Ashter Thermoforming of Single and Multilayer Laminates

Plastic Films Technologies, Testing, and Applications
1. Auflage 2013
ISBN: 978-1-4557-3186-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 352 Seiten

Reihe: Plastics Design Library

Thermoforming of Single and Multilayer Laminates explains the fundamentals of lamination and plastics thermoforming technologies along with current and new developments. It focuses on properties and thermoforming mechanics of plastic films and in particular single and multilayered laminates, including barrier films. For environmental and economic reasons, laminates are becoming increasingly important as a replacement for solid sheets and paint finishes in many industries, including transportation, packaging, and construction. Yet the processes of film formability during the extensive deformation and elevated temperatures experienced in conventional processing technologies, such as thermoforming, are poorly understood by most engineers. This book covers production processes, such as extrusion, calendaring, and casting, as well as mechanical and impact testing methods. It also describes how testing protocols developed for metals can be leveraged for plastic films and laminates, and includes a thorough discussion on methods for performing optical strain analysis. Applications in transportation vehicles and packaging, including packaging for food, medical and electronics applications, sports equipment, and household appliances, are discussed. Safety, recycling and environmental aspects of thermoforming and its products complete the book. - First comprehensive source of information and hands-on guide for the thermoforming of multilayered laminates - Covers applications across such sectors as automotive, packaging, home goods, and construction - Introduces new testing methods leveraging protocols used for metals

Dr. Syed Ali Ashter is the Founder of Ashter Enterprise, LLC, a material evaluation, product development and quality compliance consulting company, which he founded in 2020. He also works as a Senior Engineering Specialist at B. Braun Medical Inc. in Irvine California. He has served on the Board of Directors for the Society of Plastics Engineer's Medical Plastics Division (MPD) since 2012. Recently, he has completed 2-year term as the chair of the Medical Plastics Division. He has authored four Plastics Engineering books in the Plastics Design Library Handbook Series.

Autoren/Hrsg.

Ashter, Syed Ali

Leseproben

2

The Thermoforming Process

Since the beginning of the century techniques to form sheets, with materials such as metal, glass and natural fibers, have been known. The true thermoforming principles emerged as thermoplastic materials were developed, which happened during World War II. The post-war period brought about mass commercialization and rapid development of equipment and machinery capable of adapting to modern manufacturing methods, to make more useful and income-yielding products. A typical thermoforming process begins when a plastic sheet is heated slightly above the glass transition temperature, for amorphous polymers, or slightly below the melting point, for semi-crystalline materials.

The primary focus of this chapter is to provide a broad overview of the basic principles of thermoforming and the theory of forming process. An in-depth review of different thermoforming machinery is also presented.

Keywords

Laminates; forming process; vacuum forming; heavy-gauge; single-station; dual-station; multiple station; and plug-assist

Chapter Outline

2.1 Background

2.2 Basic Principles of Thermoforming

2.3 Difference between Plastic Sheets and Laminates

2.4 Theory of Forming Process

2.5 Forming Characteristics

2.6 Machinery

2.6.1 Single-Station Machine

2.6.2 Dual-Station Machine

2.6.2.1 Multiple-Station Machine

2.6.3 Heavy-Gauge Forming Process

2.6.3.1 Shuttle Press

2.6.3.2 Cabinet Press

2.6.3.3 Rotary Thermoforming Press

2.6.4 Elements of Heavy-Gauge Machinery

2.6.4.1 Sheet Handling

2.6.4.2 Sheet Clamping

2.6.4.3 Ovens

2.6.4.4 Forming Press

2.6.4.5 Pneumatic Prestretching

2.6.4.6 Plug-Assist Prestretching

2.6.4.7 Load/Unload Elements

2.6.4.8 Loading and Unloading Sheet

2.6.4.9 Vacuum Box and Vacuum Systems

2.6.5 Machinery for Light-Gauge Forming Process

2.6.6 Standard Roll-Fed Machines

2.6.7 Contact Heater Machines

2.6.8 Rigid-Form-Fill Seal Operation

2.6.9 Extrusion-Forming Lines

2.6.10 Matched Mold-Forming Machines

2.6.10.1 Foam Polymer Machines

2.6.10.2 Composite and Composite Laminate Machines

2.6.11 Wheel Machines

2.6.12 Clamping Mechanism

References 

2.1 Background

Since the beginning of the century techniques to form sheets, with materials such as metal, glass and natural fibers, have been known. However, the true thermoforming principles emerged as thermoplastic materials were developed, which happened during World War II. The post-war period brought about mass commercialization and rapid development of equipment and machinery able to adapt to modern manufacturing methods to make more useful and income yielding products.

