Thermoforming is a generic term encompassing many techniques for producing useful plastic articles from flat sheet. In its simplest concept, thermoforming is simply the manual draping of a temporarily softened sheet over a simple mold shape. In one of its more advanced forms, it involves automatic high-speed indexing of a freshly extruded sheet having very accurately known temperature into a forming and in-situ trimming station, with integral web regrind and automatic parts counting, packaging and shipping. In another, it involves automatic placement, plug and/or pneumatic stretching and pressure forming, with multi-axis router trimming.
If a polymer can be produced as a sheet, it can be thermoformed into a product [1,2]. Polymers are high molecular weight organic molecules that are produced by combining very pure carbon-based simple molecules under heat, pressure, and catalyst systems. There are more than 20 major classes of polymers available today [3] and many sub-classes, made by combining polymers with polymers, polymers with fillers and reinforcements, and polymers with additive and processing aids [4]. In order to achieve thermoformed parts having commercially interesting combinations of physical properties, it is necessary to understand the way in which basic polymer architecture affects material properties.
Once the plastic sheet temperature is within the forming window, it is ready to be stretched. There are many ways of stretching and prestretching the sheet, as detailed in Chapter 1. Vacuum, air pressure, mechanical aids such as plugs, rubber di aphragms, and combinations of these are used to shape the rubbery sheet against the mold surface. The extent to which a given polymer at a given temperature can be stretched limits the ways in which it can be thermoformed. Part design, especially local part wall thickness, depends on the extent of polymer deformation. Deep drawing, drawing into sharp corners, and replication of mold surface details such as patterns or lettering, require polymers that can be rapidly and uniformly stretched. Chapter 9 examines part design in greater detail.
Once the part has been heated and formed to shape by contact with the cool mold surface, it must be cooled and rigidified. Then the part and its web must be separated by trimming. When thermoforming a reactive polymer such as thermosetting polyurethane or a crystallizing one such as nucleated CPET, the formed shape is rigidified by holding it against a heated mold to continue the crosslinking reaction or crystallization. In most cases, rigidifying implies cooling while in contact with a colder mold. For automatic thin-gage formers, the molds are usually actively cooled with water flowing through channels. Free surfaces of medium- and heavy-gage sheet are frequently cooled with forced air, water mist or water spray. For amorphous polymers, cooling of the formed part against a near-isothermal mold rarely controls the overall thermoforming cycle. For crystalline and crystallizing polymers such as PET, PP and HDPE, the cooling cycle can be long and can govern overall cycle time.
The thermoforming mold can be as simple as a smoothed block of wood over which a heated sheet is draped. Many production molds may be as complicated as injection molds and may include:
Moving elements,
Articulated plugs,
Sophisticated means for isolating sheet over multicavities,
Means for ejecting formed sheet from mold cavities,
It is apparent that there are many variations on the basic sheet stretching process and that there are many potential polymers from which to choose. As a result, design of thermoformed parts must of necessity follow careful protocol. In this chapter, the philosophy of parts design forms the basis for the specific details on parts design. Prototyping is usually the first step in any molding program. It rarely succeeds without problems and these problems frequently reveal potential problems in production processing. Many typical processing problems and courses of action are highlighted in the trouble-shooting section and some general do’s and dont’s are presented.
To produce their final products, injection molders, extruders and blow molders begin with polymer resin in the form of pellets or powders. Thermoformers produce their final products from sheet or film purchased from a converter or the processor who converts the polymer resin. The production of the sheet or film by extrusion or one of a handful of allied processes that impart shear and heat to the polymer resin adds economic and physical property penalties to the process. In addition, a substantial portion of the sheet or film is not formed into product and that portion must be returned to the converter for additional reprocessing. While the details of the conversion process are not of paramount importance to the thermoformer, the quality of the sheet or film is. The emphasis in this chapter, then, is on an understanding of those aspects of the conversion process that most affect the performance of the sheet as it passes through the thermoforming process:
It has been observed that there have been more advances in certain aspects of thermoforming in the last few years than in the previous five decades [1]. The many reasons for this were detailed in Chapter l. Certainly new business opportunities have led to substantial improvements in heating and forming methods. In many cases, new products have required dramatic modifications in processing techniques.
The business of thermoforming depends on several fundamental concepts. Modern machines are designed to produce parts, repetitively, day in and day out, with relatively little maintenance or attention. Businesses require that these machines make money. Profitable businesses require that the machine make quality parts that can be sold at values greater than their total manufacturing costs. Profit is the expected return on investment for taking the risk of being in business. The keys to business success are quality and accountability. To make products from sheet, the interaction of the sheet with the process parameters should be thoroughly under stood. The quality of the product depends on:
• The skill of the operator,
• The selection of quality raw materials, and
• An adequate quality control of all incoming and outgoing materials
