Preliminary development of automated rapid tooling process by metal arc spraying incorporated with STL file : investigation of surface roughness and dimensional accuracy of spray

初步開拓利用金屬噴塗及 STL 檔案的自動調節快速模具工藝 : 塗層的表面粗糙及準確尺寸的探討分析

Student thesis: Master's Thesis

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  • Chi Chiu LAU


Awarding Institution
Award date15 Feb 2002


Using rapid prototyping technologies (RPTs), such as SLA, SLS, and LMT, etc., to produce a pattern of a product has advantages of: (i) fabricating the pattern in a relatively short time; (ii) quicker appearance of the product in the market: (iii) permitting rectification of any error in the design and development stages; (v) allowing marketing personnel to identify its market potential and to pre-promote the product to its potential customers; and (iv) shorter lead time for production line and tooling development. However, RPT normally has deficiencies of costly replicating several or small batch quantity of prototypes in fabricating pattern and unable to replicate with the near end-used material that subsequently not adequate for functional tests, and so 011. Techniques to preserve the advantages and yet to alleviate the deficiencies of RPTs, Rapid Tooling (RT) techniques are using a RP pattern for producing rapid mold that can be used to replicate patterns/prototypes in small batch quantity rapidly and cheaply by molding/casting processes. Utilizing metal arc spray process to spray molten metal onto a fill11 layer of PVA mould release agent coated on a RP pattern after further reinforcement to become a mold is one of such RTs. To ensure uniform quality of spray thickness of a spray layer and to release the manual operator from health hazard by breathing ill the mixture of vapor and metal particulates, a self-regulated and automated robotic metallic arc spraying system for rapid mold production is essential. In view that a RP pattern is normally built layer-by-layer following a pre-processed STL file, it was contemplated that the STL file could be used as an input to a computer program: for identifying the sequential points and boundary points on the surfaces of a three-dimensional (3D) solid RP pattern to be sprayed; and for converting the optimal spraying pat11 loci, derived from these points after incorporating with the optimal spray deposition distance, to the format acceptable to the control console of the robotic spray system. Good quality of a replicated part from the rapid mold depends 011 the smoothness of the inner spray layer and the dimensional accuracy of the spray ill relation to the RP prototype, which varies with the parameters setting of the spraying system. This project thus experimentally studied the influences of some of these parameters on both the finishing of the inner surface of spray layer and the penetration depth of the spray. Results of the spraying tests by a TAFA8830 metal arc spray system spraying metallic droplets onto a mould release agent-PVA film, with almost constant physical properties, coated on polymethyl methacrylate (PMMA) substrate indicated that: (a) the increase in arc spray pressure tended to smoothen the sprayed surface and deepen the penetration of the spray - The mechanisms involved would be due to the increase: (1) in the kinetic energy, i.e. the impacting velocity, and the ability of the metallic spraying droplets in penetrating into PVA film; and (2) ill the level of splashing and atomization of the molten droplets out from spraying nozzle before complete solidification; (b) the increase in arc spray voltage also tended to reduce the surface roughness of the inner spray - Generally the arc spray voltage increased the thermal energy of the molten metal droplets and the amount of melts of the metal wires. The increase in former thus increased the fluidity of the melt and the level of splashing of the molten droplets so that it smoothened the inner spray surface by raising the conformability of the molten droplets to the rough surface of the spray layer(s) at impingement to PVA film. The increase in latter gave higher weight of the spray to sink the spray layer downwardly into the PVA film for deeper penetration due to the increase in the spray metal density per unit spray length; (c)the decrease in spray deposition distance of spray gun increased the impinging velocity of molten droplets onto and gave deeper penetration into PVA film, and it reduced the level of further atomization of molten metal droplets that increased the roughness of the inner spray surface; (d) the increase in the traveling speed of the spray gun normally reduced the spraying metal density per unit spray length, thus reducing the penetration depth of spray layer(s) into PVA film and also decreasing the level of droplet deformation before complete solidification to give rougher inner spray surface. It is thus anticipated that suitable manipulation of the aforementioned spraying parameters enables the achievement of good quality of a spray. Besides the state of molten-solidification, the impinging velocity and the splashing of the sprayed droplets, micrographic analyses also showed that the reverse flow of the PVA film on the RP pattern also influenced the roughness of an inner surface of a spray. Minimizing such reverse flow is thus essential in achieving good quality spray. To facilitate the self-regulating of the spraying system, a very rudimental and simplified mathematical model to predict the upper bound value of inner surface roughness of the spray and the lower bound value of spray penetration depth of droplets into the PVA film (i.e. the spray dimensional accuracy in compared with the pattern) was preliminarily established in this project. Furthermore. a methodology to identify, extract and sort out the boundary points and the surface points on a 3D solid PR pattern for covering the spray from a STL tile originally for RPT was derived from this project. Conceptually, the further adding of these points by the spray deposition distance under the optimal spraying parameters predicted from the rudimental mathematical model proposed allowed the preliminary spraying path loci to be decided. The conversion of these path loci into the format acceptable by the control console of the robot mounted with the metal arc spraying gun with the optimal spray gun traveling speed paves the way for the development of the self-regulated and automated metal arc spraying system for rapid tooling to be produced.

    Research areas

  • Metal spraying, Rapid tooling, Surface roughness