In the 1950s, the volume of thermoplastic material production and its products reached impressive figures. In the 1960s and 1970s, the foundations for the future of the thermoforming industry were established. Equipment manufacturers began creating machinery capable of producing about 100,000 thermoformed individual containers per hour. Sophisticated controls were also required.

Since the 1980s, thermoformers have gone beyond those expectations and established production lines that can produce finished thermoformed products, not only from sheets but also from resin pellets. Additionally, they are able to recycle the scrap with minimum control. Equipment has been computerized and at present, it can perform auto-monitoring and diagnostic functions.

Today complex equipment relies on only one worker to control it. Thus, it is expected that the thermoforming industrial labor market will undergo a shortage of technically trained and experienced personnel, since traditional knowledge will no longer be enough. Therefore, lectures, seminars, courses, etc., are needed to further advance this well-established industry.

Many of the thermoformed products in use today have been manufactured to replace their original use forms. This has taken place so quickly that the original products have almost been forgotten. For example, it is not easy to remember what hamburgers were packed in before the arrival of the one-piece polystyrene package or what kind of material lined the interior of refrigerators.

2.2 Basic Principles of Thermoforming

A typical thermoforming process begins when a plastic sheet is heated slightly above the glass transition temperature, for amorphous polymers, or slightly below the melting point, for semi-crystalline materials. Although both amorphous and semi-crystalline polymers are used for thermoforming, the process is easiest with amorphous polymers because they have a wide rubbery temperature range above the glass transition temperature [1].

The heating is achieved using radiant heaters until the temperature reaches the forming temperature of the sheet. Once the sheet has been heated, it is forced against the mold-cavity contours, either pneumatically or mechanically. One way is by applying a vacuum in the mold cavity, which stretches the sheet until it touches the mold surface. The main issue with this forming process is the irregular thickness distribution across the part and is often corrected by using a mechanical plug.

2.3 Difference between Plastic Sheets and Laminates

Plastic sheets are produced when a continuous polymeric material is processed through an extruder. The plastic sheets vary in thickness and lengths. Laminates are composite material consisting of two or more layers bonded together to achieve improved strength, stability and appearance. These layers can be permanently assembled by heat, pressure or adhesive. However, laminates typically exist as plastic, and when an appliqué is required in the final product it is called “decorative laminate.” There are as many different types of laminates as there are possible combinations of two or more materials. Phenolic, epoxy, polyester, Diallyl Pthalate, Melamine, Silicone and Polyamide are the most common laminates in use today.

Decorative laminates are produced by applying appliqué onto the thermoformed laminates using an injection-molding process:

1. First, laminates are thermoformed when a vacuum is applied to a preheated laminate, stretching it to take the shape of the desired part. Vacuum holes or slot gaps are used for most parts, and pressure assist is used where high detail is required due to small radii or flatter part areas where it is difficult to evacuate air.

2. Second, appliqués are loaded in the cavity portion of the mold. Openings or cutouts aid in location and prevent the appliqué from moving during the injection phase of molding. Gate locations allow the melt to flow evenly to the parting lines while pinning the appliqué to the mold surface [2].

High-pressure laminates consist of superimposed layers of a thermoset, resin-impregnated or resin-coated filler bonded together by heat and pressure. A minimum of 7.6 MPa and maximum of 13.8 MPa pressure is used for high-pressure laminating. The heat and pressure during lamination creates a chemical reaction that causes the entire laminate to cure into a hard, nearly homogeneous, insoluble mass. After thermoset resins have polymerized, they cannot be re-softened or reshaped by heat or solvents [3].

The in-mold lamination process employs a multilayered laminate positioned in the parting plane to be overmolded by the polymer melt on the inner side. The decorative laminate can be placed in the mold as a cut sheet, pulled from the roll with a needle gripper or by means of a clamping frame method. By means of a thermoforming operation, the clamping frame method allows defined predeformation of the decorative laminate during the mold-closing operation.

For in-mold lamination, the outer, visible layer of the decorative laminate can be made of polyester, PA, PP, polyvinyl chloride (PVC), or acrylonitrile-butadiene-styrene (ABS) film, cotton textile or leather. This outer layer typically comes with a variety of features to create an appearance or feel. To provide the product with a soft-touch effect, there is typically an intermediate layer of polyurethane (PU), PP, PVC or polyethersulfone (PES) foam between the top layer and liner layer.

The advance composite process (developed by Advance USA, East Haddam, Connecticut) is an adaption of thermoforming for production of composite structures. It is used, for example (by Hunter Marine Corporation, Florida), for production of 5.2 m and 6.4 m sailboats, with a multilayer composite comprising: (a) an outer layer of weather-resistant acrylonitrile/styrene/acrylate (ASA), coextruded with a structural layer of ABS, (b) a rigid polyurethane foam core and flotation layer and (c) an inner layer of a polyurethane/glass cloth laminate. The hulls produced are claimed to have five times the impact...