Patent Application
20160136728
This application is related to and claims priority to Indian Patent Application No. 1173/KOL/2014 filed on Nov. 13, 2014, the contents of which are incorporated by reference herein.
The present invention relates to dough processing of polymer-metal blend compositions. More specifically, the present invention is directed to processing of polymer-metal blend compositions suitable for shape forming by way of effective controlling the rate of settling of the metal particles in polymer-metal blend under ambient conditions to generate a cost effective and simple process for producing shape formable dough. Advantageously, the present invention provides a rapid, energy saving process involving minimum material loss and utilizing non hazardous solvent system such as water. In addition to this the process of the invention and polymer-metal blend products obtained thereof are suitable for working involving wide range of polymers.
Casting and moulding are the conventional and widely practised bottom-up approach which involved melting the metal and moulding (Shivkumar et al., JOM 43.1, 1991: 26-32). Machining of metal blocks by cutting and drilling to fabricate desired shape is also practised conventionally but is not cost effective (Masuzawa et al., CIRP Annals-Manufacturing Technology 46.2 (1997): 621-628, 49.2 (2000): 473-488; Dornfeld et al., CIRP Annals-Manufacturing Technology 55.2 (2006): 745-768). Loose sintering is the simplest form of metal powder processing which can only produce porous parts and takes longer time for sintering without compaction, while isostatic pressing can produce relatively large parts of high aspect ratio with superior material properties Govindarajan et al., International journal of mechanical sciences 36.4 (1994): 343-357). High-end equipments are necessary for pressing at a very high pressure as well as extremely high temperature that are truly expensive. In roll compaction or powder rolling, metal powder is pressed between two heavy duty pressure rollers that rotate to form a continuous length of metal strip or sheet; however, does not have the versatility to produce any other shape (U.S. Pat. No. 2,033,240, 1936)). Complex solid metallic parts with intricate geometries and thin walls are mostly made through metal injection moulding (MIM) (German et al., MPIF, Princeton, N.J. (1990); Sundberg et al., Applied physics letters 57.7 (1990): 733-734.; Adv.; Johnson et al., Mater. Processes, 161 (4) (2003), pp. 35-39). Major drawback of such a process is very high volume of shrinkage during sintering owing to relatively high binder content and densification by removal of high porosity. In MIM molding the process is limited only to the thermoplastic polymer matrix which again involves temperature for processing. Spark plasma sintering (SPS) is a unique powder processing technique for producing high quality objects in a single step without use of any binder. Metal powder is sintered under pulsed DC current in sparking condition. However, SPS is only limited to produce simple symmetrical shapes and makes use of expensive pulsed DC generator.
The metal powders are difficult to disperse using only electrosteric stabilization owing to their high density, low charge density (thin oxide coating) and bigger particle size. Most of the existing processes involve viscosity change from a low viscosity to high viscous slurry after casting followed by drying and sintering (Sepulveda, 1997; Andersen et al., 2000; Angel et al., 2004; (Shimizu and Matsuzaki, 2007; Kennedy and Lin, 2011). Thickener is reported to be used as a stabilizer/viscosity-enhancer for uniform metal suspension. After casting, no further modification can be done to the formed shapes. In such process no reprocessing can also be done after that, if required.
Recently, metallic parts are also known to be fabricated by 3D printing/rapid prototyping, as an alternative to casting and machining. These techniques are useful for development of objects from computer aided design (CAD) model through layer-by-layer fabrication. However all these techniques are based on melting and deposition of metal powder or thermoplastic polymer as binder. Fugitive based techniques were commonly used for last two decades to generate highly open porous structures. Process like slurry based casting or 3D printing are limited by high drying time, surface contamination due to oxide formation during processing and significant carbon residue after sintering which makes them brittle.
Several prior U.S. Pat. No. 6,045,748, U.S. Pat. No. 7,491,356, US20100092790, U.S. Pat. No. 5,745,834, U.S. Pat. No. 4,197,118 relate to processes of manufacturing articles of metal powder from particulate material and a polymer/binder though various process steps all of which include steps of heating, melting, cooling, drying, and providing a green body before sintering.
