Browsing by Subject "Department of Chemical Engineering"

Now showing 1 - 11 of 11

Ceramic Structures by Selective Laser Sintering of Microencapsulated, Finely Divided Ceramic Materials
(1992) Vail, N.K.; Barlow, J.W.
The feasibility of producing ceramic green parts by Selective Laser Sintering from microencapsulated, finely divide ceramic powders has been reported in an earlier paper. Post-processing of a silica/zirconium orthosilicate system and an alumina system, both utilizing a polymer binder in the form of a coating, are discussed in this paper. Ceramic green parts require postprocessing to remove the intermediate polymer binder and to impart strength properties to the ceramic bodies. In this paper, the use of ceramic cements and high temperature firing to realize strengths will be discussed. The effects of cement concentration and controlled drying rates on the strengths and dimensional accuracy of the ceramic bodies are also discussed.
Indirect Metal Composite Part Manufacture Using the SLS Process
(1993) Tobin, James R.; Badrinarayan, B.; Barlow, J.W.; Beaman, J.J.
As a near term alternative process to direct sintering, an intermediate polymer binder is combined with powder to produce green preforms with the Selective Laser Sintering (SLS) process. To produce parts with desirable strength dimensional control, the binder is gradually removed from the green preform (obtained from the SLS process), and the remaining form is lightly bonded. This porous part then infiltrated. Final part density, shrinkage, and strength data are presented. An injection mold insert was fabricated from this material and used to mold PMMA, polyester, and polycarbonate parts. To date, the mold insert 176 shots at injection to 35,000 and melt temperatures up to 300°C.
Laser Sintering Model for Composite Materials
(1993) Nelson, J.C.; Vail, N.K.; Barlow, J.W.
A computer model for the sintering of ceramic/polymer composite materials has been established based on empirical sintering rate data. The model calculates sintering depths which result from variations in the operating parameters which include laser power, beam speed, scan spacing, scan vector length, and initial temperatures of the powder and surroundings. Sintering depths measured in multiple layer parts made of polymer coated ceramic powders are compared to sintering depths calculated by the sintering model.
Metal Parts From Selective Laser Sintering of Metal-Polymer Powders
(1992) Badrinarayan, B.; Barlow, J.W.
Microencapsulation of Finely Divided Ceramic Powders
(1990) Vail, N.K.; Barlow, J.W.
Polymer coated alumina particles have been prepared by spray drying alumina powder with a polymer emulsion. Powders containing a maximum of 50% wt. were obtained. The coated particles were compared to mixtures of alumina and polymer. Oven sintering tests show the coated material to compact more than the mixed and pure polymer materials. Strong, well defined parts with layer thicknesses of 0.002" were produced from both coated and mixed materials by the Selective Layer Sintering process.
Post-Processing of Selective Laser Sintered Polycarbonate Parts
(1991) Nelson, J.C.; Vail, N.K.
St\ldies were conducted·to det~rmine the effect of press\lre onpost-prpeessingofpolycarbonate coupons. No significant differences betweencou.pons processed at atmospheric •cQndidons.. andcouponsprpeessedin.vacuum were •observed. However, anisotropic shrinkage .was observedin.thez-directionnormal to thesintering plane. Thisphenomenawasfurtl1erinvestigated usingsYIllmetrical polycarbonatecubespr9(iuced by Selective LaserSintering using yariedlas~rpoweryariedpowderlayer thickness, and varied powder bed temperatures
Prediction of the Thermal Conductivity of Beds Which Contain Polymer Coated Metal Particles
(1990) Badrinarayan, B.; Barlow, J.W.
Structural parts of ceramics or metals can, in principle, be made by laser sintering polymer coated ceramic or metal powders, followed by conventional methods for removing the binder and sintering in ovens. Understanding the laser sintering of coated materials requires knowledge of the behavior of beds containing composite particles. Many correlations for predicting the effective thermal conductivity of a bed of solid particles exist in literature, but little work has been done on beds of coated particles. We coated lead shots(high conductivity) with a styrene acrylic acid copolymer (low conductivity) to study the effect of coating thickness on the thermal conductivity. The thermal conductivity ofthe coated particle·hed was found to •drop rapidly in the beginning·· and then level off with increasing coating thickness. We also developed an equation that yields the equivalent conductivity of a coated spherical particle subjected to axial heat flow. The predicted results with the experimental measurements of bed conductivity obtained by an unsteady state method.
Selective Laser of a Copper-PMMA System
(1991) Badrinarayan, B.; Barlow, J.W.
The Selective Laser Sintering process was used to manufacture green copper parts from a copper-polymer mixture. PMMA was used as the polymer binder for the metal. The green part was fired in a furnace under reducing conditions to obtain a pure metal part. The metal-polymer system and the conditions used to make parts are described in this paper. The effect of laser parameters and particle sizes on part density are also discussed. Keywords: laser sintering, copper, polymer, part density, dwell time, shrinkage.
Selective Laser Sintering of Bioceramic Materials for Implants
(1993) Lee, Goonhee; Barlow, J.W.
Selective Laser Sintering (SLS) process is employed for fabrication of biocerarnics for orthopedic implants. Hydroxyapatite and Calcium Phosphate ceramics are coated with polymer as a intermediate binder by using a spray drier. Polymer coated materials are SLS processed to make green parts, which are infiltrated and fired to remove the polymer. SLS processed green parts of hydroxyapatite have low density due to the small particle size with large specific surface area. This paper discusses the possibilities and problems in free-form fabrication of bioceramic.
Silicon Carbide Preforms for Metal Infiltration by Selective Laser Sintering™ of Polymer Encapsulated Powders
(1993) Vail, N.K.; Barlow, J.W.; Marcus, H.L.
A polymer encapsulated silicon carbide system has been developed for use with Selective Laser Sintering. Extensive studies with this material have provided information pertaining to processing and material parameters which most affect the strengths and densities of resulting green parts. The important parameters considered were particle size distribution of the powders, laser scanning conditions, and laser beam diameter. Simple and complex shapes were easily produced with this material using optimized parameters. Green objects were infused with metal by Lanxide using their pressureless infiltration process to produce both metal matrix and ceramic matrix composites. (Key Words: Silicon Carbide, Encapsulation, Polymer, Selective Laser Sintering, Composites).
Sintering Rates in the Selective Laser Sintering Process
(1990) Nelson, Christian; Barlow, J.W.
This paper presents. a procedure to determine rates of sintering as a function of temperature using an isothermaloven. The rate of height change of a powder sample in the oven at temperatures near the Tg or Tm is measured. From this information an activation energy is calculated. This activation energy is similar to activation energies calculated from viscosity versus ternp.erature curves for polymer melts. This similarity suggests that viscous sint~ring models such. as those by Frenkel and Scherer are appropriate. A comparison between sintering rates of polymer coated alumina powder and mixed powders of the polymer with alumina, suggest that better sintered products may result from the coated powders than from simple mixtures.