Brandau Bottles, Preforms and Closures

A Design Guide for PET Packaging
2. Auflage 2012
ISBN: 978-1-4377-3528-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 180 Seiten

Reihe: Plastics Design Library

As a consultant to the plastics industry, Ottmar Brandau's focus is on using his engineering knowhow and production management experience to improve quality and productivity, cut down cycle time and introduce secondary processes such as inline printing. 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. In this book, Brandau covers the engineering aspects of bottle production and the relevant production processes (focusing on blow molding), along with plant layout and organization and production management, to produce the definitive handbook for engineers and managers alike. - Learn the tricks of the trade from an experienced engineer and manager - Save money: Practical strategies to improve cycle times - Increase productivity: Improve plant layout and organization and implement secondary processes such as inline printing

Ottmar Brandau is President of Apex Container Inc. in Ontario, Canada. Brandau is a member of the Society of Plastics Engineers and was previously VP Operations at Magic North America Inc.

Autoren/Hrsg.

Brandau, Ottmar

Leseproben

Chapter 2
PET Preforms
Dr. Laura Martin formerly with Husky Injection Molding Systems Ottmar Brandau Apex Container Tech Inc. Chapter Outline
2.1 Introduction 2.2 Manufacture and States of PET 2.2.1 Manufacture of PET 2.2.2 Catalysts 2.2.3 PET is a Linear Condensation Polymer 2.2.3.1 Intrinsic Viscosity 2.2.3.2 Copolymer Content 2.2.4 Crystallization of PET 2.2.5 “Extended Chain” or “Oriented” Crystallization 2.2.6 Summary 2.3 Behavior in the Blow Mold 2.3.1 Natural Stretch Ratio (or Natural Draw Ratio) 2.3.1.1 Elastic Deformation 2.3.1.2 Yielding 2.3.1.3 Relevant Parameters 2.4 Manufacture of PET Preforms 2.4.1 Drying of PET 2.4.2 The Theory of Injection Molding of Preforms 2.5 Preforms for Single- and Two-stage Processing 2.5.1 Two-stage Process Injection Molding 2.5.2 Two-stage Process Blow Molding 2.5.3 Single-stage Process 2.5.4 Hot Runner Controls 2.5.5 Gate Mechanism 2.6 PET and Infrared Radiation 2.1 Introduction
There is probably no subject in the PET industry shrouded in more mystery than the design of preforms. There is no handbook, no course, and very little other material that a prospective preform designer may peruse to get prepared for the job. The main reason is that preform design is still a “black art,” and no calculation or simulation can guarantee a perfectly suitable preform for a given bottle design and blow machine. Our guess would be that to this day no 48 or more cavity system is built without first performing a trial run with a single-unit cavity. This is despite the fact that we understand material properties fairly well, and resin companies as well as independent laboratories offer material characterizations that detail stress/strain graphs for a variety of material conditions. The unpredictable factor is that the formation of a bottle from a preform is literally explosive, and even tiny temperature variations affect the outcome. In addition, the parameters that control the inflation characteristics of a particular preform are manifold, still too many to make perfect predictions. Here is a partial list of the parameters that make the difference between failure and success: • The preform temperature: PET can be blown at a range of 95–115 °C (203–239 °F). • The temperature distribution: The profile of temperatures both in the vertical axis as well as through the preform wall is the result of many factors characterizing the oven system in a reheat stretch blow machine or the hot runner system in a single-stage machine. We do not yet have models that can truly reflect them. • Process conditions, such as stretch rod speed, and timing of primary and secondary air pressure result in different bottle wall thickness outcomes. • Venting and the shape of mold corners affect preform inflation as well. Figure 2.1 Various parts of injection tooling. Picture courtesy of Mold-Masters. Needless to say, even the best preform designs need experienced processors to dial in a specific blow machine. Two machines of the identical model using the same preforms and tooling will require slightly different setups to account for subtle differences in the machine characteristics, including such things as screw wear and infrared lamp age. This can only be accomplished by experimental development, and experience is invaluable. In practice, less than perfect preforms are often used to make an acceptable bottle because preform designers typically add a few grams of material to allow for the factor of uncertainty and still make bottles which can be sold. The cost of the added weight is offset by a wider process window and lower scrap rates. Our guide sheds some light on the methodology that a designer might go through to come up with a preform suitable for a given bottle shape and wall thickness distribution. There are other ways of getting the same or similar results, and our methodology is by no means the only one. What most experienced designers do is to look up a similar bottle and then modify the preform to adjust for the slightly different shape of the new bottle. Rather unscientific but practical! Newcomers in the field do not have this luxury and are often stuck with whatever preforms they can buy from vendors. They then try a preform with the right or a similar weight and make adjustments to the design as necessary. With the help of our guide this task should become easier. We will explore all relevant material characteristics that are paramount to understand the inflation behavior of the preform. We then take you through the design process step by step and point out differences between preforms for the single- and two-stage process. In any case, you should always make a trial cavity and perform blow molding trials to determine whether your design works before committing to a multicavity tool! The next section gives some introductory information about PET that will help understand the terms used in the following sections. The section on Behavior in the Blow Mold is critical to understand the factors at play when designing preforms. The remaining sections in this chapter will help to give the designer and processor a feeling of the overall steps of the bottle-making process, which will be of practical use when developing or troubleshooting a container. The best practitioners of PET design and process development have to understand all the steps because every step has an influence on the properties of the final container. 2.2 Manufacture and States of PET
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 unit shown in Fig. 2.2. Figure 2.2 The ring structure makes PET tough while the ethylene component gives it flexibility. A monomer is a single unit that is repeated to form a polymer chain (Greek “mono,” one; “meros,” part). Polymerization is the name given to the type of reaction 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 comonomer) to change some of the performance properties of the resin. This can be represented by: homopolymerAAAAAAAAAAAAAAAAAAA copolymer ABAAABAAAAABAAABBAA PET is manufactured as a homopolymer or copolymer. 2.2.1 Manufacture of PET
There are a few chemical routes to manufacture PET, but basically a compound with two acids, such as terephthalic acid (TPA), is esterified with a compound with two alcohols, ethylene glycol (EG). Because 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 understand much of the behavior of PET (Fig. 2.3). Figure 2.3 An alcohol and an acid form the ester groups of PET that make it a polyester. Commercially the polymerization is done in two stages. Melt phase condensation results in molten polymer with about 100 repeat units [intrinsic viscosity (IV), as explained subsequently, is about 0.6]. The melt is pelletized and can be used for some applications such as in fiber at this point. To continue the polymerization, a process called “solid stating” is needed. Solid stating produces high molecular weight PET needed for fabricating bottles. 2.2.2 Catalysts
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 (DEG) or acetaldehyde (AA), or to improve the color. Because 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 dimethyl terephthalate (DMT) process (used chiefly by Eastman) requires an additional catalyst, which may result in a greater tendency of the resin to...