U.S. Pat. No. 4,415,528 relates to the method of forming metal alloy shaped parts from a mix of metals and/or individual compounds along with a binder to form a homogeneous mass which form green bodies. The green bodies are then processed by stripping of the binder and raising the temperature of the stripped body below the sintering temperature of the metals and finally sintering was done.
U.S. Pat. No. 2,939,199 relates to process of manufacturing articles from sinterable materials ceramic powders, metal carbides, or their mixtures, by mixing them with a vehicle comprising a thermosetting moulding material and a plasticizer. Moulding results in green shape which is hardened.
However in the above mentioned prior arts, usually most of the process steps are complex and the difficulty in making homogeneous distribution of metal particles in slurry based casting techniques since settling of the particles persists. Hence there remains a continuing need in the art to develop a simple dough processing technique which would have the ability to produce both dense and porous shape formable dough for desired metal objects in a cost-effective way. Further, the process should offer fabrication of diverse size and shape of the components with tailorable microstructure within a reasonable time frame.
It is thus the basic object of the present invention to provide the process for rapid processing of homogeneous dough of polymer-metal blend composition ranging from dense, porous, dense-porous gradient.
Another object of the present invention is to provide the processing for polymer-metal blend composition with tailorable porous structures with controlled porosity and pore size distribution.
Another object of the present invention is directed to the energy efficient simple process of processing polymer-metal blend composition by converting low viscous polymer-metal blend solution into high viscous dough under ambient conditions to generate shape formable dough with involvement of less quantity of non-toxic, aqueous/non-aqueous solvents, controlling the viscosity.
Another object of the present invention is to provide the processing for polymer-metal blend composition for formation of alloy in one step during sintering.
A further object of the invention is related to the fabrication of different metallic shapes like tapes, rods, tubes, springs, domes, sheets, laminated objects, biomedical implants along with miniaturized patterns/features like microrod arrays through embossing or micro-patterning etc. involving a common platform technology.
Another object of the present invention relates to metallic products with easy machinability in green state via tooling/laser ablation/extrusion/embossing etc. for large scale productions for diverse end uses and applications.
Another object of the present invention relates to development of dough with the much lower yield stress as compared to the metal used for processing, thus making the process of generating metallic body production involving such dough more energy efficient.
Yet another object of the present invention is related to metal foam with high permeability, high thermal conductivities (5-30 W/mK), resistance to thermal shocks, high pressures, high temperatures, moisture, wear and thermal cycling, good absorption of mechanical shock and sound, suitable for wide applications in design/architectural, automobile, biomedical etc.
Thus, according to the basic aspect of the present invention there is provided a process of dough forming of polymer-metal blend compositions suitable for shape forming comprising:
controlling the rate of settling of the metal particles in polymer-metal blend by converting a low viscous polymer-metal blend composition into high viscous dough under ambient conditions to generate shape formable dough; and
subjecting the dough to shape forming into desired metallic bodies.
Another aspect of the present invention provides a process wherein said step of controlling the rate of settling of the metal particles in polymer-metal blend dispersion by converting a low viscous polymer-metal blend dispersion into high viscous dough under ambient conditions to generate shape formable dough includes coagulation dough processing (CDP) involving coagulant with or without foaming agents or fugitives.
In another aspect, the present invention provides a process involving dense shape formable dough comprising coagulation dough processing involving coagulant and free of any foaming agents or fugitives:
i) providing metal powder dispersion in polymeric solution;
ii) increasing the viscosity of said metal powder dispersion in polymer solution involving said coagulant with or without other additives to thereby provide for said dense shape formable dough via high shear mixing with pressure ranging from 1 to 500 MPa preferably up to 100 MPa;
iii) subjecting the thus obtained dense shape formable dough to shape forming, drying and sintering.
