1.2.2 Processing
• CAD files are converted to .STL files
• .STL files converted to slices using the parameters, such as,
- various parts on a platform (different .STL files can be mixed)
- blade sweeps per layer and sweep period, and “z-wait” to wait after recoating for material mixing
• The SL process
• The part is then immersed (approx. 0.5”) with a waiting period to recoat the surface and the wiper blade is used to clear the excess fluid from the top of the surface. (Note: the sweep is optional, but it is used to get consistent thickness)
• The part is then moved, and the laser has a focal point near the surface that hardens the polymer.
• As the polymer hardens, it shrinks. This shrinkage causes a change in the volume of the fluid. To correct for this the tank has a fluid level detector that will control an adjustable reservoir that will add enough fluid to compensate for volume change.
• Layers vary in thickness from 0.002” to 0.020”. This is controlled by the amount the platform is lowered into the photopolymer. Thinner layers give smoother, higher tolerance parts (with polymer more cured) but these take longer.
• The laser is stationary, but optics and mirrors are used to guide the beam to x-y coordinates on the surface of the fluid.
• The Slice Cycle
• After the part is done, the part raises above the fluid, and resin drains out. The elevator can be tipped to drain trapped volumes.
• After removal from the bath the part is cleaned off with towels and Q-tips, and hardening of the resin is completed in a curing oven.
• The laser - is often about 10-200 mW (more power is required for faster operation)
- often He-Cd or Argon-Ion to produce UV radiation about a 320-370 nm wavelength
• The optics - the user can set the focal distance of the laser to the range of the slice thickness.
• x-y positioning - uses 2 computer controlled mirrors to reflect beam.
• polymer vat - generally holds 20-200 liters of resin. Can be interchanged to speed up change of resin (lower downtime).
• The photopolymer is light sensitive and toxic. Therefore the operation vat is often out of sight and the unit uses a ventilation unit to evacuate fumes.
• Post Curing Apparatus (PCA) - uses high power ultraviolet light to complete the curing of mostly solidified polymer.
- times are typically 1 hour and up
- after curing parts become non-toxic
• Advantages,
- can run without supervision
- high detail and accuracy
- sharp-edges tend to get “filled” by resin, thus reducing the “stepped” effect between slices
- popularity makes this process well supported
• Disadvantages,
- extra time required for postcuring (up to 16 hrs)
- polymer shrinks as it hardens - the result is stress that warps the part
- toxic chemicals (resin and cleaners)
- limited selection of chemicals (general cost $100-200 a liter)
- experts needed for process setup
- addition of supports needed
- work required after to remove supports
• Weave techniques
- used to reduce curing time and part stresses that cause warping
- most of the part is cured before, because of multiple exposures
- staggered hatching uses exposure in-between lines of previous exposure
• SL units from 3D systems (also see attached specs)
• Units are sensitive to vibration
• Application
- basic polymer is slightly brittle and therefore is best suited to conceptual models
- “Exactomer” is well suited to trial assemblies, and has been used to make secondary rubber, and spray metal tooling (being less brittle it won’t break when being removed from molds).
- Investment casting molds can be made using hollow cores (that minimize polymer expansion when melting) that won’t crack the mold.
- Dupont is creating an investment casting resin that won’t crack the mold.
• In some research fibers have been added to a stereolithography process to obtain higher strengths. [Hyer, 1991]
• An inexpensive stereolithography unit can be made using UV light guided by a fiber optic cable.
• Large parts can be created in pieces and glued together. For example, an impeller can be created in sections. The sections are glued with normal resin, and hardened with a UV lamp. Metal inserts can be added by press fitting, and the part can be machined for precision. This process might cost 1/3 of normal prototype costs.
• There are a wide variety of techniques for creating cast metal parts and molds from STL resin parts [Ashly, 1994]. These include parts cast from SLA tooling directly. For example, SLA wax parts can be used to do investment casting.
