SOME ASPECTS OF LOW MELTING POINT ALLOY CASTINGS PRODUCTION BY TEKCAST METHOD

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1 SOME ASPECTS OF LOW MELTING POINT ALLOY CASTINGS PRODUCTION BY TEKCAST METHOD Matej BEZNÁK a, Martin BAJČIČÁK b, Roland ŠUBA c a Department of Casting, Institute of Production Technologies, Faculty of Material Science and Technology in Trnava, Slovak University of Technology, J. Bottu 23, Trnava, Slovak Republic, matej.beznak@stuba.sk b Department of Casting, Institute of Production Technologies, Faculty of Material Science and Technology in Trnava, Slovak University of Technology, J. Bottu 23, Trnava, Slovak Republic, martin.bajčičák@stuba.sk c Department of Casting, Institute of Production Technologies, Faculty of Material Science and Technology in Trnava, Slovak University of Technology, J. Bottu 23, Trnava, Slovak Republic, roland.suba@stuba.sk Abstract The Tekcast method is promising technology of low melting point alloys centrifugal casting into moulds made from vulcanized rubber. The quality and properties of Tekcast castings are influenced by many parameters. The influence of some of them will be described in the paper. There will be also proposed technological operations for castings production and determined the time required for of them. The verification proved that proposed technological operations allow to produce mould and cast first castings into moulds with medium complexity of shape in less than two hours. Keywords: Tekcast, spin casting. 1. ÚVOD Foundry technology is one of the main branches of industrial production. The main development in foundry production is focused on the increasing of material s properties and the improvement of their processing technologies. The spin casting is one of casting methods significantly influencing mechanical properties of castings. The principle of spin casting is rotation of mould. The molten metal is poured into rotating mould during pouring. The mould`s rotation stops after casting`s solidification. The molten metal is affected by centrifugal force and pushed on mould`s wall and along them. The centrifuagal force affects casting` surface, structure and strength. 2. TEKCAST METHOD The Tekcast, method of low melting point alloys spin casting into vulcanized silicon rubber is one of advanced technologies. This method is high tech in spin casting technology and it allows to cast structural parts not only jewellery and artistic castings. Fig. 1 shows layout of Tekcast casting machine. The technology uses Teksil, material developed especially for this technology, for mould making. This material enables casting of low melting point alloys as zinc alloys, tin and lead alloys, Sn Sb- Pb alloys (so called white metals ), some polymer materials and in rare cases also thin walled aluminium alloy castings. The Teksil material is special silicone material, which enables to achieve good reproduction of pattern`s shape and details, good precision of castings and also good lifetime of moulds (up ten thousands castings from zinc, tin and lead based low melting point alloys) after vulcanization. It is

2 treated by pressure and temperature during vulcanization. The recommended vulcanization temperature for silicone rubber discs is 170 o C, the vulcanization pressure is from 14 to 35 MPa depending on mould`s diameter [1]. The workshop for spin casting into silicone moulds is equipped with hydraulic vulcanizer, melting furnace and casting machine. There are also necessary some auxiliary equipment for finishing operations and equipment for starting of devices as air pump, regulation and control units, etc.. The devices and necessary auxiliary equipment for Tekcast method are shown on Fig. 2. Fig. 1 Layout of Tekcast casting machine Fig. 2 Tekcast casting machine with necessary auxiliary equipment 2.1 Technological operations of production cycle The pattern for mould cavity with the shape of future casting can be produced by various technologies as machining, forming or casting from various metals. Mostly used materials for pattern are cooper, brass, zinc and their alloys. We successfully use plasma polished [2] stainless steels to get high quality of the casting's surface. The mould is made from two or more discs, depends on pattern size. The mould material is chosen dependently on pouring temperature, required dimension tolerances, mould complexity and lifetime. Bottom layers of silicon material are inserted into vulcanization frame and if it is necessary the cavities for pattern insertion are pre-cut. The parting plane is coated by lubrication spray, patterns, guiding pins and pattern of central gating system are inserted. Subsequently the parting plane is coated again to prevent the joining of patterns and moulds. After coating the top layers of silicone material are inserted and the whole mould with frame is inserted into vulcanization press, where mould is pressed and heated.

3 Fig. 3 Sketch of cavity venting Fig. 4 Close up of cavity with cut gating system and venting The final mould is tough, elastic, thermally and chemically resistant with precise dimensions. After vulcanization the gating system and venting are cut (Fig. 3). Fig. 4 shows the close up of mould cavity with cut gating system and venting. After the gating system and venting cutting the mould is assembled together and put into Tekcaster moulding machine. The mould is automatically centered and after cover closing pushed by bottom plate on top plate. Subsequently the castiing cycle parameters (rotational speed, compressing pressure and casting cycle time) are set on control panel. After beginning of cycle the molten metal is poured into gating system. The pressure caused by centrifugal force pushes molten metal into gating system and mould cavities and thus perfectly fills the whole space of each cavity. After finishing of casting cycle the mould is taken out from casting machine, disassebled and castings are removed (Fig. 5). The gating system and venting are easily broken off by hand during cleaning of castings. The castings with defects and gating systems are remelted. The finished castings have clean surface with high quality and minimal requirements of secondary machining operations. They can be further treated by electroplating (Fig. 6) or lacquered by powder or wet lacquers. The whole technological cycle with proposed times for each operation can be seen on Fig. 7. Fig. 5 Disassembly of mould and castings removal Fig. 6 Rector of Sládkovičovo university commemorative Medal down as cast, above after surface treatment

