RP's Frequently Asked Questions.

RP's Frequently Asked Questions.

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(2. What are the limitations?)

Finishes
The finish and appearance of a part are related to accuracy, but also depend on the method of RP employed. Here again, taking stereolithography as "very good," we get about the same differentiation among the methods produced by accuracy as described above. Technologies based on powders have a sandy or diffuse appearance, sheet-based methods might be considered poorer in finish because the stairstepping is more pronounced.


Surface finish, stereolithography.	Surface finish, fused deposition modeling (FDM).	Surface finish, three dimensional printing (3DP), plaster material.	Surface finish, MultiJet Modeling (MJM).	Surface finish, laminated object manufacturing(LOM).
(Courtesy, Andreas Gebhardt, CeP-Prototyping Center, Germany)	(Courtesy, Stratasys Corp.)	(Courtesy, Z Corporation)	(Courtesy, Cadem A.S., Turkey)	Courtesy, Cubic Technologies, Inc.

Resolution refers to the minimum increment in dimensions that a system achieve. It's one of the main determining factors for finish, appearance and accuracy, but certainly not the only one. For most RP systems, resolution is in the "few mils" range. Specially modified systems are available that can reproduce much finer features, but they are limited in the size of the parts they can fabricate. The inkjet-based system from Solidscape is capable of very high resolution. Note that resolutions can be different for each of the X,Y and Z axes on the same system. For example, Objet specifies the resolution of its Eden series as 300 x 600 dpi (dots per inch) with a Z-axis layer thickness of 0.0006 in.


Wax sculptural object by Bathsheba Grossman using ModelMaker inkjet technology.	Surface finish of sculpture with 0.005 inch steps.	Surface finish of sculpture with higher resolution 0.0005 inch steps.
(Courtesy, Bathsheba Grossman Protoshape, Inc.)

In any case, it will probably be necessary to sand, paint, fill, polish, infiltrate a secondary material to decrease porosity, or perform other operations on the part before it can be used.

Minimum feature size refers to the smallest detail of an object that can faithfully be reproduced. Mathematical definitions are usually based on a minimum curvature as a limit, but the table shows some anecdotal values drawn from experienced individuals' postings from the rapid prototyping mailing list and other sources.

Rule of Thumb Miniature Feature Sizes (in inches unless otherwise specified)
Material > >			Photopolymer	Plastic	Metal	Sand	Starch	Plaster	Paper
stereolithography	SLA	3D Systems et al	0.0040
(micro) stereolithography	SL	microTec	1 micron
fused deposition modeling	FDM	Stratasys		0.0050
selective laser sintering	SLS	3D Systems		0.0050	0.0050	0.0200
selective laser sintering	SLS	EOS			0.0080	0.0200
3-Dimensional Printing	3DP	ProMetal			0.0080
3-Dimensional Printing	3DP	Z Corp.					0.0080	0.0080
inkjet	MM	Solidscape		0.0050
laminated object manufacturing	LOM	Cubic Technologies							0.0080
Polyjet	J-P	Objet Geometries Ltd.	0.0060

Secondary operations
FAQ Fig

In many cases, plastic or other soft material-based rapid prototyping parts can be used directly, or after relatively minor finishing operations. Parts made by stereolithography are frequently not completely cured when removed from the machine. Final cure is effected in a box called a post-cure apparatus (PCA) where the part may be bathed in UV light while on a turntable. Before the parts are cured they must be drained of excess polymer, bathed in a solvent and any remaining excess liquid photopolymer daubed away with paper towels.

Parts from processes such as three dimensional printing (3DP) and MultiJet Modeling (MJM) can be very fragile and might not be able to take normal handling or shipping stresses. Parts made by three dimensional printing (3DP) are often infiltrated with cyanoacrylate adhesive or wax as a secondary operation to make them more durable.

All rapid prototyping methods which directly produce metal parts will almost certainly require final machining and other secondary operations before acceptable final finishes and tolerances are achieved, although there may be a few exceptions based on the requirements for a particular part. Before they can be final-machined, metal and ceramic parts made by selective laser sintering (SLS) must usually undergo a thermal baking cycle to lightly consolidate them into a "brown" part, and then they may undergo a final thermal cycle to sinter and infiltrate them with a material to make them fully-dense.


Green steel insert formed by selective laser sintering (SLS).	Inserts in crucible ready for bronze infiltration.	Polished steel insert, ready for injection molding (not the same part).
(All photos courtesy, DTM, Inc., now 3D Systems.)

Support structures must be removed
Powder-based methods of rapid prototyping are self-supporting for features such as overhangs and undercuts; the excess powder is simply shaken off or vacuumed away. All other methods require a support structure of some kind which is fabricated right along with the part. This must subsequently be removed in a secondary operation which may require considerable effort and time.


Support structure, stereolithography.	Support structure (red material), water-soluble, fused deposition modeling (FDM).	Support structure, Multijet Modeling (MJM).	Support structure, (red material) ModelMaker inkjet.
(Courtesy, Cadem A.S., Turkey)	(Courtesy, Stratasys Corp.)	(Courtesy, Cadem A.S., Turkey)	(Courtesy, Bathsheba Grossman Protoshape, Inc.)

Stereolithography parts require the supports to be cut off and the areas they were attached to finished, often by hand.

Inkjet based systems either use a hair-like support structure, as in the case of MultiJet Modeling (MJM), or a second support material which is removed with a solvent. The hair-like structures can be brushed away and the remaining down-facing surfaces cleaned up manually.

Fused deposition modeling (FDM) may use either a snap-away support structure made from a second material that doesn't stick to the part material, or a water-soluble support structure.

Removal of laminated object manufacturing (LOM) support structure.

(Courtesy, Cubic Technologies, Inc.)

Laminated object manufacturing (LOM) may require labor-intensive work to "de-cube" trapped volumes or other portions of the part that need to be removed. It may be necessary to perform this delicate operation with a hammer and chisel. Methods of LOM have been under development for several years that avoid this issue, but there is nothing commercially available yet.

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