How to use the 3D printer
The Gensys-XS 3D printer
(from Stratasys) has a printing
volume of 12” x 8” x 8”, and can print arbitrary geometries out of white
thermoplastic polyester compound (P1500).
There are three basic steps in printing a part:
Preparing a geometric specification
The standard format used to specify the geometric shape of the part to
be printed is called STL (Stereo lithography). This is a very simple format
that essentially describes the boundary of the part as a list of triangles
with normal vectors pointing to the exterior side. Typically only the pure
part geometry is described – without printing supports, bases and other
auxiliary structures that are usually added at the preprocessing stage.
Note that an STL file has no units. Most STL processors assume by default
the file to be in either millimeters or inches, but the final units of
measurements can be user specified. There are three ways to generate the
Using a CAD system
From a modeling library
Generating STL files on a CAD system
Typically a design is manually created using solid-modeling CAD software,
and then exported in STL format. Most commercial CAD systems have an “export”
or “save as” option for this purpose; for example both SolidWorks
(we have a 30 day traial version which has not been installed yet) and
Parametrics ProE (SGI/NT) support STL export. A critical parameter in generation
of an STL file is the resolution in which the part surface is facetted.
The resolution is typically measured as the maximum deviation permitted
from the true surface of the model. The maximum deviation should be, say,
an order of magnitude finer that the printer resolution. The Genesys-XS
resolution is approx 0.2 mm, so a good faceting resolution is 0.05mm.
Using a modeling library
In some cases it is easier to generate part specification from within a
program. In this case, use a library such as Unigraphics Solutions' Parasolid
or Spatial Technology's ACIS to generate
the solid definition of the object, and then use geometry inquiry functions
to obtain the precise boundary and generate an STL file. Some libraries
might provide an explicit STL Export function. In the Demo Lab we have
purchased the Parasolid solid modeling C library for NT, for which I have
written a short subroutine that uses the built-in
graphic renderer to extract a faceted description of the geometry and save
it in STL format. We have been offered (Nov-10-1999) the ACIS C/Scheme/Lisp
Library for unix/linux free of charge if there is interest.
It is always possible to generate an STL file directly. The STL file can
be generated in ASCII or in binary form. The precise file specification
can be viewed here.
A brief description follows:
BINARY STL file format is accessed by byte. The format is as follows:
the first 80 bytes are used for description, and the next 4 bytes represents
the total number of the facets(Long Int), followed by the facet information
(normal and 3 vertices), the normal and vertices are stored in floating
point format, each occupying 4 bytes. At the end of each facet information
section, there are two bytes spaces, then the next facet is repeated till
the end of the file. When BINARY format is used to describe STL file, the
data size is much smaller than ASCII format, so most STL files available
now use BINARY format.
<BINARY STL file format>::=<STL file entity name><facet number
<STL file entity name >::=<80 bytes entity name, spaces are used
to fill the blank>
<facet number N>::=<4 bytes long integer>
<facet info>::=<facet normal><facet vertices><2 bytes spaces><facet
normal ><facet vertices><2 bytes spaces> ... ...
<facet normal>::=<lx,ly,lz, float, 12 bytes>
<facet vertex coordinates>::=<x1,y1,z1,x2,y2,z2,x3,y3,z3, float,
Verifying the specification
It is strongly advised that the geometry be inspected using a third party
software before printing. Although the Printer’s software does verify and
display the STL file, it is often hard to see details, and impossible to
perform measurements or see internal/hidden geometries. There are several
STL viewers around. At the Demo Lab we have installed Actify’s
3DView on the NT machines as well as Solid
Concepts' SolidView. 3Dview is an excellent viewer (free for academic
use) that not only renders the part from STL, but also allows making various
measurements, changing transparency setting to make hidden parts visible,
as well as cross sectioning. It also has an option for computing volume
– this option is useful for estimating print time (about 1 hour per cubic
inch) and for checking for “leaks” in the boundary definition.
Setting up the printer
Before dispatching a print file the printer must be setup and turned on-line.