In another aspect, the present invention provides a process comprising:
i) mixing metal powder, polymer powder, glycerol, solvents under stifling for a period of 10-30 minutes depending upon the volume;
ii) adding coagulants under stirring, subsequently subjected to mixing for 2-3 hrs till a smooth lustre appears to the dough which facilitates removal of entrapped air bubbles and intimate mixing of all the ingredients under high shear with pressure ranging from 1 to 500 MPa preferably up to 100 MPa;
iii) after mixing, the dough was subjected to shaping through plastic deformation/yielding above the yield stress of the prepared dough;
iv) the dried green body was further subjected to heat treatment including sintering to obtain final dense components;
optionally, grinding, polishing or any other shaping/finishing may be done, if required.
A further aspect of the present invention provides a process wherein said mixing is carried out selectively involving anyone or more of roll mill, mechanical stirring/sigma blender/cone blender/V-blender for bulk volumes and after mixing, the dough was subjected to shaping through plastic deformation/yielding above the yield stress of the prepared dough, and finally the dried green body was further subjected to heat treatment (sintering) to obtain final dense components and optionally involving grinding, polishing or any other shaping/finishing
Another aspect of the present invention relates to a process wherein the polymer-blend composition (wt %) comprises based on the weight of metal powder:
1-50
3-30
In a further aspect the present invention relates to a process involving porous shape formable dough comprising:
i) providing metal powder dispersion in polymeric solution;
ii) increasing the viscosity of said metal powder dispersion in polymer solution involving said coagulant to thereby shape formable dough by high shear mixing in presence of fugitive/foaming agent;
iii) subjecting the thus obtained porous shape formable dough to shape forming, drying and sintering.
In yet another aspect, the present invention provides a process involving porous shape formable dough comprising:
i) providing polymeric solution and coagulant with or without other additives to provide a polymer-coagulant composition;
ii) subjecting said polymer-coagulant composition to vigorous stirring to increase viscosity by frothing; and
iii) thereafter gradually adding the metal powder to said mix of step ii) above to thereby further increase of viscosity of said metal powder dispersion in polymer solution and generate a porous shape formable dough;
iv) subjecting the thus obtained porous shape formable dough to cast into a predefined lubricated mold for shape forming without further high shear mixing, and drying.
v) dried green body was further subjected to heat treatment (sintering) to obtain final porous components.
In another aspect, the present invention relates to a process wherein addition of foaming agents and/or fugitive particles for porosity or increasing pore connectivity in said generated shape formable dough for porous structures, interconnected secondary macropores and wherein sintered density, total porosity, pore size distribution, microstructure, mechanical properties are selectively varied based on selective additives involved.
A further aspect of the present invention relates to a process wherein the polymer-blend composition (wt %) comprises based on the weight of the metal powder:
1-50
3-30
Yet another aspect of the present invention relates to a process wherein said step of converting a low viscous polymer-metal blend composition into high viscous dough under ambient conditions to generate shape formable dough comprises plastic dough processing (PDP) involving high shear mixing of metal powder and polymer blend composition with or without fugitive/foaming agent under ambient conditions.
A further aspect of present invention relates to a process comprising:
i) providing metal powder;
ii) adding said metal powder to polymer with or without additives to obtain a polymer-metal blend composition;
iii) subjecting the said metal polymer-metal blend composition to high shear mixing to thereby generate shape formable plastic dough.
In another aspect, the present invention provides a process wherein said high shear mixing is carried out with pressure range of 1 to 500 MPa preferably up to 100 MPa at low temperature 4° C. to 40° C. preferably at 10° C. for de-agglomeration of polymers and metal powders and wherein the high shear promotes deformation, blending of metal powders within polymer matrix in presence of very low amount of solvent, co-solvent, plasticizer and surfactant along with the applied shear force eliminating possibility of entrapped air bubbles during formation of dense components.
