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Stretch blow molding is the blow molding process used to produce bottles of the strength required for carbonated drinks. In this handbook, Ottmar Brandau introduces the technology of stretch blow molding, explores practical aspects of designing and running a production line and looks at practical issues for quality control and troubleshooting. As an experienced engineer, manager and consultant, Brandau’s focus is on optimizing the production process, improving quality and ...
Stretch blow molding is the blow molding process used to produce bottles of the strength required for carbonated drinks. In this handbook, Ottmar Brandau introduces the technology of stretch blow molding, explores practical aspects of designing and running a production line and looks at practical issues for quality control and troubleshooting. As an experienced engineer, manager and consultant, Brandau’s focus is on optimizing the production process, improving quality and reducing cycle time.
This book is a thoroughly practical handbook that provides engineers and managers with the toolkit to improve production and engineering aspects in their own businesses--saving money, increasing output and improving competitiveness by adopting new technologies.
The idea of reheating a thermoplastic material and then stretching it to enhance its properties was first employed in extruded sheet in the 1930s. However, it took until the 1970s for Nathaniel Wyeth and his staff at DuPont to blow the first polyethylene terephthalate (PET) bottle from an injection molded PET preform. At the same time, Bekum Maschinenfabriken in Germany had commercialized a similar process, stretch-blow molding an extrusion blow-molded PVC preform in what is now known as a single-stage process. Oriented PVC has oxygen and water barriers, and even carbonation retention, similar to PET. Bekum's OPVC machines featured a double carriage where one side blew a preform from an extruded parison that was then transferred to the other side where the bottle was stretched and blown (see Fig. 1.1). This yielded a lightweight bottle with superior properties and was successfully used to produce a variety of containers. However, PVC became environmentally suspect and PET is not suited to a process that requires what extrusion blow molders call 'hang strength', the ability of the material to sustain shape at melt temperature against gravity. Another problem with the PVC process was its inability to be scaled up easily.
Meanwhile, several US-based companies had developed machinery to produce stretch-blown PET bottles. Cincinnati Milacron's RHB-5 machine reheated preforms neck-side up, in four lanes, then stretched and blew them in a four-cavity mold. All molds moved at the same time and machines of this type are referred to as linear or in-line machines. Initially, output was limited to 2800 bottles per hour (bph) but later versions boosted output to 4000 bph before Cincinnati stopped producing them in the early nineties.
Meanwhile in Europe, the German company Gildameister (later to become Corpoplast and today KHS Corpoplast) and the French company Sidel were developing machines for PET production (see Fig. 1.2). Sidel produced extrusion blow molding machines using horizontal wheels. In a wheel machine, each individual mold cavity opens and closes in sequence and machines of this type are called rotary machines. In the late 1970s, Sidel started experimenting by using this concept in the PET stretch blow molding process. By 1980, Sidel had built the first prototype that would initiate unparalleled success in the blow molding industry, propelling Sidel from a mid-size machine manufacturer to a billion dollar company.
Today, companies such as Krones, Smiform, and SIPA have all developed rotary machines of their own and this competitive pressure has driven prices down, opening new applications for bottle blowing. Blow molding speeds have also driven costs down: While 1000 bottles/cavity/hour (b/c/h) was the benchmark for many years, today's machines feature outputs of up to 2200 b/c/h.
The first 'killer application' for PET was the 2-l bottle for carbonated soft drinks (CSD), introduced in 1978. The first bottles featured a dome-shaped bottom ideally suited to sustain internal pressures that routinely reach 5 bar (70 PSI). This required an additional plastic component, called a base cup, to be glued to the bottom in a secondary operation in order for the bottle to stand up. However, cost and recycling considerations (glue residue) encouraged the development of a one-piece bottle. The breakthrough came with the design of the so-called Petaloid base: a thick, mostly amorphous center disk surrounded by five blown feet. Granted as patent to the Continental Can Company in 1971, it caused controversy with three other patents and litigation ensued for several years. It took until the early 1990s before one-piece bottles came off the conveyors of reheat stretch-blow molding machines and were completely replaced two-piece bottles within a few short years.
By the mid-1990s, soft drink companies agreed to lower shelf-life requirements and so opened the way for the extremely successful launch of 20 oz and 500ml containers. At the time of writing, water and a whole new line of beverages that did not even exist a few years ago, are the key drivers for PET growth. Hot-fill juices and the so-called Neutraceuticals have raised the demands imposed on today's PET bottles and the industry has responded with a wealth of new technologies. Recent developments aim to eliminate the unsightly vacuum panels needed for controlled shrinkage of the PET bottle during cooling of a hot-filled product. Multi-layer preforms and coating technologies increase shelf life and, therefore, open the way for even smaller CSD packages and the replacement of glass in a new set of applications. At this time, it is unclear whether coatings or multilayer technologies will prevail as the preferred choice of packaging, but it is this author's opinion that a variety of methods will be required to meet an ever-increasing variety of packaging demands.
On the horizon we can see PET entering the retort arena, used for packages that typically need exposure to 125 °C (257 °F) for a number of minutes and are currently all filled in cans and glass. The PET bottle's crystallinity levels will have to be substantially increased to allow the use of PET here. At this time, the highest temperature PET is being exposed commercially is 95 °C (203 °F). Barrier enhancements will allow extended shelf life milk and other goods that require a long shelf life to be packaged in PET. Improved ways of injecting preforms and blowing bottles will extend the industry's ability to deliver a safe, environmentally sound, and economical package to consumers.
