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

Photopolymerization Reaction Rates By Reflectance Real Time Infrared Spectroscopy: Application To Stereolithography Resins

An advanced real time infrared technique for studying the isothermal in-situ cure of ultra-fast photopolymerization reactions has been developed. The method, referred to as reflectance real time infrared (RRTIR), involves time resolved IR analysis by reflected IR radiation while a resin sample is being exposed to a UV laser beam. The effect of factors such as chemical composition, radiation intensity, and temperature on reaction rate were determined for multifunctional acrylate resins exposed to a HeCd laser (325 nm). Isothermal cure profiles were monitored quantitatively through disappearance of the 810 cm-1 acrylate IR absorbance band. The dark reaction after the UV radiation was turned off also was monitored. The RRTIR method is shown to be highly effective for quantifying photopolymerization reactions in the millisecond time range. The rate data indicate that quantitative comparisons between reactivities and conversions for different stereolithography resins are possible using this method under conditions that simulate the SLA process. Also, the data show conclusively that the reaction continues for long periods of time after initial laser exposure. This is expected to be a significant factor in the development of warpage and curl during the SLA building process.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

Development of polyisocyanurate pour foam formulation for space shuttle external tank thermal protection system

Four commercially available polyisocyanurate polyurethane spray-foam insulation formulations are used to coat the external tank of the space shuttle. There are several problems associated with these formulations. For example, some do not perform well as pourable closeout/repair systems. Some do not perform well at cryogenic temperatures (poor adhesion to aluminum at liquid nitrogen temperatures). Their thermal stability at elevated temperatures is not adequate. A major defect in all the systems is the lack of detailed chemical information. The formulations are simply supplied to NASA and Martin Marietta, the primary contractor, as components; Part A (isocyanate) and Part B (poly(s) and additives). Because of the lack of chemical information the performance behavior data for the current system, NASA sought the development of a non-proprietary room temperature curable foam insulation. Requirements for the developed system were that it should exhibit equal or better thermal stability both at elevated and cryogenic temperatures with better adhesion to aluminum as compared to the current system. Several formulations were developed that met these requirements, i.e., thermal stability, good pourability, and good bonding to aluminum

Novel Liquid Crystal Monomers for Stereolithography: Reaction Rates and Photopolymerization Conversion

Liquid crystal (LC) monomers are a novel type ofresin for stereolithography that result in polymers having unique physical and mechanical properties. These monomers consist ofrigid central cores connected to acrylate functional groups by short aliphatic chains. Because ofthe rigid-rod structure ofthe monomer the cross-linked polymer networks formed have high glass transition temperatures (Tg). The high TglS result in particularly high upper-use temperatures for stereolithography parts. This paper reports on photopolymerization reaction rates and monomer conversion for two acrylate monomers measured using reflectance real-time infra-red spectroscopy (RRTIR). The RRTIR method measures the disappearance ofreactive acrylate groups in the monomer as a function oftime while the monomers are being exposed to UV light. For the two new resins, UV irradiation using an argon ion laser gives rapid photopolymerization with acrylate conversion as high as 95 %. Conversion and polymerization rates in these monomers are dependent upon photo-initiator selection and concentration. In addition, the results indicate that conversion increases with increased laser intensity and elevated temperatures.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

Linear Shrinkage of Stereolithography Resins

The linear shrinkage of an acrylate and an epoxy based stereolithography resin was measured during cure. A helium-cadmium (He-Cd) laser cured strands of resin as is done in the stereolithography process using two exposures. The exposure time was held constant while the delay time between exposures was varied. It was found for both resins that the final cure depth and linear shrinkage were a function of delay timeMechanical Engineerin

Development of a Curved Layer LOM Process for Monolithic Ceramics and Ceramic Matrix Composites

A novel rapid prototyping technology incorporating a curved layer building style was developed. The new process, based on Laminated Object Manufacturing (LOM), was designed for efficient fabrication of curved layer structures made from ceramics and fiber reinforced composites. A new LOM machine was created, referred to as Curved Layer LOM. This new machine uses ceramic tapes and fiber prepregs as feedstocks and fabricates curved structures on a curved-layer by curved-layer basis. The output of the process is a three dimensional green ceramic that is capable of being processed to a seamless, fully dense ceramic using traditional techniques. A detailed description is made of the necessary software and hardware for this new process. Also reviewed is the development of ceramic preforms and accompanying process technology for net shape ceramic fabrication. Monolithic ceramic (SiC) and ceramic matrix composite (SiC/SiC) articles were fabricated using both the flat layer and curved layer LOM processes. For making curved layer objects, the curved process afforded the advantages of eliminated stair step effect, increased build speed, reduced waste, reduced need for decubing, and maintenance of continuous fibers in the direction of curvature

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