In a further aspect, the present invention relates to a process wherein (a) for dense component dough forming composition by weight of metal powder comprises:
1-50
3-30
and (b) for porous components dough forming composition by weight of metal powder comprises:
Another aspect of the present invention provides a process wherein after the formation of the shape formable dough, the fugitive/foaming agents are removed by anyone or more of (i) formed dough is either heated at 100-600° C. for burning out of the organic/inorganic fugitive particles or (ii) dipping within a solvent for leaching out of the soluble polymer/inorganic salts which leaves spaces after removal and turned into foam after sintering.
A further aspect of the present invention relates to a process wherein said low viscous polymer-metal blend composition is converted into high viscous dough under ambient conditions to generate shape formable dough with the following specifications:
I) For coagulation dough processing:
Viscosity at shear rate 0.03 (5−1): 0.0001 to 1 MPa·s
Yield stress (MPa): 0.0001-2
II) For plastic dough processing involving high shear mixing of metal powder and polymer blend composition:
Viscosity at shear rate 0.03 (5−1): 2 to 100 MPa·s
Yield stress (MPa): 2-50
According to yet another aspect of the present invention it has been found that the advancement involving the polymer-metal blend working under ambient conditions for shape formable dough the involvement of high shear mixing except in case of dough forming involving foaming/air bubble entrapment involves a selective range of 1 to 500 MPa since it has been experimentally found that roll milling was not possible at <1 MPa, hence dough formation was not possible while attempts at above 500 MPA lead to polymer degradation which thus again affected desired shape formability of the dough.
Another aspect of the present invention provides a process wherein the dough obtained is selectively worked into dense and/or porous, graded or laminated objects with various shapes including rods, tubes, blocks, spring or other hollow structures along with required surface texturing for metal stent, drug eluting stent, device for lumen stricture, bone plates, bone screws, dental roots and crowns, dental bridges, spinal shunt, hip joints, knee joints and any other structural load bearing supports, metal foams for architectural, automobile, biomedical applications including cancellous bone analogue, dense-porous laminates and gradients for dental roots and bone.
5D-F illustrate the microstructure by Plastic Dough processing (D-E) Dense (F) Porous with fugitives.
The invention thus provides method for dough forming of polymer-metal blend composition with controlled rate of settling of metal particles which would provide for highly loaded particles with minimum powder settling leading to homogeneous green body preparation involving low energy consumption and though cost effective and use friendly route. As discussed hereinbefore the present advancement enables the following manners of preparing high viscous slurry/dough.
Method 1 (Scheme 1): Formation of viscous dough from flowable metal-polymer loaded slurry using organic/inorganic additives as coagulant for dense and/porous body (CDP).
Method 2 (Scheme 2): Formation of viscous dough from blends of polymer(s) powder, metal powder, minimum solvent, additives through high shear mixing for dense and/porous body (PDP).
The present invention by way of the present advancements and following any of the above methods enables converting low viscous slurry to a high viscous dough under ambient conditions thus favouring a cost effective and energy efficient process of metallic body forming. Processing in the present invention involves achieving high viscous dough for shape formation either mixing under high shear force or by influence of coagulant with or without foaming with characteristic viscosity ranges to make the dough flowable/castable/mouldable above yield stress of the suspension. Controlled viscosity is the characteristic parameter of the processing either for dough. A wide range of compositions are suitable for working involving the above processes of the present advancement which include binder, plasticizer, lubricant, surfactant, solvent, sintering aid etc.
Metal powders include preferably fine metal powders (˜30 nm to ˜400 micron size range) of transition/post-transition metals (Ti, V, Cr, Ru, Rh, Co, Ni, Mo, Zi, Al, Ta, Ru, W), lanthanides and actinides (Ce, Fe, Zr) and their alloys (like nitinol, CoCrMo, Ti6Al4V, steel etc.) are mixed with different weight percent of natural/semi-synthetic/synthetic polymers.