The printer is essentially a stand-alone Linux box connected directly to
the network. After switching on, it typically takes a few minutes to boot,
and then can be operated from the small 4-line LCD and keys on the front
Typically, after switching on it should be made certain that the printing
platen is clear of previously printed parts or leftovers. Especially make
sure there are no small strands of plastics left along the right edge of
the platen, where the print heads performs a print extrusion test before
every print job. Once the platen is clear, use the up/down and enter keys
to select “Clear platen and home”. This operation might take several minutes
as the platen needs to warm up. When complete, select “Put printer Online”.
After printing is completed, remove the part from the platen using the
plastic spatula provided with the printer (be careful not to scratch the
printing surface). Once the platen is clear, select “Shutdown”, wait 30
seconds while the system shuts down and then turn off the power.
Print cartridges are loaded into the machine through the top door. Open
the door to reveal 10 cartridge slots. Some of the slots may be vacant,
while others might contain cartridges, either with material or empty. New
cartridges simply insert and click in. Remove empty cartridges by pressing
them inwards, lifting slightly and pulling out. A single cartridge supports
approx 10 hours of print (equal to approximately 10 cubic inches printing
volume) and costs $90. Make sure you have enough printing material before
you start, although additional material can be loaded in mid print by pausing
the print job.
Important: When inserting a new cartridge, make sure the plastic
wafers are vertical and flush with the cartridge edge. Usually the wafers
tend to tilt forwards so that their lower edge is not flush with cartridge
end, and a "stapling error" will occur later.
To align the wafers, push and release them against the spring several times
before you insert the cartridge.
In some instances it is necessary to perform additional printer maintenance
Clean the print platen with alcohol.
Replace a printer tip when it becomes clogged or printing is uneven (or
when you get too many “stapling” problems?) – See user guide for instructions.
Recalibrating plastic pump extrusion and suck-back rates– See user guide
A stapling error occurs when the wafers do not feed well into the pump.
Typically this happens because the wafers are not exactly vertical
in the cartridge. When inserting a new cartridge, make sure the plastic
wafers are vertical and flush with the cartridge edge they are pushing
aginst. Usually the wafers tend to tilt forwards so that their lower edge
is not flush anymore. To realign the wafers, extract the cartridge, then
push and release them against the spring several times.
With any other operating problems contact Stratasys
support service at
or at their offical contact point
14950 Martin Drive, Eden Prairie, MN 55344-2020 USA
Telephone: +1.612.937.3000 FAX: +1.612.937.0070
or, contact the agent through which we bought the printer
Preprocessing and dispatching files
Once an STL file has been prepared and the printer is online, the file
can be preprocessed and dispatched. The preprocessing software used with
the Genysis printer is called Autogen, and is installed on the NT boxes.
Versions are also available for the SGI’s. Basically, the program loads
an STL file, generates supports and auxiliary structures, prepares a printer
instruction file (CMB file) and dispatches it to the printer. Operation
of the software is straightforward, but several options should be noted:
The programs generates print supports by default, these are usually good.
For particularly difficult geometries with severely overhanging heavy volumes,
set the support option to “maximum”. If you are unsatisfied with the supports,
select “none”, and generate your own support structures as part of the
By default, Autogen fills volumes with solid plastic. A “sparse” filling
option can be selected – where solid volumes are filled with a sparse structure.
The part will look the same, will print faster, use less material, but
will be more fragile. Use for visualization models.
Be sure to select appropriate units – millimeters or inches. The model
can also be scaled arbitrarily.
Manipulation and packing
The models' scale, orientation, and position on the print platen can be
When the STL file is loaded and ready for print, select the “Print” command.
Select the “preprocess only” option to prepare the CMB file but NOT to
dispatch it. This is useful to get an estimate of print time before printing,
or to preview the print. The estimated print time is correct to +/- 15%
for 85% of the models. The part can be printed or “packed”, in which case
it will not be printed now, but stored until it can be printed along with
additional parts on the same platter and print job. The nesting of geometris
in the volume is not controllable by the user.
At our request, Stratasys has provided us with a simple “Print Preview”.
This enables loading a CMB file and seeing the exact print trajectories
to be taken by the printer head, as well as the generated support structures
ahead of time. The program is called “pcview” and is installed on the NT
Hod Lipson, November 1999