Design of a Micro Injection Moulding Machine for Thermosetting Moulding Materials

Design of a micro injection moulding machine for thermosetting moulding materials

W. Michaeli, T. Kamps

Institute of Plastics Processing at RWTH Aachen University, 52056 Aachen, Germany

Abstract

Micro products made of thermosetting polymers enable innovative applications since thermoplastics do not always yield the desired properties: thermosetting moulding grades offer advantages in thermal durability and chemical resistance in comparison to thermoplastic resins. Processing smallest quantities of polymeric material for applications in micro system technology require high process accuracies, e.g. in the volume of plasticised material and injection dynamics. Alternative machine concepts especially designed for micro injection moulding meet these requirements by means of special constructive setups mostly by using a plunger for injection. An appropriate micro injection moulding machine prototype is available at IKV Aachen. A new design of the plasticising and injection unit specially designed for thermosetting moulding materials is presented in this paper.

Keywords: micro injection moulding machine, plasticising, thermosetting materials, machine concept

1. Introduction

During the last decade, micro system technology has evolved from a pure research field into an industry with immense potential for the future. Growing rates of about 16 % per year are regularly predicted. The mainstream trend in micro system technology is the increasing use of polymer parts in the micron range [1,2]. The micro injection moulding is established as one of the most common manufacturing processes for polymeric materials in micro system technology, especially for thermoplastic resins. Injection moulding is often used to produce large numbers of micro components at a high automation level, and thus at low Fig. 1. IKV-micro injection moulding cost. Complexly shaped micro components with high machine prototype function integrity are state of the art [3]. However, properties of thermoplastics are some- Main properties of thermoplastic and thermosetting times insufficient for the application, where high resins are compared in Table 1 [4,5,6]. temperature durability and a good chemical resis- Thermoset materials are of high interest for the tance are required. In this case thermosets can be micro injection moulding, particularly because of their appropriate material to substitute thermoplastic resin. flowability. Their viscosity is much lower than that of Besides extremely good heat and chemical resistance, thermoplastics once they reach the hot mould, so that advantages of thermosets are flowability, high rigidity, equivalent longer flow lengths can be taken into and a widely modifiable range of properties with the consideration when rapid injection is guaranteed [7]. help of fillers. All of them provide the opportunity to realise innovative products. 2. The IKV-micro injection moulding machine prototype

Table 1 A prototype of a micro injection moulding machine Properties of thermoplastic and thermosetting grades was built and tested at IKV in former investigations [8]. The set-up for its use with thermoplastics is shown in Material thermoplastic thermoset Figure 1. property polymers polymers For this micro injection moulding machine, a Melting concept using a two plungers unit was followed: During temperature (°C) 220–350 60–90 the plasticising phase, the upper plasticising plunger Curing pushes resin through a die heated at melting temperature (°C) – 90–130 temperature as the injection plunger is cored back at Long-term operating the same time. Injection follows when the desired shot temperature (°C) 60–240 130–200 volume is reached. A ball check valve between injection Young’s modulus plunger and metering plunger prevents the melt from (MPa) 200–3000 5000–20000 flowing back into the metering cylinder.