Polymeric binders include polymeric binders ranging from hydrophobic/hydrophilic, thermoplastic/thermosetting, wax, PVA, polyamides, nylon, polyvinyl chloride, polystyrene, polyethylene, polypropylene, polyacrylonitrile, polyvinyl butyrate, silicone, chitosan, alginate, gelatin, soy protein, whey protein, starch and derivatives, cellulose including cellulose esters and celluloids, polycaprolactone, polyolefin, polylactic acid, polyglycolic acid, their co-polymers etc.
Solvent include different solvent systems for wetting of the metal powders, partial dissolution/dispersion/lubrication of polymers and other processing aids towards homogeneous matrix formation with dispersed metal particles. Choice of solvents depends upon the hydrophobic/hydrophilic properties of the polymers and nature of the metals and their reactivity to a specific solvent. Suitable solvens include polar solvents, water, which is usually preferred owing to its environmentally friendly nature and cost-effectiveness. Besides this, aliphatic alcohols such as methanol, ethanol, propanol, isopropanol, butanol, dioxane, dimethyl formamide, dimethyl sulfoxide, N,N-dimethylacetamide, tetrahydrofuran, propylene carbonate, methyl isobutyl ketone, acid-water mixtures containing organic acids like formic acid, acetic acid, propanoic acid, inorganic acids such as hydrochloric acid, alkali/ammonical solution or any suitable noncorrosive, non-toxic solvent/co-solvent systems are useful for this process. For hydrophobic polymers, wide range of non-polar solvents are used for this process based on solubility of the polymers, like hexane, cyclohexane, toluene, carbon tetrachloride, N-vinyl pyrrolidone etc.
Lubricants that overcome stickiness and for easy removal of dough from the roll after formation, commonly involve stearic acid, stearates, sodium stearyl fumarate, sodium lauryl sulfates, oleic acid, ethylene glycol, triethylene glycol, high molecular weight polyethylene glycol, wax, paraffin, glyceryl behapate, sterotex etc.
Plasticizers include plasticizers/humectants to prevent drying of the solvents and increase the plasticity or fluidity of the blend, include glucose, sucrose, molasses, glycerol, sorbitol, glycols, sebacates, adipates, maleates, citrates (e.g., triethyl citrate, tributyl citrate etc.), benzoates, terephthalates, dibenzoates, gluterates, phthalates, azelates, trimellitates, formals, acetals etc.
Coagulants include additives such as inorganic/organic salts (e.g., nitrate, bicarbonate, chloride), polyelectrolytes (e.g., tripolyphosphate, citrate, PMMA, PAA, PMA, PEI etc.) to facilitate high viscous dough formation through coagulation, flocculation as well as gelation and thereby homogeneous distribution of the metal particles with minimized settling.
Fugitive/foaming agents include for developing porous metallic structures, usually space-holder/spacer particles ranging from polymeric fugitive particles (e.g., PVA, chitosan, styrofoam, polyurethane, camphene, naphthalene etc.), foaming agents (e.g., urea, sodium bicarbonate, ammonium bicarbonate, low melting metal powders or derivatives etc.) or soluble salts (e.g., NaCl, ammonium chloride, sugar etc.) are added during dough processing with the composition for dense fabrication. Different foaming agents involve foaming of the polymer solution through either temperature/chemical assisted foaming followed by dispersion of metal powders.
The methods under the present advancement are highly energy efficient for producing various shapes such as rods, tubes, springs, domes, sheets, sponges, biomedical implants and even miniaturized patterns/features like microrod arrays or micro gears through embossing or micro-patterning etc. as well as cellular metallic structures. Tapes produced by such process could be either laminated or pressed into bulk objects and subsequent patterning/green machining is done into various net shapes with tailorable microstructures followed by sintering under varied atmosphere. As would be apparent from the above said the method of the present advancement can be successfully utilized for forming reactive metals under controlled atmosphere as well. Compared to existing well known processes, the newly developed method of the present advancement special being economical, user friendly and time saving. This also offers high degree of homogeneity in green state and thereby sintered microstructure.