2.1 Manufacture and States of PET 5 Manufacture of PET 6 Catalysts 7 PET is a Linear Condensation Polymer 7 Intrinsic Viscosity 7 Co-polymer Content 8 2.2 Crystallization of PET 8 "Extended Chain" or "Oriented" Crystallization 10 Summary 11 2.3 Drying of PET 12 2.4 Behavior in the Injection Mold 14 2.5 Behavior in the Blow Mold 17 Natural Stretch Ratio (or Natural Draw Ratio) 17 Elastic Deformation 18 Yielding 19 Relevant Parameters 20 Property Data for PET 20 2.6 Acetaldehyde (AA) in PET Bottles 20 AA Creation 21 AA in Water Bottles 23
2.1 Manufacture and States of PET
Polyethylene terephthalate (PET) belongs to the group of materials known as thermoplastic polymers. The application of heat causes the softening and deformation of thermoplastics. In contrast, thermosets cure or solidify with the application of heat, and simply burn with continued heating.
Like all polymers, PET is a large molecule consisting of chains of repeating units. The PET used for bottles typically has about 100–140 of the repeating units shown in Fig. 2.1.
A monomer is a single unit, which is repeated to form a polymer chain (Greek 'mono' one; 'meros' part). Polymerization is the name given to the types of reactions where many monomer units are chemically linked to form polymers ('polys' many).
A resin with only one type of monomer is called a homopolymer. Copolymer resins are the result of modifying the homopolymer chain with varying amounts of a second monomer (or co-monomer) to change some of the performance properties of the resin. This can be represented by:
homopolymer AAAAAAAAAAAAAAAAAAA co-polymer ABAAABAAAAABAAABBAA
PET is manufactured as a homopolymer or co-polymer.
Manufacture of PET
There are a few chemical routes to manufacturing PET. A compound with two acids, such as terephthalic acid (TPA), is esterified with a compound with two alcohols, such as ethylene glycol (EG) (see Fig. 2.2). Since there are two functional groups on each component, they can continue to link up to form long chains. Water is a by-product of this process. This esterification reaction is reversible, and this is the key to understanding much of the behavior of PET.
Commercially, the polymerization is done in two stages. Melt-phase condensation results in molten polymer with about 100 repeat units (intrinsic viscosity, as explained below, is about 0.6). The melt is pelletized and can be used for some applications such as fiber production at this point.
To continue the polymerization, a process called "solid stating" is used. Solid stating produces high-molecular-weight PET needed for fabricating bottles.
Different catalysts are required for the two main chemical routes to manufacture PET. Special catalyst combinations can be used to influence the side reactions, to reduce the amount of diethylene glycol or allyl alcohol, or to improve the color. Since the catalyst residues remain in the PET, they are still present during drying and processing. Therefore, different grades of PET from different manufacturers react differently if not processed at optimum conditions. For example, the DMT process (used chiefly by Eastman) requires an additional catalyst, which may result in a greater tendency for the resin to oxidize or 'yellow' when over-dried.
PET is a Linear Condensation Polymer
PET does not branch: each molecule is a long "linear" chain. In addition, because it is formed by a reversible condensation reaction, it has a very simple distribution of molecular weights or chain lengths. As far as end users are concerned, the result is that the chemical structure of a grade of PET can be described by only two measures: IV (intrinsic viscosity), which is a measure of molecular weight, and the co-polymer content. In contrast, a polymer such as polyethylene can have unique molecular weight distributions and widely varying degrees and types of branching, which affect processing and performance profoundly.
The properties of the PET polymer are largely dependent upon the average molecular weight or the average number of repeat units of the polymer chains. This is usually determined by the measurement of intrinsic viscosity (IV). The relationship between molecular weight and IV is fairly linear.
High IV PET has a higher molecular weight than low IV PET. The longer chains give the resin better properties in the final product and also affect the processing in predictable ways. IVs used for PET bottle manufacturing are in the range of about 0.73–0.86.
PET co-polymers are made by replacing a few percent of one of the starting components with a different monomer. Eastman uses CHDM (cyclohexane dimethanol) to replace part of the DEG. Most other resin manufacturers use IPA (Isophthalic Acid), which is also called PIA (purified isophthalic acid), to replace part of the TPA. The co-polymers therefore have structures such as this:
PET TETETE ... PET-co-CHDM TETCTE ... PET-co-IPA TEIETE ...
DEG, a by-product of the polymerization reaction, is another comonomer which lowers the melt temperature but is not as effective at slowing down crystallization rates. DEG takes the place of EG in the chain.
Several advantages are gained by using the co-polymer, especially in preform molding applications:
(1) Co-polymers crystallize more slowly than homopolymers, making it easier to fabricate clear preforms (see Crystallization).
(2) Co-polymers are easier to melt in the extruder as a result of the lower melting point and lower maximum degree of crystallinity.
(3) Co-polymers impart better stress-crack resistance to the bottle (see Embrittlement and Stress Cracking).
Some of the generalized effects of IV and co-polymer content are outlined in Table 2.1.
2.2 Crystallization of PET
PET is a semi-crystalline resin. The word 'crystalline' refers to a region of ordered chain arrangement, as opposed to 'amorphous' where the polymer chains lack order (Fig. 2.3). Melted PET, by definition, is amorphous.
Excerpted from Stretch Blow Molding by Ottmar Brandau Copyright © 2012 by Elsevier Inc.. Excerpted by permission of William Andrew. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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1. A Short history of Stretch Blow Molding
2. Material Basics
3. Reheat Stretch Blow Molding Machines
4. Machine Details
5. Blow Molds
6. Fundamentals of the Blow Process
7. The Blowing Process
8. Injection Stretch Blow Molding Machines
9. Special Applications
10. Troubleshooting of Blowing Problems
12. Preform Design for Single and Two-stage Processing
13. Auxiliary Equipment
14. Training of Operators