shopping bags. One of the most common methods of film manufacture is Blown Film (also referred to as the Tubular Film) Extrusion. The process involves extrusion of a plastic through a circular die, followed by "bubble-like" expansion. The principal advantages of manufacturing film by this process include the ability to: • Produce tubing (both flat and gussetted) in a single operation • Regulation of film width and thichness by control of the volume of air in the bubble, the output of the extruder and the speed of the haul-off • Eliminate end effects such as edge bead trim and non uniform temperature that can result from flat die film extrusion • Capability of biaxial orientation (allowing uniformity of mechanical properties) • Blown Film Extrusion can be used for the manufacture of coextruded, multi-layer films for high barrier applications such as food packaging. 9
Blown film can be used either in tube form (e.g. for plastic bags and sacks) or the tube can be slit to form a sheet. Typical applications include Industry packaging (e.g. shrink film, stretch film, bag film or container liners), Consumer packaging (e.g. packaging film for frozen products, shrink film for transport packaging, food wrap film, packaging bags, or form, fill and seal packaging film), Laminating film (e.g. laminating of aluminium or paper used for packaging for example milk or coffee), Barrier film (e.g. film made of raw materials such as polyamides and EVOH acting as an aroma or oxygen barrier used for packaging food, e. g. cold meats and cheese), films for the packaging of medical products, Agricultural film (e.g. greenhouse film, crop forcing film, silage film, silage stretch film). Figure 4 : Product of blown film process 10
5.0 DEFECT OF BLOWN FILM PROCESS Bubble instability Not enough cooling, line speed too slow Bubble bouncing Inconsistent melt feed to die, dirty die, worn screw and/or barrel, air currents, or misalignment Die lines Inconsistent melt fed to the die, dirty die, misaligned die or air ring, dirty air ring, surging of the extruder, die heat variation, or nip rollers Gauge variations Dirty die, scratched die lips, insufficient blending of molten polymer, or inadequate purging Defect Cause 11
5.0 DEFECT OF BLOWN FILM PROCESS Streak Dirty die pin, rough roller surfaces, rough bubble guides and collapsing frames Inconsistent melt fed to the die, dirty die, misaligned die or air ring, dirty air ring, surging of the extruder, die heat variation, or nip rollers Uneven film width Air leakage from the bubble, bubble pumping or breathing, tension varies or is too high Melt fracture Splitty film Low or high blow-up ratio, die lines, high frost line, contamination, extrusion temperature too high or too low, insufficient cooling, or poor resin choice Wrinkles Non-uniform bubble, die and nip rolls not level, bubble not stable, misalignment between nip rolls and die, or improper winder tension Defect Cause 12
6.0 WASTE MANAGEMENT IN BLOWN FILM After the collection and sorting process, different recycling processes can be applied for the flexible plastic films waste treatment process. Mechanical recycling , chemical recycling and energy recovery will be discussed in this review. Methods of recycling are generally divided into four categories: primary, secondary, tertiary,and quaternary. Primary recycling is considered when the materials after recycling present equal or improved properties compared to the initial or virgin materials. On the other hand,when there cycled material obtained presents worsened properties than the virgin material, the method is called secondary recycling or down-cycling method. In the tertiary (also known as chemical or feedstock) recycling method, the waste stream is converted into monomers or chemicals that could be advantageously used in the chemical industries. 13
6.0 WASTE MANAGEMENT IN BLOWN FILM Recycling method correspond to the recovery of plastic as energy and is not considered as recycling in a true circular economy. It is important to note that there is a hierarchy in these four recycling methods, where the mechanical recycling is to be implemented first. Primary recycling is considered when the materials after recycling present equal or improved properties compared to the initial or virgin materials. When the recyclates present a decrease in the properties level, one may speak about the secondary or down-cycling method. In the tertiary (also known as chemical or feedstock) recycling method, the waste stream is converted into monomers or chemicals that could be advantageously used in the chemical industries. Finally, the quaternary (also known as thermal recycling, energy recovery, and energy from waste) recycling method correspond to the chemical recycling, offers the value chain optimal circularity, coupled with lower environmental impact. 14
6.0 WASTE MANAGEMENT IN BLOWN FILM Figure 5. Schematic representation of the recycling methods. In the context of circular economy thinking, the recycling of materials can be also categorized based on the product,which is manufactured from the secondary rawmaterials: Closed-loop recycling: Recycled materials are used to produce the same product from where they were originally recovered. Only recycled plastics or a blend between recycled and virgin plastics can be used to produce a new product. This type of recycling ensures that the product can be recycled continuously, and its recovered material can be added at the same rate . Open-loop recycling: Recycled materials are used for different applications than the product they were originally recovered from. However, this does not imply that the new application is of “lower value” . 15
7.0 ADVANTAGES AND DISADVANTAGES OF BLOWN FILM PROCESS ADVANTAGES Blown film generally has a better balance of mechanical properties than cast or extruded films because it is drawn in both the transverse and machine directions. Mechanical properties of the thin film include tensile and flexural strength, and toughness. The nearly uniform properties in both directions allow for maximum toughness in the film Blown film extrusion can be used to make one large film, two smaller ones, or tubes that can be made into bags. Also, one die can make many different widths and sizes without significant trimming. This high level of flexibility in the process leads to less scrap material and higher productivity. Blown films also require lower melting temperatures than cast extrusion. Measured at the die opening, the temperature of cast film is about 220°C, whereas the temperature of blown film is about 135°C. Furthermore, the cost of the equipment is approximately 50% of a cast line. 16
17 DISADVANTAGES Blown film has a less effective cooling process than flat film. Flat film cooling is done by means of chill rolls or water, which have significantly higher specific heat capacities than the air that is used in the blown film cooling process. The higher specific heat capacity allows the substance to absorb more heat with less change in the substance temperature. Compared to cast film, blown film has a more complicated and less accurate method to control film thickness; cast film has a thickness variation of 1 to 2% versus the 3 to 4% for blown film. The resins used for casting typically have a lower melt flow index, which is the amount of polymer that can be forced through a standard die in 10 minutes according to a standard procedure. The melt flow index for cast film is about 5.0 g/10 min whereas for blown film it is about 1.0 g/10 min[10]. Consequently, the production rates for cast film are higher: cast film lines can reach production rates of up to 300m/min whereas blown film lines are usually less than half this value. And finally, cast film has better optical properties, including transparency, haze, and gloss.
Create my own mind map 18
EXERCISE 1: STRUCTURED QUESTIONS 1. Define blown film process. 2. List the material used for blown film process. 3. State the application product of blown film process. 4. Give TWO (2) advantages and disadvantages of blown film process. 19
EXERCISE 1: STRUCTURED QUESTIONS 5. With the aid of diagram, Sketch completely blown film process. 6. Sketch the schematic diagram for recyling method. 20
21 REFERENCE Pankaj M.Patil,&Prof. D.B. Sadaphale (2018) A Study of Plastic Extrusion Process and its Defects, International Journal of Latest Technology in Engineering, Management & Applied Science (IJLTEMAS). John Goff,Tony Whelan(2000) “Extrusion Processors Handook”, Dynisco,2nd edition. “A Guide to Polyolefin Film Extrusion” , http://www.lyondellbasell.com J G Khan ,R S Dalu & S S Gadekar (2014) “Defects in extrusion process and their impact on product quality, IJMERR. Paul Waller . “Causes and Remedies to 7 Instability Patterns in Blown Film Extrusion”, Plastics Touchpoint Group, Inc., Thornhill, Can How Recycled Plastics are Used for Blown Film Extrusion https://www.pearltechinc.com
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