The details of the invention, its objects and advantages are explained hereunder in greater detail in relation to the following non-limiting accompanying figures and examples:
Ia: Coagulation Dough Processing (CDP) (Method 1/Scheme 1): (without Foaming)
¥Optional
Dough processing according to the Method 1 (
After this, the formable dough was subjected to shaping through plastic deformation/yielding above the yield stress of the prepared dough. The dried green body was further subjected to heat treatment (sintering) to obtain final dense components (
For preparation of cellular solids, the prepared dough (as per the process described above in Example 1a,) before the roll milling step (
To overcome the problem in Example 1b, all ingredients other than metal powder, as mentioned in Table 1, were mixed by vigorous stifling and subsequently, metal powder was slowly added under low shear mixing range from 0.1-100 s−1. Dough obtained through this technique can be cast by gentle tapping without further roll milling into different shapes like blocks, rods, hollow structures by using different mold, dried and sintered to obtain porous structures (
¥Optional
Dough is prepared by high shear mixing of metal powders and polymers, as described in Method 2 (
Very high loading of metal powder and deliberate removal of air bubble through high shear mixing results in high viscosity. The high shear mixing facilitates de-agglomeration of polymers and metal powders, promotes deformation, blending of metal powders within polymer matrix in presence of very low amount of solvent, co-solvent, plasticizer and surfactant. Additionally, this applied shear force eliminates possibility of entrapped air bubbles during formation of dense components. The low quantity of solvent enables to dry the components faster without significant deformation, warpage and surface contamination/oxidation and eventually offers processing of reactive metals even in ambient condition. The lubricants help in post processing and extrusion molding of the blend. The plastic deformation property of polymers was utilized for blending and homogenization of metal powders, lubricants etc. within polymer matrix. Dough obtained from PDP was further transformed into dense, porous, graded or laminated objects with various shapes like rods, tubes, blocks, spring or other hollow structures along with required surface texturing in a single step followed by drying and sintering (
Fugitives are usually added in Method 2 during pre-mixing stage before applying high shear if porosity is to be imparted (
a) Rheology of Dough
Rheology is an essential parameter to optimize the dough compositions for post processing, controlling microstructure—mechanical properties of the green and sintered metallic objects. Foam always have lower viscosity, where dough would exhibit very high viscosity all the time. Viscosity range for metal powder loaded foam composition vary with particle size of the powder, polymer percentage and molecular weight, solvent quantity and particle loading.
In this context, viscosities of all prepared doughs were measured at different solvent compositions to evaluate flow behavior of the dough. To measure dough viscosity, highly viscous dough was extruded through an extruder die using universal testing machine (25K machine, Hounsfield, UK) at different cross head speed like 1.00 mm/min and respective data was analysed to evaluate viscosity of dough (
b) Bulk Density, Apparent Porosity
The densities of metallic green and sintered compacts were measured by Archimedes principle in glycerol medium. Prior to the measurement of bulk density and apparent porosity, weight of the each vacuum dried samples (w1) was measured in a digital weighing balance. Subsequently, they were soaked in glycerol until all the air bubbles vanished. The soaked metallic samples were suspended inside the glycerol medium for few hours. Finally, suspended soaked sample weight (w2) and soaked sample weight (w3) were calculated. Density the green and sintered metallic compacts were calculated by using the following formula:
Further, the apparent porosity (%) of the compacts were calculated by using the following formula:
The density of the sintered Ti6Al4V samples obtained through PDP (example 2) was found to be achieved more than 97% and 65-70% for dense and porous components respectively.
Porosity depends upon the metal/fugitive ratio, while pore size varies depending upon the particle size of the fugitives or foaming agents. Hence, fugitives with different particle size ranging from 1-400 μm have been explored to obtain various pore diameters. Homogeneously porous objects with defined pore size or porosity graded structures were fabricated by varying the metal/fugitive ratio and particle size of the fugitives as well.
