PETpla.net Insider 06 / 2013

BOTTLE MAKING 31 PET planet insider Vol. 14 No. 06/13 www.petpla.net situations such as these the process commences with scanning the exist- ing bottle using a 3D laser scanner. In a so-called “reverse engineering” procedure a surface reconstruction is then carried out, which will result in a new CAD data record for the bottle to be produced. Creation of a new design Based on the information gathered in the discussions described in section 1.1, the designer will generate a first design concept for the new bottle on a CAD system. This process is called design engineering and results in an exact bottle geometry on the basis of the intended “look” of the bottle, using appropriate design elements, such as logos, waistlines, decorative elements, etc., and also taking into account the technical requirements described in section 1.2. In this process, the designer will also ensure that the outer contour of the bottle is designed for manufacture. For example, undercuts and sharp edges combined withnextremely thin walls and high machine outputs should be avoided to prevent the material from tearing at these spots. If sharp edges are explicitly desired, e.g., for design elements such as logos, these may not be achieved by milling. In such cases, techniques such as laser engraving should be applied. The output of this design engineer- ing stage is a 3D CAD data record that describes the outer contour of the bottle, initially without wall thick- nesses. State-of-the-art CAD systems allow visualization of the container as a 3D free-form surface model that can also include class-A surfaces. Class-A surfaces are surfaces that model not only tangentially steady (or G2-steady) transitions but also curvature-steady (or G3-steady) transitions. Such sys- tems, which are commonly used in the automobile industry, even permit a visualization of the gloss or the refrac- tion of light. Ideally, CAD systems that support the process approach are used, such as Unigraphics NX. These integrated systems also include other perfor- mance features that go beyond the mere capturing of the bottle geometry, i.e., the mere constructive design of the bottle. Apart from the output of mill- Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau 115,00 € 180 pages © Copyright Elsevier 2012 ing data for CAM, examples include the simulation of the mechanical behaviour of the bottle through finite- element method (FEM) analysis (see also section 1.5). Because Unigraphics NX, as our example, is a knowledge- based system, the expertise referred to above can be incorporated in “digital process assistants.” These systems are thus not only suitable for a descrip- tion of the cavity but also allow the design of the complete mould with all its components and the related cooling. One important aspect of the CAD output is the visualization of the bottle design in various forms for the approval process. The simplest case is a digital mock-up or photo render- ing. In this case, the finished bottle, sometimes even with its label, is dis- played in a photorealistic 3D view. This is possible on the monitor with various levels of quality (e.g., with or without transparency effect), both statically and dynamically. The bottle can be rotated against various backdrops (showroom effect). Customers who cannot attend the presentation in person, can be sup- plied with the dynamic photo rendering in the form of a digital video, e.g., in the Quick-Time or AVI format. A more demanding visualization is the output of the bottle geometry in a real, true-to-measure model, e.g., through stereolithography. For this pur- pose, a laser beam or UV light is used to harden a special resin in layers from the bottom upward so that a complete model of the bottle is generated. As a rule, this process can be completed in several hours or a single day. To get an even better impression of the optical appearance of the bottle, a 1:1 model made of solid polymethyl methacrylate can be produced. This process will take significantly longer because the model bottle is first milled on the basis of the CAD data and then has to be polished with a great deal of manual effort. On the other hand, the optical appearance of the acrylic model will be very close to that of the final bottle. High-end CAD systems are also able to map textures such as orange peel, condensed water droplets, or even animal skin, flake structures, or technical grains. Such textures, which have long been used in the automobile industry, are now increasingly finding their way into consumer goods. Often they not only result from aesthetic considerations but also have clear marketing reasons. For example, such graining may lead the consumer to grip the bottle exactly at predefined zones. Major mould makers usually have a large stock of different textures so that the customer can use sample bottles to see, touch, and select the desired effect. Of course, customer- specific ideas for the texture can be implemented as well. A high-contrast black and white picture in good quality will, as a rule, be sufficient input for the incorporation of a texture into the bottle design. If the customer is happy with the visual appearance of the container, the next step in the process can be taken, which is the transfer of the design fea- tures into a bottle design that are safe for the process. If not, the container geometry will be optimized in iterations until both the customer and the sup- plier are satisfied. *This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 1.2., Copyright Elsevier 2012