Research On Key Technologies In Rapid Manufacturing Process Of Zinc-based Alloy Moulds

The development trend of modern manufacturing industry is multi-varieties, small-lot and rapidness. The enterprises must respond to the changes of market and consumers and provide creative products to market quickly. The Rapid Prototyping (RP) and Rapid Tooling (RT) technologies are among the technical supports to solve these problems. The indirect rapid tooling technology combined RP with casting methods, which can fabricate moulds rapid and low-cost, has characteristics of rapid, flexible and broad applicable. The RT technologies by casting methods have received increasingly attention from both research and industry circles.In this paper, the RT technology by casting method based on zinc-based alloy is studied. The researches are around two themes:innovation and rapidness. A rapid manufacturing technology of zinc-based alloy moulds supporting creative product design is developed. The main research contents include five parts:product design based on patterns extracted from digital images, manufacturing and cooling simulation of conformal cooling channels of injection moulds, new zinc-based mould alloy, electroless nickel plating on zinc-based alloy and rapid manufacturing process of zinc-based alloy mould by rapid casting methods.The patterns extracting from digital pictures is researched with digital image processing techniques. The vectorization process is established for inputed image in BMP format. The algorithms are researched and sifted in gray level transformation, binary processing, median filter, outline detection and thinning. The realized by using. A program with VC++language is compiled to realize the contour extraction of image feature and output to three-dimensional design software by DXF format. By this program, the features of digital images can be applied to creative product design and manufacturing of patterns on mould cavities. The fine patterns have higher accuracy. The patterns on surface of mould cavities can be fabricated by rapid tooling technologies without corrosion or fine carving process.A processing route of manufacturing conformal cooling channels (CCC) of injection moulds by buried casting methods is proposed in RT process. This process can rapidly fabricate moulds with CCC by combing low melting point of zinc-based alloy with flexibility of casting process. The cooling effects of CCC is simulated using MPI software. The results show that conventional mold flow analysis software cannot fit the features of CCC cooling system by buried casting methods. A analytic model based on the theory of heat transfer and finite difference method is built for injection mould with CCC by buried casting methods. With the developed analog simulation program, the cooling effect between straight cooling channels and CCC are analysed, and the diameter of channels, distance to mould cavities and space between channels of CCC are evaluated. The research investigated the simulation designs of CCC cooling system and give some valuable ideas and experience to further research on fabrication and simulation of CCC.Subject of mould alloy investigated in this paper is Zn-11%Al-8%Cu-Mg alloy. Experiments are carried out to investigate the effects of RE and Zr on micro-structure and properties. The results show that:moderate addition amount of RE and Zr can refine the micro-structure and form uniform second-phase particles, but excess RE and Zr will weaken the modification effect and make worse micro-structure; the mechanical properties and corrosion resistance are improved after addition of RE and Zr, but the addition amount of RE and Zr reachs 0.06% and 0.5% respectively, the properties of alloy decrease. The heat-treatment experiments indicates:the tensile strength and elongation of Zn-11%Al-8%Cu-Mg and alloyed RE or Zr alloy after heattreatment regime"heating preservation at 320℃for 3h then water quench"+"heating preservation at 120℃for 10h then air cooling", but the hardness is decreased slightly, and the micro structure becomes more uniform. Taking all those affecting factors into account, the mould alloy used in this paper are chosen as Zn-11%Al-8%Cu-Mg-0.05%RE alloy.The electroless nickel plating on surface of Zn-11%Al-8%Cu-Mg alloy are developed to further increase the properties and facilitate the application. The analysis of characteristic of zinc alloy indicates the pretreatment of electroless nickel plating is very important. Considering the binding force of nickel platings and environmental protection, a electroless nickel plating process for Zn-11%Al-8%Cu-Mg alloy is developed, which is characterized by alkalic electroless nickel preplating plus acidic electroless nickel plating. The desirable alkaline degreasing, mild acid etching and alkalic electroless nickel preplating process are experimentally chosen. The mechanism of electroless nickel deposition on zinc-based alloy is discussed. The plating bath is optimized by investigating the influence of ingredients on deposition (?)locity. The analysis of nickel plating show that:the deposition velocity is 11.44μm/h; the phosphorus content of nickel plating is 6.03%, so the nickel plating is non crystalline structure; the nickel plating is even and compact and the corrosion resistance and combinative ability of plating coating are excellent. The electroless nickel plating shows good protective effect to zinc-based alloy, so it is expected to become surface treatment method for mould of zinc-based alloy.The rapid manufacturing of zinc-based alloy based on SLA prototype is introduced by example. The vacuum casting machine is designed and manufactured and the silicone rubber mould can be gotten for SLA prototype from CAD model. The fabricating process of plaster casting mould is investigated and casting process and solidification characteristics of zinc-based alloy is analyzed. The zinc-based alloy mould parts can be obtained by gravity casting from plaster casting moulds. This RT process can integrate above four research achievements and the rapid manufacturing process of zinc-based alloy moulds supporting creative product design is developed.Through the researches in this paper, a RT process for zinc-based alloy mould based on SLA prototype and casting process is acquire, which can provide technology applications basis for low cost, high quality and rapid zinc-based alloy mould and technology support for products innovation for rapid reaction to market.