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•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.

65.2.3 References

Hyer, M.W., and Charette, R.F., “Use of Curvilinear Fiber Format in Composite Structure Design”, AIAA Journal, 1991, pg. 1011-15.

65.3 BONDED POWDERS

•Basically a loose powder is spread in a layer, and an bonding adhesive is selectively applied to harden a slice. Layers are continually added until one or more parts are completed.

•A trademarked name for this process is 3DP (3 Dimensional Printing)

•The general sequence is pictured below,

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Layer thickness 0.005” to 0.009”

Printing head is 0.36” wide and has 128 jets

Equipment size 29” by 36” by 42”

Mass 300 lb.

Consumable materials approx $0.65 per cubic inch of finished part

No special environmental requirements

Basic unit costs $59,000

An IBM compatible PC is required to run the machine

•Advantages,

-inexpensive

-fast

-complex geometries

-suitable for desktop usage

-investment casting can be done from models

-colored parts

•Disadvantages,

-part material limited and not engineering materials

-lower part strength

65.4 SELECTIVE LASER SINTERING (SLS)

•Powdered material is fused together in layers using a laser

•The powders need fine grains and thermo-plastic properties so that it becomes viscous, flows, then solidifies quickly.

-nylon

-glass filled nylon

-somos (elastomer)

-polycarbonate

-trueform (ceramic??)

-sandform ??

-rapid steel (metal)

-copper polyamide (metal)

•invented in 1986 by Carl Deckard

•marketed by DTM corp. (Sinterstation 2000)

•The process uses a heated chamber (near the powder melting temperature)

•The product is split into slices from the .STL file and created one layer at a time by spreading

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layers of powder, sintering the powder with a CO2 laser, then adding new layers of powder and sintering until done.

• When done the part is inside a cake of powder, and putty knives and spatulas are used to remove the loose powder

x-y positioning

a roller spreads powder evenly

Powder is supplied to the system using a cartridge system

•Supports not needed as the unsintered powder supports overhangs/etc.

•powder can be reused

•slow cooling of the parts can prevent distortion due to internal stresses.

•The laser is about 50W infrared (about 10000nm) This power level is much higher than stereolithography

•Optics and x-y scanner are similar to SL

•the process chamber runs hot to decrease the power required from the laser, and reduce thermal shrinkage that would be caused by a difference in operation and cooling temperatures.

•The hot chamber is filled with nitrogen (98% approx.) to reduce oxidation of the powder.

•rate of production is about 0.5-1” per hour

•Advantages,

-inexpensive materials

-safe materials

-wide varieties of materials: wax for investment casting; polymers/nylon for assembly prototypes

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-supports not needed

-reduced distortion from stresses

-produce parts simultaneously

•Disadvantages,

-rough surface finish (“stair step effect”)

-porosity of parts

-the first layers may require a base anchor to reduce thermal effects (e.g. curl)

-part density may vary

-material changes require cleaning of machine

•DTM markets the Sinterstation 2000 for $250,000(US) to $497,000(US) depending upon the selection of 1, 2, or 3 materials (investment casting wax, nylon, or polycarbonate). The Sinterstation 2500 starts at $400,000

•Development is being done on,

-new materials

-high power lasers for metal powders/etc.

•Selected specifications for a Sinterstation 2000 are given below,