Using a plunger for injection instead of a standard 3.2. Plasticising unit screw is an appropriate method to ensure higher process accuracy, which is needed in micro injection The plasticising unit of the micro injection moulding moulding: Since plasticising screws cannot be scaled machine is designed for the processing of free-flowing down to less then 12–14 mm, the melt throughput thermosetting moulding compounds. Typical per unit stroke during injection is much smaller with plasticising temperatures of these resins lie between a plunger of 2–5 mm than with a screw. Thus, 60–90 °C (see Tab. 1). While being pushed through the inaccuracies caused by the machine control lead heated die, material is melted via heat conduction. nonetheless to a more reproductive process. As a Heat conduction in polymers is low compared to consequence of the smaller volume throughput, other materials. Therefore plasticising by conductive machine movement inaccuracies are less critical and heating is time consuming and can extend the cycle oversized compensation sprues can be avoided. time, especially when melting thermoplastics [10]. With The injection plunger diameter can generally be thermosetting moulding material, these impairments are chosen as small as required to assure exact melt less critical due to lower plasticising temperatures and dosing; the diameter is 2 mm in the illustrated case. a generally higher thermal diffusity [9]; cones-quentially, With this configuration the shot volume of the machine plasticising is less time consuming. reaches 0.1 cm³. In the plasticising unit, temperature peaks have to be avoided in all cases, for prematurely curing material 3. Machine requirements for thermoset material can put the entire machine out of action. In order to provide the best possible temperature constancy, a 3.1. Clamping unit fluid heating system is applied, as indicated in the setup sketch in Figure 2. Similar to thermoplastics, only small clamping The requirements for the plasticising unit can be forces are needed in micro injection moulding with summarised as follows: thermosets due to the small dimensions of micro parts. • adaptation on the existing machine body, In this case, a pneumatic cylinder runs at a pressure of • ensuring constant plasticising temperatures, 12 bars, which is sufficient to hold mould opening • avoidance of a peak temperature, forces [8]. • fast plasticising, For the mould design, other requirements have to • avoidance of resin fluctuation in the injection be considered, like more precise mating surfaces and system for high process accuracy, and higher temperatures than in injection moulding for • flexible shot volumes. thermoplastic grades [9]. First trials with thermosetting resins are run with an electrically heated mould for 4. Design of the plasticising unit thermoplastics. An illustration of the plasticising unit’s basic set-up is shown in Figure 2. Similar to thermoset injection moulding machines in the macro scale, a common heating via liquid medium was chosen for its temperature control [8]. The high thermal capacity of liquid heat carriers enables temperatures to be kept at a very constant level, for pre-curing resin in the machine’s injection section has to be avoided. This detail represents the main difference to the design being used for the processing of thermoplastics. It heating medium involved some constructive modification, as explicated plasticising in- and outlets in Figure 3. cylinder However, despite the more complex design significant improvements concerning temperature constancy are expected. Water is used as heating injection medium due its ease of handling and its good heat cylinder transfer. It circulates through the plasticising and the injection cylinder by means of a small-sized heating unit by E. Braun GmbH, Kammerstein (Germany). The unit can be run on internal or external temperature control. In order to keep the machine set-up flexible die regarding the part volume being moulded, the maximum shot volume can approximately be doubled by means of a modular injection cylinder. For this task, the injection plunger and the cylinder have to be ~ 10 mm adapted as illustrated in Figure 3. The exchangeable sleeve also shown in Figure 3 is provided with a bypass groove to enable plasticising at the same time as the injection plunger is pulled back after holding pressure. Fig. 2. Design for the plasticising and injection unit for thermosets injection plunger: Ø = 2 mm (A ≈ 3,1 mm²) melt inlet / ball check valve Æ max. shot volume: 1 cm³

injection cylinder

injection plunger: Ø= 3 mm (A ≈ 7 mm²) bypass groove Æ max. shot volume: 2,1 cm³ sealing

modular sleeve + plunger

Fig. 3. Modular setup of the injection cylinder

Another constructive detail is the seal preventing References resin to emerge between injection plunger and cylinder back wall. Due to the low viscosity of thermoset grades, [1] N.N. Trends in Micro System Technology 2006. a special sealing had to be considered. In this case, a Information brochure of VDI/VDE-IT, 2006 narrow fitting copper ring lowers the pressure working [2] N.N. NEXUS MST/MEMS Market Analysis III on the outward lying Teflon-wiper. 2005-2009. 4M2C PATRIC SALOMON GmbH, 2006 5. Conclusion and perspective [3] Piotter V; Ruprecht R. Im Mikrospritzguss steckt noch viel Potenzial. Kunststoff Berater (2006) 7-8, Applications made of thermosetting polymer pp. 37-41 grades are advantageous, where inappropriate [4] Becker GW, Braun D, Woebcken W. materials can be substituted, like the use of nickel in Kunststoffhandbuch, Bd.10, Duroplaste. Hanser bio-medical applications [11]. However, some Fachbuch, 1994 properties of thermosets provide the opportunity to [5] Niemann K, Schröder K. Spritzgiessen von benefit from its characteristics in the micro injection Duroplasten. Hüthig, 1994 moulding. [6] Oberbach et al. Saechtling Kunststoff-Taschen- In order to process the smallest amounts of buch. Carl Hanser Verlag, 2004 thermosetting polymer, an already existing micro [7] Paar M. Design of Injection Moulds for injection moulding machine based on a prototype Thermosets. Dissertation, RWTH Aachen designed at IKV is available. Thermoset micro parts University, 1984 with a shot weight in the area of 0.05 g to 3.0 g can be [8] Spennemann A. A New Machine and Processing manufactured with this setup. Technology for the Injection Moulding of Micro Trials will be conducted for the in-mould assembly Parts. Dissertation, RWTH Aachen University, of multi-material micro components, the so called 2000 micro-assembly injection moulding. [9] Menges G, Michaeli W, Mohren P. How to make Injection Molds. Hanser Fachbuch, 2001 Acknowledgements [10] Michaeli W, Opfermann D. Ultrasonic Plasticising for Micro Injection Moulding. In: Proceedings of The research presented in this paper is conducted as the Second International Conference on Multi- part of the Collaborative Research Centre SFB 440 Material Micro Manufacture (4M). Grenoble, “Assembly of Hybrid Microsystems” and is supported France, 2006 financially by the DFG (Deutsche Forschungs- [11] URL: http://www.tecan.co.uk/micro/custom_- gemeinschaft), to whom we extend our thanks. micro_parts/materials.php. Material Information, Tecan Ldt., Weymouth (UK), 22.12.2006