Stereolithography

The process of taking an idea from its original conception through all of the required phases that are necessary prior to its implementation out in a manufacturing environment, is both a time consuming and a costly endeavor. Since the amount of time that it takes for a company to actually reach the production/manufacturing phase of a product can be directly measured in dollars and cents, it only makes sense that the shorter this time is, the more profitable that it will be for the company. Manufacturers can reduce both the design and development time of a new product by building a prototype of an idea as early as possible in the development process. Enter the need for a concept known as rapid prototyping. Rapid prototyping not only shortens production lead-time, it can sometimes also show pending faults in the early stages of the design process before they can cause problems later on.

Rapid prototyping is rapidly becoming a successful process for quickly modeling new products by generating physical models directly from a 3-D computer drawing. Companies can utilize rapid prototyping methods to turn CAD drawings into three-dimensional objects quickly and inexpensively when compared to the conventional techniques that are usually employed. The CAD software is used as a method to define both the geometry and the dimensional requirements of the finished product while still in the modeling phase of the development. The data from this CAD file is then electronically transmitted to a rapid prototyping system. There are several different types of rapid prototyping systems available, each utilizing its own distinct process depending on such factors as required model accuracy, equipment cost, model material, type of model, and probably most important modeling time. Stereolithography is one such technique that lends itself nicely to the requirements of rapid prototyping.

Stereolithography is a process that was developed and patented by 3D systems of Valencia California in 1987.1 The stereolithography apparatus, SLA, consists of a vat of a liquid polymer in which there is a movable elevator table/platform that is capable of moving (lowering) in very precise increments depending on the requirements defining the type of model that is to be constructed. A helium/cadmium laser is then used to generate a small but intense spot of ultraviolet light which is used to move across the top of the vat of liquid polymer by a computer controlled optical scanning system. At the point where the laser and the liquid polymer come into contact, the polymer is changed into a solid. As the laser beam is directed across an xy surface, the model is formed as a plastic object point by point and layer by layer as true as is allowed by the type of photopolymer that is being used in all three dimensions: x, y, and z.2 As each layer is formed, the elevator platform is then lowered by 0.005 so that the next layer can be scanned in. As each additional layer is formed, it then bonds to the previous one and the resulting model is generated by a precise number of successive layers.

The first step is to create a 3D solid model of a part in a CAD system, and then design a structure that will be used to provide support and also to link the part to the platform during fabrication. Then the CAD file of the model and the support structure are converted to an STL format using 3D Systems' special interface specifications. The SLA special slice computer then divides the STL file into cross sections that can range anywhere from 0.005 to 0.030 of an inch thick. The slice computer then converts this slice data into an SLI format which is used by the STA to control the movement of the laser and the elevator platform. The laser is actually reflected off from a galvanometer controlled mirror which is controlled by the vector data contained in the SLI file. As each solid layer is formed, the platform lowers and a wiper blade passes over the surface to level it. Actual fabrication time depends on the thickness of the layers and the size of the part being produced. 3 The part is produced starting at the bottom and working towards the top.

After the part is removed from the SLA, it must be ultrasonically cleaned to remove any excess polymer material from crevices and openings. Then the part must undergo a post curing operation to finish hardening the polymer. This process involves bathing the part in intense long-wave ultraviolet light which causes any uncured liquid that may be trapped within the structure to harden. At the end of this process, which usually takes about 30 minutes, the part can then be removed from the support structure and finished by any number of methods until the surface finish is of the texture that is required.

At this time, there are currently two types of polymers that are being used. They are an acrylate and an epoxy based resin. The acrylate based resin is the older of the two. It has a rubbery consistence when it is removed from the SLA. Because of this, it takes longer to fully cure in the post curing process. When fully cured, it is a "hard-candy" like material which can be sanded, filled, glued, and painted without problems. If it must be machined, extreme care must be taken because it is somewhat brittle. It has a translucent yellowish brown appearance. Epoxy based resins are almost transparent and much more rugged when fully cured. They come out of the SLA almost fully cured, and therefore require very little post curing. Because of this they also require fewer and weaker support structures, which also helps with part clean-up and accuracy of dimensions. However, parts made from epoxy based resins build slower, which results in higher costs.4

At this time, parts that are made by stereolithography are capable of holding tolerances to 0.005, and even finer tolerances are possible by finish machining or precision grinding of the finished part.5 However, the complex shapes and geometry of the parts that this system is capable of producing is virtually limited only by ones imagination. The cost of a SLA may be prohibitive to some companies, but there are currently a multitude of job-shops out there who have the equipment and the know how who are just waiting for the chance to demonstrate their abilities in this new field of rapid prototyping.

REFERENCES:

1. Stereolithography: A Primer Manufacturing-Engineering

Vol. 98 December 1990 pg. 39-41

2. Stereolithography: An Introduction Chemtech

Vol. 20 October 1990 pg. 615-619

3. Make Fiction Fact Fast Manufacturing-Engineering

Vol. 106 March 1991 pg. 44-49

4. http://www.phantom.com/~pauldowd/sla.html

5. Stereolithography Automates Prototyping Mechanical Engineering

Vol. 112 February 1990 pg. 34-39