Basics The thermoforming process By Martin Barth Illig GmbH & Co. KG Heilbronn, Germany Based on several process steps, thermoforming allows the production of a dimensionally stable plastic part made from semi-finished products. At increased temperature, molded parts are created from semi-finished products, i.e., from thermoplastic plastic sheets or roll-fed material. There are different heating-up methods for heating the semi-finished products in the thermoforming process [1]. Infrared radiation by means of ceramic, quartz or halogen heater elements is applied as the universal heating-up method in most cases. The forming process of the semi-finished product takes place in the rubbery-elastic area, the so-called forming temperature [2]. The deformation of the semi-finished product is reached by pressure difference as well as partly by mechanical support [1]. Mechanical pre-forming is carried out by a pre-stretcher leading to a better material distribution in the cavity. The later shape of the molded part will then be reproduced by feeding compressed air or vacuum. The component geometry is given by a one-sided tool. Once the semi-finished product has taken on the contour of the cold tool, the oriented molecular chains freeze in their stretched position, thus under mold constraint, and the formed plastic retains its shape [3]. Demolding of the thermoforming product takes place by the holding forces on the clamping when opening the tool and/or by ejectors in the tool. If several molded parts like cups are formed from a larger semi-finished product, these will be punched out afterwards. The skeletal is what remains as production waste. Compared to other plastics processing procedures like injection molding technique, the thermoforming process offers many advantages [1]. Due to the low process forces, even large components are relatively easy to produce. In consequence of the low forming pressures and the only onesided tools, the machine and tool costs are considerably lower for the thermoforming process. In particular for small quantities, wooden tools or plastics that can be easily processed are used. The series tools mostly consist of 54 bioplastics MAGAZINE [02/12] Vol. 7
Basics (Source: CustomPartNet) temperature-controlled aluminum. Through the use of multilayer semi-finished products, the properties of a thermoforming product can easily be influenced. This allows the realization of the best possible individual solution for each packaging. The open process control shows the limits of the procedure. This makes the molding production prone to external changes. Through the use of only flat semi-finished products, no custom parts of mold can be produced and there will always be a wall thickness reduction. In principle, all thermoplastic polymers can be processed with the thermoforming method. Amorphous thermoplastics have a higher softening temperature than semi-crystalline thermoplastics [4]. The processing of semi-crystalline thermoplastics requires precise process control. As a result, only a few semi-crystalline thermoplastics are applied in the thermoforming process, e. g., PP, PE, and PET [4], but also for example, PLA. Among the most frequently processed amorphous thermoplastics are PVC, PS, ABS, SAN, PMMA, PC and A-PET [5]. Furthermore, composite materials are used as multi-layer films as well as fiber-reinforced semi-finished products and foamed semi-finished products. The production of various packaging, e. g., yoghurt cups and trays in the food sector is a major field of application for the thermoforming process. Thin semi-finished products < 2 mm in the form of rolls are the material the packaging industry usually processes. The material proportion of the product’s total costs may be 80 – 90 %, here [6]. There are many other applications exceeding the mere processing of semi-finished products into packaging. A closer look reveals that this forming technology is applied across almost all industries and areas of daily life, be it in the fridge, in the car, for furniture, in the building sector as facing or light dome, as surfboards, swimming pools or as a hull. In addition, the machine building industry produces cover parts or it packs its spare parts by using thermoforming – and flower pots for the hobby gardener or complete garden ponds are also created on thermoforming lines. References: [1] Weinand, D.: Modellbildung zum Aufheizen und Verstrecken beim Thermoformen; Dissertation, IKV, RWTH Aachen (Aachen University), (1987). 9, (1993), p. 293-305. [2] Brinken, F.: Untersuchung zum Wärmeübertrag beim Thermoformen von Thermoplasten; Dissertation, Faculty of Mechanical Engineering, RWTH Aachen (Aachen University), (1979). [3] Hegemann, B.: Deformationsverhalten von Kunststoffen beim Thermoformen, experimentelle und virtuelle Bestimmung; Dissertation, IKP, Stuttgart University, (2004). [4] Howery, M. F.: Material selection for thermoforming applications; Annual Technical Conference Proceedings (ANTEC), (1997). [5] Beilharz, F.: Einfluss der Herstellungsbedingungen von PP- Halbzeugen auf die Thermoformeigenschaften; Dissertation, IKT, Stuttgart University, (2010) [6] Albert, K. A., et al.: Acrylic modified polypropylene for thin-gauge thermoforming: Improved processing properties and economics; Journal of Plastic Film and Sheeting [7] Schwarzmann, P.: Thermoformen mit Universalmaschinen, Adolf Illig Maschinenbau GmbH & Co., (2000). [8] Schwarzmann, P.: Thermoformen in der Praxis, Adolf Illig Maschinenbau GmbH & Co., (2008). www.illig.de bioplastics MAGAZINE [02/12] Vol. 7 55
