A Computer Model for Laser Photopolymerization

A computer model for a laser induced photopolymerization process has been established which simulates stereolithography. The model couples irradiation, chemical reaction, and heat transfer equations to provide insights into rate processes occurring in the volume element contacted by the laser beam. Quantities predicted include the spatial variation in conversion of monomer to polymer, depletion of photoinitiator, and local variations in temperature in and around the spot contacted by the laser. This allows predictions to be made about the laser dwell time, depth penetration and uniformity of the photopolymer formed in the process.Mechanical Engineerin

Material and Process Parameters that Affect Accuracy in Stereolithography

Experimental real time linear shrinkage rate measurements simulating stereolithography are used in an analysis of shrinkage during line drawing in stereolithography. While the amount of shrinkage depends on the polymerization kinetics, shrinkage kinetics and overall degree of cure, it also depends on the length of time to draw a line of plastic. A line drawn slowly will exhibit less apparent shrinkage than one drawn very quickly because much of the shrinkage is compensated for as the line is drawn. The data also indicates that a typical stereolithography resin in the green state may shrink to only 65% of its maximum, thus retaining considerable potential for shrinkage during post-cure. This infonnation can be used to predict the amount of shrinkage to be expected under certain exposure conditions and to fonnulate overall strategies to reduce shrinkage and subsequent warpage that causes shape distortion.Mechanical Engineerin

A Simple Polymer Shrinkage Model Applied to Stereolithography

A simple polymer shrinkage model has been successfully applied to the stereolithography process. The shrinkage model, which computes specific volume changes from the degree of conversion of monomer to polymer, incorporates a lag between conversion and shrinkage. An overall process model used to simulate the stereolithography process was modified by inclusion of the shrinkage model. Use of the modified stereolithography process model allows prediction of the shrinkage that might be expected to occur when fabricating a strand of plastic. By varying the lag between conversion and shrinkage it is shown that faster shrinking resins should exhibit lower overall shrinkage than slower shrinking resins. This is a direct result of the fact that less shrinkage occurs after the strand has been scanned for the faster shrinking resins.Mechanical Engineerin

Simulation of Laminated Object Manufacturing (LOM) with Variation of Process Parameters

A previously developed and verified thermal model for Laminated Object Manufacturing (LOM) was used to investigate the effects of various processing parameters on the temperature profile in a LOM part during the build cycle. The mathematical model, based on 3-dimensional transient heat conduction in a rectangular geometry LOM part, allows calculation ofthe transient temperature distribution within the part during the application of a new layer as well as during other periods ofthe LOM build cycle. The parameters roller temperature, roller speed, chamber air temperature, base plate temperature, and laser cutting time were independently varied, and the LOM process response simulated. The results were analyzed in order to gain insight into potential strategies for intelligent process control.Mechanical Engineerin

Five mucosal transcripts of interest in ulcerative colitis identified by quantitative real-time PCR: a prospective study

<p>Abstract</p> <p>Background</p> <p>The cause and pathophysiology of ulcerative colitis are both mainly unknown. We have previously used whole-genome microarray technique on biopsies obtained from patients with ulcerative colitis to identifiy 5 changed mucosal transcripts. The aim of this study was to compare mucosal expressions of these five transcripts in ulcerative colitis patients vs. controls, along with the transcript expression in relation to the clinical ulcerative colitis status.</p> <p>Methods</p> <p>Colonic mucosal specimens from rectum and caecum were taken at ambulatory colonoscopy from ulcerative colitis patients (<it>n </it>= 49) with defined inflammatory activity and disease extension, and from controls (<it>n </it>= 67) without inflammatory bowel disease. The five mucosal transcripts aldolase B, elafin, MST-1, simNIPhom and SLC6A14 were analyzed using quantitative real-time PCR.</p> <p>Results</p> <p>Significant transcript differences in the rectal mucosa for all five transcripts were demonstrated in ulcerative colitis patients compared to controls. The grade of transcript expression was related to the clinical disease activity.</p> <p>Conclusion</p> <p>The five gene transcripts were changed in patients with ulcerative colitis, and were related to the disease activity. The known biological function of some of the transcripts may contribute to the inflammatory features and indicate a possible role of microbes in ulcerative colitis. The findings may also contribute to our pathophysiological understanding of ulcerative colitis.</p

A Thermal Model For Laminated Object Manufacturing (LOM)

A thennal model for Laminated Object Manufacturing (LOM) has been developed. The model is based on 3-dimensional transient heat conduction in a rectangular geometry LOM part. Heat transfer from the heated roller to the laminated part as well as heat loss to the surroundings and the base plate are considered. It allows calculation of the transient temperature distribution within the part during the application of a new layer as well as during other periods of the LOM build cycle. To verify the model performance, thennocouples were embedded every 4th layer in a 20-layer ceramic part while it was being built on a standard LOM-2030. The model predictions are in excellent agreement with the measured temperature profiles. In addition to explaining the observed thennal behavior ofLOM parts, model predictions also have direct application to on-line control ofthe part temperature during the build process, to be discussed herein.Mechanical Engineerin

Model-Based Control of Cure Distribution in Polymer Composite Parts Made by Laminated Object Fabrication (LOF) 409

A mathematical heat transfer model was used to investigate process control strategies for making thermoset polymer composite materials by Laminated Object Fabrication (LOF). The temperature of the laminator was manipulated in order to control the uniformity and overall level of cure through the thickness of a 20-layer part. When the laminator temperature was held constant throughout the LOF build process, as is normally the case in practice, the model predicted that the resulting panel would have a steep cure gradient from top to bottom. This was considered to be undesirable. The model was then used in conjunction with an optimization algorithm to determine a temperature program for the laminator which would result in panels with a more desirable spatial cure profile (i.e. constant). Computer model simulations demonstrated that it should be feasible to control both the level and distribution of cure in thermoset composite panels layed-up with LOF by simply manipulating the laminator temperature with simple and realistic heating schedules.Mechanical Engineerin