Microstructure of dense and porous samples: 1) Dense dough of Example 1a (
The green body characteristics of the metal body obtained from Example 1a and 2a could be further varied in the final compacts based on the ratio of metal to polymer. On the other hand, physical property of the dough depended upon the rheology/viscoelastic behaviour of polymer-metal powder blend which further depend on the types and amounts of polymer, solvent used along with metal powders. Dough obtained in this process was found versatile enough to be shaped into sheets (
High viscous dough of Example 1 and/or 2 can be utilized for in situ metal alloy formation during sintering and densification. Dough could be prepared by mixing individual metal powders to achieve desired alloy composition along with the other compositions as per examples 1 and/or 2 in single step process after sintering. For example, Ti6Al4V, Ni—Ti, Co—Cr were prepared through dough formation by powder processing as described in example 1 using Ti, Al, Ni, Co, Cr and V based powders in different required weight ratio. After sintering, final component would have the composition as per the requirement.
Metal foams (
Metallic components are subjected to fail under dynamic/static load due to stress accumulation inside the components causing the stress shielding effect. Gradient porous structures are required to transmit the load and minimize this effect. Therefore, a method was developed to fabricate gradient porous structures. Dough with different percentage of fugitive contents was prepared as per example 1 and/or 2 and further processed via shape forming to develop green bodies with variable porosity. These green shapes were joined one after another to fabricate porous or dense-porous gradient structures. Layers can be joined by many techniques such as lamination, solvent spraying and pasting etc. Gradient porosity of each component was measured using quantitative analysis of the cross-section.
Components developed by this process have different type of porous architecture with variable porosity. These components are used for various structural and functional applications including heat sink, heat exchanger, aerospace, biomedical industries etc. Modulus of materials is tailored by making it porous and sandwich structure to mimic cancellous bone architecture.
For symmetrical objects like rods, tubes etc., extrusion molding of the dough obtained from examples 1 and 2 was carried out for shape forming in single step under universal testing machine using extrusion die at variable compressive force. Different types of extrusion molds were designed as per requirement as well as flowability of the dough. Prior to extrusion, a rolled sheet obtained via example 1 and/or 2 was used to minimize air entrapment and inserted into the mold. Depending on rheological properties of the dough, cross head speed was set to execute smooth and continuous extrusion. The extruded tubes, cylindrical or rectangular, hexagonal rods, disks, rings were further subjected to laser/CNC machining for the fabrication of diverse shapes such as spring, screw, implants and other medical device applications.
The variable diameter tubes, rods, springs (
Products developed in example 1-9 were further re-shaped into various forms through net shape forming via CNC machining. A desktop 4-axis CNC machine (MODELA pro, MDX-540) was used to perform green machining. Machining was carried out in two modes, stationary and rotary. Prior to machining, ‘stl’ format of model was selected to fabricate physical model from a green body. Different types of tools were selected to execute green machining as per the 3D design. Based on the mechanical properties and green density of the compacts, the machining parameters like depth of cut, rpm, rate of movement of tool and distance between tool paths were optimized during surfacing/roughing. This way, rods developed in example 9 were mounted on CNC and further machined into dental roots (
Different components for structural applications as developed in example 1-10 were further transformed into various shapes with the help of laser machining in green state. Prior to laser machining, a drawing of pattern/shape was sketched in Corel DRAW software. A 2 axis laser machine (VLS 2.30, Universal Laser System, UK) with CO2 laser power of 30 W was used to perform cutting and rasting. Machining surface was focused by adjusting the z-axis. 2D file of drawn pattern was imported into software used for machining. Software controlled laser beam intensity, scanning rate, depth of engraving, mode of machining (stationary/rotary) etc. Optimized machining parameters were set to perform laser machining. A flat green metallic sheet was used for stationary machining, while extruded tubes/rods were used for rotary machining. Micromachining using laser was performed to generate micro/macro roughness on the surface of the developed components. Further, laser cutting was performed to develop shapes like metallic springs, porous architectures, surface texturing etc. (
Components developed in this way can have features from ˜50 nm to meters. Products such as springs, micro gears etc find use in aerospace industries at elevated temperature. Surface texturing generated by this technique in nano to millimeter range is used for biomedical applications to improve tissue adhesion.