4 Fig. 7 Proposed technological operations for castings production by Tekcast method 2.2 Parameters influencing production cycle The quality of casting, production cycle and mould`s lifetime at spin casting into silicon moulds is influenced by many parameters: pouring temperature, clamping force, mould`s rotation speed, time of casting cycle, mould`s temperature, pouring speed, quality and purity of molten metal. The succesfull casting with this method requires to follow four main rules: lowest possible pouring temperature, lowest possible rotation speed, smallest possible clamping force and shortest possible casting cycle. The majority of castings can be casted using the parameters reccomended by casting machine producer [7]. But very small or complex mould`s cavities can require higher rotation speed. At casting of very thin castings very high casting speeds are necessary compared to general rpm. The clamping force is set up adhering to rule that smaller moulds require smaller clamping force. To obtain the best possible dimensional stability of castings casted in each casting cycle is Fig. 8 Fins in parting plane necessary to keep constant and lowest possible clamping force to avoid large fins in parting plane (Fig. 8).

5 The time of casting cycle and metal`s solidification rate depends above all on mould`s cavities size. The casting cycle have to be shortest possible for rapid take out of castings. Thus the moulds lifetime and productivity can be increased. The pouring temperature depends on cast material composition. In most cases the pouring temperature is o C above liquidus. The pouring temperature have to be the lowest possible to avoid fins in parting plane and decreasing of mould`s. Very important at spin casting is mould temperature. The casting into overheated mould can cause significant decreasing of its lifetime, large fins in parting plane, deformation of mould cavities and instability of dimensions of castings cast in different casting cycles. The heat conductivity of silicone rubber is very low but there are several methods how to increase it [3, 4, 5, 6]. But these methods increase the mould`s production time and requirements for workers skills by adding materials with complex shape to increase the heat conductivity of mould`s part near the cavity. Fig. 9 shows crack in mould cavity caused by excessive heat stress during cyclic casting. Fig. 10 shows large defect on the sprue base also caused by excessive heat stress. This place is the area of highest heat stress because of first contact with molten metal and the largest volume of solidifying metal. Fig. 11 shows mould cavity coated with oxide layer caused by unsatisfactory metal refining after repeated remelting. Fig. 9 Crack in mould cavity caused by heat stress Fig. 10 Defect on the sprue base caused by mould heat stress Fig. 11 Layers of zinc oxides in mould cavity

6 3. CONCLUSION The Tekcast, method of low melting point alloys spin casting into vulcanized silicon rubber is method with good perspectives not only for artistic castings, but also structural parts for electrotechnical and automotive industry and tools for plastics injection moulding [3]. There must be said that proposed technological cycle and times of each operations are influenced by many factors either directly or indirectly. For example mould size, pattern complexity, complex gating system and venting, experience of worker, etc. The benefits of technological cycle proposal is mould making and casting of first castings in less than two hours or very complex mould making in less than 24 hours. [1]. The parameters influencing the casting process has great significance in Tekcast spin casting process. These parameters influence the quality of castings, silicon moulds lifetime and overall production cycle as well. ACKNOWLEDGEMENT This paper has been supported by grant agency VEGA of the Ministry of Education of Slovak Republic in the frame of the project No. 1/0354/08. LITERATURA [1] BEZNÁK, M., BAJČIČÁK, M., ŠUBA, R. The main stages of technological cycle for low melting point alloys casting by tekcast method. In: Material science and manufacturing technology - MITECH Prague: International scientific conference, June [2] PODHORSKÝ, Š. Leštenie odliatkov elektrolyticko-plazmovým procesom - vplyv základných parametrov procesu na úroveň dosahovaného lesku. Archives of Foundry, ISSN , No. 18, 2006, pp [3] GATTO, A., IULIANO L. Evaluation of inserts for modular thermoplastic injection moulds produced by spin casting. In: Journal of Materials Processing Technology, 2001, pp [4] HUAN, Z., JORDAAN G.D. Air-cooling induced from spinning of spin-casting moulds, In: Applied Thermal Engineering, 2005, pp [5] HUAN, Z., JORDAAN G.D. Galerkin finite element analysis of spin casting cooling process, In: Applied Thermal Engineering, 2004, pp [6] HUAN, Z., JORDAAN G.D. Investigation of the cooling of spin-casting moulds, In: Applied Thermal Engineering, 2003, pp [7] TEKCAST INDUSTRIES INC., NEW YORK. Spin-casting, venting, metal flow and turbulence, 2005.