Different type of porous architecture was developed through machining and lamination approach. The laser machine (VLS 2.30, Universal Laser System, UK) was used with optimized machining parameters to execute machining. A 2D design for individual tape was sketched in CorelDRAW software. Prior to machining, parameters like laser beam intensity, scanning rate, depth of engraving were optimized. A green tape/sheet developed by viscous dough obtained from example 1 and/or 2, respectively was used to create different shapes and cuts through laser machining. Further, these sheets were joined through lamination for developing different internal porous architectures (
In example 12, micro-patterned surface/texture was developed via laser machining. However, embossing could be an alternate route to laser machining for developing surface textures/roughness with varied dimension from ˜5 micron to millimeters range. Prior to embossing, master molds of PMMA/metal/silicone were developed by laser/CNC machining and coated with petroleum jelly for easy removal of the patterned sheet. Tape developed in example 1 and/or 2 was used for generating surface architecture such as micro channels, micro gears, micro arrays etc (
a) TGA Analysis
Thermo-gravimetric analysis (TGA) of the green samples is carried out to know the binder burn out temperature. Dough obtained from Ti6Al4V-chitosan system of (Examples 1 and 2) was analyzed for TGA by Thermo-gravimetric analyzer (Perkin Elmer Pyris Diamond Model, MA) using alumina crucible under argon atmosphere in the temperature range of 50 to 1000° C. with a heating rate of 10° C./min. It was revealed that binder burn out starts at ˜250° C. and almost completed at ˜450° C. (
b) Particle Distribution
Particle distributions throughout the dough were evaluated under the microscope. Prior to the characterization, samples were completely dried under vacuum at 60° C. Topography of green and sintered samples was studied by sputter coating with gold (Polaron, UK) and observed under SEM (EVO 60, Carl Zeiss SMT, Germany). Samples were also analysed under energy-dispersive X-ray spectroscopy (EDX) to observe oxide formation on the surface. The particles were found uniformly distributed throughout the polymer matrix without significant voids/defects. The particle-particle contacts were sufficient for sintering at 1400° C. for Ti and its alloy using average particle size (d50) 40 micron (
c) Hardness
Micro-hardness measurements of green compacts were carried out using a Vickers diamond indenter (UHL-VMHT, Germany) operated at a load of 5 gram force with an indentation dwell time of 10 s. Further, for measurement of micro-hardness of the sintered samples, LECO Hardness Tester (LV 700, LECO Corporation, USA) was used under 1000 kilogram force load with 10 s dwell time. Prior to hardness measurement, all the samples were polished to focus under optical microscope. At least ten indentations were taken at different positions of the samples for each specimen and the average values were reported.
d) Mechanical-Strength
The strength of a material in Example 2a is measured in terms of either the stress necessary to cause appreciable plastic deformation or the maximum stress that the material can withstand. Prior to mechanical testing, a tapes and bars with variable thickness were produced by PDP process. Further, they were cut into testing samples according to ASTM standard by laser machining and punching (
The flexural strength and compressive strength of both green and sintered samples were measured by using universal testing machine (25K machine, Hounsfield, UK) at cross head speed of 0.5-1.0 mm/min using 5000 N load cell (
The embodiments of the present invention thus relate to a novel approach to form dense and/or porous metallic bodies with various sizes and shapes for biomedical and structural applications with tailorable porosity, pore-size distribution, pore-volume and microstructure. The processing of polymer metal blend composition at ambient condition with controlled viscosity is highly energy efficient for producing various shapes such as rods, tubes, springs, domes, sheets, sponges, biomedical implants.
While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1173/KOL/2014 | Nov 2014 | IN | national |
